The primary purpose of pickling is often to remove weld oxides. During hot pickling, also known as NOX pickling (pickling acid containing nitric acid HNO3 and hydrofluoric acid HF), the acid penetrates into the natural cracks beneath the oxide and primarily attacks the chromium-depleted steel, resulting in the formation of nitrous gases. These gases rupture the oxide layer in combination with the etching. In other words, nitric acid does not dissolve the oxide.
Hot pickling provides a uniform result across the entire surface. It’s not just oxides that are removed; the result is a uniformly fine steel-gray surface. With cold pickling, certain spots or uneven results can occur because primarily the oxides are removed, leaving the rest of the surface untouched. Hot pickling also has a degreasing effect due to the high temperature. High temperature leads to quicker and more efficient pickling with shorter lead times. Approximately 5 µm or 0.005 µm is removed across the entire surface. Therefore, embedded impurities, free iron, and residues from manufacturing are removed. This reduces the risk of unwanted corrosion and discoloration later.
Calamo has its own analytical equipment for quick analyses of pickling baths. This is especially important when specific requirements for removal and pickling results apply. It also reduces the risk of downtime while waiting for analysis results. Drying spots are a risk after pickling. To avoid this, thorough rinsing in different steps is required. Therefore, we have various rinsing steps with different purities and access to deionized water. This is particularly important when there are high purity requirements. Other factors affecting the result are the steel’s alloy. The more high-alloyed it is, the longer it takes.