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topic 41930 p2

Galvanic Corrosion between Galvanized Steel and Aluminum



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A discussion started in 2006 but continuing through 2019

August 29, 2013

Q. We are a commercial builder putting up pre-engineered metal buildings with zinc-aluminum coated alloy metal 26 ga. panels. A customer recently said he "heard" finish would leach off the panel and pollute the earth.

Any truth to this ?

John Morrissey
building systems - Mattapoisett, Massachusetts, USA


August 31, 2013

A. I've heard similar questions / statements before - that zinc as a sacrificial coating will leach off and cause pollution.

But what alternatives?
Paint: flakes off and goes to earth.
No protection. Rust flakes off and goes to earth where it came from, as iron oxide.
Zinc coatings: oxidize and goes to earth where it came from.

It's a bit hard to define pollution to someone who wants to be emotional about it. Zinc is a natural element, comes from the earth, protects steel (which in turn saves energy and air pollution), and returns to the earth from whence it came.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




"Galvanize spray" on aluminum

September 15, 2013

Q. Hello Folks .

I am just an old retired fisherman with a large problem. I have a 16-foot aluminum fishing boat with a really bad transom, after so many attachment having been installed over its lifetime. The original aluminum looks like a colander or sieve. It's an a 1984 Klamath Alascan.

The wood, which was on the inside of the transom, just rotted out, so I got into it and removed the old wood and replaced it with new.

I then went ahead and put the new piece of aluminum onto the old with lots of Silicone sealant and screwed and bolted in place. Unfortunately that did not work to well and water came in as soon as I launched the boat. This was a blessing in disguise!! Because when I removed the new piece of aluminum so that I could get it welded like I should have done in the first place I found Electrolysis had occurred between the old and new. I had sprayed the old original aluminum with Yellow Chromate and the new piece with Galvanize spray. Please what should I have done?? And what should I now do before the Welder comes to weld it into place. Thank you for your time.

Brian M DAVIS
- Concord, California, USA


A. Hi Brian. Yellow chromate is fine for aluminum although it's an environmental hazard and I'm a bit surprised you were able to buy it. "Galvanized spray" (zinc-rich paint) is for protecting steel, and should not be applied to aluminum.

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
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Aluminum gutters touching galvanized gutters

November 14, 2013

Q. I was just told by a roofing contractor that my galvanized gutters were sheared off from my house when the new aluminum gutters were installed and now the galvanized/aluminum contact will corrode my new gutters. Of course he is recommending that he removes the galvanized attachment from the roof when he re roofs for an additional cost. My question is it this true and should I spend the extra money to save my new gutters?

Amy Smith
- Deerfield, Illinois, USA


November 15, 2013

A. Hi Amy. It's not clear to me if you are getting new gutters yet again with the re-roofing. If so, the small cost of a neat job should be accepted because the remnants of the old gutters will rust up some day. If you are just getting new shingles, and leaving the gutters, then leave them. Although there is somewhat of an incompatibility, a gutter is not an airliner. I see stadium bleachers at high school football games all the time where galvanized and aluminum materials are freely mixed.

Regards,

Ted Mooney, finishing.com
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October 8, 2014

Q. I recently attended an agricultural machinery field day where a machine was being demonstrated that could erect up to four kilometres a day of six wire electric fence. (Three live wires and three un-insulated earth wires) The fence posts were aluminium driven in to the ground. These were pre-drilled to cater for insulators on the live wires but the earth wires had direct contact with the posts. The wire was 2.5 mm galvanised plain wire. What sort of. Life expectancy could be expected of the un-insulated earth wires in this sort of environment?

Graham Turner
- Grafton, N.S.W. Australia


November 24, 2014

Q. Further to my question posted in October, I have since contacted the manufacturers of the aforementioned electric fence regarding my concerns as to the possibility of galvanic corrosion between the 2.5 mm galvanised wire and the aluminium fence posts. He claims that, because the wire is able to move back and forth (due to expansion and contraction following temperature changes) the wire isn't in contact in one place long enough for the corrosion to occur. Does this sound like a reasonable assumption? I have seen a situation where the un-insulated galvanised wire has actually worn through where it ran through the hole in steel fence posts but this was due the the wires being strained too tight with the result that the wires would hum like a guitar string in strong winds. The vibration caused the wire to wear through.

