1.What is the principle behind acid pickling?
Pickling works by using a chemical reaction to remove iron oxide scale (rust) from the surface of hot-rolled strip steel. Under normal circumstances, this reaction essentially stops after the iron oxide scale is removed, thanks to the protection of corrosion inhibitors and process control. However, if the pickling time is too long, the acid concentration is too high, or the temperature is too high, the chemical reaction will continue to attack the steel matrix itself, causing metal dissolution and loss.
2.What are the specific signs of over-pickling?
Complete thinning: The surface of the steel plate is excessively corroded, appearing grayish-black, rough, and dull. In severe cases, it may develop pitting or tiny holes (pitting corrosion). This corrosion evenly "eats up" a layer of metal, causing the actual thickness of the entire steel coil to be lower than the standard requirement.
3.Will excessive pickling lead to localized thinning?
For certain special steel grades, such as duplex stainless steel, improper pickling processes can even exacerbate the thinning of the material. Patent literature explicitly states that when producing thin-gauge duplex stainless steel, poorly controlled pickling processes can "easily lead to problems of width reduction and thickness thinning," ultimately resulting in high scrap rates and low yield.
4.What is the order of magnitude of thickness reduction?
The thickness of the oxide scale on the surface of normal hot-rolled strip steel is generally between 7 and 20 micrometers. Proper pickling should only remove this thickness of oxide layer. Over-pickling, however, dissolves an additional layer of the steel matrix, which can range from a few micrometers to tens of micrometers. For precision cold-rolled products, this is sufficient to cause the thickness to exceed tolerances and result in scrapping.
5.What is the core of acid pickling?
The core of pickling lies in "precision" and "moderation." Once it gets out of control and becomes "over-pickling," it is no longer a simple surface cleaning process, but an uncontrolled metal corrosion process, inevitably resulting in thinning of the steel plate and surface damage. Therefore, in industrial production, the pickling process must be controlled through strict process parameters to avoid this situation.