Because alloys are specifically formulated to meet the need for manufactured products that are durable and corrosion resistant, an important property of all alloys and metals is that they are insoluble in aqueous solutions. They can, however, react (corrode) in the presence of other media, such as air or biological fluids, to form new metal- containing species that may or may not be water soluble. The extent to which alloys react is governed by their corrosion resistance in a particular medium and this resistance is dependent on the nature of the metals, the proportion of the metals present in the alloy, and the process by which the alloy was made.
Of particular importance to dermal exposures are the potential of individual alloys to corrode in sweat. As noted under the discussion of metallic nickel, sensitization and subsequent allergic reactions to nickel require direct and prolonged contact with nickel-containing solutions or materials that are non-resistant to sweat corrosion. It is the release of the nickel (II) ion, not the nickel content of an alloy, that will determine whether a response is elicited. Occupational dermal exposures to nickel alloys are possible wherever nickel alloy powders are handled, such as in powder metallurgy or catalyst production. While exposures to massive forms of nickel alloys are also possible in occupational settings, these exposures do not tend to be prolonged, and, hence, are not of greatest concern with respect to contact dermatitis. Dermal contact with nickel-copper alloys in coinage production can also occur. The potential for nickel alloys to elicit an allergic reaction in occupational settings, therefore, will depend on both the sweat resistant properties of the alloy and the amount of time that a worker is in direct and prolonged contact with an alloy.
The European Union has adopted a Directive (94/27/EC) that is designed to protect most consumers against the development of nickel dermal sensitization through direct and prolonged contact with nickel-containing articles (EC, 1999). With the exception of ear-piercing materials, which are limited to <0.05% nickel content, other nickel-containing articles are regulated based upon the amount of nickel released into “artificial sweat.” Only metals and alloys that release less than 0.5 microgram of nickel per square centimeter per week are allowed to be used in such articles. While determination of individual nickel alloys to meet this standard requires testing on a case-by-case basis, it is worth noting that recent studies of nickel release from stainless steels (AISI 303, 304, 304L, 316, 316L, 310S, 430) in artificial sweat medium have shown that the only grade of stainless steel for which the nickel release rates were close to or exceeded the 0.5 µg/cm2/ week limit is type 303 (a special stainless steel type with elevated sulfur content to aid machinability). All other grades of stainless steel demonstrated negligible nickel release, in all cases less than 0.3 µg Ni/cm2/week (Haudrechy et al., 1994). Although the EU Nickel Directive aims at preventing dermatitis in most nickel sensitized patients, there are some extremely sensitive subjects that have shown positive patch test results with nickel alloys (non-stainless steels) that release 0.5 µg Ni/cm2/week or less (Gawkrodger, 1996). With these few exceptions, the use of 0.5 µg Ni/cm2/week seems to be protective for the majority of nickel-allergic patients.
While the EU Nickel Directive is geared toward protecting the public from exposures to nickel contained in consumer items, it may also provide some guidance in occupational settings where exposures to nickel alloys are direct and prolonged. It should be noted, however, that alloys that release greater than 0.5 ug/cm2/week of nickel may not be harmful in an occupational or commercial setting. They may be used safely when not in direct and prolonged contact with the skin or where ample protective clothing is provided. A recent comprehensive review of the health effects associated with the manufacture, processing, and use of stainless steel can be found in Cross et al. (1999).