Search

The objective of this thesis has been to develop understanding of microbiologically influenced corrosion (MIC), within the industrial context of diesel oil fuel (DOF) tanks in Royal Navy submarines. In addition, the possibility of a mitigation strategy using biotic sol-gel coatings is explored, with field trials taking place in Barrow-in-Furness (UK) and Santander (Spain) to develop knowledge surrounding biofouling. Firstly, MIC on S355 carbon steel was studied, using Desulfovibrio salexigens as a model SRB organism. This was compared to a consortium of bacteria, isolated from the DOF tank of HMS Vengeance. In both cases, electrochemical data show a non-conductive iron oxide film covering the surface developing after 12 days in sea salt solution. In growth media, a mixed film of iron oxide, iron sulfides and iron phosphate forms. Upon removal of surface films, confocal microscopy reveals pitting in all biotic samples, with the growth media having greater pitting than the sea salt solution. Furthermore, the SRB consortium shows greater pitting than the D. salexigens in both media. The consortium of bacteria were identified using 16S rRNA sequencing, with a notable absence of any species belonging to the genus Desulfovibrio, despite this genus being the mainstay of MIC research. The only culturable SRB present was of the genus Desulfomaculum. Biotic sol-gel coatings encapsulated the endospores of a number of 'protective' bacteria to study their antibiofouling properties. These coatings were applied to test panels and exposed to biofouling conditions over two trial periods of 5 and 7 months. Bacillus psychrosaccharolyticus showed the least biofouling in photographs of the panels and showed the lowest phylogenetic diversity of settled microorganisms. Bacillus baekryungensis showed the best anticorrosion performance, with no aluminium oxide spots after 37 days, and most inhibition of settled SRB.

The high temperature hydrogen attack (HTHA) of low alloy steels can result in the severe and unexpected structural failure of large petrochemical components. HTHA is a difficult to detect and monitor phenomena, of which there is poor scientific understanding of the underlying mechanism of its occurrence. The safe operation of materials in high temperature hydrogen (HTH) environments is governed by Nelson Curves, a series of plots of industrial failures related to HTHA. Nelson curves are increasingly revised downwards as HTHA repeatedly occurs unexpectedly within industry due to a fundamental lack in understanding as to its mechanism. Considerable work has been carried out previously looking at advanced HTHA exposed material but the mechanism behind the initiation of HTHA at the nano scale is not fully understood. In situ transmission electron microscopy (TEM) has become an increasingly important tool in the understanding of dynamic phenomena. Utilizing specialised in situ holders and hybrid sample preparation procedures, developed at the University of Manchester, it is now possible to examine metal-gas reactions in situ within the microscope. This work demonstrates that in situ scanning transmission electron microscopy (STEM) is an effective tool in enabling the direct observation of the chemical reaction of steel Fe3C carbides with HTH. Notably the direct reaction of carbides is demonstrated and experimental techniques refined to enable the visualisation of strain and dislocations formation at carbide-matrix interfaces. Furthermore, the initial formation of nano-scale cavitation on both steel carbides and grain boundary interfaces is demonstrated and the relationship between methane cavitation, density, and distance from a hydrogen exposed surface examined.

Cost-effective corrosion mitigation of offshore steel structures can be achieved by Thermal Spray Coatings (TSCs). These coatings, when comprised of Al, Zn and their alloys, provide a physical barrier against the environment when intact, and cathodic protection to underlying steel when damaged. As a result of the cathodic polarisation and subsequent increase in localised pH, calcareous matter (comprised of Mg(OH)₂ and CaCO₃) deposits on top of steel decreasing its corrosion. The study of thermal spray coatings onto steel with defects, such as those caused by mechanical damage or erosion remains largely unexplored. Electrochemical and surface characterization techniques were used in this PhD thesis to investigate the corrosion protection provided by TSCs on carbon steel substrates with and without defects in artificial seawater. Laboratory results showed that the presence of defects accelerates the formation of a protective corrosion product layer on the TSA coating. It was also revealed by Voltammetry and Electrochemical Impedance Spectroscopy (EIS) that both calcareous deposits and aluminium corrosion products hindered diffusion of dissolved O₂. The influence of the composition of seawater on the corrosion mechanism of TSCs was also studied. It was observed how, in order to recreate marine environments, it is crucial to prepare electrolyte solutions containing at least Ca²⁺, Mg²⁺ and SO₄², as these revealed by means of polarization resistances values to have high impact on the formation of corrosion deposits and calcareous matter. Thus, affecting the kinetics of both anodic and cathodic reaction. Lastly, validation of laboratory results was achieved by performing electrochemical measurements in situ. From the results here presented, it is expected to improve the design of corrosion protection systems with TSCs for offshore structures. This also translates in reduced costs and improved safety in marine environments.

The research reported in this doctoral thesis involves constructing physiochemical models that reproduce the transport behaviour of aqueous chemical species present in environments relevant to the oil and gas industry to gain an improved insight into the local electrochemistry near the electroactive surface (uniform corrosion) or inside the pit (pitting corrosion). The first part of the project involved constructing physiochemical models with one dimensional geometry with aqueous chemical species and chemical and electrochemical processes observed in oxygen (O2) containing brine environments to determine the changes in the local electrolyte composition and the potential within an initiated pit for a variety of external physical and chemical conditions. It was determined that the bottom of the pit suffers greatly from the effects of iR drop (Ohmic drop) if the pit geometry is taken to be macroscopic. The model was extended to include additional aqueous chemical species in conjunction with the chemical and electrochemical processes observed in carbon dioxide (CO2) rich environment to investigate the effects of CO2 on the local electrolyte chemistry at the bottom of the pit. It was found that the proton reduction electrochemical process on its own was incapable of supplying the high currents experimentally measured in CO2 environments via the buffering effect. The second part of the project was to investigate the influence of different experimental conditions on the polarisation behaviour of near static carbon steels in CO2 saturated brine electrolyte via multiple electrochemical measurement techniques. The key observation from this study was the presence of two distinct mass transport limited regions on the cathodic polarisation curve at natural pH (3.775). From the physiochemical model fitted to the experimental cathodic curve, the first mass transport limited region, occurring at lower cathodic potentials, was identified to be the direct reduction of carbonic acid while the second wave, occurring at slightly higher cathodic potentials, was shown to be the direct reduction of aqueous carbon dioxide. Based on the polarisation scans under forced convection, the rate of the direct reduction of carbon dioxide was determined to be under neither potential nor mass transport control. The third part of the project involved extending the existing one dimensional models to include the precipitation of salt films (iron chloride – FeCl2(s) and iron carbonate – FeCO3(s)) in O2 and CO2 saturated brine electrolyte respectively along with the capability to track their respective thickness. Furthermore, the ability of the underlying metal to undergo a change in its state from active to passive is implemented in the model via a set of rules based on the Pourbaix diagram. It was determined that the precipitation of salt films is greatly influenced by the mass transport with no or minimal thickness observed under even natural convection conditions. Furthermore the successful precipitation of salt film was determined to be a precursor step to the metal attaining passivation.

