Its chromed then painted mild steel. Not sure why they chromed it tbh - it makes no sense when it is then painted black afterwards. Possibly they were trying to reject heat from the exhaust manifold and then decided it looked rubbish so painted it.
Rust on steel cooling parts is usually caused by a constant leak (ie the oil cooler pipe) which then gets topped up with plain water, or, ironically, the coolant being too old and losing its anti-corrosive properties. The latter is quite rare on D2s because they have usually been in bits for something well before the coolant goes bad
