Arsthbt

Electronic connectors are often silver plated. However, silver tarnishes fairly quickly and heavily. There exists a widespread misconception that the tarnishing of silver contacts is harmless, because silver oxide has about the same conductivity as silver itself. The problem however is that silver does not oxidize under normal conditions. The tarnish on the contacts is not silver oxide, but silver sulfide that develops due to the presence of some hydrogen sulfide in the air. Unlike silver oxide, silver sulfide is not a conductor, but a semiconductor with various potential adverse effects for the connection.

I have come across a reference that suggests oxidizing silver contacts before using them in order to prevent the development of the silver sulfide layer. This implies that silver oxide does not react with hydrogen sulfide in the air under normal conditions. Is this claim correct?

EDITS

I have corrected the typo by changing "sulfur dioxide" to "hydrogen sulfide" in the title and body of the question. Thanks for pointing this out in the answer!

Silver sulfide is formed by Ag and hydrogen sulfide not sulfur dioxide. You need reducing conditions where the latter can be reduced to

$$ceSO2 +reduction -> H2S$$

Actually, sulfur dioxide chemisorbs on ultraclean silver surface, however heating can remove it. So this is reversible sorption, as suggested by Lassiter [1].

Just note that Auger electron spectroscopy is done under extremely clean environment. There is no trace of water, oxygen or any other component! Real atmosphere is far more complicated and tons of photochemical reactions occur in the atmosphere. A typical indoor air has plenty of undesirable components. How does $ceSO2$ react with Ag must be another story because we cannot avoid or control other factors.

Coming to the second part of the query: If we have surface layer of silver oxide, will it prevent sulfide formation. In principle, possibly yes, because $ceAg2O$ is decent oxidizing agent. The moment traces of $ceH2S$ come in contact with the oxide, it will reduce the oxide to elemental silver. This is my personal speculation.

As per Franey et al. [2]:

Polycrystalline silver has been exposed to the atmospheric gases $ceH2S$, $ceOCS$, $ceCS2$ and $ceSO2$ in humidified air under carefully controlled laboratory conditions. $ceOCS$ is shown to be an active corrodant while $ceCS2$ is quite inactive. At room temperature, the rates of sulfidation by $ceH2S$ and $ceOCS$ are comparable, and are more than an order of magnitude greater than those of $ceCS2$ and $ceSO2$. It appears that $ceOCS$ is the principal cause of atmospheric sulfidation of silver except near sources of $ceH2S$ where high concentrations may render the latter gas important. At constant absolute humidity, the sulfidation rate of
silver by both H2S and OCS decreases from 20 to 40 °C and then increases to 40 to 80 °C.

So may hydrogen sulfide may not be a major culprit!

References

1. Lassiter, W. S. Interaction of Sulfur Dioxide and Carbon Dioxide with Clean Silver in Ultrahigh Vacuum. J. Phys. Chem. 1972, 76 (9), 1289–1292. https://doi.org/10.1021/j100653a011.


2. Franey, J. P.; Kammlott, G. W.; Graedel, T. E. The Corrosion of Silver by Atmospheric Sulfurous Gases. Corrosion Science 1985, 25 (2), 133–143. https://doi.org/10.1016/0010-938X(85)90104-0.

When you mate connectors there is mechanical abrasion. Thus electronic connectors with silver contacts would not be resistant to silver sulfide formation because of this mechanical abrasion which would remove the silver oxide coating. That is why gold plating is often used on higher quality connectors. Gold is resistant to tarnishing of any sort and the gold layer is tick enough to resist mechanical abrasion for a significant number of matings.