Film-induced Cleavage Research Articles

Dealloying of Ni- and Fe-Based Alloys in Boiling Caustic Environments and the Parameters Influencing the Geometry of a Dealloyed Layer

Nickel alloys and stainless have been the material of choice for a wide range of industrial applications like power and petrochemical plants that involve exposure to corrosive environments. For instance, Alloy 800 (Fe-30Ni-20Cr) has been used as the tubing material in CANDU and Siemens steam generators (SGs) for decades with excellent in-service performance. However, despite their good resistance to general to localize corrosion, Ni- and Fe-based alloys are susceptible to environmentally assisted cracking (EAC) in certain environments, such as hot caustic aqueous solutions. Laboratory experiments have shown that these materials are susceptible to dealloying in hot caustic environments due to the selective removal of Fe, Cr and Mo (less noble elements). As a result of dealloying, a brittle nanoporous film forms on the surface that is enriched in Ni (the more noble element). This film has been proposed to act as a precursor to stress corrosion cracking (SCC) by a film-induced cleavage (FIC) mechanism, where the brittle film injects a high speed/energy crack to the substrate material.The fast kinetics of dealloying requires a connected percolating pathway of less noble element atoms to exist within the alloy that extends from the surface into the material. The presence of this pathway depends on the composition of the alloy. The parting limit defines the minimum amount of the more reactive element(s) in a particular alloy system that makes it susceptible to dealloying. Below the parting limit composition, dealloying will be suppressed by surface diffusion of the more noble element, even at very oxidizing potentials, and the surface of the material will show a passive-like behavior (general dissolution would eventually occur at high anodic overpotentials). Besides the alloy composition, dealloying highly depends on the electrode potential. Below a threshold potential (dealloying critical potential), surface diffusion of the more noble element will overcome the dissolution of the less noble element(s) and suppress dealloying.In this study, we examine different factors that affect dealloying and the geometry of a dealloyed layer through electrochemical measurements and nano-scale characterization techniques. Several engineering Ni- and Fe-based alloys were tested in deaerated boiling caustic solutions to understand the effect of alloy composition on the susceptibility to dealloying. Results indicate that an increase in Ni content will shift the dealloying critical potential to more positive potentials and subsequently the resistance to dealloying. Although Ni is the alloying element that provides nobility and resistance in hot caustic environments, our results show that the thermodynamic and kinetic behaviour, and the content of the less noble elements play an important role in these environments. A comparison between different alloys with similar Ni-content but different Cr and Mo contents shows the beneficial and detrimental role of Cr and Mo in dealloying, respectively. The air-formed chromia-based oxide on the surface of alloys with high Cr-content slowly dissolves in the caustic environment, delays the dissolution of less noble elements, and increases the resistance to dealloying.Changes in electrode potential and time of exposure to the caustic environments, alter the geometry of the dealloyed layer. Results show an increase in the diameter and coarsening of ligaments with increasing time of exposure to the caustic environment. Coarsening of ligaments is believed to impair the ability of a dealloyed layer to inject a micro-crack to the underlying material and initiate SCC. Also, results show that an increase in anodic overpotential of only a few millivolts in the vicinity of dealloying critical potential would change the geometry of the dealloyed layer significantly. The surface diffusion of the more noble element (i.e. Ni) cannot keep up with the fast removal of atoms of the less noble elements (i.e. Cr, Mo, and Fe) at more positive electrode potentials compared to the dealloying critical potential which results in a weak bonding at the film/metal interface. The weak bonding may not be able to support the transmission of a high-speed crack to the substrate material, reducing the likelihood of SCC initiation. Figure 1

Advanced microstructural characterization of transgranular Pb-caustic SCC in Alloy 690TT

Alloy 690TT was tested at 340 °C in mildly caustic environments containing lead (Pb) to investigate the transgranular cracking mechanism of Pb-induced stress corrosion cracking (PbSCC). Advanced electron microscopy characterization revealed that the fracture surfaces exhibited evidence of cleavage and regions with significant enrichment of metallic Ni, which maintained the same orientation as the original grain matrix. Moreover, there are some indications that crystallographic cracking along {100} and {110} planes is possible at high pH in Pb-containing environment. These experimental observations are consistent with a form of film-induced cleavage mechanism, assisted by de-alloying and exacerbated by planar slip in the matrix.

