Concrete testing - GPR, thickness, strength. Pipeline Coatings Cathodic disbonding testing Protective coatings are applied to buried or immersed metallic structures thus providing a primary line of defence against corrosion and is usually used in conjunction with Cathodic protection. To test the coatings ability to prevent corrosion attack a cathodic disbonding test is used. This cathodic disbonding test method determines the resistance to cathodic disbonding of a coating system between coating and steel substrate, resulting in loss of coating adhesion. In this test, an intentional defect holiday on a coated panel or section of the coating to be tested is immersed in a saline solution contained in a test cell and cathodic protection is applied. After a specified period, the panel is examined for evidence of coating disbondment around the intentional defect and if present, the radial disbondment and disbonded area are determined.
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The first step in understanding cathodic disbondment testing is appreciating the need for cathodic protection. Pipelines are typically quite long and the potential for corrosion is so great that passive cathodic protection i.
Therefore, active cathodic protection imparted by an impressed current cathodic protection ICCP system is almost always used. This typically includes a DC power source, an AC powered transformer rectifier, and an anode ground bed. An ICCP system essentially interferes with the corrosion process and protects the pipeline or structure. The widespread use of ICCP dictates that any coating intended for use on the pipe be tested for its resistance to cathodic disbondment.
Cathodic disbondment is the loss of adhesion between the coating and metal substrate caused by the cathodic protection system. There are a variety of causes of cathodic disbondment of a coating system, including the environment to which the pipe is exposed, and the level of impressed current on the system.
The test specimen that is submerged is a representative section of coated pipe with one end capped or sealed to allow the solution to only contact the outside of the pipe. A magnesium anode is required for this test method. The potential between the test specimen and a reference electrode is maintained at approximately The method is written for a day test duration; however, test periods such as 60 or 90 days may be used.
The area of disbonded coating is measured and recorded. This method is most appropriate when the test samples are coated pipe sections and elevated temperatures are not required.
A day test period is specified by the method; however, longer test periods may be specified. The evaluation of the coating at the end of test is the same as described for the ASTM G8 test method. This method is most appropriate when the test samples are coated pipe sections and an elevated temperature is required. The method employs room temperature test conditions; however, a modified version of the method allows the test cell to be heated on a hot plate or in an oven. The test panel can be a flat panel or a curved section of pipe.
The attached cell can be a plastic or glass tube centered over the intentional holiday and sealed to the test surface with a waterproof sealer. The anode for this test method is an impressed current anode consisting of a platinum wire in an immersion tube with a fritted disc. The test duration is 90 days; the evaluation of the coating at the end of test is the same as described for the ASTM G8 test method.
This method is most appropriate when the test samples are coated panels or curved panels. The testing can be performed on coated pipe, a test specimen cut from a section of coated pipe, or a flat coated steel plate. If the tube specimen is used, the end of the tube must be sealed similar to that described in ASTM methods G8 and G Testing can be performed at room temperature or at elevated temperatures. One advantage to using this method is the test designer can select essentially any elevated temperature desired, provided it is representative of the actual environment.
Any anode metal can be selected, provided it does not corrode in the electrolyte solution during the test period.
Platinum titanium coated wire is suggested by the method. The test duration is 28 days; other test durations may be specified. Evaluation of the test specimen must be performed within two hours of test completion, which involves making four radial cuts through the coating at the intentional and reference holidays and attempting to lift the coating away from the substrate.
For example, if the average diameter of disbondment is 0. This method allows for various solution temperatures and samples of both pipe sections and panels. It is most appropriate to use this method if the end user requires this method per a specification or wants to compare multiple test parameters by calculating the cathodic disbondment using the same formula.
In summary, there are several methods to choose from when evaluating a coating for its resistance to cathodic disbondment; each with its own set of parameters and test conditions.
These test variations can produce different results on the same coating system. Therefore, it is important to select the method that is most appropriate.
51312-01285-Cathodic disbondment test: What are we testing?
The first step in understanding cathodic disbondment testing is appreciating the need for cathodic protection. Pipelines are typically quite long and the potential for corrosion is so great that passive cathodic protection i. Therefore, active cathodic protection imparted by an impressed current cathodic protection ICCP system is almost always used. This typically includes a DC power source, an AC powered transformer rectifier, and an anode ground bed. An ICCP system essentially interferes with the corrosion process and protects the pipeline or structure. The widespread use of ICCP dictates that any coating intended for use on the pipe be tested for its resistance to cathodic disbondment.
Good test results low delamination radii indicate that the coating is expected not to delaminate to a large extent in the presence of cathodic protection and a small coating damage with the steel surface exposed to the soil. This makes the CD test essential for quality control of pipeline coatings. Potential temperature test duration electrolyte composition and diameter of the drill are the main parameters of the CD test. Due to numerous variations of the test parameters an extraordinary variety of test conditions and related acceptance criteria has been established. Inconsistent test conditions and conflictive opinions about the mechanism make interpretation of the test results difficult which raises the fundamental question: What are we testing and what is the significance of the test? By means of basic investigations factors influencing cathodic delamination have been identified and their extent on the delamination radius has been assessed. The relevance of these factors with regard to transferability of laboratory tests to field conditions has also been considered.
Evaluating a Coating’s Resistance to Cathodic Disbondment – Guidance on Selection of Test Methods