Oxidation specks on ss sheet metal

[rww88]

We are using 24 gauge 304 stainless to flash (waterproof) windows of a masonry building located a mile inland from the Atlantic ocean. After several months, exposed areas of this flashing along the face of the building were observed to exhibit a pattern of some form of surface oxidation. Specks or pits of what could be corrosion, had an appearance to that normally associated with surface contamination arising from carbon steel fabrication by-products or the use of carbon steel tools. The flashing was first installed followed by the installation of a brick & mortar and stucco façade. Incidental residual cement was allowed to remain on the flashing for a time before it was cleaned from the surface. Could these oxidation specks be some form of chloride attack stemming from the residuals produced during the masonry installation? No attempts have been made to clean the flashing.

 

[EdStainless]

Yes. (I really just should stop now)

Since you don't have a blushing or rust streaks, I would guess that the mortar etched the surface of the stainless. These areas now trap moisture and impurities leading to discoloration.
Take a ScotchBrite pad and some phosphoric acid cleaner and lightly buff the areas. Then rinse well. This may be enough to prevent the marks from coming back.
Often when stainless is used for architectural details it is coated with a plastic film to protect it in handling and installation. Though I have seen corrosion from people forgetting to remove the film.

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[arunmrao]

Is the cement a blended one or ordinary portland? Normally the order of the day is to produce blended cement where either flyash or slag is blended with clinker. The iron in slagcement can cause those rust stains.


As Ed said normally the sheets have a plastic coating and this would have reduced the risk of contamination while plastering.