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.
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. The laboratory testing can be forced by making the test harsher than the actual application but will then generate an alloy ranking rather than a precise answer on how a specific alloy will perform in the actual application. Weight loss, surface deterioration and cracking are different indicators that can be used to evaluate the results for both approaches. However, results are often treated as pass or fail. Sometimes it can be hard to separate different alloys that have both passed a laboratory or in-situ test and the result may not reveal how the alloys would respond to a slight difference in the application environment, such as varied temperature, different pH or chloride level. Variations can be both natural i.e. due to different feed stock in a refinery or on purpose for increasing productivity, and can have distinct effect on service life.1 Stainless steels and Ni-base alloys are often excellent choices for many applications where severe corrosivity is encountered. Their good corrosion properties are due to the ability to form a protective passive layer. The corrosion resistance largely depends on the speed of passive film formation, the healing ability, and its stability in different environments. Thus, it is important to study the composition and electrochemical properties of the film in order to understand the protective behavior and thereby be able to make more precise material selections for longer service life without over investing in unnecessarily highly alloyed grades. Electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis (MS) are complementary ways to evaluate the protective and semiconductive properties of passive film formation. This approach was used in this work while varying the potential, pH and chloride concentration to investigate how this affects the passive film.
