IDENTIFICATION OF FACTORS AFFECTING THE AQUEOUS CORROSION PROPERTIES OF (TI, AL)N-COATED STEEL

Carbides, borides and nitrides of transition metals are increasingly used as coating materials because of their high hardnesses and excellent wear resistances. However, these coatings can also potentially be used in applications requiring corrosion resistance because of their inherent stability under a variety of aqueous conditions. Thin film coatings based on these materials are generally produced by physical vapour deposition (PVD) methods. Such coatings often exhibit porosity which usually is thought to result from pinholes formed during deposition. If local defects form a direct path between the corrosive environment and the base material, the corrosion protection is lost. In this work the influence of different factors on the corrosion resistance of PVD (Ti, Al)N coatings on type ASP 23 high speed steel was studied. The factors evaluated were the thickness of the coating, the thickness and the type of the intermediate layer, the titanium-to-aluminium ratio of the coating and the number of heterogeneities in the coating. Electrochemical porosity measurements which are essential for the estimation of the corrosion resistance of coated components were made in 1 N sulphuric acid solution at ambient temperature. In these conditions, corrosion of a coated steel part occurs mainly in the base material. The corrosion resistance was also evaluated with immersion tests. The results of this work suggest the dominating role of the inhomogeneities on the corrosion resistance of the PVD (Ti, Al)N and TiN coatings. Preliminary analysis of the nature of the inhomogeneities is made and their possible source is discussed.