[Borek]

If anything, I would look for some well soluble Mn(II) salt, phosphate doesn't look like one.

[pcm81]

I have very limited experience actually running a phosphating bath, just started messing with it very resonantly, but here is what I have learned so far:
1. You will have allot of white/grey sludge in the bath. It needs to be filtered. Here is a video showing just how much of this waste accumulates: https://www.youtube.com/watch?v=tvaCYUvxQGw
Sludge is phosphate combined with dissolved solids in water and some free iron.
2. Total iron content needs to be monitored and should be around 0.2%. At 0.6% iron content the bath is toast. You monitor it with potassium permanganate titration. Here is a video for iron titration: https://www.youtube.com/watch?v=kxMEeDoeH4E

According to literature i read the bath PH can vary from 1.5 and higher with 2.5-2.9 range producing the most uniform coating.

I tried adding Mn-oxide but i do not see any reaction with phosphoric acid. I also tried to ball mill the Mn-oxide and still see no visible reaction. I will next try  to use metallic Mn powder...

Total acid to free acid ratio of around 6 or 8 seems to be suggested in most text i've read with free acid being between 3 and 6 points. A point is a milliliter of 0.1M NaOH solution.
I tried mixing NaOH by weight, but it does not store well and since NaOH will have unknown fraction of moisture as a hydrate you really should standardize the solution you use for free/total acid titration vs a primary standard like KHP or Oxalic acid. I went with KHP.

You will have to titrate with KMnO for iron. KMnO solution needs to be standardized as well. Literature i've seen used Oxalic Acid to standardize KMnO. I'd like to know if i can use KHP in place of oxalic acid, since  have higher purity KHP available on hand.

[Borek]

I'd like to know if i can use KHP in place of oxalic acid, since  have higher purity KHP available on hand.

No. KHP doesn't work for redox titration.

[pcm81]

I'd like to know if i can use KHP in place of oxalic acid, since  have higher purity KHP available on hand.

No. KHP doesn't work for redox titration.

I suspected that. Was hoping... since some folks used oxalic in place of KHP for acid/base standardizations. Oh well, i guess i have to wait a bit for oxalic acid to ship in.

[pcm81]

If anything, I would look for some well soluble Mn(II) salt, phosphate doesn't look like one.

In all literature that i have seen so far the Mn donor was MnO2. Interesting to note that Mn3(PO4)4 is not soluble in water. Sooo, as much as i'd like to make Manganese phosphate and just add it to the phosphate bath, i am not sure that it would even prove useful since it won't dissolve in water.

I am guessing that mangenese dioxide reaction with phosphoric acid has to happen at the surface that is being phosphate treated, to then be immediately incorporated into phosphate deposit as it drops out of solution?

[qwerty009]

I did some testing yesterday - all day and in end i had some results.
At first i was trying to add ingredients to water by formula i mentioned earlier but it gave no results. Coating did'nt adhere. There was FA/TF ratio was 1:2 and there most likely was very little dissolved manganese ions in solution.
Test piece looked like this: https://imgur.com/aKWbcsx

Then i messed around with solution, added MnCO3 + H3PO4 (several times) total amounts approx 35g + 30g (to 500mil of water).  Ended up with FA/TA 1:8,5 and test piece was like that https://imgur.com/pBfLdp8
Altho it's nice small crystals it looks most part to be iron phosphate (grey color). There was either not enough Mn ions or test piece didn't like to e inside that heavy sludge.


What produced results was: I added ~70g of MnO2 + 80g H3PO4 (85%) + 100ml of water + 1,5g steel wool and boiled that concentrate for some time. Later i added some more water so there was approx 500ml total.
I let it cool and settle and took 150ml of clear liquid (consentrate) and added ~400ml of water. PH of that solution was approx 3. FA/TA 1:5.
I got nice even coating with that. Only thing is that crystals were little on the big side that formed. But i think that because it was sandblasted part (surface roughness) it contributed to that as well little bit. https://imgur.com/R24StqJ

Those are my findings from yesterday.

