GI (galvanized) sheet vs. CRCA for rust prevention and corrosion protection

A. Hi, Prakash.

If the galvanized and powder coated surface is smooth enough for your aesthetic needs in furniture, it sounds ideal. Galvanized metal is far more corrosion resistant than CRCA because of the cathodic protection from the zinc. If it is not decorative enough, you could compromise by doing zinc electroplating rather than galvanizing before the powder coating.

Regards,

A. I agree with Mr. Mooney.

The other option You can phosphate the steel furniture before applying the power coating.

One of the important attribute of zinc phosphate conversion coating is to improve the adhesion of the top coat and to prevent underfilm corrosion, the problem that you are currently facing.

"Phosphating & Metal Pretreatment"
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TSN is clearly right that phosphatization must proceed powder coating! But sorry, I had assumed that Prakash and his competitors were already phosphating the furniture (iron phosphate for indoor furniture, zinc phosphate for outdoors?). If neither he nor any of his competitors are doing the simple and recognized pretreatment steps that are required before powder coating, yes, simply doing things right is the first step.

My personal experience is that zinc phosphate, followed by e-coat priming, followed by powder coating delivers 20-year outdoor life to expanded metal patio furniture in a salt water lagoon environment.

Regards,

A. Hi, Ashish. GI is an acronym for galvanized iron, although it actually means galvanized steel.

CRCA would not be expected to offer any corrosion resistance at all, since it's plain bare steel, whereas galvanizing is applied for the specific purpose of affording corrosion protection.

Regards,

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A. Hi, Vikas.

CRCA offers no corrosion protection except for the phosphate pretreatment plus the paint or powder coating you apply to it. If you apply a robust treatment system consisting of zinc phosphatizing followed by electrocoat priming followed by powder coating, through, you should be fine.

GI offers sacrificial protection, and the zinc coating is helpful towards retarding corrosion even on edges that are bare because of its sacrificial capability. But this is not as good as the edges being coated. In a world of possibilities it is difficult to select one coating process based on a very limited list of requirements, but powder coating on top of post-galvanized steel would be the most robust, and powder coating on top of pregalvanized steel would probably be second best. Good luck.

Regards,

A. Hi, Raghavendra.

Pre-galvanized or post galvanized sheet metal should solve your problem economically. But if CRCA has a good coating (zinc phosphate, electrocoat priming, powder coating), that should be good as well. But only metallic coatings like galvanizing and electroplating would meet your requirement for conductivity. Good luck.

Are you sure your plant uses no acids? Because zinc coatings have no resistance to acid vapors.

Regards,

A. Hi cousin Harish. Galvanizing is the application of a coating of zinc to the sheet in order to provide the steel with cathodic protection. Zinc is more active than steel, and will corrode as necessary to sacrificially protect the steel. But, in good applications, the galvanizing forms a glassy skin of corrosion products in order to not rapidly corrode away itself. Contrariwise, CRCA has no corrosion resistance. But there are circumstances, like exposure to acidic environments, where GI is no better, and probably worse, than CRCA.

If something is vvv important, you really should do whatever is necessary to get the funding to retain a finishing consultant to resolve the issue for you in a professional manner. Asking strangers on the internet, who possess little information about your circumstance and situation and capabilities, for abstract opinions is okay for general education but not appropriate at all if a specific issue is vvv important :-)

Good luck.

Regards,

A. Hi Harshad.

How hot -- just hot ambient temperature? What other factors must be considered? Cost is no object?

My understanding is that the majority of ISO cargo containers are made of Cor-Ten steel rather than CRCA or GI -- but that is based on a couple of minutes of googling, rather than actual experience. Certainly GI sheet is more corrosion resistant than CRCA or Cor-Ten.

Regards,

A. Hi Kinley.

I am not sure of the answer, but please note that "galvanized iron" is not a specification, it is merely slang. And when we attempt to read a specification into slang we can be easily misled. It would not surprise me at all if the "galvanized iron" you are referring to is actually galvanized steel also, so that your question becomes "is galvanized steel better than galvanized steel?" :-)

Some people even call galvanized steel sheet metal "tin"; trying to answer what material is better without real specifications becomes impossible. Good luck.

Regards,

A. Hi Tirth. The buyer who gave you the spec is the person you should inquire of for clarifications, especially because it's entirely possible that it was a typo or other error. But if you are just looking for speculation, I suppose that since the material is used indoors they might consider it to not be a corrosive environment that requires galvanizing, or they might feel that the galvanizing is too easily damaged by acids, or they may want to periodically repaint, and feel that CRCA makes for fewer problems. Good luck.

Regards,

A. Hi Azeem. In my opinion, CRCA is better for a laboratory/chemical environment. That is because the zinc galvanizing offers no resistance at all to the acids and acid vapors.

