Sulfur And Metals

I'm a chemist by trade, and over the last 30 years I have dealt with sulfur in many forms, mainly in the paper industry. I'm not an expert in metallurgy per se, but have seen the erosion/corrosion effects of sulfur compounds on many metals. Any other experts out there, feel free to chime in.

To start with, elemental sulfur (0 state)is a fairly stable yellow solid. Fully oxidized sulfur is also stable - in the S (minus 6) state you see it as metallic salts and sulfuric acid. Common compounds are gypsum (CaSO4), epsom salts (MgSO4) and alum (too complex for my computer skills). In these two forms sulfur is relatively benign towards metals, though long term contact will have effects (and in the case of the sulfuric acid form, there are drastic short term corrosive effects!) Sulfur is also incorporated into biochemicals, primarily proteins/amino acids, and into a host of organic chemicals including cyanide and thiosulfates.

The problems caused by sulfur in contact with metals are mainly due to the stable transitional states (minus 2 and minus 4), between the elemental and fully oxidized forms. Many metals occur naturally as minus 4 state sulfides (iron pyrites, copper sulfides). In the metal extraction process these ores are roasted to eliminate the sulfur. The sulfur released in ore roasting is dreaded SO2 gas, which forms H2SO3 (sulfurous acid) in contact with water, resulting in acid rain. If directly inhaled at low concentration, SO2 forms acid on contact with body moisure - an icky, sulfury acidic taste in your mouth (but in my experience it keeps the sinuses clean). SO2 in contact with most metals mainly causes corrosive pitting, but it is capable of darkening a metal surface.

More detrimental to metals is the minus 2 state sulfur - hydrogen sulfide, sodium sulfide, as well as organic chemicals like mercaptans. These aggressively attack most metals, especially silver, to generate tarnish (Ag2S). They are generated anaerobically, typically by bacterial action or by burning/roasting sulfur with insufficient oxygen. These compounds have a strong rotten smell.

Sulfur can "morph" from one form to another, depending on the conditions it is subjected to. Elemental sulfur subjected to heat and strong alkali will convert to the minus 2 state. If sulfur is burned it can go to any state, depending on how much oxygen is present. It can also go in the other direction. Anaerobic conditions (particularly in the presence of bacteria) can take stable minus 6 sulfur back to minus 2 state. Thus sulfur used in papermaking in the form of alum (not a tarnisher by itself) can be converted to a sulfide (a strong tarnisher) under the right conditions. I'd suspect this as a major mechanism for coin tarnish and toning in old cardboard/paper coin books, especially under damp musty conditions.