Need details of 'the 7-tank process

A. Hi Seetha. As you can see on this thread, many people have offered the general (if sometimes slightly varying) sequence of operation and the names of each of the seven steps. The '7-tank process' is a generalization of the zinc phosphatization process (which comprises the pretreatment before powder coating), but it's not a specification.

The easiest way forward is to hook up with a supplier of these pretreatment processes, and they will recommend their overall sequence exactingly, and their proprietary chemicals for each of the steps in the sequence; and they will provide technical data sheets for the operation of each step.

If you wish to proceed without relying on proprietary chemicals from a supplier, or just to learn more, there are also many excellent books about phosphating which could enable you to start trying ... but generally an applicator of the process would buy the proprietary chemicals which are the result of years of R&D rather than trying to formulate the process solutions from raw commodity chemicals.

Please introduce yourself and your situation so that people can help you most effectively. Thanks!

Luck & Regards,

A. Hi Seetha,
The pre-treatment is commonly called as 7 tank process. It varies with customer's demand. We have two tank process chemicals with proprietary formulation. The customers are expecting ASTM standards at low cost per sq.ft. of treated material. A vague reply won't help you out to implement it practically.

Mr. Ted, as you referred to the knowledgeable Mr. Shankar, he is not with us now. We lost him to Corona infection. He was awarded with honorable medal by then Indian president Ms. Pratibha Patil.

A. Hi, Shiby. A "7-tank process" is simply a process where parts are immersed in seven tanks sequentially. Normally, the zinc phosphating process is done in 7 steps while the simpler iron phosphating pretreatment process is done in 3 steps or sometimes 5 steps. So when you ask about a 7-tank process for pretreatment, you are probably asking about zinc phosphating.

To me the 7 steps traditionally meant soak clean, rinse, acid, rinse, zinc phosphate, rinse, and chrome or chrome-free passivating dip.

These days a DI rinse is sometimes used instead of the passivating dip, and an "activation" step is often inserted before phosphating. But "7 tanks" is slang more than a specification, and the zinc phosphating process could actually involve 6 or 8 steps rather than always exactly seven.

You need to find a supplier for the process, and they will give you the operating parameters for each step. But if you are looking for background info, please see Rausch's "Phosphating of Metals" [on Amazon or AbeBooks] or another book on the subject. Good luck.

Regards,

A. Hi shiby,
The 7 tank system is the following below.
1)Degreasing.
2)Waterwash
3)Acid pickling
4)Water wash
5)Phosphating.
6)Waterwash
7)Drying
This is the common steps, but now it's going up to 11 tank system also.
Based on the customer requirement and the part condition it may extend also. For example, if parts are coming with more scale, you need to introduce the Descaling and water wash then it will become the 9 tank system from the 7 tank system.
That is only the example. We will do at X number of tanks also.

A. I differ from KANNAN. The 7 tank process is:

1) DEGREASING
2) Water wash
3) DERUSTING
4) Water wash
5) SURFACE ACTIVATION
6) Zinc PHOSPHATING
7) PASSIVATION.

This is the end of your search. You can go ahead with this and this is universal. It is a foolproof method and is being practised in TAMIL NADU for more than 10 years now.

3 replies, 3 different answers, which reinforces the point that the term really has no exact meaning -- it's just slang that refers to a zinc phosphatizing process which usually has about that many steps :-)

"Water wash" is sometimes employed between Phosphating and Passivation, which would make 8 steps. Drying is usually required, but I agree with Shankar that it usually isn't counted as one of the 7 steps.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Dear,

The 9 tank process is a gimmick; if all of your tanks operate at defined levels, you need not do descaling -- your derusting will take care of it.

Now coming to pH of the solutions,

DEGREASING : take 2 ml solution from the tank, with methyl orange [on eBay or Amazon] as Indicator, titrate with N/10 sulfuric Acid, this is your Total Alkali.

Now take another 2 ml, with phenolphthalein [on eBay & Amazon] as Indicator, The solution turns pink, now titrate with N/10 sulfuric acid, the END point is Pale Pink colour, the reading is your Free Alkali

Total Alkali
FORMULA ------------ = Should be between 1.4 - 1.8
Free Alkali if >1.4 add Degreaser
If <1.8 discard Tank

I have time for only this today. If you need further details on all tanks I can provide you upon request. Leave your request here, I shall do it then...

A. By just knowing Total alkali you shall know only the bath strength, only after you find a free alkali and find out its ratio between 1.4-1.8 your degreasing will be proper.

Otherwise though your total alkali may be 33 or more than that and if free alkali is also high (30) degreasing will not occur, even with a longer dipping time.

