Untersuchungen über die interkristalline Korrosion und Spannungsrißkorrosion einer reinen AlZnMg 2-Legierung in NaCl-Lösung

Investigations Concerning the Intercrystalline Corrosion and Stress Corrosion Cracking of a Pure AIZnMg 2-Alloy in NaCl-Solution In a pure AIZnMg 2-alloy diversified fully step aged or overaged in one step intergranular corrosion (IG) and stress corrosion cracking (SCC) in airsaturated 1 N NaCl-solution or in deaerated 0,1 N NaCl-solution at 303 K were investigated under potentiostatic control. The pure AlZnMg 2-alloy had the following composition (weight percent): Zn: 4.60; Mg: 2.00; Fe: 0.005; Si: 0.006; balance Al. In the two step aged AlZnMg 2-alloys the IG-sensitivity is lowered by an increase of the first step aging time, while SCC-susceptibility and the yield point are raised. As possible mechanism of SCC hydrogen embrittlement is suspectet. At potentials noble to IG-breakdownpotential an interaction of intergranular corrosion and SCC brings about crack velocities, which are to be regarded as a sum of the velocity of intergranular corrosion, which is virtually independent of tensile stress, and the velocity of crack propagation due to SCC. In the overaged AlZnMg 2-alloy IG-attack was observed although in this state of aging no precipitate free zone anodic to the matrix can be assumed. A model to explain this variation of intergranular corrosion, that proofs to be dependent of tensile stress, is represented.     

Einfluß der Mikrostruktur und des Elektroden-potentials auf die Ausbreitungsgeschwindigkeit von Spannungskorrosionsrissen einer reinen AlZnMg3-Legierung in 1 M NaCl-Lösung

Influence of microstructure and electrode potential upon the velocity of stress corrosion cracks of a pure AlZnMg3-alloy in an 1 M NaCI-solution The influence of microstructure and electrode potential upon the stress corrosion crack velocity in a fully aged pure aluminium-zincmagnesium 3-alloy with two step ageing and quench interruption treatments was investigated in airsaturated 1 M sodium chloride solution at 303 K. For this, propagation of the crack tip was observed microphotographically under potentiostatic conditions and under constant load using specimens precracked by fatigue. The crack propagation at the corrosion potential is accelerated by denser matrix-precipitates, narrower precipitate free zone, the presence of great T-grain boundary precipitates and minor sensitivity to intercrystalline corrosion and occurs to judge from the look of the fracture surfaces without considerable metal dissolution. In contrast to the alloy, which shows no intercrystalline corrosion, the crack propagation in the alloy with intercrystalline corrosion is obviously faster above the breakdown potential than at the corrosion potential. The results suggest that the crack propagates essentially mechanically. As mechanisms are discussed a crack growth by anodic dissolution of grain boundary precipitates or of zinc- and magnesium-rich grain boundary regions and mechanical seperation of the intermediate regions and propagation by local hydrogen embrittlement of the crack tip.     

Beobachtungen zur Ausbreitung von Spannungskorrosionsrissen in einer reinen AlZnMg3-Legierung

Observations concerning stress corrosion crack propagation in a pure AlZnMg3 alloy The propagation of stress corrosion cracks has been studied (by micro fotography) using potentionstatically polarized specimens notched on one side and held underconstant load in air saturated NaCl solution. According to the results obtained crack propagation is discontinuous; the stop points visible in the crack length-time-curve may be characterized as collisions between crack tips and grain boundary triple points. At ? 1100 mV the crack propagation velocity is higher than at ?mV provided the particular potential has been given from the very beginning. If, however, the potentials is varies from?1100 to ?1300 MV during the measurement the change in crack propagation velocity is negligable (the same applies to potential changes at the time of small crack length). The discontinuity in crack propagation may be explained by assuming hydrogen embrittlement or selective corrosin to occur at the crack tip.     

Corrosion behaviour of twin belt cast EN AW 7075 alloy

The corrosion behaviour of the twin belt cast EN AW 7075 alloy is governed by intermetallic phases, namely Al₁₂(Fe,Cr,Mn)₃Si, Mg₂Si and CuAl₂, and by Mg(Zn,Cu,Al)₂ precipitates. The former are responsible for pitting activities while the Mg(Zn,Cu,Al)₂ precipitates play a key role in intergranular corrosion. The very fine dispersion of Mg(Zn,Cu,Al)₂ precipitates in samples aged to peak hardness undergo coarsening, particularly along the grain boundaries, when the hot band samples are overaged. Overageing improves the resistance to intergranular corrosion while the samples in T6 temper suffer heavy attack along grain boundaries. While ageing treatments hardly produce any change in the features of the intermetallic particles, they nevertheless seem to impact the pitting response. This may be accounted for also by the precipitation activities which in turn, change the chemistry of the solid solution matrix. Overageing to the T73 temper implies a higher purity matrix and thus changes the microgalvanic effects when exposed to neutral chloride solutions.     

