Study On Zn/Zn Al Corrosion Of WC Composite Coating And Al₂O₃-13%TiO₂ Gradient Coating

Steel materials can be corroded in natural media and the zinc/zinc-aluminum coating on its surface has been proved to be one of the effective solutions.But in the hot-dip galvanizing process,molten zinc or zinc-aluminum liquid will seriously corrode the roll and shaft sleeve,which can cause a significant impact on the production.In view of this problem,the plasma sprayed technology and supersonic flame spraying technology were used to spray corrosion resistant coating for liquid zinc/zinc-aluminum on the surface of 316L stainless steel in this experiment.The WC-10Co4Cr carbide ceramic coating was prepared by supersonic flame spraying.The results of zinc corrosion test showed that the corrosion resistance of the coating was 15days.Failure mechanism of coating:the liquid Zn can react with Co in the binder phase of the WC-10Co4Cr coating to form CoZn13 phase,which is hard and brittle and resulting in the coating being susceptible to cracking under thermal stress.Meanwhile,the blocky WC phase with corrosion protection was stripped out of the coating due to the loss of bonding.Component optimization was carried out by the analysis of failure mechanism:WC-10Co4Cr was selected as the main material.Mo and W element which have powerful corrosion resistance of zinc were added to the coating respectively.The experimental results of zinc corrosion showed that the Mo element in the coating was laid on the surface of the binding phase and connected with the ceramic phase to form a"shielding effect".It effectively prevented the infiltration of liquid zinc and reduced the rate of the reaction of zinc with the binder phase,and resistance to zinc corrosion days up to 20 days.However,because of the brittle and refractory properties of W element,there was a small amount of segregation in the coating.And the overall brittleness of the coating was increased,so that the coating was prone to cracking under the effect of thermal stress,resulting the early failure of the coating.The Al₂O₃-13%TiO₂ oxide ceramic coating was prepared by plasma spraying technology and the results of corrosion test showed that the corrosion resistance lasted for about 6 days.The failure of coating was due to the large difference between the expansion coefficient of the working layer and the bonding layer,resulting in cracking at the joint.To solve this problem,two oxide ceramic gradient coatings NiCoCrAlY/Al₂O₃-13%TiO₂ and Mo/Al₂O₃-13%TiO₂were designed.The corrosion resistance of Mo/Al₂O₃-13%TiO₂ gradient coating to zinc-aluminum was about 6 days.Mo and Al₂O₃-13%TiO₂ both have the characteristic of hard embrittlement and refractory,which cause the stress concentration at the transition layer and the transverse cracks created,leads to the falling off of the coating.The corrosion resistance of NiCoCrAlY/Al₂O₃-13%TiO₂ gradient coating to liquid zinc-aluminuml was about 9 days.Due to the addition of transition layer,the large transverse cracks caused by the mismatch of thermal expansion coefficient did not happened on coating.The liquid zinc-aluminum continuously decomposes the coating on the contact surface by penetrating cracks,which makes the coating gradually decrease with the prolongation of corrosion time.At the same time,the interior of coating also created cracks due to the existence of the pores,which eventually leads to the falling off of the work layer and the failure of anti-corrosion function.