Magnesium and magnesium alloys are susceptible to stress corrosion cracking in various environments, including distilled water. There is compelling evidence to conclude that SCC is assisted, at least in part, by hydrogen embrittlement. This paper reviews the thermodynamics of the Mg-H system and the kinetics of hydrogen transport. Aspects of magnesium corrosion relevant to hydrogen absorption are also discussed. Crack growth mechanisms based on delayed hydride cracking, hydrogen adsorption dislocation emission, hydrogen enhanced decohesion, and hydrogen enhanced localized plasticity have been proposed and evidence for each of them is reviewed herein.

Corrosion tests with gaseous H 2 S require special facilities with safety features, because H 2 S is a toxic and flammable gas. The possibility of replacing H 2 S with thiosulfate (S 2 O 3 2− ), a non-toxic anion, for studying stress corrosion cracking of stainless and carbon steels in H 2 S solutions was first proposed by Tsujikawa in 1993. H 2 S production was detected in presence of carbon steel corroding in acidified thiosulfate-containing solutions. In this paper, the kinetics of H 2 S evolution are used to estimate the range of partial pressure of H 2 S that can be simulated with thiosulfate solutions. It was determined that acid brines containing 10 −4 M and 10 −3 M S 2 O 3 2− could be used for replacing continuous bubbling of dilute H 2 S/N 2 mixtures in tests of degradation of carbon steels, with H 2 S partial pressures ranging between 0.03 and 0.56 kPa. The kinetics of H 2 S production were compared with the amount of sulfur in side reactions, like formation of iron sulfide films and elemental sulfur.

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Crevice corrosion affects the integrity of stainless steels used in components exposed to seawater. Traditionally, crevice corrosion testing involves the use of artificial crevice formers to obtain a critical crevice potential, which is a measure of the crevice corrosion resistance of the alloy. The critical acidification model proposed by Prof. J.R Galvele predicts that the critical crevice potential is the minimum potential required to maintain an acidic solution with a critical pH inside either a pit or a crevice. Application of Galvele's model requires an estimation of both the diffusion length and the i vs. E behavior of the metal in the solution inside the crevice. In this work, the crevice corrosion resistance of a 22% Cr duplex stainless steel (UNS S31803) and a 25% Cr super duplex stainless steels (UNS S32750) was investigated. The i vs. E response of the two stainless steels was determined in acidified solutions of various chloride concentrations, which simulate those found in an active crevice. Critical potentials predicted by the critical acidification model were compared with critical crevice potentials measured in simulated seawater. Results showed that despite the various assumptions and simplifications made by Galvele, the model correctly predicted the occurrence of crevice corrosion of both UNS S32750 and UNS S31803 close to room temperature in a 3.5 wt.% NaCl environment. Critical potentials obtained by Galvele's model were similar if assuming that the chloride concentration of the simulated crevice solutions was between 7 M and 12 M acidified to a pH of 0.

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The corrosion behavior of different tempers of two aluminum alloys, AA7050 and an experimental Al-Mg-Cu-Si alloy, was studied in NaCl solution by anodic polarization and scanning electron microscopy and was correlated with differences in the microstructure. Potentiodynamic polarization experiments were performed on samples from the exact sheets used by others to study the microstructure evolution during the early stages of the precipitation sequence by high-resolution characterization tools ͓i.e., high-resolution transmission electron microscopy and atom probe tomography ͑APT͔͒. The usefulness of information from these state-of-the-art tools to lead to a better understanding about the effects of nanoscale segregation on localized corrosion of aluminum alloys is discussed. APT was able to provide information about the composition of the solid solution matrix region between the fine-scale hardening particles, which is not possible by any other technique. Some of the changes in corrosion behavior, e.g., the breakdown potentials, with temper could be rationalized based on changes in the matrix composition. The formation of corrosion-susceptible surface layers on as-polished AA7050 depended on the predominant type of hardening particle. The lack of detailed knowledge of the grain boundary region limited the applicability of the microstructural information generated by previous studies for understanding intergranular Wrought precipitation-hardenable aluminum alloys are used extensively in structural applications due to their high strength/density ratio.1 Their heat-treatment normally involves a solution treatment at high temperature to dissolve the alloying elements, rapid cooling, or quenching to obtain a supersaturated solid solution ͑SSSS͒ of the alloying elements in the aluminum matrix, and controlled decomposition of the SSSS to form finely dispersed precipitates. The elements commonly added to Al for age-hardening response are Cu, Mg, Si, Li, and Zn. Age-hardenable aluminum alloys also have minor amounts of Fe, Mn, Zr, Cr, and Ti ͑added deliberately or not͒. This complex chemical composition results in the presence of three types of second phases: coarse intermetallic compounds or constituent particles ͑1-30 m͒, dispersoids ͑0.05-0.5 m͒, and fine precipitates ͑Ͻ0.01 m͒. 1,2Whereas the effect of coarse precipitates on corrosion behavior has been studied in detail, 3-8 the influence of fine-scale ͑Ͻ0.01 m͒ hardening precipitates, especially those formed during the early stages of the precipitation sequence, on the corrosion properties is not well understood. These particles have the greatest effect on mechanical properties, but their presence and metallurgical characteristics can only be assessed with advanced techniques owing to their extremely small size. They appear within grains according to a complex precipitation sequence of metastable phases during aging treatments, but equilibrium precipitates can be formed readily on grain boundaries. A precipitate-free zone ͑PFZ͒ with composition varying considerably f...

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Low-alloy steels (LAS) are extensively used in oil and gas (O&G) production due to their good mechanical properties and low cost. Even though nickel improves mechanical properties and hardenability with low penalty on weldability, which is critical for large subsea components, nickel content cannot exceed 1-wt% when used in sour service applications. The ISO 15156-2 standard limits the nickel content in LAS on the assumption that nickel concentrations above 1-wt% negatively impact sulfide stress cracking (SSC) resistance. This restriction excludes a significant number of high-strength and high-toughness alloys, such as Ni-Cr-Mo (e.g., UNS G43200 and G43400), Ni-Mo (e.g., UNS G46200), and Ni-Cr-Mo-V grades, from sour service applications and can be used only if successfully qualified. However, the standard is based on controversial research conducted more than 40 years ago. Since then, researchers have suggested that it is the microstructure that determines SSC resistance, regardless of Ni content. This review summarizes the advantages and disadvantages of nickel-containing LAS in terms of strength, weldability, hardenability, potential weight savings, and cost reduction. Likewise, the state of knowledge on the effect of nickel on hydrogen absorption as well as SSC initiation and propagation kinetics is critically reviewed.