Investigation of the Nature of Pitting Corrosion of Gold Coins

The paper presents the results of studies on the causes of defects on gold coins of Kazakhstani production—“zones of discoloration” which grow during storage. On the basis of the analysis of foreign research papers, it is shown that this phenomenon is also observed in products of mints in other countries. As shown by the results of studies, using the method of X-ray spectral microanalysis, the genesis of these defects is determined by ingress of silver microparticles, sulfur compounds, and atmospheric air moisture on the surface of gold coins. As a result of chemical reaction, hemioxide Ag₂O and hemisulfide Ag₂S are formed. Sulfuric acid electrolyte, formed on the surface of coins, contributes to the migration of silver ions and growth of defects. Depending on the time, the size, thickness, and composition of the pitting corrosion center do change, which causes formation of “discoloration zones.”