The present study reports an ongoing research to use a standardized electrochemical test for determining the degree of sensitization (DOS) of austenitic stainless steel components and equipment non-destructively in the field. DOS is understood as the higher rate of dissolution at the grain boundary region vs. the bulk grain, which is related to a chromium-depleted zone near the grain boundary. The double loop electrochemical potentiokinetic reactivation method (DL-EPR) was taken as a reference. Tests were performed in austenitic stainless steel type AISI 304 (UNS S30400). The main parameter calculated from the electrochemical test is Ir/Ia, the ratio of peak current during reactivation (backwards) and activation (forward) scans. It is assumed that DOS increases with Ir/Ia. However, cold work and inclusion content could affect the test results, hence potentially acting as false positives. These variables were included in the experimental matrix. Thermal aging time at 677 °C, amount of cold work and inclusion content were varied to understand their effect on electrochemical response. The solution used for the tests contains sulfuric acid and thiocyanate ion, and common practice is to freshly prepare it just before the test. Measurements are usually performed under deaerated conditions. However, field deployment of the technique could be facilitated if naturally aerated solutions prepared in advance in a laboratory can be used. Hence, the influence of oxygen content in solution and the effect of solution aging on measured Ir/Ia was studied. A simple electrochemical cell was developed for field measurements, based on commercial falcon tubes and 3D-printed plastics parts.
Austenitic stainless steels are widely used in refineries and petrochemical industries due to their good combination of properties such as workability, mechanical strength and corrosion resistance [1]. However, one of the most important problems they show, and which can lead to failures in service, is the susceptibility to intergranular corrosion and intergranular stress corrosion cracking (IGSCC) [2]. When these materials are subjected to temperatures in the range from 500 °C to 800 °C, the precipitation of chromium-rich carbides occurs preferentially at grain boundaries (GB) [1]. In practice, the undesired exposure to this temperature range can happen due to excursions in process variables or during welding. The precipitation of carbides at GB causes chromium-depleted zones in areas adjacent to GB. When Cr concentration falls below 12%, the formation of an adequate passivating layer is impeded and corrosion resistance decreases [1]. This phenomenon is known as sensitization.
