51316-7518-Corrosion Protection of Smooth Surfaces – Coating Adhesion

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Product Number: 51316-7518-SG

ISBN: 7518 2016 CP

Author: Catalina Hagen

Publication Date: 2016

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Coatings on machined surfaces are generally found to degrade early and this has been attributed to poor adhesion. Good adhesion normally requires blasting but not all surfaces can be blast cleaned due to other functional requirements such as seal (flange surface) or assembly (tightness of nuts). We have studied the effect of surface preparation on coating adhesion and performance in accelerated corrosion tests. Four different machining processes have been evaluated and compared with metallized and blasted reference surfaces. For the preliminary study our findings show that machined surfaces had poor resistance to cathodic disbonding and corrosion creep. It was concluded that the poor coating performance on machined surfaces is due to low wet adhesion strength which caused little resistance against corrosion creep. Cathodic disbonding seems to be the mechanism by which adhesion loss spreads over the surface. Corrosion then follows behind. Samples coated with aluminum (Al) pigmented primer were then tested to investigate the effect on the cathodic disbonding process. Al pigments will corrode sacrificially during cathodic disbonding. The result is a local exhaustion of OH- mitigating the alkalinity built-up that leads to cathodic disbonding. The Al pigmented primer decreased disbonding to about one third compared to the samples without the Al primer. Corrosion creep however did not diminish substantially. More tests are in progress to reveal the mechanisms behind the poor coating performance on machined surfaces. Cathodic disbonding corrosion creep adhesion and polarization behavior will be assessed and compared with the tortuosity of the different surface preparations.