Volume 41, Issue 2

High temperature molten salt corrosion is of utmost importance for selecting and qualifying structural materials for critical applications. Pyrochemical reprocessing of spent metallic fuels of the future fast breeder reactors in India is widely considered. One of the main processes of pyrochemical reprocessing is electrorefining. Electrorefining is generally conducted in LiCl–KCl molten salt at 500–600 °C under an inert atmosphere. Research groups worldwide are involved in developing corrosion resistant materials and investigating the corrosion behaviour of various structural materials for LiCl–KCl applications under different environments. A wide variety of materials, including metals, alloys, intermetallics, single crystals, glass and ceramics, have been investigated in molten LiCl–KCl salt. This review focuses mainly on the corrosion assessment of materials for LiCl–KCl application; a complete literature review with emphasis on the corrosion issues of materials is provided. This paper reviews the corrosion issues of metals and alloys in molten salts and the selection criteria of corrosion-resistant materials for molten salts. Understanding the molten salt corrosion mechanisms and future research scope are also discussed.

Iron-chromium-aluminum (FeCrAl) alloys are being considered for the cladding of uranium dioxide fuel in light water reactors (LWRs). FeCrAl alloys have good mechanical properties at temperatures of 300 °C and higher, and have superlative resistance to attack by steam at temperatures of up to 1000 °C and higher. A concern has been raised that the use of FeCrAl for cladding would result in a higher content of tritium in the reactor coolant as compared with the current system where the cladding is a zirconium based alloy. This review shows that the flux of tritium from the fuel rod cavities to the coolant across the fuel cladding wall will be greatly reduced by the presence of oxides on the surface of the cladding. The review of current literature and permeation data show that (a) protective oxides are expected to be present on both sides of the FeCrAl cladding, and (b) depending on the characteristics of these oxide layers it is reasonable to expect about two–three orders of magnitude reduction in tritium permeation, relative to the permeation response in clean, unoxidized condition for FeCrAl steels of interest, around 277 °C–377 °C temperatures.

Chromium-free passivation of Zn coating on NdFeB magnets becomes a research hotspot due to the serious harm of chromium ions to the human body. Chromium-based and cerium-based passivation technologies are conducted on electroplating Zn/NdFeB respectively. Morphologies, elemental compositions and phase structures of the two passivated coatings are characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The corrosion resistances of the two passivated specimens are compared by neutral salt spray test, accelerated aging test and electrochemical measurements. A complete and smooth passivation film can be obtained on the surface of Zn/NdFeB, filling the gaps and pores in Zn coating. Compared with un-passivated Zn/NdFeB, Zn(Ce)/NdFeB and Zn(Cr)/NdFeB possess excellent corrosion resistance. In comparison, Zn(Ce)/NdFeB possesses excellent anti-corrosion performance, increasing the red-rust appearing time from 288 to 432 h, which is still lower than that of Zn(Cr)/NdFeB (528 h). Therefore, the self-repair effect of cerium passivation technology during the corrosion process should be further studied to achieve the purpose of replacing chromium passivation technology.

This study investigates an environment-centered, state-driven corrosion prognosis framework to predict the long-term atmospheric corrosion loss of metal materials, and this paper takes carbon steel as an example to show the establishment process of the framework. Unlike traditional power-linear prediction models that seldomly consider environmental impacts, the proposed model quantitatively establishes the correlations between corrosion loss and dynamic atmospheric environmental factors. A comprehensive power-linear function model integrating multiple atmospheric environmental factors is constructed, following the corrosion kinetics robustness. Under the proposed framework, the steady-state start time is evaluated, followed by the long-term corrosion loss prediction under different corrosivity categories and test sites. The applicability is justified via a case study of long-term field exposure tests of metal materials in China, as well as the experimental results of the ISO CORRAG program. By comparing with the traditional power model and ISO model, the experimental results demonstrate the capability and effectiveness of the proposed prognosis methodology in acquiring accurate corrosion state information and corrosion loss prediction results with less input corrosion information.

