Corrosion is common phenomenon for metals. Iron or steel when exposed to humid atmosphere the corrosion process is initiated resulting in rust formation.
Rust is the oxide of iron which is loosely adhered to the substrate and hence it is very dangerous if overcoated by any surface coating. Rust is readily soluble in acids such as Hydrochloric acid, Sulphuric acid, Phosphoric acid and hence it can be removed by acid cleaning.
De-Rusting can be done with the following acids/acid combinations:
Hydrochloric acid is fuming acid and mostly used for “Pickling”, as it readily dissolves the mill scales formed during the hot rolling operations. Hydrochloric acid pickling is the cheapest de-rusting process. The use of Hydrochloric acid is very limited in the industry due to the corrosive nature of the Hydrochloric acid fumes which can create problem in the coating plant and secondly the carry over of Hydrochloric acid to the next pre-treatment stage (i.e.) either activation or phosphating can damage the bath permanently. In case of Pure Hydrochloric acid de-rusting the tank should be of Stainless Steel.
Sulphuric Acid is a strongest acid and it is most suitable for the heavily rusted components. Sulphuric Acid at an elevated temp.(50-60ºC) gives outstanding results in a short time. The use of hot process is limited in the industry due to the corrosive nature of Sulphuric acid fumes. The use of inhibitor is must in hot process to avoid the excess arrack of acid on the base metal. Carry over of Sulphuric Acid bath in phosphating bath can permanently damage the phosphating bath. For Sulphuric Acid based de-rusting the tank should be of either stainless steel or Mild steel with lead lining.
Phosphoric acid based de-rusting process is more popular in the finishing indusry due to its following advantages:
Phosphoric acid based de-rusting is costly as compared with Hydrochloric acid or Sulphuric acid. For Phosphoric acid based de-rusting, the bath should be of Stainless steel.