Dealloying Behavior in Ni-Cr-Fe Alloys As a Precursor to SCC in Boiling Caustic Environments

Experiments have shown that the susceptibility of Ni-Cr-Fe Alloys to dealloying, and subsequently stress corrosion cracking (SCC), highly depends on alloy composition and electrochemical potential. Dealloying is defined as the selective dissolution of the more active elements in an alloy (e.g., Cr and Fe in a Ni-based alloy). A dealloying process results in a remnant nano-porous layer on the surface that could be a precursor to SCC by a mechanism known as film-induced cleavage (FIC) where cracks propagate through this layer with sufficient velocity to promote energy transmission into the underlying metal resulting in cleavage fracture. Presence of a threshold amount of a more noble element could theoretically suppress this phenomenon by surface diffusion. This was investigated further by previous researchers, and the percolation theory of materials was hypothesized that states that the more noble element in an alloy creates a percolating mass that would limit the ability to retain a connected pathway of the reactive elements from the surface given it reaches a certain threshold of the mass of the material. This percentage was estimated to be between 50-60% for most alloying systems and termed the parting limit. In this study, percolation theory and dealloying is further explored by exposing several Ni-based and Fe-based alloys with varying percentages of Ni to boiling caustic solutions (50% NaOH) and evaluating their dealloying behavior.As mentioned, dealloying highly depends on electrochemical potential and is believed to occur above a threshold potential known as the dealloying critical potential. The dealloying critical potential is defined as the condition at which a transition occurs from a “passivated” alloy surface to the sustained formation of a bi-continuous porous structure. It can be identified by running an anodic polarization scan and initially matching anodic peaks to a pH-potential diagram (Pourbaix diagram) to evaluate the potential dissolution behavior. Bench-top electrochemical measurements were conducted to find the dealloying critical potential and make a correlation between this potential and alloys’ composition. The figure attached shows the anodic polarization scan results for all alloys included in the study in 50% NaOH at 140°C. It was observed that for the samples with high Ni content (i.e., Alloy 600 and Alloy 690) there was only a single active peak, which indicates transitioning to passivation and inhibition of dealloying. All other materials were susceptible to dealloying. Electron microscopy techniques were used to analyze all the samples used in the figure after exposure to 50% NaOH at 140°C at open circuit potential (OCP) and during a potentiostatic hold at the dealloying critical potential. Scanning electron microscopy (SEM) imaging followed by energy dispersive X-ray spectroscopy (EDS) revealed varying degrees of dealloying, with low Ni-containing alloys showing greater severity.The role of dealloying as a precursor to SCC was also evaluated by utilizing U-bend samples for Alloy 690, Alloy 800, and SS 304. The findings were in-line with previous dealloying result; the alloys with Ni content below the parting limit, SS304 and the Alloy 800, both underwent SCC after 48 hours of exposure to 50% NaOH solution at 135 °C, while the Alloy 690 sample (~60Ni) did not show any signs of cracking. A peculiar phenomenon was also noted for alloys containing Mo where it is seen that their dissolution current is approximately two folds higher than their counterparts with approximately the same Ni content. For example, when comparing SS 304 and SS 316 or Alloy C-276 and Alloy G-35. A possible explanation for that is the electrochemical behavior of Mo in caustic conditions where it is shown in its pourbaix diagram that dissolution occurs over a wider range of pH and potential in comparison to the other alloying elements. This earlier dissolution can possibly create more terrace sites on the surface that could accelerate the dissolution of iron and chromium. However, further studies are needed before a conclusion can be reached. Further work has also been conducted on Alloy 800 under the same conditions to evaluate the possibility of controlling the geometry of the dealloyed layer. The results so far indicate that the time of exposure and the electrochemical potential are critical in controlling the geometry of the dealloyed layer. Parallel studies are being conducted on SS 304 to evaluate the effect of Pb on extending the dealloying regime. The results so far have shown that the addition of Pb would shift the OCP more positively towards a region of passivity. It was also noted that dealloying kinetics is hindered in comparison to pure caustic conditions. However, SCC may still be possible with dissolved Pb present. Figure 1

Mechanisms and Kinetics of Environmentally Assisted Cracking: Current Status, Issues, and Suggestions for Further Work