Graham Turner
- Grafton, N.S.W. Australia


December 2014

A. Hi. No, that explanation doesn't sound plausible to me, but ... An electric fence is not an airplane, and galvanic corrosion issues are not nearly as big a deal. There are lots of examples of aluminum in contact with galvanizing where it is not a serious issue. Rain water is essentially non-conducting, and if this area is free from use of road salt and fertilizers, there should be essentially no galvanic corrosion. It's nearly impossible to predict life expectancy when we don't know the environment, the thickness of the galvanizing, the anodizing (if any) on the aluminum, etc. -- but galvanic corrosion probably does not seriously impact it in this case. Good luck.

Regards,

Ted Mooney, finishing.com
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November 12, 2014

Q. I am a materials and corrosion engineer. The bulk of my experience is with military wheeled vehicles.

I have come across numerous cases of galvanic corrosion between zinc plated carbon steel fasteners and aluminum panels. In this case the aluminum is the anode and the carbon steel is the cathode. The surprising thing is that very often the steel fasteners have a great deal of red rust and the aluminum is fine. From what I understand of galvanic corrosion, I would have expected very little corrosion to either material. The steel fasteners are either Yellow Zinc Plated or Tin/Zinc Alloy plated. My theory is that the zinc plating is acting as an anode to the aluminum and, given the big difference in surface area the zinc is quickly dissolved, which leaves the carbon steel un-coated. Since the aluminum and carbon steel are relatively close in the galvanic series the aluminum provides very little protection for the steel and the steel corrodes quickly due to general corrosion.

Any thoughts?

Peter Kopinski
Materials and Corrosion Engineer - Livonia, Michigan


November 2014

A. Hi Peter. Galvanic actions don't always proceed in the direction we immediately anticipate because of aerobic vs. non-aerobic conditions, localized phenomena, etc. One reason cadmium plating was such a popular finish for decades is its proven galvanic compatibility with aluminum for critical applications. If you are okay with cadmium plating despite its toxicity, it is an immediate answer to the problem.

If you are required to, or desire to, avoid cadmium plating, one expensive solution is aluminum coated hardware (Ivadized, or electroplated from molten salts, or from organic liquids with the proprietary Sigal / Alumiplate process). Good luck.

Regards,

Ted Mooney, finishing.com
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December 22, 2014

Q. I have been involved in the replacement of a cooling tower made of galvanized steel. I noticed during the tear out that the 8 inch piping which is insulated with fiberglass and covered with an aluminum skin has extreme rust and corrosion where the pipe turns down and connects to the galvanized steel top of the cooling tower. The cooling water is treated with chemicals. The insulation is often soaked and I have noticed that the iron pipe supports where they protrude through the insulation and aluminum skin are extremely rested as well. The new structural I am making to support a new tower of the same type is to be hot dipped galvanized.
I am wondering if I should coat the galvanized structural at the points of contact with the aluminum to prevent corrosion I am wondering if the final piece of insulation metal skin where it contacts the galvanized metal of the cooling tower should be galvanized sheet. The elbow covering is plastic and there would be no direct contact between the aluminum skin and the galvanized sheet at this point.

Thomas Rockriver
welding & maint. svcs. - Chapel Hill, North Carolina USA


January 2015

A. Hi Thomas. As you know, freestanding steel will rust; galvanic incompatibility is not the only cause of corrosion. For galvanic corrosion you need a liquid connection and a metallic connection. So, if there is "no direct contact" between the aluminum and the steel/galvanized components, there is no galvanic problem. So if I am properly understanding you, your plan for "the final piece of insulation metal skin" to be galvanized sounds good.

Regardless, it sounds like the "soaked" insulation is the principal problem. Maybe a change of insulation type to some kind of closed-cell foam is in order. Good luck.

Regards,

Ted Mooney, finishing.com
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Pine Beach, New Jersey
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February 24, 2015

Q. We have an aluminium frame with an aluminium tray running salt water over the tray panel for evaporation and producing distillate .
The panels are connected by aluminium clamp and galvanised bolt to galvanised support beams.
Even though the support beams are insulated from the ground by wooden posts we see sporadic corrosion of the Aluminium tray?
Can you explain what could be happening?

