Improvement of the corrosion resistance of CrN coated steel by an interlayer

The electrochemical behaviour of TiN and CrN hard coatings on steel substrate is investigated. The chromium nitride coating is compared with two double layer coatings (CrN/TiN/steel and CrN/Ni/steel) with the aim of improving the corrosion resistance of the steel substrate. The transition metal nitrides TiN and CrN are deposited by using a reactive cathodic are evaporation technology in an industrial device, while the intermediate nickel layer is deposited by an electroless process. The nitride coatings are fairly dense with a microcolumnar morphology. Adhesion is as good as usually observed with this technology. The TiN coating is characterized by a strong [111] orientation and the so-called 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 substrate. The substrate is not completely isolated from the environment and the corrosion is localized through the open porosity. This corrosion 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 system CrN/Ni strongly improves the corrosion resistance of CrN coated steel, whereas the system CrN/TiN coating has no beneficial effect compared to the single CrN layer. (C) 1998 Elsevier Science S.A. All rights reserved.