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The integral effects of Ca and Sb on the corrosion properties of E690 steel were studied in a simulated tropical marine environment through immersion and dry/wet cyclic tests, focusing on localized and long-term corrosion properties. Results showed that Ca improved the corrosion resistance of steel by modifying the MnS inclusion into MnS-CaAl 2 O 4 inclusion, and Sb enhanced the consumption of H+ to decrease the electrochemical activity on steel surface. The protectiveness of the rust layer was increased by the enrichment of Ca and Sb, and the alkalization caused by Sb promoted appositional enrichment of Sb and Ca in the rust layer. • Ca inhibited the dissolution process of inclusions and reduced the susceptibility of pitting corrosion. • Sb can consume H+ to promote the formation of Ca(OH) 2 in the rust layer. • The enrichment of Ca and Sb in the rust layer promoted the α/γ, and enhanced the protective properties of the rust layer. [ABSTRACT FROM AUTHOR]

The corrosion performance of Cu-doped Ni–Mo steel has been comprehensively studied in a simulated severe marine environment. Results have indicated that the addition of Cu accelerates the generation of corrosion products and increases the corrosion current density during the initial corrosion process, which can be attributed to the increased proportion of high-angle grain boundaries. Cu improves the long-term protection of a rust layer by forming CuFeO 2 , and stabilizes substances such as NiFe 2 O 4 and compounds containing Mo4+ and Mo6+. Rust layers optimized by Cu display improved electrochemical properties in 3.5 wt% NaCl solution, indicating the beneficial effect of Cu on their protective properties. • The effects of Cu on the corrosion of Ni–Mo steel have been comprehensively characterized. • The proportion of high-angle grain boundaries (HAGBs) is increased by the addition of Cu, leading to increased electrochemical activity of Ni–Mo steel during the initial corrosion process. • The protective effect of a rust layer is increased by the generation of CuFeO 2 , which stabilizes NiFe 2 O 4 and compounds containing Mo4+ and Mo6+. [ABSTRACT FROM AUTHOR]

The effect of Cr content on the corrosion behavior of Ni-Mo steel in the tropical marine atmospheric environment is systematically investigated. The increase of Cr content accelerates the aggregation and the enrichment of Ni element, and thus promotes the formation of an inner electronegative film which attracts positively charged ions including Mo6+, Mo4+, Ni2+ and Cr3+ ions, and repels negatively charged Cl- ions. Cr promotes the formation of more nano-FeOOH and the conversion of γ-FeOOH into α-FeOOH. A small amount of chromium promotes the formation of micro-pits at the initial stage, and with time, those micro-pits gradually accumulate into pieces. • Adding a small amount of Cr into Ni-Mo steel reduces the corrosion rate. • An electronegative inner film composed of Cr and Ni elements attracts Mo6+, Mo4+, Ni2+ and Cr3+ ions, and repels Cl- ions. • Cr 2 O 3 acts as the nucleation site of FeOOH, generating more nano FeOOH, and doping Cr promotes the transformation of γ-FeOOH to α-FeOOH. [ABSTRACT FROM AUTHOR]

The corrosion resistance of the HRB400 steel rebar alloyed with Cr and rare earths (RE) was systematically studied from two aspects, including the properties of the passive film and the protectiveness of the rust layer. The results presented that Cr increased the corrosion resistance of the steel rebar through stabilizing the passive film and was not involved in the formation of corrosion pits, while the pitting corrosion was initiated by the dissolution of (RE)2O2S inclusion, resulting in the local acidification at the bottom of the corrosion pits, which decreased the stability of the passive film. As for the long-term corrosion process, both Cr and RE decreased the corrosion rate of the steel rebar, which was related to the promotion effect on the formation of α-FeOOH in the rust layer from Cr and RE. [ABSTRACT FROM AUTHOR]

The effects of silty sand and pH on the corrosion characteristics of 1Cr steel were investigated. The results revealed that silty sand changed the nucleation behaviour of FeCO3 crystal from homogeneous nucleation to heterogeneous nucleation through increasing the critical supersaturation of FeCO3. The compactness and protection of corrosion film were improved by silty sand. The amorphous FeCO3 content decreased as the pH increased in the CO2 environment with silty sand, while the Cr(OH)3 content increased. The critical pH for the good corrosion resistance of 1Cr steel was 4.5. The amorphous FeCO3 dominated the protective properties of corrosion film when the pH was lower than the critical pH, whereas the Cr(OH)3 was predominant when the pH was higher than the critical pH. Therefore, the corrosion resistance of 1Cr steel was reduced and then enhanced with increasing pH under silty sand condition. [ABSTRACT FROM AUTHOR]

