SCC inhibition of a C-steel in acidic 5% NaCl solutions in the presence of thiosulphate

Abstract

The inhibiting effect of some organic compounds on the stress corrosion cracking (SCC) of a C-steel in 5% NaCl+0.5% acetic acid in the presence of 1 mM thiosulphate at 25°C was studied. The examined compounds at 0.1 mM concentration were: n-dodecylquinolinium bromide (DDQB), n-dodecylpyridinium chloride (DDPC), benzyl,stearyl,dimethylammonium chloride (BSDMAC), 1-(2-amminoethyl)-2 n-tridecyl-3 imidazoline (IMI-13), 2- n-hexyl-3 imidazoline (IMI-6), 2- n-tridecyl-3-imidazoline (IMI), N-phenyl-cinnamyliden-imine (PH-ATC). The inhibition of these compounds on the uniform corrosion was evaluated by recording anodic and cathodic polarisation curves after 1 and 24 h immersion of the electrode in the aggressive solutions. Slow strain rate tests were performed at a strain rate of 1×10 −6 s −1. The SCC susceptibility degree ( α) was evaluated by the ratio between percentage elongation to fracture in the test solution and that in air. The hydrogen permeation currents after 24 h of immersion of steel were determined. The examined compounds exerted an inhibiting action towards the general corrosion, ranging between 74 and 96%. The percentage inhibiting efficiencies (IE), calculated from the permeation current values of DDQB, DDPC and IMI, were almost the same as the percentage IE towards the general corrosion. In the presence of the other compounds percentage IE on the hydrogen permeation was lower than that towards the general corrosion. DDQB, DDPC and IMI were able to reduce the stimulating action of thiosulphate completely. The other compounds increased α values, but they were unable to inhibit SCC completely. The inverse linear relationship between α and hydrogen permeation current values evidenced a strict correlation between hydrogen permeation and SCC susceptibility in these solutions.