General Corrosion Summary
The two conditions that contribute to general corrosion are:
Metal and water in the same environment
Chemical reaction between the metal and water to form an oxide
The corrosion rate is affected by the following:
A temperature rise in the range of 20?F to 50?F doubles the corrosion rate
until the formation of the protective oxide film is complete.
An extremely high water velocity, 30 to 40 ft per second, tends to remove
the oxide film allowing the corrosion rate to increase.
The presence of oxygen in water to which iron is exposed increases the
corrosion rate. The reason for the corrosion rate increase is due to the
rapid reaction between the oxygen and the polarizing layer of hydrogen
absorbed on the oxide layer.
A pH between 4 and 10 results in minimal corrosion rate. If the pH falls
below or above this range, the corrosion will normally increase.
The condition and composition of the metal surfaces affects the corrosion
rate. Deposits, scale, or irregular surfaces create areas on the metal where
local corrosion can initiate and proceed at a faster rate than normal.
Dissolved solids tend to make it easier for current to flow, which results
in a higher corrosion rate.
The three products formed from general corrosion of iron are FeO, Fe O , and
Fe O .
The major points of this chapter are summarized below.