13.5 CORROSION AND CORROSION PROTECTION
13.5.1 Corrosion Mechanism
Corrosion is the destructive attack, or deterioration, of a metal by chemical or electrochemical reaction with its
environment. Corrosive attack of metals is an electrochemical process that is represented in Fig. 13.14.
In a galvanic cell, two dissimilar metals (e.g., iron and copper) are placed in electrical contact in the presence of
oxygen and moisture. Separate chemical reactions take place at the surfaces of the two metals, creating a flow of
electrons through the connecting wire. At the iron surface, or anode, oxidation of iron takes place in accordance
with the following anodic reaction:
2Fe - 4 electrons  2Fe++.
(13.2)
At the copper surface, or cathode, reduction of oxygen occurs in accordance with
the following cathodic reaction:
02 + 2H20 + 4 electrons  4OH-.
(13.3)
The actual loss of metal involved in the process takes place at the anode, as indicated by Eq. 13.2. The iron atoms
are transformed to ferrous ions (Fe++) which dissolve in the solution around the anode. They may diffuse and
combine with the hydroxyl ions (OH-), with the precipitation of ferrous hydroxide [Fe(OH) 2] in accordance with
the following net redox reaction:
2Fe + 02 + 2H20  2Fe(OH)2.
(13.4)
The hydrous ferrous oxide formed according to Eq. 13.4 (FeOH20) is further oxidized to form hydrous ferric
oxide (Fe203 . nH20), which is rust.
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Figure 13.14
Schematic representation the corrosion process: (a) simulation by a galvanic cell, (b) corrosion on metal surface
exposed to a humid environment.
Iron will corrode without the presence of a separate cathodic metal. Anode-cathode pairs can be set up on a steel
surface where different sites have different electrochemical potentials or tendencies for oxidation. An electrical
potential difference between possible anode and cathode sites can be the result of differences in composition,
differences in residual strain, or differences in oxygen or electrolyte concentrations in contact with the surface.
13.5.2 Forms of Corrosion
1.
General corrosion: General corrosion or rusting is the most familiar form of steel corrosion.
It can be considered a uniform corrosion process in
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which numerous microcorrosion cells are activated at the corroded area. The cells could be minute grains
where the boundary tends to be the anode, for example.
In atmospheric exposures, oxygen in the air is the usual oxidizing agent, and the water necessary
for the reaction is readily available in the form of rain, condensation (dew, for example), or humidity (water vapor
in the air). In the rusting of ordinary steel, the corrosion product (rust) does not form an effective barrier to further
corrosion, but permits reactants to penetrate to the steel surface beneath and continue the rusting cycle.
2.
Pitting corrosion: Ibis is a nonuniform, highly localized form of corrosion that occurs at distinct spots
where deep pits form. (A pit is a small electrochemical-corrosion cell, with the bottom of the pit acting as the
anode.) Chloride-induced corrosion is of this type and can be seen frequently in structures exposed in coastal
areas.
3.
Galvanic corrosion: When two metals of different electrochemical potential are joined or coupled
electrically in the presence of moisture or an aqueous solution, one will act as the anode and corrode; the corrosion
of steel when it is in contact with copper is a familiar example. This principle is used to advantage when steel is
protected by galvanic methods (for example, galvanized steel or the use of other surficial anodes).
4.
Stress-corrosion: Under stress, corrosion processes proceed much faster and can lead to brittle failure as
corrosion tends to be localized. Corrosion of this kind can occur in prestressing tendons in concrete.
5.
Crevice corrosion: This form occurs when moisture and contaminants retained in crevices accelerate
corrosion.
13.5.3 Corrosion Control
For most applications of structural steel, some form of corrosion control is essential, as discussed next.
Protective Coatings
Paint applied to steel functions as a barrier between the steel and the atmosphere, thereby preventing attack as long
as the coating is intact. Epoxy coatings on reinforcing bars serve the same purpose, but may not perform as
expected due to defects in the coating.
Galvanic Protection
Hot-dip galvanizing is a process in which an adherent, protective coating of zinc or zinc-iron compounds is
developed on the surfaces of iron and steel products by immersing them in a bath of molten zinc, whereas
metallizing involves the application of zinc onto the steel surface by means of a flame spray gun. The usefulness
of zinc coatings as corrosion protection depends on (1) the barrier effect of zinc and its surface oxide film, (2) the
relatively low rate of corrosion of zinc as compared with that of iron or steel, and (3) the electrolytic, or sacrificial,
protection afforded to iron by zinc (i.e., the preferential oxidation of zinc at normal atmospheric temperatures,
which acts as the anode relative to the steel).
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Cathodic Protection
This method is used for structures located below ground or immersed in water, usually in conjunction with a
protective coating. Because corrosion results from, or is accompanied by, a flow of electrical current between
anodic and cathodic surfaces, it is possible to reduce or eliminate it by controlling the magnitude and direction of
current flow. By reversing the current to the original anodic steel surface, the steel is made a cathode and does not
corrode. A reverse-current flow is obtained either by electrically connecting the steel structure to a metal of higher
electromotive energy (commonly zinc or magnesium, in the form of sacrificial anodes) or by artificially
impressing a direct current from an outside source (for example, a power line and a rectifier). Effective protection
is provided as long as the proper reverse-current flow is maintained. A protective coating, such as asphalt, tar, or
an epoxy, is commonly applied to the structure to reduce power consumption.
Corrosion-resistant Steels
These steels contain a combination of alloying elements selected to provide a special type of oxide coating after
prolonged exposure to the atmosphere (weathering). They usually contain copper and develop a resistance to
atmospheric corrosion from four to eight times that of a plain-carbon steel. In addition to copper, phosphorus,
chromium, nickel, and silicon are among the elements added (usually in combination) to achieve this special
corrosion resistance. On exposure to the atmosphere, these steels gradually develop a tightly adhering oxide
coating that acts as a barrier to moisture and oxygen and eventually almost prevents further corrosion.
Furthermore, if this coating is damaged, it will heal itself.