Page 304 - Failure Analysis Case Studies II

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3.1. Calcanic corrosion
The mixture of copper pipes and steel radiators makes up a galvanic couple. The Pourbaix
diagram for copper is shown in Appendix A. Copper will be immune from corrosion in mains water
as long as its potential is kept below 0 V. This is 0.6 V higher than the corrosion potential of iron.
The copper is in excellent electrical contact with the steel through soldered joints or compression
fittings. Because of this, the iron corrodes in preference to the copper, and acts as a sacrificial anode.
The copper is protected by cathodic protection and provides an inert surface at which the cathodic
oxygen-reduction reaction takes place. In practice, the extent of galvanic corrosion is limited by the
ionic conductivity of the water. If the conductivity is low, the electrical resistance of the galvanic
cell is high, and the steel will only corrode galvanically near to a copper surface. On the other hand,
if the conductivity is high galvanic attack will occur some distance away from the copper. The
solution conductivity can be decreased considerably if insoluble corrosion products form.
There is another way in which the copper pipes can lead to galvanic corrosion. The bore of the
tubes corrodes slowly, and releases copper ions into the circulating water. At steady state, it is quite
common to find concentrations of Cu2+ of 0.14,2mgl-’ [4]. Because steel is more reactive than
copper, the following reaction takes place where the water passes over the steel surfaces:
Cu2+ + Fe = Fe2+ + Cu. (1)
The copper metal produced by the reaction deposits on the surface of the steel as an extremely
thin layer. Galvanic cells are then set up between the islands of deposited copper and the steel in
between [3, 61. However, opinions vary as to how important this mechanism really is.

3.2. Dissolved ions
Mains water typically contains approximately 50ppm C1- and SO:- in association with Na+,
Ca2+ and Mg2+. The ions increase the conductivity of the water, and help the corrosion processes.
The metal ions migrate to the cathodic surfaces where they neutralize the OH- ions produced by
the oxygen-reduction reaction. The C1- ions are small and highly mobile: they migrate rapidly to
the anodic areas, and neutralize the Fe2+ ions produced when the iron dissolves. When mains water
enters the system to make up for evaporation, it brings dissolved ions with it, and over time the
concentration of the ions in the circulating water will increase. Both CI- and SO:- are aggressive
ions: they help to stop passive oxide films forming on the surface of steel, and this encourages
corrosion even more.

3.3. Pitting
The surface of the steel tends to divide itself into anodic areas (where the iron corrodes) and
cathodic areas (where the oxygen-reduction reaction takes place). This separation is encouraged by
anything which makes the environment of the steel non-uniform. In most places, the metal is
exposed to the flowing water (and the oxygen it contains), and behaves cathodically. However, steel
in crevices has almost no exposure to oxygen, and behaves anodically. Crevices are present in many
places: under deposits of sludge, next to welds, and in screwed connections. Obviously, the current
of electrons produced by the anodic areas must balance the current of electrons delivered to the
cathodic area. Since the anodes are small compared to the cathodes, the current density at the
anodes will be large compared to the current density at the cathodes. The steel will corrode rapidly
over a small area, and localized pits will form.

4. REDUCTION OF HYDROGEN

Figure 7 shows the line for the hydrogen-reduction reaction superposed on the Pourbaix diagram
for iron. The open-circuit potential for the hydrogen-reduction reaction in mains water is -0.4V.
The voltage difference available to drive the corrosion process is -0.4 - (- 0.6) = 0.2 V. The hydro-
gen-reduction reaction polarizes rapidly: an “overvoltage” is needed to make the reaction go at a
reasonable rate. The extent of the polarization depends strongly on the metal or alloy concerned,