The reasons for the occurrence of paint and varnish coatings defects

A frequent problem with paint and varnish coatings is the reduction of adhesion to the substrate. As a result of this phenomenon, the coating no longer fulfills its intended activities – it does not provide good protection against corrosion and its aesthetic values are significantly lowered. A question then arises – what is the cause and how to eliminate it and prevent it from occurring in the future? The answers to these questions, which are extremely important from the point of view of quality, can be provided by appropriately selected material tests and their professional interpretation.

The first examination performed in such cases is macroscopic observation. In the case of varnished fence wire, the main problem was the significant unevenness of the surface. The macroscopic image of the wire surface is shown in the photo below.

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On the surface, there are visible inclusions and impurities located in the varnish. To find out the nature of the impurities, metallographic specimens were made, which were then subjected to microscopic observations.

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The cross-section of the wire in the varnish layer shows impurities in the form of metal filings of various origins – steel and aluminum. Additional observations made on the SEM microscope and the EDS / EDX microanalysis performed allowed to determine the chemical composition of the pollutants and confirmed their metallic nature.

The presence of impurities was also confirmed in the galvanic zinc layer.

Metallic filings already present in the zinc layer may indicate contamination of the galvanic bath. It is necessary to control its cleanliness and the cleanliness of the elements placed in the bath. The presence of metal filings on the surface of the wire and contamination of the bath are the direct causes of varnish unevenness and its blistering, and thus a decrease in adhesion to the substrate.

The results of all performed tests, conclusions and recommendations were described in detail and comprehensively in the test report that was delivered to the client.