Corrosion and deposition of magnetite in ferritic steel steam generator tubes under acid chloride conditions

Abstract

Corrosion of a tube made up of 9Cr–1Mo, 2·25Cr–1Mo, and 2·25Cr–1Mo–Nb ferritic steels and containing artificial defects has been investigated under realistic steam generator conditions (355°C, 17·6 MPa) with acid chloride fault water chemistry (2 mg/kg HCl). Four regions of corrosion and magnetite deposition behaviour were observed on the tube surface. In non-heat flux regions, magnetite deposition was affected by mass transfer and probably also by surface potential. In low heatflux regions (<660 kW m^?2) observed increases in the rates of magnetite deposition and corrosion wereprobably due to rises in the degree of iron supersaturation and HCl concentration, brought about by boiling. Enhanced HCl concentrations in the normal heat flux region (660 kW m^?2) prevented magnetite deposition and caused an increase in corrosion of the three steels. Increases in corrosion and magnetite deposition were also observed at the weld between the 9Cr–1Mo and 2·25Cr–1Mo steels. In defects, accelerated corrosion was seen only in the 9Cr–1Mo steel and was confined to the top 0·15 mm. It is concluded that the corrosion behaviour observed in this work is determined by the residence time and concentration of solutions of HCl on tube and defect surfaces.     

Oxidising type inhibitors for protection of aluminium and steel surfaces in sodium chloride solutions

Abstract

Photoelectron spectroscopy was used to study the composition of surface films formed on aluminium and steel when protected from corrosion in aqueous chloride media by inorganic inhibitors of the MeO^2?₄ oxidising type and by mixtures of these and monoethanolamine (MEA). The surface condition plays a significant role in the protection of aluminium. In the adsorption of molybdate on to an etched surface reduction of Ma(VI) to Mo(IV) occurs whereas on an unetched surface, as also for a steel surface, no reduction occurs. In the case of chromate, reduction to Cr( III) took place on steel and on aluminium. Synergism was established in the inhibition of corrosion of steel by a combination of inorganic oxidisers and MEA and a study made of this phenomenon.     

Relationship between corrosion resistance,microstructure and cobalt content in a Zn–Co alloy electroplated from alkaline electrolyte

Abstract

Various protective methods may be used to improve the corrosion resistance of steel, and the application of coatings of zinc and its alloys is one of them. The efficiency of zinc in corrosion protection is due to its behaviour as a sacrificial anode. To enhance the corrosion protection, zinc has been alloyed with more noble metals such as cobalt, nickel and iron. In this work zinc-cobalt alloys were electrodeposited onto steel from an alkaline electrolyte. The investigation was carried out on electrodeposits with low and high cobalt contents. An in situ study was performed using a TEM equipped with a hot stage to relate the heat treatment with changes in structure and, consequently, corrosion behaviour. The images of the specimens as deposited and after heat treatment showed the presence of a phase that was identified as Co₅Zn₂₁. However only the diffraction pattern of the heat treated specimen fitted the strongest lines for this compound. The corrosion tests showed differences in the corrosion resistance before and after heat treatment, for alloys with low and high cobalt. The heat treated specimens displayed more active behaviour.     

Modelling correlation between alloy composition and ferrite number in duplex stainless steel welds using artificial neural networks

Abstract

Weld metal composition is thought to be an important factor in influencing the austenite/ferrite ratio of duplex stainless steel microstructures. To produce the required balance in the austenite/ferrite ratio in the weld microstructure, the chemical composition of the welding consumables should be adjusted. In the present work, Bayesian neural network analysis has been employed to predict the ferrite number in duplex stainless steel welds as a function of composition. The technique accounts for modelling uncertainty, and automatically quantifies the significance of each input variable. In this paper, the influence of variations in the weld composition on the ferrite number have been quantified for two duplex stainless steels. Predictions are accurate compared to published methods. The role of Si and Ti in influencing the ferrite number in these alloys has been brought out clearly in this study while these elements are not given due considerations in the WRC–1992 diagram.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

