The present invention relates to the field of corrosion inhibition in hydrocarbon fluid processing units. The present invention comprises a new additive for inhibiting acid corrosion comprising polymeric thiophosphate ester, which is obtained by reaction of a polymer compound having mono, di or poly hydroxyl group, preferably polymer compound which is hydroxyl-termination, more preferably said polymer compound comprising hydroxyl-terminated polyisobutylene or polybutene and phosphorous pentasulphide. Said polymeric thiophosphate ester is further reacted with any oxide selected from group consisting of ethylene oxide, butylenes oxide or propylene oxide or such other oxide, preferably ethylene oxide, capably forming ethylene oxide derivative of thiophosphate ester. The invention is useful effecting acid corrosion inhibition on the metal surfaces of a distillation unit, distillation column, trays, packing and pump around piping.