Slow strain rate testing of high-strength aluminium alloy plate in an aqueous solution of 3% NaCl + 0.3% H2O2

The stress corrosion cracking (SCC) behaviour of aluminium alloy plate materials was investigated in the short transverse direction using the slow strain rate (SSR) testing technique. The synthetic environment used was an aqueous solution of 3% NaCl + 0.3% H₂O₂. Reference tests under constant deformation and alternate immersion conditions according to ASTM G44 were performed, too. Both static and dynamic loading tests indicate high SCC susceptibility with the alloys 2024-T351, 2091-T8X51, 7050 T651 and 7475-T651. For the alloy 8090-T8171, a low SCC resistance is found in the alternate immersion tests, whereas a rather moderate sensitivity is observed performing SSR tests. The SSR testing technique fails to indicate the SCC sensitivity of the more resistant alloys 2024-T851 and 7050-T7651. As demonstrated by pre-exposure tests, the reduction of fracture energy observed with the latter alloys as well as with the immune alloys 7050-T7351 and 7475-T7351 is caused by pitting and intergranular corrosion. Using an aqueous solution of 3% NaCl + 0.3% H₂O₂, the SSR testing technique is a useful rapid method to screen wrought aluminium alloys which are quite sensitive to environmentally assisted cracking. Because chloride-peroxide solutions are also conducive to corrosion processes independent of stress, pre exposure tests are required to be incorporated into such sorting tests.     

Effects of nonhalide anions on the stress corrosion cracking behaviour of alloy 7475 in the tempers T651 and T7351

The stress corrosion cracking behaviour of 7475 plate material in the tempers T651 and T7351 was investigated performing constant load tests. Short transverse specimens were permanently immersed in aerated aqueous 0.6 M sodium chloride solutions with additions of 0.03 M sodium sulphate, 0.03 M sodium nitrate and 0.1 M sodium bicarbonate. Chloride solutions containing sulphate, nitrate or/and bicarbonate promoted environment‐induced cracking. A short transverse threshold stress below 50 MPa was found for 7475‐T651 plate material. Neutral 0.6 M NaCl solution was less conducive to stress corrosion cracking. Specimens of alloy 7475 in the overaged temper T7351 failed at applied stresses above 300 MPa. This failure resulted from overload fracture caused by a reduction of cross‐sectional area due to pitting corrosion, as confirmed by fractography showing severe pitting attack. Fractographic examinations of 7475‐T7351 specimens failed during immersion in chloride‐nitrate‐bicarbonate containing solutions with or without addition of sulphate after long exposure time periods revealed slight transgranular stress corrosion cracking, too. Pronounced transgranular environment‐induced cracking was observed on the fracture surfaces of specimens which were exposed to an aqueous solution containing chloride, sulphate, nitrate, and bicarbonate for 30 days at applied stress and were subsequently tensile tested in an inert environment.     

Stress corrosion cracking behaviour of Al‐Li alloy 8090‐T8171 plate exposed to various synthetic environments

The stress corrosion cracking (SCC) behaviour of 8090‐T8171 plate material was investigated in short transverse direction performing constant load tests and constant extension rate tests under permanent immersion conditions. At an applied stress of 100 MPa, smooth round tensile specimens were exposed to synthetic environments containing chlorides and various nonhalide anions. Environment‐induced cracking was not observed in aqueous solutions of 0.6 M NaCl, LiCl, NH₄Cl, or MgCl₂. In 0.6 M NaCl solutions containing 0.06 M Na₂SO₄ or Na₃PO₄, the SCC behaviour of 8090‐T8171 plate was similar to that observed in pure 0.6 M NaCl solution. Sodium chloride solutions with additions of nitrate, hydrogen carbonate, or carbonate promoted stress corrosion cracking. Threshold stresses below 100 MPa were obtained from constant load tests using the latter environments. When sodium sulfite or sodium hydrogen phosphate was added, values being 100 MPa or slightly higher were determined. Lithium and ammonium present as cations in mixed salt electrolytes accelerated SCC failure. Lithium chloride solutions containing nitrate, hydrogen carbonate, carbonate, or sulfite were highly conducive to stress corrosion cracking. Very low SCC resistance was found for alloy 8090‐T8171 exposed to synthetic environments with additions of ammonium salts. Constant extension rate tests were carried out using notched tensile specimens. Displacement rates were in the range 2 × 10^?6 ? 2 × 10^?5 mms^?1. Aqueous 0.6 M NaCl solutions with additions of 0.06 M NH₄HCO₃, (NH₄)₂SO₄, or Li₂CO₃ promoted environment‐induced cracking with 8090‐T8171 plate, as indicated by severe degradation of notch strength. The constant extension rate testing technique did not indicate SCC susceptibility using sodium chloride solutions containing sodium sulfate or lithium sulfate. For specimens exposed to substitute ocean water a slight degradation of notch strength was found at the lowest displacement rate applied.     

