Corrosion behaviour of a surface-treated AISI H11 hot work tool steel in molten aluminium alloy

Immersion tests in a molten aluminium alloy of a surface-treated AISI H11 hot work tool steel were carried out in order to study the corrosion mechanisms and the effect of different surface treatments on the resistance of the material. The steel has a great reactivity with molten aluminium, which causes a generalised attack on the exposed surface. Plasma nitriding and a proprietary gas oxynitriding treatment (NIPRE®) do not change the attack morphology, but improve the corrosion resistance because they reduce wettability and require the dissolution of the surface compound layer formed by the diffusion treatment, before the alloy reacts with the steel. PVD coatings are able to greatly improve the corrosion resistance. They change the mechanism from a generalised attack to a localised one. The improvement is further enhanced by a preliminary nitriding of the base material. The coating density of through-coating defects and the resistance to the thermomechanical stresses arising during immersion are the key properties of the coated surfaces.