Abstract
Two types of designs were used to investigate the effect of tin addition on corrosion behaviors of alloys. The first, based on type 430, cheap and less corrosion resistant, was the Cr-Sn (CS) alloys; the second, based on modified 20Cb3, expensive but corrosion resistant, was the S3 alloys. These tin-bearing alloys were characterized by a series of electrochemical measurements, including linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) and cyclic voltammetry (CV). LSV results show that both CS and S3 alloys become even corrosion resistant due to the presence of tin oxide on the surface of both alloys after corrosion and the cause of reinforced Ohmic polarization. The presence of tin oxide can even force the CS alloys to enter their passive region in an earlier stage (at lower potential), while intermetallic compound, (Ni, Fe)3(Sn, Cr)2, formed in S3 alloys with much amount of tin, that is inverse to corrosion resistant, causing corrosion rate to slightly increase in their passive region. Although there is improvement in corrosion resistance in both alloys, the tin oxide formed on both alloys does not become as passive as chromium oxide (Cr2O3), which is really dense and protective. LSV results show that the current density does not drop to the value of passive current density immediately after CS alloys are in passive region. EIS results also show that the formation of passive film does not change with tin content in both alloys. OCP results show that higher content of chromium and presence of nickel can improve the stability of passive film, as in the case of S3 and higher Cr content CS. If the alloys form passive films that are stable enough, addition of tin can greatly improve stability of passive films to a higher level and even repair the damaged passive films so that the alloys can stay in passive region. On the contrary, tin may have no or even inverse effect on the stability of passive film, if the passive films are not stable, as in the case of CS. Results of CV show the same individual corrosion behavior of element for both less component CS and multi-component S3. With the help of XPS analysis, one is able to make sure the existence of tin oxide, and its composition in both CS and S3 alloys is SnO2.