Mechanisms and kinetics of metal-induced embrittlement, hydrogen-embrittlement, and stress-corrosion cracking are discussed, and long-standing controversies are addressed by reviewing critical observations. Recommendations are also made regarding further work (including repetition of previous work using more advanced measurement and characterisation techniques) that should be carried out in order to resolve some of the contentious issues. The evidence to date suggests that adsorption-based mechanisms, involving weakening of substrate interatomic bonds so that dislocation emission or decohesion is facilitated, accounts for embrittlement in many systems. Embrittling adsorbed species include some metal atoms, hydrogen, and complex ions produced by de-alloying. Other viable mechanisms of embrittlement include those based on (1) dissolution of anodic grain-boundary regions, and (2) decohesion at grain boundaries owing to segregated hydrogen and impurities. The hydrogen-enhanced localised-plasticity mechanism, based on solute hydrogen facilitating dislocation activity in the plastic zone ahead of cracks, makes a contribution in some cases, but is relatively unimportant compared with these other mechanisms for most fracture modes. The film-induced cleavage mechanism, proposed especially for stress-corrosion cracking in systems involving de-alloying at crack tips, is questionable on numerous grounds, and is probably not viable. Rate-controlling processes for environmentally assisted cracking are not well established, except for solid-metal induced embrittlement where surface self-diffusion of embrittling atoms to crack tips controls cracking kinetics. In some systems, adsorption kinetics are probably rate-controlling for liquid-metal embrittlement, hydrogen-environment embrittlement, and stress-corrosion cracking. In other cases, rate-controlling processes could include the rate of anodic or cathodic reactions at and behind crack tips (responsible for producing embrittling species such as hydrogen) and rates of hydrogen diffusion ahead of cracks.

Mechanistic and fractographic aspects of stress corrosion cracking

Abstract: Basic aspects of stress-corrosion cracking (SCC) in metallic materials are outlined, followed by a summary of the numerous mechanisms that have been proposed for SCC. The characteristics of transgranular and intergranular SCC in model systems, e.g. pure metal and single-phase alloy single crystals and bi-crystals under testing conditions that facilitate discrimination between mechanisms, are then described. The applicability of the various proposed mechanisms, such as those based on dissolution, hydrogen embrittlement, film-induced cleavage, and adsorption, are discussed in detail for these systems. Mechanisms of SCC in complex commercial alloys are then considered in the light of these studies on model systems.

A revised interpretation of ultra-fast stress corrosion cracking experiments by Serebrinsky and Galvele

Reconsideration of data from stress corrosion cracking tests conducted at extreme strain rates on homogeneous noble-metal alloys leads to the conclusion that the data are more consistent with the film-induced ‘cleavage’ (in this case intergranular fracture) model than with the surface mobility model. At the highest strain rates, the data for several alloy systems are close to what would be expected from a single brittle event followed by ductile rupture. The opportunity is also taken to comment on some misconceptions regarding the predictions of the film-induced cleavage model, and more generally the correlation between dealloying and SCC in homogeneous noble-metal alloys.

Film-Induced Cleavage of Ag-Au Alloys

Single-shot fracture experiments have been carried out in a dealloying environment (perchloric acid) on Ag-20 at. pct Au and Ag-23 at. pct Au alloys in the form of thin wire and foil, respectively. Such environments produce a nanoporous metallic layer on the surface. Transgranular fracture was promoted in some of the wire experiments by inducing a bamboo grain structure. Using the wires, it was shown that complete brittle intergranular or transgranular fracture could be obtained, either by applying a very low tensile stress during dealloying, by rapidly straining the wire after dealloying without stress, or by straining to failure after first dropping the potential to a value where little or no further faradaic reaction was possible. Extremely low fracture stresses were found, especially for intergranular fracture, and all fracture events were sudden with no secondary substrate fracture. The work with foils showed that some irreproducibility noted in the response of the wires was associated with premature self-fracture of the dealloyed layer. By optimizing the condition of the dealloyed layer, extremely reproducible and complete film-induced fractures were obtained. These dealloyed layers showed properties not previously seen, such as an ability to inject deep substrate cracks even after a 5-minute immersion in deionized water. Recent studies showing extraordinary mechanical properties of dealloyed layers, such as near-theoretical strength in compression, give more credibility to this still-mysterious fracture mechanism. A detailed rebuttal of the claims of the alternative “surface mobility” model is presented.

