Time dependence of inhibiting effect of amidoamines in CO 2 saturated chloride solutions

A mixture of long chain amidopolyamine carboxylates has been synthesised as the potential active component of an inhibiting system for mild steel in CO₂ saturated chloride solutions such as are encountered in sweet oil and gas wells. The inhibitor has been tested by potentiodynamic voltammetry, by cyclic polarisation, and by electrochemical impedance spectroscopy (EIS). The testing was performed under dynamic conditions at 50°C, varying the system concentrations (20, 50, and 200 ppm) and the times of steel exposure to the model solution (1, 3, and 24 h). After only 1 h immersion, high inhibitor efficiencies are obtained at all concentrations. With a prolonged immersion time the efficiencies are increased still further. The inhibitor is suitable both economically and ecologically since it is effective at low concentrations and it contains neither phosphorus nor sulphur compounds. Testing by EIS indicates that fast chemisorption on the steel is the dominant process of inhibition followed by a slow film forming process causing a gradual increase in efficiency with immersion time. EIS testing with external polarisation reveals the tendency of the inhibitor to be desorbed at higher potentials. This may provoke localised corrosion under circumstances where the redox potential of the solution rises owing to the introduction of oxidisers (e.g. oxygen).