Originally Posted by
redlude97
That actually does look like corrosion which happens to aluminum. I've seen handlebars with similar fracturing. If the break was more clean it would possibly be overtightening, but the flaking indicates corrosion. This is coming from a chemical engineer with a PhD.
So, like galvanic corrosion that makes a carbon seatpost stick inside of an aluminum frame?
If so, maybe the mechanic meant that sweat was acting as the electrolyte between the aluminum stem and the carbon steerer.