Zinc Phosphating: Improving the Formula

A. Hi MickIT. There are hundreds of readers for every poster, and we don't want them lost or misled so bear with me if you already know this stuff ...

Although the phosphating step per se is a single step, the overall zinc phosphating process is usually 7 steps. So once you have the formula you still need to do the other 6 steps as well, which include alkaline cleaning, acid activation, grain refinement, and passivating rinse.

In the developed world it's quite unusual to try to formulate your own solutions; the process is usually bought from a supplier who markets the process as a proprietary. Rather than asking a licensing fee, they recoup their R&D costs 'by the gallon'.

Biestek & Weber (in the unlikely event you can find it) has a 6-page subchapter just on room temperature phosphating. According to that source the ideal zinc concentration is 22 g/l, but the pH must be held within a 0.1 range, but the required pH varies depending on the exact temperature, and is probably somewhere around 2.7; if zinc oxide can't keep it from dropping, sodium hydroxide in the form of drain cleaner is surely available. Commercial products also contain accelerators of nitrate + nitrite, chlorate, or copper salts, and significantly higher concentrations are suggested for cold phosphating than for hot.

It may not be possible to formulate a successful zinc phosphating solution from just zinc, zinc oxide, and phosphoric acid -- it might be better to do simple iron phosphating, which requires nothing behind a good cleaning and phosphoric acid :-)
... but we've appended your inquiry to a thread which offers several people's formulas.

Luck & Regards,

A. Hi. Apparently 22 g/l of zinc is a pretty good number, but I'm not finding evidence in the literature that a useful zinc phosphatizing solution can be formulated from just zinc, zinc oxide, and phosphoric acid --let alone a room temperature process.

Without zinc added you would be doing phosphoric acid cleaning, which is of some value. Unless you can produce for yourself some evidence (adhesion tests, accelerated corrosion tests, etc.) that someone's addition of some amount of zinc improves things over straight phosphoric acid, my guess is that it doesn't :-)

Luck & Regards,

A. Dear inquirer,

Your problem may be because of improper ratio of basic three chemicals and it may further require some special additive like nickel, etc.

Thanks,

A. Hi,

Give a few more details. Are you processing by dip and is this prepaint or an oiled finish you are referring to? The formulation is far too simple for high performance.

A. Dear Mr Sara Abiri,

From your reply I must assume you are sealing the phosphate with an oil. If you not using any seal at all then salt spray resistance will be minimal. The ratio of free acid to total acid will be temperature dependent. The higher the bath temperature the higher the ratio. Thus at 50 deg or less a 10:1 ratio is possible but on heating to 70+ the ratio will drop to 6:1.

If you want a heavier coating you could try operating on the iron-side (no accelerator. Temp will need to be above 75 °C. If you require a finer crystalline coating you will require a Ti activated predip or 2 g/l oxalic acid predip.

A. Dear Ms. Abiri:

a. If you do not use grain refiner before the phosphating stage, you won't succeed to overcome the 24h Oily phosphating requirements.
b. 24 Hours salt spray is the requirement of heavy zinc phosphate coating Military standard, the coating weight of the Zinc Phosphate might exceed 10 grams per sq meter. You must check phosphating coating weight to see if you fulfill the requirement.

A. Sorry for the delay. If iron containing a 1:1 ratio of phosphate:nitrate is OK. For accelerated baths raise 1:2 to encourage autocatalysis of the nitrite accelerator. Your nitrate content is far too low. Careful with Cu as over-addition will adversely affect corrosion resistance. Just stick with nickel!

A. Hi again!

The nitrate content is usually higher than the phosphate content. At 70 °C some nitrate will produce nitrite autocatalytically and prevent the process building up in ferrous iron. If the process switches over from nitrite containing to iron, higher temperatures and longer process times will be needed to produce complete coatings, which will be of a higher weight.

As an example of ion contents for a 20 pt bath:
Zinc 6 g/l phosphate as P2O5 8 g/l Nitrate as NO3 15 g/l

Cheers,

A. Dear Ms. Abiri,

How do you do the "Salt-Spray" test? with or without primer (E-Coat, powder,...)?
As you know doing "Salt-Spray" test only on phosphated plates is meaningless!

You should do another test methods such as: "Cyclic Corrosion" test, "VDA",...

