A spool of steel coil is an impressive sight: thousands of pounds of material, having been forced through immense pressure to form smooth, thin, gleaming metal. But this is the end product ready for sale, after the finishing process. Upon being freshly rolled, you may be surprised to find the steel’s appearance to be much more rough! This can be due to various causes from staining to rust, but most often the metal’s unpolished look will be due to mill scale. Mill scale, also referred to as scale, is a mixture of iron oxide residues which cling to the metal’s surface after rolling. It is typically a dark grayish color, with a rough and flaky texture.
Not only is scale unattractive-looking, it becomes a nuisance when left on the metal. Any coating applied over scale will be rough and uneven, and vulnerable to wear. Once water seeps under the scale, it will flake and fall off. So the effort to paint the metal is wasted; not only will the bare patch need repainting, but other scaly areas will eventually flake off and require repainting as well. For these reasons, scale is usually removed by the manufacturer before being sold.
One of the most common methods of scale removal for steel is pickling and oiling. This involves a lengthy multi-step procedure, but at its most basic, the metal is immersed in “pickle liquor” and oiled it as the final step. So what exactly happens during the pickling process?
- Loading: the material is carefully arranged on racks. Crowding the material, or allowing pieces to overlap, means the solution will not be able to reach all surfaces evenly.
- Cleaning: the rack of steel is immersed in a highly alkaline cleaning solution, which will remove dirt and oil. While this step will clear the metal of surface debris, scale still remains on the metal.
- Rinsing: the steel is carefully and thoroughly rinsed with water to remove the cleansing solution. This also helps to raise its pH level prior to the pickling.
- Pickling: the rack is then lowered into a bath of hydrochloric acid, referred to as “pickle liquor”. The immersion in the potent acid effectively eats away at the bits of scale, as well as improving discoloration on the metal’s surface.
- Second Rinsing: immediately after the pickling, the steel is rinsed to cleanse it of the acid.
- Second Cleaning: the metal is again placed in an alkaline cleansing solution. This will neutralize any remaining acid residue from pickling.
- Final Rinsing: the rack is removed from the cleaner and given one last thorough rinse.
- Oiling: after pickling and rinsing, the metal will now have the smooth and shiny appearance we associate with steel. However, if we were to stop at this step, the steel would again be vulnerable to the accumulation of surface debris. The freshly cleaned surfaces are also now more fully exposed to air, which makes it more likely to rust. So the final step of the process is oiling, which protects the metal and provides a barrier to air and contamination. This involves placing the rack of steel in an oil bath to give it an overall coating. Whether using mineral oil or a water-based oil, the cover will help preserve the steel from developing flash rust while in storage. Once the metal is selected for additional fabrication, the oil will be removed via cleansing.
As you can tell, the pickling process is lengthy and work-intensive. However, it’s also a necessary step to prevent oxidation and to prepare the metal for later processing. Without pickling, leveling a scale-covered coil to sheet would result in a product of subpar quality. And just as pickling a cucumber helps to preserve the vegetables’ shelf life, metal pickling and oiling does the same for the material. The manufacturer may not need to process their coil immediately, but it would be unfortunate to discover their stored stock has experienced “spoilage”: rust development and other damage. Properly pickled and oiled, the corrosion of your steel will be prevented for a much longer period of time.