Comments on the “Stress corrosion cracking of zirconium and zircaloy-4 in halide aqueous solutions” by S.B. Farina, G.S. Duffo, J.R. Galvele

The above paper [Corros. Sci. 45 (2003) 2497] declares a general approach to describing stress corrosion cracking (SCC) mechanism and despite of the criticism of the so-called “surface mobility mechanism” (SMM) e.g., [Corros. Sci. 36 (1994) 669, Corros. Sci. 45 (2003) 2105]. The paper [Corros. Sci. 45 (2003) 2497] demonstrates inner contradictions and mutually excluding statements of SMM when fitted calculated and measured values of crack propagation rate. As shown in these Comments, the crack propagation rate calculated using SMM is only the Nabarro-Herring creep deformation rate, which can never be numerically equal to the crack propagation rate. Thus, tests undertaken by the author of SMM cannot be accepted as experimental evidence of the SMM.     

Notes on the discussion concerning the “surface mobility mechanism” of stress corrosion cracking

A detailed analysis of the so-called “surface mobility mechanism” (SMM) of stress corrosion cracking (SCC) is given. This SMM based on “abnormally high” surface diffusivity influenced by stress in the crack tip was proposed by J.R. Galvele in 1987 and later criticized by K. Sieradzki and F.J. Friedersdorf in 1994. However, the SMM author continues to promote his concept in recent papers, which was an incentive of the present analysis. It is demonstrated that SMM contains inner contradictions and mutually excluding statements. In conclusion, it is shown that tests undertaken by the author of SMM cannot be accepted as experimental evidences of the SMM.     

Formation of quasicrystals in Zr–Pd–(Cu) melt spun ribbons and mechanically milled powders

Amorphous Zr₇₀Pd₃₀ and Zr₇₀Pd₂₀Cu₁₀ alloys were prepared by mechanical milling and melt spinnng to compare their devitrification behaviors. The devitrification of mechanically milled Zr₇₀Pd₃₀ and Zr₇₀Pd₂₀Cu₁₀ powders occurs via a single-step, first-order transformation to a stable Zr₂Pd tetragonal structure. This is in sharp contrast to the devitrification of the same amorphous alloys prepared by melt spinning, in which a primary meta-stable quasicrystalline phase forms. Since the mechanical milling process does not involve direct liquid phase formation of an amorphous structure, it is inferred that the short-range order in the solid state derived amorphous powder is different from that in the melt spun ribbon. During mechanical milling of an amorphous melt spun ribbon, crystallization of the quasicrystalline phase appears to precede disordering into an amorphous structure having an different short range order. Deformation of an amorphous melt spun ribbon by repetitive rolling at ambient temperature crystallizes the meta-stable quasicrystalline phase.     

Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions

Prestressing steels occasionally fail by a process named “stress corrosion cracking”. This process has not been fully elucidated and several theories exists in order to explain the cases in which real structures have collapsed. This paper briefly mentions the different theories and identifies the progress in understanding whether it is necessary to use a testing method, which is able to separate the different steps and mechanisms contributing to the failures.This paper presents the methodology used for inducing controlled localized attack to study the susceptibility of the high strength steels resistance to stress corrosion cracking (SCC). The method is designed to study the growth of cracks initiated from a mechanical notch; the crack is not produced by fatigue.It consists of several stages: coating of the bar with epoxy resin, generation of a small notch, constant load and controlled potential test in the media, mechanical test in air and fractographic study. It allows us to calculate the crack propagation rate and the fracture toughness in the same test.Finally, it has been possible to apply the surface mobility mechanism (SMM) in order to identify the SCC mechanism that operates.     