CORROSION/DEFORMATION INTERACTIONS IN URANIUM/WATER SYSTEMS

Aqueous Corrosion/Deformation Interactions (ACDI) in uranium and uranium alloys actually exhibit complicated mechanical damage processes. In this context, the exploration of the dominant micromechanical mechanisms vis- a -vis experimental confirmations remain an unsettled issue. Various mechanisms might be involved in such environmental effects, starting from stress corrosion cracking (SCC) hydrogen embrittlement, up to brittle film fracture or film-induced cleavage, enhanced or inhibited by diffusion barriers. These on top of hydride formation which potentially affects the local mechanical/chemical driving field. Thus, beside the driving force modifications, the reduction of the fracture resistance requires appropriate evaluations. The main objective in the current phenomenological investigation was, to provide additional findings as related to damage evolution in specific ACDI systems. In fact, the present selected systems allowed further insights into a low symmetry, orthorhombic crystal structure case. Experimentally, sustained load, slow strain rate (SSR), monotonic and fatigue tests in fracture mechanics framework were performed, supplemented by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) observations. In addition, X-ray, Fourier Transform Infrared spectroscopy (FTIR) techniques and acoustic emission (AE) tracking were utilized. Experimental findings are analyzed and discussed with emphasis to fractographic observations and kinetics associated with the crack propagation stage. These beside developments in modeling aspects founded on a stepwise decohesion processes.

Film-Induced Brittle Intergranular Cracking of Silver-Gold Alloys

Abstract Dealloying of a binary noble alloy produces a porous layer rich in the more noble element. Application of a tensile load initiates a brittle intergranular (IG) crack in the dealloyed layer that advances into the unattacked material. This study showed that the crack penetration depth (Cd) is proportional to the thickness of the dealloyed layer (t). For a given value of t, the grain-boundary crack penetration distance was shown to decrease as the dealloying potential increased. The dependence of Cd on t and the dealloying potential, as opposed to the applied potential at the time of fracture, supported the film-induced cleavage model.

Corrosion science and engineering: some recent developments

The changes that have occured and advances that have been achieved in corrosion science and engineering over the past two decades can be attributed to a number of factors. These include fundamental advances in the science of corrosion and new mechanistic insights, more sophisticated experimental equipment, particularly in the areas of the surface analysis and electron optical techniques, pressure from new service conditions and industrial requirements and the greatly increased availability and power of computers. While most aspects of materials science and engineering have benefited from similar trends, corrosion science and engineering, particularly in the UK, received the added impetus and stimulus of the publication in 1971 of the DTI Hoar Report on Corrosion. This estimated that corrosion cost the UK about £1300m(about 3.5% of the then GDP), of which about £350m could be saved by improved selection of existing materials and wider use of existing techniques for corrosion prevention and control. The corresponding figures in 1993 would be about £11bn and £2.5bn respectively. Rather than reviewing advances in the whole, very broad field of corrosion science and engineering, which is clearly beyond the scope of a single paper, this paper presents a highly selective perspective of some of the developments in three specific areas: passivityy, environmental cracking and organic coatings. In each case the paper presents a broad summary of the major developments and then highlights specific advances for more detailed discussion. Thus, in the field of passivity, the use of percolation theory and computer modelling to explain the observed compositional thresholds for active-passive transitions in FeCr alloys is discussed in some detail. Specific developments in environmental cracking include the film-induced cleavage model for the mechanism of transgranular stress corrosion crack propagation and the role of crack-tip chemistry and electrochemistry in determining susceptibility to cracking. In the case of organic coatings, there is emphasis on the potential of electropolymerization as a non-toxic alternative to chromates for improved adhesion and performance. The picture that emerge from this selective review of developments since the establishment of the Department of Materials Engineering at the University of Cape Town is one of incremental fundamental and practical advances in a relatively mature discipline.

Modelling of environment assisted cracking

A critical evaluation of mechanistically based modelling of environment-assisted cracking has been made. Models based on anodic reaction, namely slip-dissolution, anodic reaction-induced cleavage and surface mobility are evaluated from both a mechanistic and quantitative perspective. The film-rupture slip-dissolution mechanism is increasingly perceived to be relevant only to intergranular cracking though not as an exclusive mechanism for this process. The basic principle of film-induced cleavage has been demonstrated experimentally but the range of applicability of the mechanism remains contentious and requires further work. The concept of the surface mobility model is interesting but the quantitative crack growth model is based on unsatisfactory assumptions. In relation to models of environment-assisted cracking based on the effect of hydrogen atoms it is evident that for the non-hydride forming systems the hydrogen enhanced localised plasticity mechanism is gaining wider acceptance though studies of Fe-Si single crystals suggest that for some systems a cleavage mechanism of crack advance may prevail. Quantitative modelling of crack growth based on anodic reaction processes is reasonably well developed, though with a need to provide information about crack-tip electrochemistry and strain rates. These models are comparatively simple but modelling of cracking due to hydrogen atoms is more complex, for example, because of the diffusion and trapping of hydrogen atoms in the metal. Accordingly, models of crack growth in this case tend to be based on rate-limiting processes and in corrosion fatigue in particular there is a need to integrate the chemistry, mechanics, transport and materials interaction in an improved quantitative framework. Mechanistic modelling has an important role in understanding and predicting environment-assisted cracking but the application to life prediction may be limited to specific systems because of the complexities of multiple nucleation events, crack coalescence, crack-tip shielding, crack branching and complex stresses. The intelligent combination of empirical modelling supported by insight from mechanistic modelling provides the most effective approach to predicting environment-assisted cracking in metal-environment systems.

