Electrochemical methods for evaluation of the protective ability of electroplated coatings and conversion films

One of the main functions of electroplated coatings and conversion films is corrosion protection. For evaluation of their corrosion protective ability (CPA) different types of tests - natural conditions, artificial media and electrochemical methods - have been applied. The natural tests are limited because of their long duration. This is the reason for the application of accelerated tests being in a good correlation with natural ones. At present, different electrochemical methods for accelerated optimization of the CPA are applied. The multilayer Ni/Cr and Cu/Ni/Cr coatings, developed for severe corrosion media, consist of several Ni layers and a microdiscontinued Cr layer on the top. The realization of a high CPA in these coatings requires a control of the microdiscontinuity of Cr; the polarity, potential differences and thicknesses of the Ni layers, and the thickness of the Cu layer. This control is possible to be fulfilled effectively and fast by using of electrochemical methods. The corrosion protection of electrodeposited coatings with a large application - such as Zn, Zn-Fe, Zn-Ni, Zn-Co and others - is realized by chromate conversion films. The express evaluation of their protective ability is possible to be performed by the electrochemical method and apparatus developed in IPC based on an approved correlation between the values of the anodic potential maxima under galvanostatic polarization and the results from the accelerated tests in a neutral salt spray chamber. The replacement of Cr(VI), according to EU Regulations, requires the development of environmental friendly conversion coatings and methods for their test.