Volatility Diagrams for the Cr-O and Cr-Cl Systems: Application to Removal of Cr2O3-Rich Passive Films on Stainless Steel

摘要

Effective diffusional surface treatments of stainless steels require that the naturally forming Cr2O3-rich passive layer be removed to “activate” or depassivate the surface. Volatility diagrams can be used to understand the possible etching reactions in the Cr-O and Cr-Cl systems and reveal five effective methods for removal of Cr2O3-based passivating films: (1) exposure to acetylene (C2H2) at 673 K (400 °C) and higher temperatures (providing sooting is avoided); (2) exposure to atomic hydrogen at 10 to 0.001 kPa (0.1 to 0.0001 bar) at 373 K to 673 K (100 °C to 400 °C); (3) exposure to wet oxygen above 573 K (300 °C), forming the volatile species CrO2(OH)2; (4) exposure to gaseous HCl at 100 kPa (1 bar) above 473 K (200 °C); and (5) oxidation of Cr2O3 to CrO3 using ozone or atomic oxygen, followed by exposure of CrO3 to gaseous H2 or HCl. The last process takes advantage of the fact that CrO3 is removed more effectively using gaseous H2 and HCl than is Cr2O3.