A transmission electron microscopy study of stress corrosion cracking in stainless steels

Stress-corrosion cracks were produced in a 25 wt% Cr duplex stainless steel and in a 304L austenitic stainless steel by exposure to boiling chloride solutions. Samples from the steels were examined optically and by scanning electron microscopy, then prepared for transmission electron microscopy by ion-beam thinning. In the duplex steel, cracks propagated in the ferrite, and tended to be blocked by austenite. Cracking in the ferrite was transgranular and cleavage-like. Cracking in the 304L austenitic stainless steel was also transgranular and cleavage-like. Transmission electron microscopy and microprobe analysis revealed two morphologically distinct layers associated with stress-corrosion cracking: an essentially featureless outer layer which was considered to be corrosion product, and a dealloyed sponge-like region adjacent to the metal. The presence of the sponge-like layer was interpreted as being consistent with a film-induced cleavage mechanism of stress corrosion cracking.

Testing the film-induced cleavage model of stress-corrosion cracking

The effects of coarsening of porosity within dealloyed layers have been investigated from the viewpoint of the film-induced cleavage model of SCC. Tensile tests and pore coarsening measurements of Ag-20 at%Au foils were carried out in 1 M HClO 4 at room temperature. Upon ageing at potentials above 550 mV(SCE), the ability of a brittle crack to penetrate into the substrate was destroyed quickly due to rapid pore coarsening. At low potentials brittle behaviour persisted for up to 2 h. Chloride ions accelerated the coarsening and pyridine retarded it. The observed time dependence of the coarsening indicates that it occurs by the surface diffusion of Au atoms and is affected by the specific adsorption of ions or molecules.

Embrittlement of nano-scale structures

A dealloyed surface layer formed on a Cu-30 Au alloy due to a corrosion process is capable of inducing intergranular cleavage of the normally ductile substrate during subsequent mechanical loading. The structure of the dealloyed layer consists of nanoscale pores that were formed by the selective removal of copper atoms during the anodic polarization. To contribute to the understanding of environmentally — induced embrittlement, we presently compute the stress state within the porous layer and in the bulk substrate due to the eigenstraining by the dealloying. The results of the calculations shows that a tensile stress state exists in the porous dealloyed layer while a compressive stress state is generated in the substrate. Implications on the “film-induced cleavage” model of stress corrosion cracking mechanism are discussed.

Stress corrosion cracking of CMn steels in methanol-ammonia environments—II. Electrochemical and fractographic studies

The mechanism of stress-corrosion cracking of CMn steel in anhydrous methanol-ammonia solutions has been investigated using detailed fractography and electrochemical measurements. SEM and TEM examination of the stress-corrosion fractures in a large-grained material show that the opposing transgranular fracture surfaces are generally matching and interlocking; however, this was not invariably the case with certain arrays of striations. One type of striation, spaced by about 2 μm, matches peak-to-peak on opposing surfaces and is shown by periodic overloading (load pulsing) to correspond to successive positions of the crack front. Load pulsing at increasing frequency caused an increase in the overall crack velocity, and a shortening of the crack jump distance. However, no loss of correspondence between the numbers of load pulses and crack front markings was found. There is thus very strong, but not entirely conclusive, evidence that the crack advance is discontinuous. Electrochemical measurements on rapidly strained electrodes and on microelectrodes inside artificial crevices show that the highest reasonably sustained anodic current density is about 200 A m −2, which is roughly one order of magnitude too low to account for the observed crack growth rate by an anodic process. Passivation of iron microelectrodes by ammonia was shown to be necessary for stress-corrosion cracking and to coincide with the oxidation of ammonia on platinum, indicating that the passive film is probably an iron nitride. The various observations described above, together with the enhancement of stress-corrosion cracking by the addition of nitrogen to the electrolyte, are believed to provide strong support for a film-induced cleavage mechanism of cracking, wherein a thin, nitrogen-hardened layer forms preferentially (due to a mass action effect) in the presence of adsorbed molecular nitrogen and subsequently triggers about 1 μm of cleavage.

