Investigation of passive film formation on UNS N08935 in high chloride solution – Alternative method for advanced material selection

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Material selection to get “fit-for-purpose” alloys is an important task that corrosion engineers face in their daily work. Two common ways of attacking such a challenge is by testing different alloys in environments similar to the application in a laboratory environment or installing samples in actual operating equipment. In the latter, testing is usually time consuming and might require plant turnovers to get access to the equipment for sample installation.

Product Number: 51323-19266-SG

Author: Nian Zhou, Robert Mattsson Öhnfeldt, Daniel Gullberg, Ulf Kivisäkk

Publication Date: 2023

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Material selection in the chemical and petrochemical industry must consider natural variations in the process environment. This includes varying corrosion potential, pH and chloride concentration depending on process control and feedstock. Also, when trying to optimize productivity these parameters might be of interest to intentionally modify. A better understanding of how the process environment affects the passive layer formation can help make more informed material selection than classic pass/fail tests.

In this work, the protective and semiconductive properties of the passive films formed on the super austenitic steel UNS N08935 in magnesium chloride solution were investigated using potentiodynamic and potentiostatic polarizations, electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) analysis; the passivity with varied film forming potentials, chloride concentration and solution pH were compared. All electrochemical tests revealed that the anodic potentials and solution pH had strong influence on passive films formed on UNS N08935. The protective nature of the passive film reduces as the film formation potentials increase. A more neutral solution pH or increased chloride concentration increased the transpassive potential. Moreover, GD-OES analysis confirmed the dissolution of metals in the transpassive region. The results revealed large influence of the application environment on the protectiveness of the passive film formed on UNS N08935 and indicated EIS and MS measurements are useful tools for evaluating the passive film in varied application conditions.