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Critical parameters for alkaline degreasing bath in hot-dip galvanizing


 

Q. We are an environmental consulting company stated in Bogota, Colombia. We think you can help us in our effort of getting information about some critical parameters dealing with the alkaline degreasing bath in the hot galvanizing process of low carbon steel pipes of 6 meters length.

The sodium hydroxide content of the bath is about 12.13 kg NaOH per cubic meter of degreasing solution. There aren't any special substances (carbonates, phosphates, etc) added to the bath. The operation temperature is 82 °C and is kept by the injection of saturated water vapor. The solution that is carried out of the tank is replaced by the addition of fresh water at room temperature. There is only one water rinsing bath and it's carried out solution is replaced with fresh water at room temperature, too.

Our questions are referred to the following items:

Critical operation conditions of the bath.
Maximum grease content.
Maximum ferric oxide content.
Operation temperature range or interval.
Optimal temperature.
Special additives added to the bath. This is because of an actual trouble while degreasing pipes. After 15 minutes immersed in the degreasing bath, the grease remains on the pipe's ends, whilst the middle of the pipes are properly degreased. Have you any information about the type and quantity of emulsifiers (surfactants) used to solve this problem?
Bath general operation(Continuous, batching, etc).
Exhaust bath treatment.
Warming bath system.

Best regards,

Juan Carlos Matallana Arenas.
- Bogota, Colombia


A. You need a properly formulated, modern alkaline cleaning solution to solve your problem. The choice of alkalinity, complexing agents, emulsifiers, skimming equipment, etc. will affect all of the parameters you mentioned.

1. Maximum grease content - Do you want to emulsify the oil, or do you want to formulate the cleaner so that the oil can be skimmed off for recycling?

2. Maximum ferric oxide content - Not the major problem.

3. Operation temperature range or interval. Depends on #1 above, and the melting point of the oils in question.

4. Optimal temperature. See #3.

5. Special additives added to the bath. There are available hundreds of surfactants. Some will decompose within a few minutes of use in the bath, some will be useful over the life of the bath. Formulators are constantly searching for the best additives for the particular base metal, configuration of the part, the kind and amount of grease present, cost, etc.

6. Bath general operation.(Continuous, batching, etc). Depends on the selection of chemistry. -- Exhaust bath treatment. Treating the exhausted bath is complicated by the organic additives, but since you are having a problem with a straight caustic solution, we have no choice to address those issues. Warming bath system. Steam in steel coils is used extensively in the U.S.A. for alkaline cleaner tanks.

tom pullizzi monitor
Tom Pullizzi
Falls Township,
   Pennsylvania 



February 21, 2008

Faults in Hot-Dip Galvanizing


Hot Dip Galvanizing

Q. We are a galvanization plant in Lagos, Nigeria. We guess you can help us in our effort of getting information about some critical parameters, calculations and chemical formulations dealing with the alkaline degreasing bath in the hot galvanizing process of mild steel.

The sodium hydroxide content of the bath is about 48%w/v of degreasing solution(450 g/L); wetting agent (5 g/L); after 85 °C is reached sodium metasilicate (150 g/L) is added 50 kg per hour to the bath; The Capacity of Tank is 82500 L. There are two water rinsing bath and its carried out solution is replaced with fresh water at room temperature, too.

Our questions are referred to the following items:

Critical operation conditions of the bath.
Maximum grease content.
Maximum ferric oxide content.
Operation temperature range or interval.
Optimal temperature.
Special additives added to the bath.
Bath general operation,maintenance (Continuous, batching, etc).
Exhaust bath treatment.
Warming bath system.
optimum composition and formulation of the compounds of the bath
optimum calculation of the composition and formulation of the compounds of the bath

Best regards,

lilian okpala
employee - Lagos, Nigeria


June 1, 2009

A. Hi, Lilian. Please, if possible, phrase your questions in terms of the responses that Tom has already offered. After he already attempted to answer those exact questions, simply cutting & pasting the same questions in a second time is rather unproductive for all of us :-(

Perhaps the central point of Tom's answer was that the suppliers of metal cleaning solutions have invested decades in the development of optimum formulations, and they retain these formulations as trade secret. You can, if you wish, take advantage of their work by buying their proprietary cleaning solutions.

