Shot blasting vs. 7-Tank phosphating process

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,

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,

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,

A. Hi again Panchal. Yes, that is what I am suggesting, i.e., that your phosphatization process might 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,

Hi Prabeth. I think a better answer might be to phosphatize the pipes, in and out, and then paint the outsides. Iron phosphatizing may be good enough; zinc phosphatizing would be better.

Regards,

A. Hi Anil. Iron phosphating (3-step process) should suffice for components used indoors and protected from the weather. But the 7-step process is better for moderate to severe exposure for the same reason that a 7-course dinner is better than trying to make it into a 3-course dinner by putting the customer's salad into their soup and their apple pie into their coffee. When you are able to keep separate and thereby optimize the cleaning, derusting, grain refinement activation, phosphatization, and passivation steps, I think everyone would agree that it is always going to be a significantly better process.

opinion! If people worry about the environmental consequences of each ounce of phosphate while ignoring the consequences of re-mining, re-smelting, re-fabricating, re-painting, re-packaging, and re-landfilling each ton of steel every 3-4 years instead of it lasting 20 years, I think they are misunderstanding pollution & sustainability issues. It's not a matter of passing a salt spray and adhesion test, but of optimally finishing components based on decades of real-life experience which recognizes the many different types and causes of corrosion. In my own limited experience, if no galvanizing or electrocoat priming will be applied, iron phosphatizing plus powder coating will often not comprise a robust anti-corrosion sequence for outdoor exposure.

Regards,

A. Hi Devendra. The pros & cons of shot blasting vs. chemical pretreatment is mostly what the previous 33 entries on this thread have been about :-)

Please ask for clarification about anything which you did not understand or which appeared ambiguous rather than simply starting the thread over again -- because it exasperates the responders and is therefore usually the kiss of death to a thread.

- What are your parts, are they welded, what are their sizes, how heavy is the rust, hot rolled or cold finished?
- What environment will the parts see: controlled environment in an office, inside and out of the weather, lawn furniture, automobile components interior or exterior, etc.?
- Do you still have the necessary knowledge to operate your phosphating system properly?
- How much will the vendor charge you to shot blast them?

As already noted, the quality of phosphating is superior without doubt. The cost of shot blasting might well be less, but it's very hard to say without knowing something about your parts. Good luck.

Regards,

A. Hi cousin Wilhelm. There is almost never such a thing as 'best', and the most corrosion resistant processes ones are often rejected on the basis of cost anyway. If this iron is used inside a building, you really don't need best. If the shape of the castings is such that pore-free, even-thickness, powder coating isn't possible you may need electrocoating between the phosphatizing and the powder coating for outdoor exposure. If this is an architectural feature, you probably need architectural powder coating. That's why we ask for "your situation". You may also need blasting before the 'degreasing' whether you like it or not.

But for a quick answer, I think William Doherty's posting, 2 paragraphs above yours, covers most of the bases, including the need for a tight specification. Best of luck!

Regards,

A. Hi Mohan. Steel should always be phosphated. It is important for paint adhesion, to minimize galvanic hot spots, and to prevent filiform corrosion & undercutting corrosion.

Blasting before phosphating is required if the rusting & scaling is severe enough that the phosphating line cannot practically and reliably remove it. Good luck.

Regards,

A. Hi Janakar. Pretreatment is required, and zinc phosphating is best.

But you haven't yet told us who you are, or what you company does, or what the components are, or about their exposure and corrosion resistance needs, so it's hard to offer advice about whether you can practically do it in-house, or whether you must farm it out, or whether you must settle for "Plan B", a less satisfactory pretreatment. Good luck.

Regards,