Effect of cations on stress corrosion cracking: Ag-40Cd alloy in silver ion aqueous solutions containing a variety of foreign cations

Uhlig showed that cupric ions accelerated the stress corrosion cracking (SCC) of α-brass in ammoniacal solutions. When adding foreign metal cations (cadmium, zinc, cobalt and nickel) to the solutions, it was found that only the copper ion had an effect on the fracture time. The aim of the present work was to study if this is a more general phenomenon in SCC. To that purpose, a Ag-40Cd (at/o) alloy was strained in silver nitrate aqueous solutions containing a variety of foreign metal cations, at the equilibrium potential of the reaction: Ag⁺ + e⁻ = Ag. SCC was found in all cases. Silver ions specifically accelerated the crack propagation rate and the addition of foreign cations showed no significant effect on the phenomenon. Most of the cracks found were intergranular, and the specific effect of the cations could be explained through the surface mobility SCC mechanism.     

Stress corrosion cracking of pure copper in dilute ammoniacal solutions

Studies of the stress corrosion cracking (SCC) of 99.999% copper and Cu-Zn alloys containing up to 10 wt%Zn in NH₄OH solution were made with varying concentrations (0.03–0.07 M) and temperatures (40–70°C). Stress corrosion cracking occurs on pure copper and all of the alloys under the condition in which thick tarnish film (Cu₂O oxide film) forms. The path of cracking is transgranular for pure copper and alloys containing < 1.3 wt%Zn, but intergranular for alloys containing > 1.3 wt%Zn. Crack propagation rates and times-to-failure estimated by the tarnish rupture theory, utilizing experimentally determined values of the fracture strain of film and the creep rate of specimens during SCC tests, are in good agreement with those observed under constant load.     

Two distinct fracture modes of copper alloys in fluoride environments

The susceptibility of aluminum brass and aluminum bronze to stress corrosion cracking (SCC) was evaluated in fluoride environments by means of the slow strain rate technique. The effects of fluoride concentration and applied electrochemical potential on the SCC susceptibility were investigated. The SCC mechanism of aluminum brass was found to be dependent upon the de-alloying phenomenon while that of aluminum bronze on the film-rupture mechanism occurring within a certain range of applied electrochemical potentials. For aluminum brass, both the stress ratio and the time-to-fracture ratio were closely related to the logarithmic concentration of F⁻ ions. The fracture modes were ductile dimpled tearing fracture for the aluminum brass at the corrosion potential while intergranular for the aluminum bronze at various anodic potentials. The micro-galvanic effects of the constituent elements were used to estimate the anodic equilibrium potentials of the two alloys.     

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

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

Stress corrosion cracking of noble metals and their alloys in solutions containing cations of the noble metal: Review of observations relevant to competing models of SCC

The stress corrosion cracking of Ag, Au and Cu-base alloys, and of pure Ag and Cu, has been studied by J.R. Galvele and others. These authors used solutions that contained the cation of the more-noble metal, so that the tested specimen was at or close to its equilibrium potential in the given solution. The opportunity is taken to review the history of this far-from-new observation and some of its implications. The role of the exchange current density in such cracking is discussed. Observations of Sieradzki and Torchio are used to suggest that in alloys such as brass, SCC is favoured by low,nothigh,surfacemobility, in line with the film-induced cleavage model, which requires very fine nanoporosity at the crack tip - such a favourable condition can only be achieved if dealloying is either very fast or occurs under conditions of low surface mobility. Observations of very slow intergranular SCC in pure metals under dynamic loading are interesting, but not really suggestive of mechanistic continuity with the dramatic mixed-mode cracking that occurs under static loading in brass or AuAg alloys. Torchio’s observations on brass U-bends in CuSO₄ solutions of various pH and Cu²⁺ concentrations are particularly hard to interpret using the surface mobility model.     

Stress corrosion cracking of B13, a new high strength aluminium lithium alloy

The present paper focuses on the study of SCC behaviour of a new Al–Cu–Li alloy. For this purpose, two conventional media – NaCl and NaCl + H₂O₂ – were used for comparison with commercial alloys 7075 and 8090. This new alloy shows lower susceptibility to SCC than conventional alloys as it does not undergo environmentally-induced embrittlement in NaCl solutions and in 1 M NaCl + 0.3% H₂O₂ in which the 7075 and 8090 alloys, respectively, undergo environmentally-induced fracture.Solution composition was modified in order to determine the environmental conditions and strain rates under which this new alloy will crack due to a stress corrosion cracking phenomenon. The addition of 0.6 M sulphates to 1 M NaCl + 0.3% H₂O₂ solution allows the definition of a range of strain rate (between 10⁻⁷ and 10⁻⁶ s⁻¹) in which this new alloy undergoes stress corrosion cracking.     

