Influence of zinc sulphate on the corrosion resistance of l80 alloy immersed in sea water in the absence and presence of sodium potassium tartrate and trisodium citrate

Abstract

Seawater can be used in cooling water systems. L80 can be used as pipeline carrying sea water. However, this alloy will undergo corrosion. Corrosion can be prevented by addition of inhibitors such as sodium potassium tartrate (SPT), trisodium citrate (TSC), and zinc sulphate. Corrosion resistance of L80 alloy in sea water in the absence and presence of the above inhibitors has been evaluated by polarisation study and AC impedance spectra. It is observed that SPT and TSC show better inhibition efficiency in the presence of Zn2+. Further it is found that SPT–Zn system is better than the TSC-Zn system. When the SPT or TSC inhibitor is added to sea water, a protective film is formed on the metal surface. This prevents the transfer or release of electrons from the metal surface to the bulk of the solution. Thus corrosion is controlled. This is revealed by the fact that, during polarisation study, the linear polarisation resistance value increases and corrosion current value decreases. During electrochemical impedance study, charge transfer value increases and double layer capacitance value decreases. In the presence of Zn2+, corrosion resistance of the metal further increases, which is supported by the fact that the linear polarisation resistance value further increases and corrosion current value further decreases. Similarly during electrochemical impedance study, the charge transfer resistance increases tremendously and the double layer capacitance value decreases to a great extent. It is inferred that in the presence of Zn2+, more inhibitor is transported towards the metal surface as a Zn2+ – inhibitor complex. On the metal surface, an iron inhibitor complex is formed on the anodic sites of the metal surface and Zn2+ is released. The released Zn2+ combines with OH– to form Zn(OH)2 on the cathodic sites of the metal surface. Thus in the presence of Zn2+, both anodic reaction and cathodic reaction are controlled effectively. This accounts for the increase in corrosion resistance of metal in sea water in the presence of inhibitor and Zn2+.