Best Regards,

A. Dear Ms. Sara. You had better use low zinc phosphating solution.

A. To respond to the first question, your formula is missing a calcium additive. Read up on the advantages of Calcium modified Zinc Phosphate processes.

A. Dear,

FA, TA are important factors in zinc phosphating. FA is 0.6 to 1 point, TA is from 14 to 22

A. Dear Mr. Nitin Patel,

There are two problems is in the formulation:

1. The basic percentages in the formula are imbalanced.

2. For cold phosphating additional chemicals are required to be used to give the bite in the absence of the heat.

Regards.

A. Dear Mr. Praveen,

The problem is in the ratio of the basic ingredients. Please inform what is the percentages you are working with.
Regards.

A. Hi, Vijaya

As you can see, we appended your inquiry to a thread which may already answer it for you. But please introduce yourself and your situation and then follow up if you have remaining questions. Thanks.

Regards,

A. Dear Mr. Saunther,

The composition includes Phosphoric Acid and Zinc.

A. Hi, Francisco

I don't think people will be able to guess the details of your current formulation solely from the fact that the result is white. If you want suggestions for improving it to a darker color, I think you'll need to say what your formulation currently is. Thanks.

Regards,

A. Hi Chikki. I'm not understanding what you mean by the inner surface of the upper part, but is trapped air preventing the phosphatizing and other solutions from reaching that area? If you are seeing any rust shortly after phosphatizing, that area has probably not been phosphatized.

Regards,

A. Hi Chikki,

Like Ted, I am not certain exactly what you are doing, but you do need to beware that zinc phosphating on its own will not prevent corrosion, the coating is a discontinuous crystalline coating and tends to be used for a base to paint, bond or put on some other type of supplementary coating.

If you are seeing immediate flash rusting then Ted may well be right and you are not getting complete coverage, possibly due to air entrapment. It may also be the fact that the surface is remaining wet for too long, so you aren't getting things dried before flash rusting starts, also if the rinsing is not sufficient the cold phosphate solution will also promote corrosion.

If you are seeing the rusting occur after a period of time it will be because the supplementary coating is not being applied in time or no supplementary coating is being applied at all.

A. Hi cousins. Please take this charitably -- my intention is to help you get the assistance you seek by telling you what I've learned by curating hundreds of thousands of Q&A's posted on this site for decades: please fully present the specific problem you are having, YOUR ACTUAL SITUATION!,

When questions are cast in the abstract, readers rarely answer: firstly, abstract questions involve a dozen ifs-ands-&-buts that don't apply to your situation but can't be skipped without misleading other readers; secondly, flashcard-style quizzes give the impression that you want others to rifle through textbooks and provide summaries because you can't be bothered yourself; finally, readers can help you resolve a specific situation, but can't instantly elevate others to where they become experts themselves, able to independently solve more generalized problems.

Thanks & Regards,

A. In zinc phosphate coating when I use zinc nitrate and phosphoric acid it works well; but when I use zinc oxide and nitric acid instead of zinc nitrate it doesn't work. I don't understand why anyone says use zinc oxide and nitric acid.

A. Dear Shafi:
Zinc oxide is used in the formulation. It reacts with phosphoric acid to make zinc phosphate. Zinc nitrate is not required.

A. If you are maintaining tank level control, then the reason would be that water is being added to make up for the volume of sludge that has been removed. This will dilute the bath and cause FA to drop.

A. Hi Revan. Sorry, I've never heard of pinhole defects in zinc phosphating, but I'm not sure if I am quite understanding you. Unfortunately, telling us that everything is in mid range, i.e., that there is nothing wrong, isn't really very helpful because obviously something IS wrong.
It sounds to me like either your cleaning isn't working or you are generating loads of hydrogen gas for some reason. If you have additional detail, or pictures, perhaps a reader can help. Good luck.

Regards,

A. Hi again, Revan. Zinc dissolves extremely rapidly in acids. Are you putting these galvanized straps through the same acid dip which you use to activate steel parts? That will certainly cause pinholes in the galvanizing if not completely destroy it.

Regards,

? Hi Ojas. What is your zinc phosphate formulation? Are you dipping or spraying? What coating weight are you shooting for? Thanks.

