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you must use stainless fastening devices when working with aluminum (or aluminum) as stated regular steel and aluminum dont mix. corrosion starts to happen almost immediately. 
That is a common very wrong misconception. Actually SS and alum have a greater corrosion potential, mainly of the alum.
To Op, a single sheared stud like pictured is likely only caused by being defective or previously partly over tighten, and time/fatigue/corrosion finished the task, or operated when tight and all the others are loose.
Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
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