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topic 45662

Which is better surface treatment technology: "Shot blasting" or "7-Tank phosphating process"



A discussion started in 2007 & continuing through 2017

(2007)

Q. I am Dipak Thakkar working with an electricity distribution utility. All the electrical equipments are found rusted in saline atmosphere as we are close to sea in western part of India. Can anyone suggest as to which surface treatment technology is superior for electrical equipment, shot blasting or 7-tank phosphating process followed by powder coating?

Dipak Thakkar
Consumer - Ahmedabad, Gujarat, India


Phosphating of Metals
by Rausch



Phosphating Metal Pretreatment
by Freeman

(2007)

A. The 7-tank process, also known as phosphatizing, will impart greater corrosion resistance. Phosphating adds tooth, it minimizes galvanic hot spots, it offers a measure of galvanic protection, it protects in the event of a scratch, and it prevents filiform & undercutting corrosion. Shot blasting is fine when parts will be electroplated, but not if they will be painted. But you may also wish to add e-coating before the powder coating.

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


(2007)

A. 7 tanks process with good pretreatment facilities followed by powder coating is definitely better.

M P Jain
- PUNE, India



(2007)

Q. We are manufacturing MS drums i.e cold rolled steel, and for that we do degreasing, phosphating, water rinse and passivation before painting. But with this process we are facing severe metal corrosion under paint film, after only 3-4 days of painting. The paint gloss reduces drastically, and looks like 2nd grade barrel. The paint used is alkyd amino.
Could anyone give me the reasons for such type of corrosion and also how we can overcome the problem?

Amit Mitra
- Mumbai, Maharashtra, India


(2007)

A. Hello AMIT,

I propose a change in your method: (TRY IT FIRST)
Degrease (probably is not clean)
Wash Use Methanol or IPA (blow it with air jet or better dry it physically).
Then paint it.

Please let me know the good results.

Jose Castellanos
- Minneapolis, Minnesota, USA


February 28, 2012

A. Hi, Amit. I cannot agree with Jose that solvent wiping is better than phosphatization. It most certainly is not. I believe you should have your chemical supplier come in to get the pretreatment line up to snuff and tell you how to keep it there. Simply because you are going through through a series steps which you call degreasing, phosphating, and passivation is not a guarantee that you are actually achieving success in those processes :-)

Luck & Regards,

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


(2007)

A. Please note that heavily rusted ms material involves huge cost for 7-Tank process. Since, it is a pre-treatment process, and even the shot blasting is also a similar process, it is better you go for shot blasting. Shot blasting operation will cost about 25% of 7-tank process (Electrical Power, Manpower and spares cost). After shot blasting you may prefer spray painting, which will give heavy life.

SAMPATH KUMAR
- Hyderabad, Andhra Pradesh, India


(2007)

Now I think I see the problem. Some readers like myself think that Dipak wants to improve his original finish because the components rust quickly; other readers like Sampath think he wants a field repair method.

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


(2007)

A. As Ted suggested, phosphating followed by powder coating will be the avenue for you to follow. However, I will be more specific, you must use the zinc phosphate process followed by an architectural quality polyester powder (there are two qualities of polyester, industrial and architectural). You can be a little more specific as there are another two qualities of polyester to consider TGIC-free (Primid based) and TGIC-containing. If health & safety is not a consideration then TGIC-containing will outlast the TGIC-free types. Another thought, why not evaluate galvanizing prior to applying the powder or even try an epoxy powder primer -- both of these methods should be better than just phosphating alone.

Now to the other problem re- the MS drums. We have supplied a drum mfgr. for many years. They dispensed with liquid paint many years ago and changed over to powder. They only degrease (no phosphate) followed by coating the inner and outer surfaces with various colours of powder that are specified by their clients. If you wish to continue with the liquid system, I would suggest that you change over to a zinc phosphate (you do not say what you are currently using), rinse, then apply your liquid coating.
You may wish to inform us of your progress.

Terry Hickling
Birmingham, United Kingdom



TUTORIAL:

Phosphatizing is usually done in 3, 5, or 7 steps.

