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Galvanic potential of metals after conversion coating


Q. I am the responsible mechanical engineer for my company's new naval electronics cabinet. I am evaluating the compatibilities of the materials proposed by our cabinet vendor. We have aluminum and steel in the cabinet. The steel is zinc plated with chromate conversion per ASTM B633 [link by ed. to spec at TechStreet] Type II (yellow). The aluminum is finished per Mil-C-5541 [link by ed. to spec at TechStreet]. Where do the materials now fall on a galvanic compatibility chart? Do metals with chromate or other conversion remain at the same galvanic potential as the base metal?

Mark L. Whittum
- Merrimack, New Hampshire


A. The steel is covered with zinc plating, so you will be dealing with zinc vs. aluminum. This is a common construction method for computers and electronics used within an office or controlled environment and experience proves it isn't much of a problem under those conditions. But if it is to be used at sea I expect it might be a serious galvanic corrosion problem.

Any possibility of anodizing the aluminum? That would make it an insulator. Another possibility is to Ivadize or electroplate aluminum onto the steel in lieu of the zinc plating. Finally, cadmium plating is compatible with aluminum but is a cumulative toxin we are trying to get out of the environment.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


Q. Thanks for the response! We have decided to do the following in order to reduce galvanic incompatibilities and reduce the corrosion potential:

1) Tin plate the steel instead of Zinc plate
2) Use Tin-plated EMI gaskets instead of Monel (not mentioned in the original email)
3) Anodize the aluminum components in the cabinet that will be in contact with the Tin plate

Your comments were helpful in that they confirmed our concerns regarding the Zinc plating and also highlighted the advantage of anodizing the aluminum.

We did some testing here that further clarified the galvanic compatibilities of zinc plated steel with chromate conversion. An electrical engineer here set up a crude battery using de-ionized water with able salt dissolved in it. Galvanic potentials between different materials were measured and the results are as follows:


The difference in electric potential between the following materials when placed in salt water are as follows:

Copper (Cu) (MIL-F-14072 lists EMF of -0.20V)
Zinc plating (Zn) (MIL-F-14072 lists EMF of -1.05V)
Sample from vendor (unknown electric potential)
Chromium (Cr) (MIL-F-14072 lists EMF of -0.45V)

Test results (+ or - next to material indicates which pole of the battery they represented):

Cu+ to Zn- = +0.825V
Cu+ to sample- = +0.80V
Sample+ to Zn- = +0.054V

The vendor sample had the same galvanic potential to within 0.054V as the known zinc sample. The difference between the known copper and the known zinc matched the result expected from the galvanic chart to within 0.025V. FYI - the resolution of most galvanic charts is 0.05V and I assume the tolerance on these measurements is about the same. The chromate wash does not change the galvanic potential of the material surface. If it did the difference between the Cu and the sample would have been about +0.25V and the difference between the Zn and the sample would have been about -0.60V.

Conclusion: the test setup is valid and the vendor sample has the same galvanic potential as Zinc, presumably due to the zinc plating on the sample.

This result agrees with the recommendation from MIL-F-14072, "Finishes for Ground Based Electronic Equipment":

Para. 3.13.1 Use of compatible couples. The following should be considered in the selection and application of compatible couples:

a. Passivated coatings. For a compatible couple selection, passivated coatings specified herein shall be ignored and only the plating or basis metal considered. For example, all. chromate or phosphate treatments of zinc or cadmium specified in Tables III and IV shall be ignored in making couple selections and only zinc or cadmium considered as acting in galvanic corrosion. Hard anodic films on aluminum-base alloys are impervious nonconductors and, therefore, contact may be made with any dissimilar metal.

Mark L Whittum [returning]
- Merrimack, New Hampshire


thumbs up signThank you very much for the followup!

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

January 22, 2015

Q. Is it common practice to ignore chemical conversion coatings when looking at galvanic corrosion? I have a lot of chromate conversion coated aluminium parts and am not sure if I should treat it as aluminium or is there a special galvanic potential unique to the chromate conversion coating. Similarly, we have been looking at non chrome coatings like the Iridite NCP but aren't sure how this would change the metal interaction in comparison to chromate trivalent class 3.

Kosta Krontiris
- Melbourne, Victoria, Australia

February 2, 2015

A. Kosta,
I suspect the main factor here is that only exposed metal can serve as the anode in a galvanic cell. If your part were only partially coated, then the galvanic corrosion would focus on whichever of the two, substrate or coating, has the lowest reduction potential (i.e., is more prone to undergoing oxidation). If the part is completely coated, the substrate is protected as long as that coating is intact, and only the reduction potential of the coating comes into play.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

April 2016

Hi Kosta. Mark Whittum has answered this question for us by referring us to a MIL spec which says to ignore the conversion coating; I'm personally not aware of any specs that say otherwise. Good luck.


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Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Reaction of aluminum conversion coating process on steel hardware (helicoils)

October 7, 2015

Q. Good Morning, my name is David and I work in a aerospace based machine shop as the quality supervisor. I have input responsibility on job planning, NADCAP processing and assembly sequence and methods. A most recent question has come up that I cannot fully answer myself and would like to know if others can help address this question.

I have an aluminum part which ultimately gets assembled with helicoils (both steel and drylubed) and steel dowel pins. I am contemplating installing the hardware prior to the conversion coating process but I am not certain if there are any consequences from the alodine reacting with the steel, drylube or helicoils. If anyone can shed any light on this question it would be most appreciated.

David Nielsen
QC Supervisor - Placentia, California USA

October 7, 2015

These should be installed after coating. Not necessarily because of the Alodine, but the pre-treatments which can very easily attack steel. Also, press fit dowel pins are not water tight, and very well may trap chemicals that could leach out after processing causing staining of the coating..

