Electrochemical corrosion characteristics of Ni-Resist type 2 in sea water containing ferrous sulphate

Abstract

The corrosion characteristics of Ni-Resist type 2 alloy in sea water have been determined in relation to dissolved oxygen (DO) content, pH, and concentration offerrous sulphate (injected to inhibit corrosion of copper based alloy condenser tubes). The alloy underwent uniform cathodically controlled corrosion so that relatively high pH and low DO provide an acceptable environment. Compared with similar pH conditions in the absence of added iron, Fe²⁺ dosing of aerated sea water to ≤100 ppm at pH<5 increased the corrosion rate, whereas dosing at 1000-2400 ppm (pH<5) decreased it. Addition of ferrous sulphate to deaerated sea water further suppressed the already low corrosion rates in this medium. Even under aggressive conditions of anodic polarisation, the alloy showed near uniform attack of the major constituent metals, leaving, however, a graphitised surface. The possible stagnation of a concentrated ferrous sulphate solution injected into the condenser circulating cooling water system has been considered. It is also concluded that ferrous sulphate dosing does not alter the stress corrosion cracking susceptibility of the alloy.