A process for high temperature naphthenic acid corrosion inhibition and / or sulphur corrosion inhibition of metallic surfaces of hydrocarbon processing units of a petrochemical plant, with said processing units comprising distillation columns, strippers, trays, pumparound piping and related equipments, using an olefin phosphorous sulphur compound, and comprising the steps of: a) heating hydrocarbon containing naphthenic acid and / or sulphur compounds, to vapourize a portion of said hydrocarbon; b) condensing a portion of the hydrocarbon vapours, 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 olefin phosphorous sulphur compound, in corrosion inhibition effective amount to form a reaction mixture; d) allowing said condensed distillate containing said olefin phosphorous sulphur compound, to contact said metallic surfaces of said hydrocarbon processing unit, to form a protective film on said surfaces whereby each 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 the olefin phosphorous sulphur compound is a product of reaction of the olefin with phosphorous pentasulphide carried out in presence of a catalytic amount of sulphur, and under nitrogen gas purging; wherein the molar ratio of said olefin to said phosphorous pentasulphide is between 1:0.05 to 1:1.5, preferably 1:1; and wherein said olefin is polyisobutylene; wherein said reaction of the olefin with the phosphorous pentasulphide comprises the steps of: (a) stirring and heating a reaction mixture of the olefin with the phosphorous pentasulphide to 160°C under nitrogen gas purging; (b) maintaining said reaction mixture between 160°C to 180°C for a period of 1 hour to 2 hours; (c) raising the temperature of said reaction mixture to a temperature from 185°C to 250°C, preferably from 190°C to 230°C, and more preferably from 210°C to 225°C; (d) maintaining said reaction mixture with the raised temperature for a period from 1 to 24 hours, preferably from 6 to 10 hours; and (e) cooling the reaction mixture to 100°C and purging nitrogen gas into reaction vessel to drive out hydrogen sulphide gas, and thereby resulting into said olefin phosphorous sulphur compound. a) heating hydrocarbon containing naphthenic acid and / or sulphur compounds, to vapourize a portion of said hydrocarbon; b) condensing a portion of the hydrocarbon vapours, 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 olefin phosphorous sulphur compound, in corrosion inhibition effective amount to form a reaction mixture; d) allowing said condensed distillate containing said olefin phosphorous sulphur compound, to contact said metallic surfaces of said hydrocarbon processing unit, to form a protective film on said surfaces whereby each 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; (a) stirring and heating a reaction mixture of the olefin with the phosphorous pentasulphide to 160°C under nitrogen gas purging; (b) maintaining said reaction mixture between 160°C to 180°C for a period of 1 hour to 2 hours; (c) raising the temperature of said reaction mixture to a temperature from 185°C to 250°C, preferably from 190°C to 230°C, and more preferably from 210°C to 225°C; (d) maintaining said reaction mixture with the raised temperature for a period from 1 to 24 hours, preferably from 6 to 10 hours; and (e) cooling the reaction mixture to 100°C and purging nitrogen gas into reaction vessel to drive out hydrogen sulphide gas, and thereby resulting into said olefin phosphorous sulphur compound.