Galvanizing ash or flux contamination of work  

February 29, 2008

Bill

the bubbling after immersion sounds quite normal.

The white smoke sounds normal. Part of the flux is ammonium chloride - thats what fumes off. The higher the ratio the worse it is. Quadraflux most fume, triple next, double less etc. But don't let that cause you to think that quadra is no good, its actually good.

You didn't mention skimming before withdrawl.
Its normal to get ash float to the top during that bubbling. Residues of oxides etc. While material is immersed, you use a thin bladed skimmer on a handle to paddle (not vigourously) the ash to one end of the kettle, pushing it between the suspenstion wires. This creates a clean surface to withdraw through, without the ash etc depositing on the work.

In other words, creat a "clear patch" of the whole surface, then withdraw.

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland

 

Sir:
Have you had the zinc tested for aluminum? 30 ppm aluminum(e.g. 0.003% is quite normal). How many kg of "brightener bar" (usually 10% Al) was added and how many kg of zinc are in the kettle?

How many kg of quadraflux did you add and how many liters are in the flux tank? What is the baume' of the flux and what is the temperture? You are correct, quadraflux gives almost not zinc spatter as the steel enters the zinc.

Did you skim off the ash just before pulling the product out of the kettle? What type of skimmer do you use?

There is no dross initially in the zinc, the iron is fully dissolved in the zinc for at least a few days or even a few weeks. Quadaflux does not react more with aluminum than does triple salt. I do agree that the great amount of white smoke indicates aluminum (and too much of it in the zinc). At start-up it is often the case that too much aluminum is added due to a math error.

Regards,

Dr. Thomas H. Cook, Galvanizing Consultant Hot Springs, South Dakota, USA

Sir:
Over the weekend, I thought of a few questions that I wanted to ask you: 1)How many liters do you have in your sulfuric pickle tank? How many liters of concentrated sulfuric acid did you add to this tank? How many liters of inhibitor did you add to this tank? From what country did you buy the inhibitor? How are the acid fumes in the plant? Are the acid fumes: Very Slight, Moderate, Strong, or Very Strong. 2)How are you heating the sulfuric acid tank and to what temperature? 3)How are you heating the quadraflux solution and to what temperature? 4)What is the heating system for the kettle? Is it flat-flame-side-fired, end-fired, or pulsed-end-fired? How many centimeters of insulation are under the kettle (e.g. between the kettle bottom and the concrete support pad)? How many thermocouples are in the kettle and are they in the zinc or on the outside of the kettle wall?

Your observation that the quadrafluxed steel goes into the molten zinc "...with not a sign of spatter...." is exactly correct. This is also my experience and it is interesting that a drying oven is NOT required and in fact NOT desirable.

Regards,

Dr. Thomas H. Cook
Dr. Thomas H. Cook, Galvanizing Consultant
Hot Springs, South Dakota, USA

March 4, 2008

Geoff,
You say something like "quadraflux is actually good." Do you use it? If so what baume,' what ACN, and what temperature? If you do not use quadraflux what do you use? Do you have kettle enclosures?
Regards,

Dr. Thomas H. Cook
Dr. Thomas H. Cook, Galvanizing Consultant
Hot Springs, South Dakota, USA

Bill,
Your very clear description of the problem allowed a quick solution.

You need not reinvent flux usage. What is the density of your flux (in Baume' or g/mL measured at room temp.) If you get too little flux on the product it will not galvanize. If you get too much flux, the outer layer will be dry and the inner layer wet and spatter badly at the kettle. If your purpose in reducing flux on the steel is to reduce smoke (white smoke that rapidly travels upward) you cannot "win." With heavy baume' double salt you get brown/black heavy smoke at head height and it does not dissipate rapidly. With triple or quadraflux you get white smoke that goes straight up to the roof and out any open doors. The real solution to the smoke problem is a kettle enclosure and bag house.

1) I looked up "pentaerythritol." It is a white solid costing about $1 US/pound. It is listed in the pocket guide to chemical hazards with ratings of 5 to 15 mg/cu M for OSHA and NIOSH. It is an eye, skin, and resp sys irritant. I am not familiar with this product as a surfactant. It would not be my choice without testing. It is true that a good surfactant will reduce the surface tension of flux solution from 80 dynes/sq cm to 30 dynes/sq cm, which does reduce LIQUID flux solution in root threads and around weldments.

2) Coming out of the flux solution very slowly will not produce less flux on the product. The amount of flux on the product is adjusted by baume,' and temperature. Always the product should be in the flux at least 3 to 5 minutes so the product heats up and then when the product is withdrawn the flux dries from the heat within the product.

