1.What does insufficient pickling mean?
This refers to the presence of residual iron oxide scale on the surface of the steel strip after pickling. Essentially, it means the strip wasn't properly cleaned; the residue consists of iron oxides and appears as black or dark-colored spots or patches.
2.What does rust (corrosion) refer to?
This refers to the rust layer that reappears on steel strips after pickling, due to some reason. Essentially, it's "re-corrosion," a chemical reaction between the fresh steel substrate and moisture, oxygen, and other elements.
3.Why does insufficient pickling lead to "secondary corrosion"?
Principle: A properly pickled steel sheet has a clean, grayish-white or silvery-white metallic surface, facilitating subsequent passivation and oiling for rust prevention. However, in under-pickled areas, the residual iron oxide scale is loose and porous. When oiling is applied for rust prevention, the oil cannot adhere evenly and firmly to the iron oxide scale, and may even be absorbed by it.
Consequence: These areas lose the protection of the rust-preventive oil, becoming entry points for rust. During subsequent storage or transportation, moisture and oxygen easily penetrate through these weak points, causing the steel strip to rust again. In production practice, this re-emerging rust layer after pickling is called a "rust" defect.
4.What causes "indentation rust spots"?
Principle: The cold rolling process involves high-pressure rolling of the pickled steel strip. If residual iron oxide scale (under-pickling) remains on the strip surface, it will be pressed into the strip matrix under rolling pressure due to its hardness, brittleness, and poor ductility, forming black spots or black streaks.
Consequences: This pressed-in scale damages the integrity of the steel strip surface, resulting in weak adhesion to the matrix and easy detachment, forming pits. More importantly, it damages the protective layer on the finished steel surface, making these areas highly susceptible to re-rusting during storage, potentially leading to product scrap.
5.What is the relationship between insufficient pickling and rusting?
Insufficient pickling itself does not equate to rusting. It simply means that the iron oxide scale was not completely removed.
Insufficient pickling is a major cause of rusting. It undermines the effectiveness of subsequent rust-prevention treatments and allows the scale to be pressed into the surface during rolling, directly leading to defects on the finished product surface that make it prone to rusting.