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Corrosion behaviour of three commercial sintered Nd–Fe–B magnets exposed to environments containing water as vapour, pressurised vapour, and liquid was investigated in order to understand their overall corrosion performance under a range of conditions. Two types of heat humidity exposure tests, namely the 85/85 and pressure cooker test, and the immersion test combined with electrochemical measurements were used as corrosion tests. It was observed that varying the temperature, pressure, and the prevailing state of water in the exposure tests, different corrosion mechanisms were detected on the surface of Nd–Fe–B magnets. The surface finish of the magnet had an effect on the initiation of corrosion in mild heat-humidity exposure. Immersion in liquid water resulted in a corrosion topography where the Nd-rich grain-boundary phase did not corrode selectively as in the other accelerated corrosion tests but was retained intact while the matrix phase underwent corrosion. These results and the dominant corrosion mechanisms of sintered Nd–Fe–B magnets in different environments are presented and discussed in this paper. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we study the corrosion of cadmium plating by four runway de-icing chemicals using electrochemical measurements and standard (immersion) and proposed (cyclic) runway de-icing corrosion tests for cadmium plating. Besides the obtained electrochemical and gravimetric data, we analyze the exposed surfaces by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and infra-red spectroscopy (IR). The chemicals included in the tests are pure potassium formate, pure potassium acetate and the corresponding commercial chemicals. Examinations revealed two parallel and linked phenomena on the plating surface, i.e., anodic dissolution of cadmium and development of corrosion products, primarily cadmium carbonate, CdCO3. Electrochemical measurements disclosed that the formed corrosion products slightly improved corrosion resistance of the surface but definitely did not stop corrosion completely, probably due to discontinuous structure. As for the studied de-icing chemicals, potassium formates systematically introduced more segmented corrosion products than potassium acetates and, subsequently, more severe corrosion on the specimens. Immersion cadmium corrosion test did not give very reliable results due to relatively low introduced corrosion rates and very large occasional scatter in the results. Cyclic cadmium corrosion tests introduced relatively greater corrosion rates and somewhat lower deviations than the immersion test, being hence, more reliable, but also more challenging in terms of meeting the weight change criteria for passing the test. [Copyright &y& Elsevier]

Abstract: Corrosion attack in the form of corrosion product tubercles was observed in an AISI 304 (EN 1.4301) stainless steel heat exchanger only after 36 months of service. Failure analyses revealed that in one of the attacked areas corrosion had penetrated the entire wall thickness of 6.2mm, but in most of the cases it reached the depth of 2–4mm. In this paper, we report the results from a thorough microstructural characterization of the corroded heat exchanger carried out with optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffractometry (XRD). Microstructural studies by OM, SEM and XRD revealed a two-phase structure of austenite and ferrite in the bulk material, as well as the preferential attack of the ferrite phase. SEM surface studies disclosed bacteria in and close to the attacked areas. Cross-sectional SEM examinations showed the distribution and composition of corrosion products within and underneath the tubercles. TEM and XRD studies gave information about the amorphous and/or nanocrystalline nature of some of the formed corrosion products. These results are discussed in this paper and, based on them, the main corrosion mechanism for the observed attack is suggested. Further, explanations for the propagation of corrosion along the ferrite phase are presented. [Copyright &y& Elsevier]

Abstract: Coatings of two types of polyimides (PI), as poly(4,4′-oxydiphtalic anhydride-co-2,5-bis(4,4′-methylenedianiline)-1,4-benzoquinone) (AQ) and poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PM), were synthesized on galvanized steel panels and studied and compared in terms of chemical structure, microstructure and corrosion performance in 0.5M NaCl solution. Infra-red spectroscopy, scanning electron microscopy and in situ electrochemical impedance spectroscopy were employed in the investigations. The results showed that, although both studied PI coatings provided the galvanized steel substrate with corrosion protection during the test period, there were evident differences in electrochemical behaviour of the coatings, which could be primarily explained by the different nanostructures. FE-SEM examinations revealed AQ PI coating to be heterogeneous and discontinuous but PM PI coating homogeneous and continuous in nanostructure. Electrochemical behaviour of AQ PI coated galvanized steel obeyed that of a defect-containing coating and indicated gradual decrease in protectivity. In contrast, PM PI coating behaved like a defect-free coating and it provided the galvanized steel substrate with effective corrosion protection all through the 960h test. The explanations for these observations and the mechanisms of coating damage are discussed in this paper. [Copyright &y& Elsevier]

The influence of two surface pretreatments, glass-bead blasting and grinding, on the development of aluminide diffusion coatings on 9%Cr steel substrates has been studied by optical, scanning electron and transmission electron microscopies. The results showed that pretreatment of the surface by glass-bead blasting yields an aluminised layer with a more uniform structure and less thickness variation than pretreatment by grinding. These beneficial consequences of glass-bead blasting are introduced by an increase in surface roughness which, by increasing the specific surface area that is accessible to gaseous metal halides to react with the substrate, to release aluminium, facilitates the surface reactions step in the coating development process, and by an increase in dislocation density. Furthermore, changes in dislocation arrangement take place as a result of glass-bead blasting, as the dislocations form into an arrangement of cells and tangles. As such defects contribute to coating nucleation and solid-state diffusion, it is apparent that glass-bead blasting of the substrate surface affects the surface reaction and the solid-state diffusion stages of the aluminising process. However, the dislocation arrangement developed during the glass-bead blasting pretreatment changes and the density of dislocation decreases during annealing at 650°C, the pack aluminising temperature. Coating development is, therefore, influenced by the treatment only in the early stages of the aluminising process. [ABSTRACT FROM AUTHOR]

The electrochemical current noise method is widely applied for characterising the surface phenomena of bulk metals or polymer-coated metals. No studies, however, have been available on its use for the characterisation of metallic coatings. This study explores the potential of electrochemical current noise measurements to characterise the quality of chemical tin coatings on the inner surface of copper tubes. The study is carried out by using scanning electron microscopy (SEM), open circuit potential surveys and actual electrochemical current noise measurements. The results obtained by electrochemical current noise measurements were consistent with the microstructural examination and thermodynamical corrosion studies of chemical tin coatings on copper. The statistical analysis of electrochemical current noise data presented either in time or in frequency mode is capable of characterising the chemical tin coatings in terms of uniformity, surface morphology and porosity. The peak shapes of current noise transients reflect the electrochemical phenomena taking place in the tin coating–copper system exposed to drinking water. [Copyright &y& Elsevier]