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topic 16063 p3

Stainless Steel vs. Galvanized vs. Zinc Plated Fasteners



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A discussion started in 2002 but continuing through 2019

August 1, 2016

Q. Dealing with the compatibility of metal fasteners against galvanic corrosion such as galvanized steel, zinc plated and stainless steel, which metal fasteners can be used together? Thought I saw a sign in Home Depot that read "Do not use galvanized steel with stainless and zinc" or one or the other. Found out that using zinc plating in marine use is not good. Used one on a stainless swim platform and one on a jet ski port which have rusted bolt heads. Recently bought two galvanized dock cleats and galvanized lag screws so those should be okay. Stainless steel fasteners are not made in bulk. They mostly come in individual packages and limited to certain sizes.

Richard Brooks
- Montgomery, Alabama U.S.A.


August 2016

A. Hi Richard. Galvanizing and zinc plating are essentially the same material on the surface so they are galvanically compatible. But one problem is that zinc plating tends to be rather thin and therefore not suitable for service in an aggressive environment. Hot dip galvanizing tends to be significantly thicker and therefore significantly more suitable. A second problem (which may relate to the Home Depot sign) is that, because galvanized coatings are so heavy, the threads are cut differently and non-galvanized nuts don't fit galvanized screws and vice versa.

But even hot dip galvanized fasteners may not last long in marine service joining stainless parts together. The zinc is sacrificial to the stainless, and the small amount of zinc on a bolt, even if applied as a heavy coating, cannot sacrificially protect a large stainless item for long. If you must use galvanized hardware on stainless steel in a wet and salty environment you really need to isolate the hardware from the joined parts with plastic washers and bushings so there is no metal-to-metal contact.

Regards,

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



September 21, 2016

Q. We use 304 SS fasteners on aluminum hulls. The tapped holes are painted, however the paint cracks when the fasteners are installed.

As a result, two problems occur.

one being the water starts to get underneath the paint in tight area around the fasteners ( this area is deprived of oxygen), which then causes crevice corrosion.

The second issue is due to the paint failing at the threads, causing the stainless steel fastener to come into contact with the aluminum surface which initiates galvanic corrosion.

So now we have aluminum corroding away and paint that is blistering and peeling off.

Can anyone think of a solution to this problem? We are currently considering aluminum fasteners in low load areas, and using an anti corrosive compound to bed the fasteners such as Duralac.

Any thoughts?

Omar Abdulmajid
- Surrey, BC, Canada



October 19, 2016

Q. I am replacing Mild Steel set screws in a cast iron solid fuel stove [ originals so rusted they had to be chiseled out and holes re taped. What material should the new screws be ? coated mild steel - stainless steel - brass / bronze bearing in mind the high temperature and any reactions to the cast iron

David Bailes
- Sligo Ireland



Best finish for fasteners in heavy truck applications, when trucks are acid washed

December 7, 2016

Q. What plating has the best corrosion protection for use in heavy truck applications, where the trucks are periodically acid washed.

Rob Glass
Quality Manager - Columbus, Ohio US


January 17, 2017

A. I've noticed in the on-going discussion of Fasteners and the subsequent prevention of corrosion in all forms one method of coating isn't mentioned that I've noticed.

Zinc Thermal Diffusion.

Although the process has it's roots in Sherardizing historically, there have been developments to diffuse zinc into substrates below the melting point of zinc. Process temp typically around 300 °C.

When combined with passivation, and if necessary a topcoat, performance is well in excess of 1,000 hours of salt spray and also prevents the following:

- Galvanic Corrosion of dissimilar materials
- Hydrogen embrittlement since the process naturally includes a baking element to it.

Performance per dollar ratio against Stainless and other high grade coatings is quite good as well, typically with sufficient economies of scale (i.e., decent volume).

And to be quite honest I am now working for a company that is bringing their technology to North America so I am biased to a degree, but I also have data that supports the above statements.

