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Ferritic-martensitic steels are found in many elevated temperature applications due to their excellent strength properties and thermal conductivity. However, their resistance to elevated temperature corrosion, wear and combination of these is typically not at a desired level. One solution is to improve the surface properties by the application of a coating. In this study, aluminium diffusion coatings were deposited on 9Cr-1Mo steel and characterised in terms of microstructure and elevated temperature corrosion and erosion-corrosion behaviours. The two behaviours are then compared to those of an uncoated steel. The results from the tests indicate that aluminising shows great potential under the studied demanding elevated temperature conditions. The benefits and challenges of the deposition and use of aluminised coatings are discussed in this paper. [ABSTRACT FROM AUTHOR]

Abstract: Degradation of materials by a combination of erosive wear and atmospheric oxidation at elevated temperatures constitutes a problem in some power generation processes, such as fluidized-bed combustion. In this work, 9Cr-1Mo steel, a common tube material in combustion chambers, is coated by a pack cementation method from an Al-containing pack in order to improve the resistance to erosion-oxidation at elevated temperatures. The resulting coating is studied in terms of microstructure and microhardness and tested for its resistance against impacts by sand particles in air at temperatures of 550–700°C under several conditions, with thickness changes and appearance of the exposed surfaces being studied. The coating was found to contain several phases and layers, the outermost of which was essentially Al-rich and contained e.g., small AlN precipitates. The microhardness values for such coating ranged from 950 to 1100HV20g. The coating provided the substrate with increased protection particularly against normal particle impacts, as manifested by smaller thickness losses for coated specimens as compared to uncoated counterparts. However, much of the coating was lost under all test conditions, despite the fact that particle debris formed a homogeneous layer on the surface. These results are described and discussed in this paper. [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]

Abstract: Corrosion effects of five runway de-icing chemicals on aluminium alloy 2024, magnesium alloy RZ5 and cadmium-plated and subsequently chromate-treated steel 4340 were investigated by cyclic polarisation measurements, open circuit potential monitoring and cyclic chemical exposure tests. The runway de-icing chemicals included in the study contained urea, which has a long history as a runway de-icing chemical, and four new commercial de-icing chemicals, which were based on betaine and potassium formate. Corrosion effects of urea on aluminium alloy 2024 were more pronounced than those of the new de-icing chemicals. In urea, the breakdown potential, indicating the onset of pitting, was clearly distinguishable in the cyclic polarisation curve and pitting corrosion was detected on the specimen surface after all three types of tests. Weight losses during the chemical exposure tests were also higher for urea than for the other four chemicals, where pitting corrosion was only occasionally detected. The opposite was true in the case of magnesium alloy RZ5: although the alloy experienced general corrosion in each de-icing chemical included in the study, the rate of corrosion was often higher in the new de-icing chemicals than in urea. Corrosion effects of the five de-icing chemicals on cadmium-plated and chromate-treated high-strength steel 4340 were slightly different in all three tests, indicating that differences in coating quality, particularly in the thickness of the coating layers, might have influenced the results. Overall, however, partial loss of the coating layers occurred in all studied de-icing chemicals. [ABSTRACT FROM AUTHOR]

The initiation and growth of immersion tin coating from hydrochloric acid based and methanesulphonic acid based baths on the inner surface of copper tube was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The formation of immersion tin coating crystallites from hydrochloric acid based bath occurs semi-coherently with copper substrate. Tin crystallites grow actively both in vertical and horizontal directions yielding an equiaxial grain structure. The immersion tin deposition from methanesulphonic acid based bath proceeds by the formation of tin nanocrystals. These nanocrystals transform into columnar grains during the grain growth process. Differences in the coating formation characteristics between studied baths are reflected to the microstructural details and properties of coatings, such as surface morphology, porosity and adhesion. [Copyright &y& Elsevier]