Corrosion issues related to disposal of 316 SS-zirconium metal waste form under simulated repository conditions

Metal waste form (MWF) alloy of D9 stainless steel with 8·5 wt-% zirconium was cast and evaluated for the corrosion behaviour. The microstructure, phase analysis, corrosion resistance and passive film properties of the alloy were evaluated using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and electrochemical methods. The corrosion performance of MWF alloy was carried out in demineralised (DM) water at pH 1, 5 and 8 and also in simulated Kalpakkam and Rajasthan ground water systems. In X-ray diffraction analysis, the identified phases are mostly iron based solid solutions of γ-austenite, Zr–Fe type and Ni–Zr type intermetallics. The typical microstructure of MWF alloy showed the presence of solid solution and the intermetallics. The anodic polarisation results of MWF alloy in Rajasthan ground water and in DM water at pH 5 and 8 showed higher breakdown potential compared with DM water at pH 1 and in Kalpakkam ground water. The electrochemical impedance spectroscopic results of MWF alloy in different media showed the influence of ground water media. The surface morphology after passivation and polarisation revealed the possibility of different types of deposition and dissolution. The corrosion behaviour of MWF alloy in different simulated media and its relation to microstructure and surface morphology are discussed based on the results obtained in relevance with geological repository conditions

Corrosion behaviour of AISI type 304L stainless steel in nitric acid media containing oxidizing species

The corrosion behaviour of AISI type 304L stainless steel (SS) in different concentration of 0.01 M, 1 M and 5 M HNO<SUB>3</SUB> in presence of oxidizing ions at different temperatures has been evaluated. The main objective of this study is to assess the corrosion resistance of type 304L SS in non-radioactive conditions encountered during storage of liquid nuclear waste. Electrochemical impedance spectroscopy (EIS) and laser Raman spectroscopy (LRS) has clearly brought out the deleterious effect of oxidizing species on the passive film leading to increased corrosion along with increase in HNO<SUB>3</SUB> concentration and higher temperature

Corrosion assessment of nitric acid grade austenitic stainless steels

The corrosion resistance of three indigenous nitric acid grade (NAG) type 304L stainless steel (SS), designated as 304L1, 304L2 and 304L3 and two commercial NAG SS designated as Uranus-16 similar to 304L composition and Uranus-65 similar to type 310L SS were carried out in nitric acid media. Electrochemical measurements and surface film analysis were performed to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO<SUB>3</SUB> media. The results in 6 N HNO<SUB>3</SUB> show that the indigenous NAG 304L SS and Uranus-65 alloy exhibited similar and higher corrosion resistance with lower passive current density compared to Uranus-16 alloy. In higher concentration of 11.5 N HNO<SUB>3</SUB>, transpassive potential of all the NAG SS shows a similar range, except for Uranus-16 alloy. Optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO<SUB>3</SUB> with corrosion attacks at the grain boundaries. Frequency response of the AC impedance of all the NAG SS showed a single semicircle arc. Higher polarization resistance (R<SUB>P</SUB>) and lower capacitance value (CPE-T) revealing higher film stability for indigenous NAG type 304L SS and Uranus-65 alloy. Uranus-16 alloy exhibited the lowest RP value in both the nitric acid concentration. Auger electron spectroscopy (AES) study in 6 N and 11.5 N HNO<SUB>3</SUB> revealed that the passive films were mainly composed of Cr<SUB>2</SUB>O<SUB>3</SUB> and Fe<SUB>2</SUB>O<SUB>3</SUB> for all the alloys. The corrosion resistance of different NAG SS to HNO<SUB>3</SUB> corrosion and its relation to compositional variations of the NAG alloys are discussed in this paper

Corrosion behavior of Zr-based metallic glass coating on type 304L stainless steel by pulsed laser deposition method

The surface morphology and corrosion behavior of Zr-based amorphous metallic glass (MG) of Zr<SUB>59</SUB>Ti<SUB>3</SUB>Cu<SUB>20</SUB>Al<SUB>10</SUB>Ni<SUB>8</SUB> alloy and MG coated type 304L stainless steel in different nitric acid media of 1 M, 6 M and 11.5 M HNO<SUB>3</SUB> is reported. Zirconium based MG of Zr<SUB>59</SUB>Ti<SUB>3</SUB>Cu<SUB>20</SUB>Al<SUB>10</SUB>Ni<SUB>8</SUB> alloy was successfully deposited on type 304L stainless steel using pulsed laser deposition technique. The SEM morphology revealed a scattered particles of "Donut" shaped features distributed in the amorphous matrix. The atomic force microscope measurement indicated the formation of dense metallic deposited layer of agglomerate of granular clusters with negligible pores or micro-crack in metallic glass coated sample. The results of the potentiodynamic polarization shows that the amorphous MG coated type 304L stainless steel exhibited marginally lower corrosion resistance than MG alloy which is attributed to the presence of corrosion-induced defects in the coated layer. This work reports suitability of using pulsed laser deposition for the preparation of thin film amorphous metallic coating to achieve improved corrosion resistance in nitric acid medium

