Stress corrosion cracking of α–β brass in chloride containing waters

Abstract

Slow straining of an α–β brass in aqueous solutions with various concentrations of chloride ion has been used to ascertain the effects of potential, pH, and chloride content on the susceptibility to stress corrosion cracking. The ductility is at a minimum with 55 ppm Cl present and increases at higher concentrations, where dezincification effects become more significant. Anodic polarization increases the ductility loss but the effects are again complicated at high potentials by greater dezincification. The strqjn rate dependence of the embrittlement results in a ductility minimum at 2 × 10^?6. The results are assessed in terms of various cracking mechanisms but seem to be most consistent with a mechanism involving the rupture of a surfacefilm of CU₂O that forms on the alloy in the critical potential range.     

Hot ductility behaviour of Al–Li alloy AA2091–T3

Abstract

The hot ductility behaviour of the Al–Li alloy 2091–T3 was investigated using hot ductility testing of the simulated heat affected zone (HAZ) using a Gleeble 1500 machine. Alloy 2091 displayed lower ductility, greater susceptibility to HAZ cracking, and poor ductility recovery on cooling from the nil strength temperature, compared with alloy 2219. Optical metallography and electron microscopy revealed that the ductility loss was accompanied by fracture transition from ductile transgranular mode to brittle intergranular mode. It was found that the HAZ intergranular liquation cracking was caused by non-equilibrium eutectic melting of T₂ phase precipitated at grain boundaries.     

Electrochemical pitting corrosion behaviour of α-brass in LiBr containing solutions

The potentiodynamic anodic polarization curve of α-brass (70% Cu-30% Zn) in 1 M LiBr solution showed an initial active region of the alloy dissolution followed by two well defined anodic current peaks then a narrow passivation region before the pitting potential (E_pit) is reached. The initial active anodic region exhibited Tafel slope with 90 mV dec⁻¹ attributed to the formation of CuBr₂⁻ complexes. The anodic current peaks were attributed to the formation of CuBr and Cu²⁺ ions, respectively. The change of pH values of LiBr solution did not affect the anodic polarization curves of α-brass. Increasing the solution temperature from 30 to 90 °C changed the corrosion type from pitting to general one. The addition of 10⁻² M benzotriazole (BTAH) to 1 M LiBr solution is completely inhibited the pitting corrosion at 30 °C while it did not inhibit the pitting at 90 °C. The inhibition effect was attributed to the adsorption of BTAH molecules on the alloy surface, which obeys Langmuir isotherm. The presence or absence of pitting corrosion was confirmed by using SEM.     

Use of the scratching electrode technique to interpret the stress corrosion cracking behaviour of α-brass in ammoniacal copper sulphate solutions

Stress corrosion cracking (s.c.c.) of α-brass in Mattsson solutions [1 M (NH₃ + NH₄⁺); 0.05 M CuSO₄] at a given strain-rate intensifies in pH order: 4.75 < 7.35 < 11.35. There is no cracking at pH 8.9 or when tarnishing at pH 7.35 is prevented by reducing [NH₃ + NH₄⁺] to 0.25 M. A technique of continously scratching a shallow circular track on a rotating electrode has been used to investigate the electrochemical behaviour of bared metal surfaces in these environments with the object of examining the relative contributions of slip-dissolution and tarnish-rupture in the crack propagation mechanism. It is shown that the observed s.c.c. propensity correlates well with: (i) the physico-chemical nature of the reaction products, (ii) the bare surface current density at the applied potential and (ii) oxide growth kinetics on the bare surface. Thus, in non-tarnishing solutions s.c.c. does not occur if either the rate of protective oxide formation is sufficiently rapid [pH 7.35 (0.25 M NH₃)] or the whole surface is dissolving uniformly (pH 8.9); in the intermediate case (pH 4.75) cracking is observed. In tarnishing solutions [pH 7.35 (1 M NH₃) and pH 11.35] the crack propagation rate increases with the tarnishing rate. Qualitative agreement between s.c.c. behaviour and bare surface oxide growth rate, as determined by the scratching electrode method, supports a slip-dissolution model of propagation.     

Influence of sulphate and chloride ions on corrosion of 63–37 brass in aqueous ammonia containing copper

Abstract

The influence of copper added as metallic powder, cupric sulphate, or cupric chloride to 15·7M NH₄OH on the dissolution and tarnishing of 63–37 brass in this medium was studied using weight loss and electrochemical methods. Copper added as electrolytic metal powder accelerated the dissolution of brass in ammonia. The dissolution rate increased with increase in copper concentration to 4 g l^?1, beyond which the rate decreased because of the formation of a black tarnish film. When copper was added as cupric sulphate or cupric chloride it was found that at low concentrations sulphate and chloride ions had no significant effect on the dissolution of brass, whereas at higher concentrations they enhanced dissolution. Both these ions prevented formation of the tarnish film on the brass surface. When copper was present in the ammonia the corrosion of brass occurred via a cathodically controlled process. The presence of SO^2?₄ and Cl^? produced high anodic dissolution currents. Open circuit potential values of the brass in these solutions and their variation with time were measured. The results are interpreted with reference to the chemistry of the systems under study.     

