ANODIC INDEX

Galvanic Compatibility of Metals - Anodic Index

Quality engineering and design requires an understanding of material compatibility.

Galvanic corrosion(some times called dissimilar metal corrosion) is the process by which the materials in contact with each other oxidizes or corrodes.

There are three conditions that must exist for galvanic corrosion to occur. First there must be two electrochemically dissimilar metals present. Second,there must be an electrically conductive path between the two metals. And third, there must be a conductive path for the metal ions to move from the more anodic metal to the more cathodic metal.

If any one of these three conditions does not exist, galvanic corrosion will not occur. Increased variance of the two metals anodic index cause accelerated corrosion of the more anodic metal.

The variance in anodic index can be controlled somewhat through coatings (paint, epoxy, powder coat, etc)and plating (zinc, chrome, etc).

The finishing and plating selected facilitate the dissimilar materials being in contact and protect the base materials from corrosion.

The table below provides a generalize guide for predicting the anodic corrosion two dissimilar metals are prone to.

The service environment also plays a role:

For harsh environments: outdoors, high humidity, and salt environments fall into this category.

Typically there should be not more than 0.15 V difference in the Anodic Index.

For example; gold - silver would have a difference of 0.15V being acceptable.

For normal environments: storage in warehouses or non-temperature and humidity controlled environments.

Typically there should not be more than 0.25 V difference in the Anodic Index.

For controlled environments: where the temperature and humidity controlled, 0.50 V can be tolerated.

Caution should be maintained when deciding for this application as humidity and temperature do vary from.

METALLURGICAL CATEGORYANODIC INDEX (V)
Gold, solid and plated, Gold-platinum alloy0
Rhodium plated on silver-plated copper0.05
Silver, solid or plated; monel metal. High nickel-copper alloys0.15
Nickel, solid or plated, titanium and alloys, Monel0.3
Copper, solid or plated; low brasses or bronzes; silver solder;
German silvery high copper-nickel alloys; nickel-chromium alloys
0.35
Brass and bronzes0.4
High brasses and bronzes0.45
18% chromium type corrosion-resistant steels0.5
Chromium plated; tin plated; 12% chromium type corrosion-resistant steels0.6
Tin-plate; tin-lead solder0.65
Lead, solid or plated; high lead alloys0.7
Aluminum, wrought alloys of the 2000 Series0.75
Iron, wrought, gray or malleable, plain carbon and low alloy steels0.85
Aluminum, wrought alloys other than 2000 Series aluminum, cast alloys of the silicon type0.9
Aluminum, cast alloys other than silicon type, cadmium, plated and chromate0.95
Hot-dip-zinc plate; galvanized steel1.2
Zinc, wrought; zinc-base die-casting alloys; zinc plated1.25
Magnesium & magnesium-base alloys, cast or wrought1.75
Beryllium1.85