Two ferritic AISI 430 and AISI 441 and two austenitic AISI 304 and AISI 316L stainless
steels were submitted to short term oxidation in a complex atmosphere 3% O2, 16% H2O, 8% CO2,
73% N2 to simulate phenomena occurring during the rapid furnace annealing taking place after the
final cold rolling. This thermal sequence is devoted to metallurgical aims but generates undesirable
oxides which have to be further pickled. Temperature of the furnace was set to the values used in
industrial practice: 900°C for 430, 1060°C for 441 and 1120°C for both austenitics. Six different
oxidation durations were used between 30 and 300 s. For the shortest times, sample temperature
was not constant and heating rate depended on sample thickness. Oxide thickness measured by
GDOS was shown to increase monotonically for all grades whereas mass change measurements
exhibited initial mass losses for the austenitic grades. XRD and Raman spectroscopy were used for
phase characterisation and confirmed the increase of the ratio chromia/haematite with increasing
annealing time. Enrichment of manganese (MnCr2O4), silicon (SiO2) and boron (B-containing
oxide) at the external (Mn) and internal (Si, B) interfaces was observed on the GDOS profiles
(boron for austenitic grades only). Manganese spinel was responsible for blocking chromium (VI)
volatilisation after a certain time, and interface oxides for hindering chromium transfer from the
steel to the oxide scale. Ferritic grades behaved the same, except that no boron enrichment was
detected. Besides, stabilised 441 exhibited Ti and Nb enrichments as oxides at both internal and
external interfaces. External TiO2-NbO2 solid solution was assumed to be hardly dissolved in acidic
pickling baths. All these results were consistent with the different pickling behaviours of the
materials.
Effect of solution pH on the electrochemical behaviour of AISI 304 austenitic and AISI 430 ferritic stainless steels in concentrated acidic media