Effects Of Mn On The Anti-corrosion Performance Of Austenitic Stainless Steels

In recent years, the outputs of Cr-Mn-Ni series austenitic stainless steels are increasing sharply, especially that of non-standard brands. However, the anti-corrosion performance of Cr-Mn-Ni series austenitic stainless steels is very poor. Therefore, the effects of Mn on the anti-corrosion performance of three sorts of Cr-Mn-Ni austenitic stainless steels, such as LH, L1, L4, and Cr-Ni austenitic stainless steel 304 were studied detailly in this paper.In this paper, both optical microscopy and scanning electron microscopy(SEM) were used to research the non-metallic inclusions in austenitic stainless steels. The results showed that the non-metallic inclusions in the Cr-Mn-Ni austenitic stainless steels are mainly Class A strip Fe-S-Mn inclusions and class D spherical oxides Fe-Cr-S-Mn-O-Al-Mg, while that of Cr-Ni austenitic stainless steel were mainly class B oxides Mg O·Al2O3 and class D spherical oxides inclusions Mg-Ca-O-Fe-Al. With the increasing of Mn content in the steels, both the size and number of Class A non-metallic inclusions were increased. Line SEM results showed that there was an obvious chemical composition difference between the inclusions and the austenitic stainless steels matrix.The salt spray anti-corrosion performance, pitting anti-corrosion performance, crevice anti-corrosion performance and repassivation performance of four austenitic stainless steels were studied by salt spray corrosion tests, electrochemical anodic polarization curve, immersion method and sharp scratching method, respectively. The results were concluded as follows.(1) The salt spray anti-corrosion performance of Cr-Ni austenitic stainless steels was the highest, and both the corrosion degree and the corrosion rate of Cr-Mn-Ni austenitic stainless steels were sharply risen with the increasing of Mn content.(2) The breakdown potential of the Cr-Ni austenitic stainless steel was the highest, and the pitting potential of three Cr-Mn-Ni austenitic stainless steels increased while Mn content decreased. Compared with the whole sample, the part sample showed higher pitting potential. It has been also concluded that the potential difference between that two samples was raising with increasing of Mn content, which implied that the Mn element in the Cr-Mn-Ni series austenitic stainless steels influenced the pitting corrosion resistance were mainly in form of inclusions. The pits of Cr-Mn-Ni series austenitic stainless steels were mainly generated from the Class A sulfide inclusions, while that of Cr-Ni austenitic stainless steel was generated from the class B oxide inclusions.(3) With the increase of Mn/Ni, the crevice corrosion became more serious and the crevice corrosion rate was increasing. With increasing of Mn/Ni, the depassivation p H(p Hd value) of samples was increased, which indicated that the steel more susceptible to induce crevice corrosion, crevice corrosion propagation rate was also increased.(4) The ipeak flowed on the scratch of Cr-Ni austenitic stainless steel was the lowest, indicating which has the best repassivation property. The ipeak of Cr-Mn-Ni series austenitic stainless steels were raised with increasing of Mn content, which implied the repassivation ability of that three steels was decreased. Overall Consideration, the Mn content in Cr-Mn-Ni series austenitic stainless steels should be controlled within 8wt%.