How does the corrosion happen, is it present prior to assembly? That makes it a QA problem.
Maybe. Maybe not.
It could be caused by a bad (incorrectly installed or bad specification) seal or gasket allowing contaminants in. Or an error in the manufacturing process (some solvent not properly rinsed out). It could also be due to contact and galvanic corrosion between dissimilar metals, which could be an assembly or design error.
I'd be more interested in hearing what the consequences of an eventual failure due to this corrosion would be. A seized gearbox? Oil leak and possible subsequent fire?