Corrosion analysis of post-heat treatment and post-weld SS316 with electrokinetic reactivation and cyclic polarization method

Abstract

Austenitic stainless steel 316 has very high mechanical properties and corrosion resistance. This type of steel is widely used both in the nuclear and non-nuclear industries. In the nuclear industry, SS316 is used as a cladding material for uranium fuel due to its good corrosion and mechanical properties, and also low neutron absorption cross-section. In the Center for Nuclear Fuel Technology (PTBBN BATAN), it is used as material for the container of nuclear waste that is to be stored on Temporary Storage Installation of Spent Fuel (KHIPSB3 BATAN). SS316 is used as material for can to contain high-activity solid waste from the testing activity in Radiometallurgy Installation (IRM BATAN). The lid of the container is sealed with the GTAW welding process in order to tightly contain the solid waste. The main problem with the heat treatment and welding process of austenitic stainless steel is the occurrence of sensitization in a temperature range of 500-800°C. Therefore fully electrochemical analysis of SS316 stainless steel in various mediums has been conducted. SS 316 specimen was heat-treated to simulate the heat generated by the welding process. Sensitization analysis was conducted with qualitative and quantitative methods by EIS and EPR, and pitting corrosion resistance was by cyclic polarization method. The solution used for EIS and cyclic polarization test was 0 − 3.5% concentration of NaCl, and for EPR test was a mixture of H2SO4 and KSCN. Material characterization before and after corrosion testing was microstructure examination. The result of the corrosion test showed that heat treatment on the temperature range of 500-800°C caused sensitization. The corrosion test curve result showed that a heat treatment temperature of 650°C for 1 hour had the highest activation current. The lowest Rp value for SS316 specimen post heat treatment in 675°C was 69.410 ohm. The welded SS316 specimen had a higher corrosion current than that of the unwelded specimen. The microstructure of the welded specimen showed that there was intergranular corrosion particularly in the HAZ region. The effect of NaCl concentration on the cyclic polarization test showed that the higher the NaCl concentration, the more easily the pitting corrosion. The indication of pitting corrosion occurrence was evaluated by considering the Epit and Erp values. The lower Epit value meant that pitting corrosion was more easily to occur.

The Effect of Sensitization on the Susceptibility of AISI 316L Biomaterial to Pitting Corrosion

“…Kartaman et al [34] tested the corrosion resistance of sensitized (675 • C) and consequently mechanically polished AISI 316L SS specimens by PP in 0.01wt.% NaCl solution and recorded E p = 0.391 V vs. SCE and also, similarly to us, higher passive current density compared to the as-received specimen. When partially different conditions [34] are taken into account (lower temperature and chloride concentration, different method of removing high-temperature oxides), the value of the E p can be considered comparable to ours (0.36 ± 0.02 V vs. SCE)…”

“…This may indicate a lower stability of the passive state from a kinetic point of view related to the sensitization, which manifested despite the strengthening of the passive film by nitric acid. Kartaman et al [ 34 ] tested the corrosion resistance of sensitized (675 °C) and consequently mechanically polished AISI 316L SS specimens by PP in 0.01wt.% NaCl solution and recorded E p = 0.391 V vs. SCE and also, similarly to us, higher passive current density compared to the as-received specimen. When partially different conditions [ 34 ] are taken into account (lower temperature and chloride concentration, different method of removing high-temperature oxides), the value of the E p can be considered comparable to ours (0.36 ± 0.02 V vs. SCE) The impaired passive behavior and higher susceptibility to pitting of the welded AISI 316L SS in Hank’s and Ringer’s physiological solutions were confirmed by Farooq at al.…”

“…Despite the low carbon content, sensitization also occurs with AISI 316L stainless steel and induces a higher susceptibility to the pitting corrosion [ 29 , 30 , 31 , 32 ]. According to the authors of [ 25 , 33 , 34 , 35 ], chromium-depleted zones along the grain boundaries more susceptible to corrosive attack compared to the surrounding matrix become preferential sites for pit initiation. Once pitting corrosion is initiated at the sensitized grain boundaries, it can propagate deeper into the material.…”

“…Most current studies [ 17 , 30 , 31 , 32 , 33 , 34 , 35 ] evaluate the effect of sensitization on the resistance to the pitting corrosion of AISI 316L steel by electrochemical methods (often by potentiodynamic polarization and EIS) in NaCl solutions (0.01–3.5%). The above-mentioned authors record a significantly higher susceptibility to pitting and degraded quality of the passive surface film.…”

Oxidation Damage Evolution in Low-Cycle Fatigue Life of Niobium-Stabilized Austenitic Stainless Steel

“…However, when austenitic stainless steel is used in the sensitization temperature range, i.e., 400-850 • C, the affinity of chromium (Cr) and carbon (C) increases, and chromium carbide (Cr 23 C 6 ), a type of metal carbide (M 23 C 6 ), is precipitated inside the material, and the amount of chromium for suppressing the oxidation of the material decreases as carbon and chromium bond with each other for the precipitation of Cr 23 C 6 [1][2][3]. During this process, the thin chromium oxide layer (Cr 23 O 3 ), which exists on the surface of the austenitic stainless steel and serves as a protective film from the external environment, is depleted, and an oxide (Fe x O y ) is formed on the surface of the material [4,5]. When a mechanical load is subsequently applied to the material, the oxide is damaged, and the metal inside the material is exposed to the external environment through open cracks.…”