Comparison of the corrosion resistance of an Al–Cu alloy and an Al–Cu–Li alloy

ABSTRACT

In this study, the corrosion mechanisms of the AA2024-T3 and the AA2098-T351 were investigated and compared using various electrochemical techniques in 0.005?mol?L^?1 NaCl solution. The severe type of corrosion in the AA2098-T351 was intragranular attack (IGA) although trenching and pitting related to the constituent particles were seen. On the other hand, the AA2024-T3 exhibited severe localised corrosion associated with micrometric constituent particles, and its propagation was via grain boundaries leading to intergranular corrosion (IGC). Electrochemical techniques showed that the corrosion reaction in both alloys was controlled by diffusion. The non-uniform current distribution in both alloys showed that EIS was not a proper technique for comparing the corrosion resistance of the alloys. However, local electrochemical techniques were useful for the evaluation of the corrosion resistance of the alloys.     

Passivity breakdown and stress corrosion cracking of α-brass in sodium nitrate solutions

The anodic behaviour and SCC susceptibility of pure copper and four α-brasses of different zinc alloy concentration in a 1 M NaNO₃ solution was studied by means of potentiodynamic polarisation curves and constant potential slow strain rate experiments. SCC was exclusively observed when the potential was equal to or higher than a certain critical value (E_c) at which pitting initiated under slow dynamic straining. It is concluded that the same SCC mechanism should be operating during SCC of copper and α-brasses in sodium nitrate, sodium nitrite and copper (II) nitrate solutions.     

The corrosion fatigue and stress corrosion cracking of α-brass in ammoniacal solutions

A study of the corrosion fatigue and stress corrosion cracking under a constant strain-rate in 70 Cu?30 Zn brass as a function of pH has been made. Correlation between the two has been discussed. Corrosion fatigue was very dependent on pH both with respect to the lifetime and to the nature of cracks as well as those of stress corrosion cracking. A minimum in fatigue life and a large fatigue crack growth rate were observed in neutral tarnishing solutions of pH 6.5–7.0. The fatigue crack propagation rate in neutral tarnishing solution was about twice as great as that in an acidic non-tarnishing solution. Fractographic examination on the fracture surfaces of corrosion fatigue and stress corrosion cracking showed that intergranular cracking predominated both in neutral tarnishing and acidic non-tarnishing solutions, and transgranular cracking predominated in alkaline solution. It is concluded that the tarnish film is not a primary factor controlling the cracking mode in the acidic to neutral pH range and that intergranular cracking may result from the localized dissolution of alloy at grain boundaries.     

Corrosion behaviour of CoW0·013C0·001 in aqueous acidic sulphate solutions containing sodium lauryl sulphate and sodium citrate

Abstract

The corrosion behaviour of the CoW_xC_y binder alloy has been shown to be the key factor in the chemical stability of hardmetals of the WC–Co type. The anodic oxidation of the CoW_0·013C_0·001 alloy was studied in 0·1M H₂SO₄ with and without additions of 20 mM sodium lauryl sulphate or 20 mM tri-sodium citrate which were intended to act as organic corrosion inhibitors. In this investigation, use was made of linear sweep voltammetry, chronoamperometry, and ac impedance measurements (which can convey additional information on anodic activation processes occurring during corrosion). The reaction product morphology and composition were studied by scanning electron microscopy and energy dispersive spectroscopy. The inhibiting effects of the organic additives on the pseudo-passivating mechanisms were described. The precipitation of layers of corrosion products with different protective effectiveness was assessed and correlated with the electrochemical kinetics and with the morphological and chemical characteristics brought about by the presence of the organics. Sodium citrate proved more effective than sodium lauryl sulphate in inhibiting corrosion in the test conditions used in the present study.     

Dechromisation of Cu-Cr alloy in acid solutions containing Cl~-

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Pitting corrosion of single crystals of the Fe-16 Cr alloy in solutions containing Cl− ions

Pitting corrosion of a Fe-16Cr alloy in solutions containing Cl⁻ ions was studied on {100}, {110} and {111} faces of single crystals. Near the breakdown potential there occurred anisotropic dissolution of the alloy and the pits took the form of regular polyhedrons. The lowest dissolution rate was observed with the {110} face (the most closely packed crystal plane) and with the less closely packed {100} plane. These planes formed the bounding faces of regular pits. The nucleation of pits occurred at subboundaries, at spots where bubbles of O began to grow, and at the three-phase boundary between metal-solution-gas.     

