Stress corrosion cracking of Inconel 600 in aqueous solutions at elevated temperature Part II: Effects of chloride and sulphate ions on the electrochemical behaviour of Inconel 600

The influencing effects of temperature, potential and electrolyte composition on the electrochemical behaviour of Inconel 600 in aqueous solutions are presented. Considering these effects the connection between the data have been obtained from chemo-mechanical fracture investigation on CT-samples in Part I of this paper and pitting corrosion are discussed. The results have shown that chloride ions depassivate the surfaces of cracks locally and hinder the formation of a new protective oxide layer on the fracture surfaces. Furthermore, chloride promotes the dissolution of metal and initiates the cracking, respectively. The resulting crevice corrosion promotes an increase of hydrogen absorption by the metal. The increase of the hydrogen content of the metal influences the mechanical fracture behaviour. Contrary, sulphateions inhibit the initiation of corrosion mainly due to a hinderance of chloride ions adsorption on active sites of the fracture surfaces. The initiation of localized corrosion in the crevice region may be stimulated by chromate ions formed by oxidation of chromium from the oxide layer or the base metal in oxygen containing solutions.     

Stress Corrosion Cracking of Inconel 600 in aqueous solutions at elevated temperature part I: Initiation and inhibition of corrosion cracking process

The study reported here has been designed to determine the influencing effect of sulphate ions on chloride induced crack initiation of Inconel 600 using compact tension (CT) samples in chloride containing aqueous solutions at elevated temperature up to 250°. The results show that sulphate ions retard the effect of chloride ions on crack initiation in hot water up to 250°. Intergranular stress corrosion cracking (IGSCC) occurs in both chloride solution and in chloride solution with relatively low concentration of sulphate ions. The mode of cracking changes from brittle to ductile failure due to the influence of inhibiting concentration of sulphate. The results are interpreted in the light of repassivation-dissolution controlled mechanism which is the predominant mechanism. The inhibiting effect of sulphate ions on the initiation of corrosion cracking is attributed to the repassivation and the formation of Fe-Cr-spinel oxide layer which is stable also at higher temperature.     

Prediction of the remaining lifetime of stainless steels under conditions of stress corrosion cracking

The prediction of the lifetime of metal structures and equipment under conditions of stress corrosion is very complicated because of the complexity of this process of degradation. Recently a new method, based on the so‐called corrosion elongation curves, has been found, which can be used to predict the time to failure under these conditions. By upgrading of these curves (and thus obtaining Upgraded Corrosion Elongation Curves – UCEC's) it has been possible to obtain a precise definition of the time needed for the initiation of the corrosion crack, and for its stable growth. It is upon this basis that diagrams for the prediction of remaining lifetime (DPRL's) have been developed. DPRL's can also be used to predict the values of various critical parameters which have to be achieved if a stress corrosion crack is to occur.     

Oxide properties and stress corrosion cracking behaviour for Alloy 600 in leaded caustic solutions at high temperature

The stress corrosion cracking behaviour of Alloy 600 in caustic solutions with and without PbO at 315 °C was investigated by means of slow strain rate tension tests. The characterisation of the oxide that formed on Alloy 600 was derived from transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. Lead was incorporated into the oxide in a metallic lead state and a lead oxide state, which degraded the passivity and induced PbSCC susceptibility. NiB was used as an inhibitor. It reduced the lead incorporation level in the oxide layer and decreased PbSCC susceptibility.     

Caustic stress corrosion cracking of alloys 600 and 690 with NaOH concentrations

In order to evaluate the stress corrosion cracking resistance for commercial alloys (C600MA, C600TT, C690TT) and Korean-made alloys (K600MA, K690TT), C-ring tests were performed in a caustic environment of 4, 10, 20, 30, and 50% NaOH solution at 315°C, for 480 h with an applied potential of 125 mV vs. OCP. Different stress corrosion cracking phenomena were observed according to the NaOH concentration. The rate of caustic IGSCC attack did not appear to increase monotonically with caustic concentrations, but peaked at a concentration between 4 and 50% caustic, or approximately 30% NaOH. Intergranular stress corrosion cracking was found for C600MA in 10, 20, and 30% NaOH solutions, while no cracking was observed in the 4 and 50% NaOH solutions. In 30% NaOH solution, transgrnular stress corrosion cracking was detected in C690TT, which may be related with the large amount of plastic strain (150% yield) and the applied potential (125 mV vs. OCP). The overall data clearly indicate that C600MA has the worst SCC resistance while K690TT offers the best resistance. There is also fairly good correlation between the caustic SCC susceptibility and some metallurgical parameters, particularly the grain size and the yield strength at room temperature. Specifically, materials having larger grain size and lower yield strength exhibited higher caustic SCC resistance.     

