Corrosion protection of steel substrates by magnetron sputtered TiMgN hard coatings: Influence of surface morphology and Mg content on Mg release in NaCl solutions

TiMgN coatings with three different Mg contents (10, 20 and 35 at.%) were prepared by reactive magnetron co-sputtering. Three different coating microstructures have been studied by using i) polished and ii) sand-blasted sample surfaces and iii) by employing the Glancing angle deposition (GLAD) technique on polished substrates. The chemical composition was analyzed by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometry or by glow discharge optical emission spectrometry and coating morphology was characterized by SEM on focused ion beam sliced samples. To determine the Mg release from the TiMgN samples two techniques measuring the open circuit potential were employed: a) an in-situ technique by using a scanning flow cell coupled to an inductively coupled plasma mass spectrometer and b) an ex-situ technique, namely inductively coupled plasma optical emission spectroscopy. The results (measured Mg release) from these two analysis techniques are compared and discussed with respect to results obtained from corrosion studies performed in neutral salt spray tests. It was found that the lowest Mg release has been determined for polished and the highest for GLAD samples. A correlation between the amount and duration of Mg release with the corrosion behavior of the samples could be established and formerly proposed corrosion protection mechanisms could be confirmed.