The present invention relates to inhibition of high temperature naphthenic acid corrosion occurring in hydrocarbon processing units. The invention provides an effective novel non-polymeric and non-fouling additive for inhibiting high-temperature naphthenic acid corrosion, comprising an effective corrosion-inhibiting amount of a second phosphate ester wherein said second phosphate ester is obtained by reacting a first phosphate ester with an oxirane compound selected from the group consisting of butylene oxide, ethylene oxide, propylene oxide or any other oxirane compound or a combination thereof, preferably with butylene oxide, capably yielding said second phosphate ester, having a structure A or B, wherein R1 and R2 are each independently selected from the group consisting of moieties having 1 to 20 carbon atoms and R1 and R2 may be identical or different, X is H, CH3 or C2H5; and n may vary from 1 to 20, wherein said first phosphate ester is having a structure I or II, wherein R1 and R2 are each independently selected from the group consisting of moieties having 1 to 20 carbon atoms and R1 and R2 may be identical or different, said first phosphate ester being obtained as a reaction product of reaction of an alcohol with a phosphorous pentaoxide.