C-ring stress corrosion test for Inconel 600 and Inconel 690 sleeve joint welded by Nd:YAG laser

C-ring stress corrosion test for Inconel 600 and Inconel 690 sleeve joint welded by Nd:YAG laser were carried out to evaluate the applicability of the technique in the repair of heat exchanger tube of nuclear power plant. Corrosion test were carried out mainly in caustic solution. The applied stresses range between 207 and 414 MPa at 348 °C and the corrosion time is between 1624 and 4877 h. The experiment also has been done in primary water with one condition (379 MPa, 2931 h), and in sulphate solution with one condition (414 MPa, 1624 h). Stress corrosion cracking (SCC) was observed in the welded specimens immersed in sulphate and caustic solution but SCC was not observed in primary water. The length of SCC is related to corrosion time and applied stress. The length of SCC increases linearly with increasing corrosion time. SCC propagates as intergranular from base metal towards the weld of Inconel 690. Weld part of Inconel 690 appears to posses better resistance to SCC than base metal of Inconel 690 in caustic environment.     

Effect of hydrogen in Inconel Alloy 600 on corrosion in high temperature oxygenated water

Corrosion test on hydrogen charged and uncharged coupons of Inconel Alloy 600 in high temperature oxygenated water showed more weight loss of charged coupon. Observation of the oxide film by transmission electron microscopy (TEM) showed a defective, thicker oxide layer on charged coupon. Analyses of the oxide film by TEM-energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy indicated enrichment of Ni but depletion of Cr in the oxide film on charged coupon. The changes in corrosion behavior and microstructure of the oxide film were most likely due to the hydrogen enhanced preferential dissolution of Cr cations in the water.     

Primary water stress corrosion cracking in alloy 600 and intergranular crack growth rate

The effects of the stress intensity factor and dissolved hydrogen on the crack growth rate in primary water reactor environments was investigated with alloy 600 with continuous carbide films at grain boundaries. A primary water stress corrosion cracking of intergranular fracture mode was observed. The intergranular crack growth rate decreased with a decreasing stress intensity factor, and the dissolved hydrogen content influenced the crack growth rate. Such results are consistent with those reported previously.     

In-situ observation of intergranular stress corrosion cracking in AA2024-T3 under constant load conditions

A specially designed setup was used to apply a constant load to a thin sheet sample of AA2024-T3 and, using microfocal X-ray radiography, to observe in situ the resulting intergranular stress corrosion cracking (IGSCC) from the exposed edge of the sample. The growth of and competition between multiple IGSCC sites was monitored. In many experiments twin cracks initiated close to each other. Furthermore, the deepest crack at the beginning of every experiment was found to slow or stop growing, and was then surpassed by another crack that eventually penetrated through the sample. These observations cannot be explained by the theory of fracture mechanics in inert environments. The possible mechanisms underlying the competition between cracks are discussed.     

Characterization of microstructure,local deformation and microchemistry in Alloy 600 heat-affected zone and stress corrosion cracking in high temperature water

With increasing the distance from the weld fusion line in an Alloy 600 heat-affected zone, kernel average misorientation decreases and the fraction of Σ3 boundaries increases. Cr₇C₃ carbides mainly precipitate at random high angle boundaries. Chromium depletion and segregation of boron and phosphorous at grain boundaries are observed. Alloy 600 heat-affected zone specimens exhibit higher intergranular stress corrosion cracking growth rates than that in the base metal in simulated pressurized water reactor water environments. Crack growth rate is enhanced by strain hardening and by increasing temperature. The role of stress on interface oxidation kinetics is analyzed.     

High temperature corrosion of cracking tubes

Coils for pyrolysis cracking of hydrocarbons are made of centrifugically cast FeNiCr‐alloys and exposed to very severe conditions, high temperatures > 1050°C, oxidizing flue gases at the outside and carburizing atmosphere at the inside. Two tube sections have been investigated after some years operation. In the first case the tube wall was thoroughly carburized, the big size of the internal carbides indicated service at too high temperatures > 1100°C. At such temperatures the protective chromia scale fails by conversion to carbides, allowing ingress of carbon. At the outer wall surface obviously Cr was lost by evaporation as CrO₂(OH)₂ and CrO₃ which is significant at high pO₂ and high temperature. In the second case the tube had been operated at more moderate temperature, but showed creep cavities indicating loss of creep strength. This is due to repeated oxidation and loss of the scale by thermal cycling, this process has caused formation of a carbide denuded zone of considerable width at the outer side somewhat less wide at the inner wall. Besides oxide spallation, at the outer wall also evaporation will have played a role.     