Effect of high-temperature pre-precipitation on mechanical properties and stress corrosion cracking of Al-Zn-Mg alloys

1　INTRODUCTIONMediumandhighstrengthAl Zn Mgseriesaluminumalloysarethe primaryweldingstructurematerialsofaerocrafts ,transportationvehiclesandmilitaryequipments[1,2 ] ,duetoitsbetterweldabilityandexcellenttechnologicalproperty .Buttheexten siveutilizationofthesealloysishamperedbyitspoorstresscorrosioncracking (SCC)resistance[3,4 ] .Ac cordingly ,manystudieshavebeenconcernedwithsuchaproblemashowtoimprovestresscorrosionre sistanceofAl Zn Mgalloyswithnon deterioratedstrength[3,58] .Atpres…     

Die Abhängigkeit der Spannungskorrosionsanfälligkeit vom Werkstoffzustand bei homogenen Kupfer-Zink-Legierungen

Interdependence of stress corrosion susceptibility and structural state of homogeneous copper-zinc alloys Experimental investigations in ammoniacal solution of copper and ammonium sulfates have shown, that — with equal load ratio (σ/σ_0,2 = 0,9) — CuZn alloys are less sensitive to stress corrosion cracking after recrystallizing annealing than after cold working. The sensitivity increases with increasing Zn content and appears to depend primarily from the dislocation structure: parallel structures are highly sensitive while increasing transition to cellular dislocation structure reduces stress corrosion susceptibility. In the cold worked alloy the crack geometry is predominantly transcrystalline, in the recrystallized state intercrystalline. Similar relations between dislocation structure and susceptibility seem to exist in the case of austenitic stainless steels.     

Die Ausbreitung von Spannungskorrosionsrissen in der Aluminium-Legierung AlZnMg 3

Crack propagation during stress corrosion cracking of AlZnMg 3 alloy The authors have studied crack propagation in notched specimens exposed to sodium chloride solution at 30°C under constant load. Crack propagation have been followed by microphotography which has enabled gliding phenomena within the specimen to be followed, too. Starting from the rest potential and shifting it toward more cathodic values crack propagation is at first slowed down which is a type of cathodic protection, but afterwards – in the hydrogen evolution region with simultaneous cathodic corrosion – the crack propagation velocity increases. This fact can be attributed to hydrogen embrittlement. Taking into account that the industrial alloy is more resistant than the high purity alloy one may arrive at the conclusion that the impurity content acts as a decelerating factor giving rise to an extension of the incubation period.     

Research on intercrystalline corrosion,exfoliation corrosion,and stress corrosion cracking of Al–Zn–Mg–Sc–Zr alloy

The intercrystalline corrosion, exfoliation corrosion (EXCO), and stress corrosion cracking (SCC) of Al–Zn–Mg–Sc–Zr alloy were investigated by means of constant temperature immersion corrosion method, optical microscopy, transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS). The results show that intercrystalline corrosion, and EXCO susceptibility of Al–Zn–Mg–Sc–Zr alloy decrease gradually with increasing of aging time. Corrosion susceptibility order from low to high is as follows: OA > PA > UA > NA. The SCC susceptibility index of PA temper is more than OA temper at the same strain rate. According to TEM observation, with aging time prolonging, a part of η′ phases transform to η equilibrium phases, which become coarse gradually. The distribution discontinuity of the grain boundary precipitates increases. In addition, for Al–Zn–Mg–Sc–Zr alloy without EXCO, the EIS is comprised by a capacitive impedance arc at high frequency and an inductive impedance arc at low frequency. Once EXCO occurs, the EIS is composed of two capacitive impedance arcs at high frequency and at low frequency, respectively.     

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.     

