Determination of the critical pitting temperature of martensitic and supermartensitic stainless steels in simulated sour environments using electrochemical noise analysis

Abstract

The use of corrosion-resistant alloys (CRA) 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, CRA 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 13Cr and super 13Cr martensitic stainless steels in simulated production environments in equilibrium with up to 850 ppm hydrogen sulfde (H2S)(g) and 1.7% carbon dioxide (CO2)(g), bal. N2(g), at elevated pressures. In contrast to similar experimental arrays, coupons remained disconnected (i.e., exposed individually at their free corrosion potential) for most of the test duration except for intervals of 1,024 s in which ECN measurements were conducted. Temperature was increased stepwise from 25°C to 200°C after each ECN block. During ECN measurements, current fuctuations were recorded using a zero-resistance ammeter (ZRA) and the potential of the pair was recorded simultaneously under steady-state conditions. This procedure was fully automated and required minimal user intervention. © 2010, NACE International.