Galvanic Corrosion

Under these conditions the tendency of one metal to corrode will be increased, while that of the other will decrease. This Galvanic corrosion will be accelerated in the presence of sea water or salt spray, industrial and volcanic fumes, and rain. In effect the more electrically conductive the moisture is, the more rapid the galvanic corrosion.

While commonly produced Galvanic Tables are of some use, they almost always ignore the effect of the relative size of the surface areas in contact with the liquid. If the material suffering increased attack (anode is a small component such as a fastener, the attack will be concentrated and rapid failure will result.

If the fastener is the protected member (cathode) of the couple, and corrosion of the other member is spread over a large surface area, satisfactory life of both will be achieved.
Obviously fasteners must resist corrosion at least as well as other component parts of equipment or structures as they must “hold it together.”

To reduce corrosion of both fasteners and components:

• Select materials which will resist the environment. For example Hot Dip Galvanising will usually be more than adequate protection of steel against atmospheric corrosion.
• Where possible select a fastener of the same material and finish as the parts being fastened, to reduce galvanic corrosion.
• Where dissimilar metals must be used, consideration of electrolytic action at points where the dissimilar metals are in contact must be taken into account. Often an insulating barrier such as a gasket or Lanotec lanolin, with its high conductive resistance (70 Kv) will suffice.

The chart takes into account that the Galvanic corrosive effects are restricted to one of the dissimilar metals.

These recommendations assume the fastener surface area will be less than the metals joined, ensuring that the accelerated corrosion will be on the body of the structure, rather than on the fastener.

	Fastener Materials & Finishes
Metal Joined	Plain Steel	Zinc Plated Steel	Hot Dip Galv Steel	Brass & Bronzes	Stainless Steel	Aluminium
Plain Steel	R1	S	S	S	X	U
Zinc Plated Steel	S2	R	R	S	S	S
Hot Dip Galvanised Steel	U	S2	R	S	S	S
Cast Iron	R1	S	S	S	X	U
Zinc e.g Die Casting	S2	R	R	S	S	S
Brass & Bronzes	U	U	U	R	S	U
Copper	U	U	U	S	S	U
Passive Stainless Steel	X	U	U	U	R	U
Aluminium	U	S3	S3	U	R	R1

R:- Recommended       S :- Suitable     U :- Unsuitable

X :- Unlikely to be considered due to appearance.

R1: It is not intended to imply that exposure of these metals in an unprotected environment will yield satisfactory behaviour. The recommendation is essentially for elimination of galvanic corrosion.

S2: The degree of protection of the small area of the fastener depends on the amount of zinc available on the surrounding die cast, or galvanised surface.

S3: Aluminium is the protected member of the aluminium-zinc combination, causing accelerated corrosion of the zinc. This will eventually lead to the exposure of the base steel of the fastener, which in turn will accelerate the corrosion of the aluminium.
 
Corrosion can be reduced, if not eliminated, by proper selection of corrosion resistant materials and protective coatings. We hope you find this helpful. 

The team at Boltmasters hope you find this helpful.