M. Puiggali

Influence of o-phase on mechanical properties and corrosion resistance of duplex stainless steels

Abstract In order to appraise the influence of s-phase on the behaviour of duplex stainless steels, two tests are performed. The first one is the double loop electrochemical potentiodynamic reactivation (DLEPR) test that indicates the degree of sensitisation to intergranular corrosion. The second one is the slow strain rate test (SSRT) that enables us to relate the degree of sensitisation to stress corrosion cracking. A metallurgical study is also performed on two duplex stainless steels of grade UNS S31803. Sensitisation is carried out by several heat treatments at 675°C or 900°C. The influence of s-phase is clearly shown on the mechanical properties and the corrosion resistance.

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.

Application of double loop electrochemical potentiodynamic reactivation test to austenitic and duplex stainless steels

Abstract Double loop electrochemical potentiodynamic reactivation (DLEPR) tests are performed on an austenitic (317L) and an austeno-ferritic (UNS S31803) stainless steels. In order to optimise results, the standard electrochemical solution has been modified by adding NaCl to the composition and by increasing the concentration of H 2 SO 4 . Results show a high resistance to intergranular corrosion of the 317L heat treated at 675°C for 30 min to 24 h. On contrast, the duplex stainles steel corrosion resistance decreases with time of sensitisation. Low exposure time (1 h at 675°C) provides low reactivation current density whereas longer exposure time (10 h) shows an increase in the value of this reactivation current, whichis due to the formation of chromium carbides and sigma phase at grain boundaries leading to more chromium depleted areas. Heat treatment at 900°C shows a high reactivation proportional to σ -phase precipitation. The aim of this work is to validate the DLEPR technique for detecting susceptibility to intergranular corrosion of duplex stainless steels. This test is not yet normalised and could be used for on-site measurement.

Hydrogen embrittlement of 316L type stainless steel

Hydrogen embrittlement tests on type 316L stainless steel are performed including cathodic charging during slow strain rate tests. Brittle multiple cracking is observed and relationships between crack growth rate and diffusion are analysed. The influence of hydrogen on the morphology of ductile fracture is found after fractographic examination. Two aspects of ductile failure are observed in accordance with the hydrogen content of the sample; a reduced density of microvoids for higher hydrogen contents and brittle secondary cracking in addition to ductile fracture surfaces for lower hydrogen contents.

An EIS study and auger analysis on 304L stainless steel in hot chloride media before and after a sample straining

Abstract Interface development of 304L stainless steel in Scheil reagent has been studied on nonstrained samples or on samples subjected to a plastic straining. Electrochemical impedance spectroscopy (EIS) was used for this work. Continuous changes in the layer formed on the material have been observed in this electrolyte. The Auger analyses have shown a very different film constitution and thickness before and after straining. Inhomogeneities of the film displayed by EIS diagrams have been confirmed by the formation of pits observed by SEM.

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.

An impedance study of the passivity breakdown during stress corrosion cracking phenomena

Abstract The stress corrosion of 304L and 316L stainless steels has been studied in MgCl2 44 % at 153°C. Stress corrosion tests are carried out under constant load and galvanostatic control at i = 0 in order to be near free corrosion potential conditions. During the tests, the sample elongation, the electrode potential, the modulus and the phase of the impedance are recorded versus time. The electrochemical behaviour of the passive film during S.C.C. phenomena is studied. The same evolution is observed for the potential and the impedance modulus, but a maximum of the phase appears as soon as the potential exhibits an inflexion point during its increasing to more anodical values. SEM observations show that the phase maximum corresponds to the beginning of the cracks propagation. The electrochemical impedance technique seems to be more attractive to follow the evolution of the passive film and to be more precise for the detection of the beginning of the crack propagation than the sample elongation until then used.

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.