Stainless Steel vs. Galvanized vs. Zinc Plated Fasteners

A. Hi SD. It's difficult to predict the life of things even in well known environments. Even tougher when we don't really fully appreciate the nuances of the specific installation. Zinc plated hinges could last 10 years if you're lucky ...

... but users may or may not hang damp salty towels on the doors, dripping onto the hinges; janitors may or may not wipe the lockers with bleach. Although salt water pools don't use chlorine to my limited knowledge, vapors may or may not be an issue. Managers may or may not store pool chemicals in some of the lockers.

Bare zinc plating is probably inappropriate for moist, salty environments, and will probably show rusting soon in such cases. Instead of zinc plating, I think ideally the hinges should be phosphated, e-coat primed, then painted.
Luck & Regards,


Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey

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A. I have seen stainless fasteners used on architecturally anodized aluminum in outdoor exposure without any substantial problems, and suspect that it will be acceptable. I am a bit confused by your somewhat offhand reference to stainless steel brackets because these would (theoretically of course) cause the aluminum to galvanically corrode regardless of the fastener material, and being bigger in surface area than the fasteners should probably be a bigger concern.

You probably know that, were it not for its toxicity, the ideal fastener material is cadmium plated steel. It is so close to aluminum on the galvanic scale that there is no galvanic corrosion, it is corrosion resistant itself, if thick enough, and it has great lubricity. Nonetheless, cadmium is a cumulative poison, should be reserved for only the most critical applications, and even there is being rapidly phased out.

The next best material is aluminum plating or coating, again because the galvanic potential is zero. You can investigate Ivadizing for vapor deposition of aluminum, or AlumiPlate for electroplating of aluminum. But I fear that you may find the cost unacceptable for anything but airplanes.

All of the above not withstanding, you didn't mention how the aluminum is corrosion proofed, so the question remains a bit open. But zinc plating is not a good idea in my opinion, and I think it will be short lived. Stick with the stainless :-)

A. Moving from SS to zinc plating is a big transition. It would be fair to use zinc-alloy plated fasteners with zinc-cobalt or zinc-iron, which will have an equal if not better service life of SS.

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A. Stainless steel fasteners are made of solid stainless steel, not coated with it. So if the type of stainless steel chosen is corrosion resistant to the environment it is placed in, it can last essentially forever. The better grades of stainless, like 316SS, can hold up indefinitely in most environments. Even 303 stainless is an excellent choice much of the time.

Zinc plated and galvanized fasteners are made of steel with a sacrificial zinc coating. That coating is eventually consumed. Galvanized coatings are almost always several times thicker than electroplated coatings, so galvanized fasteners greatly outlast zinc plated fasteners.

I would like to agree with my extremely knowledgable friend Khozem, but can only do so with the qualification that his answer applies to Greg's question (which he was addressing) -- but not necessarily the general case.

A. Hi, Nick. There are many choices for fastener coatings including galvanizing, organic coating, mechanical plating, electroless nickel plating, zinc alloy platings, aluminum Ivadizing, and so on.

But I'm afraid that I don't understand your question, and don't understand the application in a "long tail boat" -- is this a small boat with a small outboard motor mounted on a long stick-like lever on the back of the boat? You need to determine whether there were or weren't galvanic compatibility issues (and you haven't told us what other metals are involved), and you have to determine whether the plating was defective in this particular case, or whether zinc plating is a bad specification. Zinc plating is rarely sufficient for exposed outdoor use, especially in a marine environment. For underwater use forget it.

No other corrosion resistant coating will be as inexpensive as zinc plating though.

Regards,

A. No doubt that Zinc plated fasteners are a cost effective solution. But during the acid dipping process prior to plating Hydrogen will penetrate into the fastener surface and shall remain entrapped due to Zinc plating. This will make the fastener brittle and is called HYDROGEN EMBRITTLEMENT. This will create micro cracks on the surface of the fastener which will propagate over a period of time, which will lead to failure of the fastener after a long time (say 6 months to one year). Hence you need to ensure that the plated fastener has undergone hydrogen de-embrittle treatment prior to use.

A. Hi. While Sandeep is right that the fasteners -- unless they are cheap unhardened low strength steel -- are subject to hydrogen embrittlement and require baking, and we appreciate that warning ... his post seems to imply that zinc plated fasteners would otherwise be satisfactory. I think they won't be. I feel Nick's poor experience is probably not an anomaly, but what is to be expected from zinc plated fasteners in many situations of outdoor exposure.

Regards,

A. Hi Mike. Hot dip galvanized coatings are typically several times heavier than zinc electroplated coatings. Hot dip galvanized brackets and fasteners with a heavy zinc coating will probably hold up; zinc electroplated brackets and fasteners will likely not.

If someone wants to offer a substitute to your spec, though, you would probably be best off requiring a specification number for your consideration rather than letting them be loosey-goosey about it with slang. Good luck.

