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topic 7764

Zinc Coating Thickness Control in Galvanizing


A discussion started in 2001 but continuing through 2019

2001

Q. We have continuous hot dip galvanising lines with full Zinc bath in ceramic pot. When we increase the line speed to 150 mtr per min from 120 or 130 meter per min (processing and galvanising speed) the Zinc coating thickness goes up to 150 grams per square mtr instead of desired coating of 120 grams per square mtr.

Shall be grateful if someone can suggest the method to control the coating thickness with the higher galvanising speed.

R N Mahendru
- Nagpur, Maharashtra, India


2001

A. Hi,

It sounds like you need to increase your temp.The zinc when being pulled out at a faster rate doesn't have as much time to run off and thus cools to make a thicker coating. By adjusting the heat upward the zinc will flow off faster.

Dan Scarbo
- No. Adams, Massachusetts


2001

A. Dear Mr Mahendru

The zinc coating is a function of zinc bath temp, zinc bath chemistry ( % Al, % Pb and % Fe), strip temperature entering in the bath, air knife slot geometry, air pressure of wiping blower, height of air knife above zinc bath level and its angle, formation of zinc dross, strip cleaning in chemical degreasing and / or Nonox furnace. One can use various combinations of these parameter to achieve the desire zinc coating. Unfortunately, however the higher line speed above 120 m/min, zinc coating control challenge very difficult indeed.

Since you have not indicated current practices in your internet published letter, I don't know precisely what practice modifications to recommend.

G.D.Patange
- Mumbai, Maharastra, India


2006

A. TRY TO CHANGE THE QUALITY OF YOUR WIPING PADS.
TRY TO CHANGE THE TECHNOLOGY OF THE WIPING SYSTEM.
TRY TO FOCUS ON YOUR PEOPLE WHO ARE RESPONSIBLE FOR THE WIPING.

KOSTAS TZIAMPAZIS
- GREECE


December 18, 2011

A. DEAR SIR

DO YOU HAVE THE HOT BRIDLE IN YOUR LINE TO GET THE OPTIMUM TENSION IN POT AREA? ALSO THE UP PASS COOLING TOWER DUCT TOP & BOTTOM PRESSURE TO BE SIMILAR TO AVOID STRIP FLUTTERING. USE WIPER ROLLS IN ADDITION TO STABILISER ROLL IN POT TO CONTROL THE COATING AT HIGHER LINE SPEEDS. LAST BUT NOT THE LEAST THE OPERATOR'S TALENT & DEDICATION IS A MUST.

KIRUBAHARAN MANICKAVEL
- REWARI, HARYANA, INDIA


April 28, 2013

A. Dear Sir
Of course you know that the line speed is a parameter that controls coating weight; then you have to try other parameters to reduce coating weight:
m=K dv/p
d= air knife distance
v = line speed
p = air knife pressure
m = coating weight
Also you can use close loop control, or if you need more pressure it is better to modify the air knife blower.

Jamal Ashouri
steel - Esfahan, Iran



September 5, 2019

Q. We work on hot dip galvanizing pipes and we are having hard time to control zinc thickness. We use air knives to control zinc thickness but we are not getting enough results. What would be the optimum pressure/ CFM to blow off excessive zinc.

Steven mary
- Arkansas, USA


September 15, 2019

A. Sir: I have used hot air to reduce the external zinc coatings on pipes.

To get a better understanding of fluxes and to use the 20 second dynamic test on flux, See the on-line library on this website.

Normally the zinc coating on the inside diameter of the pipes is four times thicker than the outside diameter thickness. Do you steam blow the inside of the pipes? What is the i.d. thickness of the zinc on the pipes?
Regards,

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

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