A review and development of accelerated test methods for anti-corrosive organic coatings
To enable improvements in the development of anti-corrosive coatings quick methods of evaluation are required and several are available which are both qualitative and quantitative. This investigation reviews both types of method, the first in the form of traditional salt spray exposure and the second in the form of electrochemical techniques. The emphasis in the experimental work reported here is on the Electrochemical Noise Measurement (ENM). ENM has been used to monitor coatings under immersion conditions, the aim being to assist a paint company develop a set of more environmentally friendly coatings. The immersion test has also incorporated a temperature cycle which proved effective at separating ‘good’ coatings within a short timeframe. Results showed good correlation between ENM and salt spray testing. Work is also reported which was done with the aim of making the ENM method more practically useful. The standard configuration (‘Bridge’) requires two separate specimens which is unattractive for site work. The Single Substrate (SS) arrangement was developed to get around this problem but this still requires the metal to be connected to the measuring instrument. This is avoided in the most recent development which needs No Connection to Substrate (NOCS). Results are given for immersed samples monitored using the ENM NOCS arrangement and compared with the standard ‘Bridge’ method and DC resistance. Results are also presented using sets of different electrodes (platinum, calomel and silver/silver chloride). This preliminary work has shown that the NOCS method holds great promise. In the laboratory Electrochemical Impedance Spectroscopy (EIS) is also commonly employed to assess the performance of anti-corrosive coatings. Concluding this work a comparison of the ENM and EIS techniques was undertaken on a set of laboratory samples. Results showed that both methods had the ability to rank the performance of coatings. However ENM’s advantages (as outlined above) were confirmed
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