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topic 53135

Need details of "the 7-tank process"

September 18, 2009

Q. Dear sir,

I am new in powder coating industry. in our company we powder coat HR/CR sheet. So please tell me the details of all 7 tanks.

powder coating employee - Pune, Maharastra, India

September 21, 2009

A. Hi, Shiby. A "7-tank process" is simply a process where parts are immersed in seven tanks sequentially. Normally, a 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. But "7 tanks" is slang more than specification, and the process could have 8 steps including a passivating rinse and DI rinse sequentially, or an "activator" before phosphate.

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" [link is to info about the book at Amazon] and Spring's "Preparation of Metal for Painting" [link is to info about the book at Amazon]. Good luck.


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

September 22, 2009

A. Hi shiby,
The 7 tank system is the following below.
3)Acid pickling
4)Water wash
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.

Kannan Boopathi
- Salem,Tamil nadu,India

September 22, 2009

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

2) Water wash
4) Water wash

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.

Shankar Omprakash Khandelwal
- Coimbatore, India

September , 2009

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 :-)

I have to differ from Shankar because "water wash" is surely required between Phosphating and Passivation, which would make a minimum of 8 tanks even with a dry-in-place / no-rinse passivation. Drying is usually required, but I agree with Shankar that it usually isn't counted as one of the 7 steps.


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

What is the 9-step Zinc Phosphating process?

October 9, 2009

Q. Dear Sir,
I want to know about the detail process steps of 9 Tank process of metal sheet. Please also mention the pH of the solution used for processing.

Rajkumar Pramanik
Proprietor - Kandi, WestBengal, India

October 12, 2009

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 as Indicator, titrate with N/10 Sulphuric Acid, this is your Total Alkali.

Now take another 2 ml, with phenolpthalien as Indicator, The solution turns pink, now titrate with N/10 Sulphuric 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...

Shankar Omprakash Khandelwal
- Coimbatore, India

November 11, 2009

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.

Shankar Omprakash Khandelwal
- Coimbatore

Q. Hi Shankar. You are exceptionally knowledgeable about this; but as far as I know, a DI rinse is necessary when doing phosphating for electrocoating. I don't see that in your universal method? Am I in error or is it not universal?


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

January 13, 2014

Q. I need schematic diagram and brief details of Zinc phosphating process.

Naresh Darshini
- Hyderabad, Andhrapradesh, India

May 8, 2014

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.


Surya Narayana
Process Engineer - Tumkur , karnataka , INDIA.

January 13, 2014


PRETREATMENT - Mumbai, Maharashtra, India

A. Hi Ashok. When doing electroplating on steel, which demands a clean surface, alkaline cleaning is used every time. No one goes with acid degreasing, and there is no doubt that it is a less powerful cleaner. Is acid degreasing good enough -- well, what are the norms you speak of? Good luck.


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

pH of Every Tank in a Zinc Phosphate Line

July 19, 2014

Q. Dear Sir,

Please give me information of pH of chemicals:

1. Degreasing + De rusting.
2. Water rinse
3. Activation
4. Phosphating
5. Water rinse
6. Passivation

Baiju Sathavara
Doing business - Ahmedabad, Gujarat, India

July 2014

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 of a chart of values, presented without direction or context. And it can help you because sometimes our readers correct a poster's misunderstandings ... but they can't do that for you unless you express your theory, situation, or motivation. Good luck.


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

November 7, 2014

Q. What is the effect of post-phosphating rinse with neutral pH water (pH ~ 7) at ambient temperature?
Does the rinsing temperature have any effect?

- Batam, Indonesia

November 2014

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... "


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

November 20, 2014

Q. Hi all

I wanted to know how to control the pH of conversion coating bath in pretreatment process for powder coating as it is one of the important parameters in maintaining the bath.


Snehal Patil

September 21, 2018

Q. Dear sir we want to know about how to make chemical of seven tank process, and how to check powder and its rinse, and technical process of seven tanks.

Kalyansinh devda
- Ahmedabad,Gujarat,india

September 2018
Digital version

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

A. Hi Kalyansinh. Either 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 seems to be getting 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.


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

October 14, 2018

Q. Hello everyone I own a Powder coating company. My problem is I don't have enough space for seven tank process so I have an idea to keep the seven tank in roof top and single tank in working area. My idea is to bring the chemicals one by one to the tank which I keep in the workplace. Will this technique work. I can fit NRV so mixing for chemicals can be very very less.I need to know will the process work?

Arun bhabu
- Tamil nadu, India

October 2018

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 viable economic solutions. I personally don't think it's the way forward. If you simply don't have room for 7-step zinc phosphatizing, maybe top notch 3-step iron phosphatizing with quality proprietaries is a more practical approach.


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

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