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Coating specifications for protection of steel surfaces commonly contain requirements to generate a surface profile (to anchor the coating system to the substrate), within a range (minimum and maximum depth). In the future, specifications may also invoke a requirement to achieve a minimum peak density or peak count.
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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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.
This paper describes experimental work investigating the influence of steel surface roughness on the adhesion performance of fusion bonded epoxy (FBE) pipeline coatings. The paper begins with a summary of the standards and methods that can be used to measure surface roughness. Several parameters are used to characterize the roughness of a blast cleaned steel including profile peak height and peak count. Tortuosity and rugosity indicate the proportional increase in steel surface area developed by roughening the surface. Normal pipeline coating industry practice is to specify and control a single roughness parameter termed “surface profile”. It is measured with replica tape and corresponds to the maximum peak-to-valley height.In the experimental work steel panels were abrasive blast cleaned with various steel shot and grit abrasives and the roughness characteristics of the blast cleaned surface were measured with stylus profilometers conventional replica tape and 3D imaging of replica tape.A FBE pipeline coating was applied to the prepared steel panels. The adhesion performance of the FBE coating was evaluated using the following test methods.<ul><li>Hot water immersion adhesion rating per CSA Z245.20 section 12.14 </li><li>Pull-off adhesion strength after hot water soak exposure per ASTM 4541 </li><li>Cathodic disbondment radius at 65 and 80 °C per CSA Z245.20 section 12.8 </li><li>Time before blisters were observed in Atlas Cell per NACE TM0174 modified </li><li>Average blister diameter in Atlas Cell </li><li>Pull-off adhesion strength after Atlas Cell exposure per ASTM 4541 </li></ul>The experimental data were analyzed using statistical techniques to investigate the relationship between the measured surface roughness and the adhesion test results. The adhesion results were found to be positively and linearly correlated with substrate tortuosity and rugosity. Profile peak height and peak count were found to contribute to tortuosity.