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Corrosion

Hot-Dip Galvanizing Performance in Contact with Other Metals






Where Zinc contact with another metal under atmospheric or aqueous condition, the potential for corrosion through a bimetallic couple exists. The extent of the corrosion will depend upon the position of the other metal relative to the zinc in the galvanic series, the relative surface areas of the two metals in contact, and the conductivity of the electrolyte on the surface of the two metals.

If an installation requires contact between galvanized materials and copper or brass in a moist or humid environment, rapid corrosion may occur. Even runoff water from copper or brass surfaces can contain enough dissolved copper to cause rapid corrosion of zinc coatings. If the use of copper or brass in contact with galvanized items is unavoidable, precautions should be taken to prevent electrical contact between the two metals. Joint faces should be insulated with non-conducting gaskets and connections should be made with insulating grommet-type fasteners. The design should ensure that water is not recirculated and flows from the galvanized surface towards the copper or brass surface, and not the reverse.

Any time a bimetallic assembly contains metal systems that are subject to galvanic corrosion, the ratio of the cathodic area to that of the anode must be carefully considered. The corrosion current that flows between the cathode and anode is independent of area, but the rate of penetration at the anode is dependent on the current per unit area, that is, current density. Therefore, it is undesirable to have a large cathode surface in contact with a small anode surface. The rate of penetration from corrosion increases as the ratio of the cathode to anode surface area increases. When using a stainless steel plate with a zinc rivet, the ratio of the cathode surface area to the anode surface area is large, and the rivet will fail rapidly because of accelerated corrosion. When combining zinc plate with stainless steel rivet, the area ratio between cathode and anode is reversed, although more surface area affected, the depth of penetration small; the fastener shouldn't fail because of corrosion.

Under atmospheric conditions of moderate to mild humidity, contact between a galvanized surface and aluminum or stainless steel is unlikely to cause substantial galvanic corrosion. However, under very humid conditions, the galvanized surface may require electrical isolation through the use of paint or joining compounds. The galvanic behavior of galvanized coatings in contact with various metals in atmospheric and immersion environments is summarized below.

  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
Brass 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 1 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
Steel (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

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

Related References:
1. About Zinc
2. About Hot-Dip Galvanizing
3. HDG Hot-Dip Galvanizing Last Time
4. Cost of Galvanized Steel
5. Selection of Zinc Coatings
6. Zinc Coatings-Galvanized|Electrogalvanized|Galvanneal|Galfan
7. Physical Properties of HDG Hot-Dip Galvanized
8. HDG Hot-Dip Galvanized Abrasion Resistance Resistance to Mechanical Damage
9. Hot-Dip Galvanized Corrosion Protection and the Zinc Patina
10. HDG Hot-Dip Galvanized High Temperature Exposure
11. HDG Hot-Dip Galvanized Surface Reflectivity
12. HDG Hot Dip Galvanized Coating Structure
13. HDG Hot Dip Galvanized Bond Strength
14. HDG Hot Dip Galvanized Coating Uniformity
15. HDG Hot Dip Galvanized Coating Thickness
16. Powder Coating Hot Dipped Galvanized Steel
17. Painting Hot-Dippped Galvanized Steel
18. Painting Hot-Dipped Galvanized Steel Surface Preparation
19. Surface Coatings for Corrosion
20. Hot-Dip Galvanizing Surface Preparation
21. Hot-Dip Galvanizing Galvanizing
22. Hot-Dip Galvanizing Inspection
23. Characteristics of Zinc
24. Hot-Dip Galvanizing Performance in Atmosphere
25. Hot-Dip Galvanizing in Atmosphere Time to First Maintenance
26. Hot-Dip Galvanizing Performance in Soil
27. Soil Corrosion Data for Corrugated Steel Pipe
28. Hot-Dip Galvanizing Performance in Water
29. Cause of Zinc Corrosion
30. Corrosion of Zinc Coated Steel in Selected Natural Fresh Water
31. Corrosion of Zinc and Zinc Coated Steel in Sea Water
32. Corrosion of Zinc Coating in Industrial and Domestic Water
33. Concrete Corrosion of Hot Dip Galvanizing
34. Concrete corrosion resistance of hot dip galvanized reinforcing
35. Removal of Forms Concrete Corrosion
36. Zinc Reaction in Concrete Corrosion
37. Concrete Corrosion References
38. Hot-Dip Galvanizing Performance in Chemical Solutions
39.Hot-Dip Galvanizing Performance in Contact with Other Metals
40. Hot-Dip Galvanizing Performance in contact with Treated Wood
41. Hot-Dip Galvanizing Performance in contact with Food
42. Hot-Dip Galvanizing Performance in Extreme Temperature

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