Crevice Corrosion of Corrosion-Resistant Alloys in Sour Environments

Crevice corrosion of corrosion-resistant alloys (CRAs) was studied. The crevice pH effect on initiation was investigated with the results of the linear relation between electric charge and crevice pH. The crevice pH decreased easily compared with the behavior obtained in seawater environments. Passive current density was obtained by subtracting the H2S oxidation current density measured on the Pt electrode from that on alloys and agreed well with the current density obtained by solution analysis. The relationship between the electric current passing through the alloy and crevice pH with passive current density was used to calculate the incubation time for crevice initiation based on the model proposed for a sea water environment. The incubation time increased proportionately with the alloy element concentration. The critical condition of crevice initiation is discussed with the Pourbaix diagram for sour gas environments with depassivation pH of alloy. It is concluded that corrosion-resistant alloys hardly suffer from crevice corrosion as far as pH of environments is determined by dissolved gas. The application limit of CRAs predicted from the initiation of crevice corrosion involves the pitting corrosion occurrence. The application limit determined by the initiation of pitting corrosion yields the most reliable criterion for material selection.