CORROSION-RESISTANT BLACKENING PROCESS?

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WE WANT TO BLACKEN CAST IRON AND MILD AND SPRING STEEL PARTS. WE CURRENTLY USE SODIUM NITRATE TO COLOUR THE MATERIAL BUT ACHIEVE A BLUE FINISH.
WE ARE LOOKING FOR BLACKISH FINISH ALSO THE SAME SHOULD BE CORROSION RESISTANT AS IT WILL BE WORKING IN MACHINERIES.PLEASE HELP US TO ACHIEVE BLACK FINISH.

BALKRISHNA SINGHANIA
-MANUFACTURING SMALL MACHINERY PARTS. - KOLKATA, WEST BENGAL, INDIA

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You should try cathodic electro deposited coating. It will give you 20 microns of dry film thickness which can sustain 1000 hours of salt spray test (ASTM B-117 [link is to spec at TechStreet]. It will give a semi glossy black surface. For sample coating on your components, you can contact us. Hope this helps.

Gurvin Singh
Coatec India -

Mohali, Punjab, India

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A PRE TREATMENT PROCESS SHOULD BE TAKEN CARE OF FOR CAST IRON COMPONENTS. A DESCALING BATH CONSISTING OF 200 GMS \ LTS OF CAUSTIC SODA AND 20 GMS \ LTS OF POTASSIUM PERMANAGNATE SHOULD BE USED TO DESCALE THE CAST IRON AT 70 -80 DEGREES CELSIUS.AFTER THIS THROUGH RINSING AND HCL ACID PICKLING WITH INHIBITOR AT 1% ON ACID VOLUME SOAKING FOR A MINIMUM PERIOD OF 30 MTS IMPROVES THE SURFACE. NOW KEEP 1% CAUSTIC SODA SOLUTION AT BOILING TEMP. 70 DEGREE CELSIUS AND THEN GO TO THE BLACKENING BATH WHICH CONTAINS CAUSTIC SODA 650 GMS \ LITRE , SODIUM NITRITE , 200 GMS \ LTS SODIUM NITRATE 150 GSM \ LTS AND SODIUM CYANIDE 1 GMS \ LTS. THE TEMPERATURE SHOULD BE 135 - 145 DEGREES CELSIUS WITH EFFERVESCES BUBBLING OUT IS THE RIGHT TIME TO INTRODUCE THE COMPONENT FOR BLACKING AFTER 1% CAUSTIC HOT DIP. DON'T WIPE OFF OR RINSE AFTER BLACKING ( OXIDISING ) . FINALLY APPLY DIESEL 25% + CUTTING OIL 75% DIPPING SHALL IMPROVE THE SURFACE TO GIVE A SMOOTH AND BEST FINISHING. THERE IS NO NECESSITY FOR THIS OILING BATH HEATING.

C PRAKASHAM
chemical company - KOLKATTA, CHENNAI, INDIA