Influence of Surface Segregation of Tin on the Phosphatability of Cold‐Rolled Steel Sheets

Abstract

The increasing use of iron scrap to reduce greenhouse gas emissions results in the enrichment of steel with the tramp element Sn. However, the effect of Sn on phosphatase activity remains unclear. Therefore, this study aims to investigate the effect of the Sn content on the phosphatase activity of cold‐rolled steel plates. The phosphatability of the cold‐rolled steel sheets is evaluated by adding several tens of ppm of Sn. The surface characteristics of the materials are analyzed using X‐Ray photoelectron spectroscopy and transmission electron microscopy. Additionally, the reactivities of the materials in the phosphate treatment solution are assessed using an electrochemical evaluation method. Notably, a significant decrease in phosphatase activity is observed, which is attributed to the inhibition of the phosphate reaction by the Sn oxide layers formed on the steel sheet surface, with a layer thickness of a few nanometers. These Sn surface oxides can be removed by pickling with HCl, and the initial phosphate reaction occurs rapidly in areas where the oxides are removed.