Comparison of susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys in neutral chloride solutions using electrochemical noise analysis

The susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys was investigated in aqueous neutral chloride solution for the purpose of comparison using electrochemical noise measurement. The experimentally measured electrochemical noises were analysed based upon the combined stochastic theory and shot-noise theory using the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, it was suggested that there existed two stochastic processes of uniform corrosion and pitting corrosion; pitting corrosion was distinguished from uniform corrosion in terms of the frequency of events in the stochastic analysis. Accordingly, the present analysis method allowed us to investigate pitting corrosion independently. The susceptibility to pitting corrosion was appropriately evaluated by determining pit embryo formation rate in the stochastic analysis. The susceptibility was decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition).     

On the Stress corrosion cracking behaviour of aluminium alloy sheet in an aqueous solution of 3% NaCl + 0.3% H2O2

The stress corrosion cracking (SCC) behaviour of aluminium alloy sheet was investigated in the long transverse direction using the slow strain rate testing technique. The synthetic environment used was an aqueous solution of 3% NaCl + 0.3% H₂O₂. No indications of SCC sensitivity are observed for the alloys 2024-T351, 8090-T81, and 2091 CPHK-T8X. The alloys 2091 T8X and 6061-T4 are found to be susceptible to intergranular stress corrosion cracking. At strain rates below 4 · 10^?7 s^?1, the slow strain rate testing technique indicates a slight SCC sensitivity with alloy 6013-T6. Fractography reveals transgranular stress corrosion cracking. Transgranular stress corrosion cracking is also observed with 6061-T4 specimens which are dynamically strained at strain rates below 5 · 10^?7 s^?1. Aqueous 3% NaCl solution with hydrogen peroxide addition promotes pitting and intergranular corrosion. The loss of ductility caused by these corrosion processes interferes with the evaluation of the results of the slow strain rate testing technique.     

Exfoliation corrosion testing of aluminium alloys

Abstract

The exfoliation corrosion behaviour of sheet and plate materials of various conventional aluminium and Al–Li alloys has been evaluated using accelerated tests. Results are .compared with atmospheric exposure data published in the literature to assess the applicability of the testing techniques employed. For damage tolerant Al–Li based sheet and plate, the cyclic acidified salt fog (Mastmaasis) test according to ASTM G85, Annex A2 indicated susceptibility to exfoliation corrosion, reproducing the limited outdoor corrosion data for the Al–Li alloys 8090–T81 and 2091–T84 as well as marine exposure results reported for the conventional alloys 2024–T351 and 7075–T7351. Therefore, it appears to be a promising testing technique for predicting the service performance of high strength aluminium alloys. Compared with the ratings determined following the cyclic acidified saltfog tests, the standard Exco test according to ASTM G34 indicated better exfoliation corrosion behaviour of the alloys investigated, except for 8090–T6 sheet and 7075–T7351 plate, which exhibited severe and mild exfoliation respectively. In the modified Exco test suggested by Lee and Lifka, 7075–T7351 panels were susceptible to pitting, whereas the other alloys studied generally suffered more severe exfoliation than in the standard Exco test.     