The corrosion fatigue behaviour of high strength carbon steel tensile armour wires that are used in flexible risers has been explored. An investigation of the corrosion fatigue failure mechanisms for two different sets of corrosion fatigue tested high strength steel wires has been carried out. The two different tensile armour wires were 12 mm x 4 mm and 12 mm x 7 mm. The wires had been corrosion fatigue tested in up to three different seawater environments, namely aerated, CO2 saturated to 1 bar absolute and 100 mbar absolute H2S-CO2 balance to 1 bar absolute. The corrosion fatigue failure investigation included undertaking statistical analysis of fatigue crack and corrosion pit data to establish the effects of environment, applied stress, R-ratio and microstructure due to degree of cold drawing on the corrosion fatigue behaviour. The 12 mm x 4 mm has fine grain martensite-pearlite structure with anisotropic microstructure in the transverse plane. The 12 mm x 7 mm has larger grain martensite-pearlite structure with equiaxed microstructure in the transverse plane. The corrosion fatigue crack path for the two tensile armour wires exhibits transgranular and intergranular cracking due to variations in R-ratio and microstructure. The analysis identified that a significant amount of localised corrosion pitting was present on the surface of both the 12 mm x 4 mm and 12 mm x 7 mm high strength carbon steel tensile armour wires and that many corrosion fatigue cracks had initiated from these geometric discontinuities. A method was developed in order to apply an optical image correlation technique to a sample immersed in seawater. The research has shown that digital image correlation may be applied for in-situ imaging of a corroding and dynamically deforming surface within a seawater environment. The technique demonstrated the establishment of localised surface strain around the corrosion pits during mechanical loading. The results of the surface strain mapping show that the interaction between multiple corrosion pits is consistent with a significant increase in surface strain when compared to a single surface pit acting alone. The results also show that a small single stress raiser can exhibit a high surface stress concentration when compared to a larger one as the strain is dependent upon the geometry of the pit. The highest strain concentration is at the edge of the pit, parallel to the loading direction. The results show the interaction that multiple pits have with each other, the effect they have on surface strains and how they and other types of stress raiser lead to premature failure of components. Further to this the effects of residual stress on crack nucleation were considered. Fatigue cracks initiate at the surface of the high strength carbon steel tensile armour wire therefore surface measurements were carried out to establish the effects of environment and applied load on the development of residual stress fields. The 12 mm x 4 mm wire shows some correlation between applied stress range and surface residual stress measurements with. For the 12 mm x 4 mm wire corrosion fatigue tested in aerated seawater the surface residual stress becomes increasing compressive with an increase in applied stress. For the 12 mm x 4 mm wire corrosion fatigue tested in CO2 saturated seawater the surface residual stress appears to be independent of applied stress. However for the 12 mm x 7 mm carbon steel tensile armour wire there is no correlation between the applied stress range and the surface residual stress. The differences in surface residual stress may be due to the differences in R-ratio, microstructure and level of cold drawing due to the Bauschinger effect. Surface residual stress measurements have been used to explore the effects of the shakedown process on the high strength carbon steel tensile armour wires prior to corrosion fatigue testing. They show that at a high applied stress range the shakedown process readily develops a compressive residual stress on the surface of the carbon steel wire. This is mostly the case for the low applied stress range; however care should be taken when considering the effects of shakedown on a lower stress range in so far as it may not completely remove the tensile residual stress. Through thickness residual stress measurements show a similar distribution of residual stress fields throughout the high strength carbon steel tensile armour wires independent of the applied stress range and environment.

Preferential weld corrosion (PWC) remains a major operational challenge that jeopardizes the integrity of oil and gas production facilities. It is the selective dissolution of metal associated with welds, such that the weld metal (WM) and / or the adjacent heat-affected zone (HAZ) corrode rather than the parent metal (PM). Corrosion inhibition is conventionally used to mitigate this problem however several indications suggest that some corrosion inhibitors may increase PWC. Furthermore, it is not possible to detect systems that are susceptible to PWC and or to understand the apparent ineffectiveness of some corrosion inhibitors at high flow rates. Consequently, the aim of this research is to assess the suitability of submerged jet impingement method to study flow accelerated preferential weld corrosion, which is critical to safe and economic operations of offshore oil and gas facilities. In this research, a submerged jet-impingement flow loop was used to investigate corrosion control of X65 steel weldment in flowing brine, saturated with carbon dioxide at 1 bar, and containing a typical oilfield corrosion inhibitor. A novel jet-impingement target was constructed from samples of parent material, heat affected zone and weld metal, and subjected to flowing brine at velocities up to 10 ms- 1 , to give a range of hydrodynamic conditions from stagnation to high turbulence. The galvanic currents between the electrodes in each hydrodynamic zone were recorded using zero-resistance ammeters and their self-corrosion rates were measured using the linear polarisation technique. At low flow rates, the galvanic currents were small and in some cases the weld metal and heat affected zone were partially protected by the sacrificial corrosion of the parent material. However, at higher flow rates the galvanic currents increased but some current reversals were observed, leading to accelerated corrosion of the weld region. The most severe corrosion occurred when oxygen was deliberately admitted into the flow loop to simulate typical oilfield conditions. The results are explained in terms of the selective removal of the inhibitor film from different regions of the weldment at high flow rates and the corrosion mechanism in the presence of oxygen is discussed.

Pitting corrosion is one of the most destructive types of metal loss. The purpose of this study was to investigate the evolution, or in other words, the propagation, of a single pit in carbon steel after the initiation stage. In view of the chemical and electrochemical reactions inside a single pit in carbon steel, a two dimensional model that allows the prediction of pit evolution was developed. Eleven species in aqueous sodium chloride solution and two neutral complexes were considered in the model. Given that the active-passive transition of a metal is a key phenomenon in pitting, the equations used to construct a Pourbaix diagram for iron were incorporated in the model as rules to govern passivation behaviour. By using the finite element package COMSOL Multiphysics as a tool, the Nernst-Planck equations for the mass transport and potential variations were solved. In addition, the multiphysics model was extended with Moving Boundary (ALE) mode to predict shapes of pits. The results of the study were that the model was able to investigate migration of ionic species, account for the active-passive transition of metal and also able to show the effect of solid precipitation. The model was able to show movement of the boundaries of a pit and hence, predict the shapes of pit at a given range of time. The results were discussed in comparison to the Pourbaix diagram of iron and compared with the experimental results and published models reported in literature. The principal conclusion is that modelling corrosion activities with integrated thermodynamic equations based on Pourbaix diagram methods is an outstanding way to model any other corrosion activities.

The goal of this thesis is to probe the functionality of 2-mercaptobenzimidazole (MBI) as corrosion inhibitor of carbon-steel in both strong and weak aqueous acidic solutions (HCl and H2CO3). To achieve this target electrochemical techniques have been employed, in combination with substrate analysis. Concerning aqueous HCl media, results demonstrate that MBI is an effective corrosion inhibitor, functioning essentially equally well in 1 M, 0.1 M, and 0.01 M HCl concentrations. X-ray photoelectron spectra suggest that MBI is typically bound to the surface in two tautomeric forms (thione and thiol). Furthermore, these data indicate that substrate termination varies as a function of both HCl and MBI concentration, with the interface consisting of MBI bound to film-free carbon-steel on highly inhibited substrates. In further work, the impact of dissolved oxygen, solution temperature, and immersion time on MBI performance in HCl solutions has been assessed. The latter two parameters have considerable influence on MBI inhibition efficiency. More specifically, it was found that MBI decreases dramatically its inhibition efficiency between 60°C and 70°C in 1 M HCl, and also apparently work less well as substrate immersion time increases. As regards MBI performance in deaerated CO2-saturated NaCl (0.62 M) solution, results demonstrate that MBI effectively inhibits corrosion within the parameter space explored i.e. solution temperatures of 30°C and 55°C and total applied pressures (p(H2O) + p(CO2)) of 1 bar and 20 bar. The performance of MBI does not vary greatly for different combinations of these temperatures and pressures. Post immersion substrate characterisation with XRD and SEM indicate that no significant surface scaling occurs under these conditions.

Internal corrosion of transmission tubulars is a huge concern in the oil and gas industry. Corrosion inhibitors (CIs) are often considered the first step in mitigating internal corrosion due to their high efficiency and cost-effectiveness. Yet, predicting the efficiency of corrosion inhibitors, developed and tested in a laboratory environment, in operating field conditions is very challenging. In addition, the presence of corrosion residues or corrosion products on the internal surface of tubular steels can significantly affect the inhibition performance of organic corrosion inhibitors. This aspect is only rarely considered when characterizing the performance of corrosion inhibitors. Therefore, understanding their effects on corrosion inhibition is of great benefit in applying corrosion inhibitors to tackle internal corrosion issues, particularly in aging pipelines.This work mainly focuses on evaluating the corrosion inhibition and revealing the inhibition mechanisms in the absence and presence of various corrosion residues or products, commonly found in oil and gas production. The first half of this work (Chapter 5 and 6) presents a methodology for the characterization of corrosion inhibitors and proposes several innovations to an inhibition prediction model, originally based on the work of Dominguez, et al.. An inhibitor model compound, i.e., tetradecyl phosphate ester (PE-C14), was synthesized in-house and characterized to obtain necessary parameter values required as inputs for the inhibition model. The updated inhibition model could predict steady state and transient corrosion inhibition behaviors with good accuracy.The second half of the presented work (Chapter 7, 8, and 9) focuses on the effects of corrosion residue (Fe3C) and products (FeCO3 and FeS) on corrosion inhibition and advances the understanding of the associated inhibition mechanisms. The galvanic effect caused by residual Fe3C on corrosion rate and inhibition efficiency was quantitatively evaluated using different steel types and inhibitors, and the associated mechanisms were clearly defined. The inhibition mechanisms in the presence of iron carbonate layer, considering different levels of surface coverage, were also investigated. Results showed a synergistic effect between a partial coverage of FeCO3 and inhibitor, but no additive effect was observed with a full coverage of FeCO3. The role of the presence of thin mackinawite film on corrosion inhibition was also studied. While this aspect was not studied in enough depth to draw definitive conclusions, no deterioration of the inhibition efficiency was detected in the presence of mackinawite, in the conditions tested. This work also highlighted that some of the behaviors observed through this study can be highly inhibitor dependent and that proper characterization of corrosion inhibitor performances, over a representative, albeit limited, range of conditions, is still warranted. This work advanced the understanding of the inhibition mechanisms in the presence of these corrosion residue and products and proposed a way forward to address the remaining gaps.

