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 CATEGORY||ANODIC INDEX (V)|
|Gold, solid and plated, Gold-platinum alloy||0|
|Rhodium plated on silver-plated copper||0.05|
|Silver, solid or plated; monel metal. High nickel-copper alloys||0.15|
|Nickel, solid or plated, titanium and alloys, Monel||0.3|
|Copper, solid or plated; low brasses or bronzes; silver solder;|
German silvery high copper-nickel alloys; nickel-chromium alloys
|Brass and bronzes||0.4|
|High brasses and bronzes||0.45|
|18% chromium type corrosion-resistant steels||0.5|
|Chromium plated; tin plated; 12% chromium type corrosion-resistant steels||0.6|
|Tin-plate; tin-lead solder||0.65|
|Lead, solid or plated; high lead alloys||0.7|
|Aluminum, wrought alloys of the 2000 Series||0.75|
|Iron, wrought, gray or malleable, plain carbon and low alloy steels||0.85|
|Aluminum, wrought alloys other than 2000 Series aluminum, cast alloys of the silicon type||0.9|
|Aluminum, cast alloys other than silicon type, cadmium, plated and chromate||0.95|
|Hot-dip-zinc plate; galvanized steel||1.2|
|Zinc, wrought; zinc-base die-casting alloys; zinc plated||1.25|
|Magnesium & magnesium-base alloys, cast or wrought||1.75|
- ANODIC - LEAST NOBLE
- Magnesium Alloys
- Mild Steel , Wrought Iron
- Cast Iron, Low Alloy High Strength Steel
- Chrome Iron (active)
- Stainless Steel, 430 Series (active)
- Stainless Steel 302, 303, 321, 347, 410,416, (Active)
- Ni - Resist
- Stainless Steel 316, 317, (Active)
- Aluminum Bronze
- Hastelloy C (active) Inconel 625 (active)
- Titanium (active)
- Lead - Tin Solders
- Inconel 600 (active)
- Nickel (active)
- Hastelloy B (active)
- Manganese Bronze , Tin Bronze (
- Nickel Silver
- Copper - Nickel Alloy 90-10
- Copper - Nickel Alloy 80-20
- Stainless Steel 430
- Nickel, Aluminum, Bronze
- Silver Solder
- Nickel (passive)
- 60 Ni- 15 Cr (passive)
- Inconel 600 (passive)
- 80 Ni- 20 Cr (passive)
- Chrome Iron (passive)
- Stainless Steel 302, 303, 304, 321, 347,(PASSIVE)
- Stainless Steel 316, 317,(PASSIVE)
- Incoloy 825nickel - Molybdeum - Chromium
- Iron Alloy (passive)
- Titanium (pass.) Hastelloy C (passive)
- Inconel 625(pass.)
- CATHODIC - MOST NOBLE