Surface optimization of CVD grown silicon carbide interlayer on graphite for plasma sprayed yttria topcoat

Chemical Vapor Deposition (CVD) technique is employed to deposit uniform, smooth, and dense SiC interlayer over High-Density Graphite (HDG) substrate, as an interlayer for subsequent deposition of yttria (Y2O3) topcoat by Atmospheric Plasma Spray (APS) process. In any thermal spray process, since the bonding is purely mechanical interlocking, the surface roughening of smooth CVD grown SiC surface becomes essential. Different surface preparation techniques like alumina grit blasting, plasma etching, laser ablation, and chemical etching were attempted to create rough anchoring patterns on SiC surface. Chemical etching of SiC interlayer by using molten eutectic NaOH/KOH is found to be effective in introducing the desire anchoring sites for Y2O3 splats during plasma spraying. The microstructural features and surface roughness of CVD grown SiC and after subsequent surface roughening treatments were compared using SEM/EDS and profilometer, respectively. The performance and its durability evaluation were carried out by thermal cycling studies on Y2O3 coated samples with and without SiC interlayer at 1723 and 1823 K that shows considerable improvement in the life of plasma sprayed Y2O3 coating with SiC interlayer

In situ surface investigation of austenitic stainless steel in nitric acid medium using electrochemical atomic force microscopy

Using potentiodynamic polarization and electrochemical atomic force microscope, the surface morphology of austenitic stainless steel has been investigated with the aim of understanding the passive film property in nitric acid medium. It has been found that passive film consists of platelet-like structures with a moiré pattern at lower concentrations (0.1, 0.5 M); at higher concentrations (0.6, 1 M) the platelet-like structures disappear, revealing the grain boundary structure

Corrosion investigations on metal waste form alloys of titanium-modified type 316 stainless steel-zirconium in simulated groundwater media

Corrosion assessment of metal waste form (MWF) alloys was carried out in two possible geological repository environments, simulated Kalpakkam groundwater (KGW) and Rajasthan groundwater (RGW). The corrosion resistance properties, passive film stability, and surface morphologies were evaluated by electrochemical and microscopic techniques. Studies were done in two main parts. First, different MWF alloys with varying Zr content from 5 wt% to 17 wt% were compared in KGW to evaluate better corrosion-resistant alloys. All the alloys exhibited a wide range of passivation potential, indicating passive film stability and fairly higher breakdown potentials. However, the alloy with 8.5 wt% Zr showed optimum open-circuit potential (OCP), nobler breakdown potential, optimum passive current, highest polarization resistance, and lowest capacitance; therefore, it was selected as the better alloy. In the second part of the study, the corrosion properties of this better alloy were compared in two simulated RGW media and KGW media. Results showed that because of the lower concentration of chloride in RGW media, the passive film stability was excellent. Scanning electron microscopic observation of surface morphologies after electrochemical studies in both groundwater media showed insignificant corrosion attack. Therefore, this study has clearly shown excellent corrosion performance and good applicability of MWF alloys for geological disposal

Influence of Si and Sb additions on the corrosion behavior of AZ91 magnesium alloy

Small amount of silicon and antimony additions (individual and combined) have been carried out on AZ91 alloy and its effect on electrochemical corrosion behavior is investigated. Immersion test, potentiodynamic polarization and electrochemical impedance measurements have been used to estimate the corrosion rate and the behavior. Individual addition of Si and Sb forms massive Chinese script Mg2Si and needle shaped Mg3Sb2 intermetallics, respectively. Addition of Sb to AZ91 + Si alloy is found to modify the Chinese script Mg2Si intermetallic into a fine polygonal shape besides distributing them evenly along the grain boundaries. Improved corrosion behavior is noticed with Si addition, whereas Sb addition is found to reduce the corrosion resistance of AZ91 alloy. However, combined addition of Si and Sb shows highest corrosion resistance. The Mg2Si intermetallic when present as a fine polygonal shape and distributed evenly along the boundary inhibit the corrosion more effectively compared to the bulk Chinese script. On the other hand, Mg3Sb2 intermetallic in the Sb added alloy, acts as an effective cathode to the α-Mg matrix and thereby increase the corrosion rate