Stainless steels are frequently used in the oil & gas industry due to their excellent resistance against corrosion. In seawater and produced water, however, pitting and/or crevice corrosion can occur assuming the process conditions are outside the so-called "safe operation window" for the actual alloy. In produced water, oxygen content is one important process parameter. It is well known that in electrolytes with small amounts of oxygen (O2 < 10 ppbw), stainless steels can be used at much higher temperatures than in seawater saturated with oxygen (∼ 8 ppmw O2). In this paper, the effect of temperature, pH and chloride content on crevice corrosion and repassivation potential in simulated produced water without oxygen is examined for UNS(1) S31603, UNS S31803 and UNS S32750. The tests showed that both temperature and chloride ion concentration strongly affected the results.
Corrosion Resistant Alloys (CRAs) have been widely used in oil & gas process systems since the 1980s due to their excellent resistance towards uniform corrosion in aggressive environments such as seawater and produced water containing CO2, organic acids and/or production chemicals. However, cases of localized corrosion in the form of pitting and crevice corrosion have regularly been observed. As an example, ISO(2) 21457 limits the max. operating temperature to 20°C for 25 Cr super duplex stainless steel (UNS S32750/760) and 6-Mo austenittic stainless steels (UNS S31254) in chlorinated seawater systems, to avoid crevice corrosion.1 Initiation of pitting and crevice corrosion (often called localized corrosion) are influenced by different process parameters. The most important process parameters are temperature, pH, chloride- and oxygen concentration. In addition, the actual alloy composition and microstructure plays an important role. Figure 1 shows a schematic anodic polarization curve for an active/passive alloy like stainless steels with and without chloride ions (Cl− - ions). The figure shows typical values that are used to describe the anodic curve and the four characteristic regions a complete anodic polarization curve can be divided into.