[Borek]

If i want to increase PH i could dilute solution by adding water. It also reduces TA & FA but ratio stays same.

Generally speaking yes, diluting the solution will raise the pH by a tiny bit, but that's not an effective way of changing pH here. You have a phosphate buffer present in the solution, one of the properties of buffers is that their pH almost doesn't change during dilution.

[pcm81]

If i want to increase PH i could dilute solution by adding water. It also reduces TA & FA but ratio stays same.

Generally speaking yes, diluting the solution will raise the pH by a tiny bit, but that's not an effective way of changing pH here. You have a phosphate buffer present in the solution, one of the properties of buffers is that their pH almost doesn't change during dilution.

I've read that NaOH and KOH are used as pH adjusters to raise pH. Although NaNO3 did a good job raising pH in my bath all on its own.

[qwerty009]

I did few more tests today. I took 500ml from MnCO3 based solution where i last made 3 pieces.

1) First i added 1g steel wool to it. It had hard time dissolving and it didn't dissolve completely. It looked like Mn phosphate coating formed on it (it was blackish). Since i last used like 1,5g to whole 9l patch then by adding 1g to 500ml iron content should be approx 0.2%. What i noticed is that light greysh deposition formed (iron phosphate?).
When i added test piece "2" to that solution there was active hydrogen formation. After 20min i saw no hydrogen formation so i took test piece out.
It lookes like bringing iron content up made Mn phosphate coating easier to adhere. Coating is even, nice black in color. On upper part of coating there are big crystals (notice them reflecting light back on photo). What i don't like about that is that they are relatively easy to scratch off. When coarse upper crystals are removed there seems to be even and strong coating of smaller crystals underneath.

2) I added 3g of NaNO3 to this 500ml solution where i just made test piece "2". When i added test piece "1" there was noticeably less hydrogen formation and bubbles were very small. After 20 min i took it out. Coating is dark grey, it's thinner then previous. What i've read about NO3 accelerators it that disadvantage is: "Reduction of FePO4 increases the iron content of the coating." Hence the grey color? Also more NO3 is added thinner the coating will be (i cant find source atm).

Picture: https://imgur.com/NhrYXGm




 
 

[pcm81]

I did few more tests today. I took 500ml from MnCO3 based solution where i last made 3 pieces.

1) First i added 1g steel wool to it. It had hard time dissolving and it didn't dissolve completely. It looked like Mn phosphate coating formed on it (it was blackish). Since i last used like 1,5g to whole 9l patch then by adding 1g to 500ml iron content should be approx 0.2%. What i noticed is that light greysh deposition formed (iron phosphate?).
When i added test piece "2" to that solution there was active hydrogen formation. After 20min i saw no hydrogen formation so i took test piece out.
It lookes like bringing iron content up made Mn phosphate coating easier to adhere. Coating is even, nice black in color. On upper part of coating there are big crystals (notice them reflecting light back on photo). What i don't like about that is that they are relatively easy to scratch off. When coarse upper crystals are removed there seems to be even and strong coating of smaller crystals underneath.

2) I added 3g of NaNO3 to this 500ml solution where i just made test piece "2". When i added test piece "1" there was noticeably less hydrogen formation and bubbles were very small. After 20 min i took it out. Coating is dark grey, it's thinner then previous. What i've read about NO3 accelerators it that disadvantage is: "Reduction of FePO4 increases the iron content of the coating." Hence the grey color? Also more NO3 is added thinner the coating will be (i cant find source atm).

Picture: https://imgur.com/NhrYXGm

Adding NO3 will increase pH of the bath, effectively decreasing your free acid. I am GUESSING that this also decreased solubility of iron phosphate + Mn-phosphate in the solution, resulting in slower deposition rate. What pH was your bath at after adding sodium nitrate?

[qwerty009]

I did some testing with knife blades (shiny smooth surface) as phosphate has harder time adhering there.

27.06
I used my MnCO3 based solution from earlier.

https://imgur.com/lNss3g3
"1" was only cleaned with acetone.
"2" cleaned with acetone, phosphated, phosphate removed with H3PO4 diluted solution, rinsed with water and phosphated again.
"3" Cleaned with acetone and half was kept in ~10% H3PO4 for approx 10 min (wanted to keep for minute but forgot). From acid straight to phosphate path.

28.06
500ml MnCO3 solution from 10l tank + 500ml i had from earlier combined + added 6g MnCO3 to reduce some acidity.
When solution was hot PH was 2.2 ish.
https://imgur.com/lu3uawX
left - cleaned with acetone.
middle - acetone + few minutes in dilute h3po4
right - acetone + half in dilute h3po4 but here i rised hot solution PH to 3 with NAOH.

