Chrome / chromate pretreatment of steel for FBE/3PE

Hi, Fayyaz.

I know that chromate conversion coatings are used on aluminum and zinc surfaces, but I am not familiar with their function directly on steel. Could you please tell us your situation which motivates the question? Thanks.

Regards,

Q. Dear Sir
Thanks for your response.
We are 3PE coating applicator of steel line pipes ranging from DN 6 Inch - 48 inch.
After surface cleaning of steel pipes with shot/grit blasting, Chemetall Oakite 33 wash is done and then prior to main heater, chromate solution GARDOBOND 4504 PC or ALODINE NR 6012 is applied on blasted pipes prior to main heater. It serves as promoting adhesion of FBE coating with steel substrate.
Hope it clarify your concern.

A. Hi Fayyaz.

Now that you explain it, I do recall seeing chromic acid solution being used on steel wire to promote adhesion of a plastic coating for use in bra hooks and plastic coated paperclips, and similar coated wire products.

Thank you for refreshing my memory on this (it was decades ago). I understand that the acidity and oxidation power available from chromic acid is effective as a cleaning agent for anything missed by alkaline cleaning, without being corrosive. Unfortunately I don't know the actual chemistry of the reaction between chromate and steel surfaces except at this superficial level, and we both will have to rely on some generous reader to enlighten us :-)

Regards,

A. Hi Fayyaz. This product contains hexavalent chromium, which must be reduced to trivalent chrome for disposal. Usually, the pH is adjusted to about 4 (lower than this would be faster, but will release more smelly and noxious sulfur-bearing gasses) and then a reducing agent like sodium metabisulfite is added. Hexavalent chrome has a strong characteristic "yellow" to "amber" color, and turns light green to blue when it has been converted to trivalent. Once the chrome has been reduced, raising the pH will cause the chrome to precipitate out so the solids can be clarified or filtered to separate them from the rest of the water, allowing the water to be discharged.

Different countries and locales may have different regulations about the discharge though. Good luck cousin.

Regards,

Q. Dear Sir,
Thanks for your kind guidance.
Is it possible to burn chromate or dispose of by other means or simply drain by digging a deep excavation?

Please respond with your expert advise.

Regards,

Fayyaz

A. Hi, Fayyaz. No, burning the chromate will only cause carcinogenic hexavalent chrome in the atmosphere. A deep excavation was considered a viable approach 60 years ago, but not today -- it will contaminate the drinking water and eventually will have to be dug up and treated anyway. Please try to learn from the very costly mistakes that have been made in other locales. Good luck.

Regards,

A. The way I used to treat such wastes; I'd bring the pH to about 2.5 with sulfuric, add sodium metabisulfite until a negative reaction is gotten from the carbazide color reagent, add a few pounds of ferrous sulphate ferrous sulphate [affil links], then a few pounds of lime, with strong stirring. Then, bring the pH up to about 9.5, stir a few minutes, then add flocculant.