Hydrogen storage properties of Mg100−xNix (x = 5, 11.3, 20, 25) composites prepared by hydriding combustion synthesis followed by mechanical milling (HCS + MM)

We reported the structure and the notable hydrogen storage properties of the composites Mg_100−xNi_x (x = 5, 11.3, 20, 25) prepared from metallic powder mixtures of magnesium and nickel by the process of HCS + MM, i.e., the hydriding combustion synthesis (HCS) followed by mechanical milling (MM). X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated that mechanical milling led to drastic pulverization and grain refinement of the composite produced by HCS. All the composites with different compositions showed a remarkable decline in dehydriding temperature comparing with that of the hydride mixtures prepared only by HCS. Furthermore, the hydriding rates of these composites were excellent. At 313 K the composite Mg₈₀Ni₂₀ showed the highest hydrogen capacity of 2.77 wt.% within 600 s among these four composites. The Mg₉₅Ni₅ showed maximum capacity of 4.88 wt.% at 373 K and 5.41 wt.% at 473 K within only 100 s. Some factors contributing to the improvement in hydriding rates were discussed in this paper.     

The oxidation behavior and mechanical properties of MoSi2–CrSi2–SiC–Si coated carbon/carbon composites in high-temperature oxidizing atmosphere

A MoSi₂–CrSi₂–SiC–Si multi-component coating was prepared on the surface of carbon/carbon (C/C) composites by a two-step pack cementation method. The microstructure, oxidation behavior and mechanical properties of the coating were studied. These results show that the multi-component coating could protect the C/C composites from oxidation in air at 1873 K for 300 h and withstand 30 thermal cycles between 1873 K and room temperature, respectively. The mass loss and mechanical property loss of the coated C/C composites are considered due to the worse fluidity of SiO₂ at intermediate temperatures and the thermal mismatch between the coating and C/C composites.     

Hydrogen entry behaviour of newly developed Al–Mg–Si coating produced by physical vapour deposition

A ternary Al–Mg–Si alloy was prepared by co-evaporation technique and tested with respect to hydrogen entry behaviour as an alternative to conventional zinc coating on steel. Hydrogen entry behaviour evaluated using Devanathan cell showed a smaller hydrogen entry for this new coating than conventional zinc coating. Compared to an unscratched surface, hydrogen entry increased by more than 100 times in the scratched surface, but it was lower than that for the zinc coating with a scratched surface owing to the moderate galvanic corrosion potential of the new coating. This new coating is proposed especially for high-strength steel application.     

Acoustic emission monitoring of slow strain rate tensile tests of 304L stainless steel in supercritical water environment

An experimental set-up has been developed to perform slow strain rate tensile (SSRT) tests on tubular 304L stainless steel (SS) specimens in supercritical water (SCW) environment (550 °C, 250 bar). The supercritical water is circulated inside an internally pressurized tubular specimen mounted into a universal mechanical test rig and heated by a single loop resistance heating furnace. The set-up enables in situ monitoring of acoustic emission and electrochemical potential during the SSRT test. The SCW environment is found to significantly influence the mechanical performance of the material as a result of corrosion processes. A correlation between acoustic emission response and change of electrochemical potential is revealed. The findings are compared with preliminary results of tests performed on bulk SS specimens under the same condition in a commercial autoclave. The advantages and potentialities of both set-ups are discussed.     

Hydrogen entry behaviour of hot-dip Al–Mg–Si coated steel

A ternary hot-dip Al–Mg–Si coating was formed on a steel substrate and tested as an alternative to conventional zinc coatings particularly in high-strength steel application with respect to hydrogen entry behaviour. Hydrogen entry behaviour was evaluated under wet–dry conditions using a Devanathan cell. The new hot-dip Al–Mg–Si coating shows relatively low corrosion potential during the initial stage of the wet period; however, the potential shifts in a noble direction in a short time resulting in smaller amount of hydrogen entry than that in the conventional zinc coating.     

Effect of thermomechanical history on intergranular corrosion of extruded AlMgSi(Cu) model alloy

The effect of cooling rate after extrusion, solution heat treatment, mechanical deformation and aging on intergranular corrosion (IGC) of two model AlMgSi alloys with different Cu content was investigated using an accelerated corrosion test. The low Cu extrusions (0.02 wt%) were resistant to IGC while the high Cu (0.17 wt%) variants were susceptible in certain tempers. Slow cooling in air introduced IGC. The IGC susceptibility was reduced and finally removed by artificial aging. Water quenching prevented IGC, but severe IGC was introduced by slight aging (underaging). Peak aging (T6) generally reduced IGC susceptibility regardless of cooling media. Overaging further reduced IGC, usually at the expense of introducing pitting. IGC susceptibility was related to the microgalvanic coupling between enriched Cu on the grain boundaries (noble) and the adjacent solute depleted zone (active). Cu enrichment was caused either by slow cooling in air or slight aging of water quenched samples. High IGC resistance was obtained either by keeping the Cu content low or by applying proper heat treatment to high Cu samples. For the present high Cu samples, artificial aging to peak strength provided at the same time the necessary IGC resistance.     

