CRITICAL PITTING TEMPERATURE OF MARTENSITIC AND SUPER-MARTENSITIC STAINLESS STEELS IN SIMULATED SOUR ENVIRONMENTS

The use of corrosion resistant alloys (CRAs) has allowed oil and gas operations in environments where carbon and low-alloy steels would have otherwise corroded too quickly, increasing downtime and maintenance costs. However, CRAs may suffer from localized corrosion in the form of pitting, crevice and/ or stress corrosion cracking (SCC). Therefore, it is important to determine the susceptibility to localized corrosion in service conditions with minimal or no external perturbation. In this work, electrochemical noise (ECN) analysis was used to determine the pitting corrosion susceptibility of 13-Cr and super 13-Cr martensitic stainless steels in simulated production environments in equilibrium with up to 850 ppm H2S (g) and 1.7% CO2 (g) at elevated pressures. In contrast to similar experimental arrays, coupons remained at their free corrosion potential for most of the test duration except for intervals of 1024 s in which ECN measurements were conducted. Temperature was increased stepwise from 25°C to 200°C after each ECN block. During ECN measurements, potential and current fluctuations were recorded using a zero resistance Ammeter (ZRA) under steady state conditions. This procedure was fully automated and required minimal user intervention.