The standard "ASTM G42-11 Standard Test Method for Cathodic Dissociation of Pipe Coatings Exposed to Elevated Temperatures" developed by the American Society for Testing and Materials (ASTM) describes an accelerated test method for determining the comparative characteristics of insulating coating systems applied to the exterior surface of steel pipe to prevent or reduce corrosion where the pipe will be exposed to elevated temperatures and will be cathodic protected in underground services. This test method is designed for use with coated pipe samples from commercial production and is applicable to such samples where the coating is characterized by an electrical barrier function.
This test method is designed to test coatings immersed in a high temperature test solution. When immersion of the test specimen is not practical, the test method described in “ASTM G95-07 Standard test method for cathodic detachment testing of pipeline coatings (attached cell method)” can be used, in which the test cell is cemented to the surface of the coated pipe specimen. If room temperatures are required, “ASTM G8-96 Standard test methods for cathodic detachment of pipeline coatings” can be used. If a specific test method is required without any options, “ASTM G80-07 Standard test method for specific cathodic detachment of pipeline coatings” can be used.
This test method subjects the coating on the test specimen to electrical stress in a highly conductive electrolyte. The coating is artificially punctured before the test is started. Electrical stress is produced by connecting the test specimen to the negative terminal of a direct current source and connecting an anode to the positive terminal. Electrical instrumentation is provided to measure the current flowing in the cell. The electrical potential is also measured and the specimen is physically examined at intervals throughout the test period and after completion of the test. Cathode stress is applied under conditions of a constant elevated temperature.
Damage to the pipe coating is almost inevitable during transportation and construction. Breaks or failures in the pipe coatings can expose the pipe to possible corrosion because, once a pipe is underground, the surrounding soil carries moisture and forms an effective electrolyte. Applied cathodic protection potentials can cause the coating to loosen, starting at the edges of the failure. Such potentials can also cause spontaneous failure. This test method provides accelerated conditions for cathodic failure to occur and provides a measure of the resistance of the coatings to such action.
The effects of the test are assessed by physical examination and by monitoring the current drawn by the test specimen. There is usually no correlation between the two methods of assessment, but both methods are important. Physical examination consists of assessing the effective contact of the coating with the metal surface in terms of observed differences in relative adhesive bond. It is usually seen that the cathodically separated area extends from an area of zero adhesion to an area where adhesion has recovered to the original level. An intermediate zone of reduced adhesion may also be present.
Assumptions associated with test results include:
The ability to resist separation is a desirable comparative property, but separation in this test method is not a negative indicator of coating performance. The advantage of this test method is that all dielectric type coatings currently in common use will separate to some degree, thus providing a means of comparing one coating with another.
The current density seen in this test method is much greater than that typically required for cathodic protection in natural environments.
The ability to resist separation is a desirable property in comparison, but most insulation materials will separate to some degree under the accelerated conditions of this test. Bond strength is more important for proper operation of some coatings than others, and the same separation measured for two different coating systems may not represent an equivalent loss of corrosion protection.
The amount of current flowing in the test cell can be a relative indication of the extent of areas requiring corrosion protection, however, the current density seen in this test is much greater than that usually required for cathodic protection in natural, indoor soil environments. Test voltages higher than those recommended may result in the formation of chlorine gas. Subsequent chemical effects on the coating may cast doubt on the interpretation of the test results.
Among the numerous testing, measurement, analysis and evaluation studies that our organization has carried out for businesses in various sectors, our organization also provides testing services defined in the "ASTM G42-11 Standard test method for cathodic separation of pipe coatings exposed to high temperatures" standard with its trained and expert staff and advanced technological equipment.
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