Corrosion resistance of composite coatings with nickel base And disperse ceramic phase
Maria Trzaska, Anna Wyszyńska, Magdalena Kowalewska Politechnika Warszawska, Wydział Inżynierii Materiałowej, ul. Wołoska 141, 02-507 Warszawa
Annals 2 No. 5, 2002 pages 338-341
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abstract The paper is aimed on studies of the corrosion resistance of composite coatings Ni-Si3N4 and Ni-P-Si3N4 deposited on the steel substrate (St3) by the electroless process as well as by electrochemical techniques. A poly-disperse powder of the silicon nitride has been used for the depositions of the composite coatings. The silicon nitride powder has been composed of grains of dimensions in the range of 0.1÷1 μm constituting 80% of the whole volume and in the range of 1÷2 μm in the remainder part of the powder volume. The composite coatings have been deposited in the bath containing 5 g of the powder in 1 dm3 of the electrolyte. The examinations of the morphology of the deposited coatings and their deterioration rates after the corrosion tests have been performed with using the electron scanning microscopy techniques. The corrosion studies have been realised by the potentio-dynamical method in the 0.5 M solution of NaCl. It has been detected that the content of the disperse ceramic phase in the composite coating Ni-P-Si3N4 deposited by the electroless method has reached the level of 17% of the whole volume and it is greater than that of Ni-Si3N4 layer deposited by the electrochemical method. It has reached for this last composite coating the level of 11% of the whole volume. The diagrams of the anode polarisation of the steel substrate, of the Ni and Ni-P coatings as well as of the composite coatings Ni-Si3N4 are also examined in details (Fig. 1). The potentio-dynamical curves measured for all examined coatings have quite similar forms and they differ importantly from that of the steel substrate. Results of these examinations are reported and compared. Images of the corrosion deterioration of the studied coatings are shown in Figures 2-9. In the case of Ni coatings their corrosion deterioration tested in the 0.5 M of NaCl solution are quite different (Figs 2 and 6) with respect to the Ni-P (Figs 3 and 7) as well as to the Ni-Si3N4 and Ni-P-Si3N4 (Figs 8 and 9) composite coatings. The nickel coatings deposited by the electrochemical method, which are characterised by the crystal structure, suffered the intense penetration corrosion. The images of this coating destruction, the corrosion penetration forms and their depth as well as their positions on the coating surface are shown in Figs 2 and 6. In the case of Ni-P, Ni-Si3N4 and Ni-P-Si3N4 coatings the material diminution caused by the corrosion processes is uniformly distributed on the whole exposited surfaces. It has been demonstrated that introduction of the ceramic particles Si3N4 into the Ni coatings deposited by the electrochemical process leads to important increase in corrosion resistance of the composite coatings with respect to the Ni coating only. On the other hand the presence of the ceramic phase Si3N4 in the Ni-P layer has not influences on its corrosion resistance.