If you want to formulate your own chemistry, a good source of information would be expired patents, text books, a computerized search of published literatures, etc., but there is a lot to it: surfactants, builders, buffers, detergents, sequestering agents, chelates & complexers, whether you can you employ electrocleaning, and much more. These are broad topics which must be covered in a book because it is impossible to distill it down to a length appropriate to an answer here.

Electroplating Engineering Handbook


If you can find a copy, the Garden State Branch AESF "Electroplating Course Manual" has an excellent 16-page chapter titled "Fundamentals of Metal Cleaning" which explains emulsification, detergents, diphase solvents, acidic cleaners, inhibitors, the effect of pH, saponification, wetting, deflocculation, chelating/sequestering, buffering, and testing for cleanliness.

The "Electroplating Engineering Handbook" =>
is more readily available around the world, and has a nice 12-page chapter on alkaline cleaning. Indeed most plating textbooks over at least some info on the subject.

Best of luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


August 9, 2010

Q. Please tell me why we are using more alkali degrease at High Temp.?

And Which are reactions takes place like saponification, emulsification?

Can we use acid pickling before Alkali Degrease for rust removal, as I heard that we cannot use but, for rusty part we are using one product for pickling before loading.

I want to know. Please clarify on above issues.

anand vyas
Automobile - Gujarat , India


A. Hi Anand. Please try to fully detail your situation. I don't know whether you are claiming that you have raised the temperature of your alkali degreasing process, and now it is consuming more chemicals (for one thing it may be cleaning better, thereby exhausting more cleaning chemicals) or whether you are asking why people are more likely to do their cleaning at high temperatures (because such chemical reactions proceed faster due to the extra thermal energy).

Saponification occurs if the greases are animal fats, not if they are petroleum based. As for emulsification and other mechanisms, what, if any, chemicals besides alkali are in your cleaners?

Acid dipping is usually done after alkaline cleaning, not before (except for scale-breaking & pickling by the steel mill). It is possible to do acid cleaning but, except in special situations, it is more expensive and troublesome than doing alkaline cleaning followed by acid activation.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



January 30, 2016

Q. In Degreasing, what happens during removing the grease & oil from metal body. Explain with reaction?

Atul Jarad
student - pune,maharashtra,India


Digital version
mfg_online

(No longer published, but Elsevier hasn't yet de-commissioned the online version of the Guidebook)
Download it before it disappears.
January 2016

A. Hi Atul. We've added a link above to the on-line edition of the Metal Finishing Guidebook, which will explain a lot for you. Cleaning is generally not a single reaction but a combination of many simultaneous processes as explained above, including detergency, solvency, emulsification, saponification, sequestration, etc.

But please get back to us with what grade you are in, and what your project is. Good luck.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



December 22, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I'm currently doing a study on batch galvanising and was looking into the degreasing stage of the process. I was wondering if you could possibly shed some light as to why the optimum temperature for degreasing agents to operate is between 80-85 °C?

If anyone could provide some scientific knowledge to the subject that would be much appreciated.

Martin Olowe
- England, UK


December 2016

A. Hi Martin. The thing is, 80-85 °C does not give optimum cleaning, 100 °C would be better because of greater chemical activity. But operating a cleaning bath at boiling temperature greatly increases the energy cost because of the rapid evaporation, and leads to the need for greater exhaust requirements as well, and may preclude the use of certain materials of construction like polypropylene. So somewhat lower temperatures are often employed, and 80-85 °C is a pretty common compromise value.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


December 22, 2016

thumbs up signThank you for your reply.

Martin Olowe [returning]
- England, UK

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