430SS Pitting : Chemical Etching Process- FeCl

[ChemMilling]

Hi All,

This is my first post to this forum. I am impresed with the level of help and quality of responses. Now I have a problem I am trying to solve and am seeking your input:

We chemically etch 430SS using Ferric Chloride as the etchant. Obviously, there are many steps in the process. From the picture attached, do you have any recommendations for areas to investigate as we try to eliminate the problem of small pockets opening up in the steel in the etching process? The problem is intermittent and occurs randomly on the plate. Any insight would be appreciated.

The picture is 60X magnification.

 

[mcguire]

Corrosion pits are formed at MnS inclusion sites in ferric chloride. It seems that is your problem. 430 can be etched uniformly by nitric/hydrofloric acid, the solution commonly used to pickle it.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[ChemMilling]

Thank you for your insight.

Any suggestions for a Stainless Steel that would exhibit similar physical properties to 430SS sheet and be impervious to the pitting corrosion when etching with Ferric Chloride?

 

[mcguire]

No, there is none. Ferric chloride is the standard test media for pitting resistance and eventually it pits every stainless. You can minimize the problem by switching alloys. 439 is much better. But if you can't switch etchants, this will always be a problem.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[MrEnger]

It looks, from your picture, that you are getting pitting on the top face of the channel area not in the actual etched area. If so you will need to take a look at your imaging process (assuming you are using a photo resist) the pitting will be caused by either dust particles imaging through the resist or a breakdown of the resist during etching although this is far less likely than a simple lack of cleanliness during processing.
Remember if you are using a photo resist it is a photographic process so therefore you must maintain the cleanest conditions possible.
Hope this helps and good luck.

 

[EdStainless]

This could also be related to microstructure.
What is the Carbon level?
Is the part solution annealed?
If there are any carbides you will get serious pitting at the locations. I would expect pitting in 439 because of the titanium carbo-nitrides that are part of the structure.
What temperature are you working at?
I agree that ferric chloride is probably simply too aggressive for this alloy.
If you are going to try nitric/HF I would suggest that you start with 20-25% + 0.5-1.0%.
You might look into sulfuric/phosphoric acid mixtures, they are used in electropolishing and chemical brightening applications.


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