The standard “ASTM G8-96 Standard test methods for cathodic decoupling of pipeline coatings” developed by the American Society for Testing and Materials (ASTM) describes accelerated test methods to simultaneously determine the comparative characteristics of insulation coating systems applied to the exterior surface of steel pipe to prevent or reduce corrosion that may occur in underground services where the pipe will be in contact with internal soils and may or may not receive cathodic protection.
This standard is designed for use with coated pipe samples from commercial production and is applicable to such samples where the coating is characterized by its electrical barrier function. This test method is designed to test coatings immersed in test solution at room temperature.
When immersion of the test specimen is not practical, the standard “ASTM G95-07 Standard Test Method for Cathodic Dissociation Testing of Pipeline Coatings (Attached Cell Method)” is used, in which the test cell is cemented to the surface of the coated pipe specimen. If higher temperatures are required, the standard “ASTM G42-11 Standard Test Method for Cathodic Dissociation of Pipeline Coatings Exposed to Elevated Temperatures” is used. If a specific test method is required with no options, the standard “ASTM G80-07 Standard Test Method for Specific Cathodic Dissociation of Pipeline Coatings” is used.
Both test methods described subject the coating on the test specimen to electrical stress in a highly conductive alkaline electrolyte. The electrical stress is achieved either by means of a sacrificial magnesium anode or by means of an impressed current system. The coating is punctured prior to testing.
In Method A, a magnesium anode is used without electrical monitoring during the test period. Results are determined by physical examination after the test period is completed.
In Method B, a magnesium anode or an impressed current system may be used. Electrical instruments are provided to measure the current in the cell circuit. The electrical potential is also measured and the test specimen is physically examined after the test period is completed.
In both test methods, physical inspection is accomplished by comparing the extent of loosened or separated coating in holes in the immersed area with the extent of loosened or separated coating in a new test hole made in the coating in an unimmersed area.
Breaks or deterioration in pipe coatings can expose the pipe to possible corrosion because once a pipe is placed underground, the surrounding soil carries more or less moisture and forms an effective electrolyte.
Damage to the pipe coating is almost inevitable during transportation and construction. Normal ground potentials and applied cathodic protection potentials can cause the coating to loosen starting from the edges of the failure and in some cases increase the apparent extent of the failure. Failures can also be caused by such potentials. While the apparent loosened coating and cathodic failures do not cause corrosion, this test provides accelerated conditions for the failure to occur and therefore gives a measure of the resistance of the coatings to such action.
The effects of the test can be assessed by physical examination or by monitoring the current drawn by the test specimen, and by either of these methods. 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 electrically stressed area spreads toward a boundary where the loosened coating separates for more effective contact or bond, which is attributed to the original condition across the specimen before the electrical stress was applied.
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