Pickling cathodic

Pickling—a form of chemical and electrolytic removal of mill scale and corrosion products from the surfaces of metals in an acidic solution. Electrolytic pickling may be anodic or cathodic, depending on the polarization of the metal in the solution. 

Although the majority of the hydrogen produced on the cathodic areas is evolved as gas and assists the removal of scale, some of it diffuses into the steel in the atomic form and can render it brittle. With hardened or high-carbon steels the brittleness may be so pronounced that cracks appear during pickling. Austenitic steels, however, are not so subject to embrittlement. 

Degreasing by cathodic treatment in a hot alkaline solution is desirable when the copper contains inclusions of cuprous oxide. Pickling may be carried out in either cold 50% hydrochloric acid or 20% v/v nitric acid, or in hot (70°C) 6% v/v sulphuric acid containing 1.5% w/w sodium dichromate. 

Anodic and cathodic pickling are forms of eiectroiytic pickling in which the metal is anodically or cathodically polarised in the pickle. 

Various pre-treatment protocols have been developed including pickling and anodic/cathodic pulses to remove the oxide films. It was apparent that different types of steel require different pre-treatments, i.e. cast pieces behave differently to rolled pieces. Significant success was achieved in electropolishing cast pieces and the finish obtained with the ionic liquid was superior to that with phosphoric add, however, the converse was true for rolled pieces because the oxide film is thicker in the latter samples and hence slower to dissolve in the ionic liquid. 

Summary Bleach can be made using an electrochemical process whereby pickling salt is electrolyzed. During the process, chlorine gas is evolved at the anode, and sodium hydroxide is liberated at the cathode. As the process proceeds, the chlorine reacts with the sodium hydroxide forming sodium hypochlorite (bleach). Some chlorine gas does escape, so use proper ventilation when carrying out this operation. 

Rolled or pickled steels corrode faster than machined or polished steels. The mill scale on the sample must be removed before exposure to avoid the possibility of mill scale acting as cathode. The mill scale may initially give protection against atmospheric corrosion, but scale tends to crack and spall under shock conditions, resulting in bare spots on the sample exposed to the atmosphere. The vertical plates are attacked more slowly than plates at 45°. 

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