Titanium alloys 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). 

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]. 

Crevice corrosion occurs mainly (but not exclusively) on passive materials. The most important problem is the crevice corrosion of stainless steels, nickel-base alloys, aluminum alloys, and titanium alloys in aerated chloride environments, particularly in sea or brackish water, but also in environments found in chemical, food, and oil industries. Other cases of crevice corrosion are also known such as the so-called corrosion by differential aeration of carbon steels, which does not require the presence of chloride in the environment. Also mentioned in the literature is the crevice corrosion of steels in concentrated nitric acid and inhibited cooling water and of titanium alloys in hot sulfixric environments. 

On titanium alloys, however, the metal still has an active-passive transition in the crevice environment and the surface potential must be low enough for the active corrosion to occur. Thus, IR drop is required to stabilize the active dissolution [9,74], particularly when large cathodic currents are available. 

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. 

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]. 

The surface preparation procedures described in ASTM F 86 passivate stainless steels and cobalt alloys. Titanium materials do not require this passivation. It is not clear what the ideal surface for the metal implants should be, and this will continue to be an aspect of studies relating to interfaces of these materials with the body. The condition of the surface may influence ion release. 

A major issue, for the passivation and corrosion resistance of aluminum alloys, is the existence or not of second phase inter-metallic particles resulting from alloying with elements that have low solubility in aluminum (Rynders et al., 1994 Kowal et al., 1996). These particles are detrimental to the resistance of the passive film to breakdown (the first stage of a localized corrosion process). In contrast to stainless steels, this factor often overwhelms the beneficial alloying effects. However, it must be pointed out that alloying elements such as copper in solid solution are beneficial (Muller and Galvele, 1977). Other elements, such as chromium, molybdenum, titanium, tantalum, and niobium, seem to improve the corrosion resistance of aluminum, but their solubility is too low for them to be used in conventional alloy processes, and they require the use of rapid quenching processes or some sort of nonequilibrium surface deposition. 

Anodic protection possesses unique features. For example, the applied current is usually equal to the corrosion rate of the protected system. Thus, anodic protection not only protects but also offers a direct means for monitoring the corrosion rate of a system. The main advantages of anodic protection are (1) low current requirements (2) large reductions in corrosion rate (typically 10,000-fold or more) and (3) applicability to certain strong, hot acids and other highly corrosive media. It is important to emphasize that anodic protection can only be applied to metals and alloys possessing active-passive characteristics such as titanium, stainless steels, steel, and nickel-base alloys. 

Passivity can also be readily produced in the absence of an externally applied passivating potential by using oxidants to control the redox potential of the environment. Very few metals will passivate in nonoxidizing acids or environments, when the redox potential is more cathodic than the potential at which hydrogen can be produced. A good example of that behavior is titanium and some of its alloys, which can be readily passivated by most acids, whereas mild steel requires a strong oxidizing... 

Anodic polarization of active/passive metals - alloys of nickel, iron, chromium, titanium, and stainless steel in weak-to-extiemely corrosive environments, where economy in consumption of protective currents is required. 

In order for the alloy to become passive the material must develop a protective surface to prevent reactions from occurring. Cobalt- and nickel-based alloys become passive by the formation of a chromium oxide layer on the alloy surface. Titanium develops a very tight oxide that does not require any additional alloying elements to develop passivity. 

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