In corroded steel anchorages, radial pressures generated by corrosion and bond are balanced by hoop tension provided by the concrete cover. As the concrete has negligible tension strain capacity, in the absence of confinement, anchorage failure occurs by cover splitting and consequent bond deterioration. This paper presents an experimental study of short reinforcement anchorages embedded in a strain resilient cementitious composite matrix, which were preconditioned under simulated corrosion before mechanical loading. The experiment was meant to assess the restraining effect of randomly dispersed plastic fibers on the build-up of expansive rust, by measuring the degree of rebar corrosion that led to cracking of the concrete cover, and the consequent improvements in residual anchorage capacity, by measuring the mechanical response to four-point loading. A total of eighteen short beams were tested with variables being the anchorage length extending into the shear span of the beams as well as the matrix (with and without fibers). Comparisons among artificially corroded and non-corroded coupons demonstrate the favorable effect a strain resilient matrix has in delaying metal consumption as a consequence of the fibers’ restrain against crack initiation/widening; their mechanical testing reveals even increase of local bond strength for corrosion level less than 10%.

Chloride-induced crevice corrosion ( E R,CREV ) of alloys UNS N06059 and UNS N06686 was studied at different temperatures in 0.1, 1 and 10 M chloride solutions. Crevice corrosion occurred several degrees below the reported critical crevice temperatures obtained through standard immersion tests. The repassivation potential of the tested alloys as a function of temperature and chloride concentration was given by E R,CREV  = ( A  + BT)log[Cl − ] + CT +  D for a range of environmental conditions. When temperature and chloride concentration increased E R,CREV showed a lesser dependence on the environmental variables. The repassivation potential of Ni–Cr–Mo–(W) alloys was described by a new proposed equation in terms of [Cl − ], T, Cr, Mo and W, alloys in wt%. The dependence of E R,CREV with the weight % of main alloying elements was 5–6 mV/%Cr, 17–18 mV/%Mo and ∼9 mV/%W, at 85 °C in chloride solutions. An optimal main alloying elements relationship was noted that maximizes the E R,CREV value. The optimal alloy ratio would be 1:3.3:1.65 for wt%Cr, wt%Mo and wt%W, the same factors as in the PRE equation. The optimal alloying ratio would be independent of the alloy composition since it is not a function of the content of main elements.

Sodium molybdate (Na 2 MoO 4 ) was selected as the corrosion inhibitor, compounded with benzimidazole, in order to prolong the service life of the hot-dip galvanized steel (HDGS) in the marine environment in this article. XRD, SEM/FESEM and EDS were used to characterize the micro-morphology and elemental composition of HDGS. Immersion corrosion test, Tafel polarization and EIS test were carried out to study the effect of compound inhibitor on the corrosion resistance of HDGS in the marine environment. The best proportion of compound inhibitor was added to the self-made waterborne polyurethane coating (WPUC), aiming to evaluate its influence on the coating performance in the marine environment by immersion corrosion test and EIS test. The results showed that Na 2 MoO 4 , as a passivation type corrosion inhibitor, bounded Zn at the active sites of HDGS coupons and created structural defects. Benzimidazole, as an adsorption type corrosion inhibitor, was attracted by electricity and adsorbed at the structural defects. Under the premise of 1 wt% total content, the optimal ratio of Na 2 MoO 4 & benzimidazole was 9:1 and the corrosion inhibition efficiency was 99.62%. The corrosion current density of HDGS in the simulated seawater with compound inhibitor was 5.650 × 10 −8  A/cm 2 , while that of HDGS in the simulated seawater without compound inhibitor was 1.483 × 10 −5  A/cm 2 . The WPUC containing compound inhibitor had a small decrease in corrosion resistance due to defects created by doping at the beginning of immersion, then the compound inhibitor would play an active role in the corrosion process to make more than double the service life of WPUC.