Corrosion Resistance of Cupronickels - An Overview

Cupronickel alloys have been extensively used in marine and offshore applications. These alloys offer desirable physical and mechanical properties to be used as a versatile engineering alloy. The oxidation resistance of these alloys was due to the formation of scales consisting of a thick, inner Cu2O layer and a thin, outer CuO layer, together with nickel rich internal oxide. These alloys are superior to bronzes in their corrosion resistance but fail under oxidizing conditions. In sulphide atmospheres they undergo sulphidization. Depending upon the alloy composition, they form layers consisting of alloy/nickel sulphide/CuS of various proportions. In chloride solutions, they undergo localized attack. For lower chloride concentration, an increase in nickel content worsens the resistance to passive film breakdown, where as for higher chloride concentration an increase in the nickel content favour passivation. In seawater, the prime source is dissolved oxygen and the corrosion product is Cu2O. This is formed by the hydrolysis of CuCl2-. The use of ozone enhanced the dissolved oxygen content. Localised attack occurred at those points where the seawater is in motion. The impingement and general corrosion resistance was improved by the addition of iron in the alloy. Sulphides and ammonia as pollutants adversely influenced the performance of the alloy. Microbial adhesion alters the protective properties of the complex layers of the alloy. The bacteria enhanced the corrosion rates in seawater with intergranular corrosion and denickelification. The minor additions of an element, use of inhibitors in the medium and cathodic protection are the main methods of corrosion control in aqueous media. The various aspects of the corrosion resistance behavior of cupronickel alloys in different environments are discussed in the review