Help with Pitting after Polishing up Stainless Steel sheet

[Jmanm]

I polished up a sheet of grade 304 that had a brushed appearance. First I wet sanded the surface with 1200grit sand paper to remove these machine brush marks & then machine polished to a mirror gloss using automotive type compound. After the wet sand marks were removed it become clear there was a problem. Although it was like a mirror, tiny specs that looked like metallic flake in a car paint job were visible, on close inspection they are tiny shallow holes. No amount of polishing of wet sanded prevents these, they always form.

 

I'm wondering if this caused by the compound (Meguiars M105) or the chromium oxide (black residue during polishing) or just the air it self? Maybe its just a sign of inferior made stainless?

 

I know that when polishing up aluminium that the black oxide that comes off onto the polishing pad can damage the surface causing marring. Only way to avoid is keep using a new polish pad very often.

 

I guess all I can do is try a Metal polish & another grade of SS. Thought Id ask to learn from this if I can.

 

thanks

Edited March 14, 2017 by Jmanm

[DrP]

I can't fully answer you question, but my first thoughts were that these pits must have been there already. I cannot see the polishing causing them, so the high level of polishing to such a high standard of finish has revealed them. I could be wrong as it is not my field, but I cannot see how the polishing regime you described could cause pitting in the metal itself.

 

Out of interest, please keep us posted and let us know if a better grade of steel avoids these problems or improves them. Thanks.

[StringJunky]

I think you want 316L steel as it contains molydenum - 304 doesn't - which makes it more chloride-resistant and used in marine applications; your sweat contains salts which may contribute to your problems whilst working on it and future handling. It's more ductile which would make it more amenable to polishing by making the surface layer more spreadable, I think, and it is inherently less susceptible to pitting.

 

 

he two most common stainless steel grades are 304 and 316. The key difference is the addition of molybdenum—an alloy which drastically enhances corrosion resistance, especially for more saline or chloride-exposed environments. 316 stainless steel contains molybdenum, but 304 doesn’t.

 

http://www.reliance-foundry.com/blog/304-vs-316-stainless-steel#gref

 

 

Molybdenum

 

Increases creep resistance; strength at high temperatures; and corrosion resistance, particularly in sulfite, sulfate, acetic acid and acetate solutions and in a salt-water atmosphere.

Expands range of passivity and counteracts tendency to pit.

http://www.stainlesssales.com/stainless-steel-grades.html

Reading both links will give you an overview of general properties.

 

Edit: 316L contains more molybdenum than 316 but even the latter should be better for your purposes than 304.

 

The latter link lists the grades lower down the page.

 

Edit: If you want to get really anal about the final polishing then jewellers use cerium oxide and jeweller's rouge (iron III oxide). The former can be used to polish glass and remove scratches from it but rouge is very fine and quite economically priced. i have no experience with cerium oxide but use rouge to remove fine scratches from plastic and metal.

Edited March 14, 2017 by StringJunky

[Jmanm]

I can't fully answer you question, but my first thoughts were that these pits must have been there already. I cannot see the polishing causing them, so the high level of polishing to such a high standard of finish has revealed them. I could be wrong as it is not my field, but I cannot see how the polishing regime you described could cause pitting in the metal itself.

 

Out of interest, please keep us posted and let us know if a better grade of steel avoids these problems or improves them. Thanks.

Hi,

 

Thats what I thought like swiss cheese that had hollows in it from inferior manufacturing.

 

 

 

I forgot to add an important bit: When i first got this SS sheet it had a brushed smooth slightly reflective finish. And I straight away went to polish it up skipping the wet sand stage. The end result was an almost mirror sharp finish, but with a nice brush pattern still showing. And although you could shave in the reflection, there was no signs of the pits. Not sure if the pits were hidden among the remaining brush marks? But I doubt it as there were areas free of brush marks & perfect.

I think you want 316L steel as it contains molydenum - 304 doesn't - which makes it more chloride-resistant and used in marine applications; your sweat contains salts which may contribute to your problems whilst working on it and future handling. It's more ductile which would make it more amenable to polishing by making the surface layer more spreadable, I think, and it is inherently less susceptible to pitting.

 

 

Reading both links will give you an overview of general properties.

 

Edit: 316L contains more molybdenum than 316 but even the latter should be better for your purposes than 304.

 

The latter link lists the grades lower down the page.

 

Edit: If you want to get really anal about the final polishing then jewellers use cerium oxide and jeweller's rouge (iron III oxide). The former can be used to polish glass and remove scratches from it but rouge is very fine and quite economically priced. i have no experience with cerium oxide but use rouge to remove fine scratches from plastic and metal.

Thanks. I will try some 316 when I track some down, most places that sell it tend to be big wholesalers. I think the automotive polishes have gotten this absolutely mirror like, no signs of any visible fine scratches.

Edited March 15, 2017 by Jmanm