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Modern design, manufacturing and field-testing specifications include surface profile and adhesion testing on the assumption that they are linked to long term corrosion prevention. There are a number of careful studies, dating back decades, that find no link between measurements of adhesion and (undercutting) corrosion performance, but other studies do, and the concept remains intuitively appealing and widely assumed.
Modern design, manufacturing and field-testing specifications include surface profile and adhesion testing on the assumption that they are linked to long term corrosion prevention. There are a number of careful studies, dating back decades, that find no link between measurements of adhesion and (undercutting) corrosion performance, but other studies do, and the concept remains intuitively appealing and widely assumed. Finding connections between adhesion, corrosion and surface profile requires a detailed understanding of fracture mechanics, surface energy, viscoelasticity and even nanoscience. Simple measures of surface roughness are not sufficient to correlate with the adhesive strength of coatings. In addition, common “pull-off” adhesion testing cannot characterize the forces between the coating and metal substrate that might prevent water etc. invading that interface. Modern science can dispel some contradictions but does not yet give unequivocal guidance on the choice of polymer coating for the best corrosion protection in practical situations.
Many coating product data sheets call for surface profiles of 3-4 mils. Others call for 1.5-2 miles surface profile for a very similar coating type – why should this be? To start to understand this we need to first of all look at why we measure the surface profile on steel surfaces. It pretty much comes down to two main things.
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International standards, both ISO and ASTM, require that the attributes exhibited during an adhesion test are recorded as part of the results. What are these attributes, how should they be recorded and what value are they to the inspector and applicator in assessing the reason for failure? This paper looks at all these questions and asks should we be going further than we do in defining the mode of failure.
To address questions and concerns in the protective and marine coatings market, new laboratory data has been generated regarding corrosion rates and mechanisms that will help eliminate some of the assumptions regarding the role of soluble salts. These assumptions include soluble salt types such as chloride, sulfate, and nitrate along with the resulting conjugate corrosion products.