The 3-step process, which applies to iron phosphating rather than zinc phosphating, is a combination cleaning & iron phosphating step, followed by a rinse, followed by a final 'passivating' dip. The 5-step process would usually be cleaning, rinsing, zinc phosphating, rinsing, passivating. The 7-step process is discussed in the thread.

August 26, 2011

Q. What is seven tank powder coating process and what it its importance?

Nehit Vasavada
buyer - Gandhinagar, India


February 28, 2012

A. Hi, Nehit. It's not a powder coating process, it's a zinc phosphatization pretreatment process consisting of 7 steps. Although there may be some debate and variation over those steps, generally they might be cleaning, acid dipping, phosphatizing, and a "passivating" post dip, with rinse stages in between.

Usually the more steps the better, in the same way that a 7-course dinner is preferable to shortening the process by putting the appetizers in the soup and the ice cream on the entree.

Regards,

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



February 28, 2012

Q. Is phosphating required even after doing shot blasting before CED or powder coating ?

SHOT BLASTING ALONE IS SUFFICIENT?

ABHIJIT BAPAT
- India

March 1, 2012

A. Hi, Abhijit.

It is required if the specifier specified it, and it is not "required" if not specified. However, as previously explained, a shot blasted surface is not the equal of a phosphatized surface.

Regards,

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



April 17, 2012

Q. Good day.

There is an ongoing debate where I work in regards to the proper process of shot peening and phosphating. Some believe you should phosphate first and then shot peen, but most of us argue that the phosphate has the potential to contaminate our shot peening media. What's the deal?!

Kind Regards

Adam Becklund
- Edmonton, Alberta, Canada


simultaneous April 19, 2012

A. There is no basis for debate. Shot peening will destroy the phosphate coating. Shot peen, then phosphate.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg,
      South Carolina



April 20, 2012

A. Hi Adam,

Shot peening should come before surface treatments. Firstly if you shot peen a phosphated surface you will end up removing the phosphate, secondly you will lessen the effects of the shot peening if it is not done directly on to the base material.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom


July 18, 2013

Q. We are running a powder coating plant for automobile components, we have cold 7 tank pretreatment line. Because of different problems such as Pollution, Process time, etc., we are planning to purchase Shot Blast machine. Now we want to understand the running cost difference between pretreatment line and Shot Blasting.

Arpit Tare
- India


July 2013

A. Hi Arpit. Although it is claimed above that shot blasting is less expensive, I don't think there is any way to compare the cost of shot blasting vs. phosphatizing without a careful study of volume, shape, potential for automation, amount of rust, etc. It is very easy to picture parts that can be phosphatized far more economically than shot blasted and vice versa. But if you can explain those factors for your case, perhaps someone can help.

opinion! As for pollution, remember that the purpose of pretreatment is to prevent pollution represented by early corrosion of steel components!

To try to judge "pollution" by milligrams of phosphate released to the environment, while ignoring tons of wasteful, unsustainable corrosion, and the environmental cost of re-mining & re-shipping; re-smelting & re-shipping; re-manufacturing, re-painting, re-packaging, re-shipping & re-merchandizing; and land filling of corroded components would be beyond ridiculous.

We've known for 75 years how to easily give outdoor steel components a life of 15-20 years & up very easily; but lately Chinese crap is filling the world's landfills as we commit the major environmental crime of replacing this garbage every 3 years or so. The real polluters, the environmental villains, are those who don't practice effective pretreatment regimens. Phosphatizing is far superior to shot blasting. If these parts are for interior use on autos they might, maybe, possibly be okay with just shot blasting, but if they are for exterior or under-hood use it sounds very doubtful. Phosphatize them! Best of luck.

Regards,

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



October 28, 2013

Q. Hey, I am a manufacturer of metal 4x4 bumpers. When I get my powder coating done after all the types of proper 7 tank process is being done, after a few days it starts to catch rust which shouldn't happen once powder coating is done. So I want to know how does shot blasting or shot peening overcome phosphating?

