Influence of hydrostatic pressure on the pitting corrosion behavior of API X80 steel

In this research, the effect of hydrostatic pressure on metastable and stable pitting corrosion of API X80 steel was investigated by using electrochemical method in simulated deep-sea environment. Cumulative probability distribution analysis of metastable pitting events revealed that hydrostatic pressure promoted the occurrence of metastable pitting corrosion and transformation trend of metastable pitting corrosion to stable pitting corrosion. Moreover, according to the results of scanning Kelvin probe force microscopy(SKPFM) tests, pitting corrosion of test steel can be confirmed to originate from the position of the interface between the inclusion and the substrate. Finally, combined with finite element analysis, the mechanism by which hydrostatic pressure promotes the occurrence and expansion of stable pitting corrosion of test steel in simulated deep -sea environment is explained.

STUDY OF THE PASSIVITY LIMITS OF AUSTENITIC STAINLESS-STEEL IN H2S-CONTAINING BRINES USING MOTT-SCHOTTKY AND POINT DEFECT MODEL ANALYSIS

The objective of the study is to correlate the effects of H<sub>2</sub>S, and Cl<sup>-</sup> concentration on the passivity limits and the onset of localized corrosion, in UNS S31603 stainless steel by evaluating the changes in the semiconducting behavior exhibited by the passive layer. The study is accomplished experimentally by using a combination of direct and alternate current electrochemical methods, to study the passive layer formed by the stainless steel, in equilibrium with a gas phase at 2.8 MPa (400 psi) containing up to 60% mol of H<sub>2</sub>S (bal. CO<sub>2</sub>) at 25○</sup>C. The results obtained using the Mott-Schottky analysis indicate that the decrease of the passive layer stability formed on the UNS S31603 stainless steel is consistent with the increase in the electron donor carrier density. This is observed as the consequence of the effect of Cl<sup>-</sup> and H<sub>2</sub>S. In this context, the Cl<sup>-</sup> content in the brine was found to exert a larger effect than the H<sub>2</sub>S activity. The correlation with the evaluation of the passive layer using the Point Defect model suggest that both the polarizability and the rate of annihilation of the cation vacancies at the metal / film interface increase with the H<sub>2</sub>S content in the environment. This behavior can explain the increased content of Cl<sup>-</sup> and sulfides as main electron donor species, also the observable increase in the passive layer susceptibility to both stable and metastable pitting.

Characteristics of Passive Films Formed on As-Cast Ti–6Al-4V in Hank’s Solution Before and After Transpassivation

In this work, the characteristics of passive films formed on as-cast Ti-6Al-4V before and after transpassivation by electrochemical methods will be studied. A simulated body fluid of Hank’s solution was used as the electrolyte in this work. According to the potentiodynamic polarization test, the passivation range, transpassive range, and repassivation range of as-cast Ti-6Al-4V were obtained. Afterward, the potentiostatic polarization was employed to passivate the Ti-6Al-4V in both passivation and repassivation ranges. Electrochemical impedance spectroscopy (EIS) was used to analyze the characteristics of formed passive films. Different electrochemical behavior of as-cast Ti–6Al-4V is found in passivation and repassivation ranges. The passivation current density of the sample in the repassivation range is significantly larger than that in the passivation range. Meanwhile, the growth rate of passive film in the repassivation range is also greater than that in the passivation range. Although the sample shows a higher charge transfer impedance in the repassivation range, metastable pitting corrosion is also observed, indicating the formation of the unstable passive film. Such results advance the understanding of as-cast Ti-6Al-4V polarized under different potentials for potential biomedical applications.

Effect of cerium on the initiation of pitting corrosion of 444-type heat-resistant ferritic stainless steel

The 444-type heat-resistant ferritic stainless steel is widely utilized in automotive exhaust pipes and solid oxide fuel cells, due to its excellent properties at elevated temperature. To meet the demands of significantly harsh service environments, rare earths were added in 444-type ferritic stainless steel. For the purpose of evaluating the effect of rare earths on pitting corrosion initiation, the metastable pitting corrosion behavior in 444-type ferritic stainless steel was studied through potentiodynamic polarization and potentiostatic polarization tests. The results demonstrated that pitting corrosion was initiated at the inclusion/alloy interface. The cerium alloying in 444-type ferritic stainless steel decreased the amount of preferential dissolution sites. The beneficial effect of Ce on pitting corrosion resulted from the formation of stable cerium oxides, as well as from the reduction in the amount and size of inclusions in 444-type ferritic stainless steel. In addition, electrochemical impedance spectroscopy test results revealed that cerium alloying enhanced the polarization resistance of passive films through insignificant thickness alteration.