SCC Mechanism Research Articles

Mechanism of SCC on SUS310S Stainless Steel

The author have developed a new SCC test method that enables the super Kelvin force microscope (SKFM) and the Kelvin force microscope (KFM) observations. By using this test device, the crack tip deformation and surface po- tential distribution were observed on SUS310S stainless steel by SKFM and KFM. At the same time, the distribution of hydrogen was examined by the Ag decoration method with the EDX image mapping analysis combined. Moreover, the existence of hydrogen-induced martensite was examined by the magnetic force microscope (MFM) observations. The re- sults showed that the less noble potential region existed at whole crack and Ag precipitated at less noble potential region in most cases. The MFM and KFM observation showed hydrogen-induced martensite existed at the less noble potential part. SCC is caused by the formation of hydrogen-induced martensite ahead of the crack tip.

Mechanism detection of stress corrosion cracking by acoustic emission and effect of manufacturing process on AE signals

Stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stresses coexist. In the objective to monitor online the SCC on real structures, it seems reasonable to characterize and recognize acoustic emission during static U-bend tests. The present study is concerned with static tests on 304 stainless steels in two different media (5% HCl and 5% H2SO4) in order to find a criterion to distinguish, the two different mechanism of SCC (anodic dissolution and hydrogen embrittlement) by characteristic parameters and waveform analysis of AE signals. In the next stage, effect of residual stress on stress corrosion cracking in chloride solution has been studied. Three samples have been prepared: one of them was stress relieved; the second one was made sensitive to SCC by heat treatment and the last one was cold worked by rolling. Cumulative count, amplitude distribution and waveform analysis were selected as AE-parameters. AE with amplitudes ranging from 39 to 65 dB with different counts and energy occurred during SCC of SS-304 at room temperature. According to the results, acoustic emission is able to be used as robust technique for mechanism detection of SCC. In addition it can be used to measure the effect of residual stress in manufacturing process on SCC.

Mechanism of SCC on SUS310S Stainless Steel

The author have developed a new SCC test method that enables the super Kelvin force microscope (SKFM) and the Kelvin force microscope (KFM) observations. By using this test device, the crack tip deformation and surface po- tential distribution were observed on SUS310S stainless steel by SKFM and KFM. At the same time, the distribution of hydrogen was examined by the Ag decoration method with the EDX image mapping analysis combined. Moreover, the existence of hydrogen-induced martensite was examined by the magnetic force microscope (MFM) observations. The re- sults showed that the less noble potential region existed at whole crack and Ag precipitated at less noble potential region in most cases. The MFM and KFM observation showed hydrogen-induced martensite existed at the less noble potential part. SCC is caused by the formation of hydrogen-induced martensite ahead of the crack tip.

Transgranular SCC mechanism of thermally treated alloy 600 in alkaline water containing lead

This work aims to understand a SCC failure mode of thermally treated steam generator tubing materials in high temperature water containing lead. The effect of lead contents on the anodic polarization curves of alloy 600 (UNS NO6600) and alloy 690 (UNS NO6900) has been studied in a solution of pH 10 at 200 °C and 315 °C. Lead increased the active peaks of alloy 600 and alloy 690 in mild alkaline water at high temperatures. A reduction of PbO to a metallic lead in alloy 690 is easier than that of alloy 600. When lead was added into the solution, a relative ratio of Cr from among the main metallic elements (Cr, Fe, and Ni) of alloy 600 and alloy 690 decreased in the outer corrosion film. Alloy 690 TT showed a transgranular stress corrosion cracking (TGSCC) in a 10 M NaOH solution with 5000 ppm of lead. Intergranular stress corrosion racking (IGSCC) was observed in the 100 ppm lead condition, and some TGSCC was detected on the fracture surface of the alloy 600 MA cracked in the 10,000 ppm lead solution. IGSCC seemed to be retarded by a crack blunting around the grain boundaries, and the TG cracking mode of the thermally treated alloy 600 and 690 seemed to be related to a crack blunting at the grain boundary carbide and a film dissolution by lead in an alkaline solution.

Detection of Stress Corrosion Cracking of Type 304 Stainless Steel Using Acoustic Emission and Corrosion Potential Fluctuation

This paper discusses the mechanism of chloride SCC of sensitized and non-sensitized Type-304 stainless steel based on the data obtained by simultaneous monitoring of AE and corrosion potential fluctuation (CPF). The steel produced transgranular SCC via corrosion pits filled with corrosion product (chromium oxy-hydroxides) in 35 mass % MgCl2 solution. Both the pitting corrosion and transgranular SCC did not produce primary AEs due to anodic dissolution, while they generated rapid drop (RD) type CPF. We, however, detected the secondary AEs from hydrogen gas evolution and fracture of corrosion products. Hydrogen gas was found to produce AE with single frequency component, while the fracture of corrosion products produced AEs with broad frequency components. The sensitized steel produced primary AEs due to the falling-off of grains as well as the secondary AEs. Microscopic progression of SCC was consistently interpreted by the timing and time-lag of AEs and CPFs.

Reply to E.M. Gutman's: “comments on the “stress corrosion cracking of zirconium and zircaloy-4 in halide aqueous solutions” by S.B. Farina, G.S. Duffo, J.R. Galvele”

The present “comments” are a sequel to the previously published “notes” by the same author and on the same subject. The intention of the present correspondence is to show that the surface mobility SCC mechanism is far simpler than the description made by Gutman in his comments. It must also be reminded that in 1987, by using this mechanism, it was predicted that the presence of hydrogen in metals produced an increase in the vacancy concentration in those metals. This fact was confirmed in independent studies, and it is considered of high significance in hydrogen embrittlement.

Stress Corrosion Cracking of Zircaloy-4 in Halide Solutions: Effect of Temperature

Zircaloy-4 was found to be susceptible to stress corrosion cracking in 1 M NaCl, 1 M KBr and 1 M KI aqueous solutions at potentials above the pitting potential. In all the solutions tested crack propagation was initially intergranular and then changed to transgranular. The effect of strain rate and temperature on the SCC propagation was investigated. An increase in the strain rate was found to lead to an increase in the crack propagation rate. The crack propagation rate increases in the three solutions tested as the temperatures increases between 20 and 90 °C. The Surface-Mobility SCC mechanism accounts for the observation made in the present work, and the activation energy predicted in iodide solutions is similar to that found in the literature.

Boundary characterisation of X65 pipeline steel using analytical electron microscopy

Analytical electron microscopy was used to characterise grain boundaries (GBs) and interphase boundaries (IBs) of X65 pipeline steel. There was no segregation of P or S at the proeutectoid ferrite GBs. This indicates that contrary to literature expectations, P and S are unlikely to be involved in the mechanism of SCC of pipeline steels. There was Mn segregation at IBs between pro-eutectoid ferrite and pearlitic cementite, and desegregated from the IBs between pro-eutectoid ferrite and pearlitic ferrite. This pattern of Mn segregation is attributed to diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite. A new mechanism was proposed for pearlite formation. A GB carbide first forms at an α : α GB, and then grows along the α/γ interface. Subsequently pearlite initiates from this interface carbide.

高力アルミニウム合金の応力腐食割れにおける水素脆性関与のモデル実験

Hydrogen embrittlement and SCC behavior of bending 7075-T6 aluminum alloy specimens were examined. The compression side of each specimen with both side notchs was only exposed to the cathodic charging solution or SCC environment. The notch strength was gradually decreased with increasing charging time. Prominent embrittlement was found on the tension side, but not on the compression side. Both the embrittlement and cracking on the tension side were also found in a SCC test specimen of which the compression side was only exposed. These results were only caused by the understood by the mechanism of hydrogen embrittlement hydrogen permeated from the compression side and transported to the tension side by diffusion. Thus, it was concluded that the hydrogen embrittlement was involved in the SCC mechanism.

ＳＳＲＴ法によるジルカロイ‐２のヨウ素ＳＣＣ感受性の統計的評価

A statistical evaluation has been conducted of strain-rate dependent stress-strain curves of Zircaloy-2 in air and CH3OH-1w/0 I2 solution, to study an applicability of Weibull distribution method to SSRT SCC susceptibility. The results are as follows:(1) Various random variables characterizing stress-strain curves of Zircaloy-2 during SSRT SCC process obey the single Weibull distribution function of two or three parameters. All values of the shape parameters m in this study were larger than unit corresponding to that of the wear-out failure. Thus, the SCC process failure could be described in terms of the Weibull distribution in which the extreme value statistical theory is based on the stochastic process theory.(2) It is found that strain rate dependent shape parameter of the Weibull distribution is useful for studying changes in SCC mechanism during SSRT SCC test of Zircaloy-2.

Analytical evaluation and mechanism of hydrogen embrittlement SCC in duplex stainless steel welds

Analytical evaluation and mechanism of hydrogen embrittlement SCC in duplex stainless steel welds

３種類のＡｌ‐Ｚｎ‐Ｍｇ系合金の定電位引張り試験とその破面解析

The SCC of three kinds of Al-Zn-Mg alloys in 0.6N NaCl solution was studied by the PSSRT (Potentiostatic Slow Strain Rate Technique) in connection with its polarization behavior. Specimen (A) containing Cu, specimen (B) containing Mn and specimen (C) containing Cr, Mn and Zr were used in this investigation. The potential of these specimens was regulated in the region of the pitting potential or passive potential, which was found from their polarization curves in 0.6N NaCl solution at 298K. The fracture surface of specimens was observed by Scanning Electron Microscope (SEM).At the pitting potential region, all specimens fractured transgranularly at the surface of the specimen. For specimen (A) and (B), transgranular cracking and intergranular cracking were observed. However for specimen (C), intergranular cracking could not occur under the condition of this investigation.At the passive potential, intergranular cracking occurred for specimen (A) and (B), but it could not occur for specimen (C) under the condition of this investigation. Transgranular cracking was considered as SCC induced by pitting corrosion, and intergranular cracking as the rupture of the brittle oxide film along grain boundaries. Therefore, intergranular cracking susceptibility was explained from the viewpoint of growth of oxide film. From all these results, the mechanism of SCC of Al-Zn-Mg alloy was illustrated schematically.

