White rust on galvanized pipes -- cause?

A. Do you chromate them and dry them, Muhammad Yousuf? Are the pipes touching during the chromating quench? It sounds like they are either not chromated properly or were not dry in the areas where they are touching (capillary action tries to keep that area wet) -- but please tell us what you've tried :-)

Thanks.

A. It could be that this is "white rust", and the steam might be causing it, or the water quench.
Do you have any passivating agent (like weak Chromic acid) in the water? This might help this problem.

A. Sir,
Do you use hex chrome (sodium dichromate) in your quench? What is the conductivity of your quench water (in micro Siemens)? Is this quench recycled and do you periodically dump and refill? What is the aluminum content of your zinc?
I enjoyed my UNIDO work in Lahore for PECO.
Regards,

Q. Dear Crowley / Dr. Cook,

We use Water for quench and change the water (once a Week) we consume about two tons of Zinc every day.

There is little information on pipe Galvanizing in our country, please guide me to get max. knowledge on this subject.

I appreciate you keeping us posted on your suggestions Wherever you can.

A. Quenching in plain water will surely cause white rust, and a dull finish on the galvanized finish.
You need some form of passivation. This might be a chromate type, but there are others. You should be able to purchase a proprietary brand, but there's one thing to be wary of.
Often the instructions are for a passivation chemical concentration designed for treatment at ambient temperature. Quenching hot steel straight from the zinc bath requires a much lower concentration, perhaps a tenth of that for ambient temp. Otherwise you will get severe discolouration using a strong solution. (Brown like chocolate at high strength, yellow - light brown at medium, and no visible colour at the right strength).
You might need to do a little experimentation to get the right strength.
This chemical gets consumed. You'll know when its under strength, as the visible symptoms you describe as your problem now will return. Top up the strength, and they go away.
With this system you will not need to change the water so often, perhaps every 6 months, maybe even annually. But you will need to top up evaporative losses.

Incidentally, that tank removes a lot of heat from the steel. The contents will get hot, nearly boiling. Its a low grade source of heat, but heat isn't cheap these days, you could consider capturing it and using it for something. Just pipe is somewhere and pump circulate it to something that needs some heat.

A. Sir:
Please answer the following questions. If you do not know the answer just say so.
(1) What is the aluminum content (percent) in your zinc?
(2) How many tons of pipes do you galvanize each day?
(3) Are you allowed to use hex chrome in your quench?
(4) Is your blow head manual or mechanical-automatic?
(5) What is your angle (from horizontal) of withdrawal from the zinc?
(6) What is the temperature of the super heated steam?
(7) Do you have a steam accumulator near the blow head?
(8) Do you use straight-through, focused, or spiral steam blow?
(9) How are pipes pushed into the zinc?
(10) What is the baumé (density) of your flux solution?
(11) Are you "wet" or "dry" kettle?
(12) What is the temperature of your flux solution?
(13) What is the power of your boiler as a percent of the power of the kettle?
(14) Do you have a wet season, and if so do you have fires in your zinc dust ⇦ this on eBay or Amazon [affil links] blowings?
(15) How many microns of zinc are on the outside of pipes and on the inside of pipes (the inside must be profiled)?
Regards,

Q. Dear Mr.Cook,

Sorry for late reply

1- We use very little Aluminium which is about 0.005% of Zinc (What is recommended?)
2- We galvanize about 20 tons to 35 tons per day.
3- We are allowed to use Hex Chrome but we are not using.
4- Our Blow head is mechanical Automatic.
5- Withdrawal angle is about 20 degrees.
6- Super Heated steam is at 450 degrees at Boiler but blower is 145 feet away from head.
7- Yes Steam accumulator is near the blow head, but we have few drops of water falling at every blow.
8- We use straight steam blowing head no spiral.
9- The pipe are feed into Kettle by spiral screws.
10-Flux density is 12~15 degrees.
11-We use Wet Kettle. (Please explain wet & dry Kettle).
12-Flux is not heated.
13- -So-
14-We have July~Sep. wet season but no fires (Explain it?)
15-40~50 microns only out side, not able to record inside thickness (which meter should be used to measure Zn).

