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Stainless steel and galvanized materials often are found together in industry with applications such as galvanized fasteners, stainless steel pressure vessels and roof and siding panels. Do I need to worry about these two metals corroding each other? What other concerns should I have pertaining to hot dip galvanized steel and stainless steel in contact?

Bimetallic Couple

Stainless steel and galvanized materials often are found together in the industry with applications such as galvanized fasteners, stainless steel pressure vessels and roof and siding panels. The presence of two dissimilar metals in an assembly is not always a sign of trouble but it could be a problem. When two metals are in direct contact, there is the potential for the formation of a bimetallic couple. There are four elements necessary for the contact metals to experience corrosion;

  1. One of the metals must act as the anode and generate electrons that can create an electrical current flow.
  2. The other metal must act as a cathode and collect these flowing electrons. This metal is the protected partner of the corrosion cell.
  3. There must be an electrolyte material covering these two metals at the area where they are touching to complete the electrical current path. This electrolyte material must be able to conduct ions from one metal to the other; and
  4. There must be a return current path which in almost all cases is a direct contact between the two metals. The following diagram shows all of the parts of the bimetallic couple.

As in all design and fabrication situations, the problem is not as simple as just looking it up on a chart. The zinc has been applied to the steel to provide corrosion protection for the underlying base steel. If zinc is in contact on the surface with a more cathodic metal and the zinc becomes part of a bimetallic couple and corrodes, then the zinc is not performing its designed function of protecting the base steel. The formation of a bimetallic couple needs four elements in order to form. The existence of two dissimilar metals in direct contact can be no problem whatsoever if there is no electrolytic material present.

Galvanic series of metals

In most atmospheric applications the only potential electrolytic material that can be present is rainwater or dew. Both of these forms of water are poor electrolytic materials since they do not contain many salts and ions which would make them conductive. On the other hand, marine environments and areas where the melting snow includes road salts can be very good electrolyte materials. Bimetallic couples are more easily formed in immersion situations where the assembly will be underwater when it is in service. Salt water is especially tough on two dissimilar metals in contact. The best guide as to how various metals will react in contact with zinc under different environments is the following table. 

 Environment
 AtmosphericImmersed
Metal in Contact    Rural     Industrial /UrbanMarineFresh WaterSea Water
Aluminum and aluminum alloys00 to 10 to 111 to 2
Aluminum bronzes and silicon bronzes0 to 111 to 21 to 22 to 3
Brasses including high tensile (HT) brass ( manganese bronze)0 to 110 to 21 to 22 to 3
Cadmium00000
Cast Irons0 to 111 to 21 to 22 to 3
Cast Iron (austenitic)0 to 111 to 21 to 21 to 3
Chromium0 to 11 to 21 to 21 to 22 to 3
Copper0 to 11 to 21 to 21 to 22 to 3
Cupro-nickels0 to 10 to 11 to 21 to 22 to 3
Gold(0 to 1)(1 to 2)(1 to 2)(1 to 2)(2 to 3)
Gunmetals, phosphor bronzes and tine bronzes0 to 111 to 21 to 22 to 3
Lead0 to 10 to 10 to 10 to 2(0 to 2)
Magnesium and Magnesium alloys00000
Nickel0 to 111 to 21 to 22 to 3
Nickel copper alloys0 to 111 to 21 to 22 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 silvers0 to 111 to 21 to 21 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 hard0 to 111 to 21 to 22 to 3
Solders soft000 0
Stainless Steel (austenitic and other grades containing approximately 13% chromium)0 to 10 to 10 to 10 to 2 
Stainless Steel (martensitic grades containing approximately 13% chromium)0 to 10 to 10 to 10 to 21 to 2
Steels (carbon and low alloy)0 to 111 to 21 to 21 to 2
Tin00 to 1111 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

The performance of zinc in contact with most of the common building metals is rated for most environments. This figure is easy to understand and provides a good reference sheet to fax to those who are concerned with the potential of forming a bimetallic cell.

The final answer to those who want to assemble systems with dissimilar metals that will be immersed in service is to electrically isolate the two pieces by inserting an insulating material between them. The breaking of the contact between the two metals will effectively stop any possibility of forming a bimetallic cell. Most plastics are good insulating materials. For saltwater immersion, the most common insulator is a piece of rubber.

Dr Galvfigure1

One Exception

The rate of penetration of corrosion increases as the ratio of the cathode to anode surface area increases; as it decreases, the rate of penetration decreases. This situation is portrayed using a riveted fastener as shown in Figures 1 and 2. When using a stainless steel plate with a zinc rivet (Fig. 1), 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 a zinc plate with a stainless steel rivet (Fig. 2), the area ratio between the cathode and anode is reversed, and although more surface area is affected, the depth of penetration is small; the fastener should not fail because of corrosion. The size correlation to the corrosion rate is also shown in Table 1.

  Surface area of metal in row
relative to Surface area in column 
Zinc Stainless Steel
Zinc Small
Large

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S
G
Stainless Steel Small
Large

G
G

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