Marc Green
anodizer - Boise, Idaho

Galvanic Compatibility of Aluminum to Nickel Plated Copper

April 15, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Need to know what are the effects of Galvanic Compatibility of Aluminum that has been chem-film finished when it comes in contact (attached to) a component that is Nickel plated Copper.

Alan Rust
Quality product designer - San Dimas California USA
  ^- Privately contact this inquirer -^

April 2016

A. Hi Alan. The chem-film doesn't hurt, but (at least according to Mil-DTL-14072 [link is to free spec at Defense Logistics Agency,]) it doesn't help either: see Mark Whittum's posting above, which I just re-checked and found to be current. So, you are attaching nickel to aluminum, and these are considered galvanically incompatible with an EMF differential of about 0.6 volts. If you are not obligated to conform to a particular spec the issue becomes less clear, of course, as you will see from many threads here about "copper aluminum galvanic" and "nickel aluminum galvanic". Nickel is slightly worse than copper.

We don't know your situation, but if you can tin plate the copper instead of nickel plating it, you'll have a much more galvanically-compatible joint. Good luck.


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Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

April 18, 2016

Q. I have a flange that is Aluminum chem film finish, and I am attaching it to a flange that is copper nickel plated. Is Galvanic Compatibility at risk or high?

Alan Rust [returning]
- San Dimas California USA

April 2016

A. Hi again. We still don't know what you are talking about (pipe flange? what is in the pipe?), inside a climate-controlled electronics cabinet, or out on an offshore oil platform?

The short answer is that the materials are highly incompatible and very subject to galvanic corrosion. So, unless it's a dry application in an air conditioned environment, it sounds like trouble. Once again, are you sure you can't change that nickel plating to tin? You could nickel plate the aluminum, but only if the plating is heavy (pore-free) and unlikely to be scratched through. Good luck.


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

May 2, 2016

Q. Hello Ted Mooney,

I have a flange that is Aluminum chem film finish, and I am attaching it to a flange that is copper nickel plated. Is Galvanic Compatibility at risk or high?

they are RF flanges on a unit that goes in the tail of a air craft. WR-28 & WR-42.

I was told as long as the two different metals were plated there should not be any galvanic corrosion; this is why I put it out there for more information.

Thanks for your help.


alan rust [returning]
- san dimas California USA

May 2016

A. Hi Alan. If the two flanges are plated with the same metal then there would be no galvanic corrosion because no other metal would be exposed. I think you misunderstood what you were told about this, or the person who told you misunderstood what he was told and left out a critical part of the statement.

For a galvanic current to flow and corrode something requires:
a. Two different metals with different positions on the galvanic series (the further apart they are, the more powerful the battery they create).
b. The two metals must be in metallic contact with each other so electrons can flow from one metal to the other. If insulators prevent metal to metal contact, electrons can't flow
c. A wet conductive situation that allows metal to ionize into solution.

If conditions "a" and "b" exist, electrons will want to flow from the more active metal (aluminum) to the more noble metal (nickel), but as long as condition "c" is not met, the circuit is not complete, so they can't. If condition "c" becomes met, the circuit becomes complete because electrons can flow from the aluminum to the nickel through the metal, and the aluminum atoms, now stripped of an electron, dissolve into solution as positively charged ions and migrate over to the nickel to recapture those electrons. So the aluminum corrodes.

As mentioned, aluminum and nickel are far from each other, so unless freedom from moisture is assured, this is a galvanic problem. Sorry, I don't know enough about RF flanges and aircraft fitting to know whether this is a problem or not.


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

Corrosion of Zinc blue passivated screws assembled to sulphuric anodised Aluminium Component

July 5, 2016

Q. We manufacture & supply anodised aluminium component (Sheet metal) to European customer but from past some days we are getting complaint from customer that they are facing corrosion on the Zinc blue passivated screws assembled to anodised aluminium component after some time.

Note: These zinc passivated screws are passing 240 hours of salt spray.

Is corrosion happening due to screws in contact with the sulphuric anodised aluminium? Please advise.

Packaging will be Export quality.
Shipment is through ocean

Naveen Kumar
- Bangalore, Karnataka, India
  ^- Privately contact this inquirer -^

July 2016

A. Hi Naveen. We appended your inquiry to a thread where similar issues are discussed. I do not have a full answer, but I do have thoughts for your consideration ...

1. The passivation of the screws can be ignored in terms of galvanic compatibility -- so it's a zinc vs. aluminum galvanic couple.
2. If the aluminum was anodized after fabrication, and the screws are machine screws, it would seem that there is no metal-to-metal contact, and thus no galvanic corrosion. If the screws are sheet metal screws, they cut into the aluminum, and there is metal-to-metal contact. But even still, if the whole panel is anodized there should be very little galvanic current available; that is, if the surface of the aluminum cannot corrode into solution because it is anodized, it will not steal electrons from the zinc plated screws and cause them to corrode.
3. It seems to me that either the particular screws in question are not able to survive 240 hours of salt spray, and there is something wrong with your sampling/testing plan which leaves you believing otherwise, or
4. The packaging is not doing what is intended and the assembly is being exposed to a very corrosive salt environment during transport.


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

January 2, 2017

A. Naveen,
There are many possibilities but one idea is to salt spray test not just the screws but the assembly. Testing the screws by themselves does not prove much because when it is assembled a battery is formed and zillions of electrons will go straight to the screw and in the presence of an electrolyte such as salty air, mud, etc., the screw will corrode eventually. If you can greatly improve on the screw coating it might save the day. A bandaid would be vac packing the parts and backfill with dry N2 when shipping.

blake kneedler
Blake Kneedler
Feather Hollow Eng.
Stockton, California

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