When corrosion appears faster than expected where two metals meet on a container or a body, the cause is usually galvanic. When two metals in electrical contact sit in a common electrolyte, that is water, the less noble one becomes the anode and corrodes faster than it would alone, while the nobler one is protected.
| Pairing | What happens |
|---|---|
| Galvanised coating on steel | The zinc is spent, the steel is protected |
| Aluminium profile, steel fixing | The aluminium side corrodes |
| Stainless bolt, galvanised sheet | The sheet is the anode but its area is large, so the attack spreads thin |
| Galvanised bolt, stainless body | Small anode, large cathode: the bolt goes fast |
The effect speeds up when there is a continuous film of moisture between the two metals; in a dry joint it is barely seen. Salt makes water more conductive, so near the coast and in seafood operations the attack is markedly stronger. Galvanic corrosion follows the line where the two metals meet, while general rust spreads over the surface.
Choose fasteners compatible with the body material, keep the two metals apart with an insulating washer or gasket, drain the lower corners and seal channels where water collects, and rinse off salt and dry the surface after washing.
