Critical parameters for alkaline degreasing bath in hot-dip galvanizing

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.

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.

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 offer at least some info on the subject.

Best of luck.

Regards,

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,

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,

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 very useful materials of construction like polypropylene. So somewhat lower temperatures are often employed, and 80-85 °C is a pretty common compromise value.

Regards,

Thank you for your reply.

A. Hi Essie. caustic soda [affil links] is the principal ingredient in alkaline cleaning, and could be used alone, but that is far from ideal ... as you can read in this thread which we appended your inquiry to.

Regards,

A. Hi Essie. Although you used the word 'grass', you probably are referring to a material which is more like hay or straw than soft green suburban lawn grass. If it rinses well, with no stains and no troublesome little flecks stuck to the parts, my guess is that it's not a problem.

Regards,

A. Hi AJ. There are many different ways to formulate a metal cleaning solution, but if you are speaking of a hot alkaline cleaning tank, it is rather common practice to build them with a grease trap and riffle pipe as illustrated. In this case you want to drill holes in the riffle pipe that are large enough to not plug frequently, and closely spaced enough that they sweep the grease off the whole surface, but not with such pressure that they could squirt solution out of the tank. An all iron centrifugal pump, perhaps 2 GPM for each foot of tank length might be good.

Regards,

A. Hi AJ. I've heard of such rules of thumb for filtering plating tanks, but am not familiar with any such guideline for alkaline cleaners.

Most plating lines use a soak cleaner tank followed by an electro-cleaner (sometimes with a rinse between). The pump for the riffle pipe on the soak cleaner which I described probably meets or exceeds 10 turnovers per hour, but many plating lines have no recirculation pump at all on the electrocleaning tank.

Regards,