Nickel-base 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). 

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. 

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. 

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. 

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]. 

---