Simulated Crevice Chemistries Research Articles

Metal Salt Solutions as a Simulant for the Chemistry in Crevices of Nickel Alloy 625

The development of a critical crevice solution plays an important role in crevice corrosion propagation in nickel alloy 625. In this investigation, the polarization response of alloy 625 in a set of simulated crevice chemistries using metal chloride salts has been studied. Specifically, solutions were made from NiCl2, CrCl3, FeCl2, MoCl3, and NbCl5 in the same stoichiometric ratio as they appear in the alloy and ranging in concentration from 3.0 to 5.0 molal (m). It was found that Mo and Nb contributed to lower open circuit potentials (OCP) and increased critical peak current densities (i.e. activation). Solutions that substituted NiCl2 for MoCl3 and NbCl5, such that the chloride content was equivalent to that of the NiCrFeMoNb solution, presented high OCP values and passive behavior. To better understand the polarization data, solution speciation and pH were calculated using a commercially available software package. It was found that for concentrations ranging from 3.0 to 5.0 m, the pH values were on the order of −1.0. In addition, the active to passive transition appeared to correlate with an increase of Mo3+ concentration in solution. For comparison, the polarization response of alloy 625 in HCl-based solutions was investigated. Over the range of HCl concentrations studied, the critical peak current densities were typically lower than the metal salt solutions at equivalent calculated pH. In addition, an equation for preparing HCl solutions at the needed pH to obtain an equivalent critical peak current density as observed in the NiCrFeMoNb solutions is presented.

Semiconductivity of steam generator tubing alloys in simulated crevice chemistries containing lead and sulphur

Semiconductivity of passive films on steam generator (SG) tubing alloys, Alloys 690 and 800, in simulated crevice chemistries containing lead and sulphur was investigated using the Mott–Schottky analysis. The interaction of the above species with other ions on the properties of passive film and the correlation between the semiconductivity and the breakdown of passive film were discussed. Experimental results revealed that semiconductivity could be either n or p type semiconductor; both Pb and S can incorporate into the passive layer, depending on the solution pH and alloy compositions. For n type semiconductors formed on Alloy 800, they are easily broken down but more resistant to anodic dissolution; for p type passive films formed on Alloys 690 and 800, they are not easily broken down but more prone to anodic dissolution. It is concluded that the semiconductivity of passive film on SG tubing alloys was dependent on the alloy compositions, the solutions, temperature and film formation potentials.

Pitting Corrosion Mechanism of Alloy 800 in Simulated Crevice Chemistries Containing Thiosulfate

Pitting Corrosion Mechanism of Alloy 800 in Simulated Crevice Chemistries Containing Thiosulfate

Modeling interactive effects of Pb contamination, Ca2+ and Mg2+ on passivity of alloy 800 in simulated crevice chemistries at high temperature

A phenomenological model is postulated to interpret the interactive impacts of dissolved Pb contamination, Ca2+ and Mg2+ on passivity of Alloy 800 in simulated crevice chemistries at 300°C. Passivity degradation is characterized by the retarded dehydration during passivation and increased donor density in passive film. Ca2+ and Mg2+ can cause the passivity degradation in the Pb-free chemistries but, if the Pb-contamination concentration exceeds a critical value, would reduce the detrimental effect of Pb-contamination. This behavior may be related to the extremely strong adsorption capacity of Pb contamination. Finally, the applications in the water management strategy of nuclear power plants are discussed.

Effects of dissolved Ca2+ and Mg2+ on passivity of UNS N08800 alloy in simulated crevice chemistries with and without Pb contamination at 300 °C

The effects of Ca2+ and Mg2+ on passivity of UNS N08800 alloy at 300°C are experimentally investigated in simulated crevice chemistries. In the Pb-free solutions, the impacts of Ca2+ and/or Mg2+ are similarly detrimental, but less in extent, as those caused by Pb-contamination, as indicated by the increased passive current density, the lower breakdown potential, the promoted incorporation of hydrogen into the passive films and the formation of the films with highly defective structures containing more MOH bonds. However, when Ca2+ and/or Mg2+ are/is concurrently present with Pb cation, they mitigate the detrimental impact of lead on the passivity.

Passivity degradation of alloy 800 in simulated crevice chemistries

Passivity degradation of Alloy 800 in simulated crevice chemistries was systematically investigated using cyclic polarization curve, electrochemical impedance spectroscopy(EIS), Mott-Schottky analysis, Auger electron spectroscopy(AES)and atomic absorption spectrometry(AAS). Cyclic polarization showed that the pitting potential in a thiosulfate solution was much lower than in either a chloride solution or a sulfate-chloride solution. Mott-Schottky results revealed that passive films showed n-type semiconductivity, and the presence of thiosulfate in chloride solution led to an increased donor density in the passive film. EIS spectra indicated that thiosulfate enhanced the film dissolution rate in chloride solutions. Moreover, thiosulfate enhanced the pitting propagation rate in chloride solution by stabilizing the metastable pits and forming sulfide within the pits.