Investigation of the SCC behaviour of alloy 2024 using the slow strain rate technique

The stress corrosion cracking (SCC) behaviour of 2024 plate in T351 and T851 tempers was investigated in short transverse direction performing accelerated tests under constant deformation, constant load and slow strain rate conditions. Corrosive media used were: aqueous 3.5% NaCl solution, an aqueous solution of 2% NaCl + 0.5% Na₂CrO₄ at pH 3 (according to LN 65666), an aqueous solution of 3% NaCl + 0.3% H₂O₂, and substitute ocean water according to ASTM D1141. Alternate immersion tests in 3.5% NaCl solution indicated the low SCC resistance of the alloy 2024-T351 as well as the improved SCC behaviour due to aging to T851 condition. Similar results were obtained from constant load tests under permanent immersion conditions in the acidified chloride-chromate solution, in 3% NaCl solution with peroxide, and in substitute ocean water, whereas no SCC failure was observed with specimens which were permanently immersed in 3.5% NaCl solution. Using the slow strain rate method, 3% NaCl + 0.3% H₂O₂ and substitute ocean water were found to be effective synthetic environments. The other two electrolytes did not promote severe stress corrosion cracking with alloy 2024-T351. The SCC behaviour of 2024-T851 was difficult to determine employing the slow strain rate technique. Large scatter in data, observed even in inert environment, and the low elongation of the aged material, exacerbated by a further degradation of ductility due to pitting and intergranular corrosion, precluded an evaluation.     

Spannungsrißkorrosionsuntersuchungen an Strangpreßbarren der Legierungen 8090-T851 und 2091-T851

On the stress corrosion cracking behaviour of flat extrusions of the alloys 8090-T851 and 2091-T851 The stress corrosion cracking (SCC) behaviour of flat extrusions of the Al-Li-Cu-Mg-Zr alloys 8090-T851 and 2091-T851 was investigated performing accelerated tests under constant deformation, constant load, and slow strain rate conditions. The used corrosive environments were an aqueous 3.5% NaCl solution and an aqueous solution of 2% NaCl + 0.5% Na₂CrO₄ at pH = 3. Round tensile specimens and tuning fork type specimens were alternately immersed in 3.5% NaCl solution according to ASTM G44 under constant deformation, whereas they were continuously immersed in the other tests. The investigated flat extrusions of the alloy 8090-T851 were stress corrosion resistant in the extrusion and the long transverse directions. With the alloy 2091-T851 failure was observed at 75% of the 0.2 proof stress which may be caused by stress corrosion cracking or stress assisted intergranular corrosion. In the short transverse direction the SCC behaviour of the extrusions of both Al-Li alloys is comparable with that of a 2024-T351 alloy plate revealing a high SCC susceptibility.     

Corrosion Evolution of High-Strength Aluminum Alloys in the Simulated Service Environment of Amphibious Aircraft in the Presence of Chloride and Bisulfite

The corrosion evolution of 2024-T351 and 7075-T651 aluminum alloys in the thin electrolyte layer (TEL) and wet-dry alter-nating cycle (WDAC) environment is studied in this work.The results show that in the TEL environment,the competitive effect between H+ that accelerates corrosion reactions and deposition of aluminum sulfate that impedes corrosion attacks exists during the corrosion exposure.The difference is that with increasing HSO3-,subsurface intergranular corrosion on 2024-T351 is promoted to form exfoliation corrosion eventually and the degree of exfoliation corrosion begins to decrease because the blocking effect of aluminum sulfate exceeds the expediting effect of H+.For 7075-T651,the corrosion area and the corrosion diameter decrease gradually,which is attributed to the HSO3-enhanced deposition of corrosion products and their blocking effect.In the WDAC environment,the corrosion processes of 2024-T351 and 7075-T651 are the acidic dissolution of the matrix during the soaking phase.When the HSO3-concentration is high enough (0.1 M),the inhibiting effect of aluminum sulfate becomes the dominant factor.     

