A Rapid Assessment of Non-Chromate Primers for Zn/Ni Plated Steel By Electrochemical Techniques

The pace of non-chromate coating developments is largely governed by a time-consuming coating evaluation process for its anti-corrosion properties. The most common accelerated test protocols for coating evaluation: (1) ASTM B117 Salt Spray, (2) ASTM D 5894 Cyclic Salt Fog/UV exposure, or (3) SAE J2234 Cyclic Corrosion test. Those tests are not only very time-consuming, but also do not give quantitative information about coating degradation process.

There is an increasing interest in rapid assessment methods for practical applications which provide an early indication of long-term coating performance. Several EIS-based accelerated test protocols^1-7 have been developed, including (1) thermal cycling testing¹ by impedance measurement at high temperature, (2) rapid electrochemical assessment of paint (REAP)^2-4 by impedance measurement and cathodic disbonding tests on two identical samples, and (3) the AC/DC/AC procedure^5-7 by combining impedance tests before and after a DC disbonding on the same sample. Unfortunately, above test protocols are not guaranteed to work for every coating in every environment.

In this work, we propose a different electrochemical stress cycle test protocol, in combined with coating damage tolerance technique that aims to determine quantitatively the galvanic functionality of Zn/Ni layer in non-chromate primed steel sample. The effect of coating damaged size is also investigated. It is assumed that the lowest dissolution of Zn/Ni layer, the longest lifetime of the non-chromate primer. The new test protocol has been validated as a useful method for determining a performance ranking of a series of non-chromate primers in very short times, comparing it with other different evaluation methods including salt fog test per ASTM B117⁸.

Acknowledgements

This work was performed under United Technologies Corporate R&D funding.

References

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