Titanium passivation required

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

The insertion of platinum microelectrodes into the surface of lead and some lead alloys has been found to promote the formation of lead dioxide in chloride solutions" " . Experiments with silver and titanium microelectrodes have shown that these do not result in this improvement". Similar results to those when using platinum have been found with graphite and iridium, and although only a very small total surface area of microelectrodes is required to achieve benefit, the larger the ratio of platinum to lead surface, the faster the passivation". Platinised titanium microelectrodes have also been utilised. 

Rutile possesses a high refractive index, thus making it an ideal material for passive optical wave-guides [17, 18]. Many of the mentioned applications forTi02 require the employment of thin films. CVD techniques have been used widely for the fabrication of such films. Titanium dioxide films have been prepared by the hydrolysis of TiCU [19, 20], by the reaction of TiCU with oxygen [21] and by the decomposition of titanium alkoxides, Ti(OR)4 (R = alkyl), under an atmosphere of oxygen [14, 15, 17, 18, 22]. Anastase-type SniTii 02 solid solutions have been prepared from the CVD of a mixture of TiCU and SnCU in the presence of water at 630°C [23]. 

The technique may be understood in terms of metallic passivity, i.e. the loss of chemical activity experienced by certain metals and alloys under particular environmental conditions as a result of surface film formation. Equations 15.2 and 15.3 suggest that the application of an anodic current to a metal should tend to increase metal dissolution and decrease hydrogen production. Metals that display passivity, such as iron, nickel chromium, titanium and their alloys respond to an anodic current by shifting their polarisation potential into the passive regon. Current densities required to initiate passivity are relatively high [Uhlig and Revie 1985] but the current density to maintain passivity are low, with a consequent reduction in power costs [Scully 1990]. 

Most water chillers in the chemical industry operate close to the freezing point of water in order to provide the maximum thermal potential. In our most important applications, however, there is a minimum desirable water temperature, set by the formation of chlorine hydrate, the loss of passivation of titanium surfaces in chlorine service, or the freezing of caustic soda. The water chiller can be operated at a higher outlet temperature than usual, or if colder temperatures are required somewhere, the water can be tempered for the major uses. 

FIGURE 14.26. Water content required to passivate titanium in chlraine gas [28]. 

For instance, equiatomic nickel-titanium alloy (nitinol) is a very attractive material for biomedical applications. However, the high nickel content of the alloy and its potential influence on biocompatibility is an issue for nitinol-composed devices. Corrosion resistance of nitinol components from implantable medical devices should be assessed according to regulatory processes and standard recommendations. It is now well known that nitinol requires controlled processes to achieve optimal good life and ensure a passive surface, predominantly composed of titanium oxide, that protects the base material from general corrosion. Passivity may be enhanced by modifying the thickness, topography, and chemical composition of the surface by selective treatments [46]. 

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