Corrosion protection with hard coatings on steel: Past approaches and current research efforts

This work gives a review on published results on the corrosion behavior of hard coatings deposited on steel substrates by mainly physical vapor deposition (PVD) or in some cases by plasma-assisted chemical vapor deposition (PACVD). Corrosion investigations of our own lab have been added at several sites to complete missing knowledge or to introduce new non-published data. A detailed discussion of thin film growth, microstructure and the kind of defects inherent in hard coatings has been given. Then the influence of coating thickness and some processing parameters on the corrosion resistance of coated steel is demonstrated. After this incipient part, the corrosion behavior of hard nitride coatings, multilayer coatings, alloyed or modified nitride coatings, insulating nitride, oxide or oxynitride coatings and duplex treated steel is being presented. Own investigations and published results show that single layered, conducting nitride coatings can protect steel to some extent only if very thick coatings (≥ 10 μm) are applied or if energetic deposition methods (e.g. ion-assisted deposition) are used. Distinct improvements of the corrosion behavior of hard coatings can be obtained by a proper multilayer structure deposited by deposition methods allowing energetic condensation, i.e. possessing a high ion-to-atom flux ratio. Alloying TiN by a sacrificial element, like magnesium, is also capable of significantly improving the corrosion resistance of low alloyed steel. Amazing improvements have also been reported for insulating coatings like AlN, oxynitride coatings or oxide coatings. However, the most significant improvement seems to be the application of duplex treatments. Plasma nitriding + PVD coating or PVD coating + atomic layer deposition (ALD) prolonged the time to red rust in neutral salt spray test for > 2 days up to one week, respectively. The only disadvantage of the duplex treatments is the higher cost. Recently, mapping by a confocal microscope of TiN coating surfaces before and after salt spray testing showed that open porosities in micro- and macro-scale growth defects mainly govern the corrosion behavior of hard coatings on steel. The coating microstructure – i.e. the submicron-scale morphology – seems to play only a secondary role in the corrosion behavior.