Currently, the steelmaking industry produces iron oxide waste scale resulting in pollution to the environment. It was necessary to have a good understanding of the composition, characteristics and properties of the oxide scale. This study focused on the formation of scale and its adhesion to the hot-rolled steel strip with different thicknesses. The oxide scale formed on an as-received hot-rolled steel strip was investigated by X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive X-ray (SEM-EDS). Magnetite, hematite and iron were found from the XRD results of all samples, which had the thickness strip of 8, 10 and 12 mm. The scale was thinner for the thinner strip. The adhesion test was conducted by a tensile testing machine adapted with an observation set. The strain initiating the first spallation and mechanical adhesion energy was lowest for the sample with the highest thickness (12 mm). These results indicate that the waste scale produced by hot rolled steel industry can be controlled by the final strip thickness. There was a need to control the scale of waste in a reasonable way to protect the environment.

Purpose: The purpose of this study is to investigate the formation, pickling ability and adhesion of thermal oxide scales on the hot-rolled recycled steels produced from the medium and thin slabs. Because the scale on the steel produced from the medium slab was relatively thick of about 11 µm, it contained cracks after hot-rolling. Thus during pickling, the scale was uniformly attacked with the simultaneous dissolution of the inner scale because of the penetration of acid through cracks. However, the scale on the steel produced from the thin slab was thinner of about 6 µm and thus, nearly crack-free. The pickling solution thus attacked the scale surface uniformly. At longer pickling periods, pits were also nucleated and propagated. Concurrently, the tensile testing machine with a CCD camera has been applied to observe scale adhesion. Design/methodology/approach: The formation, pickling ability and adhesion of thermal oxide scales on the hot-rolled recycled steels produced from the recycled slab, e.g. medium slab and thin slab, were investigated. The morphology and phase identification were examined by using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Furthermore, the adhesion behaviour of oxide scale was investigated by immersion test and tensile test with a CCD camera. Findings: For the scale formation, it was found that the hematite and magnetite were formed on the hot-rolled recycled steels produced from the medium and thin slabs. For the immersion test, it was found that the scale on hot-rolled recycled steels produced from the medium slab was more difficult to be pickled as represented by the longer time for the complete pickling. This was consistent with the result of tensile test; the steel produced from the medium slab had better scale adhesion as represented by the higher strain initiating the first spallation of scale. Originality/value: The effects of slab types and its alloying element were investigated to understand the scale adhesion behaviour. The empirical pickling mechanisms and the mechanical adhesion energy were proposed. It led to the understanding in the control of alloying element in the hot-rolled steel.

An investigation of intergranular corrosion (IGC) sensitization in molten nitrate salts of austenitic stainless steel welds of AISI 304, AISI 304H, and AISI321 produced by GTAW with ER 308L and ER 347 fillers was performed. The degree of sensitization (DOS) to IGC was assessed using a double loop electrochemical potentiokinetic reactivation and pitting potential. It was found that DOS levels in weld zones were quite low, not exceeding 15%, while those in HAZs were up to 60% after exposure at 600 °C for 300 h. The low DOS levels were due to low carbide precipitation. However, another cause of DOS was the delta-ferrite to sigma transformation in weld zones. Linear sweep voltammetry was used to quantify the sigma phase.