ABSTRACT

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).

INTRODUCTION

Crude oil price fluctuations and the need to improve refinery margins have driven refiners towards utilizing “opportunity crudes,” despite the challenges frequently associated with their processing. Most opportunity crudes have increased acidity and reactive sulfur compounds, making high temperature (~220–400°C) sulfidation-naphthenic acid (SNAP) corrosion a key concern for process and corrosion engineers.1,2 Naphthenic acids (NAP) are naturally occurring carboxylic acids in crude oil that are typically measured by Total Acid Number (TAN – mg of KOH required to neutralize acid in one gram of oil). Crude oil contains a wide variety of sulfur compounds, some reactive and some not - reactive sulfur compounds thermally decompose to form hydrogen sulfide (H2S) that reacts with the steel surface. It is widely accepted that the corrosion from naphthenic acids and sulfur compounds can be represented by the following reactions: 3, 4

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