Some properties of binary vanadium alloys

Conclusions Increasing the concentration of the alloying element in binary alloys of vanadium with titanium, chromium, tin, and aluminum at concentrations of 0–25 wt. % leads to an increase of hardness and electrical resistivity, particularly for alloys with tin and aluminum. The strength will be highest for alloys of vanadium with tin.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 62–63, July, 1971.     

Filiform corrosion of binary aluminium model alloys

The effect of alloy composition and microstructure on filiform corrosion (FFC) susceptibility was investigated for super-purity based binary model alloys of the systems Al-Mg, Al-Mn, Al-Fe and Al-Cu. Corrosion testing indicated that the presence of electrochemically noble second phase particles is a necessity for FFC to occur. Single phase, solid solution alloys of the Al-Mg, Al-Mn or Al-Fe systems did not support FFC. Heat treatment caused precipitation of the electrochemically noble intermetallic phase FeAl₃ in the Al-Fe alloy, resulting in extensive FFC. Precipitation of the phase MnAl₆, which has electrochemical properties similar to that of the aluminium rich matrix, by heat treatment did not impair the corrosion properties of the Al-Mn alloy. Significant surface oxidation and magnesium enrichment of the surface oxides by heat treatment did not affect the FFC properties of Al-Mg alloys. However, the solid solution binary Al-Cu alloys exhibited severe FFC. The detrimental effect of copper in solid solution is attributable to selective dissolution phenomena during the corrosion process, whereby copper was locally enriched on the surface as copper-rich particles providing efficient cathodic sites.     

On the corrosion of binary magnesium-rare earth alloys

The corrosion properties of high-pressure die cast (HPDC) magnesium-rare earth (RE) based alloys have been studied. Binary additions of La, Ce and Nd to commercially pure Mg were made up to a nominal 6 wt.%. It was found that the intermetallic phases formed in the eutectic were Mg₁₂La, Mg₁₂Ce and Mg₃Nd, respectively. Results indicated that increasing RE alloying additions systematically increased corrosion rates. This was also described in the context of the electrochemical response of Mg-RE intermetallics - which were independently assessed by the electrochemical microcapillary technique.This study is a discrete effort towards revealing the electrochemical effect of carefully controlled binary alloying additions to magnesium in order to elucidate the microstructure-corrosion relationship more generally for HPDC Mg alloys. Such fundamental information is seen to not only be useful in understanding the corrosion of alloys which presently contain RE additions, but may be exploited in the design of magnesium alloys with more predictable corrosion behaviour. There is a special need to understand this relationship - particularly for magnesium that commonly displays poor corrosion resistance.     

The steady-State corrosion kinetics of two-phase binary alloys forming the most-stable oxide

The steady-state kinetics in the high-temperature oxidation of binary A-B alloys containing a mixture of the conjugated solid solutions of B in A (alpha phase) and A in B (beta phase) with exclusive formation of the most-stable oxide BO_v have been examined, assuming that the external scale grows on top of a subsurface layer of alpha phase. The results obtained are compared with the corresponding behavior of alloys which are single phase in the whole range of composition. Under identical values of all the parameters involved the concentration of B at the alloy-scale interface is smaller for two-phase than for single-phase alloys under the same concentration of B in the alloy as a result of the restricted flux of B through the alpha-phase layer. As a consequence of this, the two-phase alloys corrode more slowly than single-phase alloys and this difference increases as the solubility of B in the alpha phase decreases. Finally, the simultaneous formation of BO_v both externally and as internal oxide is more likely for two-phase than for single-phase materials.     

On the mechanism of high-temperature sulphur corrosion of binary alloys

Taking into account the phase composition and morphology of the scales three groups of alloys may be distinguished. The first of them contains alloys forming monophase scales which constitute solid solutions of sulphides of both alloy components (AgCu type). The mte of corrosion of these alloys is a monotonous function of their composition, and the mechanism of scale formation depends on the geometrical configuration of the reaction system. The second group includes alloys the components of which form sulphides of limited mutual solubility or sulphides having spinel structures (FeCr type); the scale may then be a mono- or double phase one and the exact structure is a function of the concentration of the alloying element. The metals belonging to the third group form mutually immiscible sulphides (CuZn type). The scales form double layers, the external layer being made up by sulphides of the base metal, while the internal layer is a heterophase mixture of the sulphides of both alloy components. The kinetics and the mechanism of the corrosion of these alloys is largely independent from the atmosphere (elemental sulphur or H₂? H₂S mixture).     

