IMPROVEMENT OF THE CORROSION-RESISTANCE OF CRN COATED STEEL BY AN INTERLAYER

The electrochemical behaviour of TiN and CrN hard coatings on steel su bstrate is investigated. The chromium nitride coating is compared with two double layer coatings (CrN/TiN/steel and CrN/Ni/steel) with the a im of improving the corrosion resistance of the steel substrate. The t ransition metal nitrides TiN and CrN are deposited by using a reactive cathodic are evaporation technology in an industrial device, while th e intermediate nickel layer is deposited by an electroless process. Th e nitride coatings are fairly dense with a microcolumnar morphology. A dhesion is as good as usually observed with this technology. The TiN c oating is characterized by a strong [111] orientation and the so-calle d CrN coating is in fact a mixture of Cr(N) and Cr2N or CrN nitrides. The electrochemical behaviour of the monolayer TiN or CrN is somewhat similar to an inert but porous layer overlying an active corroding sub strate. The substrate is not completely isolated from the environment and the corrosion is localized through the open porosity. This corrosi on mode leads to more deleterious damage than corrosion occurring on a bare substrate. In order to increase the corrosion resistance of the coated steel, a decrease in porosity is necessary. The two layer syste m CrN/Ni strongly improves the corrosion resistance of CrN coated stee l, whereas the system CrN/TiN coating has no beneficial effect compare d to the single CrN layer. (C) 1998 Elsevier Science S.A. All rights r eserved.