Effect of Boronizing Process of AISI 321 Stainless Steel Surface on its Corrosion Resistance in Acid Environments (pH = 1)

The paper presents results of research under the effect of surface thermo-chemical treatment on the corrosion resistance of X6CrNiTi18-10 steel. The corrosion resistance of the surface layers of stainless steel obtained as a result of thermo-chemical treatment (boronizing process) was assessed using the method of progressive thinning, which consists in performing corrosion tests on deeper and deeper areas of the surface layer. This method allowed for the determination of changes in individual characteristic corrosion parameters read from the potentiokinetic polarization curves and the determination of the depth profiles of these parameters. In the paper, results of tests of X6CrNiTi18-10 steel resistance to general corrosion, performed in acidified sulphate solutions (pH = 1) have been presented. The thickness of the surface layer was assessed on the basis of structural tests and changes in microhardness on the cross-section of the material. It has been found that the extremely high hardness of the boron layer was accompanied by deterioration of the corrosion resistance. The general corrosion rate of the surface layer is 3-4 times higher than the corrosion rate of the material core (substrate). The characteristics of the passive state of steel are particularly worsened, which is manifested by an increase in the value of the critical passivation current, the minimum current in the passive range and by limiting the tendency to secondary passivation.