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Pull-off adhesion testing of coatings is commonly used for product testing and qualification as well as quality control / quality assurance. However, initial adhesion values do not necessarily correlate with service life of coatings or their corrosion protection performance. Adhesion of several product chemistries to steel is examined in this study before and after immersion exposure. Results are presented within the context of laboratory corrosion testing in an effort to investigate the significance of adhesion testing in modern lining systems.
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Quality assurance of coatings on steel water pipes commonly relies on tensile pull-off measurements of the coating-steel adhesion, according to ASTM D4541. These tests are performed by adhering a metal ‘dolly’ to the coating with an adhesive, then scoring around the dolly circumference, through the coating down to the steel surface, before recording the stress necessary to pull the dolly (and coating) away from the steel pipe.
The tensile pull-off test, as in ASTM 4541, in which a round metal ‘dolly’ is glued to a coating then pulled off is a very common method for measuring adhesion of a coating to its substrate. This technique is very well known to produce joint failures that are not at the intended coating steel interface but may initiate within the coating, within the glue or at the glue-coating interface.
This group of authors, thinking outside the proverbial box, and with a mind to Shakespeare’s Macbeth, presented a paper at SSPC 2010 entitled “Hubble, Bubble, Tests and Trouble: The Dark Side of Misreading the Relevance of Coating Testing”. A challenge to the status quo of coatings performance testing, the paper boldly questioned much of the conventional wisdom surrounding coating testing and how that testing is interpreted.
A long-term project such as the current Brooklyn Bridge Rehabilitation Program, known as Brooklyn Bridge Contract 6, presents a great opportunity to collect vast amounts of data and records relating to the total coating replacement on a historic landmark. On any project, especially one of this scope, the project team has to deal with many obstacles, both expected and unexpected, and significant unknowns.
Corrosion is a durability concern for any unprotected structural steel from its exposure environment and coatings are widely used for corrosion protection. Zinc-rich primer-based three-coat (ZRP) systems are the most widely used for structural steel, since 1980’s. However, these coating systems often require regular maintenance. So, there is always interest to develop durable novel coating systems to reduce the maintenance cost.
HISTORICAL DOCUMENT. The purpose of this NACE International standard test method is to specify test methods and test conditions used to evaluate thermal properties, insulation values, and performance/integrity before and after thermal aging of insulative coatings. Testing for corrosion resistance is not included in this test method.
The primary intent of this standard is to specify test conditions that would give a baseline evaluation – one that would allow direct performance comparisons between different insulative coatings. This standard is designed to have practical test procedures with limited test conditions. It also includes Appendix A (nonmandatory) which describes hot plate designs, and Appendix B (nonmandatory) which describes an alternative thermal conductivity test design.
Tensile pull-off adhesion testing is becoming a more frequent specification requirement for in situ quality assurance testing to confirm proper surface preparation and adhesion of high performance protective linings applied to concrete. It is also becoming a widely used test for forensic analysis of protective linings in existing installations. There have been numerous investigations leading to the development of different devices and test pull-off adhesion methods used to assess bond strengths of mortars and overlay materials
Pull-off adhesion testing is widely used to assess the protective coating process. It is also used to determine if a coating is fit for service in new construction and for repairs to existing structures. The result is often critical to the acceptance or rejection of a coating process, as the adhesion value quoted by the paint manufacturer can be adversely affected by aspects of the coating process. Low adhesion values are indicative of premature failure of the coating and are often due to inadequate surface preparation of the substrate.
Steel surfaces are frequently cleaned by abrasive impact prior to the application of protective coatings. The resultant surface profile must be accurately assessed to ensure compliance with job or contract specifications. But is profile height alone (the most commonly measured parameter) a good indicator of long-term coating performance?
ASTM D7234 was first published in 2005, and updated in 2012 with precision statistics. This paper will go into the history of the development of the test method, the procedure and interpretation details that are critical to using and applying this standard along with a discussion of the factors that affect the precision of this method.