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The design of sacrificial cathodic protection (CP) systems conventionally involves steady-state assumptions, which means design parameters are considered constant during the in-service life of CP systems. In contrast, it is evident by experimental observations (including field measurements) that cathodic protection is a transient process due to variations in electrolyte properties such as seasonal changes in electrical conductivity of soil, depletion of anodes, and formation of corrosion deposits on anode material surface, to name a few. The lack of practical time-dependent models on this critical issue is apparent in the literature; accordingly, in this study, a pseudo transient electrochemical model is adopted to highlight the transient behavior of cathodic protection systems and investigate key differences with steady-state behavior. For the sake of demonstration, the developed model is used to simulate the time-dependent performance of a sacrificial anode bed for cathodic protection of screw-pile foundations. The methodology proposed in this study can be used by corrosion engineers to improve and optimize the design of CP systems and numerically estimate the performance of sacrificial anodes and the level of protection over time.

On 22 April 1996, there was a catastrophic failure in a section of six-inch diameter feedwater line leading from the economizer to the steam drum on a boiler at a private industrial power plant in the Midwest. The rupture occurred immediately downstream from a chelant injection quill. Boiler water depressurized from over 900 psi and flashed to steam as it exited the ruptured pipe. A full investigation into the cause of the failure was carried out over a period of 2 years, involving experts in mechanical design, metallurgy, water chemistry and fluid flow dynamics. The boiler operating history, including water treatment methods, was carefully reviewed. Inspections were also carried out on the equivalent feedwater piping and injection quills removed from five other boilers operating with the same source of feedwater but with independently controlled rates of chelant injection. The failure occurred due to progressive thinning of the pipe wall from the water side to the point where it could no longer tolerate the operating pressures and temperature. It was concluded that thinning of the carbon steel pipe wall had been caused by a specific mechanism, termed here “flow-assisted chelant corrosion”. The difference between this and flow-accelerated corrosion (FAC) is presented and recommendations are offered to help avoid similar failures in the future. Other potential causes of the failure including direct erosion and non-flow assisted corrosion were evaluated and shown to be non-causal.

A series of mechanical and electrochemical experiments were performed on samples of titanium aluminides, Ti3Al(α2) and iron aluminide (D03-Fe3Al). Electrochemical studies of these alloys in NaCl solution showed that Ti3Al is less susceptible to pitting corrosion than Fe3Al. Both materials were susceptible to stress corrosion cracking whenever the applied potential during stressing was greater than the breakdown potential. The mode of cracking was intergranular and most often multiple cracks were observed on the surface of tested samples. At applied potentials less than the breakdown potential susceptibility to localized corrosion decreased sharply and at the open circuit potential these alloys did not show any susceptibility. Hydrogen embrittlement (HE) was studied by cathodically charging samples with hydrogen. Ti3Al was very susceptible to HE and its tensile properties after hydrogen charging dropped sharply, while Fe3Al was less susceptible and no major changes in mechanical properties were observed. Oxidation rate of both materials were low up to 800 °C, but hot corrosion resistance in molten salt was extremely low. Severe localized corrosion was observed when samples were exposed to a molten sodium sulfate salt.

The recently developed I-P-Z model is modified in order to analyze the observed enhanced permeation of hydrogen that occurs in the presence of hydrogen sulfide during cathodic hydrogen charging of iron. The modification accounts for the fact that the energy of adsorption becomes coverage dependent at the higher coverages and affects the hydrogen evolution reaction (HER) in the presence of H2S. Charging experiments were performed on Ferrovac E-iron membranes 0.5 mm thick using a Devanathan-Stachurski cell in deaerated, pre-electrolyzed solutions made from 0.1 M HClO4 and 0.1 M NaClO4 with pH values of 1 and 2. The transfer coefficient, α, exchange current density, i0, thickness-dependent absorption-adsorption rate constant, k″, recombination rate constant, k3, surface hydrogen coverage, θH, and discharge rate constant, k1o, were obtained by application of the model to the experimental results. As a result, the role of H2S has been clarified. While θH is increased in the presence of H2S, the overpo...

A mechanistic model has been developed which for the first time considers the effect of hydrogen entry into a metal on the kinetics of the hydrogen evolution reaction (h.e.r.). The model enables computation of (i) the hydrogen surface coverage and surface concentration; (ii) the hydrogen adsorption, absorption, discharge and recombination rate constants; and (iii) the h.e.r. coverage‐dependent transfer coefficient, α, and the exchange current density , from a knowledge of the steady‐state hydrogen permeation current, cathodic charging current, hydrogen overvoltage, and hydrogen diffusivity. The model predicts a linear relationship between the permeation flux and the square root of the hydrogen recombination flux, and provides an analytical method to determine the cathodic potential range for operation of a coupled discharge‐recombination mechanism of the h.e.r. With modifications the model can treat permeation data for which (i) the mechanics of the discharge step involve a (proposed) selvedge reaction, and (ii) surface hydrogen coverages are relatively high as in the presence of poisons (e.g., or ). Some of the existing literature data for hydrogen permeation in iron and nickel in acid and alkaline solutions are successfully analyzed.

A summary of a recently completed analysis of hydrogen electrode reactions during aqueous cathodic charging of metals is presented. The analysis for the first time takes into account the effect of hydrogen permeation into the metal of the h.e.r. From the model, all of the kinetic parameters are computable without use of any adjustable parameters. For the first time, surface coverages and rate constants are determinable from the measured charging and permeation currents. The enhancement of hydrogen entry in the presence of poisons, such as hydrogen sulfide, can be analyzed with the model, taking into account the Frumkin-Temkin isotherms in the discharge and recombination reaction kinetics. Keywords: Electrochemistry; Hydrogen evolution reaction; Permeation flux; Charging flux. Reprints.