JANUARY 31 2003
Temperature. Temperature is a complex external variable. Generally, as
temperature increases, corrosion increases. However this also depends on availability
of oxygen. The corrosion rate of iron in a system closed to the atmosphere has been
shown to increase almost linearly with temperature from about 40 C to 160 C (105 F
to 320 F). However, in an open system, the corrosion rate increases up to about 80 C
(175 F) and then decreases. It should also be noted that the pH of a liquid will become
more acidic as the temperature is raised, also affecting the corrosion rate. For example,
25 parts-per-million alkalinity water at a pH of 9.4 at 26.7 C (80 F), has a pH of 8.0 at
93.3 C (200 F).
Ion Concentration. Ion concentration has an effect similar to pH, except
that the corrosion rates of some metals are affected by the presence of certain ions.
For example, aluminum corrodes not only by alkalis, but shows pronounced attack by
traces of copper ions in aqueous media and is subject to rapid attack by mercury metal,
mercury ions, and anhydrous chlorinated solvents. For iron, chloride and chloride ions
may increase the corrosion rate in some electrolytes. However, bromide and iodide
ions may inhibit corrosion (slow down the rate of corrosion). The effect of concentration
of one component on corrosion is often dependent on other environmental variables,
such as the presence of salts, availability of oxygen, and differences in oxygen
solubility, pH, and temperature.
Electron Concentration. High concentrations of free electrons in the
electrolyte around the cathode can inhibit corrosion by that saturation reaching the
maximum ion migration rate of the environment. Very low concentrations of free
electrons in the electrolyte around the cathode may increase the corrosion rate by the
chemical nature of equilibrium, where the electrons readily migrate into the
pH of the Electrolyte. The pH of the soil or water electrolyte in an
electrochemical corrosion cell affects the rate of the corrosion by speeding or slowing
the chemical reactions at the anode and/or the cathode. The pH of an electrolyte is
basically the concentration of hydrogen ions. A pH below 4 increases the corrosion rate
of mild steel. At a pH of 3 the corrosion rate increases tremendously. Amphoteric
metals also show an increase in the corrosion rate in alkaline environments. Aluminum
and lead are examples of amphoteric metals.