Subsequent sealing of thermally sprayed coatings to increase corrosion resistance

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Abstract
Sealants are frequently applied to close the porosity in thermally sprayed coatings in order to improve corrosion resistance and a physical model has been developed to describe the penetration of sealant into the capillary pore system. It is concluded that a liquid intended for use as a sealant should have low viscosity, high surface tension, and a high wetting capability. However, measurements of coating adhesion have shown that gas trapped at high pressure within the pores can lead to a reduction in substrate–coating bond strength. Polarisation curves were measured for sealed and unsealed atmospheric plasma sprayed (APS) and high velocity oxyfuel (HVOF) WC–Co–Cr coatings in acid and alkaline solutions. Sealing was found to reduce the corrosion current density, but the effect depended strongly on the coating phase composition and microstructure. The WC–Co–Cr HVOF coatings showed better resistance because the higher chromium concentration in the matrix phase permitted a passive oxide layer to form, whereas when spraying Al2O3–TiO2 ceramic coatings AP S was found to give higher corrosion resistance as a result of a higher proportion of amorphous phase.

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