Although zinc coatings are a very useful corrosion fighting tool in normal environments where their corrosion products can build a good glassy skin to fight penetration and ongoing corrosion, zinc dissolves far more readily in acid than steel does, so the idea doesn't work at all in an acidic environment. My experience from a lifetime spent in plating shops is that galvanized materials should never be used in an acidic environment.

Regards,

A. Hi Parameshar. Surely it's to avoid scratches, handling damage, fingerprints, etc. But not all galvanized sheets come with plastic protection. So this question is probably best asked of the company which includes the plastic film on their product. After all, they have to charge extra for it, so they must have reasons to believe it's a good idea :-)

Regards,

A. Hi Ranito. I doubt that any 3rd-party, unfamiliar with your contract documents, will entangle themselves in your dispute regarding what must be done vs. what you can refuse to do.

Although you are correct that the galvanizing on the sheet will protect the cut edges to some extent exactly as you describe, the fact is that there is a corrosion-resisting advantage to putting zinc-rich paint, or any paint on the cut edges.

If you can think of some disadvantage to applying the zinc-rich paint, besides just the additional cost on your end, I think you have a better chance of amicably winning this one. One possible assertion is that the brand new product will look like a chewing gum repair; you can probably poorly paint a sample to emphasize your point :-)

Regards,

A. Sir:

Our publication: "Characteristics of Hot Galvanized, Metalized, and Bare Edged Galvanized Sheet Products," may be of interest to your question. It is in Metal Finishing, May 1985, pages 63-68.

Regards,

A. Hi Sahid. I would not use galvanized steel in a chemistry lab. Although zinc coatings are very useful in ordinary atmospheres, zinc dissolves much more readily in acid fumes than steel does.

When I used to design plating shops I found that it was very difficult to keep paint on galvanized steel in acidic atmospheres and felt that we were better off with painted plain steel.

Regards,

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Duplex Systems: Painting Over Hot-Dip Galvanized Steel
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A. Hi Krishna. It probably depends on the details of the furniture. What I personally found is that phosphating, followed by electrocoating, followed by powder coating, lasted 20 years on expanded metal furniture which was outdoors for all 4-seasons on a saltwater lagoon. Galvanizing was not even necessary.

But if you have crimped or spot-welded areas which paint & powder can't get to, either galvanizing or electrocoating can ensure full coverage of the steel ... and that seems key to long life. Good luck.

Regards,

A. Hi Krishna. If you are unfamiliar with this stuff you really ought to consider retaining a finishing consultant. You will probably be too busy getting started in business to read the several books which would be useful on such subjects :-)

Galvanized steel can certainly be successfully powder coated. But that is not to say that your furniture must be galvanized, or that galvanizing is the best pretreatment for you; I previously described furniture which has lasted me 20 years outdoors and wasn't galvanized. Please carefully describe the furniture that you will be making.

Regards,

A. Hi Ajay. If a piece of steel is completely covered with zinc galvanizing, it cannot get red rust because no steel is exposed. Yes, the zinc coating will, over time, corrode to white rust.

But perhaps the thing you are not yet understanding is that under good conditions the corrosion products of zinc form a tight, adherent, glassy, non-porous layer on top of the zinc which can slow corrosion to a crawl. Whereas a special problem with iron and steel is that their corrosion product (red rust) is loose, fluffy, hygroscopic, and porous. The red rust does not form a protective coating on the steel to help shield it from the environment; quite the opposite, it tends to hold moisture in contact with the steel.

Regards,

A. Hi Bhargav. It would seem to me that these racks must be painted, and that properly pretreated and painted racks, used to display closed packages of products in an air-conditioned environment would not need the extra cost and pretreatment complications of GI.

But pharmacy racks are not some new and unheard of product where we must speculate what will work satisfactorily. Surely there are racks that have been in use for decades and whose performance can be evaluated?

Regards,

A. Hi Kunal. Buffing removes a small amount of the galvanizing, but hopefully it is not significant. But my question would be why do you want to buff it? It will not stay shiny anyway, and buffing away the chromate quenching and thinning the galvanizing may actually lead to faster dulling.

Regards,

A. Dear
We have to understand that Galvanizing is not at all a cosmetic coating. It is a protective coating.

Buffing thins down the Zinc plating thickness. It is not at all recommended.

A. Hi Geetika. I don't know what you mean by "zinc polishing" ... perhaps it was a spell-check correction and you meant zinc plating? I have already expressed my personal opinion that GI or zinc plating (or any other zinc-rich finish) is inappropriate for any acidic environment. I think that a properly painted or powder coated item will be more satisfactory than a zinc coated one -- but what is it that we are talking about (hardware, tabletops, benches, steel piping, catwalk grating)?

