Bacterial biofilms formed on phosphate conversion coatings: effect on the corrosion protection of the coating.

Abstract

Phosphating is the formation of a phosphate layer (phosphate conversion coating) on a metal surface, with the aim of increasing the corrosion resistance of the metal and the adhesion strength of organic coatings. Phosphate Conversion coatings are widely used to improve carbon steel corrosion resistance particularly during transport and storage.On the other hand, it is well known that different species of environmental bacteria are able to develop biofilms on metal surfaces. Besides, some of those bacteria are capable of solubilizing phosphate. The objective of this work was to evaluate the effect of biofilms of Pseudomonas aeruginosa and Paenebacillus sp. on the stability of iron and manganese phosphate conversion coatings applied on SAE 1010 carbon steel. Both bacterial species are known for their biofilm-forming and phosphate-solubilizing capacities. The corrosion protection provided by the phosphate coatings in the presence and in the absence of bacterial biofilms was evaluated through electrochemical techniques complemented with surface observation by scanning electron microscopy. The results showed that the development of P. aeruginosa and Paenebacillus biofilms on the surface of SAE 1010 carbon steel coated with iron or manganese phosphate conversion coating provoked a shift towards more negative values of the pitting potentials. The decrease in the passive state stability of both coatings in the presence of bacterial biofilms made the materials more susceptible to localized corrosion. The degree of the compromise of passivity was dependent on the coating and the bacterial strain. P. aeruginosa biofilms were more detrimental for both coatings than Paenabacillus sp. biofilms.