Metal finishing Q&As since 1989
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Purple/grey patches on blackened steel finish
Q. Sir,
We are doing hot alkaline blackening of steel on hardened components. We are regularly getting red, violet/purple color and smoke/grey color patches on the surface among black color. We have tried cleaning up the material thoroughly but not able to get a clean black finish. We are using Sodium Nitrite along with caustic soda [affil links] ( sodium Hydroxide) in water.
Kindly advise remedy for the process or if we need to add other chemicals.
KAPIL GUPTA- LUDHIANA, Punjab, INDIA
A. The red color is from copper contamination - you either live with it or dump the bath and start again.
The other colors seem to be coming for non-uniform conditions in the bath.
Try controlling the bath with a thermometer - find a temperature where it works well and then try to keep it near there.
The temperature is going to be very hot 120-150 °C (depending on the bath chemistry).
If it is too low add more salts, too high add water TAKE GREAT CARE DOING THIS - wear face mask [on eBay or Amazon] , apron [on eBay or Amazon] , rubber gloves [on eBay or Amazon] and have two people one on the hose adding water the other stirring with a steel paddle - as the water might lie on top of the bath then mix in all at once with a lot of steam and splashing.
Better still put a piped in supply of water to the back corner and have a triangle plate over the corner to reduce splashing. Put the tap away from the bath so water can be added without being next to the tank.
The other thing is to take care on how many parts are loaded at once - load too many and the bath chills and gives poor results. This is another time to get the thermometer out.
Lastly remember this stuff is nasty I know a few people who have scars from forgetting this.
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England
Q. SIR,
WE HAVE ELIMINATED ALL THE DEFECTS STATED ABOVE. STILL WE GET PURPLE PATCHES ON THE SURFACE , BUT THEY APPEAR AFTER 2-3 DAYS AND REMAIN PURPLE. WE ARE GAS CARBURISING OUR PARTS AND THEN SAND BLASTING TO REMOVE THE SCALE. OUR COMPONENTS ARE FULLY SCALE FREE.
KAPIL GUPTA- LUDHIANA, INDIA
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