Eliminate Chemical Etching on A316L 2

[gciriani]

I need to minimize the etching effect on the surface of treated A316L, which is cleaned with various electrochemical processes.

I work on a new treatment for stainless steel used for luxury accessories (high-end watch bands). The standard steel (Aisi 316L) has a nickel content (12%) which imparts the material a bright silver-white color when mirror polished. Unfortunately the treated material has a very slight yellowish appearance (invisible to the untrained eye), which is a no-no in the industry.

The treatment diffuses carbon under the surface, for 15-30 micrometers (depending on the treatment). According to the literature available, the carbon implants itself as FCC lattice. This strains the surface and imparts hardness. Unfortunately the material needs to be cleaned to give it back its lustre. One of these post-treatments uses a solution of HNO3 with traces of HF; HF may be needed because, during pre-processing for the hardening treatment, the steel receives a metal fluoride layer.

The surface resulting from post-treatment shows the structure of the crystals, which is detrimental to the mirror-polish finishing. Under the microscope the surface appears similar to what is observed after etching for crystal structure study. I was wondering whether there are chemicals that etch the surface in a more uniform way, avoiding the creation of micro steps between one crystal and the other.

 

[SMF1964]

What is the concentration of the HNO3/HF solution? Is this an electropolish process or a 'simple' acid cleaning? There are electropolishing processes that work on stainless steel, but they would probably remove the 15-30 micron deep carburized layer. You could also try some inhibited acid solutions if all you are removing is metal oxide from the surface.

 

[gciriani]

All I know about the concentration of the HNO3/HF solution, is that HNO3 is the main component and HF is only an additive. I believe that the cleaning is done by dipping only.

I'm interested in only removing the metal oxide from the surface; clould you please expand on your suggestion of using an inhibited acid solutions? Do you have any details?

 

[SMF1964]

For ferrous alloys, I've used 1 liter of HCl with 0.5ml of Rodine 426 inhibitor (Parker AmChem used to provide the inhibitor, and may still do so. At 0.5ml a dose, we've had our 250ml bottle for many years), heated to about 50°C. In your case, however, you'll probably want to avoid hydrochloric acid with the stainless steel.

 

[gciriani]

Was the HCl pure, 50% solution with water, or anything else?

 

[SMF1964]

Sorry. 10% HCl in water.

 

[EdStainless]

I would suggest that you not use HCl on stainless.
The nitric/HF is the standard pickling solution. You may want to look at using lower concnetrations and/or lower temperatures. A slight frosting or etching of the surface is unavoidable. When you pickle you are removing both the oxide and the chromium depeted layer under it. (when the Cr oxide forms it reduces the amount of Cr in the alloy that is in contact with the oxide)
You would expect to need to re-polish after pickling.
Are you carburizing for hardness or only for color? The type of abrasive and buffs used in final polish will have an impact on the color.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[gciriani]

A paper on the treatment mentioned nitric/HF at room temperature, but I have no idea what the respective concentrations should be, and the time. Any suggestion on this?

A sub-contractor who experimented with it used some commercially available pickling product (E286, AOA, Purinox respectively for three separate electrochemical trials); it subsequently used some form of tumbling with copper powder in substitution for the buffing. The possibility of doing with tumbling is important, to be able to do it in bulk.

We are carburizing for hardness, but the color it's imperative it remains the characteristic stainless-steel-and-nickel white. Specialists have noticed a slight-yellow coloration (imperceptible to the naked eye); the lab that analyzed the surface with SEM could not find traces of copper though.