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topic 59411

Kettle Cook-Off Time in Hot Dip Galvanizing



September 25, 2013

Q. Recently I have made an interesting discovery that kettle (in molten zinc) cook-off time can be very short and produce thin zinc coatings as shown in the following table:

Time

20 sec

30 sec

45 sec

1 min

2 min

4 min

6 min

Old Flux

1.50 mil

1.51 mil

1.62 mil

2.09 mil

2.68 mil

4.34 mil

5.46 mil

Quad Flux

1.50 mil

1.60 mil

1.81 mil

2.45 mil

2.77 mil

4.19 mil

5.24 mil



Old Flux is: 29Be' 6.0%Fe+2 pH=4 ACNV=0.42 Temp=155 °F
Quad Flux is: 13Be' No Impurities pH=4.2 ACNV=1.60 Temp=155 °F


Where ACNV = ammonium chloride/(zinc chloride+iron chloride)

The two steel samples are 1 inch by 1 inch angles by 1/8 inch thick by 27 inches long. The steel was pickled, rinsed, and put into flux for 3 minutes. There was no zinc spatter from either sample upon entry into the molten zinc. After 20 seconds the kettle was skimmed and the steel was withdrawn slowly for 3 inches. There was no more zinc skimmings, (e.g. two total zinc skimmings; once just before going into the zinc and then one more time at 20 seconds. Then for each time interval the steel samples were withdrawn each an additional 3 inches. Zinc thickness for each section was measured 12 times on each side and the high and low values were thrown out. Standard deviations were between 0.1 mil to 0.4 mil for the thinner and thicker zinc coatings respectively.

All sections of both samples were perfectly galvanized with absolutely no "black" (bare) spots. At the bottom of each steel sample icicles were observed and for the old flux the icicles were ten times longer and thicker than the quadrafluxed steel galvanized sample. The old flux also gave 3.5 times more smoke and 3.5 times more ash than the quadrafluxed sample. Kettle zinc temperature was 831F. 1mil = 25.4microns

After 40 years working in galvanizing these results surprised me very much as I expected much zinc kettle spatter from the old flux and many bare spots for the old (bad) flux (actually the mono salt).

My purpose in presenting these data on finishing.com is to have galvanizers try a similar experiment and see if these excellent and (for me) unusual results are reproducible. Thus if you get good results or bad results (for 20 second cook-off time), I am very interested in your flux composition and your zinc composition. I am trying to sort out these extremely interesting results and have many ongoing experiments.

Any and all comments and results (or photos)are greatly appreciated.

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

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


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