Corrosion of Gold-Coated Contact Materials Exposed to Humid Atmospheres Containing Low Concentrations of SO2and NO2

Corrosive atmospheres containing SO2, NO2, and SO2+ NO2(NO2:0.5, SO2:0.6 ppm) have been investigated and their corrosive effects on gold-plated brass were studied. The thickness of the gold coating was 0.3, 1.0, or 2.0 µm. Corrosive gas atmosphere containing NO2(0.5 ppm) caused corrosion sites already at low relative humidity, (40 percent) and the corrosion products contained no nitrogen compounds, just oxides. SO2-containing atmospheres were aggressive at relative humidities (RH) above 60 percent and did not cause as many corrosion sites per square centimeter as did NO2. A mixed-gas atmosphere containing both SO2and NO2(0.6 + 0.5 ppm) caused the formation of suiphuric acid on the surface. This reaction was catalysed by the gold surface. This long lived liquid surface layer acts as a corrosive electrolyte. No nitrogen-containing corrosion products were produced in this atmosphere either. Atmospheric corrosion experiments have been performed with gold-coated brass used in contacts in electrical circuits. The electroplated gold coatings were 0.3, 1.0, and 2.0 µm thick, respectively. The base metal consisted of 73 percent Cu, 23 percent Zn, 3.4 percent Al, and 0.4 percent Co. The corrosive atmospheres studied contained SO2or NO2in air with different relative humidities (RH). Mixed-gas atmospheres conraining both NO2and SO2have also been investigated. At low humidities NO2 was the more aggressive gas, causing pore corrosion at 40 percent RH, while SO2mainly caused attacks at humidities of 60 percent or more. The corrosion products consist of Cu- and Zn-oxides or hydroxides in NO2atmospheres and mainly Cu- and Zn-suiphates in SO2atmospheres. At all relative humidities studied here, a mixed-gas atmosphere containing both SO2and NO2caused a surface-layer consisting of sulphuric acid. The surface layer became visible after only a couple of hours exposure. Thus, at sub-ppm concentrations of SO2and NO2the gold-coated surface seems to catalyse the formation of sulphuric acid. The formation of sulphuric ac- id decreases as the surface becomes covered with acid.