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A review of cathodic disbondment test (CDT) methods from standard organizations such as ASTM, ISO, CSA, NF and AS is presented in this paper.
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The research described in this paper was carried out with the objective of establishing any correlation between coating performance and the results of cathodic disbondment testing. Experiments were carried out using 13 coatings. Nine samples of each coating were studied in a total of 117 experiments.
The cathodic disbondment (CD) test is a well known and generally accepted laboratory test for pipeline coatings for a long time. 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.
This research compares and contrasts NACE TM0115 disbondment test method to ASTM G8, Standard Test Methods for Cathodic Disbonding of Pipeline Coatings...as applied to coatings used in water infrastructure corrosion control.
Special enhancements: (1) use the high temperature reference electrode - Ag/AgCl; (2) define the test temperature measurement procedure; (3) check the accuracy of the reference electrode; (4) develop an inexpensive anode isolation method.
The control, mitigation and prevention of corrosion in environments ranging from mildly corrosive to severe atmospheric conditions to underground exposures (such as pipelines) to chemical spill and fume exposures has long been focused on the use of thermosetting polymers such as epoxies, polyesters, vinyl esters and urethanes. For the most part these materials have worked reasonably well in applications such as structural steel and equipment coatings, architectural paints, vessel linings, concrete coatings, secondary containment linings, and floor toppings.
Most of atmospheric coatings and tank linings for offshore maintenance are routinely applied on rusted steel after dry abrasive blasting. It is well known that the salt contamination on rusted steels cannot be completely removed by dry abrasive blasting alone. Residual salt contamination, which is hidden in the corrosion pits, is difficult to remove mechanically. Depending on the rust severity, the residual salt content on the dry abrasive blasted steel surface can be in the range of 5-65 μg/cm2. Too much residual salt contamination can be detrimental to coating performance. It could cause coating blistering, adhesion degradation, and under film corrosion which will result in a shorter service life, particularly in immersion service such as pipeline coatings or tank linings. Recently wet abrasive blasting (WAB) has been used as the surface preparation in conjunction with the decontamination chemicals.
The spread of disbondment or corrosion from a scribe or holiday in a coating film, for which the terms rust creepage or undercutting are used in this paper is an important mechanism of coating degradation. The mechanism of rust creepage has been well studied by several authors who concur that the mechanism is driven by electrochemical reactions15. The reactions occurring at the discontinuity in the coating (scribe or holiday) involve an anodic reaction in which iron is dissolved. Adjacent to the anodic region, under the coating, a cathodic reaction occurs in which oxygen is reduced to hydroxyl ions.
Distress (controlled surface breaks) was created on production samples of epoxy coated rebar and some were further subject to cathodic disbondment. Samples were cast in concrete and subjected to cyclic polarization and electrochemical impedance spectroscopy measurements.
This paper will cover cathodic disbondment, dry film thickness, peak height and coating evaluation for failures.
Olin Epoxy has designed a unique low VOC and high solids epoxy system which offers superior performance in tank liner application. The formulation of these novel materials in high temperature and high chemical resistant ambient cure tank liner will be described.