Page 237 - Corrosion Engineering Principles and Practice

212 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 213

Dissolution dominant
The oxide film grows in static aqueous solutions according to the oxide growth
kinetics. Corrosion rate is a function of the bare metal dissolution rate and
passivation rate. The corrosion kinetics follows a parabolic time law.
Flow thins film to an equilibrium thickness that is a function of both the mass
Mechanical damage increases time law. Dissolution increases
transfer rate and oxide growth kinetics. The FAC rate is a function of the mass
transfer and the concentration driving force. The FAC kinetics follows a linear
The film is locally removed by either surface shear stress or dissolution or
particle impact, but it can be repassivated. The damage rate is a function of the
bare metal dissolution rate, passivation rate and the frequency of oxide
removal. The damage kinetics follows a quasi-linear time law.
The film is locally removed by dissolution or surface shear stress and the
damage rate is equivalent to the bare metal dissolution rate. The damage
kinetics follow a quasi-linear time law.
The film is locally removed and the underlying metal surface is “mechanically
damaged’’ which contributes to the overall loss rate, that is, the damage rate is
equal to the bare metal dissolution rate plus a possible synergistic effect due to
the mechanical damage. The damage rate follows a nonlinear time law.
The oxide film is removed and mechanical damage to the underlying metal is the
dominant damage mechanism. The erosion kinetics follow a nonlinear time law.
Mechanical-damage dominant
FIGURE 7.3 Summary of damage mechanisms experienced with FAC [5].

in trace quantities may accelerate the corrosion attack while at other
times they may act as inhibitors. The introduction of small amounts
of ions of metals such as copper, lead, or mercury can cause severe
corrosion of aluminum equipment, for example, corrosion of
upstream copper alloy equipment can result in contamination of
flowing water in a cooling circuit. In this example, copper can plate
out on aluminum surfaces downstream as small nodules or deposits
setting up local galvanic cells, which can result in severe pitting and
perhaps perforation.
There are many examples of the catastrophic role played by
impurities in the process industries. Conventional corrosion data on
sulfuric acid media, for example, are often based on tests in
chemically pure acid or on field exposures of indeterminate
chemistry and the effects of contaminants are often overlooked.
Serious problems can therefore arise in seemingly straightforward
applications [7].
In the manufacturing of concentrated acid by air combustion of
various feedstocks, small amounts of nitric oxides may be introduced,
depending on burner design, temperature, and so forth. Also, nitric
acid is sometimes added to sulfuric acid as antifreeze. The presence
of nitric acid can promote the oxidation of the ferrous sulfate film
which otherwise protects steel in static exposures. In glacial acetic
acid, corrosion of type 316L can be markedly increased by as little as
a few parts per million of acetic anhydride. Apparently, the anhydride
scavenges the water required to maintain a passive film.