The microstructures and passivation behavior of selective laser melted 316L stainless steel (SLM SS316L) after various heat treatments (500 °C, 950 °C, and 1100 °C) were investigated. The electrochemical results showed that the SLM SS316L sample that was heat treated at 950 °C exhibited the lowest passive current density. The microstructural characterization analysis indicated that the subgrain structures transformed from dislocation-rich subgrain boundaries into island-like cellular trace structures after heat treatment at 950 °C. This led to improved corrosion resistance due to the elimination of dislocations and the homogenization of the composition. Compositional analyses of the passive film indicated that there was no notable change in the passive film composition after heat treatment at 500 °C and 950 °C. However, heat treatment at 1100 °C promoted the formation of Cr(OH)3 in the passive film, resulting in a reduced corrosion resistance. Based on these results, heat treatment at 950 °C appears to be an adequate post-process for SLM SS316L to optimize the microstructure, while also improving corrosion resistance.

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Specimens from a failed X-52 pipeline that had been inservice for 34 years were pitted using the passivation/immersion method developed by the authors to simulate pitted pipelines observed in service. The resulting pitted samples were then cyclically loaded in an aqueous near-neutral pH environment sparged with 5% CO2 / balance N2 gas mixture at high stress ratios (minimum stress/maximum stress), low strain rates and low frequencies which were close to those experienced in service. It was found that the majority of cracks initiated from the corrosion pits and were less than 0.5 to 0.6 mm deep and were generally quite blunt. These cracks were transgranular in nature and designated as Stage I cracks and were typical of cracks found in most crack colonies. However, the further growth of these short, blunt cracks was significantly influenced by the distribution of the nearby non-metallic inclusions. Inclusions enhanced the stress-facilitated dissolution crack growth, which is the crack growth method proposed by the authors in a related paper. When the orientation of the inclusions was at a small acute angle to the orientation of the pits or cracks, and the inclusions were in the same plane as crack initiation or advance, these inclusions would enhance crack growth, or even trap hydrogen which further resulted in the formation of clusters of tiny cracks, which appeared to be caused by hydrogen. The hydrogen-produced cracks could be eaten away later by the stress-facilitated further dissolution of the blunt cracks. If these cracks can grow sufficiently however they pose an integrity risk, as they can initiate long cracks (near-neutral pH SCC). These hydrogen-caused cracks in Stage I were rare. It was nevertheless suggested that cracks deeper than 0.5 to 0.6 mm in the field should be removed to reduce or avoid the threat of rupture. If active corrosion and hydrogen generation can be prevented then smaller cracks are innocuous.

The hydrogen permeation behaviour of X80 pipeline steel exposed to corrosion environments with different H 2 S/CO 2 partial pressure ratios at room temperature was investigated through the highpressure electrochemical hydrogen permeation test. Both CO 2 and H 2 S have a hydrogen permeation promoting effect; however, CO 2 hydrogen permeation promoting effect is much weaker than H 2 S. The hydrogen permeation curves have the rising, falling then stable stage. When at a total pressure of 1 MPa, as the H 2 S partial pressure decreases, the hydrogen permeation promoting effect weakens, but the peak current of hydrogen permeation gradually increases. The main reason is that the role of corrosion products has changed from promoting hydrogen permeation to hindering hydrogen permeation during the reaction.

Low carbon steel was coated by hot-dipping into a molten bath containing Al-2 wt.%Mn. The phase composition, morphology and the erosion-corrosion behavior of the aluminide layer were characterized by XRD, OM, SEM and erosion-corrosion tester, respectively. The results show that the coatings are mainly composed of Al, FeAl3, Fe2Al5 and MnAl6 phase. The coatings consist of two-layer structure, i.e., toplayer Al-Mn alloy layer and tongue-like intermetallic compound. The thickness of the coating layer is about 800 μm and all the coating layers show good adhesion to the steel substrate. Compared with the pure Al coatings, the Al-Mn alloy coatings exhibit lower wear rate irrespective of the rotation speed. The hot-dipped Al-Mn coatings possess considerable erosion-corrosion resistance.