Improvement of the cleanability of pseudo-sensitized stainless steel surfaces by solution treatment.

Type 316L stainless steel particles were pseudo-sensitized by being heated at 700 degrees C for 100 h under a reduced pressure of 1.3 x 10(-3) Pa. Pseudo-sensitization treatment resulted in the formation of chromium-rich precipitates at the outermost surfaces of the stainless steel particles. The curve for the apparent surface charge density (sigma app) of the pseudo-sensitized particles showed a more basic character compared with the curve for the sigma app of the original particles. There was no significant difference between the amounts of bovine serum albumin (BSA) adsorbed on the original and the pseudo-sensitized particles. On the contrary, the efficiency of BSA removal from the pseudo-sensitized particles during batch-wise and continuous cleaning operations with 0.1M NaOH solution were significantly smaller than that for the original particles. It was indicated that the extremely lowered cleanability of the pseudo-sensitized particles was caused by a marked increase in the positive sigma app values after the pseudo-sensitization. Solution treatment of the pseudo-sensitized particles at 1,050 degrees C for 1 h under a reduced pressure of 1.3 x 10(-3) Pa could put again the chromium precipitates into a solid solution, thereby returning the surface chemical composition and the sigma app nearly to the original states. In addition, the solution-treated particles showed a good cleanability comparable to that of the original particles. These results demonstrated that solution treatment could improve the lowered cleanability of the pseudo-sensitized stainless steel surfaces.