Mechanisms Of Black Patina Inhibition Of Hot-Dip Galvanizing Coating By V(Ⅳ)

Black patina film on zinc coating surfaces is the result of formation of a film with a dark brown color in humid and warm environments,which is the case of corrosion in the initial stage.With the extensive use of Cr-free passivation technology and improvement of appearance quality for zinc coating,black patina brings a severe challenge in the use of galvanized products.According to the literature,vanadate exhibits intensive inhibition of black patina.However,the mechanisms remain to be determined.So it is significant to identify the mechanism of black patina inhibition of hot-dip galvanizing coating by Ⅴ(Ⅳ)(tetravalent vanadium),which facilitates the use of galvanized products.In this work,the mechanisms of black patina inhibition of hot-dip galvanizing coating by Ⅴ(Ⅳ)were investigated.Herein,the evolution of black patina was investigated using confocal laser scanning microscopy(CLSM),atomic force microscope(AFM),and scanning Kelvin probe force microscopy(SKPFM)methods.The mechanisms of corrosion inhibition by Ⅴ(Ⅳ)in aqueous solutions,conversion coatings and composite coatings were identified using X-ray photoelectron spectroscopy(XPS),scanning Kelvin probe(SKP),electron probe micro-analyzer(EPMA)and electrochemical noise techniques.Furthermore,the effects of Ⅴ(Ⅳ)on elemental composition at interface between composite coating and zinc coating were also investigated using auger electron spectroscopy(AES)methods.Finally,these results achieved in this study revealed the mechanisms of black patina inhibition of hot-dip galvanizing coating by Ⅴ(Ⅳ)in composite coatings.In humid and warm environments,potential of zinc decreased gradually,which suggests an enhanced tendency toward corrosion.Nevertheless,the composite coatings hindered the decrease in potential and gave a uniform distribution of potential,which weakens the driving force of corrosion and improves resistance to corrosion of zinc coatings.Ⅴ(Ⅳ)acts as a cathodic inhibitor in aqueous solutions for zinc coatings.Similar to hexavalent chromium Cr(Ⅵ),Ⅴ(Ⅳ)inhibits the corrosion by a mechanism involving electrochemical reduction rather than by an adsorption and electrochemical adsorption and formation of Ⅴ(Ⅳ)/Ⅴ(Ⅲ)contained layers on zinc coating surfaces.The coating layers hinder the depolarization of oxygen,and decrease the dissolution rate of zinc coatings.However,Ⅴ(Ⅳ)can not control the electron to release from zinc coatings.Ⅴ(Ⅳ)conversion coatings consist of 4+/3+ mixed oxidation state of the Ⅴfilm.Ⅴ(Ⅳ)impart active corrosion protection of zinc coatings by a mechanism involving the decrease in Fermi level position and the degradation of corrosion attack by oxygen.Ⅴ(Ⅲ)exhibits a steady chemical properties.During the corrosion attack,Ⅴ(Ⅳ)vanished and only Ⅴ(Ⅲ)retained a certain amount that played a major role in the corrosion inhibition of zinc coatings.Addition of Ⅴ(Ⅳ)markedly enhanced the corrosion resistance of polymer coatings.A thin Ⅴ contained layer presented at the interface between composite coating and zinc coating.During corrosion attack,dissoluble Ⅴ(Ⅳ)can inhibit the corrosion of uncoated surfaces.Meanwhile,Ⅴ(Ⅳ)can release towards the defect site on zinc coatings,which enhances the protection property of composite coatings.The Ⅴ(Ⅳ)contribution to protection efficacy of composite coatings mainly involves that Ⅴ(Ⅳ)can decrease the corrosion rate of zinc coatings at defect site.The research findings suggest that the accumulation of Zn(OH)2 at the interface is responsible for the black patina on zinc coating surfaces,and the V-rich thin layers at interface played the key role in black patina.Ⅴ(Ⅳ)inhibits black patina by a mechanism involving Ⅴ(Ⅳ)reduction to Ⅴ(Ⅲ)and formation of 4+/3+ mixed oxidation state of the Ⅴ film at the interface during the corrosion of zinc coating surfaces.Finally,the film on zinc coatings inhibits the oxygen oxidation reaction.OH-ions are combined with Ⅴ(Ⅲ)instead of Zn2+,which can inhibit the accumulation of Zn(OH)2.In this manner,the inhibition of black patina on hot-dip galvanizing coating is achieved.This work investigated the mechanisms of corrosion inhibition of zinc coating by Ⅴ(Ⅳ)and revealed the mechanisms of black patina inhibition.The findings achieved facilitate the understanding of protection mechanisms by Ⅴ(Ⅳ),and development of a new eco-friendly pigment.