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PHOSPHATING

IN DIP PHOSPHATING, SEVEN TANK PROCESS, I AM FACING YELLOW STAIN PROBLEM. AFTER HOT SUL.ACID PICKLING, THE COMPONENTS GOES TO DIP HOT CLEANING. AFTER PICKLING COMP. SOME AREA LOOKS LIKE ETHCHED ( POLISHED). THESE AREAS GOT YELLOW COLOUR IN DIP HOT CLEANING. AND ALSO LOOKS YELLOW AFTER PHOSPHATING. HOW TO AVOID THIS PROBLEM. WHAT IS THE INCREDENT MAKE IT YELLOW IN CLEANER.

GOVINDAN.R
TI CYCLES OF India - Chennai, India

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You say you are acid cleaning, then alkaline cleaning. If you are not rinsing properly between the two stages and the acid is reacting (forming salts) in the alkaline, this could be leaving sulfur stains. Why are you not alkaline cleaning/rinse/acid pickle/rinse/phosphate/rinse/seal/oil, this is the usual sequence. The etched/polished you see may actually be an oily surface which has not been acid cleaned.

Wayne McKee
McKee Specialty - Mississauga, Ontario, Canada

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It sounds like a rinsing problem to me. Check water rinses and dwell times between each stage. Also check the hot H2SO4 is not drying out between processing and rinsing. Check Fe content or accelerator content of phosphate (if Zn) if Fe check free acidity.

Good Luck,

Alex Jennings
- UK

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