The Mad Plater
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Preparation of soluble zinc phosphate from ZnO
Well, this was most unexpected.
It appears that, after a few failed attempts, I have finally managed to prepare a solution of soluble zinc phosphate - which is one of 2 key
ingredients for the Zn phosphating of steel surfaces (the other being Zn nitrate).
The target compound, in this instance, is zinc dihydrogen phosphate; a Zn analog of the manganese dihydrogen phosphate (which I'm already familiar
with).
It's substantially soluble in water, given suitably acidic pH - as opposed to the totally insoluble Zn orthophosphate.
Of course, normally I just wouldn't bother with any of this, and just buy the compound from a commercial supplier, as is the case with eg. the Mn, Zn
and Ni nitrates.
But, since we can't have nice things, this compound - as well as the Mn analog - doesn't appear to be available locally, at least not in lab
quantities (~1kg).
Now, with the Mn dihydrogen phosphate, there are 2 feasible routes:
1. Reaction of MnCO3 with phosphoric acid (VERY fizzy - lots of foam!), or
2. Dissolution of MnO2 in a phosphoric acid-H2O2 mixture.
Method (1) is what I used to make my first "real" 1L batch of the bath concentrate, because I didn't have the cheaper method (2) properly worked out
yet at that time.
From now on, though, I'll save my (more expensive) MnCO3 for adjusting the free acid value, and stick to method (2) in the future. Cheaper,
and far less hassle.
And here we reach the crux of the problem with the Zn phosphate: cost.
ZnO is dirt cheap, even analytical grade. IIRC, it's even cheaper than MnO2 of decent purity.
On the other hand, zinc carbonate is hideously expensive - to the tune of 5-6x more than MnCO3.
Also, putting the cost aside for a moment, it'd be nice to not have to deal with the huge quantities of foam involved in the reaction between a metal
carbonate and conc. phosphoric acid, since it really slows things down an awful lot.
Several weeks ago, armed with a big bottle of 75% orthophosphoric acid, and half a kilo of analytical grade ZnO, I set out to investigate this angle -
of course, on a gram scale first.
Well, whatever I tried, it just didn't work.
The ZnO just didn't want to dissolve fully - only roughly half of it would dissolve, the remainder of it sitting unreacted on the bottom of the flask,
silently mocking me.
There was a ray of hope, when I managed to get it to dissolve fully in highly dehydrated acid (roughly the consistency of warm road tar).
However, instead it turned out to be the methaphorical light of an oncoming train in the tunnel  , since upon diluting the extremely viscous goop with water, a whole bunch of the insoluble Zn orthophosphate crashed
out almost immediately.
Looking back at it, it appears that I have made some totally stupid mistake in the stoichiometry at that time - because I decided to try it again
today, and to my amazement, it worked flawlessly! 
Here's what I did to make it work:
1. Weigh out 1g of ZnO (as a fine powder), and add it to a 100mL Erlenmeyer flask.
2. Weigh out 4g of 75% orthophosphoric acid, add to flask.
3. Start to reconsider that last part, just as the contents of the flask begin to get VERY hot, and the inside gets fogged up in a few seconds. (NB:
next time, do it in the opposite order!)
4. Use a stirring rod to break up the curiously hard, crusty aggregated ZnO clumps, which resulted from step 3.
5. Add ~15 mL of DI water, because the flask is far too big for the amount of reagents involved. Now I know what these ridiculously tiny 5mL
and 10mL flasks are for.
6. Boil vigorously for a few minutes, until the ZnO is completely dissolved. Most of the water was boiled away in the process, leaving only a few mL
of a slightly viscous, completely clear liquid.
7. After cooling back to RT, rediluted to ~100mL with DI water (resulting in a roughly similar conc. as in the Zn phosphating bath) - no change was
observed in the appearance of the solution, even after a few hours.
Now, all of this can only mean one thing: tomorrow, it's SCALE UP TIME! 1L scale! 
