ASTM D 6676-2021 “Determination of Cathodic Exfoliability of Pipe Wall Coatings upon Heating by Internal Heating Method”

Meaning and Purpose
4.1 Damage to pipe coatings is almost inevitable during transportation and construction. Breaking or peeling of the pipe coating may expose the pipe to possible corrosion because after the pipe is installed underground, the surrounding soil will be moist and will constitute effective electrolytes. The cathodic protection potential applied may cause the coating to loosen, starting from the holiday edge. Spontaneous holidays may also be caused by this potential. Typically, external pipe coatings coated on pipes carrying thermal media (oil, gas) are exposed to high temperatures inside the pipe and low temperatures outside, and are subject to temperature gradients. The heat flux is directed from the metal (substrate) to the coating. The test method provides accelerated conditions for cathodic stripping that occurs under simulated heating and provides a measurement of the resistance of the coating to such effects.

4.2 The effect of the test will be evaluated by physical examination and monitoring the current consumed by the test sample. Usually there is no correlation between the two evaluation methods, but both are significant. The physical examination involves assessing the effective contact of the coating with the metal surface based on the observed difference in relative adhesive bonding. It is often found that the cathodic debonding region propagates from the region where adhesion is zero to the region where adhesion has reached the original level. There may also be intermediate areas of reduced adhesion.

4.3 Assumptions related to the test results include:

ASTM D 6676-2021 “Determination of Cathodic Exfoliability of Pipe Wall Coatings upon Heating by Internal Heating Method”

4.3.1 The maximum adhesion or bonding force is found in the coating not immersed in the test liquid, and

4.3.2 The reduced adhesion in the immersion test area is the result of cathode debonding.

4.4 On a comparative basis, the ability to resist stripping is of professional quality, but stripping in this test method is not necessarily an unfavourable indication of coating performance. The advantage of this test method is that all the dielectric type coatings that are now commonly used destick to some extent, thus providing a way to compare one coating with another.

4.5 The current flow in the test unit is a relative indicator of the extent of the area requiring corrosion protection; However, the current densities present in this test are much greater than those normally required for cathodes protection in natural inland soil environments.

4.6 Test voltages higher than the recommended voltage may cause chlorine gas formation. Subsequent chemical effects on the coating may call into question the interpretation of the test results. A filter tube with a sintered disk (see test method G95) or a sand layer (40 mesh) on the coated surface can reduce this effect.

ASTM D 6676-2021 “Determination of Cathodic Exfoliability of Pipe Wall Coatings upon Heating by Internal Heating Method”

Scope of
1.1 This test method describes an accelerated procedure for determining the comparative characteristics of a coating system applied to the exterior of a steel pipe to prevent or mitigate corrosion that may occur underground or in water where the pipe carries a heating medium and is under cathodic protection. The test method is intended for use on samples of coated tubes, or specimens cut from coated tubes or plate parts, and is applicable to such samples when the coating has an electrical barrier function.

1.2 The test method is designed to simulate the exposure of the outer coating to the high temperature inside the pipe and the ambient temperature outside, thus being affected by the temperature gradient. If high temperature is required but no temperature gradient is available, see Test Method G42.

1.3 Values expressed in SI units or inch-pound units should be treated separately as standard values. The values specified in each system are not necessarily the same; Therefore, to ensure compliance with the standards, each system should be used independently of each other and the values of the two systems should not be combined.

ASTM D 6676-2021 “Determination of Cathodic Exfoliability of Pipe Wall Coatings upon Heating by Internal Heating Method”

1.4 This standard is not intended to address all safety issues, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health and environmental practices and to determine the applicability of regulatory restrictions prior to use.

1.5 This international Standard has been developed in accordance with the internationally recognized standardization principles established in the Decision on Principles for the Development of International Standards, Guidelines and Recommendations issued by the Committee on Technical Barriers to Trade of the World Trade Organization.

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