Coating and Lining Failure

DESCRIPTION

Coating failures are usually the result of problems associated with either coating application (i.e. surface preparation, primer, or topcoat), chemical or environmental durability or wear and abrasion. Such failures are typically related to loss of adhesion, cracking or wear.

PREVENTION OR REMEDIAL ACTION

1. surface preparation
   • surface must be properly cleaned to remove all dirt, corrosion and contamination.
   • surface must be roughened to provide an adequate anchor pattern for coating.
2. selection of primer
   • provide corrosion inhibition
   • provide additional barrier between metal environment.n\
3. selection of topcoat
   • need adequate coating thickness to cover all of the surface (free of defects or holidays). This may require multiple coats.
   • must have chemical or moisture resistance and temperature resistance.
   • in some applications, it must also have abrasion resistance.n\
4. applications involving cathodic protection
   • must have resistance to cathodic disbonding.

STANDARD TEST METHODS

• NACE TM0175 - visual standard for surfaces of new steel centrifugally blast cleaned with steel grit and shot.
• NACE TM0174 - laboratory methods for the evaluation of protective coatings used as lining materials in immersion service.
• NACE TM0183 - evaluation of internal plastic coatings for corrosion control of tubular goods in an aqueous flowing envrionment.
• NACE TM0375 - abrasion resistance testing of thin film basked coatings and linings using the falling sand method.
• ASTM G-8, G-80, G-95 - tests for cathodic disbonding of pipeline coatings.
• ASTM G-42 - test for pipeline coatings subjected to high or cyclic temperatures.
• ASTM B-117 - salt fog testing.
• ASTM G-85 - modified salt spray (fog) testing.
• conversion coatings: ASTM B-449 (Al-alloys); ASTM B-201 (Zn and Cd alloys); ASTM B-281 (Cu-alloys).
• ASTM G-7 - practice for atmospheric environmental exposure of nonmetallic materials.
• water resistance of coatings in 100% humidity.

Evaluation of Coatings and Lining

In general, coatings and linings work to prevent corrosion by imposing a dielectric barrier between the corrosive environment and the material surface. Therefore, an evaluation of coating performance is one of determining the chemical compatibility and thermal stability of the coating in the service environment and the adhesion of the coating on the substrate. Many coatings tests use post exposure visual examination and/or mechanical performance to rate its resistance to the corrosive environment. Newer evaluation techniques such as electrochemical impedance spectroscopy (EIS) can be used in-situ to assess changes in coating parameters such as film capacitance and pore resistance well in advance of the point where damage can be determined visually. Another important aspect of coating evaluation is the use of service life testing methodologies. In their simplest form, these take the form of evaluations of coating performance after multiple exosure period instead of the commonly used single point testing. This allows for the tends in coating performance versus time to be obtained. Additionally, the use of theoretical and empirical models can be used to accelerate the test by increasing temperature or by changing other parameters (e.g. applied potential). Short term failures obtained at multiple conditions can be used to predict longer terms limits of serviceability in various service environments.