INTRODUCTION
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.
High pressure and high temperature (HPHT) environments containing high concentrations of hydrogen sulfide (H2S) represent a challenge for traditional corrosion management strategies (i.e. carbon steel + corrosion inhibitors). In such conditions, using corrosion resistant alloys (CRAs) become cost-competitive [1, 2]. At present, common materials used by oil and gas companies for both onshore and offshore production include martensitic and super-martensitic steels, such as 13Cr and super-13Cr alloys, duplex- and super duplex-stainless steels of up to 25% Cr, and Nickel-base alloys [2].
The use of CRAs has allowed 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 and crevice corrosion, which may lead to stress corrosion cracking (SCC). Therefore, determining the localized corrosion resistance of CRAs in well conditions is crucial to the successful application of these materials in HPHT wells.
The NACE MR0175/ISO 15156-2:2003 international standards as well as the European Federation of Corrosion (EFC) Publication 17 are widely used as guidelines for selection of CRAs for sour service [4, 5]. The standards propose a number of laboratory tests including 4-point bend, constant slow strain rate testing, and statically loaded C-ring specimens to determine sulfide stress cracking (SSC) and stress corrosion cracking (SCC) resistance [4, 5]. In addition, the ASTM G-48 standard and modifications of it are other tests commonly used to determine the susceptibility of a CRA to pitting and crevice corrosion [6].
The main limitation of these methodologies is that they are either time consuming, invasive, or may not represent the actual service environment. For example, NACE/ISO standards suggest testing for SCC resistance at the highest downhole temperature and chloride concentration, ignoring the possibility of synergisms between SCC and SSC modes at intermediate temperatures. Such effect has been recently reported for duplex stainless steels [3].
