Hello, thanks all for your comments. I've just ordered a new cap just to be on the safe side...
So, the corrosion experiment is officially under way.
However, the idea of putting grease and bits of metal in the kitchen oven at 100°C for several days was received at home with a rather ironic smile, so I took the hint and moved the operation to the sailboat, where i have a gas oven.
What I’m trying to imitate is the basic principle of the ASTM D4048 copper corrosion test for lubricating greases. In the proper test, a clean polished copper strip is immersed in or completely surrounded by the grease and kept at an elevated temperature for a defined period. Afterwards the strip is cleaned and its colour and surface condition are compared with a standard corrosion scale.
The point is to see whether the grease contains additives (particularly certain active sulphur compounds) that react with copper. A good result leaves the copper more or less unchanged or only lightly stained; a bad result can produce strong darkening, black or grey films, staining, and eventually more obvious surface attack.
I’ve used a piece of plumbing copper, which should be reasonably pure, and put it in the SIL GREASE L-EP 00 at around 100°C. The manufacturer actually states a copper corrosion result of Class 1 max. after 24 hours at 100°C, so in theory it should behave well.
The only slight flaw in the plan is that, being a gas oven on a boat, I’m not particularly keen on leaving it running unattended for 24 hours… so this may turn into a rather more hands-on ASTM D4048 than the standard probably intended.
I’ll keep heating it for as long as practical and see whether the copper shows any staining, darkening or obvious attack.
My partner remains unconvinced.