Current obstacles that prevent commercial implementation of amine-scrubbing CO₂ capture are the high costs. Reducing capital costs by appropriate selection of construction materials, which requires knowledge of material corrosion performance for the process, will improve the economic feasibility of this technology. Corrosion was evaluated in three pilot plant campaigns using aqueous piperazine with the Advanced Stripper (PZAS). 316L stainless steel (SS) experienced higher corrosion than 304 SS and 2205 duplex SS, and the corrosion rate showed strong dependence on the temperature. 304 and 2205 performed well at all locations and should be good construction materials for PZAS. Degraded PZ exacerbated 316L corrosion, and removal of PZ degradation products using a carbon adsorption bed significantly reduced corrosion. Carbon steel (CS) corrosion showed a weak temperature effect because the corrosion was more dependent on the protective siderite film. The protectiveness of the films was related to fluid velocity. Ni-based alloys corroded in PZ, and the rate increased with temperature. Corrosion of C1010 CS and SS (304, 316L, 430) was measured at absorber and water wash conditions on the bench-scale. Corrosion rate decreases with increasing PZ. With more than 0.003 m PZ in solution, CS has acceptable corrosion performance. Corrosion of CS increases with increasing partial pressure of CO₂, suggesting loading is another dominant parameter for carbon steel corrosion. Temperature has a less significant effect than PZ concentration and loading. CS corrosion increases with increasing flow velocity at both absorber and water wash conditions. SS had little corrosion at this lower temperature. Performance of siderite (FeCO₃) protective films on CS was studied at representative stripper conditions on the bench-scale. Siderite films can deposit on the surface of CS in CO₂-loaded PZ solution at temperatures >100 °C and protect CS from corrosion. The protection may fail in degraded PZ. Ethylenediamine (EDA) is one of the major contributors for the loss of film protectiveness or can be the surrogate for the effect of PZ degradation on siderite film protection. A link between protectiveness and the apparent density of siderite films was discovered. The apparent density of siderite films decreases with increasing flow velocity and decreasing CO₂ loading, resulting in higher corrosion of CS.

Gas hydrate formation and corrosion can cause serious safety and flow assurance problems in subsea environments. One aspect that has been given less attention is the corrosion behavior of materials in gas hydrate formation environment (GHFE). This work introduces a new technique/method for corrosion testing of materials exposed to low temperatures GHFEs. This technique allows pH monitoring, and control of test conditions like temperature. In this work, GHFE is defined as an environment that includes water, methanol and its degraded products in the presence of corrosive agents like CO2 and chloride salt at gas hydrate formation temperatures (GHFT). After 20 hrs immersion in CO2-saturated salinity environment at GHFT, as-deposited Ni-Mo alloy coating has the highest corrosion resistance of 33.28 kΩ cm2. The corrosion resistance dropped to 14.36 kΩ cm2 and 11.11 kΩ cm2 in the sweet low-salinity and sweet high-salinity test solutions respectively. The combined results of SEM/EDX showed that the Ni-Mo coating oxide layer broke down quicker in sweet high-salinity environment than sweet low-salinity environment. When carbon steel was immersed in a CO2-saturated high salinity environment at GHFT, there was slight overall change in corrosion rate (CR) as salt concentration increase from 3 wt% to 25 wt%. In degraded methanol environment, methanol showed an inhibitive effect on the corrosion of carbon steel. Higher methanol content (up to 50 vol. %) increased the corrosion rate of carbon steel at gas hydrate formation temperature, however, the corrosion rates were lower with methanol contents between 10 to 20 vol%.

As geologic environments associated with oil and gas production have become increasingly aggressive, aqueous corrosion at high temperatures in the presence of hydrogen sulfide (H2S) is more frequently encountered. The understanding of sour corrosion mechanisms is an important but still largely elusive target, especially at high temperatures. The purpose of this project is to explore the thermodynamics and kinetics of H2S corrosion at high temperature, and to develop a thermodynamic model to predict the corrosion product layer formation, as well as a mechanistic kinetic model to predict the corrosion rate of mild steel at high temperature in the presence of H2S.The first part of the project focused on the development of experimental and safety procedures to investigate layer formation and corrosion mechanisms in high temperature environments. This included the development and validation of a water chemistry model for a closed system especially designed to properly control the experimental parameters.In the second part of this project, the effects of temperature (80~200°C), exposure time (1~21 days), and partial pressure of H2S (0.1~2.0 bar) were thoroughly investigated. Significant and somehow unexpected findings were obtained: A Fe3O4 layer was always identified on the steel surface although this type of corrosion products was thermodynamically less stable than FeS. Fe3O4 formed very fast at the initial stage of corrosion and was responsible for the quick decrease of the corrosion rate. The Fe3O4 layer experienced a continuous process of formation (due to corrosion at the steel/Fe3O4 interface) and conversion to iron sulfide (at the Fe3O4/FeS interface). The transformation of the outer iron sulfide layer was also observed at high temperature and thoroughly documented for the first time. The general transformation sequence was identified as mackinawite, troilite, pyrrhotite, pyrite. With the increase of temperature, time, and partial pressure of H2S, iron sulfide transformed to thermodynamic more stable state. The roles of these different layers in corrosion were also examined and discussed. The outer FeS layers also formed via a precipitation mechanism from the bulk solution.An overall mechanism for the corrosion of carbon steel in high temperature H2S environments was proposed based on the experimental observations. Existing thermodynamic and kinetic models were adapted following a mechanistic approach and validation was performed with experimental data. By keeping the formation region of Fe3O4 in an H2S environment, the main modification to the formation region of Fe3O4 in an H2S environment. The kinetics of Fe3O4 formation and conversion were determined experimentally and were successfully incorporated into the mechanistic kinetic model. Corrosion trends and rates, as well as layers thickness could be predicted with relative accuracy (less than 20% error).

Corrosion of carbon steel is a potential hazard to oil and gas production infrastructure. The total annual cost of corrosion in the oil and gas production industry is estimated to be $1.372 billion in the USA (Simons, 2008). Over 20% of the total corrosion cost is caused by microbial induced corrosion (MIC) or biocorrosion (Javaherdashti, 2008). MIC of the carbon steel infrastructure in oil and gas production facilities is most often associated with the activity of sulfate-reducing bacteria (SRB). However, a different biocorrosion mechanism was presumably observed in the pipelines in the Putumayo Basin, southwest Colombia, South America and production water storage tanks in the Barnett Shale, north Texas, USA. Both sites typically experienced very specific types of corrosion and/or MIC. CO2 corrosion was considered as the corrosion mechanism in Putumayo Basin sampling sites, while SRB induced corrosion was presumed to be the main corrosion mechanism in Barnett Shale storage tanks. However, in the Putumayo Basin production water samples, the pH of all samples was circumneutral which didn’t support typical CO2 corrosion mechanism. The total dissolved iron ions species (ferrous and ferric species) concentration was 6 to 8 orders of magnitude higher than the equilibrium iron concentration with respect to solid-phase Fe(OH)3. The Shewanella genus, which is known to contain iron-reducing bacteria (IRB), comprised 30% of the microbes detected in Sucombio production water sample from the Putumayo Basin. In Barnett Shale production water samples, only one sample indicated it was impacted by the typical SRB corrosion mechanisms. The four samples lacked sulfate reducing activity and had no FeS precipitate or had sulfate reducing activity but FeS was only 14% to 21% of the corrosion solid phase products. These four samples had dissolved iron ions concentrations 2 to 5 times higher than the equilibrium iron concentration of solid-phase FeCO3 and contained bacteria orders with species that can act as both SRB and IRB. As such, the Putumayo and Barnett Shale results indicated that there were other microbial induced corrosion mechanisms in the sampling sites besides that facilitated by SRB. The research questions of this study are: 1) what caused the high total dissolved iron concentration in the production water samples from Putumayo Basin and Barnett Shale, and 2) can the high total dissolved iron concentration shift the function of microorganisms from sulfate respiration to ferric respiration. It is hypothesized that organic ligands chelated with Fe(III) in the ferric oxyhydroxide layer to form soluble ferric-ligands complexes, resulting in the high total dissolved iron concentration. The increased availability of dissolved ferric ions shifted the function of microorganisms from sulfate respiration to ferric respiration, thereby allowing microbes with iron reducing abilities to take advantage of the thermodynamic and kinetic gains in energy to thrive. Gas chromatography/mass spectrometry analysis showed a positive relationship between the concentration of organic metabolites and the total dissolved iron concentration in Putumayo production water samples, and a positive linear relationship was found between iron-chelating molecules and the total dissolved iron ions concentration in Barnett Shale production water samples. These results support the hypothesis that organic ligands chelated with Fe(III) and caused the high dissolved iron concentration. Evidence supporting the hypothesis that the high concentration of the total dissolved ferric ions shifted the microbial community to favor microbes capable of respiration ferric ions was supported by the atomic ratio of S/Fe in particulates collected from biofilms, as well as sulfate reduction assays and microbial community analysis results.

