D. Desjardins

Effect of microstructure on stress corrosion cracking of an Al-Zn-Mg-Cu alloy

The susceptibility to stress corrosion cracking (SCC) of the Al-Zn-Mg-Cu French 7010 alloy of different temper has been investigated by Slow Strain Rate Testing. The tests have been carried out at strain rate ranged between 10−5 and 10−7s−1, at corrosion potential and at cathodic polarisation, in NaCl and NaCl + As2O3 solution. The lowest susceptibility to SCC of overaged alloy has been explained by presence of large phase precipitates which increased the alloy resistance to pitting and intergranular corrosion, changed the conditions of crack initiation and trapped the absorbed hydrogen.

Effect of molybdenum on passivity of stainlesssteelsin chloride media using ex situ near fieldmicroscopyobservations

Abstract Combined AFM/STM investigations of passive films formed on 316 Land304 L stainless steels immersed in NaCl media were performed. From these experiments,thestructure of the first atomic layer of passive films was described. The formation ofmolybdenumchlorides was then shown and mechanisms of Cl− adsorption werediscussed.Therefore, assumptions were proposed on mechanical properties of passive films andeffect of Moon pitting processes.

The influence of the strain-rate on the stress corrosion cracking of alloy 600 in high temperature primary water

Abstract Alloy 600 used as a material for steam generator tubings in pressurized water reactors is susceptible to intcrgranular stress corrosion cracking. Laboratory tests have shown that on tensile smooth specimens alloy 600 stress corrosion cracking in the thickness (1.27 mm) involved first an initiation period depending mainly on electrochemical processes, then a slow-propagation stage with crack depth less than a critical size—highly influenced by stress and mechanical state of the surface—and finally a rapid-propagation stage leading to failure. Tests separating stress and strain-rate contributions have shown that strain-rate is the most suitable parameter to describe the different stages of propagation. Stress is only needed to generate a creep rate. Correlation between SCC susceptibility and creep strain-rate was investigated at 360 °C on six mill-annealed tubes showing different chemical compositions, mechanical properties and grain sizes. The tubes with high creep strain-rate exhibited the greatest susceptibility to stress corrosion cracking and several tubes with low creep strain-rate demonstrated a low susceptibility, but one slow-creeping tube evidenced a relatively easy cracking. Heat treatment at 700 °C, which has resulted in an increased creep rate, was found to improve stress corrosion cracking resistance. This lack of correlation between creep and stress corrosion cracking may be a consequence of differences of metallurgical structure contribution to creep and (or) of an influence of primary water on the creep rate of alloy 600. An assessment of the durations of the initiation and the slow-propagation stages of cracking was attempted for the uniaxial tensile tests, using the macroscopic strain-rate. A crack-tip strain-rate damage model is still in progress to predict these durations for constant-load tests from constant extension rate tests (CERTs).

Quantification of the hydrogen produced during corrosion fatigue crack propagation

Abstract Modelling of environmentally assisted cracking depends on the ability to provide quantitative information on the anodic dissolution and on hydrogen production at a crack tip. The amount of gaseous hydrogen produced during corrosion fatigue crack propagation at the open circuit potential was measured on 316L stainless steel in MgCl 2 solution at 117°C. In situ monitoring of natural cracks was performed by using long focal length video optical microscopy to measure hydrogen bubble diameters at the crack mouth. A boundary condition of the hydrogen evolution inside a crack was obtained and, by considering the electrochemical reactions occurring inside a crack, a minimum volume of metal dissolved per cycle at the crack tip was estimated.

The influence of the mechanical test conditions on the corrosion fatigue behaviour of austenitic stainless steel in chloride solutions

The corrosion fatigue behaviour of the 316L alloy in chloride solutions is analysed and compared to the stress corrosion cracking behaviour of the same alloy in the same electrochemical conditions. The kind of corrosion fatigue test (cyclic tensile stress, tension-compression strain) is shown to have a very sensitive effect on the resulting damage. The tension-compression test is much more severe than the cyclic tensile stress and induces damage which can occur even if stress corrosion cracking is not observed.

