Which ions in flint water caused the protective layer inside the pipes to break down?

The answer is Oxygen ions & Chloride ions.  Although Dissolved Oxygen (DO) can not interact with the Lead, Orthophosphates have to be added continually or the barrier breaks down. If the barrier does break down, DO combines with Lead atoms, oxidizing them. It allows Lead to drain into drinking water. Once oxidized, Lead dissolves into the water instead of sticking to the pipe. Flint’s water treatment plant did not add
orthophosphates,  allowing the pipes to corrode, and Lead quickly contaminated the drinking water. Additionally, Flint River water had high levels of chlorides, which accelerate corrosion.