Definition - What does Cathodic Pickling mean?
This is the chemical removal of high-temperature scale and the adjacent low-chromium layer of metal from stainless steel metal surfaces. After welding, or any other heat treatment method, a colored oxide layer forms, and underneath is the chromium-depleted layer on the surface of the steel. When the content of the chromium layer is low, resistance to corrosion also reduces. For the restoration of the full resistance condition, the layer must be removed to expose the pure alloy.
Cathodic pickling is a practice that is common on the stain steels to increase the effectiveness of pickling. In the process, electric current is used for the effective activation of the surfaces so that plating can be done satisfactorily.
Corrosionpedia explains Cathodic Pickling
Pickling starts with applying sulphuric acid to soften the scale for easier removal by subsequent pickling in nitric and hydrofluoric acids. These acids remove the scale and depleted layer of chromium to restore the resistance to corrosion. These solutions also remove ferrite oxides and other contaminants from the metal surfaces.
It should be noted that pickling should be carried out on fully annealed stainless steel. This is due to the risk of attack on the grain boundary. Since the process involves strong acids, proper and controlled handling of these acids should be followed and all pickling treatments done through rinsing. When using pickling paste, care should be taken since it is prepared through the mixture of several strong acids. It is applied on small, overhanging or vertical areas.
Pickling pastes, made by mixing with inert carriers, are used to treat some selected areas such as welds. This method also removes red rust that results from the corrosion of steel or contaminant iron or steel particles. Scale type and the method of removal are dependent on the grade of steel and the involved heating conditions.
Since pickling results in removal of the metal, the product's sheen becomes dull, and its size and dimensions are reduced.