Preventing Coating Failure by Surface Contamination

Coating failures happen when the protective coating fails to protect the substrate, it no longer provides an esthetic look or serves its specified function. This can occur for a multitude of reasons, but the most common failures are caused by insufficient surface preparation. This can lead to adhesion failure where a paint or coating delaminates from the coating or surface below. Not properly preparing or cleaning the surface prior to coating can leave surface contamination such as rust, mildew, soluble salts, dust, dirt, or other abrasives making the coating application fail by blistering or peeling. These coating failures exemplify the importance of specifying the appropriate surface preparation and profile. Coating failures can also occur throughout the application, curing stage, or after application. The first step of prevention is determination.

How to Determine Surface Contamination

To identify potential causes of coating failure, an independent investigation should be performed. A site visit including access to the structure with the coating failure and a forensic laboratory investigation is warranted.

Investigative tasks during site visit may include:

Visual Examination - Visually examine the coating in areas where the coating failure occurs as well as the non-failing areas to determine the presence of surface contamination, improper application or improper formulation.
Presence of Pattern Failure - Determine whether or not there is a noticeable pattern of failure such as paint defects occurring in specific areas like low spots or welds.
Paint Adhesion Tests - Test the coating adhesion in both failing and non-failing areas, keeping in mind that the adhesion tests are destructive and should be limited. These tests should be performed in accordance with:
- ASTM D3359, “Measuring Adhesion by Tape Test”
- ASTM D6677, “Standard Test Method for Evaluating Adhesion by Knife”
- Or alternatively with the pull-off adhesion tests
Coating Thickness - Measure the film thickness of the coating and individual layers to determine if the thickness is in range of the manufacturers recommendations or if it is related to the failure.
Sample Collection - Samples from both failing and non-failing areas along with photographs of the sample site should be collected and sent to a laboratory that specializes in coating failure analysis. An opinion on the cause of the coating failure, based on objective evidence, is then formed by evaluating the combined results of the laboratory analysis, data collection and site investigation.

Testing for surface contamination is also done with such products as test patches, or sleeves.

Test patches measure water soluble salts on steel. If they aren't removed prior to painting, chemical reactions can result in blister formation and accumulations of rust that destroy the adhesion between the substrate and the applied protective coating.
Test Sleeves are an easy to use, efficient field test and were developed to prevent outside and cross contamination. They offer consistent, safe, and reliable testing results for contamination.

Both types are used to extract or measure and analyze the soluble salt content on a substrate.

When to Incorporate Salt Remover

If the content is above the acceptable range specified, you would need to incorporate the use of a salt remover or maintenance wash to remove the salts, then retest prior to application. The wash reduces the content of dirt and salt. It contains no volatile organic compounds (VOC), is non-flammable and also biodegradable. Maintenance wash is recommended for use anywhere that these contaminants are identified and need to be removed prior to the coating process.

Summary: The best way to keep your coating from failing is to prevent surface contamination in the first place. Test, analyze, and clean your substrate prior to coating to prevent premature coating failure which can result in serious corrosion and operation issues resulting in economic loss. Ensure proper surface preparation by incorporating salt remover when warranted into your procedure. By following this process, you can ensure that you surface is clean of dirt and salts that cause premature coating failure.

For more information on coating failure see: General Coating Failure Cause and Prevention

For information on specific paint defects see: Blistering Paint, Cracking Paint and Other Paint Defects and Solutions

Visit our Protective Coatings Industry Page for Surface Preparation and Protective Coating Test Instruments.