Ted Tsikhlakis
- Jackson, Missouri, USA


January 2017

thumbs up signHi Ted. Thanks. Yes, thermal diffusion went unmentioned on this thread previously, as did the very popular zinc-rich dip-spin coatings and a number of other technologies. When "acid washing" is involved, as in the most recent posting, we should probably emphasize electroless nickel plating since zinc coatings have no acid resistance. Each technology has disadvantages to go along with their touted advantages, which is why no method has obsoleted the others.

Regards,

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



January 24, 2017

Q. Aloha,

We are attaching fascia to 4x6 rafter tails. Both materials are fir PT with Borates. Directly over the fascia will be copper gutters. The fasteners for the fascia will be countersunk and puttied over so there should be no contact between the copper and the fasteners unless the fastener corrodes. My question is what is the best fastener for this application, (whether screw or nail), to make sure that there is no corrosion of the fasteners. The area is very wet and has considerable salt in the air. While the top of the fascia is covered by the roof, we have had a problem in the past with galvanized nails corroding after a few years and staining the fascia. Thanks.

Sean Nunokawa
- Honolulu, Hawaii, USA



March 12, 2017

Q. I need to strap plastic barrels onto my cedar swimming raft. What would be the best strapping material? I was thinking of using galvanized metal strapping with galvanized screws.

Joe Happe
- Buffalo Minnesota


March 2017

A. Hi Joe. Galvanized should be okay, but not as good as stainless.

Regards,

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


March 15, 2017

A. Plastic strapping is strong enough for most purposes, doesn't rust, and is less expensive than steel strapping and much, much less expensive than stainless steel.

tom_rochester
Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
plating systems & technologies banner ad


July 6, 2017

A. This answer really applies to many of the questions here. In the late 90's I began testing various Stainless bolts while attached to aluminum. The results were staggering. You should never use raw stainless steel bolts with aluminum. If you do need to use stainless steel to connect to aluminum, it should be coated with a coating specifically designed for this type of connection. My company uses millions of 410HT SS screws for an aluminum connection due to the very high strength of this alloy. 410 does however contain carbon and will surface rust over time. This is generally cosmetic and the coating takes care of that as well as the dissimilar metals and galvanic reaction. Using 304 SS is the most common but you need to know that 304SS is 20-35 weaker than the standard carbon steel generally used for bolts and 304 is one of the most corrosive alloys when combined with aluminum.

Tef-Gel

On a separate note, I have never seen any galvanized bolts last 20 years in a salt water environment without constant maintenance. If you are dealing with salt water, stick to Stainless if you do not want to do the maintenance.

My last thought is that if you are having an issue with Stainless and aluminum on bolted connections and you have to remove the nut on a regular basis, try Tef-Gel. I believe it was developed by an engineer working on the space shuttle program specifically to stop galvanic corrosion between dissimilar metals. I have used on my boat and had no issues removing bolts years old. Just my two cents...

Thomas Johnston
- Pensacola Florida



April 13, 2017

Q. Hi, I have a 1970 Shasta travel trailer and am replacing the butyl putty tape around all windows and body seams. The window frames appear to be aluminum and all of the original screws are completely covered in rust. I want to replace all of the screws but not sure if stainless steel is needed or zinc coated will be ok, or a better choice..... I need approximately 550 of them so cost is a consideration.

Terry Higgins
retired - Rolla, Missouri USA


16063-3
July 12, 2017

Causes and Control of Fastener failures =>


Galvanic corrosion- HASCC- Hydrogen Assisted Stress Corrosion Cracking- dissimilar metals-Delayed failures

Gregg Melvin
Fastener manufacturer - Decorah, Iowa USA



October 2, 2017

Q. Hi,

What would be the ideal treatment for the screws used to fasten PCB (without any copper pad below - just the FR4) to meet 240+ hours of salt fog test. PCB's are enclosed in mechanical housing (not sealed - has multiple pockets through which fog can move)

What would be the ideal treatment for the screws used to ground PCB (ENIG Finish on PCB) with same 240+ hrs of salt forg requirement.