• Newly designed low-cost steel bar is well used in tropical marine atmospheric environment. • Corrosion product films of newly designed low alloy steel bar and carbon steel bar is studied by outdoor exposure and immersion test. • Fixture for simulating concrete steel rebar is designed. • Improvement mechanism of Cr and Mo on the compactness of corrosion product films of steel rebar is proposed. Carbon steel rebar seriously corrodes in severe tropical marine atmospheric environment, threatening the safety of building structures. Hence, it is of great significance to design and manufacture low-alloy steel rebar which can be used in tropical marine atmospheric environment. In the paper, both of the self-designed low alloy (LA-1) steel rebar and carbon steel (HRB400E) rebar has been exposed in Sriracha for two years. The corrosion rate of LA-1 steel rebar is only half of that of carbon steel rebar after two years. However, it is difficult to study the root cause of low alloy steel rebars' enhanced corrosion resistance by alloy elements due to the presence of a large amount of CaCO 3 in the rust layer. Therefore, the mechanism of improving the protective effect of alloy elements on rust layer is further studied by dry/wet cycling corrosion tests to simulate a tropical marine atmosphere environment. The formation of dense product film on LA-1 steel surface is mainly because of the addition of Cr and Mo elements, leading to the corrosion rate of LA-1 steel rebar far less than that of HRB400E steel rebar. The protective film formed by molybdate in alkaline environment hinders the further corrosion of steel rebars. Cr(OH) 3 formed by hydrolysis of Cr in low alloy steel rebars inhibits the growth of FeOOH in rust, resulting in the generation of smaller size of FeOOH particles. The experiment proves that the newly designed LA-1 steel rebar can be well applied in tropical marine atmospheric environment. [ABSTRACT FROM AUTHOR]

This study compares the weathering performance of steel with three microstructures: ferrite, ferrite + bainite, and bainite. The rolling and cooling processes of a novel and cost-effective Ni–Mo weathering steel (WS) were manipulated to achieve these microstructures. Validation of these microstructures was performed through field exposure tests in Trat, Thailand, representing a typical tropical marine atmospheric environment. The bainitic steel exhibited better weathering resistance. After 12-month of exposure, the corrosion rates for ferritic + bainite and bainitic steels were 105.6 % and 97.9 % respectively, compared to ferritic steel. The variation in corrosion rates confirm that modifying the microstructure, with emphases on bainite, improves the WSs' resistance to atmospheric corrosion. Changes in corrosion rate mainly depend on the rust layer's adherence. Interestingly, α-FeOOH predominantly characterizes the rust layer on the skyward side of the samples following 12-month of exposure, while the earthward side is mainly composed of Fe 3 O 4 /γ-Fe 2 O 3. • Field tests in Thailand compared microstructure's impact on Mo–Ni WS corrosion. • Bainitic micro-structured weathering steel exhibited better corrosion resistance. • Skyward: α-FeOOH dominates the rust layer; Earthward: Fe 3 O 4 /γ-Fe 2 O 3 dominates. [ABSTRACT FROM AUTHOR]

Purpose: The salinity of the oilfield produced water has a significant effect on steel corrosion. The purpose of this paper is to study the influence of salinity on corrosion behavior of X60 steel and it also provides basic for material selection of gas wells with high salinity. Design/methodology/approach: The weight loss experiment was carried out on steel with high temperature and high pressure autoclave. The surface morphology and composition of corrosion scales were studied by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry. Findings: The results show that as salinity increases, the corrosion rate of X60 steel will gradually experience a rapid decline stage and then a slow decline stage. X60 steel is mainly exhibiting uniform corrosion in the first rapid decline stage and pitting corrosion in the second slow decline stage. The increase in salinity reduces gas solubility, which, in turn, changes the morphology and density of the corrosion scales of X60 steel. At low salinity, loose iron oxides generated on the surface of the steel, which poorly protects the substrate. At high salinity, surface of the steel gradually forms protective films. Chloride ions in the saline solution mainly affect the structure of the corrosion scales and initiate pitting corrosion. The increased chloride ions lead to more pitting pits on the surface of steel. The recrystallization of FeCO3 in pitting pits causes the corrosion scales to bulge. Originality/value: The investigation determined the critical concentration of pitting corrosion and uniform corrosion of X60 steel, and the new corrosion mechanism model was presented. [ABSTRACT FROM AUTHOR]

