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How to do oxlate coating for stainless steel cold forging?

We are doing Oxlate coating to stainless steel parts for cold forging. For that we are preparing bath consists of 1000 liters R.O Water, 75 kgs of Oxalic acid, 40 kgs of sodium chloride, 3 kgs of sodium thiosulphate [on eBay or Amazon] Still the coating is not satisfactory and we are not having any checking method. Please advice whether the bath preparation is OK ? And suitable checking method.

NARAYANAN
FASTENERS - HOSUR, INDIA
2006

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2007

Is this a new process or a used bath no longer functioning properly? What is the recipe source? Was the oxalic acid used anhydrous or the dihydrate?

If the bath has never worked, try activating the SS in nitric-hydrofluoric pickle solution, rinse, dilute NaOH rinse (10%, at 45 C), rinse, immerse in the oxalate solution 5-15 minutes at 60 C, rinse, and dry.
Note: First store some unused solution in a glass bottle for later analyses.

If the bath previously worked, analyze and replenish ingredients. Also, D. B. Freeman in Phosphating & Metal Pretreatment [affil link on Amazon] mentions Fe(+2) should be no more than 0.4%. Any green color to indicate Fe(+2)?

Re analysis. I suggest the following:
1. Get a good quantitative analysis chemistry book.
2. Titrate for oxalic acid using 1.0N NaOH [1N NaOH on Amazon] , using either a pH meter or phenolphthalein [on eBay & Amazon] indicator.
3. Titrate iodometrically for thiosulphate using 0.1 N potassium dichromate. Hint: Use a known amount of dichromate solution as the sample, then titrate with bath solution in the buret, as if doing a Cr(+6) analysis, with starch indicator. Correct for oxalic acid & any Fe(+2) also oxidized by the dichromate.
4. Titrate for Cl(-) using 0.1 N silver nitrate solution, with potassium dichromate indicator (the Mohr titration).
5. Determine Fe(+2) colorimetrically [other ingredients interfere with redox titrations]. Use the natural green color of (Fe+2), or if too dilute, add 1,10-phenanthroline which forms a ferrous complex that strongly absorbs at a wavelength of 510 nm.

Best: buy a commercial product, e.g., OXICOAT from Heatbath. Learn to use and maintain their bath.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.