plating, anodizing, & finishing Q&As since 1989
"Over dose of Nitric Acid in passivation"
My part is Stainless Steel 4 series. In the process it is sent for tumbling and then goes for passivation. After those processes, my parts are found to have rust in peel off surface. In the passivation, I use nitric acid. Will this acid bring this effect to my part if the acid is over dosed. Currently I got 2 tanks are using nitric acid which is mix with Sodium Dichromate (7.5kg) + Nitric Acid (100liter) + 150 liter DI Water. Another tank is Copper Sulphate (5.5kg) + Nitric Acid (100liter) + 150 liter DI Water. Can I know that is it my mixture is over dos? If over dos, will it bring the above mentioned situation towards me part?Vivien
Stampling - Johor, Malaysia
Ed. note: Full name please Vivien. Thanks.
400 Series stainless steels are not easy to passivate using nitric acid and dichromate. 400 series alloys generally contain high amounts of sulfur for free machining properties and sulfur is not removed from the surface by nitric/dichromate mixtures. Unremoved sulfur will give the appearance of rust spots during testing, especially bioling water testing.
There are commercially available pretreatment and passivation chemicals that will provide excellent passivation results for all 400 series stainless steels. These materials are environmentally friendly and non-hazardous for workers and waste disposal considerations.
We have been passivating large quantities 400 series SS with a non-nitric system with excellent results.
- Lake Zurich, Illinois, USA
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