Development and evaluation of aluminium-based thin films as potential alternatives to cadmium coatings

Electroplated cadmium coatings have been extensively used for many years in the military, marine and aerospace industries to provide corrosion protection to steel substrates. Cadmium coatings act as a barrier layer, protecting the substrate from corrosion attack. They continue to offer protection even when damaged. Cadmium deposits also offer sacrificial protection by preferentially corroding and galvanic protection to coated components in contact with other metals such as aluminium. However the hazardous nature of cadmium which is harmful to both humans and the environment, is a major issue. Started over 30 years ago, research into finding more environmentally-friendly alternatives continues as no substitute coating has yet been developed which could replace cadmium over the full range of applications. In this study, metallic AlCr(N) coatings containing 4 – 16 at% Cr and two different nitrogen contents (11-12 at% and 18 at%), were deposited by magnetron sputtering on stainless steel substrates - following initial coating deposition trials by both magnetron sputtering and electron-beam PVD. The coatings were then analysed to evaluate their mechanical, tribological and structural properties before being corrosion-tested using a novel AC/DC/AC cyclic test equipped with a pH monitoring system. The results indicate significant improvement in the mechanical (hardness range: 2 – 3.9 GPa) and tribological properties of the coatings with increasing alloying element content. In terms of corrosion performance, AlCr(N) with the lowest Cr content (i.e. 4 at%) displayed the best corrosion resistance properties owing to the formation of a thick and stable passive film within the pores as well as on the coating surface, providing excellent barrier corrosion protection. These promising results could be used to design graded Al-rich coatings in which individual layers are tailored to meet specific property requirements depending on the intended application, thereby enabling the replacement of cadmium on a range of steels and other engineering materials.