Civil Engineering Reference
In-Depth Information
Water
Corrosion Scale
Me
+
OH
-
OH
-
e
-
+
1
/
4
O
2
+
1
/
2
H
2
O
Me
+
Cathode
e
-
Anode
Base Metal
Fig. 21-1.
Schematic of a corrosion cell
service life and utility of the metal. In generic terms, we speak of corrosion mor-
phology as being either uniform or localized.
Localized Corrosion
In localized corrosion the anodic processes are fixed in one
location on the plumbing surface. Metal oxidation occurs only in that area, with the
surrounding area serving as the site of the cathodic reactions. In copper plumbing
systems, this type of corrosion is often called pitting corrosion and it can lead to rapid
penetration of the pipe wall and leakage. In some circumstances, penetration is com-
plete within three months of the inception of the pitting process (see Figs. 21-2 and
21-3). On iron and steel systems, penetration of the pipe wall is of concern, but an
even greater problem is the tuberculation that accompanies the localized corrosion
processes. The tuberculation is the result of the accumulation of corrosion products in
the immediate vicinity of the localized corrosion. The tubercles generally have a dis-
tinctive structure, can be quite large, and ultimately occlude the pipe dramatically,
reducing flows (see Fig. 21-4). The intense localized corrosion produces heavy metal
loss. Oxidized metal in different mineral forms often accumulates immediately above
the pit site, creating a tubercle.
Pipe wall penetration can be rapid, often requiring less than six months. The pro-
tected and largely enclosed area of the pit creates a chemical environment that pro-
motes the pitting process; hence passivation processes that would retard corrosion are
not effective, and the pit becomes self-propagating. The chemistry of the pit also leads
to the formation of unusual mineral deposits, as seen in the micrograph. The tuber-
culation on this pipe section has developed over many years. Due to occlusion and
increased surface roughness, the hydraulic capacity of the line has been reduced by
over half.
Uniform Corrosion
As the name implies, on some metals the corrosion process
appears to be uniformly distributed over the entire surface of the metal so that it is
not possible to distinguish distinctive anodic or cathodic sites. When uniform corrosion
occurs, it is because the metal is acting as a ''polyelectrode,'' meaning that any one
