The introduction of modifying elements such as chromium and nitrogen into aluminum coatings applied by magnetron sputtering opens up prospects for a significant improvement in their corrosion resistance. The amorphous structures obtained as a result of the sputtering process contribute to the formation of a uniform passive layer that prevents the penetration of aggressive ions to the substrate. This study is devoted to a comparative analysis of the corrosion behavior of Al–Cr and Al–Cr–(N) amorphous coatings obtained by direct current magnetron sputtering in order to determine the optimal composition for protecting low-carbon steel from corrosion in aggressive environments.
As a substrate, low-carbon steel of the St3 grade was used. Before coating, the samples were subjected to mechanical grinding and polishing, and then cleaned by ultrasound in acetone and ethanol. Al–Cr and Al–Cr–(N) coatings were deposited by magnetron sputtering on a vacuum sputtering unit. The targets were Al–Cr alloys with different chromium and Al–Cr contents with additional nitrogen supply to the reaction chamber for the formation of Al-Cr (N) coatings. Sputtering parameters such as power, working gas pressure, and substrate temperature were kept constant.
Electrochemical methods were used to evaluate the corrosion resistance of coatings: polarization measurements in 3.5% NaCl solution and impedance spectroscopy (EIS). The surface morphology and elemental composition of the coatings were studied using scanning electron microscopy (SEM) and energy dispersive analysis (EDA). The phase composition was determined by X-ray phase analysis (XRD).
X-ray diffraction confirmed the amorphous structure of the obtained Al–Cr and Al–Cr–(N) coatings. SEM images showed that the coatings have a dense and uniform microstructure with a minimum number of defects. EDA confirmed the presence of aluminum and chromium in Al–Cr coatings, as well as the presence of nitrogen in Al–Cr–(N) coatings.
Polarization curves obtained in 3.5% NaCl solution showed that Al–Cr and Al–Cr–(N) coatings significantly improve the corrosion resistance of low-carbon steel. In particular, the corrosion potential shifts to the positive side, and the corrosion current density decreases by several orders of magnitude. The addition of nitrogen to the Al–Cr coating leads to a further decrease in the corrosion current density, which indicates a higher corrosion resistance of Al-Cr (N) coatings.
The EIS results showed that the impedance of Al–Cr–(N) coatings is significantly higher than that of Al-Cr coatings, which indicates the formation of a denser and more protective passive layer on the surface of Al-Cr – (N) coatings. Analysis of equivalent electrical circuits used to model the impedance spectra revealed that Al-Cr – (N) coatings have a higher charge transfer resistance and a lower capacitance of the electric double layer, which also confirms their higher corrosion resistance.
Amorphous Al–Cr and Al–Cr–(N) coatings applied by direct current magnetron sputtering effectively protect low-carbon steel from corrosion in 3.5% NaCl solution. The addition of nitrogen to the Al–Cr coating leads to a further improvement in corrosion resistance, which is associated with the formation of a denser and more protective passive layer. The results obtained show that Al–Cr–(N) coatings are promising for use as protective coatings for steel in aggressive environments. Further research will focus on optimizing the spray parameters and coating composition to achieve maximum corrosion resistance and adhesion to the substrate. In addition, it is planned to study the influence of various factors, such as temperature and pH of the medium, on the corrosion behavior of these coatings.
Author: J. Creus, A. Billard, F. Sanchette
Institute: Laboratoire d’Etude des Matériaux en Milieux Agressifs, Université de la Rochelle, F17042 La Rochelle Cedex 1, France, Laboratoire des Procédés de Traitement de Surface, CEA DTEN/SMP, F38054 Grenoble Cedex 9, France, Laboratoire de Science et Génie des Surfaces, Ecole des Mines de Nancy, Parc de Saurupt, F54042 Nancy Cedex, France