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.     

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.     

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.     

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.     

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.     

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.     

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).     

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.     

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。     

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.     

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.     

Effect of microstructure on stress corrosion cracking of alloy 600 and alloy 690 in 40% NaOH

Stress corrosion cracking (SCC) behaviors of Alloy 600, Alloy 690 and the Ni-10Cr-10Fe alloy have been studied using a C-ring in 40% NaOH solution at 315°C. The current density of Alloy 690 in polarization curves was higher at 200 mV above corrosion potential than that of Alloy 600. SCC resistance increased with Cr content for the chromium carbide free alloys, probably due to facilitation of SCC crack tip blunting with an increase in Cr content. Both thermally treated Alloy 600 and sensitized Alloy 600 have a comparable amount of intergranular carbide. But the former is more resistant to SCC than the latter, which might be attributed to the presence of the slight Cr depletion around the grain boundary in the former one. Sensitized Alloy 600 showed higher SCC resistance than the solution annealed one due to intergranular carbide in sensitized Alloy 600. This implies that the beneficial effect of intergranular carbide overrides the harmful effects of Cr depletion for sensitized Alloy 600. SCC resistance of Alloy 600 increased with grain size. This article based on a presentation made in the symposium “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18 under the auspices of Far East and Ocean Fracture Society (FEOFS),et al.     

Impact of temperature excursion on stress corrosion cracking of stainless steels in chloride solution

The impact of a temperature excursion on the subsequent stress corrosion crack growth at the normal operating temperature has been investigated for 321 stainless steel (UNS32100) and 316L stainless steel (UNS31603) using precracked compact tension specimens. Although the data are preliminary the indication is that once crack growth has initiated in 321 SS at the elevated temperature, 130 °C in this study, the crack growth may be sustained at the lower temperature (40 °C), at least over the exposure time of about 700 h. However, the growth rate of 316L SS at the lower temperature was significantly lower than for 321 SS and tended to zero after 2000 h. For the 316 SS a temperature transient should not impact on structural integrity, provided it is short in duration.     

Role of Bayer solution concentration and temperature in stress corrosion cracking susceptibility of steel

To improve the efficiency of the Bayer process for the extraction of alumina from Bauxite ore, there is a push for increasing processing temperature and caustic concentrations, which has also led to an increased concern for caustic embrittlement. In this study, the caustic cracking behaviour of steel in Bayer solutions of 2.5, 5, 7.5 and 10 mol dm⁻³ “free caustic” concentrations have been studied at different temperatures using pre-cracked circumferential notch tensile specimens. It has been observed that at 100 °C, steel is susceptible to caustic cracking in each of the four Bayer solutions. Caustic cracking has also been observed at temperatures as low as 55 °C. Tests were also conducted using only the notched specimens (i.e., without pre-cracking) in a 7.5 mol dm⁻³ “free caustic” Bayer solution at 120 °C to study the stress corrosion crack formation and propagation behaviour in blunt notches.     

Mechanistic aspect of near-neutral pH stress corrosion cracking of pipelines under cathodic polarization

This paper investigates mechanistically stress corrosion cracking (SCC) of an X70 pipeline steel that is under cathodic protection (CP) in a near-neutral pH solution. It was found that there is a critical potential range, i.e., ?730 and ?920 mV_SCE, where the steel is in a non-equilibrium electrochemical state, and anodic dissolution (AD) reaction may occur when the steel is polarized cathodically. When the applied potential is more positive than this range, SCC is AD-based; while the applied potential is more negative, SCC of pipelines is under hydrogen embrittlement (HE) mechanism. When the polarization potential is within the range, SCC of the steel is under the combined effect of AD and HE. Therefore, AD may still occur on pipeline steel that is under CP with the potential within this critical range, contributing to the cracking process.     

Predictive approaches to stress corrosion cracking failure

The methods available for predicting the occurrence of stress corrosion cracking are discussed in detail. Current decay measurements and potentiodynamic methods are considered in relation to kinetic factors controlling crack initiation and propagation. Prediction from structural aspects of an alloy are reviewed and are considered to be applicable only where there is some preexisting active path and not for a strain-generated path. Localized changes within cracks and the distribution of potential are discussed. In many cases the magnitude of potential drop in a crack is of minor concern. The nature of the specimen surface may be very important since some oxides of iron exhibit a very low current reduction efficiency. Cracking conditions need to be carefully defined by employing both kinetic and thermodynamic data as has been done for a C-Mn steel in phosphate solutions.