On a road wheel that he build/laced himself, my brother has recently had a few spokes break. Or rather spoke nipples. All on different spokes. The breakage is happening right at the rim. The spoke head parts from the rest of the nipple/spoke. Actually...the spoke is remaining in tact. That seems like an odd place for the break to be happening. He thinks it's possibly being cause by extreme corrosion there due to the dissimilar metals...brass nipple, aluminum rim, steel spoke. I think it could possibly be a result of over tensioning. He's very meticulous about even tensionion when he builds a wheel. So I'm sure the tensioning is even...but just maybe excessive. The wheels are a couple of years old. Never ridden in wet conditions. Bike is stored indoors. Any ideas what could be the issue?

Dan