Korrosionsprobleme in chlorhaltigen Medien: Lösungsmöglichkeiten durch einige nichtrostende Spezialstähle

Corrosion problems in chloride containing media: possible solution by some stainless special steels The increasing water pollution forces the chemical industry to use water with increasing chloride content for cooling and other purposes. This trend brings about increasing corrosion danger, in particular pitting, stress corrosion cracking and corrosion fatigue as well as crevice corrosion. The present paper deals with some steels characterized by resistance to these specific corrosion phenomena. A steel containing (%) 21 Cr., 7.5 Ni, 2.5 Mo, 1.5 Cu, to 2 Mn, to 1 Si and 0.06 C is particularly resistant to stress corrosion cracking. It contains 30 to 50% ferrite in an austenitic matrix. Even in Mg chloride solutions it may be kept under a load of 7 kg/mm² without stress corrosion occurring (with a steel of the 18 10 CrNiMo type the admissible load is only 2 kg/mm²). A steel containing (%) 25 Ni, 21 Cr, 4.5 Mo, 1.5 Cu, to 1 Si, to 2 Mn, and 0.02 C has a broad passivity range and is resistant to general corrosion in acid reducing media and phosphoric acid of all concentrations. A ferritic steel containing (%) 26 Cr. 1 Mo and minor additions of C, Mn, Si, Cu, Ni and nitrogen is resistant to stress corrosion cracking in neutral chloride solutions and general corrosion in oxidizing and neutral media, even against hydrogen sulfid and organic acids; it is beyond that lergely resistant to pitting in chloride solutions.     

Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

Beitrag zum Spannungsrißkorrosions-Verhalten der AlZnMg-Legierungen

The stress corrosion cracking behaviour of aluminium-zinc-magnesium alloys The stress corrosion cracking of AlZnMg3 is a two-phase process where the preparation period may also take place without tensile stress. With decreasing pH, the service life is reduced, the reduction being confined to the preparation period. The same applies to anodic connection associated with heavy pitting corrosion where the fracture consists of stress corrosion cracking zones and transcrystalline force fracture. Even with cathodic connection, stress corrosion cracking is encountered. Since, in this case, the current density does not decrease prior to fracture, it must be assumed that the propagation of the crack may have nothing to do with the electro-chemical dissolution at the tip of the crack. Even minor quantities of water — e.g. in carbon tetrachloride — are sufficient to cause stress corrosion cracking, probably through adsorption of atomic hydrogen which has the effect of reducing the surface energy.     

Atom probe tomographic study of elemental segregation at grain boundaries for a peak-aged Al–Zn–Mg alloy

Atom probe tomography (APT) has been used to characterize the element segregation at the grain boundary (GB) for a peak-aged Al–Zn–Mg alloy with high stress corrosion cracking (SCC) susceptibility. The results show that Mg segregates along the GB with a peak concentration of 1.38 at.% and width of 3 nm. Zn does not segregate at GB. However, segregation of Zn and H atoms at oxide-containing clusters on GB has been observed. APT atom maps also reveal that Mg₂Si is the H trapping site, but MgZn₂ is not.     

铸造AZ91镁合金在CO32-/HCO3-体系中的应力腐蚀行为

用电化学实验、浸泡实验与慢应变速率拉伸实验研究铸造AZ91 镁合金在不同浓度比例的CO₃^2-/HCO₃^-溶液体系中的应力腐蚀(SCC)行为。结果表明,随着溶液pH值的升高,AZ91 镁合金的应力腐蚀敏感性降低。点蚀是AZ91镁合金在该实验体系下的裂纹萌生源,随着实验时间的延长,萌生于点蚀底部的微裂纹会逐渐扩展形成主裂纹,主裂纹靠吞并其前端的微裂纹向前扩展,直至发生SCC失效。     

Corrosion characteristics of the wrought Ni‐Cr‐Mo alloys

This paper concerns the wrought, nickel‐chromium‐molybdenum (Ni‐Cr‐Mo) alloys, a family of materials with a long history of use in the chemical process industries. Their attributes include resistance to the halogen acids and resistance to pitting, crevice attack, and stress corrosion cracking in hot, halide salt solutions. The purpose of this paper is to characterize the performance of the Ni‐Cr‐Mo alloys in several key chemicals, using iso‐corrosion diagrams. These indicate the expected corrosion rates over wide ranges of concentration and temperature. Furthermore, the differences between individual Ni‐Cr‐Mo alloys, and their behavior relative to the stainless steels are defined. The data indicate benefits of both a high chromium content and a copper addition, as used in Hastelloy® C‐2000® alloy.     

