Ditto Gareth J, they were electroplated steel, screw fixed to the stainless steel frames as staking angles to hold the mitred corners together, just like with picture frames.
Dumb idea, mixing two different materials. Even on cars without rust issues the lower front corner ones will eventually rust (water gets into the mitre joints as they are unsealed) therefore they blow up & expand & force the mitred joints apart.
Remove them, make up fresh new ones in galv or SS & install them so as to pull the mitres tight together. Paint them heavily in POR-15 or cold galv or whatever, & seal up the frame joints, before reinstalling the windows, & they should be good for many years to come.
I wouldn't weld them as I'd want to be able to disassemble the frames if I needed to in the future.
Edited by gilltech, 21 March 2017 - 06:38 PM.