Effect of aging on fatigue crack growth behaviour of Cu bearing HSLA-100 steel

Abstract

The effect of aging on fatigue crack growth rate (FCGR) of Cu bearing HSLA-100 steel has been studied. The steel was solution treated, water quenched and aged at various temperatures in the range of 350–700°C. The fatigue crack growth resistance of the steel decreased for the initial stages of aging from 350–500°C. Further aging up to 650°C resulted in an improvement in the crack growth resistance. Beyond 650°C, once again an inferior crack growth resistance was observed. This nature of variation of FCGR behaviour was similar to the trend portrayed by the strength properties with aging treatment. The results are related to the changes in the microstructural constituents owing to the aging treatment.     

Influence of Si,Ca and Ag addition on corrosion behaviour of new wrought Mg–Zn alloys

Abstract

The development of new wrought magnesium alloys for automotive industry has increased in recent years owing to their high potential as structural materials for low density and high strength/weight ratio demands. However, the poor mechanical properties and low corrosion resistance of the magnesium alloys have led to searching a new kind of magnesium alloys for better strength, ductility and high corrosion resistance. The main objective of the present research is to investigate the mechanical properties and the corrosion behaviour of new magnesium alloys, Mg–Zn–Ag (ZQ) and Mg–Zn–Si–Ca (ZS) alloys. The ZQ6X and ZS6X–YCa alloys were prepared by using hot extrusion method. Hardness AC and DC polarisation tests were carried out on the extruded rods, which contain different amounts of silver or silicon and calcium. The potential difference in air between different phases and the matrix was examined using scanning Kelvin probe force microscopy. The microstructure was examined using optical and electron microscopy (TEM and SEM), X-ray analysis and EDS. The results showed that the silver addition improved the mechanical properties but decreased the corrosion resistance. The addition of silicon and calcium also affected both mechanical properties and corrosion behaviour. These results can be explained by the effects of alloying elements on microstructure of Mg–Zn alloys such as grain size and precipitates caused by the change in precipitation and recrystallisation behaviour.     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

Effect of heterogeneous deformation on precipitate distribution in Al–3·7Cu and Al–3·8Cu–1·8Mg alloys

Abstract

Cold rolling and hot rolling of solution treated Al–3·7Cu and Al–3·8Cu–1·8Mg alloys were performed to test the effect of inhomogeneous precipitation in shear bands during following aging of the material. The most effective inhomogeneous distribution of particles was observed for samples hot deformed and aged within the temperature range 473–573 K. It was noted that magnesium addition intensifies coarse shear bands development and following coarsening of particles within sheared area.     

Effect of grain size on grain boundary strengthening of copper bicrystals under cyclic loading

Abstract

To clarify the strengthening effect of grain boundaries (GB), cyclic deformation behaviour of really grown [4¯79] ∥ [1¯25] copper bicrystals with different widths (4, 6, and 8 mm, denoted RB-4, RB-6, and RB-8) of com-ponent crystals and a combined copper bicrystal (denoted CB-6), obtained by sticking component single crystals G1 [4¯79] and G2 [1¯25] together, was investigated. The results showed that the cyclic saturation stresses increased in the order of bicrystals of CB-6 < RB-8 < RB-6 < RB-4. It is indicated that the GB effect caused different degrees of strengthening, which increased with the decreasing width of the RB bicrystals. By surface observation, it was found that only the primary slip system was activated in the combined bicrystal during cyclic deformation. However, an additional slip system appeared near the GB within the crystal G2 [1¯25] in the RB bicrystals (except in the primary slip system), and formed a GB affected zone (GBAZ). The width of the GBAZ was about 400 and 600 μm at plastic strain amplitudes of 0·1% and 0·2% respectively. Meanwhile, using an electron channelling contrast (ECC) technique in the SEM, the dislocation patterns near the GB and within the component crystals were observed. It was found that a two phase structure of persistent slip bands (PSBs) and matrix (or veins) can form in these bicrystals, similar to that in copper single crystals. But these PSBs cannot transfer through the GB during cyclic deformation. Based on the results above, the effect of grain size on GB strengthening of copper bicrystals was discussed.