Zinc Phosphating Problems & Solutions

A. Hi Shaikh,

You need to give more details about your operation before a precise answer can be given. Specifically, is this system spray or immersion? What are the contact times in the cleaner and phosphating stages? What exactly do you mean by a bad result?

From what you have written, I can do a little troubleshooting. You say that the phosphate coating comes out uneven. Are you getting adequate cleaning in Stage 1? Is the work coming out water-break-free? The metal surface must be free of all oils and contaminants before a homogenous phosphate coating can be deposited. You also said that the phosphating bath was turning brown. Is something being carried over from Stage 1?

Are you overflowing the rinses in Stages 2 and 3 sufficiently? Poor rinsing will make the problem of carry-over even worse. Remember, water is your cheapest chemical!

Is the phosphating bath made up properly? Are you breaking in a new bath by dissolving some iron in it? Are your total acid and free acid numbers in the proper ratio? Your chemical supplier will be able to tell you the proper ratio for your application. An out-of-balance phosphating bath will produce poor coatings.

You say that your phosphate solution has a catalyst; I assume this means accelerator. Is the accelerator present at the proper concentration? Not having the accelerator at the right concentration can also have a detrimental effect on the coating.

Would it be possible to convert the rinse in Stage 3 into an activation stage? Your chemical supplier should be able to provide you with a Ti activator, which will help in the formation of a uniform phosphate coating.

I have presented a lot of "what-ifs" here. Without knowing more about your system, it will not be possible to pinpoint the problem.

A. Hi,

From your scarce info I would say you are running a dip bath. The catalyst is probably sodium nitrite.

1. The brown colour is due to the build up of ferrous iron.
2. I would say that the temperature needs to be 60+ °C
3. You are either putting too much steel into the bath or you are not adding enough accelerator. If it is a liquid add 500 mls per load per 1000 liters tank capacity for the first few loads. Control with starch iodide test papers.
4. The low temp will favour the build up of ferrous iron hence higher additions of nitrite will be required. The new bath will work at first until ferrous iron builds up.

A. Shaikh,

May I suggest after your 2 rinses from the cleaner, dip your parts into phosphoric acid. This will pickle the metal, remove any unwanted oxides from your metal, and make the part more susceptible for the phosphate (which is really a glorified form of phosphoric acid). After that, rinse first in cold water then hot water. Next, after the hot rinse use a titanium activator to get more uniform coverage on your part. Next would be the zinc phosphate (165 °F): use 1 pint of sodium nitrite for every 400 gal. to remove unwanted iron (this is important... you want this bath to be free of iron, where you may be picking up the brown stains).

Then rinse in cold water, and last, rinse in a solution of chromic and phosphoric acid (160 °F). This last rinse serves 3 functions:
1. dries part off quickly
2. prevents short term corrosion
3. leaves the surface slightly acidic which is the best condition for paint or any other organic coating to adhere to.

A. Well, how I can understand your process have a problem on the final stage. The most of times the final stage it's occupied a bath called "sealing" that have a chrome solution and other stage (rinsing) to eliminate the excess of chrome.

The first two degreasings steps are made it with NaOH to clean the surface and eliminate the scale and rusting of steels.

If you have a final stage with this compound you can have a detrimental phosphating coating because NaOH to undo the crystals and the paint not have a good adhesion.

Check your final stage.

A. Hello, Nader. You have already identified what the principal problem is (carryover from one tank to another), and you know that it must be fixed.

Just one person's opinion: Don't talk yourself into wasting time on further "investigation" rather than confronting this difficult problem :-)

A. If bath solution shows brown color, it means your cleaning and rinsing is not proper. To avoid this, improve the rinsing and cleaning processes.
Also check that if you are adding chemical in old bath then check the pointage of phosphate and iron contamination.

A. HI, CHECK THE PHOSPHATE LAYER WITH A SOLUTION OF 20 G/L SODIUM CHLORIDE AND 7.5 G/L potassium ferrocyanide [affil links] .
DIP A PIECE OF FILTER PAPER INTO THE SOLUTION AND AFTER DRAINING EXCESS LIQUID , PUT IT ON THE PHOSPHATED PART. AND AFTER 20 MINUTES IF YOU OBSERVE BLUE SPOTS ON THE PAPER LOOK FOR PROPER SUPPLIER.
BEST REGARDS,
REZA.

