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CHROMATE CONC. TO PASS 90 HOURS SALT SPRAY IN G. I. SHEET

OURS IS CONTINUOUS GALVANIZING LINE. WE MAINTAIN PASSIVATION BATH CHROMIC ACID HEXAVALENT CONCENTRATION AS 5-7.5 % . OUR CUSTOMER REQUIREMENT IS 25 mg/M2 LAYER OF HEXAVALENT CHROMIC ON EACH SIDE OF SHEET AND PASSES 60 HOURS SALT SPRAY TEST. SO, PLEASE TELL US HOW TO MEASURE CHROMIC ON SHEET IN mg/M2 AND WHAT PASSIVATION BATH CONCENTRATION WE SHOULD MAINTAIN.

NITIN BHUSANGE
JAI CORP LTD. ,NANDED MAHARASHTRA, INDIA - NANDED MAHARASHTRA, INDIA
2005

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2005

In general (batch) galvanizing a sodium chromate concentration of 200 ppm (light yellow solution at 0.02%) to 1,000 ppm (light orange solution at 0.1%) is commonly used. By adding acetic acid ⇦this on eBay or Amazon [affil links] the chromate coating is virtually colorless. Drips of chromate on the bottom of the products are a pleasant light sky blue color. The pH of the solution MUST be kept at the proper level by titration (using a pH indicating dye; pH meters and paper are useless) using 1/100 HCl. pH adjustments are made with hydrochloric acid. The resulting chromate treatment easily passes a 40 hour salt spray test. Sheet galvanizing is more difficult to pass a salt spray test because the aluminum content in the zinc is so much higher. When the conductivity (due to zinc, ammonium, and chloride ions) reach a certain level the solution must be changed out. Chromium trioxide (CrO3)is NOT a good source of hex chrome because it badly stains the zinc coating.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA