All aluminum and aluminum alloys, whether coated or seemingly “bare” metal, are susceptible to chemical attack if the soak times are long enough. We’ve seen it happen after several hours on some chrome conversion-coated carburetors (with the motley/splotchy yellowish finish), while others are unaffected for days. The speed of attack depends on the type of coating, the thickness of the coating, how long the part has soaked in the past, etc. And when we say “coating,” we don’t just mean something that has been visibly altered; even regular aluminum—like they make drink cans out of—can be thought of as having a coating because of its natural oxide layer. Good rule of thumb is to soak aluminum for 30-60 minutes, check it, and repeat if necessary, limiting the total soak time as much as possible.