Stress—corrosion cracking of austenitic stainless steel in hydrochloric acid media at room temperature

Stress—corrosion cracking of solution quenched, type 304, stainless steel can occur at room temperature in HCl solutions ranging between 5·10^?1M and 1M HCl. The cracking observed in HCl solutions is similar to that previously observed in H₂SO₄ + NaCl and HClO₄ + NaCl solutions. Cracking occurs at ? 0·200 V (NHE), in the active potential region, it is under cathodic control, and it develops in conditions under which the corrosion rate of the external surface area is more or less constant and independent of the HCl concentration, in the range 10^?1 M?1 M HCl. At higher HCl concentrations, corrosion rates increase and uneven, general corrosion occurs instead of cracking. The development of pitting and stress—corrosion cracking under active conditions precludes the conclusion that active—passive cells always play a role in localized corrosion and, in particular, in stress—corrosion cracking. Under these conditions, it has been shown that sensitized and non-sensitized specimens behave similarly (giving rise in both cases to transgranular cracking); active—passive cells, due to chromium depletion at the grain boundaries, are not involved. Active—passive corrosion mechanisms can however arise at more noble potentials (0·100?0·200 V NHE), as in the case of HClH₂O₂ solutions of specific concentration, producing intergranular corrosion of the stainless steel in the sensitized condition.     

Susceptibility of Ti–6Al–4V alloy to stress corrosion cracking in a Lewis-neutral aluminium chloride–1-ethyl-3-methylimidazolium chloride ionic liquid

The susceptibility of Ti–6Al–4 V alloy to stress corrosion cracking (SCC) in a Lewis neutral AlCl₃–EMIC ionic liquid (IL) was investigated employing U-bend tests. The results indicated that cracking initiated at the α/β phase boundary, propagated intergranularly, followed by transgranular cracking and failed in dimple mode under overloaded stress.     

Effect of welding speed on microstructure and corrosion resistance of Al–Li alloy weld joint

The corrosion resistance of a weld has a great impact on the service life of the joint. Changes in welding parameters can cause changes to the heat input, which affect the formation of the weld bead and the precipitation of the second phase, which determines the corrosion resistance of the weld. In this paper, the effect of a change in the welding speed on 2195 aluminium–lithium (Al–Li) alloy joints welded by laser and metal inert gas (laser-MIG) hybrid welding using Al–Si welding wire was studied. The macrostructure and microstructure of the weld were characterized by optical microscopy, X-ray diffraction, and scanning electron microscopy. The results show that the predominant precipitates in the laser-MIG hybrid welded Al–Li alloy were the θ (Al₂Cu) and T (Al–Li–Si) phases. As the welding speed increased from 11.5 mm/s to 16.5 mm/s, the heat input decreased, and the amount of the precipitated phase increased. Intergranular corrosion and electrochemical experiments were carried out on the weld seam, and the corrosion resistance was tested. With increasing welding speed, the corrosion resistance of the weld decreased. The high potential of the precipitated phase decreased the corrosion resistance of the weld joint.     

Dechromisation of Cu-Cr alloy in acid solutions containing CIˉ

The phenomenon, characteristics and influence factors of dechromisation of a Cu-Cr alloy in acid solu-tions containing Clˉ were studied by means of metallographic observation, SEM/EDX, TEM and XRD. The mecha-nism of dechromisation of Cu-Cr alloy was discussed. The results show that dechromisation of the Cu-Cr alloy oc-curs at a certain temperature in solutions with H and Clˉ simultaneously. The corrosion starts initially at the inter-faces between Cu phase and Cr phase, then develops gradually into the centers of Cr phase. It is also revealed that dechromisation rates of the Cu-Cr alloy increase with increasing concentration of H or Clˉ， and temperature of the solutions.     

Corrosion of aluminium in chloride solutions containing some anions—I. In 0.01M NaCl solutions

A study has been made of the corrosion behaviour of Al in solutions containing 0.01M NaCl alone or mixed with silicate, phosphate, dichromate, permanganate or sulphate ions covering concentration ranges of several orders of magnitude. Chloride ions become adsorbed on cracks in the pre-immersion oxide film. Silicate, phosphate and permanganate play a dual role. At small concentrations they modify the rates of nucleation and growth of the crystalline oxide layer. At higher concentrations they become incorporated in this layer in progressive amounts and hence impair its protective nature. The reduction product of dichromate improves the properties of the oxide film. At small concentrations, sulphate ions simulate chloride ions, but at higher concentrations they are adsorbed on the growing oxide nuclei hindering its growth.     

Thermomechanical treatment and corrosion resistance correlation in the AA2198 Al–Cu–Li alloy

ABSTRACT

The influence of T3, T8 and T851 thermomechanical treatments on the microstructure and corrosion resistance of the AA2198 was investigated. Differential scanning calorimetry and scanning electron microscopy were used for microstructural characterisation, whereas electrochemical methods were employed to analyse the corrosion behaviour of the alloy. The morphology and composition of constituent particles were similar for the T3 and T8 thermomechanical treatments but varied in the T851. There was an inverse relation between T1 phase density and corrosion resistance. The T3 treatment with the highest corrosion resistance was the one with the lowest density of T1 phase. The mechanisms of corrosion varied with the thermomechanical treatments.