To specify pacification or not 1

[pennpoint]

I have a requirement to electropolish an interior surface of a 304 stainless steel box, 24" cubed. Should I have my shop passivate all welds prior to EP?
Is there a degree of polish I can specify? or do I specify to my vendor;"best brite surface finish attainable"? It's the interior surface thats important to my customer.
The exterior of the box will have carbon steel members for reinforcements (vacuum service), I realize the these exterior members will be corroded (rusted in color and finish) but without lacking in strength.

Thanks For your Input
pennpoint

 

[kenvlach]

No, it is redundant/unnecessary to passivate prior to electropolishing. Electropolishing (includes a nitric acid rinse) gives a superior passivation.

However, you must descale, e.g., wire brush (SS wire only), pickle, beadblast, mechanical polish, buff, etc. Electropolishing can handle some heat discoloration

http://www.ableelectropolishing.com/pdf1021.htm

but not heavy scale.

Also, electropolishing gives brightness via micro-smoothing, but doesn’t remove macro-roughness such as scratches. It generally gives a 50% improvement in RA on polished surfaces. See

http://www.electromatic.com/process.html

for illustrated process description.

In corrosive environments, the smoother the surface finish the better. Of course, in vacuum, this gives less outgassing. Typically, polishing with 320 grit abrasives (SiC used with lubrication) will achieve a finish better than 0.5 micrometres Ra. Electropolishing then further improves corrosion resistance and brightness.

http://www.assda.asn.au/Teastain.pdf