This time, though, I'll be adding ZnO to the hot acid, not the other way around...
[Edited on 8-4-2017 by The Mad Plater]
Current chemistry WIP:
1. Mn phosphating of carbon steels:
- the phosphating step by itself works perfectly fine;
- currently working on surface prep & activation.
2. Zn phosphating of carbon steels:
- on backburner ATM;
- current status: brand new bath concentrate is ready for first phosphating tests!
3. Electroless nickel plating (>10% phosphorus content):
- on double backburner ATM;
- initial results (on carbon steel) VERY promising;
- plating on aluminium needs a lot more R&D (surface prep - zincating).
4. Electropolishing of stainless steels:
- IT WORKS! At least "well enough" for now;
- the process parameters could use a bit more refinement.
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Corrosive Joeseph
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I had never herd of this this now - Please post more results and information when and as you can please
Fascinating - https://en.wikipedia.org/wiki/Phosphate_conversion_coating
https://en.wikipedia.org/wiki/Zinc_phosphate
/CJ
Being well adjusted to a sick society is no measure of one's mental health
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The Mad Plater
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Yeah, I find it rather strange.
The Zn phosphating process is decades old - the better part of a century, even - and yet there seems to be a paucity of meaningful data
available regarding it.
Now, of course there are all the proprietary formulations and so on, but given its age, shouldn't there be at least some useful information
available in the public domain?
As you've already found, this process involves the conversion of a soluble metal phosphate into the insoluble form at the surface of a steel
part.
(There seem to exist also (different) phosphate conversion processes for substrates other than steel, but I do not concern myself with that. Simply
not useful for my purposes.)
Put simply, this involves operating the bath at a careful balance of conditions, such that it's very close to the point where insoluble precipitate
starts to form spontaneously.
The specifics are of course different for the 3 major forms of phosphating (Fe, Mn or Zn), but the overall idea is the same.
If anyone wants to try this right away (it'll be many weeks before I can run any serious tests myself), knock yourself out.
The following is the formulation I came up with by reverse-engineering the MIL-P-50002B Type Z specification.
Concentrate solution ingredients:
Orthophosphoric acid, 75% - 250g/L
Zinc oxide - 56g/L
Zinc nitrate hexahydrate - 220g/L
Nickel(II) nitrate hexahydrate - 3g/L
The ZnO will dissolve with some difficulty (and it's quite exothermic!). Do NOT add the nitrates until the solution is cooled back down to roughly
<40C!
80mL of this concentrate makes 1L of phosphating bath.
This is different (more dilute) than the MIL spec, but it's the same ratio as for my Mn phosphating concentrate. Purely for convenience.
Refer to MIL-HDBK-205A for information on controlling and operating the bath itself. This is CRUCIAL.
NOTE: correct surface preparation before phosphating is ABSOLUTELY CRITICAL.
MIL-HDBK-205A is right on the ball in this case.
Don't even THINK about using acid pickles or alkaline dips, unless you have a VERY good activation bath figured out! (I don't, not yet at least)
Actually, do try this at least once on a small piece of scrap steel, just to see what a failed coating looks like. If it's very glittery and
feels like medium-coarse sandpaper, it's no good at all.
For sandblasting, use a fine grit at relatively low pressure, the surface should have a fine, satin-like finish.
Alternatively, on otherwise clean parts (free of rust, grime and other mung), dipping in (or rinsing with) volatile hydrocarbon solvents seems to give
acceptable results most of the time, too.
NB: MIL-P-50002B is clearly in error: the Zn % called out for in the Type Z specification is WAY off the mark - using the specified amount of Zn would
result in the bath TA/FA ratio being far too low.
I had to recalculate the required Zn % from first principles (the Mn % for Type M is correctly specified, though).
FWIW, another error is in Table 1 on page 32 - they totally screwed up the conversion to metric units for Type M, roughly by a factor of 2. This one
tripped me up but good at first!
