Corrosion of Wrought, Cast, and Nitrided Ni/Fe-Cr-Mo Alloys in High-Temperature Brines in Equilibrium with Wet Supercritical CO₂ Having Oxygen or Hydrogen Sulfide

Corrosion-Resistant Alloys (CRAs), among commercial Ni/Fe-Cr-Mo alloys for surface, subsea, and downhole equipment, and among traditional and proprietary cast Fe-Ni-Cr alloys for pressure pumping operations have been corrosion evaluated and ranked for their environmental resistance in two static carbon-dioxide environments with liquid brines in equilibrium. The selected CRAs were tested not only in their API Spec 6ACRA heat-treated conditions, but also after liquid nitriding, with specific attention given towards general, pitting, and crevice corrosion. All environmental tests were completed over a 30-day period in 4000psi (276bars) pressurized autoclaves at a temperature of 425°F (218°C) with either oxygen (0.5% O₂) or sour gas (20% H₂S). The tested CRAs were exposed to both a denser 90,000-120,000ppm NaCl-rich liquid brine, purposely supersaturated with oxygen or sour gas, and a lighter and supercritical carbon dioxide fluid containing water along with oxygen or hydrogen sulfide. For the two selected environments, the test results have revealed the following: (1) corrosion in liquid brines and supercritical fluids turned out to be comparable and rarely alarming, (2) all tested Ni/Fe-Cr- Mo wrought alloys corroded less than the cast alloys; i.e., ~0.5 to ~1.1mpy (~12 to ~28µm), (3) nitriding, while generally inconsequential to the Ni/Fe-Cr-Mo alloys, produced thin layers that corroded faster and delaminated, (4) the novel cast alloys, having a general corrosion resistance closer to the wrought Ni- Cr-Mo alloys, outperformed the traditional pump cast alloys; i.e., ~4.0 to ~40mpy (~0.1 to ~1.0mm), (5), general corrosion and pitting were the dominant corrosion mechanisms with (6) oxygen accelerating corrosion more than hydrogen sulfide. For the nitrided alloys, the progressive chemical conversion of the nitrided layer into a delaminating corrosion byproduct is proven to occur rapidly, resulting in the exposure of the base materials without noticeable accelerated corrosion over equipment lifespan.