What i'm planning to do is find optimum ratio between H3PO4, MnO2 and water to make concentrated solution that i can later dilute and use as i need - i'm open for suggestions  .
Also some testing with accelerators. NaNo3 & HNO3
Does anyone know some conditioner that would make Mn crystals small and even?
 

[qwerty009]

I did one additional test with MnCO3.

i took 70g MnCO3, 110g 85% H3PO4, 150ml h2o, 1,5g steel wool. Mixed togather, boiled until reaction stopped (took quite a while).
I let it sit and i extracted clear liquid. Settement (<-spelling?) remaining was light grey.
I added 100ml of this concentrate to 850ml water + ~0.2g NaNO3.  For some reason white flaky settement appeared. https://imgur.com/tecwCGm

Added test piece and i left it there for 40 min. When i returned there was no bubbling remaining. Coating was very fine grain.
https://imgur.com/CPN2flw
PH was ~2,7 FA was ~4, TA 30 ratio 7,5. <- those are approx values i have 10y old uncalibrated Chinese ph meter.

[qwerty009]

I coated 2 things with solution i made last and even tho in test piece it was fine (fine grain but on thin side) then on test pieces coating was too thin (different steel). It is specially notable that when steel has low surface roughness then coating is extra thin.
I will try to mix MnCO3 and MnO2 solutions and see what it gives.

[pcm81]

The white sludge you ended up with in your last mix is similar to what i had and as far as i understand it needs to be filtered off. Your initial mix (the one that did not plate) had too much free acid. You should have FA around 4 and TA 24-40. The white sludge in my earlier experiments would actually partially dissolve when solution is heated up. Liquid would go from dull white/yellow to clear. Large sludge precipitate would settle.

Here are the baths parameters to play with:
PH between 1.5 and 4. Mine is 2.75
FA around 4
TA 6 to 10 times higher than FA
Iron content - around 0.2%. At 0.6% bath is about done.

I read that addition of Ni+ ions will help with grain structure, but i do not know quantity or preferred salt.
Nitrates are known as accelerators and bath stabilizers. Sodium nitrate as well as nitric acid are the sources of nitrates i read about.
Last night i started to try to dissolve MnO2 in dilute phosphoric acid. With some heating and active stirring (magnetic stirrer) i now have clear liquid with purple/violet tint to it; but most of MnO powder is still present. Hoping to use this as phosphoric acid replenishing tank.

[pcm81]

I did one additional test with MnCO3.

i took 70g MnCO3, 110g 85% H3PO4, 150ml h2o, 1,5g steel wool. Mixed togather, boiled until reaction stopped (took quite a while).
I let it sit and i extracted clear liquid. Settement (<-spelling?) remaining was light grey.
I added 100ml of this concentrate to 850ml water + ~0.2g NaNO3.  For some reason white flaky settement appeared. https://imgur.com/tecwCGm

Added test piece and i left it there for 40 min. When i returned there was no bubbling remaining. Coating was very fine grain.
https://imgur.com/CPN2flw
PH was ~2,7 FA was ~4, TA 30 ratio 7,5. <- those are approx values i have 10y old uncalibrated Chinese ph meter.

Getting a closer look at your final piece i see some red tint in the middle. Iron phosphate is yellow-ish and Iron-oxide (Haematite) is red. I am wondering about the iron content in your final bath... Do a KMnO titration for iron content. Should be no greater than 0.6%.

[qwerty009]

Now that i think about it i know where it came. When i was dissolving MNO3 in concentrated acid i added steel wool there too (should have added it later). In that strong concentrate it didn't dissolve completely. It turned red and that must have contaminated solution.
Regarding white flakes precipitate i noticed that it appears when PH increases.
 
pcm81 can you share how you prepare your solution + composition and what results you are getting?

