Galvanizing is Peeling

A. Sir: There are generally two types of hot dip galvanizing peeling problems: (1)The peeled coating is bendable and the cause is improper cleaning or fluxing of the steel. (2)The peeled coating is brittle (snaps if one tries to bend it to a right angle) and the cause is reactive steel that is too long in the zinc at too hot of temperature. Often too long in the zinc is caused by bad flux (especially one with sodium chloride in it). Any zinc coating thicker than 6 mils (152 microns) is quite subject to strike impact peeling. For reactive steels (containing silicon and especially phosphorus) at usual galvanizing temperature the growth rate of zinc is about one mil for each minute the product is in the zinc. Thus longer than 6 minutes gives more than 6 mils and strike impact causes peeling. I think in the USA the hammer test has been discontinued.
Regards,

A. You should try with higher concentration of Al in Zn bath.
>From 0.16 to 0.20%.

Dear Sirs:

My apologies, it is "Sandelin" and not "Sandolin"; that explains why I am not seeing many facts/writeups about it on the net or other resources. I understand the steel reactivity profile was coming from a curve plotted by a metallurgist, Dr. Sandelin, way back in the 1940's(?) showing zinc coating thickness plotted against silicon mass in steel. I am assuming that during his era silicon-killed steels were not that predominantly manufactured or the technology (most likely borne in search for cost-effective products) might have not existed yet. I had the notion therefore that his presentation was deeply related if not very much reflective to what I call the real culprits behind steel's "reactivity" that are phosphorus and sulfur present with silica in the iron ore. My sources for these "detrimental impurities" dated back to 1959 (Leicester Hamilton and Stephen Simpson of Talbot's Quantitative Chemical Analysis).
and they have not written anything about silicon steel or silicon killed steel in the book.

Regards.

A. Sir:

There are basically two types of peeling:

(1) The product is not properly pickled and properly fluxed. In this case the peeling comes off as a flexible sheet. This type is rare and I saw it once. The steel was re-phosphorized and aluminum semikilled steel.

(2)The other type relates to "reactive steels" and thick zinc alloyed coatings form (above about 6 mils (150 microns)). The zinc coating is brittle and easily breaks with a "snap". Thus knowledge of what makes a reactive steel is important. The following is my list which I consider to be reactive elements that cause a steel to be reactive:

(A) Silicon between 0.05% up to about 0.15% and then silicon above about 0.3%,

(B) Phosphorus above about 0.015%.

(C) Aluminum semikilled steels (likely above about 0.06% aluminum).

(D) Various cast irons.

(E) Copper (as in Cor-10) above about 0.1%.

There are others and other combinations, but I do not have at hand the galvanizing book by Heinz Bablik (1940) or a recent article on this subject.

A. I've seen galvanize peeling on several different substrates, and some unexplained cases.
In addition to the above:

On 3 mm sheet steel, where the zinc peeled off. Not a thick coating, probably not reactive steel, but shotblasting prior to pickling solved this one.

On 10 mm wall 100 x 100 SHS, where a <100µ coating peeled. No explanation, and no cure found.

A. Please tell me whether you are quenching the material after galvanizing or not? If yes, what is the process and with what?
This will solve your problem.

Hi Ehsan. This forum can often be helpful and I hope it helps you. But sometimes it takes days or weeks before everyone sees it, and even then not all are willing to freely share their knowledge. And even if they did, they probably don't have enough info.

What I am getting at is that while you are very welcome to post here, if I had 2 million pounds of product at risk I would do my best to retain a world class galvanizing consultant immediately :-)

Regards,

A. Sir:
For a flux do you use zinc ammonium chloride solution or a gas flux with 5% hydrogen and inert gas? Do you use caustic? Do you use HCl? Does the delaminated zinc coating bend easily or is it brittle? How thick, in microns, is the zinc coating? What is the steel composition? What is the zinc composition (additives like Al, Sn, Bi, Pb) etc.?
Regards,

Q. Dear Mr. Mooney & Dr. Cook,
First of all, I really would like to thank you for your attention.
Our line is Hot-Dip Galvanizing and we do not use Flux in our line. In addition, we do not use caustic and HCL in our line. we just have used HCl in pickling line and we sure it would be cleaned at the end of line. Delaminated zinc coating bends easily and thickness is about 8 microns in each side.
St12 is steel composition, and zinc composition is: Al= 0.21, Pb= 0.12, Fe= 0.0047

A. Hi,

In order to help you, I need some more information.