Regards,

A. Hi Andrew. From a theoretical viewpoint, it certainly seems that CRCA ought to be significantly cheaper because galvanizing involves a costly extra operation. But it's also possible that the steel warehouse doesn't stock CRCA in the size, shape, and volume you need, meaning they'd have to order, handle, and cut it, thus driving up the price. I've seen that issue arise when trying to obtain non-galvanized roofing panels for plating shop construction. So discussing pricing in the abstract isn't very satisfying; but a steel warehouse would be happy to quote actual prices :-)

Regards,

A. Hi Ibrahim. I don't know what you are implying when you say "We have now some pollution control issues". Please explain what you are referring to, recognizing that galvanized sheets, like CRCA sheets, must be phosphatized before powder coating.

Regards,

A. Hi again, Ibrahim.
1. Your assumption that GI sheet does not require phosphating is incorrect. It does: the powder coating will peel right off without it. Sorry that you did not understand my previous response :-(
If you continue to believe that GI does not require phosphate, please tell me what is the basis for that belief. Thanks!

2. It's okay to ask as a matter of general curiosity whether powdercoated GI will look different than powdercoated CRCA (I think it will unless you are using textured powder or a thermoplastic), but you can't use such opinions as engineering guidelines. You must buy a few sample GI sheets, powder coat them, and judge for yourself or have the customer judge for himself. Good luck.

opinion!  The worst pollution going on in the sheet metal business these days involves skipping of the required phosphatization -- thus polluting the planet by requiring re-mining, re-smelting, re-fabricating, re-painting, re-packaging, re-retailing, and re-landfilling steel 5 times over ... instead of simply phosphating it once and done so it will last 5 times as long :-(

Regards,

A. Hi Gabriel. As this thread reveals, GI means the steel is galvanized with a corrosion fighting zinc coating, so it will be more corrosion resistant than plain steel. It's probably not necessary for changing room lockers which are indoors, but could be beneficial for letter boxes because they are out in the elements.

You may be able to find general pricing for these alternates on Alibaba or a similar site.

Unfortunately, it's not quite as simple as indoors vs. outdoors, however, since there are so many other corrosion resistant treatments you could consider as alternatives, like electrocoating followed by powder coating -- which would probably be less expensive than galvanizing followed by powder coating and might well be perfectly acceptable if the letter boxes don't get banged around. Selecting any finish for mass production usually involves retaining a finishing consultant if you don't have finishing expertise in house, followed by a testing program. Good luck.

Regards,

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A. Hi Heena. It's difficult for an end user to properly prepare a substrate. If it's galvanized, the paint might not stick well. But if it's plain mild steel, without a good phosphate pretreatment, the corrosion resistance won't be very good.

I'd go with the galvanized, and try a primer made for galvanizing. Good luck.

Regards,

A. Hi Shelesh. In general, yes, CRCA sheet metal is commonly used for such purposes. Unfortunately, "sheet steel" says nothing useful, just that it is some kind of steel and is in sheet form. But if you tell us the application details, and the specs for two different steels, someone may be able to suggest which of the two is better for that purpose.

Pretreatment of steel for powder coating is the subject of numerous books ... but you could probably start with topic 1136 for your own education, and if you give us full details of the application somebody will probably offer a suggestion regarding possible good pretreatments. Best of luck.

Regards,

A. Hi cousin Manikandaprabu. Actually, no two materials nor any two finishes have "equivalent properties". What is needed is for you to carefully describe your situation so people can tell you whether a less expensive material can meet your needs. Painted sheet might feel your needs, or powder coated sheet, or zinc electroplated sheet, or electrocoated then powder coated; stainless sheet might even be cheaper if very thin.

Regards,

A. Hi Fisan. I've seen mile after mile of galvanized cyclone fence, and mile after mile of painted/powder coated cyclone fence. I've seen mile after mile of galvanized highway guardrail, and mile after mile of painted/powdercoated highway guardrail. I've seen acres of galvanized metal roofing, and acres of painted/powder coated metal roofing and siding. You probably have as well. The fact is that in many applications it's a wash or it's hard to say.

If you can tell us exactly what product you have in mind, and offer a good description of its very specific environment rather than merely general outdoor use, someone may be able to suggest a reason for one being better than the other. What may be much more important than which finish you choose, is insisting that it be done with highest quality. Good luck!

Regards,

A. Hot dip galvanizing gives better corrosion resistance than powder applied coatings, but is aesthetically boring!
But remember that powder coating is a method of application. It does not define the polymer used, the thickness, corrosion protection or anything like that. "To powdercoat" is similar to specifying "to brush", or "to spray".
Many polymers can be applied by that method we call powder coating, but they have quite different performance.
And of course remember that most commonly applied paint or powder coatings are porous, so will not give the longevity of galvanizing. And that performance is influenced by the form of pretreatment used. But how long do you want it to last? If you want 30-70 years maintenance free, the galv is probably the answer. But if you only need a few years, and you want colour, then Powder might be better.