Regards
Subash

Subash Rai
- Bangalore, Karnataka, India


December 14, 2017

A. Hello,

Zinc flake coating would suit your function.

Regards

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser 
Bengaluru, India
Saify Ind



January 3, 2018

Q. Hello, I'm trying to figure out what the best fasteners would be for an assembly of aluminum framed windows and doors. I try to get 18-8 stainless steel whenever possible. But sometime the choices are 410 Stainless steel or Zinc plated steel.

My questions is what will last longer in terms of corrosion, 410 or zinc plated?

Some of our products will be used on the coast so I would like our fasteners to hold up as long as possible. Any info would be greatly appreciated.
Thank you.

Jeromy Kirsch
- Palm Desert, California, USA


January 2018

A. Hi Jeromy. Although it is possible to electroplate zinc fasteners for severe or very severe service, the zinc fasteners you are liable to find will surely have the minimum zinc plating thickness, 5 microns or perhaps not even, and would not be suitable for such service even if there were not a galvanic compatibility issue (which there is).

Regards,

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



Automotive fastener plating

January 19, 2019

Q. Hi, I'm a perfectionist who's locked up and needs help! I am restoring a car for eventual daily use in Florida Environment. I have powder coated frame ready to assemble the chassis with parts and was going to have all fasteners (below grade 5 due to H embrittlement ) cad plated with conversion coating for best corrosion resistance. Appearance isn't an issue since not visible and I rather like the gold appearance of passivated cad anyway. Though this sounds great for corrosion resistance of the fastener itself it may not be great due to possible galvanic corrosion when connected to the steel frame (assuming the frame is steel!) or the parts being attached. Some fasteners will go into a threaded hole in the steel frame, others might just go through a hole and the threads are in the actual part being attached.

I realize teams of engineers designed the car and I'm just a dentist so what do I know, I don't want to re-engineering the car but the car probably wasn't designed to last more than a decade and I would like to improve upon it since I will have my lifetime then the kid's plus this is an expensive project taking years. This site is largely where I have learned so much about this and thank you!

Jackson Sullivan
Amateur auto restorer - Oklahoma City, Oklahoma
  ^- Privately contact this inquirer -^


January 2019

A. Hi Doc. Cadmium is not considered environmentally friendly, but if you can get the plating done, cadmium plus gold colored chromate is probably perfect. It is galvanically protective for the bolts and the steel, it's compatible with aluminum if there is any, it's less susceptible to hydrogen embrittlement than zinc or zinc alloy plating, and it's consistent torquing characteristics make it ideal for auto fasteners.

As for its 'environmental unfriendliness', don't feel too guilty. The government's response to the twin scandals of solid cadmium children's jewelry and cadmium paint on children's drinking glasses was just partial & ineffective recalls (see thread 57468 and 58085). Since an ounce of cadmium can promote sustainability by protecting a ton of steel from having to be re-mined, re-smelted, re-cast, re-fabricated, re-heat treated, re-finished, re-sold, and re-landfilled, I say go for it. Good luck!

Regards,

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


January 30, 2019

Q. Thank you so much for the response, after reading through so many articles on this forum throughout the last year and learned so much I had settled on cad with conversion plating. I then looked at a galvanic comparability chart and noticed that when cad fastener and steel or cast iron base were in contact that the cad fastener would corrode; so you are saying that the galvanic interaction is at an acceptable level? I guess I was looking for no corrosion but that may not be practical? Thanks again, Jackson

jackson sullivan [returning]
- oklahoma city, Oklahoma USA


January 2019

A. Yes, the cadmium plating is sacrificial to iron, that's the main reason it's used (to protect the steel from corrosion). Nothing last forever, and eventually all of the cadmium can be consumed, so you can arrive at bare steel ... but if a lot of the iron & steel is painted, the corrosion is slower; and under-hood components tend to be oily and don't stay wet, so that helps, too, because an electrolyte (salty water) is required for corrosion.

