Surface Protection and Stainless Steel

It shouldn't matter for protection, however, many times these items are painted a particular color, (color coded), to identify the material that is running through them.

Example:

http://images.google.com/imgres?imgurl=http://meritkaplanpaints.com/images/engineer_pipe_colors_sm.jpg&imgrefurl=http://meritkaplanpaints.com/paint.htm&usg=__AzFt5EgxoD08KSQJLt94tkRhTl8=&h=354&w=263&sz=13&hl=en&start=10&zoom=1&um=1&itbs=1&tbnid=zsMCV1YzuLQjEM:&tbnh=121&tbnw=90&prev=/images%3Fq%3Dindustrial%2Bcolor%2Bcodes%26um%3D1%26hl%3Den%26sa%3DN%26tbs%3Disch:1

the problem you run into when coating stainless steels is that you can have pinholes or scratches/dings in the coating and this will allow water to leach under the coating and generate a pit, which can get rather large before being detected. Best practice is to not coat stainlesses if possible.

I agree. Can it be done? Yes. Should it? Not best practice.

Stainless resists corrosion by renewing its impervious oxide layer thanks to access to oxygen. cut off access to oxygen, and it can corrode quickly.

See the second bullet under chloride corrosion here:

http://www.mcnallyinstitute.com/04-html/4-1.html

And here is another reference:

Stainless steels get their corrosion resistance by the formation of a very thin surface film, called the passive film, which forms on the surface in the presence of oxygen. Therefore, stainless steels usually have poor corrosion resistance in low-oxygen environments, such as under deposits, in mud, or in tight places, called crevices, where structures or hardware are attached. This is particularly true in seawater, where the chlorides from the salt will attack and destroy the passive film faster than it can reform in low oxygen areas. All of the stainless steels except the best of the specialty alloys will suffer from pitting or crevice corrosion when immersed in seawater. One of the best 300-series stainless steels is type 316. Even this alloy will, if unprotected, start corroding under soft washers, in o-ring grooves, or any other tight crevice area in as little as one day, and it is not unusual to have penetration of a tenth of an inch in a crevice area after only 30 days in seawater. If water flows fast past a stainless steel, more oxygen is delivered to the stainless steel and it corrodes less. For this reason, stainless steels have been successfully used for impeller blades and propellers. These need to be protected from corrosion when there is no flow.

Painting stainless steels usually does not stop the crevice corrosion; it will occur any place where there is a scratch or nick in the paint. For this reason, I usually recommend against using any stainless steel except certain specialty alloys in seawater for more than a few hours at a time. There is a strong tendency to use in seawater the same materials that work well in fresh water or sea atmosphere, so that types 303, 304, and 316 stainless steel are often used for undersea applications. They will also usually fail if the exposure is long enough, unless they are in continuous solid electrical contact with a material that will provide them with cathodic protection such as steel or aluminum. As soon as the electrical contact is broken, the steel will corrode.

Source: http://www.dieselduck.ca/machine/04%20auxiliary/corrosion.htm

Milo