A. Thougiq,

We had the same problem years ago and weren't able to find a solution. The only thing we could think of was there was excess water in the rubber and during the molding process, the water flashed into steam and rusted the metal. There weren't any blemishes or any indication of water in the rubber after molding, except the redness.

A. 1: -maybe you are adding excess of Accelerator in the Phosphating bath.
2: -This might be excess of sludge on the wire rods or you are maintaining the Lime/Borax temp on a lower side then specified.
3&4: -This might be acidic atmosphere/ because of detrimental Phosphate coating you're observing rustiness on the wire rods.
5: -I don't exactly get what you mean blue colour observed on wire rod after phosphating, kindly specify this observation found on any particular grade/ found in all grades.

Regards
Pravin H.

A. Hi Vishal. Is there any chance of abrasive blasting or tumbling? Chemically removing scale is difficult, and chemically removing burned-on oil can be next to impossible. But if you were able to blast these parts, it seems to me that the rest of your process is very robust.

Regards,

A. Mateus,

At a glance this looks like you have some sludge depositing on the work while it is being processed. This sounds like an older bath from your appearance description; have you been de-sludging regularly? It is an unavoidable by-product of the reaction that you need to take care of to keep the bath running optimally.

Sludge gets kicked up from the bottom when you add work to the tank and it can stick to parts, causing powdery coating in random spots, thus resembling stains or blotches.

A. Hi Mateus,
Streaking on your phosphated panel is due to:
Poor rinsing after HCl pickling
Dip time in Ti activator should be 2 mins
Accelerator titration should be 2 to 4
Increase nitrite
Temperature is fine and TA is fair. Increase immersion time

Hope this helps.

? Hi Abdul. Are your parts the same 'silver grey' as the photos from Mateus? Please mail some photos to and tell us what you find objectionable. Are your parts made of aluminum or of steel?

Regards,

A. Typically a yellow appearance after surface finishing is a result of iron contamination. Iron is required for this process to be properly "seasoned", but an excess can prematurely corrode causing the staining you see. Using the following titration you can check for that level. At above 11.0 "points" a decant is typically necessary:

1. Remove 10 ml sample of operating solution
2. Add 5 - 10 drops of 50% Sulfuric Acid
3. Titrate with 0.2N potassium permanganate [on eBay or Amazon] (KMnO4)
4. The change to a pink/purple color is the end point
Since the pink color will fade, titrate until the color persists.
5. Number of ml of 0.2N Potassium Permanganate = "Points Iron"

A. Hi Vea. Unfortunately I haven't really learned much from that photo :-(

But most finishing processes, including phosphating and its associated cleaning and activation, do not have magical powers to penetrate underneath the rack contact points. At best we can minimize rack marks, we cannot eliminate them. You may need to process in barrels, find a different racking point, move the part on the rack mid-process, or whatever.

Luck & Regards,

A. Hi Mohd. Between this thread, topic 10242 and topic 50266 you will see well over 100 Q&A's about zinc phosphating defects ... and we have many more threads about zinc phosphating defects, operating parameters, etc., as well :-)

Please send photographs of the defect to mooney at finishing.com for posting here, and supply as much about the operational parameters as you can, and hopefully you can get some help. Unfortunately, the cause of craters in the general case could be virtually anything, starting with inadequate cleaning.

Luck & Regards,

A. Hi Sahar. I Brown suggests below that iron contamination may be responsible for this. Please try to follow his/her instruction. But if it doesn't seem to be your situation, then send us photos for posting here and a description of your pretreatment sequence. Thanks.

Luck & Regards,

A. Hi Dayanidhi. I think it means that you are not doing the de-rusting properly or in the correct sequence. If you have a serious rust problem on the parts your sequence should probably be: de-rust (or maybe abrasive blast), alkaline clean, MILD activation, phosphating. If you are doing alkaline clean, STRONG derust, phosphating I think patchy phosphating could result.

Please try to flesh out your inquiry with some details.

Luck & Regards,