8/7/2019 HCL Corrosion

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HCL Corrosion

The characteristics of an electrochemical reaction are that oxidation (or the generation of electrons) and

reduction (or consumption of electrons) both occur, and there is an electron transfer from the anode to the

cathode. The characteristics of an electrochemical reaction are illustrated by considering the behavior of ironin hydrochloric acid. Iron reacts vigorously with HCl; hydrogen is evolved and the iron gradually goes

completely into solution. The reaction is:

Fe + 2HCl_ FeCl2 + H2 (Eq 4)

The solid iron gradually disappears and a gas is evolved. This can be seen with the naked eye. The solution inthe above reaction is ionized and contains positively and negatively charged ions. The HCl contains hydrogen

ions (H+) and chloride ions (Cl). One drop of acid contains millions of each of these ions. Likewise, ferrous

chloride in solution can be considered as iron ions (Fe2+) and chloride ions (Cl). Therefore, Eq 4 can be

written as the following:

Fe + 2H+ + 2Cl_ Fe2+ + 2Cl+ H2 (Eq 5)

The iron converted to an iron with two positive charges. By definition, the iron is said to have been oxidized

(loss of electrons). On the other hand, the hydrogen ions have each gained an electron. By definition, they

have been reduced. The overall reaction can be considered as two separate ones:

Oxidation: Fe

_

Fe2+ + 2e(Eq 6)Principles of Aqueous Corrosion 77

Reduction: 2H+ + 2e_ H2 (Eq 7)Equations 6 and 7 both take place on the surface of the metal. The areas where oxidation occurs are defined as

anodes, and those where reduction takes place are defined as cathodes. An electrical potential exists

between the anode and cathode areas. The electrons produced in Eq 6 flow through the metal to the cathode

areas to take part in the reaction of Eq 7. Hydrogen ions in the vicinity of the anode areas are not needed

there, and they flow (under the influence of the potential difference) to the cathode to sustain the reduction

reaction. A complete electrical circuit exists, and a current flows from anode to cathode. The faster the

solid is converted to iron ions (i.e., the greater the corrosion), the larger is the current flowing in this corrosion

cell. The mechanism for the preceding case is shown schematically in Fig. 15. Many such corrosion cells

occur on a corroding metal surface. Anode and cathode sites can switch roles so that uniform corrosion can

occur. Equations 6 and 7 must occur at the same rate to conserve the electrical neutrality of the metal. If Eq 6

stops, then so must Eq 7, and vice versa.

In summary, corrosion in the electrochemical sense occurs by solid metal being oxidized to

positively charged metal ions in solution. This occurs at areas called anodes. The resulting excess of

electrons passes through the metal to surface areas called cathodes where electrons are removed bya reduction reaction. The corrodent must contain a species that can be reduced at the cathode and

ions capable of completing the electrical circuit between anode and cathode areas (i.e., the

corrodent must be an electrolyte). A current flows through the solution from anode to cathode areas.

As described subsequently, the driving force in the reaction is the electrical-potential difference thatcauses current flow between the anode and cathode.