Civil Engineering Reference
In-Depth Information
Fig. 21-4.
Tuberculation on a cast-iron pipe
site on the metal surface may be an anode one instant and a cathode the next. Why
this should happen is not well understood, but it clearly relates to the heterogeneity
in the crystalline structure of the metal and possible rapid variations in oxidant and
reductant concentrations on the metal surface.
Uniform corrosion is generally a desirable condition, since the metal loss from the
plumbing systems is distributed over the entire surface, ensuring a relatively long
service life. Corrosion scales that develop on these surfaces are usually compact and
evenly distributed. Unlike the tuberculation associated with localized corrosion, they
do not reduce the hydraulic capacity of the plumbing system. Often, the scales on
uniformly corroding surfaces contribute to the passivation of the surface and serve to
limit the underlying corrosion processes (see Fig. 21-5). Metal systems that frequently
corrode in a uniform pattern include copper, lead, zinc, and most bimetallic solders.
Effective passivation layers need only be a few microns thick. The passivation layer
on this copper specimen consists largely of the cuprous and cupric oxides (Cu
2
O and
CuO).
Pourbaix Diagrams
The most important chemical factors influencing the corrosion of a particular metal in
an aqueous environment are the pH of the water and the electrical (redox) potential
established between the water and the corrosion surface. These two parameters largely
determine the magnitude of the driving force for corrosion, as well as which corrosion
products are thermodynamically stable. Using electrochemical principles outside the
scope of this chapter, it is possible to graph the relationship between electrical potential
for a particular metal and the solution pH. Such a graph is known as a Pourbaix
