MB20B Base Stock as Tarnish Promoter or Preventor

摘要

Electrical noise develops in association with visible staining of gold surfaces during operation of slip-rings lubricated with MB20B base-stock oil. Attempts were made to duplicate these stains in the laboratory using model metal systems and controlled conditions. Visual evaluation and surface-sensitive analysis were used to study and compare the laboratory-induced stains and those that developed in operational slip-rings. Matched pairs (MB2OB-coated versus bare) of model electroplated Au/Ag/Cu and Au/Cu coupons were exposed to various atmospheres (ambient, H2S, H2O) and temperatures. Although the MB20B film provided corrosion protection initially, it subsequently promoted a visibly more aggressive attack than on the uncoated surface. Visual evidence was supported by surface compositional data obtained by Auger spectroscopy and other analytical techniques. Thus, in addition to the C, Ag, and S present as the main tarnish ingredients on bare, gold-plated coupons, tarnish on the MB20B-coated coupon also contained Cu, Cl, and O. The latter, more complex constituency resembled that on discolored areas of operational, MB20B-lubricated slip-rings. To develop comparable constituencies on model coupons in the absence of MB20B, it was necessary to use highly aggressive atmospheres. Silver-to-antimony ratios were established as valid monitors of tarnishing and were used to confirm surface migration rather than mass transfer through the gold coating as the principal source of silver in the tarnish. (Author)