Corrosion Resistance and Thermal Conductivity Enhancement of Reduced Graphene Oxide-BaSO4-Epoxy Composites.

The results of studies on the corrosion protectiveness and thermal conductivity of reduced graphene oxide-BaSO4 epoxy composites are reported here. A commercial epoxy resin and reduced graphene oxide (rGO) were blended with a hardening reagent and then mixed with prepared BaSO4-epoxy resin (B-epoxy). The reduced graphene oxide-BaSO4-epoxy composite (rGO-B-epoxy) paste was used to coat the surfaces of Al 7205 alloy and the corrosion and thermal properties were investigated. A corrosion test in a 3.5 wt% synthetic sea water solution showed that the composite coating containing BaSO4 had the best corrosion resistance. Moreover, the rGO-B-epoxy composite showed better protection against corrosion than the epoxy alone. The rGO-B-epoxy composite with 5 wt% BaSO4 had an in-plane coefficient of thermal conductivity of approximately 165.0 W/m K, and the in-plane thermal diffusivity was 71.38 mm2/s. In standard thermal conductivity tests, all three samples had values below 40 W/m K. The rGO-B-epoxy composites showed good surface corrosion protection and in-plane thermal conductivity.

Oxidation Behavior Characterization of Zircaloy-4 Cladding with Different Hydrogen Concentrations at 500-800 °C in an Ambient Atmosphere.

This study simulated the after-burned zirconium cladding oxidation in air at temperatures between 500 and 800 °C. The weight changes of Zircaloy-4 cladding with hydrogen contents of 100-1000 ppm continuously measured through thermogravimetric analysis (TGA) during oxidation tests at different temperatures in an air atmosphere. The TGA results indicate a transition of oxidation kinetics from a parabolic rate law to a linear rate law for as-received and hydrided Zircaloy-4 cladding. The hydrogen concentration of Zircaloy-4 had a marked effect on its pre-transition oxidation in air between 500 and 800 °C. For all samples, the linear oxidation (post transition stage) at 650 °C, which is the critical oxidation temperature, displays a similar trend. In addition, scanning electron microscopy and transmission electron micros-copy examinations indicated the presence of a few and numerous discontinuous micro-cracks in the oxide layer in the pre-transition and post-transition stages, respectively.

The Corrosion Susceptibility of 304L Stainless Steel Exposed to Crevice Environments.

The present study focuses on the corrosion behavior of 304L stainless steel in crevice corrosion environments. The specimen with a salt deposit of 0.1 g/m2 was assembled with a crevice former made of Poly-tetra fluoroethylene (PTFE) to make a test device. The assembled test devices were kept at the ambient temperature of 45 °C in combination with a relative humidity of 45%, 55%, and 70%. After testing for 5000 h, the corroded area of the specimen exposed to 70% humidity was three times larger than that subjected to 45% humidity. For the specimen sustaining a tensile force, the crack growth rate was approximately 1.4 mm/year at the stress level of 300 MPa in a crevice corrosion environment with 0.1 g/m2 of sea salt deposited on the surface. The small portion of intergranular cracking occurred at the surface due to the existed strain on the surface. As cracks propagate in a grain, the grain undergoes a greater localized deformation, and some secondary cracks would develop inside the grain; transgranular cracking was vigorous due to the path corrosion that nucleated at the slip steps.

The Effect of Deposited Dust on SCC and Crevice Corrosion of AISI 304L Stainless Steel in Saline Environment.

Crevice corrosion has become an important issue of the safety of AISI 304L austenitic stainless steel canister when exposed to the chloride environments located in coastal areas. Moreover, dust deposited on the canister surface may enhance the corrosion effect of 304L stainless steel. In this work, white emery was adopted to simulate the dust accumulated on the as-machined specimen surface. To investigate the effect of deposited white emery, chloride concentration, and relative humidity on the crevice corrosion behavior, an experiment was conducted on 304L stainless steel specimens at 45 °C with 45%, 55%, and 70% relative humidity (RH) for 7000 h. The surface features and crack morphology of the tested 304L stainless steel specimens were examined by SEM equipped with energy-dispersive spectrometry (EDS) and electron back scatter diffraction (EBSD). From the experimental results, a threshold RH for the stress corrosion cracking (SCC) initiation of AISI 304L austenitic stainless steel with different concentrations of chloride was proposed.

Surface Structure Analysis of Initial High-Temperature Oxidation of SS441 Stainless Steel.

Chromia-forming ferritic stainless steel (FSS) is a highly promising interconnect material for application in solid oxide fuel cells. In this study, initial oxidation of chromium oxides was performed at 500-800 °C to understand the evolution of materials at an early stage. The structural variations in oxide scales were analyzed through scanning electron microscopy, energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffractometry (XRD), laser confocal microscopy (LSCM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Surface electrochemical properties were investigated through electrochemical impedance spectroscopy to understand how the heat treatment temperature affected surface impedance. Treatment temperatures higher than 700 °C facilitate the diffusion of Cr and Mn, thus allowing ferritic spinels to form on the surface and leading to high electrical conductivity.

Influence of Chloride Concentration on Stress Corrosion Cracking and Crevice Corrosion of Austenitic Stainless Steel in Saline Environments.

Stainless steels are used as canister materials for interim storage of spent fuel. Crevice corrosion has proved to be a safety concern of 304L stainless steel spent fuel canisters, when exposed to the saline environments of coastal sites. To study the effects of chloride concentration and test duration on the crevice corrosion behavior, and the effect of relative humidity on the initiation of discrete SCC cracks, a test program was conducted on the 304L steel specimens sprayed with synthetic sea water of 3.5 wt.%. The salt-deposited specimens, wrapped up with a crevice former to form a crevice configuration, were then exposed to an environment at 45 °C with a pre-set 45%, 55%, and 70% relative humidity (RH), for 400 h and 10,000 h, respectively. The surface features and crack morphology of the tested 304L stainless-steel specimens were examined by energy-dispersive spectrometry (EDS) and electron back scatter diffraction (EBSD). For the specimens deposited with a chloride concentration of 1 g/m2, no cracks were found in the corroded regions after 400-h exposure, whereas SCC cracks were observed with the specimens tested for 10,000 h at all three pre-set relative humidity. The specimens tested at the pre-set relative humidity 45% are characterized with discrete SCC cracks, but, on the other hand, those exposed to the environments of 55% and 70% relative humidity show SCC cracks of distinct features. From the results of 10,000-h tests, it is inferred that the chloride concentration threshold for SCC initiation of 304L stainless steel at 45 °C is between 0.1 g/m2 and 1 g/m2.