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Finnish nuclear waste disposal program proposes to dispose spent nuclear fuel in tightly sealed copper canisters in a geological repository; here, copper acts as a corrosion barrier. The key argument for such materials selection is that copper is resistant to corrosion in oxygen-free water. However, the presence and colonization of microbial species in ground water in the geological repository may initiate, facilitate and accelerate corrosion of copper under such conditions. In this study, the results from experiments that were designed to simulate the final stage of the deep geological nuclear waste repository, when the temperature has already stabilized to the level of the surrounding bedrock and all oxygen included at the construction stage has been consumed, are reported, both in the presence and absence of micro-organisms retrieved from the repository site. The experiments were performed for 10 months, during which the copper specimens were subjected to electrochemical measurements. After the tests, the specimens were investigated in terms of microstructure and weight changes, while the test media was characterized with respect to selected chemical species. Analysis concentrated on EIS data in the light of results obtained by other methods. In the presence of micro-organisms, e.g., sulphate-reducing bacteria, the development of Cu 2 S is the primary surface process on copper specimens, with its corrosion protection properties being regulated by the growth of the biofilm. As compared to two-layered Cu 2 O films that grow on the specimen surfaces in the absence of bacteria, Cu 2 S films were essentially less susceptible for the attack by Cl − . Under abiotic conditions, passivity breakdown by Cl − resulting in the localized type of attack detected on the surfaces is the probable reason for an essentially higher corrosion rate than in the biotic system. [ABSTRACT FROM AUTHOR]

In this study, types 440B (martensitic) and 304 (austenitic) stainless steel were subjected to microstructural characterisation and a series of tribocorrosion tests in a pin-on-disc facility equipped with a corrosion cell, in order to obtain understanding on their tribocorrosion behaviour in 0.01 M NaCl solution under the loads up to 5 N. The results from tribocorrosion tests revealed that, as compared to pure corrosion, open circuit potential of the materials under the applied load shifted towards negative direction and the measured current density correspondingly increased. Potentiostatic tests enabled tribocorrosion behaviour of the materials to be investigated at constant potential values that represent different oxidising capacities of the solution. It was shown that volume losses by wear were independent of potential and test material. In contrast to this, volume losses by corrosion increased with increase in potential for type 440B stainless steel, while those by synergetic effects of corrosion and wear grew with increase in potential for both test materials and become the key damage mechanism under oxidising conditions. However, corrosion attack was local, causing actually more dramatic effects on material losses (through synergy) than disclosed solely by volume losses by corrosion, and clearly linked with the material microstructure. In this paper, these results are presented and discussed in the light of alloy repassivation capability. [ABSTRACT FROM AUTHOR]

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]

Corrosion behaviour of four commercial aircraft materials, Al alloy 7075-T6, high strength steel AISI 4340, Ti alloy 9046 and Mg alloy RZ5, was studied in six commercial antiskid chemicals, one sodium acetate based, three potassium acetate based, one sodium formate based and one potassium formate based, using potentiodynamic polarisation measurements (polarisation curves) and chemical exposure tests (weight change data), combined with microstructural examinations (appearance of surfaces). The results show that among the four materials, the Ti alloy experiences least corrosion, showing pitting corrosion during the polarisation measurements and significant weight gains and discoloration in the exposure tests only in Safeway SF. In contrast to this, the Mg alloy and the high strength steel underwent corrosion in all chemicals, with severe pitting corrosion (polarisation measurements), significant weight changes and discoloration (exposure tests) being observed for the Mg alloy and discoloration (exposure tests) for the high strength steel. Accordingly, sodium formate based Safeway SF is the most corrosive of the chemicals, introducing some indications of corrosion on all materials, and potassium acetate based Safeway KA HOT and potassium formate based Meltium are the least corrosive chemicals: no signs of corrosion could be observed on the Al and Ti alloys. The results also show that based only on the salt, pH value and conductivity, corrosivity of the antiskid chemicals cannot be reliably evaluated. In this paper, explanations for these observations are presented and the similarities and differences between the results from the two types of corrosion tests are discussed. [ABSTRACT FROM AUTHOR]

A microstructural characterisation was carried out for Al–Co–Fe–Cr feed powder and the coatings sprayed with a high velocity oxy-fuel method using different operation conditions. The aims of the study were to explore the structural development of thick Al–Co–Fe–Cr coatings and the influence of the spraying parameters on the microstructure of produced Al–Co–Fe–Cr coatings. X-ray diffractometry, scanning electron microscopy and analytical transmission electron microscopy were the techniques used in the phase identification and in the microstructural exploration of the study. The results show that Al–Co–Fe–Cr feed powder and the coatings sprayed with low and high operation temperature are composed of a dodecagonal quasicrystalline phase. The composition of this new dodecagonal phase approximately corresponds to that of the feed powder, being A170.6Co12.5Fe9.4Cr7.5. The dodecagonal phase does not decompose during the spraying process. Instead, it orientates to form a lamellar coating structure. When a lower spraying temperature is used, the incomplete melting of powder particles introduces a partly orientated coating structure. Due to this incomplete melting of powder particles, porosity is also involved in these coatings. Higher spraying temperature, in turn, promotes oxidation, leading to the incorporation of an oxygen-containing film on the splat boundaries. While the feed powder and the coating deposited with a lower spraying temperature are one-phase quasicrystalline structures, the coating sprayed with a higher operation temperature is comprised of a dodecagonal phase and an oxygen-containing phase. This oxygen-containing phase is not pure aluminium oxide but contains all the elements present in the alloy. [Copyright &y& Elsevier]

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]