Effects of Na2O-SiO2 molten oxide on Cu precipitation at steel/scale interface

The formation of liquid oxide phases within the oxide scale may suppress copper precipitation at the steel/scale interface during high-temperature oxidation which causes hot shortness. In this study, the quantitative effect of liquid oxide phases in the scale on copper precipitation behavior at the steel/scale interface was investigated. Oxidation experiments were performed at 1373 K in air using Cu-containing steel samples with 25Na2O-75SiO2 and 40Na2O-60SiO2 (mol%) powders applied to their surfaces. The applied powders formed liquid oxide phases within the scale during oxidation, exhibiting morphologies similar to the FeO-Fe2SiO4 liquid phase observed in actual scales. A novel method using an embedded Pt wire as a reference was developed to measure the oxidation amount as the iron loss on the powder-applied surface. The amount of copper precipitated at the steel/scale interface was quantitatively evaluated by image analysis and compared with the oxidation amount. The results revealed that samples with a liquid oxide phase formation exhibited a significantly lower amount of copper precipitation per oxidation amount. Furthermore, Cu was detected in the liquid oxide phase at up to 5 mol% (cation ratio). These findings indicate that the liquid oxide phase suppresses interfacial copper precipitation by promoting the transfer of Cu from the steel into the scale. The suppression effect was less effective for 25Na2O-75SiO2, which is attributed to greater amount of liquid oxide phase transformed into Fe2SiO4 within the scale.