Surface activation/preparation has big role in end result. 

[pcm81]

Received some metallic Mn today ($12/lb). As expected the reaction with dilute phosphoric acid is much more vigorous than MnO2 had. Observing the quantity of MnO2 which i had in dilute phosphoric acid for almost a week i am questioning the quantity of Mn that actually gets picked up by phosphoric acid from MnO2 in phosphating bath. I am going to try to run a bath with metallic Mn as the donor of Mn. More specifically i am dissolving Mn and steel wool in dilute phosphoric acid, will add nitrate before adding additional water and using it in phosphating bath.

[pcm81]

Did a very quick test just to compare MnO2 vs mettlic Mn as the source of Mn. The top nail is hot black oxide (so its true black) the middle nail is mangenese being sourced from MnO2 while the bottom nail is Mn being sourced from Mn metal. I believe that using MnO2 as a source of Mn is difficult and often we fool ourselves into thinking that we are depositing more Mn that the bath actually does.

[pcm81]

Pretty good write up on Mn phosphating bath here: http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1870-249X2013000400010

[qwerty009]

Did a very quick test just to compare MnO2 vs mettlic Mn as the source of Mn. The top nail is hot black oxide (so its true black) the middle nail is mangenese being sourced from MnO2 while the bottom nail is Mn being sourced from Mn metal. I believe that using MnO2 as a source of Mn is difficult and often we fool ourselves into thinking that we are depositing more Mn that the bath actually does.

How did you prepare nails prior to phosphating? I assume you did abrasive cleaning but anything chemically to activate surface? I've found that placing items prior to phosphating to dilute H3po4 solution and from there straight to phosphating path results better Mn adhesion.
Can you also make some tests with carbon steels? I've found that i can make satisfactory coating with mild steel test piece but some other stuff i need to coat get's poor uneven coating.
Better thing to test on is utility knife blade: https://images.homedepot-static.com/productImages/e97155cb-129f-4999-b0b0-fb13ac13de87/svn/olfa-replacement-blades-9281-64_1000.jpg - you can still use them after
If you can make good coating on that with pure Mn based solution i'm going to buy it also. But as i need to order it from other country i would wait for your results first.

[pcm81]

My cleaning process consists of mechanical cleaning, followed by acid etch in a mixture of sulphuric/phosphoric acids at pH of 1.2, followed by a cleaning in citric acid at ph of 2.2. Then a rinse in water. I need to do a better / proper job of surface activation prep, but so far i have not established a full process, just playing with some basic steps. The sulphuric acid is doing all of the heavy lifting in cleaning process, with citric acid cleaning up the sulphate deposits from sulphuric acid bath.

[pcm81]

Every time i try to post with an embedded image with or without IMG tags the post does not go through...

[Arkcon]

This one: 
http://www.magicmrv.com/Pictures/DPR/blade.JPG

That I resized and attached:

[pcm81]

Thanks for your help. I'd never figure out what was bugging it down.
Here is the funny part: that 2MB image is the reduced size version. Just checked the file on cameras card and the original jpg is 17.3MB.

[qwerty009]

It's made with metallic Mn solution (bottom nail and blade)?
It looks thin and uneven, even nail. Previous nail with metallic Mn looks better.
But how does it looks when it's wet? Imo when it's wet you get best visual info. 
If i'm not mistaken proper thickness Mn coating is only in bottom left corner of blade 0.5mm x 15mm strip.

[pcm81]

It's made with metallic Mn solution (bottom nail and blade)?
It looks thin and uneven, even nail. Previous nail with metallic Mn looks better.
But how does it looks when it's wet? Imo when it's wet you get best visual info. 
If i'm not mistaken proper thickness Mn coating is only in bottom left corner of blade 0.5mm x 15mm strip.

This is almost the same solution with metallic Mn as was used on previous nail. Actually this one has more Mn dissolved, 1g/L. The image with the blade is before oiling, so it is grey-ish. oil makes it darker almost black.

Here is a side note: the digital cameras will alter colour depending on lighting conditions (energy spectrum of incident light). White balance is meant to compensate for it, but does poor job. That is why i am including the black oxide coated nail to serve as a "true black" reference point in every image.

These images were straight out of the tank after they were rinsed in water and allowed to dry the goal was to see if the blade will receive phosphate, which it did. Still need to play with cooking time, this was 30 minutes., iron content and pH to optimise this bath. The purpose of this bath mixture was to see if darker coating can be achieved using metallic Mn than using Mn-Oxide; which it did, confirming that Mn-Oxide as a source of Mn, may be lacking in its capacity as Mn donor.