In all honestly, a bit of 5mm (or larger) bit of m/s angle would last quite a while in the ground naturally anyway. Plenty of RSJ's buried as footings and the like, they only have red 'never dry' paint on them.
Btw, couldn't find anything in our shop, have some 70mm x 5mm 316 bar, but I'm fooked if I can get that out the back door
sorry!