Surface Mobility Stress Corrosion Cracking Mechanism Research Articles

Application of the surface-mobility stress corrosion cracking mechanism to h.c.p. metals in iodine solutions

Stress corrosion cracking (SCC) susceptibility of some h.c.p. metals: titanium, zirconium and one of its alloys, Zircaloy-4, was investigated in iodine-methanol solutions. Slow strain rate tests were carried out until the rupture of the specimens, in some cases interrupted strain rate tests were conducted. SCC was observed in all the systems studied, but preceding crack propagation, intergranular attack was found in all cases, and this intergranular attack was due to anodic dissolution of the grain boundaries. The rate controlling step is the diffusion of iodine to the tip of the advanced crack. It was also demonstrated, that it is necessary to locally disrupt the passive film to induce SCC in these materials and this is done by the chemical effect of the iodine molecules on both, titanium and zirconium oxide film. Once the intergranular attack reaches a certain value, a transition to a transgranular cracking mode occurs. This last step was identified as the true SCC process. The surface-mobility SCC mechanism was applied to explain the results obtained.

Stress Corrosion Cracking of Zirconium and Zircaloy-4 in Iodine-Alcoholic Solutions

Abstract Stress corrosion cracking (SCC) susceptibility of Zr (UNS R60702) and one of its alloys, zircaloy-4 (UNS R60804), was studied at room temperature in 10 g/L iodine dissolved in various alcohols: methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, and 1-octanol. SCC was observed in all the systems studied and the crack propagation rate was found to vary depending on the size of the solvent molecule. As the solvent molecular weight increased, the crack propagation rate decreased. Prior to crack propagation, intergranular attack was found in all the solutions tested. The intergranular corrosion rate also varied according to the size of the solvent molecule. Since the corrosion rate of Zr single crystals was similar in all the solutions tested, it was concluded that the decrease in both the intergranular attack rate and the crack propagation rate was attributable to a steric effect that hindered the access of the corrosive species to the tip of the crack. The surface-mobility SCC mechanism accounts ...

Stress corrosion cracking of zirconium and Zircaloy-4 in halide aqueous solutions

Zirconium and Zircaloy-4 in 1 M NaCl, 1 M KBr and 1 M KI aqueous solutions were found to be susceptible to stress corrosion cracking (SCC) only at potentials above the pitting potential. In all the tested systems the following steps were found: first electrochemical breakdown of the passive film, followed by intergranular attack due to anodic dissolution assisted by stresses; and finally a fast transgranular propagation. This last step was identified as the “true” SCC process. The analysis of the possible mechanisms involved during this process led to the conclusion that the surface-mobility SCC mechanism can be used to explain the experimental results found in the present work.

Effect of Ag + ion concentration on the stress corrosion cracking susceptibility of Ag–Cd alloys in AgNO 3 and AgClO 4 aqueous solutions

Silver–cadmium alloys, with compositions ranging from Ag–10Cd (a/o) up to Ag–40Cd (a/o) were found to be susceptible to stress corrosion cracking (SCC) in silver nitrate and in silver perchlorate aqueous solutions, at the equilibrium potential of the reaction: Ag⇌Ag ++e −. The effect of Ag + ion concentration on the stress corrosion cracking propagation rate (CPR) was studied. A linear relation between the logarithm of the CPR and the logarithm of the Ag + concentration was found. The slope of this relation was very close to that reported for the silver exchange current density and the silver ion concentration: (δ log i 0/δ log [ Ag +]) . Most of the cracks found were transgranular, and the crack propagation mechanism was explained through the surface mobility SCC mechanism. In the present work, it is suggested that Ag–Cd alloys could be used as model alloys to study the SCC behaviour of technically more relevant alloys, such as Cu–Zn brasses.

The effect of Cu 2+ concentration on the stress corrosion cracking susceptibility of α-brass in cupric nitrate solutions

The stress corrosion cracking (SCC) susceptibility of α-brass in aqueous cupric nitrate solutions was studied. The effect of the Cu 2+ ion concentration on the stress corrosion crack propagation rate (CPR) was measured. A linear relationship between the logarithm of the CPR and the logarithm of the Cu 2+ concentration was found in all the alloys studied. The slope of this relationship was very close to that reported for the cuprous ion exchange current density and the cupric ion concentration: δ log i 0(Cu + → Cu) δ log [Cu 2+] . All the cracks found were transgranular, and the crack propagation mechanism was explained through the surface mobility SCC mechanism.

Application of the surface-mobility stress corrosion cracking mechanism to nuclear materials

The surface-mobility stress corrosion cracking mechanism is of recent development, and one of its most promising properties, besides that of being a deterministic model, is that it allows the prediction of crack velocities at any temperature, if the value at a given temperature is known. This is particularly important when crack velocities as low as 1 × 10−12 m s−1 or even lower are relevant, as it happens in nuclear pressure vessels, nuclear repositories, nuclear steam generators, etc.

Stress corrosion cracking of Ag30Cd in silver nitrate solutions

Stress corrosion cracking (SCC) of Ag-30Cd (at/o) was studied in mixtures of silver nitrate and sodium nitrate solutions. Transgranular SCC was found in all the solutions tested, and the crack propagation rate was a function of the silver ion concentration. The surface mobility SCC mechanism assumes that the silver ion increases the metal-ion interchange, and therefore, there is an increase in the surface mobility.