Untersuchungen zur transkristallinen und interkristallinen Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißen Magnesiumchlorid-Lösungen

Investigations Into transgranular and intergranular stress corrosion cracking of austenitic stainless steels In hot magnesium chloride solutions The stress corrosion cracking (SCC) of austenitic stainless steels in hot magnesium chloride solutions is known to be transgranular. Therefore the slip-step-dissolution model is most favourable when explaining the failure mechanism. Constant load and constant extension rate tests (CERT) show that both methodes are almost equivalent. Moreover constant extension rate tests in more concentrated magnesium chloride solutions at 135°C reveal a small potential range of intergranular stress corrosion cracking more negative than the range of transgranular SCC. Observations of crack nucleation and crack propagation make plain that crack nucleation is a localized corrosion process. Pitting produces crack nucleis in the elastic range whereas cracks start along slip lines after plastic deformation. Fractography of specimens which failed by intergranular and transgranular SCC show macroscopically brittle fracture surfaces. Therefore a model is proposed which explains crack propagation by hydrogen-induced intermitted cracking at high-stressed sites at the crack tip.     

不同晶粒尺寸5083铝合金晶间腐蚀和应力腐蚀开裂的差异性

通过冷轧和退火获得具有不同晶粒尺寸(8.7～79.2μm)的5083铝合金板.研究其微观结构、晶间腐蚀(IGC)、应力腐蚀开裂(SCC)和裂纹扩展行为.结果表明,粗晶粒样品表现出更好的抗IGC性能,其腐蚀深度为15μm.慢应变速率测试结果表明,细晶粒样品表现出更好的抗SCC性能,敏感性指数ISSRT为11.2％.此外,...     

Correlations among stress corrosion cracking,grain-boundary microchemistry,and Zn content in high Zn-containing Al–Zn–Mg–Cu alloys

The correlations among the corrosion behaviour, grain-boundary microchemistry, and Zn content in Al–Zn–Mg–Cu alloys were studied using stress corrosion cracking (SCC) and intergranular corrosion (IGC) tests, combined with scanning electron microscopy (SEM) and high-angle angular dark field scanning transmission electron microscopy (HAADF-STEM) microstructural examinations. The results showed that the tensile strength enhancement of high Zn-containing Al–Zn–Mg–Cu alloys was mainly attributed to the high density nano-scale matrix precipitates. The SCC plateau velocity for the alloy with 11.0 wt.% Zn was about an order of magnitude greater than that of the alloy with 7.9 wt.% Zn, which was mainly associated with Zn enrichment in grain boundary precipitates and wide precipitates-free zones. The SCC mechanisms of different Zn-containing alloys were discussed based on fracture features, grain-boundary microchemistry, and electrochemical properties.     

SCC initiation for X65 pipeline steel in the “high” pH carbonate/bicarbonate solution

The initiation of stress corrosion cracking (SCC) was studied using scanning electron microscope observations of linearly increasing stress test specimens. SCC initiation from the following surfaces was studied: (i) initiation from the commercial pipe surface covered by the Zn coating, (ii) initiation from a mechanically polished surface with a deformed layer, and (iii) initiation from an electro-polished surface. SCC initiation involved different features for these surfaces as follows. (i) For the Zn coated commercial pipe surface, a crack in the Zn coating led to the dissolution of the deformed layer and when the deformed layer was penetrated, intergranular SCC initiation became possible. (ii) For a mechanically polished surface with a deformed layer, cracks in the surface oxide concentrated the anodic dissolution to such an extent that there was transgranular SCC in the deformed layer. SCC was intergranular when the deformed layer had been penetrated. Transgranular stress corrosion cracks were stopped at ferrite grain boundaries (GBs) oriented perpendicular to the SCC propagation direction. (iii) For an electro-polished surface, the surface oxide film was cracked at many locations, but intergranular SCC only propagated into the steel when the oxide crack corresponded to a GB. An oxide crack away from a GB is expected to be healed. The observed SCC initiation mechanism was not associated with simple preferential chemical attack of the ferrite GBs.     

Bildung und Einfluß von α'-Martensit auf die Spannungsrißkorrosion von Chrom-Nickel-Stählen

Formation of martensite and influence of the latter on stress corrosion cracking of chromium nickel steels Martensite formation depends from alloy composition, deformation and undercooling. By contrast to an industrial alloy corresponding to (DIN) X 10 CrNiTi189 a pure Fe 19 Cr 10 Ni alloy becomes partially martensitic upon deformation and quenching to ?190 °C. Undercooling and subsequent deformation give rise to the addition of deformation and quenching martensite. The formation of α′-martensite is largely suppressed by addition of 2% Mo. Since corrosion resistance in aqueous MgCl₂ solutions increases with the α′-martensite contents, cracks grow predominantly along the martensite plates. Addition of molybdenum reduces stress corrosion resistance because of the suppression of martensite formation. In this type of alloy cracking is intercrystalline nature.