Corroded gearbox casing

[RupertW]

Hello all, 

Just been cleaning up the gearbox of the box saloon and underneath all of the grime I've discovered quite a lot of corrosion to the casing - picture below hopefully:    

Can anyone shed any light on what might have caused this? My only thought is perhaps a reaction with the steel of the bulkhead, but that seems unlikely given it had a good coating of oil and grime!

It's not obviously leaking or porous so I'd imagine safe to continue using?

Thoughts welcomed!

[Chris KC]

Was probably left sitting in a puddle / scrapyard at some point in its life. 
I've had reasonable success (cosmetically) repairing this sort of pitting with aluminium-filled epoxy putty in the past (Wilko used to sell it). It won't blend perfectly but doesn't look too bad and will stay stuck. 
I doubt you will encounter strength / leakage issues with that degree of pitting.

[Tony Griffiths]

The damage appears to be more or less in a line - so is it just impact damage from stones or gravel, etc? Aluminium is protected by the oxidised coating on its surface. Might the casing have been, at one time, subjected to impacts by gravel and salt water over a winter allowing the pitted corrosion to continue unabated? Or, could it be galvanic corrosion where the box has been used before in a situation (a boat?) where it was in contact with another metal? Read all about it here: https://www.thyssenkrupp-materials.co.uk...inium-rust

[RupertW]

From memory the car spent a significant amount of it's life in a lockup and is fairly original so I'm not sure if the damage would have happened from sitting in water or a scrapyard etc. but can't rule that out for certain. There's also damage in the other side but lower down and not in a similar line. Perhaps a poor casting from the factory?