Comments on “notes on the surface mobility mechanism of stress-corrosion cracking”, by K. Sieradzki and F. J. Friedersdorf

Sieradzki and Friedersdorf's work is analysed. It is made evident that these authors have assumed thermodynamic equilibrium conditions for surfaces undergoing a corrosion process. It is also shown that vacancies are being injected into the considered metal surfaces during the corrosion process, and that the equations used by Sieradzki and Friedersdorf to calculate the vacancy surface concentration are not applicable. The equations used in the surface-mobility stress corrosion cracking mechanism are reviewed, and it is concluded that these equations are valid.

Stress corrosion cracking of Ag-20Au in HCLO4, AgCLO4, and KCL solutions by surface mobility

Slow, intermediate, and ultrafast strain-rate experiments were performed on Ag-20Au (atomic percent) wire samples in 1 M HC1O4, AgClO4, and KCl solutions. Intergranular stress corrosion cracking was found in all of the solutions tested. In the ultrafast strain-rate experiments, 9.6 s-1, in HC1O4 and in AgClO4 solutions, the size of the cracks proved to be a function of the electric charge circulated before straining. AgClO4 was also found to specifically induce stress corrosion cracking (SCC) in the Ag-20Au alloy. The surface mobility SCC mechanism was concluded to be the only one that accounted for all of the experimental observations made in the present work.

The surface mobility-stress corrosion cracking mechanism: Ag-15Pd alloy in Na 3PO 4, Na 4P 2O 7 and NaOH solutions

The stress corrosion cracking (SCC) susceptibility of an Ag-15Pd alloy was studied in 0.2 M Na 3PO 4, 0.2 M Na 4P 2O 7 and 0.1 M NaOH solutions. Phosphate and pyrophosphate ions were found to have no specific effect on the SCC susceptibility of the Ag-15Pd alloy in alkaline solutions. If they did have any influence, it was overshadowed by the action of the hydroxyl ions. The predictions of the surface mobility-SCC mechanism were supported by the present experimental results.

Surface mobility-stress corrosion cracking mechanism of steels for steam turbine rotors

Crack velocities measured by Magdowski and Speidel for steels in hot water, covering a wide range of temperatures, Yield Strength and impurity levels, were compared with the Surface-Mobility Stress-Corrosion-Cracking Mechanism predictions. A very good correlation was found between the predicted and the measured values.

Experimental confirmation of the surface mobility-stress corrosion cracking mechanism: Ag-15Pd, Ag-15Au and Ag- 30Cd alloys in halide and sulphate containing solutions

The stress corrosion cracking susceptibility of Ag-15Pd and Ag-15Au alloys was studied in I M KCl, KBr, Kl and Na2SO4 solutions. These studies were complemented with measurements on an Ag-30Cd alloy in 1 M KCl, HCl and 10% FeCl3 solutions, where only de-alloying was detected; and on an Ag-15Pd alloy in iodine-saturated organic solvents, where crack velocities similar to those in a 1 M Kl solution were measured. The predictions of the surface mobility-SCC mechanism were supported by the present experimental results. De-alloying was found to be either a parallel or a competing process rather than the cause of SCC since de-alloying is dependent on surface mobility.

Stress corrosion cracking of α-brass in phosphate, triphosphate and pyrophosphate solutions

The constant potential slow strain rate technique was used to study the susceptibility of α-brass to stress corrosion cracking (SCC) in phosphate, triphosphate and pyrophosphate solutions. α-Brass was found to be immune to SCC in alkaline Na 2HPO 4 solutions, but susceptible to SCC in alkaline Na 4P 2O 2 and Na 5P 3O 10 solutions. Only intergranular cracking was observed. The presence of Cu 2+ ions in the corrosive environment expanded the range of electrode potentials where SCC was found. At constant potential, the presence of Cu 2+ ions had no effect on crack velocity. The Cu 2+ ions acted by reducing the IR drop in the crack, thus allowing for higher electrode potentials at the tip of the crack. The present results fit better into a surface-mobility SCC mechanism than into an anodic dissolution mechanism.

The influence of temperature on the susceptibility of type AISI 304 stainless steel to transgranular and intergranular stress corrosion cracking in LiCl solutions

Stress corrosion cracking (SCC) of type AISI 304 stainless steel in 6.5 and 11.8 N LiCl solutions in the temperature range 25–130°C and at constant potential was studied by using intermediate strain rate and slow strain rate techniques. At high temperatures transgranular cracking predominated at potentials close to the pitting potential, while intergranular cracking increased as the applied potential decreased. Only transgranular SCC propagation rates could be predicted by the intermediate strain rate technique. At room temperature, both transgranular cracking and pitting were found at potentials close to the pitting potential. The results are discussed from the point of view of the surface mobility SCC mechanism. This mechanism gives a good prediction of the effect of temperature on SCC velocity.

The susceptibility of type AISI 304 stainless steel to transgranular and intergranular SCC in 40% MgCl 2 solution at 100°C

Stress corrosion cracking (SCC) of Type 304 austenitic stainless steel in 40% MgCl 2 solution at 100°C was studied with constant potential intermediate strain rate techniques and slow strain rate techniques. Loss of HCl from the solution, during SCC tests was found to have a strong effect on crack morphology. In fresh solutions potential regions with transgranular cracks and with intergranular cracks were observed. No critical potential for SCC could be found in those solutions. In preboiled solutions only the high potential cracking, predominantly transgranular, was found. In these solutions a critical potential for SCC could be defined. Contrary to what was reported in the literature, no evidence of anodic dissolution was found during the intergranular cracking. The results are discussed from the point of view of the surface mobility SCC mechanism.