Stress corrosion cracking of a supermartensitic steel in acidic 5% NaCl solution in the presence on thiosulphate and its inhibition

Supermartensitic steel (SM) was developed as substitute for martensitic steel in OCTG. The mechanical characteristic of the steel are very interesting, its resistance to pitting and general corrosion in brine in the presence of CO2 is much higher than that of martensitic steel, but its resistance to sulphide stress corrosion (SSC) is not very high. Laboratory tests indicated that the highest partial pressure of H2S, which SM can support, was 1 kPa. Moreover the SM can be used in sweet wells. In this paper we studied the behaviour of SM UNS S41425 in 5% NaCl, buffered with acetic acid and sodium acetate in the pH range between 2.0 to 6.0, in the absence or in the presence of thiosulphate at different concentration. Slow Strain Rate Tests (SSRT) were performed at a deformation rate of 2x10-6 s-1. Anodic and cathodic polarisation curves were recorded on cylinders of the same steel, starting from the corrosion potential of the electrodes after two hour immersion, at a scan rate of 0.2 mV x s-1. All the tests were conducted at 25°C. The results of the SSRT were compared with those published by other authors on the same steel in solutions of the same pH, but saturated with different partial pressure (pp) of H2S. The results obtained at pH 2.7 and 3.5 were comparable. At pH 2.7, 1kPa pp of H2S gave the same results of 10-5 M thiosulfate; at pH 3.5 10-4 M thiosulphate produced the same effect of 5 kPa pp of H2S. At pH 4 or higher, due to the higher stability of thiosulphate, SM was not susceptible to SSC. In the most aggressive solution - 5% NaCl + 0.5% CH3COOH + 10-3 M S2O32- pH 2.7- the inhibiting efficiency of some organic compounds was tested. Dodecylpyridinium chloride (DDPC), Dodecylquinolinium bromide (DDQBr), Benzyl,stearyl,dimethyl ammonium chloride (BSDMAC), 1-(2aminoethyl)-2-n-hexyl-2-imidazoline (IMI-6), and 1-(2aminoethyl)-2-n-tridecyl-2-imidazoline (IMI-13) were examined as inhibitors at 10-4 and 5 x 10-4 M concentrations. DDPC and DDQBr were the most efficient substances. Not only reduced these additives, at 10-4 M concentration, the corrosion current but they also inhibited completely the SSC susceptibility of SM in SSRT. At 5 x 10-4 M concentration IMI-6 also was shown to be a very good inhibitor. The influence of thiosulphate and the action of the inhibitors were discussed on the basis of the polarisation curves.