Corrosion Resistance Of Nickel Free High Nitrogen Austenitic Stainless Steel

Nickel free high nitrogen austenitic stainless steels keep high plasticity and toughness and without magnetic compared with the traditional austenitic stainless steels. But the strength improves obviously. At the same time they have excellent biocompatibility and corrosion resistance. They become the focus of competition and development in the world because they can save the precious resources of nickel and reduce the costs of production. At present, the production of nickel free high nitrogen austenitic stainless steels adopts high pressure metallurgy method in the world. This method has high production costs and high danger, so it is not suitable for mass production. Thus seeking a way of smelting nickel free high nitrogen austenitic stainless steels at atmospheric pressure is the key of material scientific research. The solubility of nitrogen in steels is very low at atmospheric pressure, so compared with high pressure metallurgy, there are lots of nitrogen porosities in the organization of high nitrogen austenitic stainless steels smelted at atmospheric pressure and the distribution of nitrogen is uneven. But the nitrogen porosities will be welded in subsequent rolling process. The distribution of nitrogen is uniform after solution treatment and the organization is full austenite. In this state, the tested steels smelted at atmospheric pressure have excellent corrosion resistance or not is the key of whether they can be applied in practice.In this paper, (Cr18Mn18Mo2NbN0.6) nickel free high nitrogen austenitic stainless steel smelted in an atmospheric pressure was studied. The microstructures of casting, rolling state and solid solution treatment state were analyzed through scanning electron microscope, optical microscope etc. Acid pickling and polarization curve testing of nickel free high nitrogen austenitic stainless steel after solid solution treatment and 1Cr18Ni9Ti were done in acidic medium (10%H2SO4 and 10%HNO3); Salt spray corrosion testing and polarization curve testing of tested steel after solid solution treatment and 1Cr18Ni9Ti were done in neutral medium (3.5%NaCl); The microstructure analysis, FeCl3 soak testing and polarization curve testing of tested steel after precipitation of nitrides, cold deformation 10%,20%,30%,40% and rolling state were done. The experimental results show that the cast structure of nickel free high nitrogen austenitic stainless steel was equiaxial dendritic crystals. Undissolved sheet nitrides and nitrogen porosities existed between dendritic crystals. The nitrogen porosities will be welded in subsequent rolling process. The microstructure of rolling state of tested steel was obvious layer structure. The matrix was austenite. Layer nitrides distributed on the interface of layer structure and between layers. The microstructure of the tested steel after solid solution treatment was full austenite. Lots of annealing twins existed in the microstructure. The grains of austenite were very small. The corrosion rate of nickel free high nitrogen austenitic stainless steel was less than 1Cr18Ni9Ti in 10%H2SO4 and 10%HNO3.The natural corrosion potential and pitting corrosion potential of nickel free high nitrogen austenitic stainless steel was higher, the passivation was very wide. This was because nitrogen improved corrosion resistance. The corrosion rate of nickel free high nitrogen austenitic stainless steel was less than 1Cr18Ni9Ti in 3.5%NaCl. 1Cr18Ni9Ti appeared serious intergranular corrosion while the surface of nickel free high nitrogen austenitic stainless steel only appeared a small amount of smaller pitting corrosion pits. The natural corrosion potential of nickel free high nitrogen austenitic stainless steel was higher and the current density of passivity was smaller. Therefore nickel free high nitrogen austenitic stainless steel had stronger ability of resisting Cl- corrosion than 1Cr18Ni9Ti. The natural corrosion potential and pitting corrosion potential of tested steel after different cold deformation approximately equal with rolling state. But when distortion was greater, the number of pitting corrosion pits increased, and pitting corrosion pits deepen. The pitting corrosion potential of tested steel after precipitation of nitrides obviously reduced and the pitting corrosion pits appeared vastly.