stain-less steel or rustrproof steel ?

[StainlessBloke]

Stailess Bloke, what exactly is electropolishing?

Electropolishing is a method of treating stainless steel (and some other metals) to ensure that the surface is consistent and free of contamination. If you think of the process as a reversal of electroplating then you won't be too far from the truth. Basically, the part to be polished is immersed in a bath of electrolyte (a blend of acids) where it acts as the anode. The tank being used has cathodes around its perimeter and an electrical current is passed through the solution causing the acids to remove a very, very small layer of metal from the surface of the part.

This is the critical difference between EP and a manual polishing process - whereas manually, what is occuring is a 'folding over' of the 'peaks' on the substrates surface, EP is actually removing these peaks. Thus, when you manually polish, dirt and contaminants will as part of the process, become trapped under these folded peaks - this is not the case with EP.

This isn't a criticism of manual polishing however: this process can produce a shinier finish than EP can achieve and it is fine for a lot of work. But where the surface cleanliness is absolutely critical, as a rule of thumb, it's better to opt for EP - hence it's often used in medical components. It can also be a lot, lot quicker than a manual process too on large surface areas. I think it's fair to say that for the majority of purposes, EP is used as tool to aid corrosion resistance and dirt entrapment - the fact that it makes a lot of components 'bright and shiny' is a secondary effect.

One other thing to bear in mind though - because EP removes a layer of metal, if you have marks, gouges or deep scratches on the surface then the polishing process won't help them; generally it will make them look worse. Again though, this is a generalisation and not always the case.

Hope you find this helpful mate!

 

[fingers99]

That's great, thanks.

One other thing to bear in mind though - because EP removes a layer of metal, if you have marks, gouges or deep scratches on the surface then the polishing process won't help them; generally it will make them look worse. Again though, this is a generalisation and not always the case.

So -- I guess -- you could manually polish out the scratches, then EP it?

 

[StainlessBloke]

That's great, thanks.



So -- I guess -- you could manually polish out the scratches, then EP it?

No problem.

And yes; quite often parts which are electropolished receive some kind of pre-treatment prior to immersion. This can vary from a simple degreasing operation, through to pickling and indeed, manual polishing. Consistency in the surface to be processed is key.

Just as an additional point - EP is often used to polish the interior of pipes and such like - obviously, these areas can sometimes not be reached by manual techniques. It's important to note though that electropolishing only occurs where there is "throw" from the cathodes (i.e. the surface has to face a cathode with no obstruction) so often they have to be individually designed and constructed in order to do the job. It's a fairly simple process if you're polishing a straight length of 40mm pipe but it can get rather complicated when you're presented with components incorporating all sorts of twists and turns!

 

[andypocock]

That's about as stainless as a geriatrics pants

Here a stainless exhaust after 20K miles. The pipe is discoloured from the heat but the silencer is still near mint.

 

[StainlessBloke]

I agree that whether or not that exhaust is stainless is somewhat unclear.

Having dealt with a number of exhaust manufacturers over time, I'd recommend that you bear in mind the different grades of ss - some are better suited to this purpose than others. One prominent manufacturer told me that "if it's run of the mill stuff for everyday cars, we just ship in any old crap from China. If it's someone with a classic car, or a prestige job, we build it ourselves in 316"

It doesn't matter what grade you do build it in though - exhausts get hot and if you get enough heat through the metal it will discolour.