Corrosion behaviour of AlN and TiAlN coatings on iron

We have studied the corrosion behaviour of AlN and TiAlN coatings deposited by magnetron sputtering on pure iron substrates. The samples have been subjected to two different corrosion tests: (i) a Prohesion test and (ii) an accelerated corrosion test in constantly humid SO2-polluted atmospheres. The transformations induced in the coatings by the aggressive environments have been followed by X-ray photoelectron spectroscopy (XPS). The results indicate that under the corrosion conditions of the Prohesion test, the TiAlN coatings confer on the substrate a more efficient protection than the AlN coatings. Under the conditions of the accelerated corrosion tests in SO2-polluted atmospheres, the behaviour of both coatings is very similar in the initial stages since in both cases the corrosion of the underlying iron occurs just after 1 day of exposure to the aggressive conditions. However, it seems that once the corrosion has started it progresses faster under the AlN coating than under the TiAlN coating. The XPS results also show that the TiAlN coating is more chemically inert than the AlN coating, which suffers chemical transformations under the influence of the two different aggressive conditions considered. The better performance of the TiAlN coating is attributed to its denser microstructure since these AlN coatings have been observed to grow in a more open columnar structure. Copyright (C) 2006 John Wiley & Sons, Ltd.