A process of high temperature naphthenic acid corrosion inhibition of metallic surface of hydrocarbon processing unit of a petrochemical plant, used for processing a stream containing naphthenic acid, wherein said processing unit comprises distillation columns, strippers, trays, pump around piping and related equipments, and wherein the process comprising the steps of: a. heating said hydrocarbon containing naphthenic acid to vapourize a portion of said hydrocarbon; b. condensing a portion of the hydrocarbon vapors, passing through said hydrocarbon processing unit, to produce a condensed distillate; c. adding to said distillate, before said condensed distillate is returned to said hydrocarbon processing unit or collected as a product, from 1 to 2000 ppm of an oxirane treated phosphate ester in corrosion-inhibition-effective-amount, capably forming a reaction mixture; d. allowing said reaction mixture to contact said metallic surface of said hydrocarbon processing unit to form a protective film on said surface whereby said surface is inhibited against corrosion; and e. allowing said condensed distillate to return to said hydrocarbon processing unit, or to be collected as said product; wherein said oxide treated phosphate ester is obtained by reacting a phosphate ester with an oxirane compound selected from the group consisting of butylene oxide, ethylene oxide, and propylene oxide, and a combination thereof yielding said oxide treated phosphate ester having a mixture of compounds of structural formulae A and B, wherein R 1 and R 2 are each independently selected from the group consisting of moieties having 1 to 20 carbon atoms, and R 1 and R 2 may be identical to or different from each other, X is H, CH 3 or C 2 H 5; and n is 1, wherein said phosphate ester at least contains a mixture of compounds of structural formulae I and II, wherein R 1 and R 2 are each independently selected from the group consisting of moieties having 1 to 20 carbon atoms, and R 1 and R 2 may be identical to or different from each other, wherein said phosphate ester is obtained as a reaction mixture of reaction of an alcohol with a phosphorous pentaoxide. a. heating said hydrocarbon containing naphthenic acid to vapourize a portion of said hydrocarbon; b. condensing a portion of the hydrocarbon vapors, passing through said hydrocarbon processing unit, to produce a condensed distillate; c. adding to said distillate, before said condensed distillate is returned to said hydrocarbon processing unit or collected as a product, from 1 to 2000 ppm of an oxirane treated phosphate ester in corrosion-inhibition-effective-amount, capably forming a reaction mixture; d. allowing said reaction mixture to contact said metallic surface of said hydrocarbon processing unit to form a protective film on said surface whereby said surface is inhibited against corrosion; and e. allowing said condensed distillate to return to said hydrocarbon processing unit, or to be collected as said product; wherein R 1 and R 2 are each independently selected from the group consisting of moieties having 1 to 20 carbon atoms, and R 1 and R 2 may be identical to or different from each other, X is H, CH 3 or C 2 H 5; and n is 1, wherein R 1 and R 2 are each independently selected from the group consisting of moieties having 1 to 20 carbon atoms, and R 1 and R 2 may be identical to or different from each other,