Current chemistry WIP:
1. Mn phosphating of carbon steels:
- the phosphating step by itself works perfectly fine;
- currently working on surface prep & activation.
2. Zn phosphating of carbon steels:
- on backburner ATM;
- current status: brand new bath concentrate is ready for first phosphating tests!
3. Electroless nickel plating (>10% phosphorus content):
- on double backburner ATM;
- initial results (on carbon steel) VERY promising;
- plating on aluminium needs a lot more R&D (surface prep - zincating).
4. Electropolishing of stainless steels:
- IT WORKS! At least "well enough" for now;
- the process parameters could use a bit more refinement.
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unionised
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Don't forget that hot conc phosphoric acid attacks glass.
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The Mad Plater
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unionised, can you please elaborate?
It's not the first time I hear about this - but despite my flagrant disregard of this issue (the fact that I keep forgetting about it doesn't help),
it never seemed to cause me any problems.
Is it something like the corrosion of borosilicate glass by strong bases, in that it's "relatively" slow at modest temperatures, and would require
repeated/prolonged contact to cause significant damage?
I'd think it's much slower than that, since my analytical-grade (!) 85% phosphoric acid comes in perfectly ordinary glass bottles, and they
show no visible degradation even after 1-2 years of storage @ RT. And it specifies a 2-year shelf life, too.
I think these bottles might be regular flint glass, too - I don't think that they would be borosilicate at that price, unless the acid is about as
cheap as tap water.
Whereas, OTOH, I had a mason jar become corroded to hell and gone after just a few months, from storing a modestly concentrated (~50g/L?) solution of
sodium metasilicate (IIRC - but definitely NOT the hydroxide!). That was long before I knew about the whole glass-alkali incompatibility, mind you.
In any case, about the most extreme thing I did with the phosphoric acid, was boiling down ~25mL of it to roughly road tar consistency (!), slowly
adding a bunch of ZnO, and heating some more. That reaction didn't work (totally wrong stoich!), but the 100mL borosilicate beaker showed no visible
degradation afterwards.
Current chemistry WIP:
1. Mn phosphating of carbon steels:
- the phosphating step by itself works perfectly fine;
- currently working on surface prep & activation.
2. Zn phosphating of carbon steels:
- on backburner ATM;
- current status: brand new bath concentrate is ready for first phosphating tests!
3. Electroless nickel plating (>10% phosphorus content):
- on double backburner ATM;
- initial results (on carbon steel) VERY promising;
- plating on aluminium needs a lot more R&D (surface prep - zincating).
4. Electropolishing of stainless steels:
- IT WORKS! At least "well enough" for now;
- the process parameters could use a bit more refinement.
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unionised
International Hazard
Posts: 5102
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Location: UK
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If memory serves, it's only a problem at 200C and above
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The Mad Plater
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Ah, OK then. Seems to be consistent with observations, too.
As for boiling down the conc. acid to thick goo - I don't intend to do that again anytime soon. In fact, I don't normally have any need to heat any
mixture containing phosphoric acid beyond 100 deg.C.
Even the electropolishing mix (~30-50% phosphoric acid content) doesn't get even nearly that hot; normal operating temperature is ~70-90C.
In any case, the phosphating baths are rather tame, on the order of 2-3% phosphoric acid content. Would be essentially harmless, if not for the rather
unhealthy metal salts.
Current chemistry WIP:
1. Mn phosphating of carbon steels:
- the phosphating step by itself works perfectly fine;
- currently working on surface prep & activation.
2. Zn phosphating of carbon steels:
- on backburner ATM;
- current status: brand new bath concentrate is ready for first phosphating tests!
3. Electroless nickel plating (>10% phosphorus content):
- on double backburner ATM;
- initial results (on carbon steel) VERY promising;
- plating on aluminium needs a lot more R&D (surface prep - zincating).
4. Electropolishing of stainless steels:
- IT WORKS! At least "well enough" for now;
- the process parameters could use a bit more refinement.
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