Can We Chemically Black Duplex?

A few weeks ago a customer came to us with a number of screws that they wanted to be Chemically Blacked. They were not 100% sure on the grade of material, assuming it was Stainless Steel yet after trying both of our Chemical Blacking processes (for mild and stainless steels) neither would adhere and so we came to the conclusion that the screws must be either Duplex or Super Duplex. So why is it that Duplex can’t be chemically blacked?

What is Duplex?

Duplex Stainless Steels are used because of their high mechanical strength at lower weights and due to their lower Nickel content they have a more stable price than many Stainless Steel Grades. Grades 2507 and 4501 are regarded as ‘Super Duplex’ due to their higher corrosion resistance properties. The term Duplex comes from the fact that the metal has a two-phase microstructure which consists of grains of both Ferritic and Austentic Stainless Steels and herein lies our explanation on why Duplex Steels can’t be Chemically Blacked.

What is Chemical Blacking?

Chemical Blacking is a conversion coating where a protective layer is formed through a reaction with a Ferrous Alloy. As Duplex is an even mixture of both Ferrous and Austentic Stainless Steels, there is not enough material for this conversion process to take place and so the coating does not adhere to the part. Silchrome Plating are able to carry out Chemical Blacking for both Mild and Stainless Steels.

Why Choose Chemical Blacking?

As a conversion coating, Chemical Blacking has a very negligible effect on tolerances, it offers mild corrosion resistance on its own but when combined with Wax or Oil this corrosion resistance is enhanced. It produces a very smooth and appealing matte black finish.When compared to Painting or plating Chemical Blacking is more cost effective. Although Chemical Blacking can't be carried out on Duplex or Super Duplex, their process is suitable for Carbon and Stainless Steel Alloys.

Silchrome are ISO 9001 and 14001 certified and based in Leeds, West Yorkshire. We offer a wide variety of metal finishing options and can help you determine which process is right for your application.