A shelving & logistics company requested SGS investigate problems with powder coating adhesion to steel items produced at their plant. The steel components are used as structural supports in storage shelves and are produced from rolls of sheet metal.
An experienced chemist attended the site and together with the plant personnel, closely examined each step used in the production process. Samples of the supports, semi-finished intermediates, phosphatising and washing solutions were collected for further testing and laboratory analysis.
The investigation found the failure of the powder coated surfaces was not due to a single cause but a combination of factors. The most probable mechanism which led to the failure was the phosphate bath and rinsing process.
Over time, the rinse water accumulates a high amount of dissolved solids, and a 2.3% by weight of dissolved solid was found to be present in a sample of spent water collected. During the dying process, the water evaporation concentrated the salt until the solution was saturated and the salts began to crystallize out, forming a powdery film on the metal surface. The powder coating to these areas did not adhere to the surface and resulted in the coating blistering and coming off after a short time. Additionally, the high concentrations of salts in these areas also promoted corrosion of the sites.
To remediate these problems, SGS suggested appropriate cleaning of the surfaces prior to coating. Keeping the pH of the phosphatising solution low was beneficial in removing the oxide film formed during the welding process and using deionised water in the final rinse prevents salt deposition.
After the above recommendations were implemented the production line had a zero percent failure rate on the products over the next 12 months of operation.
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