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 arewidely usedin industry,iswhy it isinteresting to know itscorrosionresistancein aggressivemedia, such asmediaormarine environmentschlorinateddrinking water.The method of productionofalloys andthecontent thereofplay an important roleinthe final propertiesof the resulting material.In this connection, it was observed that introducingmolybdenum to the stainless steelalloyisbeneficial to corrosion process inchloride media[1], also improving the passivityof these materials[2].In contrast, themanganesehas a detrimental effectoncorrosion resistancein 3.5% NaCl. [3] Hasalso been studiedthe development ofdendritic microstructureduring solidification316L steelandotheralloys[4,5], correlating the results withmathematical expressionsavailable in the literatureto predictthesecondarydendritic spacing[6]. Additionally, electrochemical testson316steelsusingElectrochemicalImpedanceSpectroscopy (EIS) technique [7,8]are the basis forthis investigation.Thiswork aims tostudytheoverallinfluence of the variationof the structure(equiaxed, columnarandcolumnartoequiaxedtransition-CET) on thecorrosion resistanceof316Lstainless steelinaqueous 3% NaCl(pH=5.5),at room temperature(Figure 2), usingcyclicpotentiodynamic polarizationandelectrochemical impedance spectroscopy(EIS)techniques and investigate the relationship betweenthecorrosion resistanceandthesecondarydendriticspacing evolution in the material.From the present research the principal conclusions are:- 316L stainlesssteels had defined areas of passivity with variable repassivation capacity.- The stainless steel cylindercontaining 8% Mo (specimen C) had a good repassivation capacity after pitting in all its solidificationstructures, 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 athigh frequencies and a diffusion process at low frequencies.- Impedance results indicated that the 316Lstainless steel with lower content of Mn has a low resistance to charge transfer in equiaxed and CETstructures, making it more susceptible to pitting.- In general the sample with higher content of Mo in equiaxedand columnar structures was the most resistant to both charge transfer and the dissemination of vacanciesin the film.- Areas with equiaxed structure showed the highest values of secondary dendriticspacing in most of the studied samples, and a good ability of repassivation after pitting, while areas withcolumnar and CET structures showed passive zones were larger than the case of equiaxed structure.