Free and total acidity in phosphating

A. Hi Gautam.
1. We appended your posting to a thread which explains the meaning and effect of total acid to free acid ratio, and how to check it.
2. Sorry I'm not personally familiar with any formulas for calculating the amount of sludge formation, although minimizing sludge formation is obviously very important.
3. You cannot successfully powder coat on top of loosely adhering sludge.

As a general learning strategy for those new to phosphating (I don't know if you are or aren't), we have hundreds of threads about phosphating if you search the site with carefully selected topics of interest, but ...
Learning from a well written book where the author spent months or years arranging things in proper tutorial sequence will save countless hours compared to endless bouncing back and forth between stuff you don't yet have the background to understand and stuff you've already read a dozen times, but ...
Getting help from a reputable supplier of phosphatizing chemistry is the best approach. Today's better phosphatizing processes are quite complex, with various accelerators, inhibitors, and trace ingredients; and the technical data sheet from the supplier will tell you about the optimum FA/TA ratio, P ratio, etc. for the particular process.

Luck & Regards,

A. Without knowing the details of the phosphate that you are running it is impossible to tell you whether your operating parameters are sufficient. What you need to let us know is what your pre-treatments are, the times, temperature and make up of your phosphate, whether it is proprietary or self-formulated and what coating weight you are aiming for plus if there are any post phosphate treatments such as a dichromate rinse.

I would not expect to get too long a life out of phosphate in salt spray, it is not designed as a corrosion resistant coating, but one that supports corrosion resistant coatings such as oils, preservatives and paints. What I normally see quoted for unprotected phosphate salt spray resistance is one and a half hours when tested in accordance with ASTM B117.

A. Dear Tudor Silaghi,

I do agree with Brian Terry that the phosphate coating is not expected to give a long life in salt spray test.

It is important to note that phosphate coating is only a pretreatment and not a finish. The corrosion resistance offered by phosphate coatings, when tested in accordance with ASTM B117, will be limited to only a few hours.

The free acid, total acid, free acid to total acid ratio and, the pH of the phosphating bath depend on the formulation and the type of surface to be coated. Excess free acid will lead to heavy metal dissolution and affect the conversion ratio.

I am not sure about the type of rubber material that you are using in your assembly and to what extent it can withstand in the phosphating bath at 72 °C.

A. Two things come to mind: (1) 72 °C is not hot enough, (2) a ratio of 7.5/1 is good, however, that ratio must be determined freshly right at tank side when you are ready to put the work in the tank. You cannot titrate on Monday and phosphate on Tuesday, the free builds very fast in an empty solution. Take the burettes to the tank side, start heating, when you get to about 180 °F, titrate, adjust the T/F preferably with manganous carbonate (do not put that nasty sodium in the solution), by the time you do that the solution will be up to 195 °F, put the work in, and I do not like over 200 °F. But 72 °C (161 °F) is not hot enough.

A. Hi.
I believe, in phosphating, when titrating for Total Acid (TA) and Free Acid (FA), the mls of the titrant (either 0.1N or 1N Sodium Hydroxide ) refers to the points. 1 ml of titrant = 1 point.
Good Luck.

A. Hi Nick,

Not from pH, but from titration, you can measure free and total phosphate points. You use two different indicators so your "final point" cuts the titration in pH=9 (phenolphthalein [on eBay & Amazon] ) or pH=4,5 ( bromophenol blue [on eBay or Amazon] ). In the first one you titrate only free hydrogen (or phosphoric acid) and in the second one free hydrogen plus monobasic phosphate.

Hope I could clarify something! Regards,

A. Hi Vijith. You are a student and in the learning process, so it is important that we not mislead you. Practical and effective phosphatization requires more than zinc phosphate and water; and people usually buy their phosphatizing solutions from specialist formulators rather than trying to mix their own.

As a good experiment for you then, can we suggest that you attempt to determine the free acid and total acid in the mixture you have on hand, and then people can go deeper into this for you. Get the solution to 180-195 °F as Robert tells you, and perform the free acid and total acid titrations following Eric's or Daniel's instruction. If it is unclear to you how to do those titrations, please request clarification on the specific issues that are still unclear to you.

If it's a bit of a struggle to clarify the vague areas, try your library for a book covering the subject. Biestek & Weber's "Conversion Coatings" or the ASM Metals Handbook Vol. 5.
have quite clear instructions.

Reading a chapter of a book, where the author has spent countless days working out exactly how to best present the subject in sequential, cogent, tutorial fashion is usually far less frustrating than surfing the web, drowning in a suffocating mix of snippets of stuff that you don't yet have the background to understand yet, thoroughly mixed in with stuff you've already read a dozen times. Biestek & Weber make it clear in minutes that dissolution of metal drives the precipitation of phosphate coatings, and the importance that free & total acid thus play in this. :-)

Luck & Regards,

A. Hi Sarang. The "why" of free acid vs. total acid checks is ... (quoting Biestek & Weber)
"Primary divalent iron, zinc, and manganese phosphates are readily soluble, whereas secondary phosphates are insoluble in water, except for sparingly soluble zinc phosphate. Precipitation of these phosphates at the metal surface results in the formation of a coating insoluble in the medium"

I'm not implying that I fully understand the fine details of this reaction and the required balance of free vs. total acid -- only that the overall principle is that dissolution of a small amount of metal from the surface consumes acid, thus changing the balance and causing insoluble phosphates to form.

Regards,