Regards,

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


January 30, 2019

Q. Last question ... is there a specific Cad plating thickness required and hex chrome or tri chrome if I can get it? And thank you so much for the advice and also the education I have received on this forum ... AWESOME SITE!

Jackson sullivan [returning]
- oklahoma city, Oklahoma USA


January 2019

A. Hi again. Common thicknesses are 5, 8, or 13 microns of cadmium. I'd try to get the 13 if possible, and wouldn't settle for the 5. Chromates usually aren't specified by thickness but only by color. Tri-chrome is not (to my knowledge) available for cadmium, so it would be gold colored hexavalent.

Thanks for the kind assessment.

Regards,

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



May 7, 2019

Q. My name is James Donnelly, I am an Electrical Inspector for Mining, in Saskatchewan Canada.
We find we have early corrosion and failures of electrical equipment and wiring support structures in dry fertilizer storage buildings. We have found plated screws corrode and electro-galvanized clamps corrode. SS304 seems to be an issue around moisture.
Can you provide / suggest an appropriate metal to use for screws , fasteners, cable trays, cable straps, etc.

James Donnelly
Electrical Inspections - Mining - Melfort, Saskatchewan, Canada


May 2019

A. Hi James. Of the materials you mentioned, 304SS would be the most expensive but the most corrosion resistant. Somewhat more expensive and more corrosion resistant would be 316SS. There may not be a practical plating/coating metal to completely stop the corrosion, but if the screws and other items are large enough for hot dip galvanizing, that would be a lot more corrosion resistant than plated/electrogalvanized finishes because the zinc coating is much thicker.

The multi-layer metal+paint coatings typically used on deck screws and similar construction fasteners probably offer the best corrosion resistance but may not be acceptable for electrical applications because of poor grounding capability (you could check with the manufacturers about that).

Regards,

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


May 7, 2019

thumbs up sign Thank you Ted, appreciated very much!
I will contact the manufacturer for his advise as you suggest.

Take Care out There.

James Donnelly [returning]
Inspections - Melfort , saskatchewan, Canada



May 21, 2019

Q. Wonderful thread! I just time-traveled through them all! I'm custom making exterior raised garden beds for my community. What sets my boxes apart are the three thick galvanized carriage bolts I place in each corner after boring holes into a 4x4 corner post for increased stability of the 2" thick untreated Doug Fir (and Cedar). The add'l cost of this stability is worth the lower profit margin, but to be wise, I'm seeking lower cost bolts without compromising strength. My lumber yard charges me $1.50 for 3/8" x 6" galvanized carriage bolts + nut + washer. On Amazon, I found a source for 150 of each for a total of $65. Great price, but they are green and quote "triple coated to outlast hot-dipped galvanized bolts". After searching, I can't find any information on this miracle process. [deleted by editor] is noted in some places, but no definitive info. Your advice on whether these green triple coated carriage bolts will serve our vegetable boxes well would be greatly appreciated.

Joe Wachs
- Maplewood, New Jersey, USA


May 2019

A. Hi Joe. We deleted the specific brand name of the fastener coating (why?) but there are a good number of suppliers of generally similar coatings, and there is no reason to believe that this one is better or worse.

Carriage bolt sets

If you go to a hardware store you will see deck screws and such with generally similar multi-layer organic or organic/inorganic coatings. My opinion is they are probably superior to galvanizing for this application, where the screws are in wood (and cedar is considered a corrosive wood). Many bolts are used in metal, and in applications where we need all of their strength, or where vibration might chip away organic coatings and expose the substrate to corrosion or make the fit too loose. But this application, in deliberately wet wood, even though it uses nuts, seems like a case where deck screw style coatings would be better than galvanizing.

Regards,

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


May 30, 2019

thumbs up sign  Thank you! Considering I already bought 150 count, your response is greatly appreciated. Best,

Joe Wachs [returning]
- Maplewood, New Jersey, USA

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