Improvement of corrosion resistance of materials coated with a Cr2O3/NiCr dilayer using a sealing treatment

A sealing treatment has been applied to diminish the effects of the pores and micro-cracks on the corrosion properties of the plasma-sprayed Cr2O3/NiCr dilayer coatings. This treatment decreased the porosity both in the ceramic and in the NiCr dilayer and effectively improved the corrosion resistance. The sealing treatment involved the use of polymethyl silicon and vinyl methyl silicon. Two compositions of plasma sprayed Cr2O3/NiCr dilayer coated materials (Cr2O3/80NiCr/316 and Cr2O3/50NiCr/316) were studied. Optical microscopy and scanning electron microscopy demonstrated that all of the open pores in Cr2O3/NiCr dilayer were blocked by the sealant resin and only closed pores were remained. The ceramic and sealing resin formed a compact layer without open pores, which protected the materials beneath against corrosion. Corrosion tests of the sealed and unsealed specimens showed that the corrosion resistance of the Cr2O3/NiCr dilayer coated materials increased markedly after the sealing treatment. Electrochemical experiments showed that after the sealing treatment the difference in corrosion resistance between the sealed Cr2O3/80NiCr/316 and Cr2O3/50NiCr/316 was very small probably because the corrosive environment served to mask the effect of the 80NiCr and 50NiCr on the corrosion properties of the dilayer coated materials. Sealed specimens showed small weight grains due to the absorption of water in the resin.