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topic 42140

Stainless or zinc plated a better galvanic couple with aluminum?


A discussion started in 2006 & continuing through 2017 . . .

(2006)

Q. I have a question regarding the appropriate choice between zinc plated steel and stainless steel part when in contact with aluminum.

We are designing an electronic enclosure which has Aluminum Extruded Side walls. The aluminum sidewall has grooves running the length of the part. The lid is currently a stainless steel plate which runs through the groove and in contact with the sidewall. The aluminum side wall is anodized on the outside surfaces but the grooves are left natural. The reason for this is to ensure there is an electrical connection between the side wall and the lid (this is essential for EMC Shielding). We are now considering changing the plate from stainless steel to zinc plated steel to reduce galvanic corrosion. Both these materials will serve our mechanical requirements, we are only concerned about the galvanic corrosion issue.

Is zinc plated steel a better choice than our current stainless steel part?

Sarva Thurairatnam
- Kensington, Victoria, Australia


Fastener Design Manual

(2006)

A. Due to their relative closeness in the nobility of metals chart, zinc plating would be the better choice to mate with aluminum. Stainless steel will actually cause the aluminum panel to corrode in an attempt to protect the stainless steel fastener (because it is more noble). If you wanted to take it a step further, I recommend using a zinc lamella dispersion coating instead of standard zinc plating. Zinc dispersions usually offer superior corrosion protection (around 500-1000 hours ASTM B117 [link by ed. to spec at TechStreet] ), and many times offer an aluminum rich topcoat to further diffuse bi-metallic corrosion.

King Drummond
- Cleveland, Tennessee, USA



(2006)

Q. Using zinc plated coach screws to fix aluminium frames,is there a problem? I am fixing an aluminium framed building to a concrete base.I intend using zinc plated coach screws.Question:- Will there be any chemical reaction or corrosion possible.

John keenan
- Belfast Northern, Ireland


(2006)

A. There can only be galvanic corrosion if there is moisture (or some electrically-conducting aqueous solution) present.

In the absence of an electrolyte, it doesn't really matter what two metals are in contact, they won't react.

Bill Reynolds
Bill Reynolds
   consultant metallurgist
Ballarat, Victoria, Australia

We sadly relate the news that Bill passed away on Jan. 29, 2010.



May 6, 2011

Q. I had a lot of problems finding suitable screws for aluminum screens doors in the area where bottom seals made of some type of aluminum and rubber are been added, the area around the screws goes to a fluffy white corrosion almost bonded to the frame and difficult to unscrew, and far worse close to sea areas. Any suggestion would be welcome. Many Thanks.

Lucio Rizzo
- Sydney NSW Australia



October 2, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am currently working on a project where a contractor has fixed a powdercoated aluminium louvre to an aluminium bracket using 2 different types of fixings, one being a stainless steel bolt connection and the other a galvanised tech screw. What is the likelihood of this corroding?

Simon Kelly
Commercial Builder - Melbourne, Victoria, Australia


December 16, 2012

A. It all depends on which is the larger surface area that will ultimately start the galvanic corrosive action. In all likelihood the Galvanized screw may be the first chap to go, assuming that the Aluminum in contact with it is larger in area, If its powder coated from both sides with no exposure of the Aluminum to the weather of any kind, they may both live happily ever after till death do them part.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind supporting advertiser
Bangalore, Karnataka, India

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Rust observed on the zinc plated screws when in contact with the Anodised Aluminium parts

October 20, 2016

Q. We Manufacture & supply Anodized aluminium components (Sheet metal) to European customers but from past some days we are getting complaint from customer that they are facing corrosion on the zinc blue passivated screws assembled to the anodised aluminium component after some time.
Also note : Plated Screw passes 120 hours of salt spray.
Packaging will be Export quality.

My doubt is is due to contact between Anodized aluminium surface and plated screws is there any reaction occurring and thus rust forming on the plated screws. Please advise.

Naveen Kumar
Product Designer - Bangalore , Karnataka & India


A. Hi Naveen. Perhaps you should ship a couple of extra screws loose or in the identical packaging you ship your assemblies in, and see if that gives a clue. Although zinc plating and anodized aluminum are not quite galvanically compatible, it should not be a huge issue for non-critical parts. 120 hours is not a lot (5 days); what is the exposure condition, and the plating thickness?

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



Zinc bolt corrosion in Aluminum outdoor sign

January 20, 2017

Q. We made a sign for the corporate business center, we choose aluminum as the building material. All internal and external parts are aluminum (6061). The outer parts are clearcoat anodized, the inner pieces are bare. It was assembled with zinc plated steel fasteners, my online research pointed to this being the best option, even over SS fasteners. The sign is bolted to a brick base with a limestone slab top. And it is 100% exposed to the elements. Corrosion is taking place, orange coloration is evident. I need a recovery plan.

Since the Zn plate fasteners did not work, what is the correct choice?

Once this is known, I plan to pull the sign to the assembly area and remove all fasteners and replace them. The rust staining will have to be removed at this time. My hope is that the clearcoat anodize will provide a level of protection where this will be easy to clean.

JOHN STEFANICK
- Mason, Illinois


January 2017

A. Hi John. We appended your inquiry to a thread with rather similar questions, but I don't quite agree with the previous answers. My own experience (just observation) shows that stainless works very well with anodized aluminum; further while some rusting on the surface of some grades of stainless is possible, it is unlikely to ever run down onto the aluminum. One final point that is often missed, and goes unmentioned in your inquiry, is that different thicknesses of zinc on the fasteners and anodizing on the sign are possible.