Corrosion influenced or driven by the presence or activities of microorganisms is known as biocorrosion or microbiologically influenced corrosion (MIC). MIC has been recognized as a significant problem in many industrial systems such as power plant cooling systems, oil and gas transportation, and water utilities. In nature, different types of microorganisms live in a community that attaches to materials such as metals to form a biofilm. Microbes in the biofilm are responsible for MIC. Thus, it is critical to understand the corrosion mechanisms that are caused by different microbes and find better ways to treat biofilms.The main importance of this work is listed below:(1)H2S is not the primary contributing factor in carbon steel MIC by sulfate reducing bacteria (SRB).(2)Sulfate reducing archaeon (SRA) is corrosive against C1018 carbon steel at 80oC because SRA can utilize extracellular electrons for sulfate respiration just like SRB.(3)A nature-inspired anti-biofilm peptide at ppb (w/w) levels are found in lab tests to enhance a popular commercial biocide in treating a corrosive oilfield biofilm consortium on C1018 carbon steel.Many people mistakenly believe that biogenic H2S is the major contributing factor in sulfate reducing microbes (SRM) MIC on carbon steel. In this project, a nitrate reducing Pseudomonas aeruginosa biofilm that does not have H2S corrosion complication was evaluated for its corrosivity against carbon steel. It was found that P. aeruginosa grown as an NRB (nitrate reducing bacterium) was very corrosive against C1018 carbon steel because starved sessile cells switched from organic carbon to elemental iron as electron donor, thus causing more corrosion. A causal-relationship experiment was designed to prove a hypothesis that the Fe2+ acceleration of SRB MIC of carbon steel is primarily attributed to reduced H2S toxicity that leads to a higher sessile cell count, rather than an elevated [H2S] during incubation. A higher Fe2+ concentration in the culture medium can counter the toxic effects of H2S to promote SRB planktonic and sessile cell growth with increased dissolved [H2S]. A previous work demonstrated that lower [H2S] due to H2S escape to a larger headspace led to accelerated corrosion because of better sessile cell growth. In the two cases, [H2S] had opposite trends, suggesting that H2S was not the primary causal factor in the corrosion. In both cases, the increase of weight loss was due to increased electron uptake as evidenced by the increased sessile cell count in both cases. The combination of the headspace work with the Fe2+ work provides conclusive evidence that H2S is not the primary contributing factor in carbon steel MIC by SRB, thus solving a long-time mystery in SRB MIC of carbon steel.There are lots of data on MIC at mild temperatures such as 37oC. However, in a deep reservoir, much higher temperatures (e.g., 80oC) are encountered. Archaeoglobus fulgidus, a thermophilic SRA, was found to be corrosive against C1018 carbon steel at 80oC. It was also found that under different levels of carbon source starvation, the thermophilic SRA became more corrosive against carbon steel. This work proved that SRA also can utilize extracellular electrons for sulfate respiration just like SRB. This will inspire new research into the extracellular electron transfer (EET) mechanisms in archaea. In enhanced oil recovery (EOR), polymers are usually injected to increase the viscosity of injection water that is needed the push out viscous crude oil. EOR polymers, which are organic molecules, may be utilized by microbes as an organic carbon source. In this project, a commercial EOR polymer was degraded by an oilfield biofilm consortium (labeled as Consortium II) in an artificial seawater medium during a 30-day incubation period resulting in reduced viscosity by 34.5%. An efficient biocide treatment in the presence of EOR chemicals and other oilfield chemicals is desired. An equimolar D-amino acid mixture (D-mix) containing four different D-amino acids (D-tyrosine, D-methionine, D-leucine, and D-tryptophan) was tested to enhance tetrakis hydroxymethyl phosphonium sulfate (THPS) against Consortium II in the presence of EOR chemicals and other oilfield chemicals in this work. The combination of 100 ppm (w/w) THPS + 100 ppm (w/w) D-mix achieved further logarithmic reductions of sessile cell counts compared with the 100 ppm THPS alone treatment, thus further reduced carbon steel coupon weight loss and pitting corrosion. The test results suggest that D-amino acids are compatible with the other oilfield chemicals.Facing escalating biocide dosages, it is desirable to find biocide enhancers that make biocides more effective in biofilm treatment. Peptide A, a 14-mer cyclic peptide, was inspired by the anti-biofilm Equinatoxin II protein found in a sea anemone that exhibits biofilm-free exteriors. Peptide A at ppb levels was found to enhance 100 ppm THPS in treating biofilm Consortium II on C1018 carbon steel in an enriched artificial seawater medium. The enhanced biocide treatment reduced sessile cells on the carbon steel thus decreasing the coupon weight loss and pitting corrosion. Properly conducted electrochemical measurements such as linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and potentiodynamic sweep (PDS) can be used to support weight loss and pitting data and provide transient corrosion trends in MIC mechanistic studies and in biocide efficacy assessment.

Microbiologically influenced corrosion (MIC), also known as biocorrosion, is a major problem in the oil and gas industry and water utilities. The same problem also exists with biomedical implants inside the human body. Although these medical implants are usually made of corrosion resistant stainless steel alloys or titanium, they still can experience MIC. MIC problems that occur on implants can be even worse sometimes because some places inside the human body (e.g., mouth) contain more bacteria and are more ideal growth environments than pipelines. Among all the microorganisms that cause MIC, sulfate-reducing bacteria (SRB) are often blamed because sulfate is ubiquitous in nature. Chapter 1 discussed various MIC issues facing different industries. Chapter 2 provided a review of various microbes that cause MIC problems on carbon and stainless steels.In order to mitigate MIC, the underlying mechanism must be analyzed. The Ohio University MIC Research Group proposed the biocatalytic cathodic sulfate reduction (BCSR) theory, discussed in Chapter 2, which not only explains the bioelectrochemistry of the SRB MIC process but also overcomes the shortcoming of the previous cathodic depolarization theory (CDT) that is only applicable to hydrogenase-positive SRB. Chapter 4 confirmed the BCSR theory with SRB starvation and overfeeding tests using 304 stainless steel. It was confirmed that SRB MIC is due to SRB harvesting electrons from extracellular iron oxidation for sulfate reduction in order to generate energy when there is a local shortage of organic carbon molecules. Chapter 6 investigated the impact of liquid/headspace volume ratio on MIC by Desulfovibrio vulgaris. It was found that a large headspace allowed more H2S to escape from the culture medium to the headspace and this increased culture medium pH, however, did not statistically influence the weight losses of coupons in 100 ml culture medium and in 50 ml culture medium in the 125 ml vials. It was recommended that future tests should use a fixed volume of the culture medium with different vial sizes.Biofilms cause MIC, thus, MIC mitigation is about biofilm treatment. Biocides have been used to treat biofilms. However, prolonged use of biocides leads to microbial resistance and thus dosage escalation. One way to combat this is to use biocide enhancers that make biocides more effective. D-amino acids are naturally occurring chemicals that have been found to disperse biofilms. Some of them, such as D-tyrosine and D-methionine, have been proposed as biocide enhancers in the literature. In Chapter 3, D-leucine, D-aspartic acid, and D-asparagine were investigated individually to see whether they could be enhancers of tetrakis (hydroxymethyl) phosphonium sulfate (THPS) and determine their efficacies against D. vulgaris biofilms on carbon steel. The results showed that D-leucine and D-aspartic acid demonstrated a promising enhancing ability for THPS at concentrations of 1000 ppm and 400 ppm, respectively. A D-alanine substitution test was conducted to confirm the mechanism suggesting that D-amino acids work as biocide enhancers through substitution of the D-alanine terminus in the peptidoglycan molecules in the bacterial cell wall. A different way to mitigate biofilms other than using biocides is to use anti-bacterial stainless steels. Chapter 5 investigated the copper-containing 2205 duplex stainless steel’s biofilm inhibition abilities. It was found that this novel metal inhibited the nitrate-reducing Pseudomonas aeruginosa biofilm. However, it promoted the growth of the D. vulgaris biofilm.