• O 2 has a greater effect on 3Cr than X70 before 24 h, and then the trend reverses. • The self-repairing effect of Cr is much less than the O 2 destruction. • O 2 promotes the generation of Fe 2 O 3 , γ-FeOOH and Fe 3 O 4 and degrades FeCO 3. • The effect mechanism of oxygen on the corrosion product layer is proposed. This study investigates the impact of oxygen contamination on corrosion products formed during CO 2 pre-corrosion of X70 and 3Cr steels. The results show that O 2 promotes the generation of Fe 2 O 3 , γ-FeOOH and Fe 3 O 4 and the degradation of original FeCO 3 , accelerating the corrosion process. The pores on the original surface and the hydrolysis of Cr(OH) 3 lead the general corrosion rate of 3Cr steel to be much higher than that of X70 steel in the first 24 h. Cr's self-repairing can mitigate the corrosion failure of 3Cr steel, and thus the general corrosion rate of 3Cr after 24 h is lower than that of X70. However, this self-repairing effect is quite less than the destructive effect of oxygen. Therefore, in the actual oil and gas production process, it is recommended to use deoxygenation treatment, anti-oxidation corrosion inhibitor and steel with high chromium content. [ABSTRACT FROM AUTHOR]

• Corrosion resistance of 3Cr steel was first reduced, followed by enhancing and then reduced as the rotation speed increased. • Silty sand changed from participating in the corrosion film to destroying the corrosion film. • Increasing rotation speed converted a factor that played a dominant role in corrosion resistance from mass transfer process to shear stress of silty sand. • Critical rotation speed for corrosion mechanism transformation of 3Cr steel was 600 r/min. The 3Cr steel used as the tubing has serious corrosion damage in a flowing CO 2 environment with silty sand. Hence, the investigation for the influence of flow velocity on the corrosion behavior of 3Cr steel has an important and practical significance. It could not only unveil the corrosion damage mechanism of the steel under dynamic conditions, but also provide a theoretical basis for the corrosion protection of the tubing. In this paper, the effect of flow velocity on the corrosion film of 3Cr steel in a CO 2 environment with silty sand was investigated by adjusting rotation speed. The results revealed that the corrosion resistance of 3Cr steel was first reduced, followed by enhancing and then reduced as the rotation speed increased. There was a critical rotation speed for the transformation of corrosion mechanism of 3Cr steel, i.e., 600 r/min. When the rotation speed was lower than the critical speed, the mass transfer process of corrosive medium dominated the corrosion resistance of 3Cr steel, and silty sand participated in the formation of corrosion film. The increasing rotation speed accelerated the mass transfer process, which first reduced and then improved the corrosion resistance. When the rotation speed was higher than the critical speed, the shear stress of silty sand played a predominant role. At the same time, silty sand impinged on the corrosion film, and then destroyed the stability of the film, leading to a significant decrease of the corrosion resistance. Silty sand changed from participating in the corrosion film to destroying the corrosion film with increasing rotation speed. [ABSTRACT FROM AUTHOR]

To optimize Ni, Cu, Mo element content of low Cr steel rebars in tropical marine atmosphere environment, five newly steel rebars were exposed in Dalat to carry out the outdoor exposure tests including weight loss method, surface characterization method and electrochemical test. The results show that when 0.15 wt% Mo is added in the steel rebar, the corrosion resistance is 31.8% higher than that of the steel rebar with 0.3 wt% Cu. The corrosion resistance of 0.3 wt% Cr added steel rebar is 63.4% higher than that of 0.2 wt% Ni added steel rebar. With the addition of 0.3 wt% Cu and 0.3 wt% Cr, the corrosion resistance is 46.3% higher than that of the steel rebar with 0.2 wt% Ni and 0.15 wt% Mo. Mo and Cr are the main elements to improve the protection of steel rebar rust layer. The corrosion products of the five steel rebars are composed of α-FeOOH, γ-FeOOH and Fe 2 O 3 or Fe 3 O 4. The higher the content of Cr is, the higher the ratio of α - FeOOH to γ-FeOOH (α/γ) in the rust layer becomes. The protective layer composed of CaMoO 4 and Fe 2 (MoO 4) 3, formed by molybdate in alkaline environment hinders the further corrosion of steel rebars. Cr(OH) 3 produced by hydrolysis of Cr in low alloy steel inhibits the growth of FeOOH in rust, resulting in the decrease of FeOOH particle size and the rust layer more compact. On the whole, the newly designed low alloy steel rebar with a little of 1.6Cr-0.15Mo elements can be well used in tropical marine atmospheric environment. [ABSTRACT FROM AUTHOR]