A review of the crystallography of stress corrosion cracking

Some analogies in transgranular stress corrosion fracture were found among the three crystal systems reviewed. All fracture surfaces exhibit similar fractographic morphologies, such as parallel and relatively smooth fracture facets separated by crystallographic or fan-shaped steps. Also, most fracture surfaces are cleavage-like in appearance. In both BCC and HCP metals it has been shown that the slip system geometry has a determining effect on the orientation of the cracking plane. It seems, therefore, that in the SCC of metals with these two crystal structures a strong mechanical factor is involved. Another common characteristic of the transgranular SCC in metals with BCC and HCP structures is that cracks prefer not to follow slip planes. This was observed to a great extent also in FCC metals and appears to suggest that mechanical parameters may influence the SCC mechanism in all metals. In reviewing the extensive work in this area, one is struck by the lack of any broad systematic study of the crystallographic nature of SCC. Such a program would lend much to the overall understanding of SCC phenomena in these systems.

Stress corrosion cracking of alpha-beta brass in distilled water and sodium sulfate solutions

Specimens of a Cu-42 wt pct Zn alpha-beta alloy have been tested to failure in uniaxial tension at constant extension rates in a variety of environments. Testing in distilled H25O and 1 N Na2SO4 solutions over a range of potential-pH conditions results in large ductility losses and an interfacial + transgranular cleavage-like fracture mode. The electrochemical potentials of the specimens have been monitored and/or controlled during testing, and a shift to significantly more active open circuit potentials on straining is observed in environments which cause large ductility losses. The appearance of the transgranular beta fracture surfaces and the effects of potentiostatic polarization on the observed SCC behavior are similar to those seen for alpha phase Cu-Zn, which suggests that SCC of alpha and beta phase Cu-Zn are mechanistically similar. It is concluded that the observed behavior is most consistent with an SCC mechanism based upon the adsorption of a damaging species from the environment.

Initiation of stress corrosion cracks in aluminum alloys

Studies on initiation of stress corrosion cracking (scc) were conducted on notched rods of aged Al-8.6Mg, Al-21.5Zn and Al-2.6Mg-6.3Zn alloys tested in aqueous solutions and water-containing ethanol solutions under free corrosion conditions. It was found that there is a definite scc initiation period (t) preceding crack propagation. The crack initiation rate (1/t) increased with increasing initial stress intensity (KI,) at the notch. The crack initiation rates were thermally activated with apparent activation energies of Q ≈ 40 to 60 kJ/mol in the ethanol based solutions and Q ≈ 109 kJ/mol in aqueous solutions. Cathodic hydrogen precharging prior to scc decreased the subsequent crack initiation period. The results were discussed in relation to a hydrogen mechanism of scc and shown to be entirely consistent with the lattice decohesion model developed by Oriani.

Investigation of the Kinetics and Mechanism of Subcritical Crack Growth by Delayed Failure of Titanium Alloys In and Out of Corrosive Environments

Abstract The kinetics of crack growth by SCC of titanium alloys in some solutions have been studied by the electrical resistivity method. The effect of cathodic polarization on crack growth rate was used for investigation of the mechanism of SCC of titanium alloys in different conditions. The hydrogen diffusion model is proposed to describe the kinetics of delayed failure of titanium alloys in the absence of a corrosive environment. According to this model, it is possible to determine DH from kinetic data. The subcritical crack growth mechanism in the absence of a corrosive environment is shown to be internal hydrogen embrittlement.

Mechanism of scc of α-brass in Mattsson's solution under potentiostatic conditions

SCC of α-brass in Mattsson's solution has been studied under potentiostatic anodic and cathodic polarization. In annealed samples, anodic polarization brings in a minimum in cracking time accompanied by a transition in cracking mode from intergranular to transgranular. The range of potential giving maximum susceptibility to cracking has been found to be independent of Cu content and pH of the test solution, but dependent on cold work. Cathodic polarization increases the resistance to cracking, and in cold-worked samples, where the cracking is initially transgranular, a transition to intergranular has been observed on cathodic polarization. SCC has been encountered also in a non-Cu-containing solution. These observations strongly suggest a mechanism based on a shift in active anodic and cathodic sites on polarization and adsorption playing an important role in the cracking process.