Evidence for film-induced cleavage in rhodium plated nickel

R. E. Ricker, J. L. Fink, J. S. Harris, and A. J. ShapiroMetallurgy DivisionNational Institute of Standards and TechnologyU.S. Dept. of Commerce, Technology AdministrationGaithersburg, MD 20899Introduction Transgranular stress corrosion cracking (T-SCC) is a form of environmentally induced subcritical crackgrowth which produces fracture surfaces that are cleavage-like in appearance. These fracture surfaces resemblecleavage because: propagation occurs on flat crystallographic facets, steps or ledges exist between the flat facetsforming river patterns, these facets and ledges match precisely on opposite sides of the fracture, andundercutting occurs at the ledges in the fracture surface. A variety of different mechanisms have been proposedto explain this form of crack growth, but no one mechanism is universally accepted. In general, the mechanismsthat have been proposed can be classified in two groups based on whether they assume that dissolution ormechanical fracture is responsible for crack growth.A dissolution mechanism for T-SCC needs to explain why dissolution is localized to the crack tip, why thefracture surfaces generated resemble cleavage and why opposite sides of the fracture surface match precisely (1).Even with these difficulties, dissolution based mechanisms attracted a number of advocates until Bertocci et al.(2) demonstrated that T-SCC cracks can propagate in a 70-30 α-brass exposed to a solution where Cu metal is inequilibrium with Cu

Brittle fracture of an Au/Ag alloy induced by a surface film

The film-induced cleavage model of stress-corrosion cracking (SCC) has been tested using an Ag-20 at. pct Au alloy in 1 M HClO4 solution. Brittle cracks, both intergranular (IG) and transgranular (TG) in nature, were formed by high-speed loading of a thin foil covered with a dealloyed (nanoporous gold) layer. These cracks were found to propagate through the dealloyed layer and into the uncorroded bulk face-centered cubic (fcc) material for a distance of many microns. Hydrogen embrittlement (HE) can be excluded on thermodynamic grounds; thus, only film-induced cleavage can explain the observed decoupling of stress and corrosion in the fracture process.

Cyclic loading effects in the environment sensitive cracking of α-brass

Cyclic loading tests have been carried out on an α-brass exposed to sodium nitrite or soluble arsenic salt solutions to test the hypothesis that environment sensitive crack growth in such systems should be dependent upon the frequency of cyclic loading if the mechanism of crack growth involves film-induced cleavage. No significant effect was observed for changes over a few orders of magnitude of the frequency of load cycling. The results show that continuing micro-plastic deformation is necessary for the initiation and growth of environment sensitive cracks in these systems.

Metallographic contributions to understanding mechanisms of environmentally assisted cracking

Mechanisms of environmentally assisted cracking can often be deduced from a knowledge of the relationship between cracking and microstructure, the fracture planes and directions, the detailed appearance of fracture-surfaces, the slip planes that are active during cracking, and the strains associated with crack growth. Metallographic and fractographic techniques for determining these characteristics are briefly reviewed, and the advantages of studying cracking in single crystals are outlined. Transcrystalline, cleavage-like cracking in various materials is then discussed, and it is concluded that the metallographic observations strongly support the following: 1) a mechanism of hydrogen embrittlement based on repeated formation and fracture of brittle hydrides for materials such as niobium, vanadium, and zirconium; and 2) a mechanism of liquid-metal embrittlement, hydrogen embrittlement, and stress-corrosion cracking, based on an adsorption-induced localized-slip process for materials such as aluminium alloys, nickel, and high-strength steels. There is, however, less support for other proposed mechanisms of transcrystalline stress-corrosion cracking, such as those based on film-induced cleavage or localized dissolution at crack tips, and the mechanism of cleavage-like cracking in some materials such as copper alloys is still unresolved.

Rupture fragile des matériaux ductiles

The stress corrosion cracking of ductile materials is observed to occur generally by discountinuous cleavage or by a brittle intergranular mode. Recent models have been published to explain this surprising type of failure : the film-induced cleavage model, the dissolution inducing vacancies model, the adsorption model and the enhanced surface mobility model. Nevertheless these models show conflicting mechanisms. In the present paper a new physical model is proposed for both trans and intergranular stress corrosion cracking. The effect of a localized dissolution on the dislocation behavioural the crack tip is taken into account. Experimental results are described and discussed to confirm the basic ideas of this new model. The application of this model to the stress corrosion cracking behaviour of the 316L alloy in CL- solutions is presented.