Titanium, spontaneous passivation

Figure 6. Spontaneous passivation of titanium by galvanically coupling to platinum...

An interesting variation of the effect of galvanic coupling occurs with metals that exhibit active-passive transitions. When noble metals such as platinum, which are good catalysts for hydrogen reduction, are coupled to a metal with an active-passive transition below the reversible proton-hydrogen potential, spontaneous passivation may ensue (Fig. 7). Thus, a porous coating of noble metal on titanium, chromium, or stainless steels will result in anodic protection of the substrate. 

RG. 7—Spontaneous passivation of an active-passive metal, such as titanium, by galvanically coupling to a noble metal such as platinum. The noble metal has a high rale constant for the proton-hydrogen reaction thus, the corrosion potential of the system Is near to the reversible potential for this reaction [7]. 

Titanium metal has a pronounced tendency for spontaneous passivation and this gives the metal a very good corrosion resistance. The passive film consists of TiOj with a thickness of 10-100 A. The film has a remarkable stabihty in chloridechemical plants commercially pure and palladium-containing titanium lead to an increased Hfetime, reduced maintenance and reduced iron contamination of products. This has made titanium a very important alternative material in equipment for the production of cellulose. 

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.)... 

The oxide film on metallic implants is usually very thin (5-10 nm). It is formed as a result of a spontaneous reaction between the metal and the environment. In spite of the common use of immersion treatments in nitric acid solutions, usually known as passivation treatments, they are not necessary to form an oxide. They are often responsible for an increase in corrosion resistance due to removal of surface contaminations or inclusions, as in the case of stainless steels. As indicated in section 9.4.1, there have been reports suggesting that this acid treatment may decrease the corrosion resistance of titanium. 

Table 6.8 provides the Gibbs free energy of formation of different oxides and the corresponding standard potentials for aqueous solutions. Generally, the oxides of less noble metals exhibit a lower standard potential of formation. These metals passivate spontaneously in the presence of protons. Furthermore, many oxides exhibit good chemical stability in acidic environments. This explains the higher eorrosion resistance of metals such as titanium, tantalum and chromium. 

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. 

This is indeed found in acid and alkaline solutions, but around neutral pH the oxide formed is very dense and non-conducting, and oxidation is effectively stopped after a thin layer of about 5 nm has been formed. This thin layer of oxide permits aluminum to be used as a construction material and in many other day-to-day applications. There are, of course, additional ways (e.g., anodizing and painting), to protect aluminum that is exposed to harsh environments, beyond the protection afforded by the spontaneously formed oxide film. However, the unique feature of this metal (and several others e.g., titanium, tantalum and niobium) is that it re-passivates spontaneously when the protective layer is removed mechanically or otherwise, as long as the pH of ihe medium in contact with it is in the appropriate range shown in Figure 18.7. 

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