Corrosion Protection in Chloride Environments of Nanosilica Containing Epoxy Powder Coatings with Defects

This paper describes the use of innovative, nanosilica containing epoxy powder coatings for the corrosion protection of steel. Two types of nanosilica particles (hydrophilic -HL- and hydrophobic -HB-) were mixed by ball milling with the powders (0.75 wt.%). The adequate homogeneity and embedding of nanoparticles were verified by transmission electron microscopy. The corrosion performance of the coatings as-received, and with HL and HB additions, were analyzed in 3.5 wt.% NaCl solutions. The mechanism and rate of delamination of defective coatings under drops simulating atmospheric conditions were analyzed by Scanning Kelvin Probe measurements for 30 d. The results show that the corrosion attack progresses through a cathodic delamination mechanism. Besides, fully-immersed samples, with and without defects, were monitored by electrochemical impedance spectroscopy. In defective coatings under these conditions, the occurrence of anodic undermining is proved. The results obtained reveal that the corrosion driven coating failure is delayed in the case of the epoxy coatings containing nanosilica. This delay is larger in the case of HB additions than HL additions in both atmospheric and immersion conditions. The corrosion mechanism observed is dependent upon exposure conditions. It is proposed that the nanoparticles delay water absorption, thus delaying corrosion attack.