Home /
Search 🔍
the Site
pub Where the world gathers for
plating, anodizing, & finishing Q&As since 1989


Need help making 6 stage iron phosphate bath last more than 6 weeks

April 5, 2010

My 6 stage wash system starts with a potassium hydroxide solution 2% up to 5%, then city water rinse. Next is my iron phosphate bath. Then city water rinse we dropped sealer bath do to carry over to R.O. bath. My question: my iron phosphate only lasts 5 to 6 weeks before I need to recharge. I run 4.30 to 4.60 on pH in my iron phosphate with a 5% charge. The steel I run varies from 16 ga to 18 ga to 20 ga CRS and 20 ga SS.

p.s. I am getting zinc in the waste water; is this from cheap SS?

Robert sumrall
employee at casket factory - Richmond, Indiana

April 9, 2010

Hi Robert,

A properly maintained phosphating bath should last indefinitely. Your supplier will know the details of bath maintenance better than anyone reading about your situation on this site. But there are aspects that are common to all phosphating baths.

How often do you titrate this bath and measure pH? This should be done once per shift at minimum. If you perform these tests and add product or replenisher to the bath to keep it within spec, it should run indefinitely. Are the titrations and pH within the ranges specified by your supplier? If not, then there's the cause of your problem.

How often is the rinse in Stage 2 dumped? Ideally, this should be done daily. Many plants think they can save money by dumping weekly, but water is your cheapest chamical. If you have significant drag-in from Stage 1, the offending material will find its way into the phosphating bath and compromise its quality.

Hope this helps.

George Gorecki
- Naperville, Illinois

April 12, 2010

Unless I missed something, the iron phosphate sticks to the processed metal and depletes the tank of chemical, hence the need to recharge after a certain period of time(depends on amount of product treated)
Have a nice day.

Sheldon Taylor
Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina

April 15, 2010

I titrate every two hours. I have a constant drip at one gallon per hour. I do only dump stage two once a week. I also clean the slug out of the bottom of stage 3 once a week. My issue is a solid blue to gray patch. It can be a square to a extended line. If possible, will get a picture.

Robert Sumrall
- Richmond, Indiana

April 16, 2010

I not sure I understood the last part of that last response. There isn't any way that you should have to clean sludge out of your phosphate tank once a week, I can tell you by looking at your first post again that you are running your phosphate concentration way too high, should be 1.5-2.0% max. Your sludge is probably excess phosphate dropping to the bottom of the tank like when you put too much sugar in the tea. Adjust your pH with a proper reducer, not phosphate concentration. You're aware that this system doesn't do anything to the SS except clean it?

Sheldon Taylor
Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina

April 22, 2010

Hi Robert,

It sounds like you're paying close attention to your bath's parameters. Are your pH and tirations within the ranges specified by your supplier? If so, the bath should perform without any problems.

George Gorecki
- Naperville, Illinois

May 11, 2010

George I am trying the dump of stage 2 every day. I cut back the overflow, but pH is still running 9.2 to about 9.7. on stage 2. It is still to early to tell about the sludge in stage 3, but is taking less pHc. Will keep trying and appreciate your input!

Robert Sumrall
- Richmond, Indiana


"Phosphating of Metals"
by Werner Rausch
from Abe Books

Affil. Link
Your purchases make finishing.com possible

Ed. note: We had an older thread on this subject, which we've now merged into this one in lieu of maintaining separately . . .


Phosphate concentrations in iron and zinc phosphate baths I am interested in the phosphate concentrations of iron and zinc phosphate baths. I am under the impression that the phosphorous is consumed so that there would be a range from a fresh bath to the time it is discarded. Am I right? Any help would be appreciated

Tom Mort


Dear Tom, The components of both Iron and Zinc Phosphate baths are consumed fairly evenly while processing. Chemical replenishment (i.e., Additions) replace the consumed components. In most cases Iron baths are discarded only to remove mud & particulate sludge, in most cases with proper filtration, Iron Phos. baths do not require dumping. Zinc Phosphate baths which contain soluble iron are discarded to remove High soluble iron contaminant and by-product sludge. Zinc phosphate baths that operate on the "Zero-Iron" side can be maintained much like an Iron Phos bath. Simply filter or remove sludge and try not to discard. I hope this information proves useful.

Dick Sarlund
finishing.com is made possible by ...
this text gets replaced with bannerText
spacer gets replaced with bannerImages

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

Chemicals &
Consult'g, Train'g
& Software

About/Contact    -    Privacy Policy    -    ©1995-2023 finishing.com, Pine Beach, New Jersey, USA