Regards,

A. Hi again Mike. "Zinc chromate" is an ambiguous phrase. Some say it refers to the zinc chromate primer ⇦this on Amazon (affil link)] paint that was once popular, whereas others will say it means zinc electroplating followed by a chromate conversion coating (zinc electroplating is virtually always chromate conversion coated).

The steel and cast iron innards of automobile engines don't rust -- because oil is not corrosive. Zinc electroplating with a chromate conversion coating is probably suitable for parts that are not exposed to the moisture and salt air, but they won't last if they are exposed.

Regards,

A. Hi Vivek. Those materials are corrosive, and most plated fasteners will not be resistant. I would consider zinc plating and chromating, then painting the exposed surfaces after assembly with an acid and alkali resistant paint or perhaps epoxy.

Regards,

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A. Hi again.
1. Billions of fasteners are zinc or zinc alloy electroplated followed by chromating. Although hydrogen embrittlement is an issue to be dealt with, in most cases it's simply a matter of immediately and properly baking the parts for embrittlement relief.
2. If loosening the bolts for disassembly or adjustment is a frequent thing, painting the exposed parts becomes less practical.
3. It's possible that zinc plating or galvanizing will be reasonably resistant to some of the materials you are handling. But not to dishwasher detergent (which is extremely alkaline), nor highly acidic fertilizers like ammonium sulphate ... and no one can offer a general solution to "etc." :-)

Are stainless fasteners prohibitive cost-wise or strength-wise? If so, I'd just use zinc plated and chromated fasteners and replace them when necessary; it seems that you will be removing them frequently anyway. Good luck.

Regards,

A. Hi again cousin. I do not agree that zinc-nickel plating is brighter than plain acid zinc plating. But it is more corrosion resistant (and more expensive).

The idea is that if you add the right amount of nickel (something like 6%), the surface becomes slower to corrode (probably partially due to greater nobility and partially due to "tighter" corrosion products) -- but is still active enough to cathodically protect steel. Yes, hydrogen embrittlement is usually manageable in hard and high strength steels, but not in the very hardest and highest strength steels.

But zinc-nickel being longer lasting in plain water and slightly salty water, for the reason explained, does not mean that it is necessarily resistant to the chemicals you may be packaging. Good luck.

Regards,

A. Hi Paresh. It's not galvanic corrosion because stainless steel is cathodic to zinc plating, not anodic to it. I suggest that you double check the specifications for your fasteners as properly passivated fasteners of a proper type of stainless steel should not be corroding. Do you know what type of stainless steel you are using?

Regards,

A. Have you thought about using a heat cured epoxy based dry film lubricant such as MIL-PRF-46010 [from DLA] ? Using a spin dip application can get you a thin coating on all surfaces of the bolts and nuts that will stay lubricated but doesn't require any preventative maintenance. (Tip: Don't use one that contains carbon black as the graphite will accelerate the corrosion greatly)

While zinc could work as it will provide lubricity, there may be issues with zinc corrosion product that could lock the nuts onto the bolts due to the voluminous nature of the corrosion product.

A. This is a common problem if the bolts and the mount that they are screwed into are the same alloy of stainless steel.

It is less of a problem if two different alloys are used, for example, use 316SS for the bolts and 304 for the mating thread.

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A. Hi Brian. Best fasteners would be hot dip galvanized if the z-bars are actually hot dip galvanized (I doubt it since the z-bars are only 20 ga.); or zinc plated fasteners if the z-bars are actually just zinc plated (electrogalvanized).

Regards,

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A. Hi Kris. Before attempting to remove bolts, you should apply WD-40 ⇦this on eBay or Amazon [affil links] to the area and tap it to try to vibrate it into the joint, then let it sit for an hour or so. It makes a big difference. When you re-assemble, use some teflon thread sealant. If it's a poor design for a corrosive situation, I think you just want to minimize the problems it causes you, rather than trying a re-design for less corrosion and galvanic issues :-)

Regards,

A. Good advise from Ted. A dab of anti-seize when reinstalling is a good idea. Plain old Vaseline will do the job.

A. Hi Rachel. Yes, zinc plating is galvanically similar to galvanizing but thinner. I think the zinc plating will last at least several years in your non-seaside environment (but don't let pressure treated wood be in contact with it).

Regards,

A. Hi Hitesh. Those terms are not specifications and it's hard to put a fine point on slang. "Plating" usually is an abbreviation for electroplating, which is the electrodeposition of a metal coating onto a substrate. But coating is a term with almost no delimitations -- fruit can have a coating of chocolate, a road can have a coating of ice, and a fishing net can have a coating of algae or slime.

