Dichromate problems for hot-dip galvanizing

A. I would expect that since it will have trace amounts of chloride,sulfides and other nasty anions vs tap water.

A. I feel the Raw water / Softened / DM water does't make any difference in quenching or does not influence the quality of the finishing of Hot dipped steel. the chlorides/carbonates or any minerals of the quenching water does not have time to react with the quenched metal parts, which are suspended while quenching that too only for a shorter period.
Water may have its influence only when the hot dipped metal is allowed to stay into quenching bath for a much longer duration.

A. Sir:
I recently had a workshop here on the ranch and a company employee came here, for which I consulted many years ago. They had put two ion exchange canisters for the water for their quench (no chromate). It was a fairly expensive operation costing about $0.04/gallon (I think around $10,000/month). Since that time this company has been purchased by another company and now they use hex chrome in their quench and the ion exchange tanks have been removed. I had the guy bring tap water and the conductivity meter registered 200 microSiemens (about 160 ppm total dissolved solids). I doubt seriously that they needed DI water. In your case if you take the conductivity or TDS of your tap water you may get a high (or low) value and help you decide if DI water is worth the money.
Regards,

A. Sir:

Most North American galvanizers use sodium dichromate dihydrate, likely it is less expensive than the corresponding potassium salt. Also the usual concentration is around 500 to 1000 ppm and I like to use the color of the solution (e.g. standards in long, big test tubes) to estimate concentration. In use the pH of the solution drifts upward and must be carefully controlled using the proper chemicals. It is quite wise to have a water tank for quenching prior to the hex chrome tank.

Chromium trioxide (chromic acid) flake (dark purple flakes) is NOT suitable for hex chrome treatment of galvanized product. This chemical causes terrible stains on the product. Keep in mind that Europe has banned the use of hex chrome and with good reason. Hex chrome is a terrible cancer causing agent, and it easily comes out of solution into the air (e.g. the higher the oxidations number (e.g. +6 in this case)) the more covalent (more volatile) is the compound). The biggest problem is when a massive galvanized product is pulled out of hex chrome quench while it is still quite hot. At 300C, I measured 17000% the maximum CDC hex chrome in the air 4 feet above the hex chrome tank as a massive steel piece was being withdrawn.

Regards,

A. I've seen the same. Using a combined quench passivate, various ash drops off the work and settles in the tank.
Accumulation rate dependent on ash present, tonnage processed, and perhaps other factors too?

One method of removal is to use your drossing equipment, and at about half the frequency.
So if you dross fortnightly, then every second drossing, use the dross spoon to clean the tank. Saves pumping!
And it will settle using the same down time that the zinc requires after drossing.

A. Donavan:

At 500 ppm (0.05%) % by weight and % by volume are virtually identical. I also used a few hundred ppm of acetic acid . Another guy thinks using phosphoric acid to control pH is better, especially if the product is to be painted. He is likely correct.

Bad flux (especially low ACNV and high baumé) causes more flux carry-over into the quench resulting in more chloride in the quench tank (at a faster rate).

Additional comments on my new process are not possible at this time.

Regards,

A. Hello Maxim,
I think silane based passivation systems are not mature for consistent results yet, so I would go for trivalent chromates for hot dip galvanization process.

And, it's only my opinion, for hot dip galvanized parts the customer is not looking for "better white corrosion resistance" but a good thickness for long-lasting parts in the field.

So, I hope you take the best choice for your product and your customers!!

Best of luck!

A. Not much detail to go on to answer here..
Is all work affected by the white marks after quench? Or just these handrails?
What's in the tank? 0.02% what?
What else is in there? NaCl? KCl?
What temperature does this tank run at?

A. Hatta: You are running chromate at 200 ppm so it should be a light yellow color. Is the solution clear or cloudy? How old is the chromate solution? Is it made up from sodium dichromate or potassium dichromate or hopefully NOT chromic acid (CrO3) which stains the product. Hopefully you adjust the pH using conc. HCl. pH 4.2 is better. What is the TDS (total dissolved solids) in the chromate quench. Above about 4,000 TDS white rust is common. What is the TDS of your tap water? Sodium or calcium in the chromate quench can cause white rust. Do you use a water quench prior to putting the product into the chromate quench, so as to wash off the flux residue in water rather than the chromate quench?

Regards,

A. Hatta: The Ka for acetic acid is about 10 to the minus 5 which means that no matter how much acetic acid you put in your quench the quench will not have a pH lower than 5. acetic acid "buffers" the quench to pH 5. You will need a small amount of HCl to lower the pH to 4.2. Perhaps 1/2 liter or 1 liter will do the job.

Regards,

  Really appreciate on your valuable advice sir. I will try to work on it

A. Hatta: To calculate the exact amount of conc. HCl to add to the chromate tank, put 50 ml of chromate solution in a conical glass flask, add 15 drops of 0.1% bromophenol blue ⇦this on eBay or Amazon [affil links] sodium salt and titrate with 1:100 HCl (1 ml conc. HCl + 99 ml of distilled water). When the solution changes from a dark color to the normal yellow chromate color stop adding the 1:100 HCl. By proportion calculate how much conc. HCl to add to the chromate tank.

Regards, Tom Cook

A. Zinc Oxide (a/k/a 'white rust') is white. That's not the answer. If the black material is magnetic, then it's either iron or iron oxide (magnetite).