Chronoamperometric studies of pitting corrosion of Al and (Al-Si) alloys by halide ions in neutral sulphate solutions

The pitting corrosion of Al, (Al + 6%Si), and (Al + 18%Si) alloys in neutral 0.50 M Na₂SO₄ solution in the absence and presence of NaCl, NaBr and NaI under the influence of various experimental variables has been studied by using potentiodynamic and chronoamperometric techniques. The results showed that the pitting corrosion resistance of the three Al samples decreases in the order: (Al + 18%Si) > (Al + 6%Si) > Al. The current/time measurements showed that the overall process can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involving pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increasing halide concentration, temperature, and applied potential. The pit growth current density (j_pit) increases linearly with t^1/2, indicating that the pit growth can be described in terms of an instantaneous three dimensional growth under diffusion control. The effect of Cr₂O₇²⁻, CrO₄²⁻, WO₄²⁻, MoO₄²⁻, NO₂⁻ and NO₃⁻ as inorganic inhibitors on the pitting corrosion inhibition of pure Al and its alloys in (0.5 M Na₂SO₄ + 0.2 M NaCl) solution has also been studied. The presence of these anions (except NO₃⁻) results in an increase in the incubation time and a decrease in the pit growth current density of the three samples to an extent depending on the nature and the concentration of the inorganic inhibitors and the composition of the Al samples. The inhibition efficiency of these inhibitors decreases in the order: Cr₂O₇²⁻ > CrO₄²⁻ > WO₄²⁻ > MoO₄²⁻ > NO₂⁻.     

The electrochemical behaviour of leaded brass in neutral CL and SO4 media

The electrochemical behaviour of lead brass with different leaded content in neutral chloride and sulphate solutions was investigated using the EIS technique. For comparison, the behaviour of the pure components of the alloy was investigated under the same conditions. The corrosion process was found to proceed via oxygen reduction following a diffusion controlled mechanism. The Cu electrode showed a higher polarization resistance due to film formation during oxygen reduction. Zn and Pb showed markedly lower impedance values due to continuous dissolution. The two investigated brass alloys (1.8% and 3.5% Pb, respectively) showed higher impedance values indicating the passivation of the surface in the Cl⁻ or SO₄⁻ media. Brass II was found to be more stable against corrosion indicating the beneficial effect of the lead content in the alloy.At cathodic potentials, the only process is the oxygen reduction. Anodic polarization leads to selective dissolution of Zn. At more positive potentials simultaneous dissolution of the alloy components with the deposition of Cu(I) salt takes place leading to the passivation of the alloy surface. At higher potentials, film breakdown occurs producing Cu(II) compounds whose diffusion control the corrosion process. At potentials higher than − 0.1 V, pitting corrosion was observed and a transmission line type in the impedance spectra was recorded.     

Spannungsrißkorrosion an Kupferlegierungen

Stress-corrosion cracking of copper alloys Stress-corrosion cracking (SCC) occurs if three factors are simultaneously present: a susceptible material, a specific corrosive medium, and tensile stresses. All copper alloys and copper itself are susceptible to SCC – but in a different extend. The most susceptible alloys are brasses with copper contents below 80%. For a long time only ammonia and its derivatives were considered to cause SCC of copper alloys. Only in recent years other mediums have been reported to produce SCC – especially nitrites. With the exception of some rare SCC-cases of copper and zinc-free copper alloys SCC-failures of brasses prove the highest importance in practice. This is due to the high susceptibility and the large use of brass. Besides clear failures by ammonia and nitrite increasing cases of SCC influenced by outdoor environment - mainly industrial or urban atmosphere - can be stated. The reason could be the general atmospheric pollution by sulphur and nitrogen oxides. Failures of this kind may appear after long times of service - e.g. after several years. The risk of SCC can be reduced by minimizing tensile stresses or by choosing other materials than brass.     

