Grinding polished 304 Stainless surface

[ralphi]

After the welders weld the stainless steel.. i saw him grinding the welds to make it shiny silver.. but I also noticed he grinds with sandpaper the other part of the stainless steel that doesn't have any welds (for example those farther area that are hits with some burn marks). But can you really do this to polished stainless surface in the say 304. It's not pure stainless (but has carbon).. If your grind it with sandpaper.. what would happen to the surface, theoretically? Is the surface of normal 304 stainless steel coated with pure stainless (without carbon) or is it all the same as inside? thanks..

 

[MikeHalloran]

Even the fanciest stainless steel is still ~60 pct iron.
That iron, if left on the surface, will corrode if given the opportunity, absent pickling and/or passivating, which are chemical processes for removing iron near the surface.

There is another reason for grinding: The brown discolored area around a weld, formally called 'heat tint', should be removed by pickling or by abrasive machining. Leaving it severely compromises the corrosion resistance, and seems to catalyze an odd sort of brittle failure.



Mike Halloran
Pembroke Pines, FL, USA

 

[ralphi]

So can one directly grind the surface (area outside of weld region) with sandpaper and then leave it without painting or putting some kind of coating? Or a more obvious example.. if you have a stainless steel railing at any finished mall you visited at weekend and you sand paper it.. what would happen to it? Would it attract rust or destroy any coating that factory of 304 stainless steel put? or no effect?

 

[MikeHalloran]

In common environments where 304 would be a good choice, there is no good reason to apply a coating. Most of the sanding and polishing is done to provide a pleasing and/or practical surface texture.

Mike Halloran
Pembroke Pines, FL, USA

 

[Lnewqban]

For most SS, I have seen no rusting consequences from sanding, grinding.
The internal parts of a SS plate are exposed after drilling and cutting.
Same happens for machined parts from a bar.

When SS rusts, the factory and worked on surfaces rust alike.
There is no way the factory finish can be conserved near to welded areas; the welders just try to blend color and texture by doing what you have seen.

"Engineering is achieving function while avoiding failure." - Henry Petroski