Effect of Simulated Annealing Conditions on Scale Formation and Neutral Electrolytic Pickling

Abstract

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.

Oxide Scale Formation of EN 1.4622 and EN 1.4828 Stainless Steels during Annealing and Descaling Behavior in Neutral Electrolytic Pickling

“…The previous study focused on oxide scale formation on austenitic AISI 304 oxidized in the same low water vapor annealing conditions as EN 1.4828. [22] In the most demanding conditions, i.e. in five minutes at 1200 C, the oxide layer and a net-like-structured under layer due to breakaway oxidation, and the total thickness of oxide was over 30 μm.…”

“…More detailed information on annealing and pickling methods, and schematic figures of annealing and pickling devices are presented in a previous study. [22] After pickling, the amount of oxide-free and oxide-covering sample surface was examined by FESEM-EDS. The efficiency of pickling was evaluated visually classifying pickling results and by image analysis from FESEM images.…”

“…These methods, including, e.g., binarization of 8-bit grayscale FESEM images, were based on our previous study. [22] The image analysis was used to compare the thickness of the oxide scale formed during annealing and the pickling efficiency, and to assess the separability of the visual classification.…”

From Fossil‐Fueled to Hydrogen‐Fueled Annealing Furnaces: Effects on the Oxidation of Stainless Steels

“…[29] The longer time of 5 min in the annealing furnace could cause some problems during the pickling process because the oxide scales thicken considerably and iron oxide-containing nodules form. The electrolytic pickling highly dissolves the chromium-rich oxide layer [33,34] and increasing the pickling time would improve the pickling result, [17,35] but the removal of large nodules and internal oxide pockets, which would provide a smooth and uniform surface, would require more effort to be put into the pickling process. Thus, it could also be assumed that the methane-air atmospheres are more suitable for use under longer annealing times at 1100 °C than hydrogen-oxygen and methane-oxygen atmospheres.…”

“…Similar enrichment of silicon and manganese in Cr-rich oxide layers was detected on the surface of the grade AISI 304. [17] A water vapor-containing atmosphere has been found to influence the oxidation of Fe-Cr alloys [18][19][20] and stainless steels. [21][22][23] After long-term oxidation, Issartel et al [24] reported that the oxidation of ferritic 4509 stainless steel was similar using a water vapor content of 7.5% in air compared with dry air at 900 °C, but by increasing the temperature to 1000 °C, the water vapor atmosphere began to affect the amount and composition of the oxide.…”