Phosphorus is typical impurity in steelmaking process. Phosphorus was tended to segregate at grain boundaries. Phosphorus segregation causes solidification clacking at welding and defects at forming. In the steel making process, Phosphorus cannot refine completely. We focused on the effect of phosphorus segregation and precipitation to corrosion resistance. Phosphorus in steel was expected solid solution, segregation, and precipitation and the size was submicron order. Therefore, we need microelectrochemical measurements to analysis. Solid solution carbon was improved corrosion resistance was known¹⁾. To analysis only the effect of phosphorus, we use Fe‑P binary alloy. In this study, we assessed pitting initiation of Fe‑P binary alloy using microelectrochemical measurement.

Two types of heat-treatment were conducted to Fe-P binary alloy. The one was heat-treated at 1100 ℃ at 1 h and quenched in water. The other was tempered at 1373 K for 1 h. As-tempered steel was aimed to precipitate of Fe₃P. The specimen surface was polished down to 1 μm using a diamond paste. Pure iron was used as reference materials.

Potentiodynamic anodic polarization curves were measured in Boric-Borate buffer with 5 mM NaCl solution. The pH value of the solution was adjusted to 7.0. Size of electrode areas were 1 cm × 1 cm, 1 mm × 1 mm, and 0.5 mm × 0.5 mm.

When the size of electrode area was 1 cm × 1 cm, pitting potential of Fe-P was increased above pure iron. In 1 mm × 1 mm electrode area, as-tempered steel was shown the higher pitting potential than as-quenched steel. Pitting was only observed from edge of masking. In 0.5 mm × 0.5 mm electrode area, no pitting was observed on both as‑quenched and as- tempered steel.

Reference;

1) Kadowaki, I. Muto, Y Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 165, C711 (2018).