Scale formation of AISI 304 stainless steel during annealing at temperatures between 1100 and 1200 °C under a water vapor‐containing atmosphere is studied. Characterization of the oxide scale is performed with field‐emission scanning electron microscopy–energy dispersive spectroscopy (FESEM–EDS) and glow discharge optical emission spectroscopy (GDOES) and removal of oxide scale is done via neutral electrolyte pickling. The pickling conditions are kept constant and the effect of the annealing conditions and scale properties on the pickling result are examined. The effectiveness of pickling is evaluated using analysis FESEM images taken on polished sections of pickled surfaces. Research shows that the thickness, morphology, and composition of the oxide scale are dependent on annealing temperature and time. The thicknesses of the scale formed under the established conditions vary from 0.2 to over 30 μm, and morphologies between the chromium rich oxide layer and layered scale structure formed by breakaway oxidation. The pickling response of oxide scales remains good at all annealing temperatures with the shortest exposure time.

Because of their good corrosion and oxidation resistance, formability, and mechanical and physical properties, stainless steels are used in many applications. Examples of the most common usage applications include automotive and transportation, residential, chemical, and petroleum industries. [1] Austenitic stainless steel grades are widely used in high-temperature applications. However, nickel-free ferritic stainless steel has lower thermal expansion coefficient and is cheaper as a material. As such, it has been developed especially for applications with repeated thermal cycles such as automotive exhaust systems. [2][3][4][5] In the industrial manufacturing process, the annealing of cold-rolled stainless steel is performed by a continuous treatment line that has a high temperature, short annealing time, and an oxidizing atmosphere dependent on the fuel and oxidizer used. The annealing is followed by a pickling line. During annealing, an oxide scale layer is formed on the surface of the steel; the thickness and the composition of the oxide scale depend on the annealing conditions and the composition of the steel. [6,7] To restore the corrosion resistance properties of the stainless steel, both the formed oxide scale and the chromium depleted layer below it must be removed. This is usually conducted with sequential electrolytic and mixed acid pickling. [8] Thus, the aim of annealing is to produce the desired microstructure and mechanical properties for the steel [1] in such a way that the material losses are minimized during further processing while ascertaining a good pickling result. [8] This can be achieved by controlling the oxide layer formed on the stainless steel surface to be optimal in terms of thickness and pickling efficiency.In short-term oxidation studies of ferritic stainless steel, temperatures usually vary between 1000 and 1150 C, focusing mainly on nonstabilized AISI 430 and dual-stabilized AISI 441 steel grades. Saeki et al. [9] determined that the oxide scale composition of the AISI 430 stainless steel was (Fe,Cr) 2 O 3 after 3 min of oxidation at 1000 C. In addition, the spinel (Mn,Fe, Cr) 3 O 4 was detected for the same grade with 10 times higher Mn content than in their previous study. When a higher partial pressure of oxygen in the atmosphere is used, the spinel oxide

As the production of stainless steel increases, environmental concerns due to an increasing demand of production capacity, require an efficient use of energy and materials. Efficient removal of otherwise slow-to-dissolve chromium oxide layer can be achieved with electrochemical pre-pickling before final mixed acid pickling. Neutral electrolytic pickling can be used to rapidly dissolve chromium oxides but suffers from low current efficiency of both the reaction and the system. In order to study the effect of critical parameters for the current efficiency of the system, a pickling device was assembled. A system current efficiency factorial analysis of bipolar neutral electrolytic pickling was conducted for temperature, electrolyte concentration, cell potential, and electrode-to-sample distance. Lowering the concentration of electrolytes shows potential to increase current efficiency significantly as conductivity can be used to reduce electrode-to-electrode short-circuiting, which is the biggest cause of decreased efficiency in neutral electrolytic pickling. Increasing the temperature has a positive effect on efficiency, despite increased conductivity, possibly from increasing reaction kinetics. K E Y W O R D S bipolar electrode, current efficiency, factorial analysis, neutral electrochemical pickling, stainless steel This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

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