On the corrosion behavior of TiN-coated AISI D2 steel

An extensive investigation on the corrosion behavior of the TiN-coated D2 tool steel was performed. It is demonstrated that the corrosion properties of the TiN-coated D2 steel are primarily determined by the synergetic effect of the packing factor and thickness associated with the coating. The packing factor, a normalized mass density, reflects the denseness of a coating; whereas the coating thickness relates to the length of the diffusion path of corrosive media. These factors are beneficial to corrosion protection as they are increased in magnitude, thus leading to a requirement of minimum [packing factor] x [TiN thickness] value to safeguard against the corrosion in a certain environment. This minimum value has been evaluated to be approximately 2.4 in a 5% NaCl solution and a 20% H3PO4 solution. Above the critical value, the corrosion current density of the TiN-coated D2 tool steel is practically nil. The beneficial effect of both packing factor and TiN thickness is consistent with the result of a 500 h salt spray test, which reveals that there exists a boundary separating the regime of corrosion immunity from that of corrosion attack in the [packing factor] vs. [TiN thickness] plot. It is also demonstrated that a Ti interlayer of approximately 0.2 mu m thick between TiN and metal substrate is useful in reducing the overall corrosion speed of the TiN-coated D2 steel, in addition to its being able to enhance the adhesion strength of the coating. (C) 1999 Elsevier Science S.A. All rights reserved.