Aluminium-based Coatings for Cadmium Replacement

Abstract

Cadmium electroplating is widely used in the aerospace industry for the corrosion protection of high strength steels. Cadmium is also used as compatible coating to reduce the galvanic corrosion generated in the assembly of components manufactured with different materials. However, environmental and safety concerns over the high toxicity of cadmium has led to the investigation of suitable replacements. Aluminium coatings are promising coatings for the replacement of electroplated cadmium. Previous studies have shown that the use of SermeTel 984, a commercial aluminium sprayed coating, is beneficial in eliminating the hydrogen direct embrittlement without increasing the risk of re-embrittlement. However, the coating has shown to be prone to passivation in the mild corrosive environment. The addition of active zinc and magnesium particles are thought to avoid the passivation of the aluminium. A range of modified SermeTel 984 coatings, containing 0.5%, 3%, 10% and 50% of zinc, and SermeTel 984 modified with the addition of 30% and 50% of Mg/Al alloy particles in weight have been evaluated as possible alternatives. Chromium free SermeTel 984 and a SermeTel 984 modified with aluminium alloy 7075 particles together with IVD aluminium coatings have also been included in the project. The sacrificial protection of the new coatings has been evaluated together with the associated risk of hydrogen re-embrittlement and compared with the electroplated cadmium. Zinc modified SermeTel 984, containing 1% of zinc and 5% of zinc electroplated aluminium, IVD aluminium and electroplated Zn-Ni coatings have been studied as alternative coatings to cadmium as compatible coatings. Slow strain rate testing has been performed to study the effect of hydrogen on the re- embrittlement of steel substrate as a result of the corrosion of the aluminium-based coatings in 3.5% NaCl. Linear polarisation testing in 3.5% NaCl has been used to evaluate the self-corrosion rates of the coatings; galvanic coupling measurements have been used in the case of steel substrate to evaluate the sacrificial properties or, in the case of bronze/aluminium assembly, to evaluate the compatibility properties of the coatings. Polarisation behaviour tests have been used to study the anodic or cathodic control of the corrosion mechanisms. Total corrosion, calculated as the addition of self-corrosion and galvanic corrosion, has been calculated to evaluate the duration of the coating in service compared to electroplated cadmium.