While hot dip galvanized zinc coated bolts might be fine, in my own experience most commercial 'zinc plated' bolts, having 0.0002" of zinc plating (or even less), are going to rust fairly soon outdoors on their own even if insulated from all other metals.

I'd suggest type 316 stainless fasteners. Type 303/304 would probably be okay if you can't find 316 easily.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


Using zinc-plated screws to fasten aluminum to steel

February 27, 2017

Q. We have an application where we're hanging 1/8", 5052-H32 aluminum panels off of 20 gauge galvanized steel clips. This is indoors, in the northern mid-west of the US (so not near the ocean).

Two questions:

1. I am wondering if I can use zinc planted, 410 stainless, or 18-8 stainless screws for this, as opposed to screws with a coating to protect against "salt spray," since for my application, there seem to be more choices for zinc, 410, or 18-8.

2. Also, we are not planning to put any sort of electrical insulating tape between the aluminum and the steel clip. I assume this is okay, since the application is indoors and not near the ocean. Am I correct?

Thank you!

Carl Wedell
Designer - Minneapolis, Minnesota, USA


February 2017

A. Hi. If the screws are visible I'd go with stainless. If not, zinc plating should do. You can't have galvanic corrosion without conductive moisture, so it sounds like no problem to me. We have dozens of threads and hundreds of postings on this subject if you want a second opinion. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



Galvanic Corrosion And Where It Occurs

March 28, 2017

Q. Hello,

I am working on a research project in which I am creating a block of anodized aluminum with a stand cut inside the middle with thin glass to see through and ports for chilled liquid and vapor to come into the block.

Basically I am looking for information on galvanization between my fasteners (screws and washers to keep the glass eyehole in place and connectors for the ports) and the anodized aluminum block. This system will likely have water as an electrolyte as it will be difficult to prevent complete condensation on the chilled block.

To my understanding, if I used cadmium or zinc plated alloy steel fasteners, these are more anodic and will preferentially corrode. I am wondering once the finish has corroded, will the aluminum then corrode as it is more anodic than steel alloy of the fasteners?

Or if I use passive stainless steel, as this is more cathodic, will the aluminum block corrode?

And where will the corrosion occur? Does it occur where both the washer touches the aluminum block and where the screws are threaded inside of the block? This is my big question as I have not found any info on corrosion inside of a block/plate due to screws.

Thanks for your time and I appreciate any help that I can get.

Jared Hale
Research and Desire to learn more to be more competitive when entering the job market - California, Anaheim, United States


March 2017

A. Hi Jared. Galvanic corrosion is dissolution of metal into a conductive electrolyte, so it's usually concentrated near the interface of the two materials. If a thread sealant is used, it should keep moisture out of the threaded area, preventing corrosion there.

Cadmium is highly compatible with aluminum, but is a biocumulative poison and is no longer used as a plating except in rare and ever diminishing critical cases. The ideal finish would be aluminum plating on the screws, but this is unusual and expensive; you won't find aluminum plated hardware except on aerospace components.

Thus I think your choices are zinc alloy plating or stainless steel screws. My preference is stainless steel and I suspect they'll be fine. Neither anodized aluminum nor stainless are very conductive due to oxides on their surface, so galvanic corrosion isn't often a major thing in non-critical applications

Can automotive antifreeze (with its corrosion inhibitors) be used in the application? Condensation water will not be very conductive if it occurs in a benign environment.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


March 31, 2017

thumbs up signHello Ted,

I appreciate the quick response. And that is good information to know about the thread sealant.

You are right about the anti-freeze. We will be using an ethylene glycol and water solution, flowing through the block to keep it below freezing. With this, there will be condensation onto the block and fasteners from moisture in the air.

This is my main concern as this system will be running for days at a time, covered in the water due to the condensation. We have a system to prevent this over the glass eyepiece but will not be able to apply it to the entire apparatus.

That is why I was concerned about possible corrosion issues based on material and finish selection due to the time exposed to the electrolyte.

Thank you for your time and information.

Jared Hale [returning]
Research - Anaheim, California, USA



Zinc lamellar coating

October 25, 2017

Q. Hi please help me about zinc lamellar ... what is zinc lamellar coating?
How is it made and what equipment is required?
What will be the cost per square feet to coat?
I am new to this so help me. I don't get any info in google.

chandu talluri
- vizag A.P India


October 2017

A. Hi Chandu. This coating is probably better known as "zinc-rich dip spin" coating, so you'll make better Google progress with that term ... in fact you'll find many threads about it by searching this site with that term. Generally the coating is not a "home brew" but something where the coating shop sets up a licensing agreement with one of the half-dozen or so world-wide suppliers of the coating chemistry.

You are unlikely to find pricing on the Web, and will need to offer some particulars about your parts to coating shops to get quotations. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



November 1, 2017

Q. Hi, I'm building an aluminum-frame screen porch. I live in Florida just a few blocks from the ocean.

Would it be ok to connect the aluminum frame with aluminum L-brackets and stainless steel screws? From my understanding the aluminum frame has a larger area than the stainless screws, thus there will be a more limited interaction between the metals.

Or is it better to special order aluminum screws?

Thanks! Mike

Mike Lipsky
- SATELLITE BEACH, Florida, USA


November 2017

A. Hi Mike. Stainless screws are the usual architectural choice; I think they'll be okay. A no-no for an airplane, but okay for a screen porch. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



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