Dissimilar Metals in Contact

Hot-dip galvanized steel is well suited for use in a variety of environments and fabrications, and sometimes is placed in contact with different metals including, among others, stainless steel, aluminum, copper and weathering steel.

When two different metals are in contact in a corrosive environment, one of the metals experiences accelerated galvanic corrosion while the other metal remains galvanically protected.

Metals near each other in the galvanic series have little effect on each other. Generally, as the separation between metals in the series increases, the corroding effect on the metal higher in the series increases as well.

Relative surface areas of contacting dissimilar metals is also relevant in determining which metal exhibits accelerated corrosion. It is undesirable to have a large cathode surface in contact with a relatively small anode surface.

Galvanic corrosion occurs when two different metals are in contact in a corrosive environment: one of the metals experiences an accelerated corrosion rate. The contacting metals form a bimetallic couple because of their different affinities (or attraction) for electrons. These different affinities create an electrical potential between the two metals, allowing current to flow.

The metal higher in the galvanic series of metals, the “anode,” provides protection for the metal lower in the series, the “cathode.”

As can be seen from the galvanic series, zinc protects the lower-order steel.

With respect to contacting surface areas of the two metals, although the corrosion current that flows between the cathode and anode is independent of area, the rate of penetration at the anode does depends on current density. Thus, a large anode area in contact with a relatively small cathode area is generally not problematic. Regardless, environmental conditions remain large determinants of corrosion rates.

  Environment
  Atmospheric Immersed
Metal in Contact     Rural      Industrial
/Urban
Marine Fresh
Water
Sea
Water
Aluminum and aluminum alloys 0 0 to 1 0 to 1 1 1 to 2
Aluminum bronzes and silicon bronzes 0 to 1 1 1 to 2 1 to 2 2 to 3
Brasses including high tensile (HT) brass ( manganese bronze) 0 to 1 1 0 to 2 1 to 2 2 to 3
Cadmium 0 0 0 0 0
Cast Irons 0 to 1 1 1 to 2 1 to 2 2 to 3
Cast Iron (austenitic) 0 to 1 1 1 to 2 1 to 2 1 to 3
Chromium 0 to 1 1 to 2 1 to 2 1 to 2 2 to 3
Copper 0 to 1 1 to 2 1 to 2 1 to 2 2 to 3
Cupro-nickels 0 to 1 0 to 1 1 to 2 1 to 2 2 to 3
Gold (0 to 1) (1 to 2) (1 to 2) (1 to 2) (2 to 3)
Gunmetals, phosphor bronzes and tine bronzes 0 to 1 1 1 to 2 1 to 2 2 to 3
Lead 0 to 1 0 to 1 0 to 1 0 to 2 (0 to 2)
Magnesium and Magnesium alloys 0 0 0 0 0
Nickel 0 to 1 1 1 to 2 1 to 2 2 to 3
Nickel copper alloys 0 to 1 1 1 to 2 1 to 2 2 to 3
Nickel-chromium-iron alloys (0 to 1) (1) (1 to 2) (1 to 2) (1 to 3)
Nickel-chromium-molybdenum alloys (0 to 1) (1) (1 to 2) (1 to 2) (1 to 3)
Nickel silvers 0 to 1 1 1 to 2 1 to 2 1 to 3
Platinum (0 to 1) (1 to 2) (1 to 2) (1 to 2) (2 to 3)
Rhodium (0 to 1) (1 to 2) (1 to 2) (1 to 2) (2 to 3)
Silver (0 to 1) (1 to 2) (1 to 2) (1 to 2) (2 to 3)
Solders hard 0 to 1 1 1 to 2 1 to 2 2 to 3
Solders soft 0 0 0   0
Stainless Steel (austenitic and other grades containing approximately 13% chromium) 0 to 1 0 to 1 0 to 1 0 to 2  
Stainless Steel (martensitic grades containing approximately 13% chromium) 0 to 1 0 to 1 0 to 1 0 to 2 1 to 2
Steels (carbon and low alloy) 0 to 1 1 1 to 2 1 to 2 1 to 2
Tin 0 0 to 1 1 1 1 to 2
Titanium and titanium alloys (0 to 1) (1) (1 to 2) (0 to 2) (1 to 3)

Key:

  • 0 = Zinc and galvanized steel will suffer either no additional corrosion, or at the most only very slightly additional corrosion, usually tolerable in service.
  • 1 = Zinc and galvanized steel will suffer slight to moderate additional corrosion that may be tolerable in some circumstances.
  • 2 = Zinc and galvanized steel may suffer fairly severe additional corrosion and protective measures will usually be necessary.
  • 3 = Zinc and galvanized steel may suffer severe additional corrosion and the contact should be avoided.
General Notes: Ratings in brackets are based on very limited evidence and hence are less certain than other values shown. The table is in terms of additional corrosion and the symbol "0" should not be taken to imply that the metals in contact need no protection under all conditions of exposure.
Source: British Standard Institute, pp 6484: 1979, Table 23