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I understand the difference between a #3 finish on a piece of stainless steel compared to a #4 finish being that the surface roughness is different. Specifically, a #4 finish is classified as having an Ra value of less than 25 micro-inches where as a #3 finish has an Ra value of less than 40 micro-inches.

The basis of my question is if I have two cabinets made from 304 SST, one has a #3 finish, one with a #4 finish in the same outdoor environment, which will rust faster? Are there any other concerns between the two cabinets as far as performance of the cabinet (welds, hinges, bends, etc.)? Have there been any studies conducted?

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Surface Finish

I have read about a couple of corrosion concerns with stainless steel surface finish. I don't have any direct experience with differing rates of corrosion, but below are some considerations:

  1. The smoother the finishes are less likely to collect dirt and water on the surface. Each of these will help to increase the local rate of corrosion. Some smooth (mirror-like) finishes are even considered "self cleaning" from an architectural stand point.

  2. This is related to #1 above. When finishes have a defined brush pattern, the brush lines should be oriented vertically. This helps to allow water and dirt to flow downward instead of collecting.

Fasteners and Hinges

Fasteners and hinges will have some of the same issues. Any location where water and dirt will collect will likely show increased corrosion.

Another consideration is galvanic corrosion. This is increased corrosion that occurs when two different types of metals are in contact. This can be common if hinges and fasters aren't also made of stainless steel.

References

A manufacturer's website that has more information about the different finishes can be found here.

In aggressive environments, such as in the nuclear or offshore industries, a correctly polished stainless steel surface has a better resistance to corrosion than a surface that is roughly or badly polished. A smooth surface is less susceptible to an accumulation of deposits and stainless, which often become focal points for localised corrosion.

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In addition to hazzey's answer I'd just like to add that passivation of stainless is very important to reduce the risk of corrosion, especially in welded products.

Typically the steel is cleaned then dipped into various chemical baths following welding. This allows the steel to restore its chromium-oxide rich "passive" layer.

Depending on the application it may be possible to remove surface contaminants from welds with a stainless wire brush rather than using the chemicals but this will affect the surface finish.

Some of the pickling chemicals used in the passivation process can dull a very shiny finish. The #3 and #4 brushed finishes may be affected so it would be worth consulting the passivation company before going ahead with it. For high shine finishes it is worth considering electro-polishing.

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  • $\begingroup$ What is the impact of this on the question itself? Does this change or impact the finish? $\endgroup$ – VulgarBinary May 8 '15 at 1:50
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    $\begingroup$ This answer partly answers the "are there any other concerns?" section of the question. I've edited the answer to take into account potential effects on surface finish. $\endgroup$ – switchtizzer May 8 '15 at 5:32

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