The influence of yttrium (Y) on the corrosion of Mg-Y binary alloys

Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direct observations. There were two important effects. In 0.1 M NaCl, the corrosion rate increased with increasing Y content due to increasing amounts of the Y-containing intermetallic. In 0.1 M Na₂SO₄, the corrosion rate decreased with increasing Y content above 3%, attributed to a more protective surface film, despite the intermetallic. The corrosion rate evaluated by electrochemical impedance spectroscopy was somewhat smaller than that evaluated from H evolution as expected from the Mg corrosion mechanism. A mechanism is proposed for filiform corrosion. Direct in situ corrosion observations revealed that a predominant feature was hydrogen evolution from particular parts of the alloy surface.     

The steady-state corrosion kinetics of single-phase binary alloys with a solubility gap forming the most-stable oxide

The steady-state, high-temperature oxidation kinetics of single phase alloys rich in a most-reactive componentB in binaryA-B systems presenting a limited solubility of the two components (beta phase alloys) have been examined assuming the exclusive formation of the most-stable oxideBO _v. Alloys sufficiently rich inB can form externalBO _v scales directly in contact with the beta phase, while below a criticalB content the growth ofBO _v involves also the appearance of an intermediate layer ofB-depleted solid solution ofB inA (alpha phase). The parabolic rate constants for the oxidation of single-phase beta alloys are lower than those of alloys of identicalB content which are single-phase over the whole range of composition (solid-solution alloys) but higher than for two-phase alpha + beta alloys under the same values of all the relevant parameters. Moreover, the tendency of single-phase beta alloys to form the most-stable oxide simultaneously as an external scale and internally to the alloy is greater than for solid-solution alloys but smaller than for two-phase alloys.     

The acid corrosion of Fe-P alloys

The effect of the P-content in iron (0.002-0.12% P) and the stirring rate on the corrosion rate of five Fe-P alloys in H₂SO₄ and HCl solutions (pH = 0) has been investigated. It was assumed that phosphine generated during corrosion process acts as a stimulator of the cathodic process (at low concentrations) or as an inhibitor of both partial electrode processes (at high concentrations). A mechanism of the corrosion of Fe-P alloys, including the transfer of phosphine from the surface into the bulk of the solution by stirring has been discussed.     

The oxidation of binary iron-manganese alloys

Oxide scales formed on iron-manganese alloys are generally considered to be similar to those formed on the parent metals. Metallographic examination is used in conjunction with EMPA, XRD, and X-ray imaging to reveal the presence of the additional phases Mn₂O₃ and Mn₃O₄ in the scales formed on binary alloys containing up to 40% manganese at 700 and 800°C in 200 Torr oxygen. A mechanism is proposed to explain the apparent change in growth from iron-rich to manganese-rich scales during oxidation of Fe-40Mn at 800°C.     

Short-range ordering in binary liquid alloys of rare earth metals with copper and silver

The thermodynamic properties of binary liquid alloys of rare earth metals with copper and silver indicate that these systems possess chemical short-range ordering. Ideal associated solution (IAS) model calculations of the thermodynamic properties including the concentration correlation function S_cc(0) for Cu-Y, Cu-La and Cu-Lu melts are in good agreement with experimental data. The results of a model analysis on the dependences of S_cc(0) vs. concentration including the percolation theory approach are proposed in order to facilitate the calculation of S_cc(0) from experimental thermodynamic activity data. The Faber—Ziman partial structure factors in the zero wavevector limit and the charge-transfer effects for Ag-La melts are evaluated; the parameters which are not available were shown to be represented using the IAS approximation.     

Gelatin as corrosion inhibitor for aluminum and aluminum silicon alloys in sodium hydroxide solutions

The corrosion behavior of aluminum and aluminum silicon alloys in 0.1 M NaOH solution in the absence and presence of gelatin was studied using potentiostat polarization, electrochemical impedance spectroscopy (EIS), cyclic voltammetry and potentiodynamic anodic polarization techniques. The percentage inhibition efficiency increases with increasing the concentration of gelatin and with decreasing temperature as well as Si content. This was attributed to a lower affinity of the inhibitor to adsorb on Si than on Al. The inhibitory action of gelatin was explained in terms of adsorption of gelatin on the surface of Al or Al–Si alloys forming a barrier of mass and charge transfer leading to protect the metal surface from the aggressive ones. The adsorption of gelatin on the metal surface follows Freundlich isotherm. Some activated thermodynamic parameters were calculated and discussed.     

