This chapter describes the general corrosion processes of iron and carbon steel (not stainless steels) in aqueous environments. Of particular interest is the formation of the oxide film and the effects of system variables on the corrosion process.

EO 1.7 LIST the two conditions that contribute to general corrosion.

EO 1.8 DESCRIBE how the rate of corrosion occurring in the plant is effected by the following:

a. Temperature

b. Water velocity

c. Oxygen

d. pH

e. Condition and composition of the metal surface

f. Dissolved solids

EO 1.9 LIST the three products that are formed from the general corrosion of iron.

EO 1.10 IDENTIFY the action taken for initial fill of a reactor system to limit general corrosion.

EO 1.11 STATE the four methods used to chemically control general plant corrosion.

EO1.12 LIST the six water chemistry conditions that limit corrosion of aluminum.

Conditions Contributing to General Corrosion

General corrosion is the process whereby the surface of a metal undergoes a slow, relatively uniform, removal of material. The two conditions typically required for a metal to undergo general corrosion are: 1) metal and water in the same environment, and 2) a chemical reaction between the metal and water that forms an oxide.

Corrosion of Iron

Unless noted otherwise, the following discussion applies to deaerated water at room temperature and approximately neutral pH. The affects of temperature, oxygen, and pH are discussed later in this chapter.

The oxidation and reduction half-reactions in the corrosion of iron are as follows.

The overall reaction is the sum of these half-reactions.

The Fe⁺² ions readily combine with OR ions at the metal surface, first forming Fe(OH)₂, which decomposes to FeO.

Ferrous oxide (FeO) then forms a layer on the surface of the metal. Below about 1000F, however, FeO is unstable and undergoes further oxidation.

Atomic hydrogen then reacts to form molecular hydrogen, as described previously, and a layer of ferric oxide (Fe₂0₃) builds up on the FeO layer. Between these two layers is another layer that has the apparent composition Fe₃0_a. It is believed that Fe₃0₄ is a distinct crystalline state composed of O^-2, Fe ⁺², and Fe⁺² in proportions so that the apparent composition is Fe₃0₄. These three layers are illustrated in Figure 5.

Once the oxide film begins to form, the metal surface is no longer in direct contact with the aqueous environment. For further corrosion to occur, the reactants must diffuse through the oxide barrier. It is believed that the oxidation step, Equation (2-3), occurs at the metal-oxide interface. The Fe⁺² ions and electrons then diffuse through the oxide layer toward the oxide-water interface. Eventually, Fe⁺² ions encounter OH^- ions and form FeO. The electrons participate in the reduction reaction with hydronium ions. These latter reactions are believed to take place predominately at the oxide-water interface, but some reaction may occur within the oxide layer by the diffusion of H⁺, OH^-, and H₂0 into the layer.

Figure 5 Simplified Schematic Diagram of Oxide Corrosion Film on the Surface of a Metal

Regardless of the exact diffusion mechanism, the oxide layer represents a barrier to continued corrosion and tends to slow the corrosion rate. The exact effect of this layer on the corrosion rate depends on the uniformity and tenacity of the film. If the film is loosely attached, develops defects, or is removed, the metal surface is again exposed to the environment and corrosion occurs more readily.