Microelectrochemical Investigation of Pit Initiation Site on Austenitic Cast Stainless Steel

      Austenitic cast stainless steels are widely used as valves, pumps, and connecting tubes but sometimes suffer from pitting corrosion in chloride-containing environments, such as sea water. The corrosion behavior of cast stainless steels is being studied by immersion tests in acidic FeCl₃ solutions; however, electrochemical investigations in near-neutral solutions are need to ascertain the corrosion mechanisms in the actual environments.

      For austenitic cast stainless steels, the primary ferrite solidification mode is effective for reducing the susceptibility of solidification cracking ¹, so that the steels generally contain delta-ferrite phases. In this case, the solidification starts with the formation of primary delta-ferrite, and then the ferrite transforms to gamma-phase (austenite) during cooling. It is known that phosphorus and sulfur segregate at the delta/ gamma grain boundaries, and the segregation tends to cause corrosion, such as intergranular corrosion and pitting. Microelectrochemical techniques are successfully applied to investigate the pit initiation site on stainless steels.^2-4 A small electrode area is suitable for studying the initiation site and morphology of corrosion, since the ratio of the dissolution to background currents is relatively high, and the polarization can be stopped at the very beginning of the corrosion processes. In this study, a microelectrochemical approach was applied to elucidating the initiation site of pitting for austenitic cast stainless steels. The effect of grain boundary segregation on pitting was also studied.

      A 20-kg ingot of austenitic cast stainless steel (0.007%C, 0.81%Si, 0.82%Mn, 0.028%P, <0.001%S, 8.1%Ni, 18.7%Cr, <0.01%Mo, <0.01%Cu, 0.001%Al, 0.0042%N, 0.0029%O was prepared by vacuum induction melting.  To eliminate the effects of sensitization and sulfide inclusion on pitting, carbon and sulfur contents were lowered. The volume of delta-ferrite was around 15% at room temperature, and a small amount of Al-Mn oxide inclusion existed. The specimens (15 × 25 × 5 mm blocks) were taken from the center of the ingot and were used as-cast without any heat-treatment. The specimen surface was polished down to 1 μm diamond paste. The potentiodynamic anodic polarization curves were measured in a naturally aerated 0.1 M NaCl solution at 298 K. The electrode area was changed from 0.02 mm² to 100 mm².

      The pitting potential of the steel was 0.35 V (vs. Ag/AgCl, 3.33 M KCl), when the electrode area was 1 cm². In the macroscopic measurements, the initiation site of pit was not identified, since a large pit with a diameter of ca. 100 μm was generated. The electrode area was then reduced to 0.02 mm² (ca. 150 μm square); however, no pit was observed even when the electrode area contained the delta/gamma grain boundaries. It was suggested that the initiation sites for pitting were sparsely distributed in the steel.

      On the basis of the size of the delta-ferrite, the electrode area was adjusted to 1 mm². Figure 1 shows the anodic polarization curves and the surface appearances of the sites of pitting before and after the polarization. A meta-stable pit was generated. In Fig. 1, the pit was initiated at the oxide inclusion located at the delta/gamma grain boundary. It was confirmed in the separate experiments that the metastable pit was also initiated at the delta/gamma grain boundary at which no inclusion was located.

      A scanning electron microscope (SEM) and a scanning transmission electron microscope (STEM) equipped with an energy-dispersive X-ray spectroscopy(EDS) system were employed to analyze the phosphorus and sulfur segregation at the grain boundary. Figure 2 exhibits the SEM/STEM images and EDS analysis of the grain boundary at which the metastable pit was initiated (Fig. 1). It was clear that phosphorus and sulfur were segregated at the delta/gamma grain boundary. Figure 3 shows the results of segregation analysis of the delta/gamma grain boundary at which no pit was initiated. No segregation of phosphorus and sulfur was detected in this case. It was clear that the pit initiation sites of austenitic cast stainless steel were the grain boundaries of the delta/gamma phases, and the segregation of phosphorus and sulfur was proven to cause the pit initiation in chloride environments.

References;

1. V. Kujanpää, N. Suutala, T. Takalo, and T. Moiso, Weld. Res. Int., 9, 55 (1979).

2. I. Muto, Y. Izumiyama, and N. Hara, J. Electrochem. Soc., 154, C439 (2007).

3. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 159, C341 (2012).

4. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C511 (2013).