Pre-Galvanized Sheet vs. Post-Galvanized Components

A. Hi Gopinath. Well, yes, pre-coated sheet rusts on the edges because there is no zinc coverage there. The galvanizing on the rest of the sheet offers some degree of cathodic protection from the rusting, but it's far from perfect. There would no post-coated products if pre-coating, which is cheaper and easier, performed as well as post-coating; it doesn't.

Full name please! This is both a place of camaraderie and a forum where we can best understand your situation by reviewing your previous postings, but we have many posters by the name of Gopinath, and don't know which of them, if any, is you. Thanks!

A. Hi Jason. Steel coil can be pre-coated via hot dip galvanizing or via zinc electroplating (which is usually called 'electrogalvanizing' when done in continuous style on coiled strip); or the components can be post-coated via hot dip galvanizing or via zinc electroplating. (There are other ways of applying a zinc coating as well, of course).

When doing pre-coating on continuous strip via hot dipping, it becomes practical to limit/control the zinc thickness via 'wiping' away any excess thickness from the sheet, and it is always very easy to limit the thickness of zinc electroplating/electrogalvanizing by reducing either the plating time or applied current.

When doing post-coating, again it is easy to control/limit the zinc electroplating thickness by controlling plating time or current. But with post-coating via hot dip galvanizing, whatever zinc doesn't drip off doesn't drip off.

All four approaches are viable, with post-fabrication via hot dipping usually being much thicker (and more corrosion resistant) than the other processes.

So, yes, the basic differences are whether the sheared edges have a coating, but also the thickness of the coating. There are some metallurgical differences between hot-dipped zinc and electroplated zinc but the most generally accepted idea is that the corrosion resistance of the coating is basically proportional to its thickness, summarized as "zinc is zinc" :-)

A. Sir:

My publication, "Characteristics of Hot Galvanized, Metalized and Bare Edged Galvanized Sheet Products," METAL FINISHING,pages 63-68, May 1985, may be of interest to you. You can obtain a copy through interlibrary loan from any large library for the copying cost.

There was a major white rust problem when a cooling coil exchange unit changed from post galvanizing frames to galvanized sheet formed before bending. My research showed that the much higher aluminum content (e.g. 0.15% in the sheet) was the cause. Galvanizing after forming the product normally has only about 0.002% aluminum in the zinc coating.

Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA

? What do you mean by "pre galvanized" and "post galvanized" steels?

Steel before it is galvanized and steel after it is galvanized?

Or items that are galvanized after fabrication, compared with materials made of steel galvanized before fabrication?

Clarifying your question will help obtaining answers.

A. Pre-galvanized usually is in sheet form, though sometimes pipe. It has a much thinner zinc coating than items fabricated first then hot dip galvanized.
The thickness of the zinc is very carefully controlled in pregalvanized sheet, etc., but has little control in post fabrication galvanizing. "Post galv" has a wide variability of coating thickness as well as a much higher average thickness.
The life expectancy of a galvanized finish is directly proportional to the thickness of the zinc coating, so in general, post galv will last longer, perhaps by as much as 5-10 times, but as thickness of coating is also dependent on immersion time which in turn is proportional to thickness of the steel, then the difference in life expectancy is hard to compare.
Very few items of thin sheet metal are fabricated first then galvanized as distortion becomes an issue. Pregalv is typically stretched and leveled after galv, making it very smooth, flat finish. That's not possible with post galv.

A. Sir:

I recommend that you get my article through interlibrary loan from any good sized library:

Characteristics of Hot Galvanized, Metallized and Bare Edged Galvanized Sheet Products, Metal Finishing, 63-68, May 1985.

