Influence of phosphorus, sulpher, and oxygen on 65 percent nitric acid corrosion resistance of super high-clean 316L stainless steel

摘要

For the purpose to verify the effect of decreasing phosphorus on corrosion properties of stainless steels, corrosion resistant tests in 65 percent nitric acid solutions were carried out for super-low phosphorus content 316L type stainless steels which were manufactured by the cold crucible type levitation melting method using Ca-CaF_2 flux for aiming at the dephosphorization. By decreasing phosphorus content in these steels from 0.026 to 0.0002 mass percent, the corrosion rate in 65 percent nitric acid solutions decreased remarkably. Especially even under 0.001 percent phosphorus content, the improvement of corrosion resistance against nitric acid solution could be observed. As for the non-metallic tramp elements, there were linear relations between corrosion rate and the very small amount of sulfur and oxygen contents in the super-low phosphorus stainless steels containing under 0.0002 mass percent phosphorus. So the super high purification of these elements is also very effective for the improvement of corrosion resistance against nitric acid solutions. It is known that the corrosion of austenitic stainless steels containing over 0.003 mass percent phosphorus in nitric acid solutions is intergranular corrosion. In the present work, it was confirmed that the corrosion of super-low phosphorus content 316L type stainless steels even at 0.0002 mass percent was not uniform corrosion but intergranular corrosion.