Ritmesh, just to be clear: are the iron parts cleaned in MEK before zinc coating, or are the zinc-coated iron parts corroding after being exposed to MEK during service as drum clamps? I ask because the the usual process for pre-zinc dip coating is an initial acid wash and rinse/dry, not an MEK cleaning dip.
Also please note 35μm of zinc is not very thick and may not be able to cover up all the pits & scratches in the iron.
Also I notice that the corrosion is pitting corrosion associated with the sheared edges of these parts which suggests pinholes in the zinc in the roughened zones. Two things to try: a tumble deburring of the parts before zinc coating to remove any sharp edges, and a thicker zinc coat.
Finally, if the problem is pinholing then you should be able to rapidly produce it by dipping the parts into a warm, well-oxygenated weak acid solution with a little salt in it and then watching closely for gas evolution at the zinc surface, where the pinholes are. You can also do the pinhole test by assembling an electrochemical cell where one electrode is copper and the other is your zinc-coated part and the solution is a weak acid. this will resolve pinholes in seconds if you do it right.