Page 273 - Corrosion Engineering Principles and Practice

246 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 247

7.5.1 Pitting Mitigation
Since pitting is an electrochemical process, it can be stopped by
cathodic protection. It can also be prevented by the use of inhibitors
to alter the electrode reactions of the local cell and remove their
driving force. In some cases, agitation of the environment will prevent
environmental differences from developing and will prevent pitting
that otherwise would occur.
Penetration by pitting is often prevented by coating the surface of
a metal with a sacrificial layer of another alloy, such as zinc on steel or
Alclad* on aluminum, or by applying protective coatings. A zinc-rich
paint is sacrificially active and will prevent the pitting of either steel
or aluminum.
Another way to prevent pitting corrosion is simply by choosing a
more corrosion resistant material. An empirical ranking scheme that
describes the corrosion pitting susceptibility of austenitic and duplex
stainless steels can provide some useful guidance. The pitting
resistance equivalent number (PREN) predicts the pitting resistance
of these alloys as a function of their content in chromium, molybdenum,
tungsten, and nitrogen [Eq. (7.2)]:

PREN = Cr + 3.3 (Mo 0.5 W) 16N (7.2)
+
+
7.5.2 Crevice Corrosion Mitigation
Crevice corrosion is ideally prevented at the design stage by avoiding
them in the first place or during construction by filling uncorroded
dry crevices with a durable jointing compound that will exclude
moisture and remain resilient.
Crevices are present in many types of equipment. Examples are
gasketed flanges, rolled joints between tubes and tube sheets in heat
exchangers, faying surfaces between tanks and supporting structures,
and so forth. Moisture and chemical solutions may be trapped within
crevices and held stagnant. The use of angle iron construction, skip
welding, high outlets in tank bottoms, “dead” flow areas in piping
stubs, heat exchangers, and vessels should be avoided. A number of
procedures may be used to avoid crevice corrosion at the design stage
(see Chap. 11 for more details on this topic).
1. Use butt joints in preference to lap joints
2. Eliminate or carefully seal lap joints so that they will not
“open up”

* A composite wrought product comprised of an aluminum alloy core having on
one or both surfaces a metallurgically bonded aluminum or aluminum alloy
coating that is anodic to the core and thus electrochemically protects the core
against corrosion.