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Galvanic Corrosion, Zinc and Stainless Steel

 

Hi,

Is it true that a corrosive reaction will take place if a a hot-dip galvanised and stainless steel products are clamped together?

Basically I sell steel channels and special beam clamps for suspending utilities eg. cable management systems, pipings, .........etc.

I was once cautioned by an engineer at a site by the shore oil plant project that we can't clamp a fastener (in my case it refers to a beam clamp) to a hot-dip galvanised steel beam. The thing was, we did not really get in detail about it as I thought he probably could have heard it from someone else. So, as it is important to my business, I need a professional view from you.

I have checked with a local galvaniser in Malaysia, they seem to have also heard/seen it somewhere but still haven't got any firm/documented reply pertaining to this.

In reference to the surrounding conditions, I was hoping we can evaluate this issue based on a general basis ie. sea-side, outdoor and indoor.

Nevertheless, is it true that a corrosive reaction will take place at the contact area of these two named metals?

In my knowledge, if the corrosion do occur, it will not spread as Zinc is self sacrificial.

Thank you very much.

Jeremy Yap
Hokomo Trade Centre Sdn Bhd - Selangor, Malaysia


 

I wouldn't do it, especially in very corrosive environments, because as you say, the zinc is sacrificial, and it will dissolve. After it is gone, there is nothing stopping the remaining steel from corroding with the stainless as the cathode.

Tom Pullizzi
Falls Township, Pennsylvania

 

I think the problem you fear, and which Tom confirms, would be real: spot corrosion of the beam in the area near the clamp. However, your closing sentence about self-sacrificial zinc probably isn't accurate. The fact that the corrosion occurs near the contact point rather than across the whole beam is probably a practical issue rather than a theoretical one. In theory, the corrosion would not be localized to the area of contact; rather the anodic current would be spread across the entire surface of the beam. But in the real world, a bit of water probably collects at the beam clamp rather than immersing the entire beam, so the galvanic current is restricted to the wetted area.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


 

Jeremy,

As Ted and Tom infer, you could well have problems especially if the contacts got wet!

If you are concerned, why don't you play safe and, if you can, place some plastic all around the contact area? In other words chemically isolate that area. Another option is to 'paint' the contact points of your clamps with a suitable paint.

You will definitely have some corrosion or preferential corrosion if you join dissimilar metals together, that's the nature of the beast!

I am slightly biased towards plastics (40 odd years in this field) but the plastic clamps have nowhere the load bearing capacity of metal ones.

Food fer thought, as they say in bad English ... but you have an excellent command of this universal 'linqua franca'.

Cheers !

freeman newton portrait Freeman Newton
- White Rock, British Columbia, Canada
freeman newton died


 

Zinc in contact with stainless steel is extremely dangerous in time of fire. There is a chemical reaction and that can cause an explosion. That is why stainless steel thermowells having union-nipple-union construction should NEVER be specified with galvanised union-nipple-union fittings, only black iron or stainless.

Michael Oser
- Sydney, Australia


+++++++

Zinc will cause cracking of SS in case of fire (high temp.)

M Alabbasi
- Kuwait


July 17, 2008

If you couple zinc with stainless steel near the sea, you can get embrittlement of the stainless steel. This is due to the formation of atomic hydrogen when seawater breaks down and releases chlorine molecules.
As zinc is amphoteric it can be attacked by either very low or very high pH solutions. The hydrogen gas that forms can enter the molecular matrix of the stainless steel and expand, causing the steel to crack.
But please note that this normally only happens when the steel is under stress.
I have coated stainless steel with a zinc silicate many times over the years, and once the silicate was coated with a polyurethane primer and paint there was never any problem as the paint was a 'salt barrier' to the metals.

Rick Simpson
- Spalding, England
  ^-- this reader rates this thread: winner
November 7, 2011

I was just told by a dockmaster that nothing will happen to stainless steel props if they sit in the water. We are brackish here and I contend that eventually your zincs will get eaten and electrolysis will begin.......even with stainless steel. please tell me I have not been thinking wrong for 30 some odd years.

Audrey Crandall
- Palm City, Florida

November 8, 2011

Hi, Audrey.

Electrolysis isn't quite the right word to use in talking about the situation. Electrolysis is about two pieces of metal (your prop and something else) being in solution (seawater in this case) and exposed to an externally applied source of current (non-existent in the case under discussion), so that's not exactly what we have here whether the zinc anode is present or absent.

What we have instead is stainless (but not 100% corrosion proof) steel which can perhaps benefit from the sacrificial protection of zinc anodes in the same way that steel or aluminum do, although to a much smaller extent. In the real world there can be various factors, like two different stainless steels, or even a heavily cold worked area of one piece of stainless steel, that can lead to minor galvanic corrosion forces. A zinc anode will sacrificially protect against this. There can also be occasional electrical currents in marina waters that accelerate corrosion. I'd say zinc anodes can help even if most of the time the stainless steel props don't terribly need them.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
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