Fatigue behaviour of 8090-Al-Li alloy in chloride containing environments

The effect of an environment containing chloride on the fatigue behaviour of a 8090 Al Li Mg Cu alloy was studied. Results were compared with those obtained on a traditional 2024 Al Cu Mg alloy. Fatigue and corrosion-fatigue tests were carried out both on smooth specimens and riveted samples. In order to assess the corrosion behaviour of the materials and their susceptibility to stress corrosion cracking, potentiodynamic polarization and slow strain rate tests were carried out. Results showed a remarkable effect of the aggressive environment on the fatigue behaviour both of the innovative 8090 alloy (in T8 ageing conditions) and on the traditional clad 2024 alloy (in T3 natural ageing conditions), though the former showed slightly better behaviour. Nevertheless, in the presence of rivets the reduction in fatigue strength in the aggressive environment was negligible. Slow strain rate tests showed premature fractures under anodic polarization above the pitting potential and with a strain rate of 10⁻⁶ s⁻¹, only for the 8090 T8 alloy.     

Inhibition of stress corrosion cracking of alloy AA8090 T-8171 by addition of rare earth salts

Aluminium-lithium alloys are suitable for aeronautical purposes because of their good mechanical properties and high damage tolerance. Although these alloys are less susceptible to stress corrosion cracking than conventional alloys, Al-Li-Cu-Mg alloy (8090-T8171) still experiences this problem in a NaCl + H₂O₂ solution.In this work it has been demonstrated that the addition of 10,000 ppm of CeCl₃ to the medium inhibits the stress corrosion cracking of 8090 alloy by precipitation of cerium oxides/hydroxides. The deposition of these compounds on the alloy surface decreases the pit density and slows the crack growth through the grain boundaries by hindering the anodic dissolution of T phases.     

Effect of aging time and temperature on exfoliation corrosion of aluminum alloys 2024‐T3 and 7075‐T6

Two types of aluminum alloys, 2024‐T3 and 7075‐T6, have been selected in this study to investigate the effect of metallurgical aspects on exfoliation corrosion. To determine and evaluate the metallurgical effects of heat treatments on corrosion behaviour of these alloys, G34 ASTM test was selected to investigate the exfoliation corrosion behaviour. The results showed that with increasing the aging time for the aluminum alloy type 2024‐T3 the susceptibility to exfoliation corrosion increases, while for type 7075‐T6 decreased. These results refer to precipitation of the intermetallic compound phases such as CuAl₂, and MgZn₂, in 2024‐T3 and 7075‐T6 respectively. The amount of these phases increases with increasing the aging time for both alloys. The investigations showed the phases that initiate in 2024‐T3 act as anode sites while in 7075‐T6 they act as cathode sites.     

Stress corrosion cracking behaviour of 2091 Al—Li alloy in sulphate and chloride containing solutions

Abstract

The stress corrosion cracking resistance of 2091 Al—Li alloy in underaged, and in peak aged condition was investigated in chloride solutions with or without sulphate addition using both the static load technique and the slow strain rate technique. It is shown that the underaged material is more resistant than peak aged material. Sulphate additions to chloride solutions increase the stress corrosion cracking susceptibility. Metallographic cross-section observations show the simultaneous occurrence of other kinds of corrosion: generalised. corrosion, pitting corrosion, intergranular corrosion, and exfoliation corrosion. It appears that stress corrosion cracking susceptibility increases as the extent of intergranular corrosion decreases.     

The effect of nitrate on the corrosion behavior of 7075-T651 aluminum alloy in the acidic NaCl solution

The effect of nitrate on the corrosion behavior of 7075-T651 aluminum alloy in an acidic NaCl solution is investigated by electrochemical investigation and morphology characterization. Localized corrosion initiated from intermetallic particles could be observed in the solution with and without NaNO₃. The nitrate plays a controversial role in the corrosion of 7075-T651 aluminum alloy. It could enhance the performance of passive film and reduce the probability of pitting corrosion initiation. However, the pitting corrosion would be promoted by nitrate, once stable pitting corrosion is initiated.     

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.     

The fractography of long-transverse stress corrosion cracking in AlZnMgCu alloys

Precracked TL-oriented double cantilever beam specimens from 7075-T7351, 7050-T75651 and 7049-T7351 alloy plates were exposed to a marine atmosphere for over four years. Most of the stress corrosion cracking occurred in a plane perpendicular to the precrack, i.e. SL-oriented. These cracks had an intergranular morphology and assumed the shape of the plastic zone ahead of a crack tip. There was also some transgranular cracking which initiated on the surfaces of the short-transverse cracks. The transgranular fractures were characterized by well-defined striations and an extremely slow growth rate ($\text{? 4 × 10}{}^{\mathrm{?12}}\text{m s}{}^{\mathrm{?1}}$).     

