Effect of phosphate ions on electrochemical behaviour of copper in sulphate solutions

Abstract

The effect of phosphate ions on the corrosion behaviour of copper in sulphate solutions was investigated at different temperatures (293, 313, 333, and 353 K) and at different pH values (pH = 2·1, 7·2, and 12·3). A three electrode electrochemical technique was used for the measurements, using a saturated calomel electrode (SCE) for reference purposes. Both anodic and cathodic polarisation curves were established, from which the anodic and cathodic Tafel constants were determined. Corrosion potentials were determined by extra trapolation of the anodic and cathodic Tafel lines to the point of zero current density. The results showed that for a pH value of 2·1, the corrosion rate of copper showed a marked tendency to increase with increasing temperature in solutions containing either sulphate alone or sulphate plus phosphate. However, at pH values of 7·2 and 12·3, this tendency became less pronounced in solutions containing sulphate alone and in solutions containing sulphate plus phosphate together, max imum corrosion rates were greatly reduced and showed less tendency to increase with increasing temperature. Anodic and cathodic reaction mechanisms are proposed to explain the observations for the corrosion of copper.     

Reformulation of time averaged Joule heating in presence of temperature fluctuations

Abstract

Strong temperature fluctuations might exist in non-isothermal turbulent flow. When a RANS approach is used for simulation it is necessary to time average all properties that are temperature dependant. Here we focus on the time average of the Joule heating released within the turbulent slag region of an electroslag remelting process (ESR). For that the average temperature dependant electric conductivity of the slag was expressed as a function of the time averaged temperature field and its standard deviation. The results using this new approach are compared with the results given by the classical approach using only the electric conductivity at the time averaged temperature. It will be shown that the temperature fluctuations decrease strongly the amount of electric current flowing directly to the mould, and increase the efficiency of the remelting process by 50%.     

Application of minielectrochemical cell to corrosion studies of welded joints of austenitic stainless steel

Abstract

A specific electrochemical cell for small scale electrochemical testing has been built. The electrochemical cell has been made bearing in mind the microstructural features of the stainless steel weldments and the performance of the design has been verified. Two polarisation methods (potentiodynamic anodic polarisation measurements and cyclic potentiodynamic polarisation measurements) and two reactivation electrochemical techniques (the electrochemical potentio-kinetic reactivation test, EPR, and electrochemical potentio-kinetic reactivation double loop test, EPRDL) have been applied to two austenitic stainless steels: AISI 304 and AISI 316L. The results are reliable and have been compared to large scale experiments. Scanning of the welding joints has been performed and the results have been correlated with their microstructural features. It, thus, makes it possible to study the intrinsic heterogeneous microstructures such as the heat affected zone of a welded joint.     

Corrosion properties of nickel free austenitic stainless steels in 0·5M H2SO4 and 0·5M H2SO4 plus 0·4M NaCl

Abstract

The corrosion behaviour of four nickel free austenitic stainless steels were investigated in 0·5M H₂SO₄ and 0·5M H₂SO₄ plus 0·4M NaCl solutions by means of potentiodynamic and potentiostatic anodic polarisation testing. The performances of the nickel free alloys are compared to those of an experimental intermediate nickel alloy (4%Ni) and a standard AISI 304 steel grade. Once passivity was reached all alloys displayed similar current densities i _p in 0·5M H₂SO₄, independent from alloying. Mo and Cu were shown to be beneficial in decreasing the active dissolution currents and i _crit values. The commercial AISI 304 steel displayed superior resistance to pit initiation during potentiodynamic testing, and AISI 304 steel displayed the highest E _pit value of all alloys tested. When tested potentiostatically the N and Mo alloyed nickel free alloys showed excellent resistance to pit initiation and growth. The dominant effect of N was associated with repassivation of incipient pits, while Mo appeared to act at an earlier stage, suppressing initiation.     

Photocorrosion inhibition -of n-GaAs in acidified aqueous iodide solution

Abstract

The photocorrosion of n-GaAs electrodes in aqueous solutions represents one of the most important factors limiting their application in solar energy conversion. In continuation of previous studies, the corrosion of n-GaAs has been investigated in 0·5M H₂SO₄ using a pllOtoelectrochemical cell. Illumination of the n-GaAs anode with white light of intensity 20 mW cm^-2 generated a saturation photocurrent of 27 mA cm^-2 at biasing voltages more positive than +0·45 V(SCE). This current corresponded to hydrogen discharge at the platinum counter electrode and to oxidation of the GaAs surface. When 0·01-6·0M LiI was added to the H₂SO₄ solution, 12 was formed at the n-GaAs electrode and dissolved in the iodide solution. The onset potential of the photocurrent (E_p) shifted from -0·36 to -0·73 V(SCE) as the concentration was varied from 0·01 to 6·0M I^-. Addition of I^-₃ shifted E_p to more positive values. The extent of corrosion was studied by comparing the ratios of photogenerated I₂ and H₂. In 0·01, 1·0, and 6·0M I^- solutions, I₂/H₂ was 0·38, 0·87, and ～1 respectively. These ratios imply that, at lower I^- concentrations, the holes photogenerated at the n-GaAs surface were consumed in the simultaneous oxidation of I^- and GaAs. However, in 6·0M I^-, where I₂/H₂ = 1, surface oxidation of GaAs was completely suppressed owing to total consumption of the photogenerated holes by the oxidation of I^-. The results are considered on the basis of the band model of the semiconductor/electrolyte interface.     

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.     

Identification of flow instabilities during hot working of powder metallurgy superalloy IN 100

Abstract

A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of powder metallurgy materials. This criterion has been applied to the existing flow stress data of powder metallurgy superalloy IN 100 and compared with the reported microstructural observations. The optimum hot working conditions for the PM superalloy IN 100 are suggested.     

Consolidation and green body characteristics of gelcast metallic powder

Abstract

A preliminary study concerning the compaction of metallic powder was carried out in order to investigate a rarely explored route in powder metallurgy, to form complex geometry parts, known as gelcasting. Green bodies produced with as supplied stainless steel powder showed a tendency to form foam, which affected the surface finishing. The mechanical behaviour of green compacts was also affected by the processing additives present in the metallic powder. Organics in the as supplied powder were removed by thermal treatment at 500°C and additional samples were produced. Although no difference in green density was observed, these samples displayed better surface finish and mechanical characteristics, as a result of improved adhesion between the polymer network and particle surface. The results showed that the gelcasting process is able to produce green parts suitable for subsequent thermal treatment.