Passivation Film Quality Control And Process Optimization Of The Products In Shougang Jingtang Galvanized Production Lines

In order to improve surface properties and quality and to prolong service life,metal products generally require proper surface treatment.As an important surface anti-corrosion treatment,passivation treatment is widely used in the surface treatment of galvanized products.Since 2012,several galvanizing production lines in Shougang Jingtang have been gradually adopting chrome-free passivation products,which replace the original chrome passivation products.In the process of product substitution,a series of problems have also emerged.For example,since 2014,the failure events in the 3#galvanizing line(hereinafter referred to as 3#CGL)salt spray test is about 50%,and those in 4#CGL(hereinafter referred to as 4#CGL)neutral salt spray test reach 56%or so.For this reason,some customers have raised quality objections on the inconsistency of the neutral salt spray test of Jingtang galvanized sheet,which seriously affected the company’s production,sales and economic benefits.In order to solve the problem of poor corrosion resistance of hot-dip galvanized sheet,it is necessary to study the quality of passivation film from microscopic view and passivation process to improve the product quality.Passivation film thickness is one of the important factors affecting the corrosion resistance of chromium-free passivation products.Excessively thick passivation film will cause waste of raw materials,and too thin passivation film will cause the phenomenon of salt spray failure of chromium-free passivation products.The emphasis was firstly put on Jingtang 2#CGL and 3#CGL in this thesis.The thickness of the passivation film in 4#CGL chromium-free passivation product was then statistically analyzed.It was found that the thickness of passivation film of 3#CGL and 4#CGL was significantly thinner than that of 2#CGL,resulting in poor anti-corrosion ability in 3#CGL and 4#CGL passivation film.In order to deeply study the difference in corrosion condition of two different passivation solutions,electron microscopy analysis and passivation film structure analysis of passivation films in two different passivation solutions were performed.Moreover,the passivation process parameters was analyzed for 3#CGL and 4#CGL production lines in depth,which include the coating pressure,belt speed,rotation speed ratio,pressure difference in coating roller(OS-DS)and the specification of the strip steel.Finally,by reducing the passivation section tension to improve the cracking of the passivation film,and by standardizing the photosynthetic liquid conductivity,cleaning water pressure,rinsing temperature and coating roll pressure process parameters,improving the surface quality of the coating roller prior to the production,optimization of the passivation process parameters was made after strict inspection and composition adjustment of the passivation solution.The salt spray test results have been greatly improved by using the optimized passivation process.More than 95%of the products in 3#CGL and 4#CGL have reached the quality standard,which produced huge economic benefits for the company and won the trust of customers.