RESISTANCE TO CORROSION AND PASSIVITY OF 316L STAINLESS STEEL DIRECTIONALLY SOLIDIFIED SAMPLES

CLAUDIA M. MENDEZ; COVINICH, MÓNICA M.; ALICIA E. ARES

DEVELOPMENT IN CORROSION PROTECTION

INTECH

Lugar: Rijeka, Croatia; Año: 2014; p. 41 - 63

Stainless steels are widely used in industry, is why it is interesting to know its corrosionresistance in aggressive media, such as media ormarine environments chlorinated drinking water. The method of production of alloys and the content thereof play an important role inthe final properties of the resulting material. In this connection, it was observed that introducing molybdenum to the stainless steel alloy is beneficial to corrosion process inchloride media [1], also improving the passivity of these materials[2]. In contrast, the manganese has a detrimental effect on corrosion resistance in 3.5% NaCl. [3] Has also been studiedthe development of dendritic microstructure during solidification 316L steel and other alloys [4,5], correlating the results with mathematical expressions available in the literature to predict thesecondary dendritic spacing [6]. Additionally, electrochemical tests on 316 steels using Electrochemical Impedance Spectroscopy (EIS) technique [7,8] are the basis for this investigation. This work aims tostudy the overall influence of the variation of the structure(equiaxed, columnar and columnar to equiaxed transition-CET) on the corrosion resistance of 316L stainless steelin aqueous 3% NaCl(pH = 5.5), at room temperature (Figure 2), using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques and investigate the relationship between thecorrosion resistance and the secondary dendritic spacing evolution in the material. From the present research the principal conclusions are: - 316L stainless steels had defined areas of passivity with variable repassivation capacity. - The stainless steel cylinder containing 8% Mo (specimen C) had a good repassivation capacity after pitting in all its solidification structures, while 3% Mo and 2% Mn alloy composition (specimen B) reached more anodic pitting potentials. - Stainless steel alloy composition corresponding to 0.50% Mn (Specimen D) is susceptible to pitting corrosion. - Impedance diagrams obtained at corrosion potentials of each specimen exhibited a capacitive behavior at high frequencies and a diffusion process at low frequencies. - Impedance results indicated that the 316L stainless steel with lower content of Mn has a low resistance to charge transfer in equiaxed and CET structures, making it more susceptible to pitting. - In general the sample with higher content of Mo in equiaxed and columnar structures was the most resistant to both charge transfer and the dissemination of vacancies in the film. - Areas with equiaxed structure showed the highest values ​​of secondary dendritic spacing in most of the studied samples, and a good ability of repassivation after pitting, while areas with columnar and CET structures showed passive zones were larger than the case of equiaxed structure.