The standard "ASTM G95-07 Standard test method for cathodic dissociation testing of pipeline coatings (attached cell method)" developed by the American Society for Testing and Materials (ASTM) describes an accelerated test method for simultaneously determining the comparative characteristics of coating systems applied to the exterior surface of standpipes to prevent or reduce corrosion that may occur in underground service where the pipe will be in contact with natural soils and will 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.
.jpg "ASTM G95-07 Standard Test Method for Cathodic Dissociation Testing of Pipeline Coatings (Attached Cell Method)")
This test method is designed to facilitate testing of coatings where the test cell is cemented to the surface of the coated pipe sample. This is appropriate when it is impractical to immerse or dip the test sample as required by the test methods ASTM G8-96, ASTM G42-11 or ASTM G80-07 standards. Coating sample configurations such as flat plate and small diameter pipe may be used as long as the test procedure remains unchanged. This test method allows for options that must be identified in the report.
The test method described subjects the coating on the test specimen to electrical stress in a highly conductive alkaline electrolyte. The electrical stress is obtained from a direct current system. An intentional break in the coating shall be made before starting the test. Electrical instruments are provided to measure current and potential throughout the test cycle. At the end of the test period, the test specimen is physically examined. The physical examination is accomplished by comparing the extent of loosened or separated coating in the intentional break in the immersed area with the extent of loosened or separated coating in a reference break in the coating in an unimmersed area.
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 break. Such potentials can also cause spontaneous failure. This test method provides accelerated conditions for cathodic breakage to occur and provides a measure of the coatings resistance 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 property on a comparative basis, but separation in this test method is not necessarily 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 extent, thus providing the opportunity to compare 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.
When preparing the test specimen, during the coating process, the applicator may be directed to cut a section of coated pipe of sufficient length for laboratory evaluation of the coating. When cutting the pipe, precautions should be taken to prevent spatter from damaging the coating at the location where the test is to be performed. Wet cloths should be placed on both sides of the torch-cut area to minimize thermal changes and spatter damage to the coating. The test specimen should be taken at least 76,2 mm from any torch-cut edge. Special precautions should be taken to prevent damage to the coating during handling, transportation, or further cutting. Test specimens may be a cut coupon, a flat sheet, or a ring specimen. Testing may also be performed on an actual in-service pipeline or any other suitable coated surface.
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