The corrosion of aluminium in solutions of sodium fluoride and sodium chloride

Abstract

The corrosion of 99·997% aluminium by 0·1 N-sodium fluoride and chloride has been examined in the presence of air over the pH range 2—13. When a solution of either salt was saturated with aluminium oxide, corrosion was arrested.

Under anaerobic conditions, 0·1 N-sodium fluoride attacked the air-formed oxide film and corrosion was rapid, in contrast no corrosion was observed in 0·1 N-sodium chloride solution.     

The corrosion performance of anodised magnesium alloys

The corrosion performance of anodised magnesium and its alloys, such as commercial purity magnesium (CP-Mg) and high-purity magnesium (HP-Mg) ingots, magnesium alloy ingots of MEZ, ZE41, AM60 and AZ91D and diecast AM60 (AM60-DC) and AZ91D (AZ91D-DC) plates, was evaluated by salt spray and salt immersion testing. The corrosion resistance was in the sequential order: AZ91D ≈ AM60 ≈ MEZ ? AZ91D-DC ? AM60-DC > HP-Mg > ZE41 > CP-Mg. It was concluded the corrosion resistance of an anodised magnesium alloy was determined by the corrosion performance of the substrate alloy due to the porous coating formed on the substrate alloy acting as a simple corrosion barrier.     

The oxidation and corrosion resistance of nitrided iron alloys

The chemistry and mechanical properties of nitrided iron alloys are well understood but their resistance to oxidation and corrosion has received less attention. Thermogravimetric and metallographic results are presented of the oxidation in air of nitrided iron, FeMo and FeCr laboratory alloys, and mild steel. Oxidation resistance is improved after nitriding by formation of a fine grain oxide which has a higher fracture strain than the coarser oxide on the corresponding annealed alloys. No void formation occurs at the oxide/metal interface. The results are discussed in terms of nucleation of oxide on dispersed incoherent nitride particles in the metal surface. The improved cohesion of the oxide on nitrided alloys is however not reflected in the results of thermogravimetric studies as the reduced interfacial voidage on the dispersoid-containing alloy allows unrestricted cation transfer from metal to oxide and the conditions are insufficiently aggressive to cause spalling on un-nitrided alloys. Experiments on nitrided mild steel demonstrate the resistance to fracture of oxides formed on nitrided alloys. The criteria for resistance to aqueous corrosion are different from those for resistance to high temperature oxidation but nitriding is shown to produce significant improvements in resistance to general and pitting attack. Potentiokinetic measurements are reported for nitrided laboratory alloys and a commercial austenitic stainless steel containing nitrogen in solid solution. This behaviour is not fully understood but is not simply related to oxide nucleation and cohesion as is the case in high-temperature oxidation.     

The localized corrosion of resistant alloys in chloride solutions

A potentiostatic scratching technique has been used to study the breakdown potentials of stainless steel 304, Incoloy, Inconel 600, Monel 400 and Hastelloy C in chloride solution at pH 8 as a function of chloride activity, temperature and metal cold work. For the first four alloys breakdown occurs through pitting. For Hastelloy C there is no pitting process, and breakdown occurs by a transpassive reaction, probably involving oxidation of Mo and Cr in the film to soluble anions.     

Grain-boundary segregation in binary alloys

Results of experimental studies of grain-boundary segregation in binary alloys have been considered and analyzed. The model of grain-boundary segregation and an isotherm that allows for the possibility of the formation of complexes in grain boundaries have been suggested.     

The localized corrosion of resistant alloys in chloride solutions

A potentiostatic scratching technique has been used to study the breakdown potentials of stainless steel 304, Incoloy, Inconel 600, Monel 400 and Hastelloy C in chloride solution at pH 8 as a function of chloride activity, temperature and metal cold work. For the first four alloys break-down occurs through pitting. For Hastelloy C there is no pitting process, and breakdown occurs by a transpassive reaction, probably involving oxidation of Mo and Cr in the film to soluble anions.