Passivation high corrosion resistance

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

It is interesting to see that chromate ions, CrO, which are strongly hard in the Lewis base and an oxidizing agent, oxidize the iron atoms or ferrous ions on the metallic iron surface and reduce the chromate ions themselves forming a passive film of chromic-ferric oxide, which is extremely thin and highly corrosion resistive ... 

In a finely divided form, Hf and Zr metals are pyrophoric, but the bulk metals are passivated the high corrosion resistance of Zr is due to the formation of a dense layer of inert Zr02- The metals are not attacked by dilute acids (except HF) unless heated, and aqueous alkahs have no effect even when hot. At elevated temperatures, Hf and Zr combine with most non-metals (e.g. equation 22.11). 

Corrosion rates and those of mechanochemical wear of stainless steels and alloys widely applied in friction joints of chemical equipment are presented in Table 4.2 [30]. Owing to the formation of passivating protective films on contact with hostile media, these materials display high corrosion resistance. As can be seen from the table, corrosion rates grow during friction by a factor of thousand. Under such conditions, the material wears largely due to corrosion even in a weak solution of sulfuric acid for both sliding friction over a softer material (PE) and under abrasive action (ceramics). 

Tungsten (together with Al, Ti, Zr, Bi, Ta, and Nb) belongs to the group of the so-called valve metals, which passivate and show a very high corrosion resistance in most common aqueous media. The composition of naturally or anodically produced oxide films is essentially identical to WO3. 

Tin improves the mechanical properties of Pb—Ca—Sn alloys. It reduces the passivation phenomena that proceed on the positive battery plates and improves the corrosion resistance of the positive grids. Tin increases also the creep resistance of the Pb—Ca—Sn alloys and thus sustains a good contact between the CL and the PAM. The combination of high corrosion resistance and high creep resistance of the grids prolongs the cycle life of the batteries. 

Titanium 2illoys are being used in increetsing quantities in the aerospace industry because of their high corrosion resistance. The reduction in the passivating films caused by particle erosion as observed in steiinless steel has been investigated for titanium and titanium alloys [2]. 

Titanium is a popular material in industrial and medical applications because of its high corrosion resistance and biocompatibility. These properties are enhanced by the formation of a protective passive oxide film which spontaneously forms on titanium surfaces [11-16], It has been shown that titanium oxide l er enhances the body s ability to incorporate the implant, and reduces the risk of rejection [11, 15, 17-26], In this study. X-ray photoelectron spectroscopy was used to investigate the chemical compositions of titanium thin films on glass and silicon, and to compare these to the surface compositions of bulk titanium disks after different polishing treatments. 

Stainless steels, as well as A1-, Ni-, and Ti-based alloys have been studied extensively as possible candidates for bipolar plates. One of the most well-studied materials for bipolar plates is SS 316/316L (16-18% Cr, 10-14% Ni, 2% Mo, rest Fe) other candidates are 310,904L, 446, and 2205. Bare stainless steel plates form a passive 2-A nm chromium oxide surface layer under PEMFC conditiOTs that leads to unacceptably high ICRs. A similar trend is observed for the other alloys and therefore surface modification or surface coatings on selected substrate material has to be considered as a pathway to meet the technical targets of low ICR and high corrosion resistance. 

Many metals such as titanium, zirconium, tantalum, and niobium have been known to provide high corrosion resistance in acid media. This is mainly due to their ability to passivate on the surface of the bulk material by forming metallic oxides (Antolini and Gonzalez, 2009). Thus, these oxide materials have potential as corrosion-resistant materials in PEM PCs. Zirconium oxide (ZrOj) can be easily modified with sulfonic acid groups (SOJ to increase the acidity and proton conductivity (5 x 10 S cm at 60-150°C) of the material. In fact, sulfated-ZrOj (S-ZrOj) is the strongest solid acid among well-known super acids (H < -16). For these reasons, much research has been conducted with S-ZrOj in fuel cells as... 

A good level of understanding of the surface reactions involved in the formation of passive films (passivation/repassivation) is necessary for designing new, highly corrosion-resistant alloys, and this is achieved by active, continuous, and worldwide research efforts in this area. 

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