Regards,

A. Hi Ojas. Thank you for the data.

I was asking for it as the site curator because I felt it would be hard to go further without it. I am not actually a zinc phosphating expert, but my limited understanding of zinc phosphating (pending a better answer from another reader) tells me that efforts toward reducing grain size will reduce coating weight -- so please see Yehuda's suggestion above. For further detail on activator/grain refiner please see topic 14221, "Surface Activation for Phosphating. Grain Size Issues", especially P.M.Saravanan and TSN Sankara's outstandingly informative entries. Good luck.

Regards,

A. Hi Ojas!

The Zinc Phosphate solution to be used should be 3 % in water and maintain a temperature of 70 °C.
If you are coming with your own formulation then share it so that it can be corrected.

Also add Titanium based salt bath before phosphating to give fine etching and hence finer and low weight coating. The adhesion also will be excellent.

Please try this and share the results.

A. Hello,

My advice would be to use a grain refiner; this may be enough to get you to within your limits alone if that is not enough reduce the time in the phosphate bath until you get within your range required. I have just been doing a project on some small base plates and managed to achieve 3-5g M2 using a grain refine and only 1 minute in the zinc phosphate after 2 1/2 they were fully phosphated and I was achieving 17g M2 !!

A. Hello Pradeep!

Please tell us more about your process:

- Is it a barrel process?
- How do you process the parts? (degreasing, how many rinses, pickling, how many rinses, phosphating, how many rinses, sealing?)
- What shape are the parts you process?

I think we can help you if you tell us more.

Regards!

Daniel

A. Hello Rathin!

Do you formulate your own bath? If not, you should ask your vendor, as the products are balanced to mantain the relation between FA and TA in the operational values.

If you are formulating your own bath, my advice is to add one formula to replenish the drag-out and one formula to mantain the TA in process (adding product by parts/barrels/racks processed). The second formula should have more FA than the first one to mantain the relation. There is no magical formula, you should try and adjust it until you have the one that mantains the relation in your optimum range.

Hope you can solve your problem! Regards,

Daniel

A. Hi Ali. Although the page may be confusing, the answers are here, very insightful answers.

Because the thing is, it's easy enough to make a zinc phosphating solution with nitric & phosphoric acid and zinc oxide, but what most users are actually seeking is the composition which will work best for them in terms of corrosion resistance, salt spray, grain size, undercutting prevention, paint adhesion, minimal sludging, robustness against small process variations, freedom from toxic ingredients ... and which will work on their steel, cast iron, zinc, and aluminum products ... and will be inexpensive.

Thus the page introduces the ideas of controlling total acid vs. free acid, employing additional ions like calcium and nickel, and grain refiners like titanium, and accelerators, etc. In the West these days, shops do not try to formulate their own zinc phosphating solutions, but purchase proprietary formulations which are the result of decades of research into all those factors. Good luck.

Regards,

A. Dear,
It is recommended to add Di water in your bath about 50% to reduce 50% of your bath values. Then add about 20% sodium hydroxide aq sol in phosphate bath to adjust the free acid in between 0.6 to 1.0.

A. Hi Parag,

There are many many formulations for many many different uses, and there are a large number of vendors offering proprietary formulations that no one but them can tell you that formula.

I think that if you want to use a zinc phosphate bath, you should buy some proprietary phosphate from a vendor. If you want to sell a phosphate as you product, you should make your own formula by experimenting and getting information from old patents or some actual phosphate producer.

Hope you can make something of this recommendations! Best of lucks!

A. Hi Santosh. Regarding free acid vs. total acid, if you don't quite understand the discussion on this thread, then thread 47351, "Free and total acidity in phosphating vs. salt spray performance", and thread 31949, "Zinc Phosphating process control & analysis" may help clarify the subject.

There are several good books that will tell you what is involved in phosphatizing chemistry, including Biestek & Weber's "Conversion Coatings", Rausch's "Phosphating of Metals" [on Amazon or AbeBooks], Samuel Spring's "Preparation of Metals for Painting", and Freeman's "Phosphating & Metal Pretreatment" [on Amazon or AbeBooks].

Or you can do a search through patents.google.com to see what people claim are the critical elements, or scholar.google.com to see what researchers have determined.

Luck & Regards,