Without risking this becoming an additive discussion, I do have experience with all these sulphur compounds through research into odour compounds in septic sewage and developed a method to capture and analyse them.
Many sulphur compounds have distinct odours at very low concentrations. Hydrogen sulphide can be detected by odour as low as 10 parts per billion in air. Thiols and organic sulphides are all odour compounds.
MarineBob is correct on mercaptan, otherwise known as methanethiol.
To quote Mr Google " Mercaptan, also known as methanethiol is a foul-smelling gas that is added to natural gas. Since natural gas is colourless and odourless, mercaptan acts as an odorant to make it easier to detect. It is added as a safety measure to ensure that natural gas leaks do not go undetected. "
Where Bob is slightly off on that one, is it's hydrogen sulphide that has the rotten egg smell and mercaptan is more like the odour given off by well rotted dead animal bodies and for that reason, many reported gas leaks turn out to be the latter. Just out of interest, hydrogen sulphide goes through several odour stages. At low levels, it's the rotten egg smell. At higher concentrations it becomes a sweet smell. Above that, it can be fatal and in the sewer and agricultural industries, it causes fatalities.
I have a good nose for these gases and have reported four gas leaks in two years alone.
I've also worked with methane in calibrating instruments used by workers in confined spaces.
Dry hydrocarbon gases obviously do not corrode metals. But adding sulphur compounds and maybe some moisture does increase the risk of of the sulphur compounds reacting with metals. If you have ever smelt old brass or copper fittings you will know what I mean.
It wasn't so long ago that there was an incident in the UK where a water main managed to discharge into a gas main. Gas appliance users suddenly found water coming out of their cookers etc.
In conversations with the Southern Gas Networks fitters where I have reported leaks, they are almost always due to corrosion of metal fittings. The corrosion was probably external, but it can give a route in for moisture.
In all my work with gases in laboratories, moisture was guarded against by passing the gases through molecular sieves to ensure absolute removal of water vapour.
Of course the plastic mains should reduce that problem but there are many thousands of miles of old steel and iron mains around.
A far greater worry is the proposal to add hydrogen to the mix. Hydrogen through it's much smaller molecular size, will find leak sites that methane will not pass through. There is already a requirement for the equivalent of a 4" round ventilation hole at ceiling level, permanently open for premises using hydrogen mixes. It took a lot of effort to get leak-free installations into laboratories using pure hydrogen in analytical instruments, hence the high quality fittings mentioned before.
There have been several houses totally destroyed recently by natural gas leaks and that will probably get more frequent when a mix or worse still 100% hydrogen is used. Another safety issue is that pure hydrogen burns with a totally colourless flame. The only way to "see" the flame is by collecting the water vapour on a cold surface like a mirror.