Atomistic simulations of stress corrosion cracking

Stress corrosion cracking (SCC) of stainless steels and nickel alloys in pressurised water reactors (PWR) has been studied for many years but the mechanism at atomic scale is still under debate. The purpose of this paper is to use atomistic calculations, molecular statics (MS) to describe the sequence of phenomena occurring at the crack tip of an SCC fracture. MS simulations with EAM potentials have been carried out on Ni bicrystals containing hydrogen. The calculations show that compression force applied on the crack lips with H at the GB causes brittle rupture. A theoretical model of SCC cracking has been proposed which fits particularly SCC of irradiated stainless steels (IASCC).     

Micromechanical testing of stress corrosion cracking

Abstract

It is known that grain boundaries with differing chemistry, misorientation and structure have varying susceptibility to stress-corrosion cracking (SCC). However, up till now it has not been possible to obtain mechanical property data on individual grain boundaries as they fail under SCC. A novel method of using focused-ion beam machining to manufacture test specimens containing single grain boundaries, combined with loading in a nano-indenter, allows threshold stress levels and crack growth rates in 304 stainless steel to be directly measured. This technique opens up a new field in being able to validate atomistic scale and dislocation models of intergranular SCC. Combining this information with recent advances in microcharacterisastion, modelling and thermomechanical treatment engineering promises to provide a more complete understanding of inter-granular SCC failure and a better approach to reducing SCC susceptibility and predicting component lifetimes.     

Volume diffusion and stress corrosion cracking

The dissolution behaviour of radioactive labelled brass in cuprammonium solutions is interpreted as solid-state diffusion process. The fast rate of this process is facilitated by the large number of excess vacancies near the metal/electrolyte interface, which are generated on the corroding surface by preferential dissolution of the less noble metal. Excess vacancies are preferentially annihilated at lattice imperfections (like grain boundaries) and the voids formed decrease the tensile strength along the perturbed region. A mathematical model is developed to calculate the steady-state rates of intergranular crack propagation of homogeneous binary alloys. Results of the numerical calculations correlate closely with empirical findings in respect of the dependence from bulk alloy compositions, current densities, temperature, tensile stress, as well as threshold values for crack propagation. Basic characteristics of the mechanism are applicable for transgranular crack propagation as well as for SCC of inhomogeneous alloys.     

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,...     

Correlation between the degree of sensitization and stress corrosion cracking susceptibility of type 304H stainless steel

Austenitic stainless steel 304H is extensively used in the super heater tubes of power boiler due to its superior mechanical properties at elevated temperatures. However, its relatively high carbon content increases the susceptibility to sensitization and subsequent stress corrosion cracking.This work is concerned with investigation of the sensitization and stress corrosion cracking (SCC) of austenitic stainless steel grade 304H. Electrochemical potentiokinetic reactivation (EPR) test was used to evaluate the degrees of sensitization (DOS) of the studied alloy at various temperatures and periods of time. DOS increased with increasing sensitization time and temperature. This was confirmed by microstructure examination after EPR test. Boiling magnesium chloride (MgCl₂) test was used to evaluate the susceptibility of 404H stainless steel to stress corrosion cracking. DOS and test stress level had negative effects on time to failure in boiling MgCl₂. The correlation between DOS and SCC was also discussed.     

Primary water stress corrosion cracking (PWSCC) mechanism based on ordering reaction in Alloy 600

A PWSCC mechanism based on an ordering reaction in Alloy 600 is proposed. An activation energy for the ordering reaction in Alloy 600, Q = ～46 kcal/mole (～190 kJ/mole), are determined by a differential scanning calorimeter (DSC). The ordering reaction in Alloy 600 is an indispensable process during reactor operating conditions. The ordering reaction in Alloy 600 causes an anisotropic lattice contraction. This anisotropic contraction produces an additional stress. The stress level would be the maximum value about 70 and 300 MPa according to the lattice planes in Alloy 600 and Weld 182, respectively. In addition, the anisotropic contraction forms the micro cracks in the high angle grain boundary where the difference in lattice contraction is large. The formation of crack induces stress intensification at the crack tip, and this causes crack growth. The initiation and propagation of PWSCC is controlled by the formation, growth, and coalescence of micro cracks due to anisotropic lattice contraction by ordering. These whole processes are governed by the kinetics of the ordering reaction. This is the reason why the activation energy for PWSCC, Q pwscc = 40–50 kcal/mol, is consistent with that for the ordering reaction, Q ordering = 46 kcal/mol. This mechanism can be proved by the comparison of the initiation behavior in the ordered and the disordered specimens.     

应力腐蚀裂纹试验研究

通过对聚合釜应力腐蚀裂纹附近材质的硬度、金相组织进行试验制定等方法，指出了压力容器应力腐蚀的特征及产生的原因。