SUS 304鋼におけるCr欠乏層による耐食性の劣化

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  • The Decline of Corrosion Resistance by Cr-depletion Layer in SUS 304 Stainless Steel

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When stainless steels are annealed in gas burning furnace to form surface oxide scale enriched in Cr, Cr depletion zone is generated due to the delay of Cr diffusion from matrix to surface. Therefore, insufficient pickling causes a decline of corrosion resistance. In this work, the relation between corrosion resistance and the degree of Cr depletion was studied quantitatively. SUS 304 stainless steel was annealed in siliconit furnace set at 1, 150°C, in which exhaust gases generated from the burning of propane were filled up, and immersed in alkaline molten salt (NaOH 80%, NaNO320%, 450°C, 5 sec), and then immersed in the pickling solution (HF2%, HNO37%, 50°C) to etch the Cr-depleted layer for 5, 10, 20, 30, 40 and 50 sec respectively. After pickling, depthprofile of Cr and corrosion resistance against Cl- ion were measured by electrochemical method. Surface Cr concentration and the corrosion resistance increased with increasing pickling time, but etching speed slowed down when it became 16-17%, and the roughness of surface increased to cause the decline of corrosion resistance. Therefore, it is concluded that a pickling time of 35-45 sec is suitable in this work.

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