The characterization of damage in SCC by an electrochemical impedance and statistical study of multiple cracking

Abstract To build a realistic predictive model, it is essential to understand how the damage is developed. The evolution of this damage on 304L steel in Scheil reagent during slow strain rate tests has been studied for 4 various strain rates. An observation of the distribution of crack traces (number, size and morphology) has shown that the damage is closely linked to the strain rate. Interrupted CERTs have been realised for two strain rates. For each strain rate, electrochemical impedance measurements at fixed frequency (100 mHz) and histograms of the distribution of the crack traces have been carried out. The linked use of two techniques to characterize the damage in stress corrosion studies gives very interesting results. The evolution of the phase of the electrochemical impedance is closely linked to the initiation rate of cracks. Density of cracks that have reached a given size could be a new definition of the critical damage in stress corrosion studies when multiple cracks exist.

The electrochemical behaviour of stainless steels in hot nitric solutions as related to the SCC phenomenon

Abstract Boiling diluted nitric acid solutions are sometimes used for industrial chemical cleaning, but in the presence of chloride pollutions corrosion phenomena can occur, in particular stress corrosion cracking. The electrochemical behaviour of AISI 304 and 316 stainless steels in boiling nitric acid solutions in the concentration range of 0.1–5 N, containing chloride (0.1–2 wt%) is presented. The modifications of the corrosion potential, of the passivity range and of the electrochemical material response during mechanical depassivation, recorded in these various solutions are in good agreement with the corrosion observed after stress corrosion tests.

A critical study of stress corrosion cracking testing methods for stainless steels in hot chloride media

Abstract The selection of a stress corrosion cracking (SCC) test which procures a reliable classification of stainless steels with regard to their SCC resistance in chloride media requires the choice of a suitable electrolyte in which the passive film is similar to those built in industrial conditions and the choice of a reliable mechanical testing mode. The same classification is obtained by the slow strain rate technique (SSRT) and the constant load technique. It appears that SCC mechanisms are little modified in the two testing modes. The effect of some alloying additions (Mo, Ni, Ti, Cu) to austenitic stainless steels and the behaviour of a duplex austeno-ferritic steel has been examined by these two modes.

Effects of tempers on environmental-assisted failure of an AlZnMgCu alloy

Abstract The AlZnMgCu alloy was investigated by slow strain rate tests in sea water. The influence of tempers on the strain rate dependence of fracture energy was observed. The effects of the strain rate, addition of As 2 O 3 , pre-exposure and room temperature storage were explained by a change in the grain boundary microstructure following more advanced ageing that affected the susceptibility to pitting corrosion, hydrogen absorption and morphology of cracking.

The stress corrosion cracking of austenitic stainless steels in H3BO3 + NaCl solutions between 100°C and 200°C

Abstract The stress corrosion cracking of 316L austenitic stainless steel inH3BO3 + NaCl aqueous media between 100 and 200°C has been studied. The results obtained from slow strain rate and constant load tests, the polarization curves, the rapid tensile straining tests and the fractographic investigation of the fracture surface by scanning electron microscopy, permit the examination of the nature of mechanisms at work. The experimental crack propagation velocities and those calculated from Faradays law have been compared. The analysis of the electrochemical behaviour in the absence of stress has revealed the potential ranges of maximum stress corrosion cracking susceptibility. Etch-pitting and stereographic observations are employed in order to determine the cracking crystallography. The transgranular fracture surfaces show crystallographic morphology and though the cracking is cleavage-like and is discontinuous in some cases, the cracks propagate mainly under the control of localized anodic dissolution. This process seems to be the limiting factor of the phenomenon whatever the proposed mechanisms may be. In fact, the crack propagation velocity seems to be clearly consistent with a film rupture-dissolution-repassivation process, although it proves insufficient to quantify the phenomenon. The influence of the various environment parameters (temperature, concentration of chemical species, oxygen) which have been studied, has confirmed the importance of the localized anodic dissolution on the cracks propagation.