Regards,

Sir:
My published article, "Characteristics of Hot Galvanized, metallized and Bare Edged Galvanized Sheet Products," in METAL FINISHING, pages 63-68, May 1985 can be obtained through a large library via interlibrary loan for the cost of copies. In the slitting process some zinc is smeared across the cut "bare" edge. Also some reaction products (zinc+2 ions) get onto the edge to help protect the "bare" edge. Also there is an electrolytic reaction in which the zinc is the anode and is preferentially corroded. The real problem about corrosion relates to the fact that galvanized sheet has much more aluminum in the coating and can cause white rust under certain conditions like cooling coils on the top of a building.
Regards,

A. Hi Asit. We appended your inquiry to a thread which previously answered that not all pre-galvanized sheet is electrogalvanized -- sheet can also be pre hot dip coated.

I think one of your issues will be how thick you want the coating to be. You might review thread 7666 "Hot Dipped Galvanized Coatings: G90 vs. G60" as a starting point for that, and for a discussion of your question about standards. Good luck.

Regards,

A. Hi John. I would conjecture that a cold galvanizing compound (zinc-rich paint), widely used on chain link fencing installations, would be a better way to treat exposed edges than a clear acrylic because of the galvanic protection, but I can't quote a study to prove it.

Regards,

A. Hi Adam. If you are going to powder coat the galvanized parts, you should zinc phosphatize the items after galvanizing them anyway -- and this should provide fine corrosion resistance. If you are talking only about cut edges, rather than holes in the items, you might be able to 'roll the edges'. Good luck.

Regards,

----
Ed. note: We appended your inquiry to a thread where Dr. Cook mentions his article which attempted to determine the value of 'smeared' zinc from shearing/punching.

A. Hello,
Once you have punched a hole via laser or press you have penetrated the coating. So both the pieces are subject to corrosion. The difference is insignificant laser vs press. Ideally, fabrication is done first and then the steel is Hot dip Galvanised or Coated as per process requirement.

A. The main differences between pregalvanized sheet and post fabrication galvanized are that pregalv is very flat sheet (having been stretch leveled during the process) and with thin coating. Post galv more likely to have some distortion, not be float, and have much thicker coating.
The thicker the coating the more the so called self healing effect. Actually this is just the zinc acting as a local anode, and it's effective to about 3-4 mm away from the zinc. Cut edges should be protected by the zinc either side, but the thicker that coating the better it will be. The chemistry of the steel doesn't normal have any direct effect on speed of rusting, but could have an indirect one by way of affecting the steel reactivity with zinc, causing either thinner or thicker coating. (I'm not allowing for alloyed steels or Corten, etc.)

A. Hi. Although I don't have the deep experience with galvanizing of Dr. Cook and Geoff Crowley, I can add that cut edges and uncoated steel will always represent somewhat of an additional problem in that dependance on galvanic protection of bare steel inherently interferes with the ability of corrosion products on the rest of the component to slow or limit corrosion :-(

Consider, for example, that nickel plating can provide corrosion protection to steel even though it is cathodic to it. It does so as a "barrier layer coating", encapsulating the steel so it is not exposed to the environment. To a certain extent the carbonate corrosion products on galvanized steel form a barrier layer coating on the zinc which reduces its exposure to the environment and slows its corrosion; but obviously a zinc atom which is being consumed in the protection of a bare steel edge cannot simultaneously be forming a barrier layer on itself. Dr. Cook and presumably others have attempted to quantify the issue but it is a complex one, such that we can simply say that bare edges are always disadvantageous.

Luck & Regards,

A. This sounds more an issue of poor specification that poor coating.
Pregalvanized sheet always suffers from cut edge lack of protection.
If its too late and someone has specified this in error and you are trying to salvage the situation, then using a zinc rich paint could solve the problem. If its fresh galvanizing and clean, then applying a painted coating to the edges will help to protect them. It's will add nothing to the aesthetics, and be a very obvious repair, but will have functionality.

Thank you Geoff Crowley for the clarification. As of now we have requested our supplier to do some modification in the shearing process to reduce the rough edges area. I am waiting for their observation response. I will check with them regarding the surface coating quality as well.