Effect Of Chromium Carbide (Cr23C6) Formation On The Microstructure And Corrosion Passivity Behavior Of AISI 316

Mohd Ashaari, Omar (2010) Effect Of Chromium Carbide (Cr23C6) Formation On The Microstructure And Corrosion Passivity Behavior Of AISI 316. Project Report. UTeM, Melaka, Malaysia. (Submitted)

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Abstract

AISI 316 is known as austenitic stainless steel which has Face Centre Cubic (FCC) structure. AISI 316 has good corrosion resistance at the ambient temperature. However, the corrosion behavior of AISI 316 is change due to the environment condition. AISI 316 normally used particularly for sour service applications where corrosion resistance is required in chloride environments. While these steels show many superior characteristics, limitations are associated with the welding of these steels, particularly controlling the weld structure and properties and understanding how the weld metallurgy may influence the susceptibility to intergranular attack. The focus of this study is to report on the findings from a detailed study of the various heating time within AISI 316 as a function of heat input and type of sensitization occurred, in terms of the metallurgical structure, composition, and mechanical properties and to assess the susceptibility to intergranular attack. Heating procedure was performed using the manual Oxyacetylene fuel cutter technique for 5 seconds, 10 seconds, 20 seconds, 40 seconds, and 60 seconds as a function indicate that at both long and short heating time. Structural analysis consisted of optical microscopy of heated specimens, chromium carbide (Cr23C6) and ferrite determination by using X-ray diffraction (XRD), including Vickers hardness measurements. Cyclic Potentiodynamic Polarization (CPP) test was used to determine pitting corrosion at susceptible intergranular attack towards AISI 316 structure. The electrolyte solution used was 3.5 % NaCl as alternative of aggressive chloride environments. The test was conducted at ambient temperature. The potential was scanned from -500 mV (SCE) to +200 mV (SCE) and back to -500 mV (SCE) at a rate of 1.67 mV/s and the ratio of the reactivation charge to the passivation charge was calculated. From the results obtained, it can be shown that it was correlated with the local variations in material compositions and microstructure caused by the heat sensitizing procedure. From the XRD analysis, the Cr23C6 is detected at the XRD graph pattern by following the reference point obtained from International Center for Diffraction Data (ICDD) library. Thus, it indicated the presence of Cr23C6 at the tested samples. The present of Cr23C6 is obtained during microstructure analysis which is found the Cr23C6 structure at the tested samples. CPP analysis indicates that the corrosion passivity behavior of as received material of AISI 316 had noblest pitting nucleation resistance (Epit) with the value of 301 mV and showed the widest immunity zone or protection range which is 192.8 mV. The Epit and immunity zone of AISI 316 is decrease with increasing heating time towards it. The worst corrosion passivity behavior is occurred to specimen heated for 60 s. Epit of this specimen was shifted to active potential by getting -84 mV and present lowest immunity zone with it protection range only 173 mV. Since 60 seconds show the best view of the three heated structure zone, the results show significant value of hardness results. Fully heated zone get 223 kgfmm-2 higher value of hardness compare to HAZ with 193 kgfmm-2 and base metal 175 kgfmm-2. As a conclusion of this study, when heating time towards AISI 316 is increase, the grain boundaries of AISI 316 became broader and the Cr23C6 formation in the grain is also increase makes it hardness properties is increase. However, at the region rich with Cr23C6 is easily attack by environment containing chloride ion.

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