Acid pickling of carbon steel

Abstract

This study reviews the possibilities of recovering the pickling waters from carbon and galvanised steel. Acid pickling with hydrochloric acid (HCl) is the most widely used chemical process to remove iron oxides from the metal surface without any significant attack on the steel itself. The acid pickling bath contains mainly ferrous chloride (FeCl2) produced by the reaction between the steel and free hydrochloric acid. However, zinc chloride (ZnCl2) is also found in the pickling of carbon steel parts prior to galvanisation, as the hooks and tools used to hang the carbon steel parts are also galvanised and reuse again polluting with Zn the pickling waters. Pickling water recovery or recycling technologies primarily seek the reuse of HCl in two ways. Partially by recovering the unreacted HCl or fully by breaking the FeCl2 bond through Pyrolysis technologies such as fluidised bed and spray roasting which in turn produces another iron oxide by-product. However, the most common by-product produced by pickling water recovery and recycling technologies is ferric chloride (FeCl3), as it is a coagulant widely used in wastewater treatment. However, if the pickling water contains ZnCl2 or other metals, the production of FeCl3 becomes unattractive and the pickling water is neutralised and deposited in landfill sites. This study also discusses a wide range of technologies capable of recovering all or part of the pickling water, including galvanic pickling water, that are usually excluded from circular economy strategies.

Comparison of Different Surface Pre-Treatment on Mild Steel for Cobalt-Nickel-Iron Electroplating

“…Pickling provides deep cleaning while also activating the metal surface by removing the oxide layer from the surface of the material. The pickling process has resulted in a pure metal surface, which is free of contaminants and oxides [14]. Subsequently, the specimens were immersed in different solutions for varying periods, as shown in Table 1.…”

“…The most common pickling used in mild steel surface treatment is corrosive fluid such as hydrochloride acid [11]. Another corrosive acid used in pickling could be sulfuric acid [12], or a mixture of acids with toxic inhibitors [13]- [14]. Alkaline such as ammonia fluoride was also proposed in the pre-treatment of mild steel.…”