A. Sir:
1)30 years ago I thought 0.003% aluminum was good (for bright product). However, there was much white rust. With quadraflux and IQM, I now recommend 0.0005% to 0.001% Al and the product is very bright and almost never white rust. IQM would likely give too thin of zinc on pipes.
2)You said you "consume about 2 tons of zinc every day." Thus your GZU is 7.3%. e.g. ((2/27.5) times 100 = 7.3%) Unless your pipe wall thicknesses are very thin you should be able to do better. I think you have too much zinc inside your pipes.
3)Hex chrome (in quench) at a few hundred ppm and the correct pH normally prevents white rust. Hex chrome causes cancer and is given off if the solution is hot.
4)Is the blow head like a vice grip and air operated? Is it heated?
5)Withdrawal angle is excellent.
6)A little hotter steam would likely be better.
7)Steam accumulator is excellent if it is big enough.
8)Spiral or focused blow head works well on larger diameter pipes.
9)Best pipe zinc entry for smaller pipes is ventilated, progressive, small diameter screws. Ventilated means the screws are spiral rail twisted around the drive core and the rail is connected via spokes. Progressive means the pitch is greater at the zinc entry and less in the zinc. Small diameter means a foot or less and the pipes do not pass under the screws.
10)This flux density is on the low side.
11)"Wet" kettle has black bubbly layer of top flux on zinc. "Dry" kettle is just zinc surface. Usually "dry" kettle gives better zinc usage. Proper equipment is required for "dry" kettle.
12)Most pipes flux solutions are gently heated. Do you dry the flux on the pipes in a drying oven? What temperature?
13)For the boiler to supply 400 pipe volumes of steam per blow it must have about the same power as the kettle.

A. Sir:
Quenching in plain water is much more likely to cause white rust than not quenching. There seems to be trivalent chrome for quenching which is said to be much safer than hex chrome. The worst case is HOT hex chrome, which emits hex chrome from a boiling solution far above USA EPA standards. I did many, many lab tests years ago and made a 1 hour video during the tests.
The main problems in not quenching is that the steel retains the heat and the outer pure zinc layer continues to alloy with reactive (silicon semi killed) steels and thus turns gray. Also the steel is hot and difficult to unrack. The turning gray problem does have solutions.
Regards,

A. It is possible to use a chrome based quench at high temperatures (like 90 °C) and stay well within the UK exposure limits. (I do not know how USA compares with UK limits).

Its all a matter of concentration, but then wrong concentration produces wrong results anyway. Too strong = brown staining. Too weak, and you might as well use water and suffer the resultant white rust.

The chromic acid is consumed, so it must be tested and concentration adjusted about twice a week. (dependent on production level).

In the Taranaki boondocks, maybe weekly would do! (Cheekiness from a Kiwi of the migratory sort)

A. We are presently galvanizing pipes from 3".Firstly the gross zinc consumption should be in between 3% to 4%. Secondly you can use sodium dichromate for quenching along with some sulfuric acid. Are you sure these are not rubbing marks because of the internal die. The pressure of the air should always be checked as because it will cause oxidation and drying up of zinc fast.

A. Dear Aman;

Actually, quenching in water would have dull finish. I don't really understand how your pieces become brighter with water quenching.

I can think of one possibility; you might have very "thick" pieces like 20-30 mm. Then heat content of the material will result in gray finish. There is nothing to do for that but just using high Ni content on bath ( 0.6% ^^0.06%). But then you will face reduction of coating thickness (maybe under any given standards).

If you give more detail about your production then we can help you more efficiently.

Al %	Pb %	Ni %	Sn %	Fe %	Cu %	Cd %

0.005	0.391	0.01	0.001	0.036	0.027	0.0031	

0.005	0.65	0.01	0.002	0.026	0.03	0.0035	

0.005	0.64	0.01	0.002	0.128	0.029	0.0035	

0.005	0.474	0.01	0.002	0.019	0.029	0.004