PH Effect on Sulfur-Induced Passivity Degradation of Alloy 800 in Simulated Crevice Chemistries

The effects of pH on sulfur-induced passivity degradation of Alloy 800 in simulated crevice chemistries were evaluated using electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis method. Experimental results reveal that the detrimental effect of impurities containing sulfur at the reduced or intermediate oxidation level (Sx) on the passivity of Alloy 800 depends significantly on the solution pH. In neutral crevice (NC) chemistry containing chloride ions, the Sx obstructs the healing process during repassivation, retards the dehydration reaction and also incorporates H, –OH and S into the passive film. However, the detrimental effect of Sx is insignificant in a basic crevice (BC) solution due to a change in surface adsorption and surface charge. Impurities containing sulfur at the reduced or intermediate oxidation level would cause Alloy 800 to lose its passivity and become active in an acidic crevice (AC) chemistry. Experimental evidence also indicates that the solution pH alters the semiconductor type of the surface film —from n-type in NC solution to p-type in BC solution.

PH Effect on Sulfur-Induced Passivity Degradation of Alloy 800 in Simulated Crevice Chemistries

The effects of pH on sulfur-induced passivity degradation of Alloy 800 in simulated crevice chemistries were evaluated using electrochemical impedance spectroscopy (EIS), secondary ion mass spectroscopy (SIMS), and Mott-Schottky analysis methods. Experimental results reveal that the detrimental effect of impurities containing sulfur at the reduced or intermediate oxidation level (Sx) on the passivity of Alloy 800 depends significantly on the solution pH. In neutral crevice (NC) chemistry containing chloride ions, the Sx obstructs the healing process during repassivation, retards the dehydration reaction and also incorporates H, –OH and S into the passive film. However, the detrimental effect of Sx is insignificant in an alkaline crevice (AKC) solution due to a change in surface adsorption and surface charge. Impurities containing sulfur at the reduced or intermediate oxidation level would cause Alloy 800 to lose its passivity and become active in an acidic crevice (AC) chemistry. Experimental evidence also indicates that the solution pH alters the semiconductor type of the surface film —from n-type in NC solution to p-type in AKC solution.

Effects of pH On S-Induced Passivity Degradation of Alloy 800 in Simulated Crevice Chemistries

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Passivity degradation of nuclear steam generator tubing alloy induced by Pb contamination at high temperature

Effects of Pb contamination on the passivity of a Ni-based alloy (UNS N06690) in a simulated crevice chemistry of steam generator with near-neutral pH at 300°C are elucidated using electrochemical measurements and surface analysis techniques. The experimental observations reveal that Pb impurity can enter anodic film, which results in substantial changes in the films structure via hindering the dehydration during the passivation and retarding the formation of spinel oxides. The presence of Pb-contamination can also increase hydrogen content in anodic film. Finally, the mechanism of passivity degradation induced by Pb contamination is described on the basis of the experimental data and established theory.

Effects of dissolved calcium and magnesium ions on lead-induced stress corrosion cracking susceptibility of nuclear steam generator tubing alloy in high temperature crevice solutions

The effects of Ca 2+ and Mg 2+ ions on the stress corrosion cracking (SCC) susceptibility of UNS N08800 are investigated using constant extension rate tensile (CERT) tests at 300 °C in simulated crevice chemistries. The presence of lead contamination in the crevice chemistries increases significantly the SCC susceptibility of the alloy. The lead-assisted SCC (PbSCC) susceptibility is reduced markedly by the addition of Ca 2+ and Mg 2+ ions into the solution and this mitigating effect is enhanced by increasing the total concentration of Ca 2+ + Mg 2+. The CERT test results are consistent with the types of fracture surfaces shown by Scanning Electron Microscopy (SEM). There is a reasonable correlation between the SCC susceptibility and the donor densities in the anodic films in accord with the role of lead-induced passivity degradation in PbSCC.

Investigation of passive films on nickel Alloy 690 in lead-containing environments

Passive films formed on Alloy UNS N06690 were investigated in simulated crevice chemistries. It was found the role of lead in corrosion processes is strongly dependent on the pH value of the testing solutions. At pH 1.5 the effect of lead is narrowly noticeable; while at pH 12.7, lead has a significant influence on the electrochemical performance of alloy UNS N06690. The lead alters the surface morphologies at both pH and account for higher hydroxide content in the surface film at pH 12.7. The lead incorporation hinders the formation of spinel oxides during the passivation in alkaline solution. Nanoindentation tests indicate a significant lead-induced degradation in the mechanical properties of passive films. The passivation degradation is attributed to detrimental effects of lead via interrupting the dehydration process and hindering the formation of protective layers on the alloy surface.