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51318-11071-Corrosion Behavior of Naphthenic Acids Isolated from Vacuum Gas Oil Crude Fractions

Picture for Corrosion Behavior of Naphthenic Acids Isolated from Vacuum Gas Oil Crude Fractions
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Corrosion behavior of native naphthenic acids in two VGO fractions are compared with white oil solutions of the acids isolated from them by solid phase extraction (SPE). Tests are per the in-house “pretreatment-challenge” protocol on carbon steel and 5Cr steel samples.

Product Number: 51318-11071-SG
Author: Yathish Kurapati / Winston Robbins / Gheorghe Bota / David Young
Publication Date: 2018
Industry: Petroleum Refining
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Opportunity crudes frequently have increased acidity, making high temperature naphthenic acid (NAP) corrosion a key concern for their processing in refineries. Lab corrosion rates are hard to interpret due to either variable composition of crude fractions or the use of commercially available model carboxylic acids that may be unrepresentative of NAP species present in vacuum gas oils (VGO). Here, the corrosion behavior of native naphthenic acids in two VGO fractions are compared with white oil solutions of the acids isolated from them by solid phase extraction (SPE). Corrosion tests are conducted per the in-house “pretreatment-challenge” protocol on carbon steel and 5Cr steel samples. Corrosion rates for isolated acid solutions in mineral oil are lower than those for the corresponding VGO. Corrosion product scales formed by the isolated acids are more resistant than VGO to a high severity acid-only challenge. Characterization of corrosion product scales by cross-section electron microscopy techniques confirm that the isolated acids generate dense oxide-rich layers under thin iron sulfide (FeS) layers, in contrast to the oxide layers observed under thicker sulfide layers for the VGO. The resistance of the oxide layers to the acid challenge is consistent with previously reported formation of nano-particulate magnetite (Fe₃O₄).

Key words: Naphthenic acids, solid phase extraction, magnetite, NAP extraction, sulfidation

Opportunity crudes frequently have increased acidity, making high temperature naphthenic acid (NAP) corrosion a key concern for their processing in refineries. Lab corrosion rates are hard to interpret due to either variable composition of crude fractions or the use of commercially available model carboxylic acids that may be unrepresentative of NAP species present in vacuum gas oils (VGO). Here, the corrosion behavior of native naphthenic acids in two VGO fractions are compared with white oil solutions of the acids isolated from them by solid phase extraction (SPE). Corrosion tests are conducted per the in-house “pretreatment-challenge” protocol on carbon steel and 5Cr steel samples. Corrosion rates for isolated acid solutions in mineral oil are lower than those for the corresponding VGO. Corrosion product scales formed by the isolated acids are more resistant than VGO to a high severity acid-only challenge. Characterization of corrosion product scales by cross-section electron microscopy techniques confirm that the isolated acids generate dense oxide-rich layers under thin iron sulfide (FeS) layers, in contrast to the oxide layers observed under thicker sulfide layers for the VGO. The resistance of the oxide layers to the acid challenge is consistent with previously reported formation of nano-particulate magnetite (Fe₃O₄).

Key words: Naphthenic acids, solid phase extraction, magnetite, NAP extraction, sulfidation

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