Carbon steels and low-alloy steels are often used in various stages of the refining process in petrochemical industries and power plants where they are susceptible to graphitization after prolonged exposure at temperatures of 800°F (427°C) or above. Graphitization is a result of solid-state phase transformation of metastable iron carbide to form iron and graphite structure. The formation of graphite results in the loss of tensile strength, ductility, and creep strength, which may result in untimely catastrophic failure of the component. The current study focused on developing a further understanding of graphitization on ex-service welded carbon steel components, which were removed from service after 17 years at 800°F (427°C). The materials received were examined using metallography, hardness, creep-testing, bend tests, Gleeble thermal simulation, Cryo-cracking, and heat-treatment for further fundamental studies of graphitization. In the materials examined, graphitization was primarily observed in a narrow band just outside of the visible HAZ which corresponds to a weld thermal cycle just below the lower-critical transformation temperature (Ac1). A hypothesis to support for the increased amount of graphitization observed in the base metal just outside of the HAZ was proposed. The materials examined also exhibited increased amount of graphitization with the increase of aluminum content. A hypothesis to support this increase in graphitization with aluminum content based on thermodynamic principles was also proposed in this study. Creep studies using accelerated stress and temperature conditions were performed to evaluate the remaining service life of the graphitized components. An accelerated heat-treatment studies were performed to obtain the time-temperature behavior of graphite nucleation and growth. A potential in situ inspections studies using both non-destructive (ultrasonic inspection) and destructive methods (core extraction) were studied. In situ studies showed possible viable approach towards the inspection of graphitized carbon steel components for run, repair or replace decisions. Repair welding performed on core extraction sites using friction hydro tapered pillar processing (FTHPP), showed viable approach.

Sulfate reducing bacteria (SRB) are common culprits of microbiologically influenced corrosion (MIC) that has been reported to cost $138 billion annually in the United States. Most literature reported the study results of SRB-induced corrosion when organic nutrients were provided to bacteria. But SRB-metabolizable organic substrates are not always available in the field conditions. There is a clear need to identify how SRB can induce pitting under the condition of long term starvation of organic substrates. The goals of this work are to elucidate the corrosion mechanisms of organic starving SRB on carbon steel (C1010) and propose possible MIC mitigation approaches. The specific objectives are: (1) monitoring MIC by correlating results of electrochemical impedance spectroscopy (EIS) and potential difference (PD) with measurements of bioactivities, biofilm and corrosion deposits; (2) development of a faster methodology for pit characterization to significantly reduce characterization time; (3) explication of the survival and corrosion mechanisms of a well-known SRB, D. vulgaris under long term starvation; and (4) development of possible MIC mitigation approaches. Coupons and sulfate-reducing biofilms were examined by confocal laser scanning microscopy (CLSM), infinite focus microscopy (IFM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Bioactivities in the liquid phase were measured by high-performance liquid chromatography (HPLC) and various biological assays. Metal-biofilm interfacial layer evolution was monitored by EIS and PD. Our results supported the following conclusions: (1) EIS may be used for online monitoring of biofilm and corrosion product evolution, but the signals are complicated and require more systematic studies to improve understanding and ensure correct interpretation. Under certain conditions, pitting occurrence might be detectable from inner layer porous resistance and PD profiles. (2) Empirical correlations that enable fast estimation of maximum and average pit depths and pitted area percentage from the standard 3D surface parameters obtainable with IFM were established. (3) D. vulgaris, a common H2-utilizing SRB can survive on carbon steel up to 46 days under organic starvation by coupling direct electron uptake from steel surface with sulfate reduction. Direct cell attachment to metal is essential for this survival mechanism and the resultant pitting corrosion. (4) Future mitigation approach should target at preventing direct cell attachment or killing all the cells of biofilm including innermost layer of cells. KH2PO4 was demonstrated to be a promising MIC inhibitor by significant reduction of sessile SRB number and inhibition of pitting and intergranular corrosion.

Transportation of multiphase fluids using carbon steel pipelines is ubiquitous in the oil and gas industry. These pipelines are prone to internal corrosion when exposed to an aqueous CO2 environment. The mitigation of CO2 corrosion can be achieved by the use of organic corrosion inhibitors or by reliance on formation of protective corrosion product layers. However, the mechanisms for their potential failure in flow conditions are still being debated.Wall shear stress induced by turbulent multiphase flow is considered an important parameter in CO2 corrosion as it has been claimed to be responsible for the removal of protective corrosion product layers and corrosion inhibitor films. In this study, a floating element wall probe was used to directly measure the wall shear stresses in single-phase and horizontal gas-liquid two-phase flow, instead of the more common indirect measurement of the wall shear stress. Wall shear stress measurements were complemented by high speed video camera recordings of the flow field. In single-phase pipe and channel flow, the wall shear stress was in good agreement with empirical shear stress calculations. In two-phase pipe flow, video-recorded observations confirmed that the high wall shear stress pulses captured by the probe in the slug flow pattern were in sync with the passage of liquid slugs; the highest measured wall shear stress values were of the order of 100 Pa. Wall shear stress values in the slug body varied along the inner pipe circumference with the top of the pipe having the highest values and the bottom of the pipe having the lowest values. The maximum wall shear stress measured was about 2 to 4 times higher than the calculated mean wall shear stress in the slug body, obtained by using the mixture velocity, which can serve as a guideline for slug flow modeling. Findings suggest that the wall shear stress alone, produced in single-phase and multiphase flow patterns covered in the present study, is insufficient to mechanically damage the protective corrosion product layers or corrosion inhibitor films. In addition, corrosion experiments with an imidazoline-based inhibitor were performed on X65 pipeline steel in a thin channel flow cell (TCFC). Local flow velocities were up to 28 m/s with wall shear stresses up to 4.8 kPa. Electrochemical measurements, surface analysis and computational fluid dynamics (CFD) were used as diagnostic tools. Localized corrosion was observed on a protrusion in the flow cell and depended on local flow conditions and inhibitor concentration. Overall, the wall shear stress was unable to affect performance of corrosion inhibitors. However, the low static pressure at the protrusion caused cavitation with bubble collapse leading to accelerated desorption of inhibitor from the steel surface, which explained the localized corrosion. An excess amount of inhibitor was found to mitigate localized corrosion by cavitation.

