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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?
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;
- One of the metals must act as the anode and generate electrons that can create an electrical current flow.
- The other metal must act as a cathode and collect these flowing electrons. This metal is the protected partner of the corrosion cell.
- 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
- 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.
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.
Table 1: Bi-metallic Effect on Galvanized Steel in Various ApplicationsThe 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.
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 | ---- | S G |
| Stainless Steel | Small Large | GG | -- -- |
For more detailed information about galvanic corrosion and hot-dip galvanizing in contact with dissimilar metals, see Dissimilar Metals in Contact.
© 2023 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.
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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
(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?
(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.
(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.
Seth Sugimoto
Would it be okay to attach an aluminum roof to a galvanized purlin with stainless steel screws? I believe the galvanized purlin and the aluminum roof connection should be adequate, but the main concern is the stainless steel screws. This structure is also located near the ocean so it is subjected to some saltwater.
(AGA)
Hi Clancy,We explain different methods to prevent corrosion between different metals here:https://galvanizeit.org/knowledgebase/article/dissimilar-metals-in-contact-with-hdg
(AGA)
Hi Seth,Mixing more than two metals results in corrosion behavior which is very difficult to predict. This is especially true for sea/coastal environments. Where possible, use all one material. Where this is not feasible, utilize the mitigations we provide here: https://galvanizeit.org/knowledgebase/article/dissimilar-metals-in-contact-with-hdg
Sudhkar
We do have requirement for hot dip Zinc Galvanizing for SS tanks.Is it ok??? To go head for process??Please advise us.
(AGA)
Hello Sudhkar -- Many stainless steels of the 300 series can be galvanized because they contain nickel, which is necessary to initiate the reaction between the steel and the zinc. Alternatively, stainless steels of the 400 series do not contain nickel and cannot be galvanized successfully.
Austin Turner
Hi, I have some raised garden beds made from corrugated galvanized steel. The formed sheets are connected using zinc-coated steel nuts, bolts, and washers. I had initially planned to use stainless steel hardware, but was concerned that dissimilar metals in contact could reduce the protective effects of the galvanized coating and cause the corrugated panels to rust. After 1 year there are no signs of corrosion, either on the panels or the zinc-coated hardware, but I'm unsure if I've done the right thing for longterm durability or if there's more I could so to protect the beds. Great article, and I appreciate any advice you may have!
(AGA)
Hello Austin, it sounds like the all-galvanized solution is working as intended. For more information, see our guide on the use of HDG with dissimilar metals: https://galvanizeit.org/knowledgebase/article/dissimilar-metals-in-contact-with-hdg
Charles Miller
I drilled holes in galvenized post and would like to paint the post then thread with stainless steel cable as horizontal balisters 4" apart. Wil I have any issues?
(AGA)
Hi Charles,Stainless steel wire will be acceptable for long-term contact with hot-dip galvanizing in many mild or low corrosive environments. In aggressive or salty environments, it is preferred to use all one metal or electrically isolate the two different metals such as placing an inert spacer/wrap at the area where the metals contact each other.
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(AGA)
Hello Mithun, It is recommended to isolate galvanized and stainless steel in marine/C5 environments. For more information, see our guide on the use of HDG with dissimilar metals: https://galvanizeit.org/knowledgebase/article/dissimilar-metals-in-contact-with-hdg
Benchmark Fabricated Steel Inc.
Is it okay to attach 5/8" treated plywood to galvanized decking using stainless steel screws?
(AGA)
Hi Benchmark,Both Stainless and HDG fasteners are appropriate for use in treated plywood. As this is a fairly aggressive environment, there may be some concerns with galvanic corrosion depending on the overall design life you are looking for. If possible, use HDG fasteners which are both suitable for use in treated wood and also will not present a galvanic corrosion issue.
PT Bambang Djaja
We have a transformer tank with Galvanized Steel material. While the material for the bolts and nuts that are in contact with the tank is stainless steel series 300. I have three questions as follows:1. Will this bimaterial cause corrosion?2. Can I galvanize bolts and nuts whose material is made of stainless steel series 300?3. If I have galvanized the bolts and nuts, can the corrosion problem be solved?
(AGA)
Hello PT Bambang Djaja,When used in mild-to-moderately corrosive environments and/or mild-to-moderate humidity, contact between a galvanized surface and stainless steel is unlikely to cause substantial incremental corrosion. However, under very humid conditions or corrosive environments (including atmospheres close to bodies of salt water), these materials may require electrical isolation from each other for the structure to perform as intended.It is possible to galvanize 300 series stainless steels because they have nickel. There is no fear of galvanic corrosion when galvanized steel is in contact with other zinc coatings.
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