When steel is heated during annealing or welding, an outer layer called "oxide scale" typically forms. Oxide scale will always form unless the steel is fully protected against oxidation when heated above its reaction temperature. Rust in the environment is usually considered a separate issue. Once oxide scale forms on the steel surface, it typically requires a comprehensive descaling process to remove it.
Chemical Descaling While oxide scale can be removed mechanically, chemical methods are the most common. Chemical descaling is typically very thorough. Several commonly used acidic solutions are employed for descaling.
These solutions usually require heating to improve quality and efficiency. Regardless of the chemical method used, monitoring and controlling the solution's temperature and concentration is crucial.
Mechanical Descaling Although chemical descaling is more common, many companies also employ mechanical descaling processes. This process typically involves bombarding the oxide scale with particulate matter, commonly known as "sandblasting." There are three main types of sandblasting: sand, steel grit, and steel shot. The primary difference between these three methods lies in the type of medium. Shot blasting is generally the most controllable and effective method. It should not be confused with shot peening.
Post-treatment After descaling, steel is susceptible to corrosion. Due to the clean surface and direct exposure to the environment, corrosion is highly likely. Various coatings can be used to prevent re-oxidation of newly descaled steel. The most commonly used and effective coatings include lime, polymers, and oils. Each coating has its advantages and disadvantages, depending on the application.
Chemical Descaling vs. Mechanical Descaling In the past, chemical descaling was considered more efficient than mechanical descaling. However, new shot peening technologies have narrowed the efficiency gap between the two descaling processes. Many forging companies now prefer shot peening because it avoids the corrosive solutions commonly used in chemical descaling. Shot peening is a line-of-sight operation, meaning the shot must travel a specific path to reach the target surface. For products with hard-to-reach areas, such as coils, chemical descaling can be challenging, while this problem is generally not an issue with chemical descaling.