Does the problem happen in hot rolled strip only, or both hot and cold rolled condition? In which thickness range?

Does it flake all over the width or just near the edges?

Best regards,

A. Sir:

I have worked for only three sheet line galvanizing companies (North America, Panama, and Istanbul). In each case they had a zinc ammonium chloride water solution flux with high baumé and an ACN between the monosalt and the double salt. I am aware of sheet lines that use hot gas of 95% nitrogen and 5% hydrogen as a flux, but I have not seen them. I do not think sheet galvanizing can be economically accomplished using only HCl.

You are using 0.2% aluminum in your zinc to prevent zinc/iron alloy build-up on the surface of your sheet. As I remember the past (quite some time ago), they may have used closer to 0.15% aluminum for this purpose. Your observation of only 8 microns of zinc is quite thin indeed. I do not understand your use of 0.2% lead in your zinc, perhaps it is to give you a slight spangle in the coatings. I recommend that you review your process and look more closely in the literature to have a properly functioning sheet line.

Personally I do not believe you can galvanize in a sheet line without any flux at all.

Regards,

A. Interestingly, letter 13235 is about copper immersion deposit onto the pickled steel strip if Cu is present in the pickle liquor above the threshold limit. There is a possibility that there is a connection between Cu deposited on the steel strip and the galvanized strip adhesion failure. Has anyone experienced this before?
By the way, Ehsan, we are waiting for your response to Mr. Joao Pires's question.

Best regards

YKVoo

A. Dear all, some years ago, our customer had similar problem, and reason was high acid drag-over in the flux solution. After pH adjustment with ammonia [on eBay or Amazon] solution, zinc flaking problem disappeared.

A. Dear Mr. TC Khoo,

Your problem of zinc flaking is because of High coating on very smooth surface.Pl, try to minimize immersion and withdrawal time and dip in quench tank as soon as possible.

A. Material is peeling off
1. Please ensure the quality of the steel which you are Galvanizing.
2. If steel is Ok. then in Zinc Bath do not add anything even Aluminium.
3. Keep the Bath Temp 445-450 Max.

A. It's a bit hard to tell from the picture. Both incidences seem to be on the same face, and both black. Is it on top of the zinc? Or is there bare steel underneath the black and no zinc?

This could be bare patches caused by insufficient cleaning in pre-treatment, possibly as a result of paint or other contamination on the steel. It would result in such a black sooty deposit.
But that's a guess based on a single image and not much info.

A. Sir:

Looks like incomplete pickling or bad flux to me.

Regards,

A. Dear
Pictures are not telling true story. What I can say:
1. The work is not properly cleaned.
2. The Zinc Coating is not proper. Peel off Zinc coating can be easily identified.
3. Before further alkaline painting the Galvanized surface is treated with etching chemicals. So that the Paint can stick with GI surface properly.

Regards,

A. Zinc peeling problem is because of two reasons the first one is improper flux chemistry. The second one is if the material has not been pickled properly. This should not be possible n your case. Flux seems to be the problem.

A. This thread suffers from misunderstanding.
Some parties are talking about continuous strip galvanizing, and others are speaking of post fabrication batch galvanizing.
The arrangements of the two processes are quite different.

A. Hi cousin Ganesh. Please tell us what has led you to raise this question? Do you have other 'galvanizing defects' in mind besides peeling?

Please tell us your own situation, and maybe your hypothesis, rather than casting your question in the abstract. As previously noted, our helpful readers repeatedly tell us that they don't want us to post hypothetical questions which ask them to speculate open-endedly while also covering a dozen "if, ands, & buts". They want to straightforwardly try to help you with your individual situation, not to write white papers or lecture the world. They are far more likely to respond to you if you introduce your own particular situation. Thanks for your understanding!

Regards,

A. Sir: To learn more about flux see the on-line library on this website. Also check the 20 second dynamic flux test also in the on-line library.

Regards,

Lawrence.. what thickness of steel plate is this (hard to tell from image)?
How is it cut? Gas? laser? plasma?

I have found that gas cut changes the chemistry of the steel for a few microns into the steel from the cut. Grinding or blasting that cut surface changes the galv thickness on that surface. You mentioned machining. That would also remove this effect of the cutting method.
Is there any measurable zinc alloy thickness in the part where the galv peeled? If not this looks like very unreactive steel (too low in Si).