Mechanism of High Temperature Corrosion by Model Naphthenic Acids
Naphthenic acid (NAP) corrosion is one of the major concerns for corrosion engineers in refineries, particularly with the increasing acidity in “opportunity crudes”. However, complex compositions of NAP in crude oil make it difficult to evaluate accurately their corrosive behavior. In the present study, several model acids, including 4-phenylbutyric acid, 1-naphthoic acid, and 4-cyclohexyl pentanoic acid, have been tested in lab scale experiments previous applied to real crude fractions. Pretreatment/challenge protocol tests the corrosive behavior of the model acids under temperature and velocity ranges close to those encountered in refineries. This work demonstrates that the pretreatment of carbon steel and 9Cralloy with model acids yields iron oxide scales with different morphology and chemical composition as determined by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). It is postulated that the formation of the iron oxide scale resulted from the decomposition of iron naphthenate on the surface at high temperatures.
Analysis of corrosion scales formed on steel at high temperatures in hydrocarbons containing model naphthenic acids and sulfur compounds
Corrosive naphthenic acids and sulfur compounds in crude oils present a major challenge for refineries from a corrosion perspective. Although it is accepted that some sulfur compounds may form protective FeS scales on the metal surface and deter corrosion, attempting to correlate the characteristics of FeS scale with its protective properties has not been successful. Given the complex chemical compositions of real crudes, model sulfur compound and model naphthenic acids were used to mimic the corrosion by crude fractions in the present study. The iron sulfide scale formed by the model sulfur/acid compounds was challenged by naphthenic acids under high‐velocity conditions to examine its protectiveness against corrosion. Moreover, the scale was analyzed with transmission electron microscope/energy dispersive X‐ray spectroscopy technique, and a layer of iron oxide formed on the 5Cr steel was found when naphthenic acids were present in the solution. The iron oxide layer appeared to be important for maintaining protection against naphthenic acid corrosion, and further analysis revealed that it was composed of magnetite. Copyright © 2015 John Wiley & Sons, Ltd.
High-resolution scanning transmission electron microscopy (HRSTEM) techniques: high-resolution imaging and spectroscopy side by side.
This work presents an overview of high-resolution scanning transmission electron microscopy (HRSTEM) techniques and exemplifies the novel quantitative characterization possibilities that have emerged from recent advances in these methods. The synergistic combination of atomic resolution imaging and spectroscopy provided by HRSTEM is highlighted as a unique feature that can provide a comprehensive analytical description of material properties at the nanoscale. State-of-the-art high-angle annular dark field and annular bright field examples are depicted as well as the use of X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy for probing samples properties at the atomic scale. In addition, promising techniques such as cathodoluminescence, confocal HRSTEM, and diffraction mapping are introduced. The presented examples and results indicate that HRSTEM-related techniques are fundamental tools for comprehensive assessment of properties at the atomic scale.