A report by Professor. John Zotos,
Materials & Metallurgical Consultant & Lecturer
Pure gold has a low chemical affinity for most elements and compounds, and its resistance towards corrosion and/or interaction with the human skin arises from this fact rather than from the formation of passive films on its surface. Additions of silver, and particularly copper, lower the nobility of gold somewhat. At room temperature levels, gold alloys can go down to as low as 50 atomic per cent gold, i.e., 65 weight per cent gold in the case of silver alloys and about 75 weight per cent gold for the copper alloys, and still possess good passivity towards the human skin. Thus, 18 karat gold alloys (75 weight per cent) would be very safe and should not be hypersensitive when they come into contact with the human epidermis. (1)
Surgical, non corrosive grades of stainless steels are derived from a family of ferrous alloys classified as being austenitic and their crystal structure is face centered cubic. The normal analyses of the various grades of austenitic stainless steels are delineated in the literature. (2) Completely homogenized austenitic stainless steels possess optimum resistance to corrosion and in this state they will not interact with the human skin. The condition is achieved by rapidly cooling all of these alloys from their solution temperature ranging from 1900 F to 2000 F. The presence of annealing twins in the micro-structure of this material proves that it is a fully homogenized state and thus is in an optimum condition for sustained hypoallergenic behavior, i.e., it should be non hypersensitive when it comes into contact with the human epidermis. A special etching solution can be used to reveal such a structure in this family of stainless steel alloys. (3)
Since pure gold is classified as being 24-karat, the 14-karat gold alloys contain only 58.3 weight per cent gold. This simply means that all 14-karat gold alloys are lacking sufficient passivity towards the human skin whether they are used in earrings and/or other types of contact jewelry products since they are 6.7 weight per cent gold below the critical level for silver alloys and 16.7 weight per cent below for copper alloys.
Thus, fully annealed, surgical grades of austenitic stainless steels quenched from their solution temperatures should have a homogeneous microstructure and a far greater hypoallergenic potential than any 14-karat alloys.