Papers to be presented at Eurocor '77

Abstract

Despite numerous studies on atmospheric corrosion of copper and copper based alloys, the corrosion induced release processes of individual alloy constituents suffer from significant knowledge gaps. This investigation comprises metal release rate measurements of copper, zinc and tin from some copper based alloys including brass (20 wt-%Zn) and bronze (6 wt-%Sn), and their pure alloying metals, copper, zinc and tin. Data have been generated during a 2·5 year urban field exposure in Stockholm, Sweden and parallel laboratory investigations in a specially designed rain chamber using artificial rain. Brass shows significantly lower annual release rates of both copper and zinc compared to pure metal sheets of its alloy constituents. Zinc is preferentially released compared to copper. Dezincification of brass occurs both at field and laboratory conditions, a process influenced by rain characteristics. Alloying with tin does not largely reduce the release rate of copper from bronze compared to pure copper. No measurable amount of tin is released from the bronze surface.     

High temperature corrosion properties of ionic liquids

The corrosion behaviour of several metals and metal alloys (copper, nickel, AISI 1018 steel, brass, Inconel 600) exposed to a typical ionic liquid, the 1-butyl-3-methyl-imidazolium bis-(trifluoromethanesulfonyl) imide, ([C₄mim][Tf₂N]), has been investigated by electrochemical and weight-loss methods. Corrosion current densities have been determined by extrapolation from Tafel plots and by polarization resistance measurements and 48 h immersion tests were performed at 150, 250, 275 and 325 °C. Room temperature results show low corrosion current densities (0.1-1.2 μA/cm²) for all the metals and alloys investigated. At 70 °C, the corrosion current for copper dramatically increases showing a strongly dependence on temperature. At 150 °C copper shows significant weight-loss while nickel, AISI 1018, brass and Inconel do not. At higher temperatures (?275 °C), the copper sample crumbles and localized corrosion occurs for the other metals and alloys.     

Effect of common water contaminants on the corrosion of aluminium alloys in ethylene glycol-water solution

The effects of common water contaminants of chloride (Cl⁻), cupric (Cu²⁺) and ferric (Fe³⁺) ions, in four different mixture combination of Fe³⁺ + Cu²⁺, Cl⁻ + Fe³⁺, Cl⁻ + Cu²⁺ and Cl⁻ + Fe³⁺ + Cu²⁺, were examined on the corrosion behaviour of aluminium alloys in ethylene glycol-water solution, using mass loss technique. The highest material losses were recorded for the two alloys in ethylene glycol solution containing the combination of the chloride and the two heavy metal ions. The corrosivity of the solution in the presence of the combination of ions was in the order of Cl⁻ + Fe³⁺ + Cu²⁺ > Cl⁻ + Cu²⁺ > Cl⁻ + Fe³⁺ > Fe³⁺ + Cu²⁺. The results gave first-order kinetics with respect to aluminium in ethylene glycol solution-ion systems. Alloy 3SR exhibits maximum corrosion in all the solutions. It is concluded that the two commercial alloys in the solution polluted with all the three ions would not be able to survive for reasonable period of time without corrosion inhibitor.     

Effect of carbon on corrosion and passivation of iron in hot concentrated NaOH solution in relation to caustic stress corrosion cracking

Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H⁺ and HFeO₂⁻, thereby favouring the formation of Fe₃O₄. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.     

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

Abstract

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

Effect of the strain rate on stress corrosion crack velocities in face-centred cubic alloys. A mechanistic interpretation

Constant extension rate tests on smooth samples, with strain rate (SR) values from 10⁻⁶ s⁻¹ up to 20 s⁻¹, were used to study stress corrosion cracking (SCC) systems in face-centred cubic alloys. It was found that by increasing the SR a monotonic increase of the (crack propagation rate) takes place. It was also observed that the slope α in vs. plots had different values for different SCC morphologies. Intergranular SCC is more steeply accelerated by SR, α_IG=0.5-0.7, than transgranular SCC, α_TG=0.2-0.3. The differences found between intergranular SCC and transgranular SCC were analysed under the light of the available SCC mechanisms.     

Effect of Co additions on B2 phase stability of Ni-poor NiTi-based alloys

Based on the generalized atomic site preference model and the quasi-chemical bond approximation, the atomic distribution state free energy of the third addition in a binary alloy with the B2 lattice is derived analytically. The configurational energy coefficients are independent of the alloy composition and can be determined directly by the first principle calculation. The calculated B2 ↔ bcc transformation temperature of the Ni-poor NiTi-Co alloys changes at 200 K at.%⁻¹ by the increasing of Co concentration. The experimental results about the effect of the third Co additions on the martensitic transformation procedure of the Ni-poor NiTi-based alloys are well explained.     

