Could it be rust? Need help in identifying the type of corrosion. 1

[JHWC]

Hi,

Please see picture attached. The picture shows the gate material (Stainless Steel AISI 410) coated with Tungsten Carbide.

By right, we should not expect any corrosion or rust taking place. We try to touch the "rusting" part on the surface, but it was still smooth.

Any comments?

Thanks,
Jeffrey

 

[EdStainless]

What has it been in contact with? It looks like deposit on the surface, likely rust that is dissolved in the system.
But why wouldn't you expect 410 to rust? It is barely stainless unless in a very clean environment.

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P.E. Metallurgy, Plymouth Tube

 

[JHWC]

Hi EdStainless,

My understanding of SS is that it has a layer of oxide to protect from corrosion.

On the other hand, it is coated with Tungsten carbide. Therefore, the chances of it to rust is slim.

Could it be due to the grease that is pressurized and resulted in the stain there? Note that the surface (brown area) is still smooth.

Thanks,
Jeffrey

 

[Compositepro]

The brown stain appears to be varnish build-up. "Varnish" in this case could be any organic material, which leaves a hard residue on the surface. Typically it is caused by oils that contain some double bonds, which react with oxygen to polymerize and harden. Metal surfaces catalyze the oxidation process. It is quite possible that rust particles from elsewhere are trapped in the varnish.

 

[EdStainless]

410 has so little Cr that the passive oxide layer is not very strong.
Is the WC pure WC or does it have a binder (Co or Fe)? How was it applied (how porous is it)?

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P.E. Metallurgy, Plymouth Tube

 

[JHWC]

Hi EdStainless,

Thanks. Information as follows:

Composition of the coating:
WC - 86%
Co - 10%
Cr - 4%

Applied through HVOF, Porosity to be maintained at 1%.

Hi Compositepro,

Thanks. Same thought here too. =)

 

[mrfailure]

You should be able to dissolve and collect some of the discolored material and have EDS analysis performed to identify the discoloration. Then do the same collection process in a nondiscolored area to identify what might be different.

 

[weldstan]

See mrfailure. Without knowing the service environment, it could be rust, rust carryover or a varnish.

 

[itsmoked]

I agree completely with Compositepro's assessment. I see what looks exactly like that frequently on machines I work on and it IS just a deposit. I've never needed to put a name to it but varnish is probably the correct name.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[JHWC]

Thanks all for the wonderful information. =)