I have seen that with coolant only, dumm ka pani coolant. - the well was salty/brackish water.
you dont want to know the amount of corrossion I saw - I tried to repair it - and as usual my first job is to descale it - when I put the acid in, water ended in oil and oil in coolant, water in combustion chambers, gases in coolant and oil.
when torn apart - there was nothing remaining worth saving at all - the dumm wala pani did its perfect job.