Influences of Inorganic Coatings on Corrosion Resistance of Invar Alloy

Invar alloy (64% Fe & 36%Ni) has low coefficient of thermal expansion (CTE). Invar’s CTE is the lowest of any metal. This alloy is Iron based, so it can be corroded and need protection from corrosive species. In this research, corrosion prevention of Invar by inorganic surface coatings was investigated. Protective influences of electroless nickel plating, electroless nickel/zinc plating double layer, and zinc rich ethyl silicate coatings were studied. Electrochemical polarization test and Tafel extrapolation were used to characterize the corrosion behavior of each specimen. Results showed that the corrosion current density (icorr) of bare Invar was about 4.08 μA/cm2 and the electroless nickel coating could not decrease icorr of Invar, suitably. In addition, the specimen with electroless nickel/zinc coating showed low icorr about 5.30×10-2 μA/cm2. Also, with zinc rich ethyl silicate, the corrosion protection mechanism was cathodic and after 20 days, it changed to barrier mechanism with icorr about 6.71×10-2 μA/cm2 in icorr in 3.5% wt sodium chloride solution.