Darshil Mehta
auto tech - Mumbai, India


October 30, 2013

A. Hello Darshil. I don't think powder coated steel is really an acceptable finish for a bumper whether the bumper is phosphatized or shot blasted. Metal bumpers should be galvannealed steel, and then phosphatized and powder coated. But if the phosphatization and powder coating were done right, they certainly wouldn't rust in a few days.

Shot peening is not so much a preparation step for finishing as a process to reduce fatigue failure in components. What happens is that a stressed component that has not been shot peened can fail by getting a tiny crack in it that propagates and enlarges every time the load cycles. But if you shot peen it, you can put the surface into compression so that when a load is applied the surface stays in compression or only very moderate tension, so cracks do not form and propagate. A good example of shot peened components is the landing gear on aircraft.

Regards,

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



January 14, 2016

Q. What is the rust resistance after shotblasting before any pre-treatment process.

Mohakrishna Gaddipati
- Jamshedpur, Jharkhand, India


January 2016

A. Hi Mohakrishna. Zero! Flash rusting can occur in minutes.

Get it to phosphatization immediately.

Regards,

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


October 21, 2016

We are making 5 & 6.5 Gallon fuel tanks using 3 mm HR sheets. Before welding the panels are degreased and post welding the fuel tanks are shot blasted.

Post shot blasting tanks are cleaned by air pressure and then powder coated.

We are facing problem of painting peel off on the larger surfaces of tanks after 30-45 days and find the surface below the coating rusty.

Need help in assessing the possible cause of rusting & coating peel off.

Thanks,

Bharat Bhushan
- Delhi, India


A. Hi Bharat. In my opinion, and that of some other people on this thread, your process consisting of shot blasting followed by powder coating is no good. As a minimum, the tanks should be phosphatized before powder coating. Good luck.

Regards,

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



Can we skip cleaning and de-rusting before phosphatizing if the parts are blasted?

January 8, 2017

Q. Dear sir,
We are doing 9-tank phosphate process using annealing over components after shot blast; no oil/rust, is it necessary to do degreaser & de-ruster & activator before phosphating?

Prahalada RAO V K Karanam
- India, Karnataka,Bangalore

January 2017

A. Hi Prahalada. If I am understanding correctly you would need to do the activator because this is also a grain refiner pretreatment.

Whether you need to degrease and de-rust will probably have to be determined empirically rather than from first principles. But actually, you'll consume very little cleaning chemicals if the parts are already clean, and you'll consume very little de-ruster if the parts are free of rust and the de-ruster is inhibited, so there may actually be little value in experimenting to see whether those steps can be safely skipped anyway.

Regards,

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



August 1, 2017

Q. Hello,
I'm a manufacturer of hospital furniture in India. I use Cold Rolled Mild Steel material for hospital beds and other furniture. My question is, Is it advisable to sand blast the material before undergoing the 7-tank pre-treatment process (for better finish) which is then followed by powder-coating of the CR MS material?

D Panchal
- Gujarat, India


August 2017

A. Hi Panchal. Phosphatization of cold rolled steel without sandblasting is probably adequate, but it is easy to picture, for example, welding slag not being thoroughly removed and then breaking off after phosphatization and powder coating, or one fabricated bed being set aside for a few weeks and becoming too rusted to handle without blasting. Although metal finishing is a science, some aspects must still be resolved empirically rather than from first principles :-(

Regards,

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


August 12, 2017

Q. Hello Ted,

Your response is highly appreciated.
So let me get this straight ... if I am fabricating the bed and keeping it aside for a couple of weeks, then it is advisable to first sandblast the fabricated bed (since it will probably be too rusted), then undergo the 7-tank pre-treatment process which is then followed by a powder coating process.
Am I getting this correct?

D Panchal [returning]
- Gujarat, India


August 2017

A. Hi again Panchal. Yes, that is what I am suggesting, i.e., that your phosphatization process may not be capable of removing heavy rust. It's obviously hard to say from half a world away exactly how much rust your phosphatization system can reliably handle.

Regards,

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



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