Analysis of causes for surface tarnishing of 304 pickling plate

The surface tarnishing phenomenon was analyzed. The main inclusions are type A and type C determined by metallographic analysis. Serious intergranular corrosion was found in surface tarnishing 304 plate after annealing and pickling. After solution treatment and pickling test,the amount of ferrite decreases significantly,little carbide precipitation happens,and surface tarnishing phenomenon decreases. The result shows that the improper annealing results in carbide enrichment on the grain boundary and sensitization of material,which lead to intergranular corrosion when pickling,and then surface tarnishing of 304 pickling plate.