Corrosion Behavior of Active-Screen Plasma Nitrided 17-4 PH (H1150D) Steel in H2S/CO2-Containing Environments

The presence of hydrogen sulfide (H2S) in typical oilfield environments promotes hydrogen absorption and subsequent failure of high-strength steels by sulfide stress cracking (SSC). Plasma nitriding is as a potential method to increase the resistance of the 17-4 PH to SSC, although further investigation is required to evaluate the corrosion resistance of the modified layer when it is exposed to H2S-containing environments. The aim of this study was therefore to evaluate the corrosion resistance of the 17-4 PH in typical oilfield environment. Samples were plasma nitrided at low (420°C) and high (500°C) temperatures and immersed in produced water with mixed H2S and CO2. The electrochemical data and scanning electron microscopy (SEM) micrographs showed that there were no detrimental effects on the corrosion resistance when plasma nitriding was performed at low temperature (420°C), whereas the integrity of the modified layer was compromised when an elevated temperature (500°C) was applied. The enhanced resistance to localized corrosion of the nitride case obtained after the low-temperature surface modification was attributed to the formation of a compound layer of mixed M4N/M2-3N, the inner section being more corrosion resistant than the outer part, as revealed by SEM micrographs.