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Coatings of metal tend to have better adhesion to metal substrates than paints or plastics do. If a metal plating is anodic to the substrate it can afford it sacrificial protection; if a metal plating is cathodic to the substrate it can accelerate the corrosion if there are scratches or porosity. If there is no abrasion or scratching action, an organic coating like paint will often outlast a metallic coating; if there is abrasion or possibility of scratching, a cathodic metal coating will usually offer more corrosion protection than painting.

High quality organic coatings are probably usually better than plating for wood screws, but often aren't applicable for traditional nut & bolt construction because of clearance issues, vibration, a need for repeated unfastening & refastening or other issues.

Regards,

A. Hi Steve. I think I'd use the 316SS for the aluminum situation, although you could perhaps order the set screws to be Ivadized or electroplated with aluminum. For the galvanized situation, I'd probably use galvanized steel setscrews. I don't think it's possible to galvanize stainless steel, but it can be electroplated with zinc if you wish. Good luck.

Regards,

? Will the applications allow the use of nylon set screws?
Regards,
Bill

A. Hi Mate,
I do not have to hand the mechanical properties of nylon fasteners.
Your first point of reference could possibly be a Blackwoods catalogue to identify Aussie manufacturers?
Regards,
Bill

A. Hi Jim. I doubt that it's critical, and suspect that galvanized bolts would be okay, but stainless just sounds like a nicer idea, especially considering that the system is going to have some stainless fasteners anyway.

Regards,

A. Mr. Jim King, according to Mr. Mooney hot dip galvanising is okay, but further a 5% H3PO4 dilute coating shall convert zinc into zinc phosphate coating, which has a long shelf life in mud & rough environment.

A. Hi Chris. Insulation is always a good idea because if there is no metallic path between the two metals to convey the electrons from one to the other, there can be no true galvanic corrosion. The more common question is whether it is necessary when it isn't easy and practical :-)

Stainless steel fasteners are a commonplace on aluminum and experience seems to show that it's not much of a problem for non-critical applications like shower doors. Galvanic accelerated corrosion is not the only corrosion through. Even anodized aluminum sliding shower doors for indoor use, and all-aluminum patio chairs eventually start corroding.

Regards,

A. I found stainless bolts, nuts, and cap nuts off the internet. I could not find any stainless turn buttons. I found them in nylon and tried them. Unfortunately, they all broke. I ended up settling for zinc plated turn buttons on eBay from the closing of a hardware store in Florida. They work fine, just not exactly what I wanted.

A. Hi Elton. Theoretically, aluminum screws or aluminum plated screws would be the first choice because they'd be galvanically compatible, but aluminum screws might not be strong enough and aluminum plated are not very available (pretty much restricted to aerospace uses). Second choice traditionally was cadmium plating, but this is toxic and restricted these days to critical applications (if any).

So you are right that your practical choices are hot dip galvanized or stainless steel. Galvanized would be less expensive and probably approximately as good, so that's what I'd probably use for large fasteners as suggested. But if the hardware is smaller than about 1/4", it's probably zinc electroplated rather than hot-dipped even if it says galvanized because the thickness of the zinc plating on a small screw is limited -- so I'd probably use stainless steel.

Regards,

A. Hi Elton. I am no roofing expert and there seem to be variations in these standing seam roofs -- but isn't it the case that the aluminum roofing snaps directly onto galvanized clips, so there is no preventing the dissimilar metal-to-metal contact anyway? Insulating is not always done in the case of dissimilar materials, but it is always recommended. If you have true insulation (no metal-to-metal contact), you can't get galvanic corrosion -- but a washer alone (no bushing/tubing) will not prevent metal-to-metal contact unless the hole is oversized and the screw is centered with no movement.

I'm having a hard time envisioning how you prevent metal to metal contact, and I thought the idea of pancake screws was to prevent an impression in the panel above it, which a washer under the screw head would seem to aggravate. I can talk about galvanic corrosion, but I can't talk knowledgeably about that roof :-)

Regards,

A. Hi Andrew. If you could get aluminum or aluminized clips, and aluminum plated or aluminized hardware, that would be the ideal solution. Short of that, or cadmium plating which is no longer available due to toxicity, you have galvanic incompatibility whether you choose galvanizing or stainless, and it becomes a matter of avoiding the one which is worse :-(

The thing is, there is far more aluminum area in the roof than galvanizing or stainless steel, so it's generally considered best to make the larger area component the anode, because that way the corrosive current can be diluted over a large area than concentrated on a small area. That's why stainless (cathodic to aluminum) is probably better for small fasteners and clips -- just book knowledge, and would be happy to hear from roofers with contrary experience.

Regards,

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,

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.

Hi 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,

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

Regards,

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.

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.

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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...

A. Causes and Control of Fastener failures .

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

A. Hello,

Zinc flake coating would suit your function.

Regards

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,

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,

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,

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,

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,

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A. Hi Joe. We deleted the specific brand name of the fastener coating (huh? 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.

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). When bolts are used in metal, 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, pure metallic coatings can be better than organic coatings. 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,