Microstructure and mechanical properties of Ti 45Al 5Nb + (0–0.5C) sheets

The possible application of gamma titanium aluminides in aerospace industry requires a detailed understanding of the microstructure–property relationship of sheets made from this material. This paper reports the mechanical properties of sheets up to 1000 °C, based on alloy concepts with high Nb concentrations and small additions of C. Sheets were manufactured by rolling powder metallurgical compacts with compositions Ti 45Al 5Nb and Ti 45Al 5Nb 0.5C. The microstructures of both sheets are “near gamma” and consist of γ-TiAl and ₂-Ti₃Al phases. The texture of both phases is very weak. The strengths levels are very high and that of the C-containing sheet exceeds that of the C-free material at RT by 200 MPa. The mechanical properties of Ti 45Al 5Nb are independent of the direction in the sheet, in the whole temperature range from RT to 1000 °C. However, for the C-containing sheet this is true only in the upper temperature range.     

Influence of non-reactive particles on the microstructure of NiAl and NiAl–ZrO2 processed by thermal explosion

Nickel aluminide intermetallic alloy and NiAl–ZrO₂ composites were synthesized in a hot press by sintering reaction and thermal explosion (or Self-propagating High temperature Synthesis, SHS). The addition of a post-SHS heat treatment allows a control of the microstructure and an enhancement of the mechanical characteristics. Thus, NiAl properties processed by self-combustion are above those obtained by reactive sintering (RS). For all these syntheses, the role played by the non-reactive particles is determining. Indeed, the product granulometry is a function of the diluent size distribution since this latter acts as nucleation sites during the reactive processes. These particles can also enhance mechanical properties by specific reinforcement mechanisms and exercise an influence on SHS reaction parameters by controlling its reactivity and the thermal exchanges during self-combustion.     

Environment-induced cracking of Al–Cu alloy (AA2017P-T3) under constant load in distilled water and sodium chloride solutions

The environment-induced cracking (EIC) of a commercial Al–Cu alloy has been investigated as functions of applied stress, chloride ion concentration and test temperature in distilled water and sodium chloride solutions by using a constant load method. The effect of chloride ion on EIC is complex. The EIC susceptibility increased, unchanged and then decreased with increasing the chloride ion concentration. However, whenever EIC takes place with and without chloride ion, the fracture appearance and the value of t_ss (transition time to deviation from linear elongation)/t_f (time to failure) are the same, and further the relationship between log t_f and log l_ss (steady state elongation rate) becomes the identical straight line irrespective of applied stress, chloride ion concentration and test temperature. The latter means that l_ss becomes a relevant parameter for predicting t_f. It has been concluded that EIC of Al–Cu alloy takes place by hydrogen embrittlement (HE) associated with the fracture of hydride, and a HE mechanism is qualitatively proposed to explain the results obtained.     

Characterisation of anodic layers on Cu-10Sn bronze (RDE) in aerated NaCl solution