Comparison of the Fracture Toughness and Stress Corrosion Behaviour of Four Aluminium Alloys

Abstract

The fracture toughness and stress corrosion properties of forgings in four aluminium alloys (AA2014–T6, AA7075–T73, L83 WP and DTD 5084WP) have been compared, using precracked specimens to define K_Iscc (the critical stress intensity for stress corrosion cracking) for each alloy in the short transverse direction when tested in 3% sodium chloride solution. In the T73 condition of heat-treatment, alloy AA7075 was resistant to stress corrosion whereas the other three alloys all showed susceptibility to stress corrosion, the ratio K_Iscc to K_Ic being about 0·5 for the L83 WP and about 0·7 for the other two alloys.     

Stress-corrosion cracking of aged AlCuMg alloys in NaCl solution

Pitting potentials and stress corrosion life-times of AlCuMg alloys (mainly 2024 alloy) with various ageing structures have been measured in a de-aerated 1M NaCl solution under conditions of controlled potential. The aged alloy, which has the higher susceptibility to stress-corrosion cracking, showed two pitting potentials corresponding to pitting at the grain boundaries and within the grains. The susceptibility of the alloys to intergranular stress-corrosion cracking occurred at potentials above the pitting potential of the grain boundaries. The intergranular stress-corrosion cracking is caused not by the dissolution of the grain boundary precipitates (S phase) but by the pitting dissolution of the solute-denuded zones along the grain boundaries. Aspects of SCC in the alloys are similar to those in the Al-4%Cu alloy without Mg.     

Zum Spannungsrißkorrosionsverhalten rekristallisierter Bleche der AlLi-Legierung 8090-T81

On the stress corrosion cracking behaviour of recrystallized 8090-T81 Sheets The stress corrosion cracking behaviour of a recrystallized sheet of the Al-Li-Cu-Mg-Zr alloy 8090-T81 was studied performing accelerated tests under constant deformation, constant load, and slow strain rate conditions. The used electrolytes were an aqueous 3.5% NaCl solution, an aqueous solution of 2% NaCl + 0.5% Na₂CrO₄ at pH = 3, and synthetic seawater according to ASTM D1141. Alternately immersed in 3.5% NaCl solution according to ASTM G44 the investigated alloy was found to be susceptible to stress corrosion cracking was not promoted by continuous immersion in aerated 3.5% NaCl solution, 3.5% NaCl solution saturated with carbon dioxide, and in acid chromate inhibited 2% NaCl solution. Using the slow strain rate technique with continuously immersed flat tensile specimens stress corrosion cracking was only observed in synthetic seawater. Under specific environmental conditions hydrogen embrittlement can occur in the investigated material.     

Untersuchungen zum Korrosionsverhalten von AlLi-Legierungen

Corrosion behaviour of aluminium-lithium alloys The AlLiMgCu alloy 8090 was studied in its texturated fine grained version “A” and in its recrystallized coarse grain structured version “C” in different artificially aged conditions in reference to several other AlLi alloys, each in its heat treatment condition of practical interest, and to the convetional alloy 2024 T3. The subject of research was the general corrosion behaviour of semifinished AlLi products, particularly sheet material, under alternate and permanent immersion conditions in neutral 3.5% sodium chloride solution; the stress corrosion behaviour was studied under constant load in the long transverse direction according to ASTM G44 and G49. The underaged conditions, which are the relevant conditions for technical application of the 8090 “A” and “C” sheets, showed an approximately equivalent or even better corrosion behaviour in comparison to the lithium-free alloy 2024 T3 in the corrosion tests with unloaded specimens. The threshold above which the AlLi alloy 8090 in some heat treatment conditions is attacked by stress corrosion cracking within the 30 days lasting constant load test depends on alloy composition, testing direction, grain size, stretch-forming, artificial ageing condition, surface pretreatment and the specimens' dimensions.