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Can weathering Steel be left unpainted? Are there any environments when the properties of weathering steel will prevent the steel from performing as designed, i.e. surface rust forming to encapsulate the remaining steel core.

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  • $\begingroup$ I would question how well it does with repeated (tidal) immersion in salt water, and if cathodic protection may help. $\endgroup$ – Brian Drummond Jan 5 '18 at 13:16
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    $\begingroup$ You may find this answer (and its links) provide a lot of your answers. Did you find any information out yourself? I ask because some of your information is on the wikipedia page. $\endgroup$ – hazzey Jan 5 '18 at 14:18
  • $\begingroup$ This depends upon exposure to surface contaminants and moisture. As long as there is continued exposure, corrosion will progress; there is no stopping it unless you apply protection. You can also get corrosion from dissimilar metals and oxygen starvation. NASA has done a lot of work on this subject. $\endgroup$ – AsymLabs Feb 5 '18 at 8:48
  • $\begingroup$ Surface contamination is not just limited to chloride. Any halide or sulphates, acidic industrial atmosphere (hydrogen sulphide for example) could be considered a contaminant. $\endgroup$ – AsymLabs Feb 5 '18 at 8:56
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Anywhere within about 5 miles of a sea coast , they should be coated to resist corrosion . Otherwise they are pretty good. They do slowly rust and turn reddish brown , I have heard of them being painted because important people don't like the color , but otherwise it is generally not necessary to paint them . They have tiny amounts of P , Cu , Cr , and a few other elements depending on the manufacturer.

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  • $\begingroup$ Thinking globally : They should coated within 5 miles of any third world industrial site . $\endgroup$ – blacksmith37 Feb 5 '18 at 21:22
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Yes

The reason that weathering steel was developed was so that it would have better weathering characteristics than regular carbon steel. It was specifically developed so that it creates a patina of rust that is durable and keeps the remaining steel from rusting further.

When not to use it?

In certain environments, the protective patina will not form. In these locations, additional protective coatings are recommended. These can be paint or zinc coatings.

The Wikipedia page lists more specific details about locations where uncoated weathering steel is not recommended.

In general, locations that are corrosive (e.g. pollution or swimming pools) or locations that do not experience wetting and drying cycles are not good locations for weathering steel.

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