Regarding source of Mn i found this:

The dissolved manganese cations required for necessary
component (A) may be obtained from any soluble manga
nese salt or from manganese metal itself or any manganese containing compound that reacts with aqueous acid to form
dissolved manganese cations. Normally preferred sources,
largely for economic reasons, are manganese carbonate and
manganese oxide. (If manganese oxide is used to prepare a
concentrate composition according to the invention, the
presence of reducing agent component (E) as defined above
is usually preferred, because without it the dissolution rate
of MnO in phosphoric acid is very slow. Reducing agents
appear to act in a catalytic or at least partially catalytic
manner to speed the dissolution process, inasmuch as the
amount of reducing agent needed to make the dissolution
rate of MnO practically fast is far less than the amount that
would be stoichiometrically required to react with all the
manganese present.)
Source: https://patentimages.storage.googleapis.com/6e/84/60/36a21d2dfe05b6/US5728235.pdf

[Borek]

Regarding white flakes precipitate i noticed that it appears when PH increases.

Trivial - higher pH means converting more H₃PO₄ into dissociated forms (H₂PO₄^-, HPO₄^2-, PO₄^3-), which form the precipitate.

Do you know what a solubility product is?

[pcm81]

Regarding white flakes precipitate i noticed that it appears when PH increases.

Trivial - higher pH means converting more H₃PO₄ into dissociated forms (H₂PO₄^-, HPO₄^2-, PO₄^3-), which form the precipitate.

Do you know what a solubility product is?

I can confirm this. I got heavy precipitate after my initial addition of sodium nitrate and it increased ph to 2.75.

[pcm81]

 
pcm81 can you share how you prepare your solution + composition and what results you are getting?

I am not sure if my solution qualifies as "a recipe worth sharing" I am still playing with chemistry of it, but basically I started with 80ml of 85% H3PO4 (my intention was to make 8L of solution). I diluted the 80ml of acid with about 800ml of water. I added about 15 grams of steel wool and 12 grams of MnO2 and let the solution sit for 24 hours. Grey precipitate formed. I decanted half of solution and dissolved in 4L of water. First set of test pieces turned out to be grey and non uniform. Ph of about 1.5-2 (going by memory) I added 92 grams on NaNO3, which pushed ph to 3.2. I then added more of the "acid steel wool solution" (the original 800ml mix) to get pH down to 2.6. This created grey coating. Interesting to note that prior to adding NaNO3 i could see allot of bubbles coming off the parts being treated. After adding NaNO3 bubbling stopped and new precipitate formed. My initial titration for iron of that bath revealed 0.06% concentration (but i was using uncalebrated KMnO solution for the titration. I weighed it out on a scale, but did not titrate to calibrate). I then added more steel wool to get iron content up to 0.22%. This is the solution I have now. I am considering this as a learning batch and playing with testing the iron as well as free vs total acid. At the moment my FA is 3.75 and my TA is 29.

[qwerty009]

NaNO3 interacts with H2 formed on steel surface (H2 bubbles are interfering with phosphate deposition) that accelerates phopspating process as i understand.
Regarding playing with acidity am i on right track?:
If i want to increase PH i could dilute solution by adding water. It also reduces TA & FA but ratio stays same. If i add MnCO3, it would react with free acid thus reducing it and increasing TA also PH would increase. To lower PH i add concentrated solution.

Some pictures from pieces i made 24.06
I made 3 pieces in 10l tank (all were together and same time).
Regular test piece (i coated it with oil before taking photo): https://imgur.com/Locd55B regular S235 steel
1 key for some old lock i made. It's also oiled, but notice its dark grey not black. Altho coating is nice uniform but in thin side, should be similar S235 steel. https://imgur.com/Z1goFNx (not best photo).
Machinist square <- don't have picture atm, but that took very thin coating. Of course it's some sort of hardened alloy steel - what i've read is that phosphate coating works best with regular mild steel.