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topic 12409

Black oxide more "reddish" than black


My company produces parts out of powder metal & we have a group of low-alloy steel (4650) parts that when black oxided, show a reddish tint on primarily one surface. The rest of the part is fine. The surface that doesn't take the blackening well is in contact with our alumina trays in sintering, so there is a chance that either this surface is picking up small amount of alumina, or that it's higher in carbon than other areas of the parts.

Could either of these conditions cause the oxide to be more reddish than black, and are there any suggestions?


Matt Bulger
Net Shape Technologies - Solon, Ohio



That reddish tint on your parts may be due to the temperature of the hot black oxide bath. Typically, these baths run @260-288F, and immersion time depends on the mass of the part to be blackened. Red tint usually is seen on surface when the caustic concentration of the black oxide bath is in excess. Another aspect of this problem you might want to investigate is the powder metal itself. Sometimes when blackening a sintered metal that is not impregnated, the caustic salts tend to bleed out afterwards. This could also be a problem, and is solved by going through a heated water displacing oil after the black oxide bath.

Brian Nardella
AJ Tuck Company - Brookfield


We have a problem when using hot blackening process to blacken cast iron material ( FC200 ). It did not turn out as black as S45C material.It is rather look like redish color.
Is there any solution to solve this color problem ?

Leong Kum Soon
machining shop - Thailand

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