Our firm is involved in a project where the through-wall masonry flashing is copper. When a 2-day-old portion of the wall was taken down to correct a problem with the coursing, I noticed that the copper flashing had turned black wherever it was in contact with mortar. Is this a problem?
No. The black color is a copper oxide that forms in the high pH environment of mortar. After this black oxide forms, there is essentially no further corrosion. The copper flashing is then stabilized and protected by the portland cement in the mortar.