Relativenobility of precipitated phases in stainless steels : Evaluation with a combination of local probing techniques
Abstract: Stainless steels often exhibit complex transformation and precipitation behaviour due to a high content of alloying elements. Secondary phases can be formed in the temperature range of 300-1000°C and are generally undesirable due to their detrimental effect on mechanical properties and corrosion resistance of stainless steels. Of all precipitate types, sigma phase is the major concern due to its effect on both toughness and resistance to corrosion. However, the effect of the phase itself cannot be separated from that of associated changes in the surrounding matrix if macroscopic techniques are used. The situation is similar for investigations of chromium nitrides, which are the precipitated phases frequently observed in high nitrogen stainless steels. High resolution techniques are required to characterize such small individual precipitates to be able to examine their actual effect on the corrosion resistance of stainless steels. In this work, magnetic force microscopy (MFM) and scanning Kelvin probe force microscopy (SKPFM) were used to investigate the magnetic properties and the Volta potential difference of precipitated phases. The magnetic domain distribution was used to distinguish between ferrite (ferromagnetic), austenite (paramagnetic), and sigma phase (non-magnetic). The Volta potential differences reflect the relative nobility of the phases and thus their tendency to corrode. The MFM and SKPFM techniques are shown to achieve a high lateral resolution of at least 100 nm. This means that small particles or precipitated phases can be characterized separately from the surrounding matrix. Two grades of duplex stainless steels, the standard 2205 and the super duplex 2507, and an electroslag strip weld with a multi-phase microstructure were investigated using a combination of local probing techniques. The duplex stainless steels underwent various heat treatments to precipitate a sufficient amount of secondary phases. Scanning electron microscopy in backscattered electron mode and energy dispersive X-ray spectroscopy yielded information on the phase distribution and chemical composition of individual phases. Detailed marking of the surfaces was used so that exactly the same areas could be investigated with the MFM and SKPFM techniques. Transmission electron microscopy (TEM) was also employed to characterize the chemical composition of depleted phase boundaries. The general observation is that, austenite exhibited a higher Volta potential compared with ferrite, most likely due to a higher nickel content in the austenite phase. When sigma phase was present, it showed an intermediate Volta potential between the austenite and ferrite phases. This indicates that austenite is, in general, more noble than sigma phase, and that ferrite is the most active phase. However, austenite showed a lower Volta potential than sigma phase when a long-term isothermal heat treatment at 800°C was used. This is attributed to the depletion of alloying elements in the austenite phase as a result of the formation of chromium nitrides and sigma phase. Synergistic interactions between chromium, molybdenum, and nitrogen may contribute to the effect on the Volta potential, since such small changes of these elements result in reversed Volta potentials of the austenite and sigma phase. Results from SKPFM and TEM analyses are in concordance and indicate local drops in Volta potential at the phase boundaries due to the depletion of alloying elements caused by sigma phase formation. Immersion tests in acidic mixtures also confirmed that these depleted regions are more susceptible to selective corrosion. Precipitated chromium nitrides showed a higher Volta potential compared with the other phases. This indicates that any deterioration in the corrosion resistance is unlikely caused by the nitride particles themselves, but rather by the alloying element depleted regions surrounding the nitride particles. The size of nitride particles affected the measured Volta potential, and the measured Volta potentials of small particles are tended to be concealed by the surrounding matrix. When the size of nitride particles is below the resolution limit of the SKPFM technique, the Volta potential differences of these particles relative to the matrix could not be detected. Volta potential measured in air with the SKPFM technique correlated better to the tendency to active dissolution than to pitting corrosion in acidic mixtures. The magnetic force showed a certain influence on the electrostatic force, thus Volta potential measurements are recommended to be performed with a non-magnetic tip. Although many factors may affect the measured Volta potential, the SKFPM technique combined with other local probing techniques is a promising approach to evaluate corrosion tendency of precipitated phases in multi-phase alloys. With optimal conditions, the detectable size was down to approximately 100 nm.
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