Corrosion Special Topical Papers
Protective Coatings: Performance Evaluation and Life Prediction*
Dr. Jianhai Qiu
Methods used to evaluate the performance of protective coatings and their capability to predict the service life in real environments are discussed in the context of the degradation process of a coating system. The accelerated tests based on existing standards (ASTM) are compared with some non-standardized methods such electrochemical impedance and electrochemical noise methods. Fairly good qualitative correlation of accelerated test results with service performance has been reported. It is noted that these "accelerated" tests may not really yield results in an "accelerated" way as most methods require several thousands of hours of exposure and the they are often destructive in nature. On the other hand, electrochemical impedance and and electrochemical noise methods are non-destructive and non-accelerating in nature yet they can produce quantitative or semi-quantitative results within a few days or even hours. When real-life exposure tests are used in conjunction with the non-destructive electrochemical impedance/noise methods, realistic models for life prediction of protective coatings may be developed.
Protective coatings is probably the most widely used method for combating corrosion. Steel structures exposed to atmospheres, buried in the soil or immersed in the sea water are commonly protected with coatings either alone or in combination with cathodic protection. This broad term - "protective coatings" encompasses metallic coatings, inorganic coatings and organic coatings as shown in Table 1.
Table 1 Types of Protective Coatings
No matter how complicated the design and formulation of a coating system may seem to be, its protective properties all boil down to two basic functions:
Any factors that may influence how the above two functions work will have
an influence on the protective property of a coating system (for better
or for worse). Some of these factors are coating selection, surface preparation,
application condition, inspection and routine maintenance. When a coating
system failed prematurely, the coating's supplier, the contractor and the
facility owner often do not agree with each other over the causes of the
coatings failure and who should pay for cost of rectifying the problem.
Sometimes lawsuits do arise from dispute of this nature. It was reported
that a coating system expected to last for 25 years failed only after 12
months in service ! While it is possible to
compare the relative salt spray resistance of different coating systems,
there is no straightforward answer to the simple question of how long the
coating will last. It is hoped that this paper will bring the coatings
designers/specifiers, suppliers, contractors and facility owners a step
closer in the understanding of the pros and cons associated with various
methods of coatings evaluation and life prediction.