Galvanized vs. electroplated bolts, nuts and washers

A. Hi, Velayudham.

Hot-dip galvanized coatings are usually far heavier than electroplated zinc and will exhibit greater corrosion resistance. It is never permissible to make a substitution without approval because one never knows the intent of the designer. For example, zinc electroplating is susceptible to zinc whiskers whereas galvanized coatings are not believed to be, or at least far less so. If these items are intended for a computer room or hospital life-support equipment, for example, you could have huge liability if you substitute without pre-approval. Good luck.

Regards,

A. I agree with Ted.

Hot Dip coatings are usually thicker, better bonded and will probably perform better than electroplated zinc, which are commonly more used for cosmetic purposes only. We find these fasteners are only well protected in stored conditions. For structural fabrications, if atmospheric corrosion is your concern, I would get the client to agree on a design change.

But does it ever actually rain in Bahrain?

A. Mr Ravishanker

Hot dip galvanising puts a thick zinc layer all over the part and does protect well. There are Electro Galvanising (electroplated) Zinc iron alloys that perform very well and compete with Hot dip galvanised coatings for Corrosion resistance. These bear a look .

Regards,

A. Zinc plating is fine for some applications, Michael, mostly indoors -- but it's corrosion resistance isn't enough for rough outdoor service. The ACQ manufacturers have highly specific recommendations about fasteners (electroplated fasteners are not acceptable but they may claim that hot-dip galvanized fasteners are).

opinion! Yes, copper in the wood greatly accelerates corrosion due to the galvanic couple. Zinc plated screws lasted a few years on my old pressure treated deck, but rusted completely through in less than a single season on the replacement ACQ boards. That stuff is absolute murder, and I personally wouldn't trust even hot-dip galvanized screws for commercial use; I'd use only stainless steel.

A. Investigate replacing it, N. "Electro galvanized" is a euphemism for zinc electroplated. This is usually inadequate for outdoor service. But the "Cold Galvanizing Compound / Zinc Rich Paint" that you mention (which is actually zinc-rich paint) is pretty good at deterring corrosion. It is not nearly as attractive as electroplating though.

A. Hi, David. The thickness of electroplated zinc on hardware is typically controlled at anywhere from from .00015" on up to a practical limit of about .0007" according to specific needs. The upper limit is essentially based on practicality in that it takes over an hour of zinc plating time to reach .0007" in barrel plating and it becomes uneconomic, and you're approaching beating the fastener to death in the plating tank :-)  The thickness of the steel has no effect on the thickness of electroplated coatings; it's a function of the electroplating current and time.

Hot dip galvanizing is never that thin. Typical thicknesses for fasteners are .0015" to almost .004". In general you can't readily control the thickness; rather it depends on the surface condition of the steel. Thicker steel takes longer to heat up and usually takes a heavier coating.

In all, it might be theoretically possible for a very thick electrodeposited zinc coating to approach the thickness of a very thin hot dipped coating, but it is rarely heard of. Rather, a "rule of thumb" might be that galvanizing is about 5x as thick.

A. Hi, Syed.

The phrase "electrogalvanized", which means zinc electroplated, has become pretty common usage. To my mind GI means "galvanized iron" and "galvanized" means hot dipped. Unfortunately, it's hard to put a fine point on words like this because, although I can claim that "electroplated is not galvanized", a supplier of the electroplated finish can always say "I've always called it galvanized; try and stop me!"

That's why we must require adherence to clear and unambiguous specifications, rather than relying on slang and acronyms like "GI". Good luck.

Regards,

A. Hi, Serkan. I'm sorry but I can't figure out what parts of this thread need clarification. We've said several times that galvanized coatings are applied by hot dipping and are much thicker and more corrosion resistant than electroplated zinc. We've said that they are less susceptible to metal whisker growth. Is that important for your application? They are not as attractive and they are significantly more expensive that electroplating. Is that important to you? Thanks.

Regards,

A. Hi, Savas.

Mixing hardware like that is difficult and may not be useful. It's difficult because the dimensions on galvanized fasteners are different and they tend to not join up well with black or electroplated fasteners.

The reason that it may not be useful has to do with the protection mechanism. Zinc plating or galvanizing does two things symbiotically. First, it protects the steel cathodically, with the zinc corroding/dissolving to keep the steel from corroding/dissolving. Secondly it protects the steel by coating it: while the corrosion products of steel are a fluffy non-adherent rust that does not deter continued rusting, the corrosion products of zinc can be highly adherent and build a good impervious skin against continuing corrosion. But if you have black hardware in contact with the zinc plated hardware, the zinc cannot protect the black steel by coating it, only by quickly dissolving away. Regards,

A. Hi, Adeniyi.

Cermet means ceramic-metal, like titanium nitride, but not necessarily titanium nitride. Most cermets are probably applied in PVD (physical vapor deposition) vacuum chambers. Fluoropolymer means an organic coating similar to Teflon. If only the threads are topcoated, I'd have to guess that the topcoating is a Teflon-like lubricant rather than a corrosion protection barrier.

It is difficult to answer your question properly in the abstract, though. What is your situation?

Regards,

Q. Thank you for your assistance

My situation is that I want to supply fasteners to an oil company in Nigeria; part of their requirement is have the fastener to be cermet base coat and fluoropolymer top-coating; at the same time some requirements is to have other sets of fasteners with bichromate treatment.

A. Hi, again. Steel does not accept bichromate treatment, so the fasteners you are speaking of must be zinc plated or cadmium plated, or something similar. It seems to me that you are trying to quote or supply fasteners when you have only vague instructions as to what is needed. There should be tight and definite specifications for what they want. Good luck.

Regards,

"Nothing lasts forever but the earth and sky" -- Kansas, 'Dust in the Wind'

A. Hi, Chris. In normal and proper application, Galvanized surfaces may take on a rust stained look relatively early on because of the iron content in the galvanized coating. This is not indicative of true rusting, which hopefully does not set in for a few decades. But galvanized coatings can also get "wet storage stain", which is white and crusty, and not considered proper. They can also get black spots, which some say is just an early manifestation of wet storage stain.

Please try your best to put your question into the full context of your own situation; otherwise the answers you get can be more misleading than informative. Thanks.

Regards,

A. Hi Amol. If you can provide additional facts like what size bolts & nuts you have in mind, what the application is, and why you want to consider galvanizing, you will get better answers. We can answer your questions, but without understanding the situation, we may lead you in the wrong direction :-)

No, you cannot galvanize zinc plated hardware; but you can easily strip the zinc plating in muriatic acid first. My understanding is that your galvanizer will spin off the excess galvanizing to prevent bridging of the threads. Even after spinning, however, the galvanizing will be quite thick and will not work on very small hardware (under 8 mm or so). Plus, it is usual for galvanized nuts to be tapped oversize to allow for the zinc thickness, whereas your zinc plated nuts will not have that oversize tapping, so you will need to have the threads of the nuts free of galvanizing in order to fit, and even then they may be too tight (it is usual to cut the threads of nuts after galvanizing, leaving no zinc). Good luck.

Regards,

A. Hi. My understanding is that M12 bolts are large enough to accept galvanizing and spinning.

However, the nuts will not fit, and they must be chased. For that reason it is probably better if possible to start with nut-shaped blanks and just tap them after galvanizing. Good luck.

Regards,