Abstract: This thesis is focused on the studying the role that model sulfur-containing molecules have in corrosion reactions on relevant metallurgies. This was done by using a design of experiments where a number of sulfur-based compounds were reacted with various metallurgies. The 8 model sulfur compounds were chosen to represent a broad set of structures: two mercaptans (M1 and M2) with M2 having a longer chain than M1, an aliphatic sulfide and disulfide (LS and LS), an aromatic sulfide and disulfide (RS and RD), and two cyclic thioethers (C1 and C2) with C2 containing more hydrogen than C1. The 5 metallurgies used were: 5 chrome alloy, 9 chrome alloy, 410 steel, 316 stainless steel, and carbon steel. In the first round of experiments, the compounds were tested by placing 5000 ppm by weight of sulfur of a respective sulfur compound into 9 mL of Paraflex white mineral oil along with a rectangular carbon steel coupon in a sealed reactor, pressurized to 40 psig, and held at a medium temperature for a fixed set of time. The next round of experiments replaced the rectangular coupons with 1-inch extruded wire sections 200 µm in diameter of the five metals. Each of the eight compounds were reacted with the carbon steel wires at two process temperatures, a medium temperature and a medium-high temperature, and two experiment times, a short experiment time and a long experiment time. The remaining four metallurgies (5Cr, 9Cr, 410 steel, and 316 stainless steel) were reacted with each of the eight sulfur compounds only at the most extreme conditions of the medium-high temperature and reacted for the long experiment time. Once again, for these experiments the compounds were measured to 5000 ppm by weight of sulfur and placed with 9 mL of Paraflex white mineral oil into the sealed and pressurized reactor along with the extruded metal wire. After reacting, these wires were mounted in nickel-infused epoxy and polished for SEM and EDXS analysis, or left unmounted and used for XRD analysis. Overall, in these corrosion experiments, the 316 stainless steel remained nearly pristine after reacting with any of the sulfur compounds, while the carbon steel samples were heavily corroded. To react the metals with pure H2S, a sealed tube furnace was sparged and filled with 5000 ppm H2S gas (5174 ppm H2S, 10.19% hydrogen, argon balance) and heated to either a medium or medium-high temperature with the extruded metal wires supported vertically inside in the tube. After reacting for either a quick, short, or long experiment time, these wires were then removed and mounted in nickel-infused epoxy and polished for SEM and EDXS analysis. The decomposition temperature of each sulfur compound was recorded as the lowest measured temperature that H2S was detected in the headspace gas of the sealed reactor after reacting for a long experiment time at the respective temperature in pure Paraflex mineral oil. This was determined by either increasing or decreasing the reaction temperature in 5 °C increments until the temperature was found such that H2S was first detected. As expected given their thermal stability, the cyclic thioethers, C1 and C2, did not decompose into H2S in any of our experiments, up to the limit of temperatures we were capable of reaching with our equipment. On the other hand, the aliphatic disulfide, LD, readily decomposed into H2S at temperatures lower than the medium corrosion experiment temperature, likely due to the overall structure and properties of the compound.

H2S localized corrosion, occurring at discrete sites on a steel surface, can result in fast penetration of pipeline walls and loss of containment. This mode of metal attack is generally considered to be the main cause for catastrophic corrosion failures of facilities in the oil and gas industry. Hence, the prediction and control of H2S localized corrosion is a significant challenge for assuring asset integrity in oil and gas fields containing H2S. The purpose of this dissertation project is to explore, heretofore poorly understood, localized corrosion mechanisms related to the formation of iron sulfide polymorphs and related phases.In order to make this investigation possible, a thermodynamic model was initially developed as a tool to determine experimental conditions that could potentially replicate localized corrosion associated with iron sulfide polymorphism in an aqueous H2S system. Equilibrium expressions for H2S solubility and dissociation constants were reviewed and compared. Models to predict water chemistry of an H2S-H2O system were built and verified against experimental measurements. In order to predict the formation and dissolution of iron sulfides, their solubility limits were experimentally measured in an H2S-H2O-Fe2+ system. At 25oC, the measured pKsp,2 values were observed to change with time as the identity of the observed iron sulfide type changed. The pKsp,2 of mackinawite at 25oC was measured as 3.6 ± 0.2. Pyrite and greigite were observed at 60oC. Greigite was dominant around pH 5 with a pKsp,2 9.8 ± 0.5, while pyrite was dominant around pH 3.5 with pKsp,2 6.5 ± 0.5.The electrochemical thermodynamics of an H2S-H2O-Fe system were then investigated, with iron sulfides selected in relation to the oil and gas industry. Mackinawite, pyrrhotite, greigite, and pyrite were taken into consideration for Pourbaix diagram generation, accompanied by a complete accounting of all the assumptions, underlying thermodynamic data, and reaction mechanisms. Generated Pourbaix diagrams were validated by long-term experiments at different temperatures (25oC and 80oC) and by adjusting solution pH. Following the establishment of the thermodynamic model, experimental conditions leading to the formation of different iron sulfides as corrosion products in a sour environment were established. A strong correlation between the formation of greigite and/or pyrite on a steel surface and onset of localized corrosion was observed. Localized corrosion was absent when neither greigite nor pyrite formed. Consequently, the formation of greigite and/or pyrite was hypothesized to play an important role in the initiation of localized corrosion. Novel experiments involving deposition of pyrite on the steel surface were then designed and conducted. It was found that the galvanic coupling between pyrite particles and steel is the dominant mechanism for this type of localized corrosion.Finally, a descriptive model was built to answer when, where, and how this type of localized corrosion occurs in a sour environment. This model can provide guidance for the mitigation of localized corrosion in field conditions.

The mechanism of carbon steel corrosion in H2S environment (sour corrosion) has been investigated since 1950s and still poorly understood now. This obstacle hinders the development of an effective protocol for corrosion control and protection in sour environment. The goal of this thesis project is to understand H2S corrosion mechanism through systematic experimental studies and to build a mechanistic corrosion model to simulate the corrosion process at different conditions, including H2S partial pressure, pH, flow rate, and temperature. The first part of the project investigated electrochemical behavior of carbon steel corrosion in pure H2S environments. Initially, the uniform H2S corrosion mechanisms were experimentally studied in short term corrosion experiments (lasting 1-2 hr) before any significant interference from iron sulfide corrosion product layers occurred. Corrosion rates were obtained by linear polarization resistance (LPR). Mechanisms related to H2S/CO2 corrosion were investigated using potentiodynamic sweeps and by comparison with electrochemical modeling. LPR results showed that corrosion rates increased with increasing temperature, partial pressure of H2S, flow rate and decreasing pH. Results of potentiodynamic sweeps show the presence of H2S could affect both cathodic reactions and the anodic reaction. An electrochemical model was developed and can be used to predict the effect of temperature, pH, pH2S and flow on corrosion mechanisms of mild steel in aqueous solutions containing H2S in the absence of protective iron sulfide layers.In the second part of the project, the combined action of H2S and CO2 corrosion was investigated. Experiments were conducted at the different H2S concentrations ranging from 0 to 10% in the gas phase at 1 bar total pressure at pH 4.0 and pH 5.0. Results showed that the presence of H2S slowed the charge transfer kinetics related to H2CO3 and H2O reduction reactions at the steel surface. An electrochemical corrosion model was developed for a mixed H2S/CO2 system which was calibrated with new experimental results and compared to data in the open literature.The third part of the project investigated the effect of the iron sulfide corrosion product layer on H2S corrosion and kinetics of iron sulfide formation. The existence of the thin “inner” iron sulfide layer and its effect on H2S corrosion were clarified based on literature research. The effect of the “outer” iron sulfide layer was investigated using a new experimental set-up which permitted continuous replenishment of fluid to control the surface water chemistry, especially the pH. The effect of pH, flow rate, and temperature on iron sulfide corrosion product layer growth and corrosion rate was examined. High pH, low flow rate and increased temperature lead to a higher precipitation rate of iron sulfide on the steel surface and to the formation of a protective iron sulfide layer. Finally, a comprehensive mechanistic transient model of uniform CO2/H2S corrosion of carbon steel has been developed, covering three main processes underpinning corrosion: aqueous chemical reaction in the bulk solution, electrochemical reactions including the mass transport between the bulk solution and the steel surface, and a corrosion product growth model for iron carbonate and iron sulfide layers.

Zinc rich epoxy primers are an effective corrosion protection method to protect the steel structures against an aggressive environment. An aggressive environment like marine environment (high content in chloride ions) degrades the metal structure until a complete failure of the metal system. A way to avoid the degradation is to isolate the system form the environment. Zinc rich primers protect the steel by the sacrificial action of the zinc dust particles. A zinc corrosion products barrier layer protects the steel by blocking the passage of the aggressive species like oxygen and chlorides that favor the cathodic reaction. In this thesis, the corrosion protection properties of a commercial formulation (zinc rich primer with CNTs) system and prototype formulations (Zn/CNTs ratios) were studied as a function of the immersion time. Electrochemical corrosion measurements were implemented in non-dearated electrolytes of 0.6M NaCl (3.5 wt% NaCl) in distilled water with a pH=6. Besides the NaCl solution, alkali-halides solution identify the chloride effect with the change in the cation. The transport properties of these solutions defined the 0.6M NaCl the more aggressive solution to the bare metal and the coating-metal system..Electrochemical Impedance Spectroscopy is the main AC technique to characterize the system and the DC Polarization curves, potentiostatic (galvanic couple) and OCP. Localized electrochemical impedance characterizes the commercial system to define the anodic and cathodic zone once the sample is immerse in the solution. ASTM-B117 was employed to evaluate the commercial and the prototye system. The samples were divided into two sections and one with 1 inch long indentation and tested by 30 days. High resolution techniques like an atomic force microscope (AFM) and scanning electron microscopy (SEM), characterizes the surface before and after immersion. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) identify the corrosion product formation to validate the corrosion protection properties. A long-term immersion of 360 days was followed for the commercial samples. Intact and pre-damage samples compared the corrosion protection mechanism of the zinc rich primers with CNTs. The potential change after the final day was close enough to consider the two types of samples were protected by means of the coating system. EIS shows the system with the pre-damage develop the cathodic protection mechanism once immersed in the electrolyte. On the other hand, the intact coating follows a barrier effect and then the cathodic protection system. A short-term immersion of 10 days was followed for the prototype systems. A barrier and cathodic effect are in effect once the electrolyte touches the system. This mixed mechanism of protection changes with the Zn/CNTs ratio. Low or non CNTs with high zinc protects the system differently than the non CNTs and low zinc and the high CNTs and high zinc. The differences reside in the Zn/CNTs matrix network they created. This network works as a physical and electrochemical barrier. Physical that repels water from the bare metal and electrochemical because the CNTs might act as a second cathode (instead of the carbon steel). This mixed effect protects the system with a barrier and a cathodic effect.