If we isolated the panel from the galvanised rail with nylon insulators do you think the problem is resolved?
Thanks.
Peter

Peter Johnstone
Water processing - Melbourne, Victoria, Australia


February 25, 2015

A. Hi Peter,

Insulating the galvanising from the aluminium won't hurt, but to be honest I don't think galvanic corrosion is your issue, I think it is simple atmospheric corrosion. If you aluminium is completely unprotected the aggressive nature of salt solutions probably means the natural oxide layer is insufficiently adequate to protect it. Consider having the sheet anodised, this will give better corrosion protection (although this will reduce thermal conductivity).

Brian Terry
Aerospace - Yeovil, Somerset, UK



March 2, 2015

Q. We have completed construction of a home that has an external aluminum stairway supported by primed steel struts sticking out of the building at the bottom and top landing. The struts are directly bolted to the aluminum landings with galvanized bolts. There is not any material separating the steel from the aluminum.

Aluminum staircase a Aluminum staircase b

The home is located 15 miles from the ocean in an area that gets about 20 -30 inches of rain a year during the winter. Is there a potential corrosion issue that can cause a structural problem? Can a sacrificial anode be used as on a boat? Can I obtain cathodic protection by connecting the aluminum to the negative pole of an exterior DC voltage source? How do I do this correctly? Removing the stairs to put a barrier between the steel struts and the aluminum stairs would be a big job.

Glenn Fricker
- Sebastopol, California


March 3, 2015

It is always unwise to put dissimilar metals in direct contact with each other and in an environment where corrosion could occur. In the case of aluminium, it is best to use anodised aluminium, as the hard anodised layer (oxide) is electrically insulating and will help prevent galvanic corrosion. If it is not possible to use anodised material, then you need to use an insulator between the two metals - this can be as simple as a plastic (or rubber) washer or insert. Simply, if the two dissimilar metals are not in direct electrical contact with each other, there can be no galvanic corrosion.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK



March 13, 2015

Q. Hey guys,

I'm looking at buying a builders trailer 8 x 5. A few of the trailers I have looked at, the base is hot galvanize dipped and the top is checker plate aluminium.

Does hot galvanize dipped and checker plate aluminium react with each other? The suppliers of the trailers insist that they have no problems with it. but I wasn't sure!

Is there something I should be checking for/asking that will prevent the reaction?

Thanks in advance for the advice!

Brandan Holas
- brisbane, queensland, australia


March 2015

A. Hi Brandan. If the aluminum is not metallically connected to the galvanizing you can't have galvanic corrosion. If they are connected, then we're into the zone of everything is relative. If it was a boat trailer and the aluminum/galvanized junction could get wet, I would not go for it. If it's a builders trailer that doesn't get salt water on it, then you need to look at how bad the road salt situation might be for your geography and usage. The thing is, the two metals are not grossly incompatible, like galvanized and copper would be, but they're not compatible either.

Regards,

Ted Mooney, finishing.com
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March 16, 2015

Q. Hi Ted,

Thank you for your assistance, what do you mean in terms of metallically connected?

The aluminium top is on plastic or rubber packers but is bolted down to the trailer with galvanised bolts and nuts and also the front and rear ladder racks are galvanised and in direct contact with the ally checker plate top. Will this be a problem and if so will plastic spaces fix the problem and is there an alternative to galvanized bolts that would reduce reaction?

I currently live in the city so it wouldn't be likely to be exposed to salty environments on a frequent basis.

I just don't want to spend all the extra money to prevent rusting of a zinc primed trailer and create more problems for myself in doing so.

Thanks very much in advance

Regards,
Brandan

Brandan holas
- Brisbane, queensland


March 2015

A. Hi again. Metal conducts electricity whereas plastic & rubber don't. So by "metallically connected" I mean any metallic path from one to the other, not interrupted by plastic or rubber. If the bolts are sleeved with plastic washers and bushings, the parts are not metallically connected. If the bolts are metal and touch both the aluminum and the galvanized surfaces, they are metallically connected.

But the unfortunate truth is that it's very difficult for a consumer to determine the corrosion resistance or suitability of materials from any sort of first principles. That's why we must rely on the reputation that a manufacturer has earned, testing by Consumer Reports, Better Business Bureau files, etc.

Regards,

Ted Mooney, finishing.com
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Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey
We need "Aloha" now more than ever


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