So I had to do some more research today. As for corrosion of aluminum from chemicals there is little risk; a PH level between 4 and 9 is ideal, and most photographic chemicals are within that range. Then there is galvanic corrosion between metals. Some metals have a positive electrode potential (cathode) and some a negative potential (anode). Positively charged particles move from the cathode to the anode, and the anode gets the major share of corrosion. The larger the potential difference is, the more corrosion. Both aluminum and stainless steel have a negative potential and there is little difference in potential, hence little corrosion. The aluminum has the larger negative potential of the two, so if there is corrosion it's the aluminum that gets eaten. I checked online with a manufacturer of fasteners who says that stainless steel fasteners are increasingly used for aluminum.
Now I understand the worries about drilling holes in the lid, but I think the problems of getting this thing welded is underestimated. The laws and regulations here are such that mechanical workshops can only sell their services to registered companies. Independent car body shops are almost non-existent, and they are not cheap. I haven't seen anybody repair pots and pans since the early seventies and that was done by travelling gypsies. They have been marginalised and harassed by the state.
As I said yesterday I will use Liquid Silicone Gasket and try to minimise contact between the two metals. If the aluminum rivets gets eaten, they can be replaced. The biggest worry is space for the rivets, the margins are small on both sides.
