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Corrosion Special Topical Papers |
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Protective Coatings: Performance Evaluation and Life Prediction*Dr. Jianhai Qiu
Electrochemical Impedance Spectroscopy (EIS)
In conjunction with ASTM D610 and D714 for visual evaluation of coating's performance after immersion in sea water, electrochemical impedance spectroscopy was used to study the correlation between the breakpoint frequency of a coating system and its performance in immersed condition for up to a period of 550 days [6]. For a typical coating thickness of 100 um, it took about 2 ~ 9 days for the chemical species such as oxygen, sodium ions and chloride ions to reach the metal/coating interface (Fig.1) and to initiate corrosion. The breakpoint frequency measurements after the initiation corrosion at the metal/coating interface can be used to predict the coating performance for up to 1.5 years into the future. Impedance measurements taken at exposure times less than the time required for the diffusion/permeation/migration of sodium ions to the metal/coating interface proved to be less successful in correlation with the long-term coating performance. The electrochemical impedance spectroscopy is not a standardized method yet. It is non-destructive and non-accelerating in nature. The time required to take one measurement on a 100 um thick coating can be as quick as 1 hour. The quick response and the non-destructive nature of the technique make it also a powerful tool for quality control/assurance in steel coating lines and other manufacturing processes involving coatings. Electrochemical Noise
Another electrochemical technique that has shown promise in coating's evaluation and life prediction is the electrochemical noise. The fluctuations of current and potential for a given system can be monitored simultaneously, leading to the potential or current or resistance noise methods. When a coating start to deteriorate, the potential of the system tends to shift towards the active/negative direction, eventually approaching the potential value of a bare steel. Mirroring the potential changes, the current tends to increase with time for the low performance coating systems. The noise resistance (Rn=Vn/In) for a good coating system was found to be above 1010 Ohms.cm2 upon immersion and decreases gradually to 109 Ohms.cm2 after 2000 hours testing. AC impedance and electrochemical noise techniques were also successfully used to rank the performance of several coatings systems (polyurethane, epoxy-polyamide and alkyds) after 12 month exposure to an industrial atmospheric environment [7]. It was observed that the electrochemical measurements after exterior exposures showed the same general trend as those generated by laboratory immersion tests. Concluding Remarks
The availability of various standardized and non-standardized test methods means that the search for the magic conversion factor where X hours of test (accelerated or non-accelerated, in laboratory or in field) can be extrapolated to Y years of service life is continuing. In many consulting projects, the author is often asked to prove how long a particular coating system will last. It takes a lot of effort to explain to the clients that no such test methods exist yet. The standardized ASTM methods are accelerated tests which may require several thousand hours of continuous operation for a typical coating system (ASTM D5894 requires more than 4000 hrs). These tests are all destructive in nature and the test results are qualitative. In contrast, the electrochemical impedance and electrochemical noise methods, though not standardized, can provide rapid and quantitative measurements of the protective properties of a coating system. These measurements are non-destructive and non-accelerating. It may be possible to find a magic conversion factor for life prediction if one monitors the long-term electrochemical responses of coating system under exterior exposure condition. The coatings designers/specifiers, suppliers, contractors and facility owners should consider the available options and agree on the specific method for performance evaluation of a coating system. They must also realize the risks involved in extrapolating the accelerated test results. If you want to know more about this topic, here are three short courses:
Life Predication of Corrodible Structures and Components Protective Coatings: Inspection, Maintenance and Repair Electrochemical Impedance Spectroscopy -Measurement, Modeling, Data Interpretation & Applications
These corrosion short courses can be taken as in-house training courses, course-on-demand, online courses or distance learning courses.
REFERENCES:
1. E. D. Thomas and A. A. Webb, Journal of Protective
Coatings and Linings, Feb. 2001
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