This study quantitatively analyzed rust-streak formation under controlled droplet supply and its relationship with the rust-removed surface profile of the substrate. A NaCl aqueous solution was dropped at a constant flow rate onto SPCC steel plates inclined at 70° to observe the temporal development of the rust streak. Surface line profiles before and after the removal of red rust were measured, and profile changes were quantified relative to the initial surface. Rust layer height hrustx and rust-removed surface profile zr*x were determined, and their distributions and integrated values were compared. The rust width reached approximately 2.5–3.0 mm, comparable to the droplet diameter under the present conditions. Downstream, rust layer height increased with the extension of test duration, whereas the integrated profile of the rust-removed surface remained relatively small. Rust layer height and rust-removed surface profile were not directly related at each observation position L. These results suggest that rust streak formation within the tested parameter window involves not only locally formed rust but also rust carried from upstream by liquid flow, and indicate that visible rust morphology alone cannot adequately represent substrate-side profile changes under these specific conditions.
Ishida et al. (Fri,) studied this question.