Hydrochloric acid pickling reactions

Hydrochloric acid is of great importance and is widely used in industrial processes. The largest use is in the metallurgical industry for the pickling of metals to remove oxide scales. Other uses include the production of inorganic halides from metals, oxides or carbonates, extraction of metals from ores in metallurgical processes, and various organic reactions. 

Most pickling inhibitors function by forming an adsorbed layer on the metal surface, probably no more than a monolayer in thickness, which essentially blocks discharge of H+ and dissolution of metal ions. For example, both iodide and quinoline are reported to inhibit corrosion of iron in hydrochloric acid by this mechanism [19]. Some inhibitors block the cathodic reaction (raise hydrogen overpotential) more than the anodic reaction, or vice versa but adsorption appears to be general over all the surface rather than at specific anodic or cathodic sites, and both reactions tend to be retarded. Hence, on addition of an inhibitor to an acid, the corrosion potential of steel is not greatly altered (<0.1 V), although the corrosion rate may be appreciably reduced (Fig. 17.3). 

About half of the hydrochloric acid produced annually in the United States (3.0 billion pounds) is used in metal pickling. This process involves the removal of metal oxide layers from metal surfaces to prepare them for coating, (a) Write the overall and net ionic equations for the reaction between iron(in) oxide, which represents the rust layer over iron, and HCl. Identify the Brpnsted acid and Br0nsted base, (b) Hydrochloric acid is also used to remove scale (which is mostly CaC03) from water... 

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