Contact Corrosion

WARNING: This page is about a different car, the 2016 Volkswagen Beetle, 2015 Volkswagen Beetle, 2014 Volkswagen Beetle, 2013 Volkswagen Beetle, and 2012 Volkswagen Beetle. However, it is still accessible from the selected car via links, so may be relevant.

The transmission housing and clutch housing consist of a magnesium alloy.
Bolts and other components which come into direct contact with the transmission have a surface coating adapted for this.
Contact corrosion occurs when incorrect components (such as bolts, nuts, washers) are used. The transmission housing and clutch housing are damaged.
Only install the correct parts.