Page 24 - Concrete-Technology-Interacted_Book_Prof-MIK_Neat

Reinforcement corrosion 9/3

Positive ions Positive ions

O2 + 2H2O + 4e– → 4OH– Fe → 2e2+ O2 + 2H2O + 4e– → 4OH–
e– e–

Cathode Anode Cathode

Figure 9.3 Schematic diagram of the electrochemical processes occurring during corrosion.

where oxidation occurs is called an anode. For example, iron dissolves (is oxidized) to
form positive iron ions. The residual electrons in the metal are consumed by a reaction
which produces a more negative product. Such a reaction is termed reduction and the
location where reduction occurs is called the cathode. For example, oxygen is reduced to
form negative hydroxyl ions. An electronic conductor on which an oxidation or reduction
reaction occurs is termed an electrode.

Between the anode and cathode, ionic and electronic current must flow to prevent the
build-up of charge. The ionic conductor is termed an electrolyte and is usually an aqueous
solution of ions (e.g. Fe2+, OH–, Na+, Cl–). The positive ions are termed cations and the
negative ions are anions. The metal provides the electronic conductor. If there is no
external source of electrons (provided by an external power supply), the anodic oxidation
reaction must generate electrons at exactly the same rate as the cathodic reduction reaction
consumes them and the flow of electrons in the metal and ions in the environment
between the anode and cathode must prevent any accumulation of electric charge.

Two important ionic species present in all aqueous environments are hydrogen ions
(H+) and hydroxyl ions (OH–). When combined these ions produce water. Most common
cathodic reactions result in the generation of hydroxyl ions. An excess of hydroxyl ions
is termed alkalinity (a more alkaline environment is produced at the cathode). By contrast,
the iron ions produced at the anode may react with water (hydrolysis) to produce hydrogen
ions (acidity) which will lead to acidification of the local environment at the anode. The
processes involved in the formation of rust include the precipitation of iron ions as iron
hydroxide and the oxidation of the iron hydroxide by further reaction with oxygen.

A potential difference (voltage) is the driving force for an electrochemical reaction. A
more positive potential results in a higher oxidation state and the release of more electrons
(positive ions are repelled from a more positive electrode surface). Likewise, a more
negative potential will drive negative ions away from an electrode. An oxygen electrode
has a more positive potential than an iron electrode. Thus, iron dissolves as positive ions
because its potential is raised by the couple to an oxygen electrode, and oxygen is
reduced to negative hydroxyl ions because its potential is lowered by the same couple.
The potential of an electrode supporting an electrochemical reaction can be measured
relative to a stable standard reference electrode.

The stability of compounds in a given environment and potential range is determined
by thermodynamics. In the case of iron, a number of possible electrochemical reactions
exist. The most stable products as a function of potential and pH for a fixed set of
environmental conditions are given in a Pourbaix diagram illustrated in Figure 9.4. This
diagram does not indicate the rate at which the most stable state will be achieved.

24