Influence Of Phase Composition And Structure Of Oxide Scales On Corrosion Resistance Of SS400Hot Rolled Strip

In this paper, oxide scales with different phase composition and structure arefabricated on the surface of SS400hot rolled strip by various heat treatment processes.The phase composition and structure of oxide scale are analyzed by a series of analysismeans, such as, XRD, EDS, LRS, EBSD, SEM, OM. Accelerated cyclic wet-dryimmersion corrosion test, salt spray test and electrochemical test （polarization curves,electrochemical impedance spectroscopy, corrosion potential test, scanning Kelvinprobe test） are used to investigate the corrosion behaviors and mechanism of strips withdifferent oxide scales under the same test condition. The experimental results as follow:The scale prepared by the process A （heating at560oC for3h in the air and thencooling down to room temperature in the furnace） is very dense and continuous, withthe existence of few micro-pores at the scale. It has a great many white globular phaseon irregular surface. The oxide scale composition consists of a large quantity of theinner layer Fe₃O₄phase and a small amount of the outer layer Fe₂O₃phase. The averagegain size of Fe₃O₄phase is0.285μm.The scale fabricated by the process B （heating at560oC for5h in the air and thencooling down to room temperature in the furnace） is dense, divided into two layers.However, many defects existe in the joint between the inner and the outer layer scale. Ithas a large amount of white globular phase densely distributed on irregular surface. Theoxide scale composition consists of a large quantity of the inner layer Fe₃O₄phase and asmall amount of the outer layer Fe₂O₃phase, comparing with the process A, it has morecontent of Fe₂O₃phase. The average gain size of Fe₃O₄phase is0.376μm.The scale prepared by the process C （heating at560oC for3h in the air and thencooling down to room temperature in the furnace, followed by the vacuum heattreatment （heating at560oC for3h, cooling in the vacuum furnace）） is dense, mainlycomprises of gray color phase with a small distribution of light-gray color phase and hasfew micro-pores. It has very few white globular phase dispersedly distributed on theregular surface. The scale mainly consists of Fe₃O₄, as well as a spot of Fe₂O₃.Theaverage gain size of Fe₃O₄phase is0.464μm.The scales fabricated by the processes D/E （heating at500oC for6h in the air andthen cooling down to room temperature in the furnace, followed by the vacuum heattreatment （heating at580oC for1.5h, cooling with a slower cooling rate or furnacecooing in the vacuum furnace）） is dense and continuous, mainly comprises of light-graycolor phase with a small distribution of gray color phase and white granule phase. The original phases of the scales fabricated by the processes D/E are Fe₃O₄/Fe₂O₃/FeO/Fe.Comparing to the former, the latter has more content of FeOphase and less content ofFe3O4phase. Both the two scales have little content of Fe₂O₃phase. Both the two scalesare irregular and have no obvious white globular phase. The scale prepared by theprocess E has many micro-pores. The average gain sizes of Fe₃O₄phase are0.668μm,0.853μm respectively （D/E）.The corrosion resistance of different hot rolled strips is close to the phasecomposition and structure of the scales, which is indicated by the experiment results ofaccelerated cyclic wet-dry immersion corrosion test, salt spray test, polarization curves,corrosion potential test, electrochemical impedance spectroscopy test, and scanningKelvin probe test. The corrosion behavior and corrosion mechanism of hot rolled stripswith different oxide scales is various. The oxide scale which consist of a small mount ofthe outer layer Fe₂O₃phase distributed continuously and a large quantity of the innerlayer Fe₃O₄phase with the fine grain size, and possess nice compactness, continuity,integrity in the morphology structure, has very excellent corrosion resistance property.