Corrosion Behavior of Ferritic Stainless Steel in Hydrochloric Based Acid with Ferrous and Ferric Ions

The effects of ferrous(Fe²⁺) and ferric(Fe³⁺)ions on pickling behaviour of originally hot-rolled and blasted ferrite stainless steel（FSS） in HCl^- based electrolytes were investigated in this work through weight-loss test,macro profile,SEM morphology,self-corrosion potential test, polarization curves and electrochemical impedance spectroscopy measurements.The results demonstrated that the accumulation of Fe²⁺ in the electrolytes for pickling FSS would lower both the self-corrosion potential and the weight-loss rate,meanwhile Fe³⁺ had the opposite functions with higher self-corrosion potential and weight-loss rate.In the electrolytes of 3 mol·L^-1HCl,0.5 mol/L Fe³⁺ enhanced the weight-loss from 0.22% to 1.23%,while the same amount of Fe²⁺ reduced it to 0.14%.By promoting the molar ratio of Fe³⁺ and Fe²⁺(C_Fe³⁺/C_Fe²⁺) in the pickling electrolytes,the weight-loss rate of FSS would also increase accordingly.In the pickling electrolyte with 1.37 mol/L HCl and total ferrous and ferric ions of 1.79 mol/L,the weight-loss rate of FSS were enlarged 7.2 times or more than that without any ferrous and ferric ions when C_Fe³⁺/C_Fe²⁺≥2.It is concluded that in the pickling electrolyte with HCl,Fe³⁺ acts directly in cathodic reactions to accelerate the pickling of FSS,and Fe²⁺ replaces originally attached H⁺ in the interface of FSS and the electrolyte to partially inhibit the pickling rate.