In another instance, if your total alkali is just 12 and free alkali is just 8.5 or so, then also you shall get a perfect degreasing with a longer dipping time.

I emphasize on the ratio between the total and free alkali you should maintain the ratio to get perfect degreasing.

Hi Shankar. You are exceptionally knowledgeable about this; thanks.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Naresh , The schematic flow diagram of zinc phosphating :

Start -> SURFACE PREPARATION : Pre-Cleaning -> Degreasing -> Rinsing -> Rinsing -> Pickling -> Rinsing -> Rinsing -> Activation -> ZINC PHOSPHATING -> Drag Out Rinse -> Rinsing -> POST TREATMENT : Passivation -> Air Drying -> INSPECTION -> Oiling -> Packing.

Regards,

A. Hi Ashok. When doing electroplating on steel, which demands a very clean surface, alkaline cleaning is used without exception. No one would even even attempt just acid degreasing, so there is no doubt that it is a less powerful cleaner and doesn't leave the surface truly clean.

But does acid degreasing leave it good enough for your phosphatizing process? It's hard to say; it probably depends upon what are the norms you speak of? Good luck.

Regards,

A. Hi cousin Baiju. The zinc phosphating bath operates at a pH of 3.1 to 3.4 per AES illustrated lecture "Theory and Practice of Phosphatizing", but optimum pH actually depends on phosphate concentration and other factors. The process is actually controlled more by "points" and free acid vs. total acid, rather than by pH. The final rinse should be acidulated. The passivation will probably be at a pH of 4-5.

Please try to tell us what you are up to with your question. It helps the readers because they are rarely interested in dry tabulations of values, presented without direction or context. And it can also help you because our readers can correct a poster's misunderstandings ... but they can't do that for you unless you express your theory, situation, or motivation. Good luck!

Regards,

A. Hi Donno. This is zinc phosphating you are speaking of? Any dried salts left from the phosphatizing process will cause the paint to blister. Warm water tends to rinse more quickly and effectively than ambient or cold water.

"Please try to tell us what you are up to with your question... "

Regards,

A. Hi Kalyansinh. Any of the books I mentioned earlier will answer a lot of your questions, and there are other other good phosphatization books as well, but the Metal Finishing Guidebook can at least get you started on those questions. Download it TODAY as availability is always shaky.

You didn't introduce yourself so we don't know what you are aware and unaware of, but if you don't know, it is customary to buy proprietary processes rather than to try to formulate such solutions from commodity chemicals; that way the vendor will also give you technical data sheets explaining how to check things. If you want to formulate your own phosphating solution you will need to make some decisions about the type you wish to make. Good luck.

Regards,

A. Hi Arun. There have been a number of attempts towards that approach in metal finishing, but they mostly have seemed to be a flash in the pan rather than an enduring viable solution. I personally don't think it's the way forward.

If you don't have room for 7-step zinc phosphatizing, maybe top notch 3-step iron phosphatizing with quality proprietaries is a more practical approach?

Regards,

A. Hola Alejandro. Perdon: No hablo espanol...

Iron phosphatization is a much thinner coating, and is not the equal of zinc phosphatization, but it usually considered sufficient or equal -- except for exterior use in the weather; even then it is often used, and it is cheaper and easier. If the component will be severely bent after powder coating, iron phosphating is actually often preferred for better adhesion.

Regards,

A. Hi Prit.

Thread 11573 is a good start on a discussion of the components in degreasing processes; and hydrochloric acid is the most common building block for derusting (but inhibitors or acid extenders are usually added), so thread 32106 is probably a good start for derusting.

But if you're working in a finishing shop it does little good to engage in theoretical discussions about saponification, wetting & emulsifying, de-flocculating, chelating/sequestering, buffering, and inhibiting because people have spent decades optimizing & testing these processes, and you need to purchase a proven proprietary solution, not waste time fooling around trying to mix commodity chemicals.

But if you are a student studying the chemistry, it doesn't help you to be told that finishing shops purchase proprietary cleaning formulations from specialist suppliers rather than attempting to home-brew them; you'll want us to name books and reference materials...

And if you are a chemist working for a supplier trying to improve a formulation, you'll need to start by building a handy library of patents as well as the books.

So please start with who you are and why you want to know, and then we can probably help you much more efficiently. Meanwhile, best of luck!

Regards,

A. Hi Navin
You can probably give the pipes a quick dip in your alkaline cleaner after phosphatizing if pH concerns you. But remember that phosphatizing is a pretreatment and is not intended to be a final finish.