The primary objective of this work is to investigate the corrosion behavior of carbon steel in simulated CO2-EOR environments when O2 is present in the CO2 supply. A preliminary study was first conducted at low pressure to investigate the effect of O2 on the protectiveness of iron carbonate (FeCO3) corrosion product layers in mild steel CO2 corrosion. Carbon steel (UNS G10180) samples were immersed in a CO2 saturated 1 wt.% NaCl electrolyte for 2 days to facilitate formation of a protective FeCO3 layer on the steel surface. Temperature and pH were maintained at 80°C and 6.6, then 1 ppm O2 was introduced to the electrolyte. The impact of the oxidant(s) was studied after samples were exposed for one week to test conditions. Electrochemical measurements indicated increased corrosion rates over the first two days of O2 exposure, with a decrease in corrosion rate thereafter due to corrosion product formation that conferred some degree of protection to the steel surface. When O2 was introduced after carbonate formation, the corrosion rate did not increase. Although the final corrosion rates of all tests were relatively low (less than 0.2 mm/y), localized corrosion was observed. Surface analysis showed attack of iron carbonate crystals and formation of iron (III) oxides. This degradation of initially formed FeCO3 occurred concurrently with the development of localized corrosion features as deep as 80 µm. High pressure experiments were then conducted at CO2-EOR simulated downhole conditions. The effect of O2 (4 vol. %) on the corrosion performance of mild steel (UNS G10180) in CO2-saturated brine was investigated using a 4-liter autoclave at two different temperatures (25 and 80°C) and pressures (40 and 90 bar). Experiments at 25°C are categorized as `FeCO3-free’ while experiments at 80°C are termed `FeCO3-forming’. The work included electrochemical measurements, weight loss determination, and characterization of the corrosion products. Severe corrosion was observed on the steel specimen after 48 hours of exposure to the corrosive environments. Tests at FeCO3-forming conditions exhibited localized corrosion, while the FeCO3-free experiments displayed severe general corrosion. Corrosion prediction using Multicorp© software was performed and the output corrosion rate data were compared against experimental results. Reasonable correlation was observed with the experimental data in anoxic conditions.

The subsurface and surface microstructures during pin-on-disk fretting wear of A216 steel disks under various loading conditions and times are investigated. The corresponding pins are fabricated from 410 stainless steel to simulate in-service conditions found in such engineering components as the Siemens W501FD engine row-2 diaphragm of a Siemens turbine engine, which is known to be prone to failure by fretting wear. Loading conditions range from 2N to 15N and times from 1 hour to a maximum of 69 hours, when steady state is confirmed. Wear track depth is quantitatively determined by optical profilometry, and found to range from 3 to 11 microns dependent upon load. Wear depth increases from 2N to 10N load, but decreases when increased to 15N load, due to heavier transfer of pin material to disk, as can be seen by EDS images of chromium transfer on A216 disk. Microstructures are evaluated by transmission electron microscopy of samples prepared by focused ion beam machining to pinpoint wear tracks and expose them in cross-section. EDS is used, in conjunction with TEM, to elucidate primary wear mechanisms at each stage of fretting wear. Microstructures in the subsurface of wear tracks are found to be heavily dislocated and layered, features that vary with both applied load and time. The microstructure eventually evolves into stable dislocation cells with cell walls aligned parallel to the surface. Penetration depth of the damaged layers increases with applied load, associated with a non-uniform maximum shear stress distribution that varies with depth. Primary oxide appears to evolve from Fe2O3 to Fe3O4, with increasing fretting time, leading to a uniform oxide on the surface of the A216 disk. Oxidation rate may be increased with the evolution of this subsurface dislocation cell structure. It is concluded that fretting wear failure is likely associated with a synergy between oxidative wear and crack initiation and propagation along dislocation cell walls under high strain accumulation at sufficiently high loads or sufficiently long times.

There have been many reported failures due to biocorrosion, also known as microbiologically influenced corrosion (MIC), throughout the years. Additional research is needed in this area focusing on mechanisms and mitigation. Treatments for MIC rely primarily on biocides and pigging. Biocide enhancers are an excellent way to improve upon already existing biocides. D-amino acids as biocide enhancers for tetrakis (hydroxymethyl) phosphonium sulfate (THPS) have shown promise in recent years. In this work, D-tryptophan was combined with THPS at various concentrations to determine its efficacy as a biocide enhancer against sulfate-reducing bacteria (SRB) Desulfovibrio vulgaris biofilms. It was determined that D-tryptophan could be an effective biocide enhancer for THPS at high concentrations.Though not as well studied as SRB, nitrate-reducing bacteria (NRB) can also cause MIC failures. Additional research is needed to further understanding and mechanistic study in this area. Anaerobic experiments were conducted with Pseudomonas aeruginosa grown as an NRB. Three different strains were tested under the same conditions to determine if there was any significant difference in their MIC. It was determined that there were no significant differences in the MIC caused by the three strains.

Carbon steel is susceptible to the liquid-air interface (LAI) corrosion in liquid nuclear waste and in simulants, but a precise understanding of the mechanism is still missing. This work aimed to expand the understanding of the LAI corrosion mechanism.LAI corrosion was found to initiate at the very top of the meniscus in the form of pits under potentiostatic polarization condition. Gas bubbles were often observed to collect on corrosion product in the meniscus after extensive LAI corrosion. Additionally, LAI corrosion initiated later than crevice corrosion in the presence of a crevice. The effect of meniscus geometry on LAI corrosion initiation depends on the precise triple phase boundary rather than the opening angle of the meniscus geometry. Deaeration accelerated LAI corrosion. LAI corrosion was not enhanced by wetting the surface immediately above the meniscus. A pre-existing passive film boundary in the bulk solution could not eliminate LAI corrosion.Cyclic potentiodynamic polarization measurements were conducted, but could not distinguish the solution aggressiveness in terms of passive current density and pitting potential at varied alkaline pHs and concentrations of nitrate and nitrite ions. In addition, linear polarization resistance and electrochemical impedance spectroscopy measurements at OCP in the meniscus and bulk solution could not confirm the hypothesis that the meniscus solution would become increasingly aggressive to initiate LAI corrosion.Efforts were made to generate direct supporting evidence for several hypotheses. The use of a small pH electrode did not confirm existence of a pH gradient before initiation of LAI corrosion. Similarly, no IR drop was present in the meniscus prior to LAI corrosion initiation. In situ Raman spectroscopy was applied to monitor concentration changes of nitrate, sulfate and nitrite with time in the meniscus during LAI corrosion. A proposed mechanism of nitrate ion enhancement and nitrite ion depletion at the meniscus was not verified. Results showed that both aggressive ion (nitrate or sulfate) and inhibiting ion (nitrite) concentrations exhibited no variation before LAI corrosion initiation, as well as the nitrate-to-nitrite concentration ratio.The cathodic reduction kinetics in simplified waste simulants were also studied electrochemically. Both cyclic voltammetry and cathodic polarization measurements indicate that the nitrite reduction reaction exhibits faster kinetics than the nitrate reduction reaction at larger cathodic overpotential. However, the cathodic reactions at OCP are not dominated by the reduction of nitrate and nitrite. The varied availability of oxygen to the electrode surface clearly distinguishes the cathodic kinetics among varied immersion conditions.Lastly, nitrogenous gases, such as NO2, NO, N2O and sometimes NH3, were detected using gas chromatography-mass spectroscopy and gas detector tube methods, but only after extensive LAI corrosion. The reduction of nitrate or nitrite to the gases is probably promoted by the local acidification in the meniscus region generated by the hydrolysis of cations after extensive LAI corrosion propagation.Based on this work, it is likely that LAI corrosion is related to the local acidification at the top of meniscus. This mechanism is consistent with all known facts about LAI corrosion.

