Role of Surface Nanocrystallization on the Corrosion Resistance and Stress Corrosion Cracking (SCC) Behaviour of Stainless Steels


Student thesis: Doctoral Thesis

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Award date9 Jul 2020


Improving the overall strength and hardness of metallic materials is always the dream of many scientists and engineers. This has been achieved over the years, thanks to surface nanocrystallization process which helps in the preparation of nanostructured materials with enhanced mechanical properties that can withstand the test of time. However, due to the continuous usage and exposure over time, failures caused by corrosion and stress corrosion do occur on the surface of most metallic materials which ultimately influence their properties. In this research, the corrosion and stress corrosion mechanisms as well as the nanoscale structures of surface mechanically treated stainless steels (SS) in different corrosive media and concentrations were studied. More importantly, the influence of temperature change during surface treatment by a novel surface mechanical attrition treatment (SMAT) technique on corrosion resistance and the resulting microstructures has been extensively investigated in all SMAT treatment conditions.

To achieve this aim, multilayer-structured 17-4 precipitation hardening SS (SS17-4PH), 301 SS, and twinning-induced plasticity (TWIP) steels were firstly designed and fabricated via SMAT and magnetron sputtering techniques. The material microstructure characterized by transmission electron microscopy (TEM) revealed the formation of nanostructured layer, dislocations, and twins on the TWIP steel. In addition, by TEM, a grain refinement and the formation of a longitudinal lath-like martensite grain of about 50 nm thick was formed on SS17-4PH sample. The mechanical properties and the anti-wear performance were thereafter determined.

The corrosion properties of the nanostructured steels facilitated by SMAT were then studied using electrochemical measurements. By the combination of SMAT and low-temperature annealing treatments, the potentiodynamic polarization study and X-ray photoelectron spectroscopy (XPS) spectra demonstrated that the corrosion resistance of 17-4PH, 301, and TWIP steels is not deteriorated after treatments which is evidenced by the higher corrosion potentials, higher chromium (Cr) contents, and reduced corrosion current densities of 0.241 mA/cm2, 1.308 mA/cm2, and 3.535 mA/cm2, respectively. Furthermore, the stress corrosion cracking (SCC) behaviour of TWIP steels was investigated using slow strain rate test (SSRT) approach in terms of mechanical properties, nucleation and growth of cracks.

While investigating the influence of SMAT temperature change on the corrosion resistance, the microstructural and corrosion behavior of TWIP steel sample was studied and compared in no-SMAT, room-temperature (RT) SMAT, low-temperature (LT) SMAT (-100°C), and high-temperature (HT) SMAT (200°C) conditions. The microstructural evolution revealed grain refinement on the LT SMAT-treated TWIP steel sample, thereby forming more grain boundaries for chemical diffusions during electrochemical behaviour. The lower current density and larger impedance exhibited by the LT SMAT treated samples also strengthen the fact that SMAT treatment in LT condition can relax the surface layer, create more diffusion path, and nucleation sites through which Cr can easily move from the matrix to the material surface, thereby producing more protective dense oxide layer (passive film) on the material surface layer.

After SMAT and polarization processes, an advanced material characterization was carried out using atom probe tomography (APT) technique to further study the nature and the atomic-scale elemental distribution of the passive film as well as the concentrations of all elements from the material surface. The APT characterization revealed the presence of high-density Cu-rich precipitates in the SS17-4PH sample with observable elemental segregation in the volume obtained from the atomic-scale distribution of all elements, indicating an uneven distribution of elements. However, the case is different for the TWIP steel with no sign of precipitate and noticeable periodic fluctuations of elemental fractions.

To sum up, the corrosion resistance of the treated steels is not deteriorated and low-temperature SMAT process is more beneficial to passivation and could improve the corrosion resistance of metallic materials compared to other SMAT temperature conditions. This study will find applications in many automobile and aerospace industries where excellent corrosion resistance is required. For future study, the influence of cell size evolution and misorientation during SMAT on the corrosion resistance could be further explored.

    Research areas

  • Surface mechanical attrition treatment (SMAT), corrosion resistance, stress corrosion cracking (SCC), atom probe tomography (APT), polarization; passive film