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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
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
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. 

Bi Metallic Effect On Galvanized Steel Graph Large 3
Table 1: Bi-metallic Effect on Galvanized Steel in Various Applications

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 ZincStainless Steel
ZincSmall Large

-- --

S G
Stainless SteelSmall Large

G G

-- --

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Comments

Charles Redfield

We have 1.5" x 4" x stainless steel tubes designed to slide within larger galvanized steel tubes. The separation all around is about 1/8" but there is no doubt that there is contact along the length of the tubes. They are exposed to thye weather. Is there a problem of corrosion? Is there a way to avoid corrosion? CR

Alana Fossa (AGA)

Hi Charles, Depending on the corrosive of the environment and zinc to stainless ratio, their combination may be tolerable in service. For more information on impact to longevity and available mitigation methods, please refer to our guide on Dissimilar metals in contact with HDG: https://galvanizeit.org/knowledgebase/article/dissimilar-metals-in-contact-with-hdg

Rajan Varghese

I am fixing Stainless steel enclosure mounting ears to Hot Dip Galvanized surface. Do we meed to isolate both material? Can we use Stainless steel hardware 300 series with isolation of mounting ear and surface of contact? Can we use HDG bolt to fix S/S box mounting ear with isolating material on HDG surface?

Jeff Cunningham (AGA)

Stainless steel is only an issue when the parts are exposed to air containing chlorides. If this occurs isolating the two materials can protect from dissimilar metal corrosion.

Fábio Ghessi

I'm a manufacturer of LGS structure for houses and 4 floor buidings in Brazil. We use Z275 galvanized steel with galvanized rivets for ordinary situations and Z350 for marine environments with stainless steel rivets (316). Please, let me know more about the possibility of galvanic corrosion putting together Z350 GS with SS316. Thank you.

Alana Fossa (AGA)

Hello Fabio, Galvanizing and stainless could be suitable in contact for environments with low chlorides (non marine and/or lower humidity). For more detailed information on the impact of connection geometry: https://galvanizeit.org/knowledgebase/article/dissimilar-metals-in-contact-with-hdg

Clancy Warner

Hi, i am a sculptor and i will be welding bronze sculptures onto a 12mm thick 304 stainless steel baseplates. The baseplates will then be bolted onto galvanised steel baseplates for installation. Could i coat either the stainless baseplate or the gal baseplate with a rubber sealant to prevent corrosion prior to installing? Will this hold up over time? Both baseplates will be buried in the ground near a fresh water river. Thanks for your help.

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