FORMABILITY AND CORROSION-RESISTANCE OF TIN-COATED STAINLESS-STEEL SHEET

Refractory binary nitrides such as TiN used as coatings can improve the corrosion resistance of stainless steels. However, the necessary prerequisite is that the coatings are defect-free. Recently it has been proposed that TiN should be deposited directly on stainless steep strip and that moderate plastic forming of the TiN-coated steel may be possible without appreciable cracking. Results from uniaxial and biaxial tensile tests with TiN-coated stainless steels are reported. The surface integrity and possible formation of cracks were studied both visually and by using scanning electron microscopy and scanning tunnelling microscopy. Corrosion tests were carried out to evaluate the performance of the samples after forming. The tests showed that although TiN is a brittle material its adhesion to the substrate is surprisingly good. The coating thickness affected the crack behaviour. The number of cracks decreased with increasing thickness. The crack pattern changed from transverse to oblique cracking at a particular strain level. The corrosion resistance of the TiN-coated steel was decreased by subsequent forming but it still exceeded that of the stainless steel substrate.