Effect of thiosulphate concentration on corrosion mechanisms of 13 Cr martensitic stainless steel corrosion in chloride solution

摘要

The use of brine containing thiosulphate solution to simulate the sour gas environment was recently evaluated in the scientific literature to overcome the problems related to H2S use for laboratory testing of CRA for Oil & Gas industry. On the other hand thiosulphate induced corrosion on stainless steel in chloride containing environments was deeply studied in the past. At high concentration thiosulphate has an inhibiting effect on pit propagation while at low concentration not enough thiosulphate can reach pit nucleus so inhibiting its propagation. Stress corrosion cracking is the competitive damage mechanism occurring in sodium chloride solution. Corrosion behaviour under tensile load of 13% Cr martensitic stainless steel in a modified NACE TM0177 standard solution at room temperature in presence of thiosulphate ions at different concentration (from 10~(-3) to 10~(-1) M) was evaluated by Acoustic Emission (AE) technique. Different corrosion attacks have been observed on specimen surfaces starting from tiny surface cracks at low thiosulphate content, passing through few large pits, precursors of mixed brittle fractures, for medium thiosulphate content, up to extended generalized corrosion for high thiosulphate content. Changes in steel ductility was also observed. AE techniques allowed to clearly highlight the different corrosion mechanisms activating during time.