Does phosphating hurt stainless steel?

A. Hi M.F. As a rule, stainless steel cannot be successfully phosphatized. The metal is slightly attacked in phosphatizing, so it probably is correct to say it is "detrimental". However, the degree of attack is probably very low (phosphatizing tanks are made of stainless steel, and last decades). So we'd need to know a lot of details before being able to hazard an opinion as to whether it is actually harmful in a particular case.

A. I hope someone else can shed some light on this, Marcel, because I have never heard of such a problem, and I am having difficulty envisioning how it could be harmful -- but I can't swear that it isn't :-(

A. Hello Mr MFK,

There is no issue in painting SS tanks with a zinc phosphate primer [on eBay or Amazon] or a zinc chromate paint/primer [affil links]; the SS will not be attacked and the paint will serve well.

However, why not use a self etch primer instead first; they bond well onto SS and even onto nickel - chrome plating.

Follow up the self etching primer with a PU paint for a lifetime solution.

A. I don't know of any way that S.S. can be phosphated.

A. Rusting of stainless after sandblasting is usually due to iron contamination of the sand -- from cleaning steel, cast iron grit, etc. Use clean sand or else follow by cleaning and passivation per ASTM A380 .

What is the composition of Chemlok? Etchants, especially those with ferric chloride [on eBay or Amazon], can cause rusting.

A. Why use stainless if you're going to alter the surface? Why don't you use a cheaper material if you have to go through so much prep and are destroying the mechanical properties of the stainless in the process?

A. Cathodic electrochemical treatment can be used to deposit a phosphate based coating on stainless steels. The electrolyte solution consists of a zinc salt, phosphoric acid and an accelerator. The deposit on stainless steel will be a zinc - zinc phosphate composite coating.

A. A mild Nitric acid wash will clean SS quite well. Please rinse well afterwards.

Hi, Maruti. Sankara has answered your question, but each time people ask it, we all ask in return "Why?" :-)

So please return the favor by telling us why you would want to phosphatize stainless steel. Thanks!

Regards,

Q. Phosphatizing is our client requirement. However, at this stage we do not have any details of phosphatizing. Hence I am asking if there is any typical cycle for an equipment with SS304L material.

A. Hi again Matuti.

Okay, then we must ask: "Are you sure that your client knows what they are doing?" :-)
… because this situation introduces the frequently repeated question of who is supposed to be the expert.

If an aircraft or automobile manufacturer specifies a process, it is your obligation as the plating shop to simply do what they ask because they are the experts -- although it might still be prudent to inquire about it if it sounds off the wall to you. At the other extreme if a consumer asks you to process a part, you are the expert that he came to and relied on; so if he asked for something stupid it's your obligation to educate him. In your case, as we've already said, we don't understand why your client would want to phosphatize stainless steel, and suspect that it may be an error or misunderstanding.

We know that phosphatizing of 304L stainless steel can't be done via the process commonly recognized as phosphatizing, but that a zinc-zinc phosphate coating can be achieved through a special electrochemical process explained by Sankara. Good luck in talking to your client about it.

Regards,

A. Yes, pickling will produce a more chemically active surface which will promote faster and better phosphate coverage. I'd suggest you try:

1. Clean in elevated temperature alkaline cleaner.

2. Double rinse in clean water, ambient temperature.

3. Acid pickle, 20% HCl, ambient temperature, 1-3 minutes.

4. Double rinse

5. Phosphate, concentration and temperature as recommended by supplier. Pre-activation as/if recommended by your supplier.

Time depends on how thick you need, but likely 5 - 10 minutes.

4. Rinse, Rinse, dry. The second rinse can be heated to promote drying.

A. No, it is common info that acid pickling will cause a coarser, rougher and a skipping crystal. Is your "activator" a titanium oxide powder that is heated and vigorously agitated? The titanium oxide laying on the steel causes an increase in the cites of crystal nucleation and results in a finer grain.

 Bob Probert and I will be interested to hear what you learn from a brief HCl pickle.

A. Zinc can lead to Liquid metal embrittlement in Stainless steels so is not recommended to Zinc phosphatize SS.