Carbon steel is susceptible to stress corrosion cracking (SCC) in fuel grade ethanol. Dissolved oxygen and corrosion potential have been identified as the critical factors. The threat of SCC prevents the use of the cost-efficient pipeline system for long distance transport of ethanol. Simulated fuel grade ethanol (SFGE) was used in the laboratory. Due to the high electrical resistivity of SFGE, adding non-complexing supporting electrolyte is considered to be the most practical method for accurate potential control. TBA-TFB was found to be the best suitable supporting electrolyte in deaerated FGE among the salts that were tested. Carbon steel exhibits passivity and high corrosion potential in aerated SFGE. Deaerated conditions are of interest so that the effects of high potential on SCC susceptibility can be determined separately from other possible effects of dissolved oxygen. In slow strain rate (SSR) tests, cracking was reproduced at applied potential without oxygen in the deaerated SFGE + TBA-TFB, which indicates that the role of oxygen in ethanol SCC might be a simple oxidizing agent. However, the passivation effect of oxygen is also required to prevent lateral corrosion at crack tip. A potential range for ethanol SCC was determined by SSR tests at different potentials. No experimental evidence was found to support the proposed role of oxygen to react with ethanol to form an aggressive oxidation product. The presence of chloride causes decreasing corrosion potential and enhanced pitting corrosion. The chloride effect on ethanol SCC was investigated both in aerated SFGE at open circuit and deaerated SFGE at applied anodic potentials over a wide range of chloride concentration. A minimum concentration of chloride is required for SCC of carbon steel, but it is not the controlling factor for crack growth. There is no upper limit of chloride concentration for cracking in aerated SFGE, but a window of chloride concentration in deaerated SFGE at applied potential. The dissolution based SCC mechanism has been identified for carbon steel in ethanol environment. SSR testing with periodic potentiodynamic scans at different stains and strain rates show that the anodic current difference between plastic and elastic region has a peak in the cracking potential range. The notched SSR testing is more sensitive to ethanol SCC susceptibility, and it generates more consistent results of current evolution. A high R-ratio and low cyclic frequency crack growth rate (CGR) test was intended to mimic the actual loading condition of pipelines in service at accurately-controlled fracture mechanics conditions. The measured CGR at applied potentials matches the cracking susceptible potential region. An oxygen depletion induced dissolution model and the traditional film rupture induced dissolution model were proposed to explain the mechanism of ethanol SCC. A few models based on oxygen diffusion and consumption were considered in an attempt to explain the differences of the two proposed mechanisms. Complete oxygen depletion is less likely to occur at crack tip.

This work focused on NaCl particle-induced marine atmospheric corrosion of carbon steel. Corrosion does not initiate under deliquescent NaCl droplets smaller than a critical size. The mechanisms of corrosion initiation and propagation were identified. The effects of diverse natural environments on the formation and aging of iron oxide products were identified. Part I On carbon steel that were exposed to Hawaiʻi's marine test sites for 30 min, corrosion did not initiate under small seawater droplets (D < ~30 μm) but occurred under larger droplets. Similarly, laboratory study showed that corrosion did not initiate from small NaCl droplets (D < ~45 μm) formed by the deliquescence of pre-deposited NaCl particles upon exposing to high humidity (RH > 80%), while occurred under larger droplets and sometimes propagated in the form of filiform corrosion. In addition, anodic polarization showed the steel passivated under small NaCl droplets and corroded actively under larger droplets. In situ and ex situ Raman spectra from the rust species that formed during the corrosion process showed that green rust (GR) formed close to the corrosion pit (i.e., anodic site) while lepidocrocite (γ-FeOOH) clusters precipitated in regions outside of the GR region (i.e., cathodic site). Similarly, the rust formed in Evans' droplet experiments was also indentified as GR and lepidocrocite using In situ Raman spectroscopy. Part II The corrosion products formed on 1008 steel that were exposed to Hawaiʻi's diverse micro-climates for 1 year period were characterized using micro-Raman analysis, energy dispersive X-ray analysis (EDXA), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analysis. Lepidocrocite was identified as the major rust in the surface layers on samples exposed to industrial, rainforest, dry and agricultural test sites, while goethite was detected in the inner rust layers. Akaganeite with high concentration of Cl was also found on the face-down sides of samples from dry and agricultural test sites. In addition to the dominant rust phases--lepidocrocite in the surface rust layers and goethite in the inner rust layers--more akaganeite was detected on samples from marine test sites, both in the surface and inner rust layers, due to high airborne chlorinity.

The internal corrosion of flowlines and pipelines made from carbon steel is encounted in the oil and gas industry. The corrosion process is primarily associated with the presence of free water in offshore or onshore production, particularly when it is accompanied by carbon dioxide gas. Corrosion inhibitor injection in oilfields is a very common and useful method for pipeline internal corrosion prevention. There is a need to understand the physics of phase wetting, carbon dioxide corrosion and the effect of corrosion inhibitor on those two processes in oil-water two-phase flow.In this study, the flow patterns and phase wetting regimes of oil-water two-phase flow were experimentally investigated in a large scale flow loop. Five major flow patterns were observed within the test flow conditions by flow visualization. Three types of phase wetting regimes (water wetting, oil wetting and intermittent wetting) were found by using a carbon steel test section consisting of wall conductance probes, wall sampling and corrosion monitoring. Based on experimental results, phase wetting maps were generated for model oil and five crude oils in the flow loop tests.Following the large scale flow loop tests, a small scale flow apparatus, called a doughnut cell was developed to simulate phase wetting occurring in oil-water two-phase pipe flow. Phase wetting maps were created for model oil and five crude oils in the doughnut cell tests.Two generic inhibitors, quaternary ammonium chloride and fatty amino were studied in this work. Corrosion inhibition tests showed that a direct exposure to oil phase enhances the performance of fatty amino inhibitor. Both inhibitors produce not only a reduction in oil-water interfacail tension, which promotes easier water entrainment by the oil, but also changes the wettability of the steel surface from hydrophilic to hydrophobic, which may reduce the possibility of CO2 corrosion.A mechanistic phase wetting prediciton model was developed to predict the transition between water breakout and full water entrainment. The model was validated with the experimental results of model oil and crude oils obtained from flow loop and doughnut cell tests. The model predictions have a good agreement with empirical results of model oil, but the model overpredicts the critical oil velocity of full water entrainment for all tested crude oils. The phase wetting prediction model is also used to correct the geometry difference between doughnut cell and flow loop. A corrosion inhibition model was built based on fitting corrosion inhibition test results with the Langmuir adsorption isotherm. A strategy is proposed to predict uninhibited and inhibited CO2 corrosion rate in oil-water two-phase pipe flow by integrating phase wetting, CO2 corrosion and inhibition models.

Erosion-corrosion occurs in pipelines that transport both corrosive liquids and erosive solid particles. This study has tested the erosion-corrosion behavior of mild carbon steel under conditions where there is no protective iron carbonate film. High and low salt concentrations were studied in order to determine the effect of salt concentration on the erosion-corrosion process. The effect of erosion-corrosion on mild steel was tested under disturbed flow by using a specially designed test section consisting of three flow alterations: a flow constriction, protrusion, and expansion. Under the tested conditions it was found that there is no synergistic effect between erosion and corrosion and that for an unprotected base metal the rate of metal loss is equal to the sum of erosion loss and corrosion loss. The higher salt concentration led to a lower corrosion rate and erosion rate but did notaffect the interaction between erosion and corrosion.

The two-dimensional (2-D) stochastic model, which describes the balance of two processes: corrosion (leading to metal loss) and precipitation (leading to metal protection), is able to predict localized corrosion, which is the most serious type of corrosion attack found in practice. The model uses uniform corrosion rate and surface-scaling tendency predicted by a 1-D mechanistic corrosion model as the inputs and can predict the possibility of localized corrosion as a function of primitive parameters such as temperature, pH, partial pressure of CO2, velocity, etc. The maximum penetration rate as well as uniform corrosion rate can be predicted and used to describe the severity of the localized attack.