The anodic surfaces formed on Cu-10Sn (wt.%) alloy (α-bronze) are investigated in aerated 0.1 M aqueous chloride solution, using electrochemical reduction and characterisation methods such as scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). On the whole anodic domain, investigations performed on a bronze rotating disk electrode (RDE) reveal the systematic formation of a uniform oxidation layer. It is evidenced that the chemical composition of the layer varies with the applied anodic potential, but also that the latter always exhibits a poorly crystallised (probably nanocrystalline) hydrated and hydroxylated nature. Close to E_oc, the compounds are mainly (hydroxide) oxides of tin and copper, incorporating very low amounts of chlorides. At intermediate oxidation potentials corresponding to the active-passive transition, the first oxidation peak corresponds to the formation of hydrated tin oxyhydroxide chloride species which transforms in a more stable one - probably related to the Sn(II) → Sn(IV) oxidation. At higher anodic potential, on the current plateau, the layer contains hydrated tin (IV) oxyhydroxide and copper chloride (mainly CuCl). However, XRD and XPS results reveal that the barrier layer has a complex nature, including unidentified products and different spatially distributed charged surface zones. The corrosion mechanism involves an internal oxidation of the alloy linked to a preferential dissolution of copper, namely a decuprification. A decuprification factor f_Cu is defined and calculated. Both f_Cu and the layer thickness increase with the applied potential. We show unambiguously that the tin compounds remain in the corrosion layer, acting as stabilizing species. It is suggested that the tin species promote the formation of a network as for tin oxide xerogel, through which copper ions and anions migrate. Both the layer microstructure and the decuprification factor (f_Cu) are in agreement with those found in Type I patina of ancient bronzes.     

Transient stability of passive films in aqueous solutions

This discussion is concerned with the fundamental interactions of mechanical and chemical processes as they affect transient breaking and re-formation of passive films. These combined effects are important in many technical modes of degradations including: stress corrosion cracking, corrosion fatigue, wear, machining, grinding, fretting, erosion with and without entrained abrasives, and cavitation. These interactions of mechanical and chemical processes occur at surfaces of metals and involve the mechanically-induced breaking of films followed in sequence by transient film-free dissolution and repassivation. There is no useful theory for these transient electrochemical processes. This discussion provides a framework for advancing the fundamental understanding of such processes.     

Reactive extrusion synthesis of mechanically activated Ti–50Ni powders

In the present study, an alternative approach to the synthesis of TiNi alloys through powder metallurgy was successfully conducted by mechanically activated reactive extrusion synthesis (MARES) using elemental powders. The production of dense bodies was essentially dependent on the amount of intermetallic phases formed prior to reactive extrusion and on the densification temperature. The mechanically activated powders yielded a well controlled synthetic reaction during heating up to 900 °C with formation of multiphase products. This was possible due to the powder structure developed during mechanical activation. The best densified products were obtained at 700 °C although without a complete conversion into NiTi phase. More homogeneous microstructures and an effective reduction in the amount of secondary intermetallic phases were achieved after heat treatment at 950 °C/24 h followed by water quenching, yielding TiNi as the predominant phase, a relative density of 97%, and a Vickers micro-hardness of 682 H_V.     

Microstructure and intergranular corrosion resistance of UNS S17400 (17-4PH) stainless steel

UNS S17400 or 17-4PH is a precipitation hardening martensitic stainless steel with many industrial applications. Quite different mechanical properties can be produced in this material by varying the aging temperature. In this work, the influence of aging temperature on the intergranular corrosion susceptibility was evaluated by electrochemical and metallographic tests. The microstructural features were investigated by X-ray diffraction, optical and scanning electron microscopy. Intergranular chromium carbide precipitation occurs in specimens aged at high temperatures, although NbC carbides were also observed. The results obtained by double loop electrochemical potentiodynamic reactivation tests (DL-EPR) show that the susceptibility to intergranular corrosion resistance increases with the increase of aging temperature. Healing due to Cr diffusion in the 600-650 °C range was not observed by DL-EPR tests.     

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

Oxidation mechanism of an Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy at 470 °C (Part II)

This paper is the second part of a global study on the oxidation process of an Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi. It focuses on the growth mechanism of a duplex oxide scale. The oxide layer has a duplex structure composed of an internal Fe-Cr spinel layer and an external magnetite layer. The magnetite layer grows by iron diffusion until Pb-Bi/oxide interface whereas the Fe-Cr spinel layer grows, at the metal/oxide interface, inside the space kept “available” by the iron vacancies accumulation due to iron outwards diffusion for magnetite formation. This growth mechanism is close to the “available space model”. However, this model is completed by an auto-regulation process based on oxygen supply.