3) Using half a lift and going in half the kettle and then tractoring to the other end can be done. Remember however that the total time the product is in the zinc determines the thickness of the zinc. (e.g. longer in the zinc means thicker zinc on the steel).

I asked several unrelated questions so as to avoid common start up problems. Recently I was in a plant that used 3 barrels of sulfuric acid inhibitor per tank and the fumes were so bad that I could not even work properly. I used one US gallon per tank of good inhibitor and the fumes were reduced to ZERO. About 25 years ago I tested about 100 sulfuric acid inhibitors. Only about 5 were good. The others caused more problems that they solved. Some vaporize out of the acid, some coat the tanks with black goo, some are horribly toxic and smelly, some just plain do not work (e.g. protect the bare steel from further acid attack after pickling AND greatly reduce fumes in the plant).

If the heating coils are made of the wrong material then they will fail in a few weeks or months and can destroy the boiler.

Quadraflux until recently is not a commonly sold flux and I was curious if it is available in Australia. Is it? It can easily be made from double or triple salt or even from zinc chloride and ammonium chloride.

I trust that the 4% aluminum diecasting alloy contains only aluminum and zinc. Other elements (e.g. copper, Mg, etc.) would be very harmful. If the entire plant is filled with white smoke it is possible that you have too much aluminum in the zinc. It would be useful to know how many kg of casting alloy was used.

I am still wondering how you will use spray rinses to rinse the inside of the product and what you will do with this spray rinse water? (especially when production increases)

If you get by this way please stop in and catch some rainbows. They are now 2.5 pounds and will be 4 pounds by June.

Regards,

Dr. Thomas H. Cook
Dr. Thomas H. Cook, Galvanizing Consultant
Hot Springs, South Dakota, USA

Bill,

Skimming is uanavoidable. Don't try to dilute your flux, nor to get less flux on the job, or you'll encounter other problems. The flux is there for the time of immersion, not so much as a pretreatment before immersion. Insufficient flux crystals on the job (dried in place there after the water carrying it evaporates) and you'll get bare patches, lumpiness and other quality issues.

The zinc is continually oxidizing on the surface of the molten kettleful also. Not so much at startup and for a week or two, but in production use, you'll see the skin forming all the time. To get good clean bright galvanizing, you must skim before withdrawl.

It can be difficult if you have not arranged the suspension wires to facilitate, so think about this when wiring up the steelwork. Try to get the wires in straight lines across the bath. They can be as close as 200mm apart between rows of wires and still aloow skimming.

My design of skimmer is a blade of flatbar steel, about 40x3mm and about 400-500 long. Attached to the middle of this is a 10-12mm roundbar coming off at rightangles, then curving back to be about an angle of 30degrees off parallel. This handle must be long enough to hold from alongside the kettle where you walk along sort of floating the skimmer about half submerged and half exposed along the blade length. Push the skimming from one end of the kettle to the other. When you encounter a line of wires, give an extra push and the ash will float past the wires, to be picked up by the skimmer on the other side. Continue the full length.

After several dips, you could have a shovel full (or 3) of ash floating about at one end. Use the skimmer as a chopping knife to "chop it up" to dislodge any metallics, then use a perforated spoon (make one for this) to remove the ash. Put it in a drum or other suitable container (remember its hot). You can sell this and get perhaps 50% of LME Zinc price for re-use in a variety of industries.

Judicious arrangement of suspension wires will help faciliate skimming. With structural steel you'll be using multiple wires to get the combined tensile strength to support the steel. (If you need more info on this, ask)

Depending on kettle width, you might be tempted to make a single skimmer to span the full length. I don't advise it, its too hard for the operator. Rather have someone else attend and help at this stage, one each side, each with a skimmer. They should be able to skim the full length up to about a 12m kettle. Longer than that and you might need two goes, from the centre to each end, as the ash buildup will be too much to skim easily.

This design of skimmer also makes a good tool for getting off those drips of Zn that due to low angle, don't drain naturally. Try to get as much vertical component in the hanging angle as possible, but there'll always be something that doesn't drain well, doesn't hang well, and has drips. Left to solidify, they become spikes, and it takes labout to remove them. Minimise this work by removing before solid, as the steel withdraws from the Zn.
You'll also find that on some jobs there's ash forming on the surface again after a first layer has emerged, and you can skim between layers of steel as the emerge from the zinc, giving a nicer finish.

Hope this helps.

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland