Chassis Strengthening Plates

[Nigeyboy]

Just been cleaning the underside of the T350 and took off the chassis strengthening plates for better access. Can anyone tell me what they are made of and what finish was applied to them.

They appear to have a bot of rust starting around where the washers sit and there appears to be a scratch which is starting to look a little corroded.

Also, should stainless steel bolts/washers be OK to replace the original passivated M12 x 20 ones.

Thanks, Nigel