The effect of various halide ions on the passivity of Cu, Zn and Cu-xZn alloys in borate buffer

Cu and Zn metals and four of their alloys, Cu-10Zn, Cu-20Zn, Cu-30Zn and Cu-40Zn, were studied in borate buffer, pH = 9.2, in the presence of the halides, NaF, NaCl, NaBr and NaI. Electrochemical polarization and SEM/EDS methods were used in the study. The mechanism of corrosion of copper and copper alloys induced by iodide ions differed fundamentally from that induced by other halide ions. Iodide ions promote the general type of corrosion during which very slowly soluble CuI is formed. F⁻, Cl⁻ and Br⁻ ions promote localized corrosion attack. All halide ions induce localized corrosion on the passive layer on zinc. The breakdown potentials of Cu, Zn and four Cu-xZn alloys were measured as a function of concentration and type of halide ions.     

Effect of underpotential deposition of lead on polarization behavior of nickel in acidic perchlorate solutions at room temperature

The effect of Pb²⁺ on polarization behavior of nickel has been investigated in 0.1 M NaClO₄ + 10⁻² M HClO₄ + x M PbO solutions (x = 0, 10⁻⁵, 10⁻⁴, 10⁻³) at room temperature. The cyclic voltammogram has suggested that Pb²⁺ degrades the stability of the passive film on Ni. The corrosion potential of Ni shifted to the more noble direction and the anodic current peak of Ni dissolution decreased with increasing Pb²⁺ concentration in solution, indicating that Pb²⁺ suppresses significantly the anodic dissolution. The underpotential deposition (UPD) of lead on Ni in the potential range more noble than −0.215 V (SHE) corresponding to the equilibrium potential of the Pb²⁺ (10⁻³ M)/Pb electrode was confirmed by XPS and GDOES analyses. The anodic Tafel slope, b⁺, of Ni dissolution changed from b⁺ = 40 mV decade⁻¹ in the absence of Pb²⁺ to b⁺ = 17 mV decade⁻¹ in the presence of 10⁻⁴ or 10⁻³ M Pb²⁺, which was ascribed to the increase in active sites of Ni surface emerged as a result of electrodesorption of Pb adatoms. The roles of Pb adatoms in active dissolution and active/passive transition of Ni were discussed from the above results.     

Environmental effects on the stress corrosion cracking susceptibility of 3.5NiCrMoV steels in high temperature water

Constant elongation rate tests (CERTs) were carried out to investigate the effects of environmental factors of dissolved oxygen and temperature on the stress corrosion cracking (SCC) susceptibility of 3.5NiCrMoV turbine steels. Tests were conducted in pure water of various dissolved oxygen concentrations at temperatures of 50 °C-200 °C in the range of strain rates from 5 × 10⁻⁸/s to 1 ×  10⁻⁶/s. Dissolved oxygen significantly affected the SCC susceptibility of turbine steels in water. The SCC susceptibility of the turbine steels increases as the dissolved oxygen concentration in water increases. The elongation of the turbine steels tested in aerated water at 150 °C at a strain rate of 1 × 10⁻⁷/s decreased to half of that of the steels tested in deaerated water in the same test condition. And the SCC susceptibility of the steels increased with decreasing strain rate, and with increasing temperature. The increase of the SCC susceptibility of the turbine steels in the higher dissolved oxygen environment is considered to be due to the higher content of dissolved oxygen enhancing the reduction reactions of oxygen on the metal surfaces (cathode) and accelerating the dissolution rate at the crack tips (anode) by galvanic attack of an aeration cell.     

Stress corrosion cracking of copper in swollen bentonite simulating nuclear waste disposal environment

Stress corrosion cracking (SCC) of pure copper in bentonite clay was examined using a slow strain rate test (SSRT). Bentonite was swollen with pure water or aqueous solutions containing NH₃ of 5 and 10 mM. Thick corrosion films and particulate deposits were formed on the copper surface after the SSRT. Typical tarnish rupture-type SCC occurred on pure copper in swollen bentonite with and without NH₃. The crack propagation rate was enhanced by NH₃. It is confirmed that a thick oxide layer was formed on copper during plastic deformation, resulting in tarnish crack-type SCC. Many particulate deposits observed on the surface were formed due to the rapid dissolution of Cu²⁺ ions to form porous CuO at local deformed sites, regardless of the SCC occurrence.