Page 50 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

28 INTRODUCTION AND FORMS OF CORROSION

improved the repassivation behavior without any effect on pit initiation. Total depen-
dence on metallurgical factors was noted in the corrosion resistance of aluminum
alloys (42).
1.5.1.7 Internal or Subsurface Attack (Oxidation) It is usually identified by sim-
ple visual observation of the sample surface. However, subsurface phenomena within
the matrix of the alloy sample, as well as obscured relations at the interface of the alloy
with the surface films, formed in high temperatures may be noted as in Figure 1.11.
Electrochemical corrosion at high temperatures at the interface also involves the dif-
fusion of the aggressive gas phase to the vulnerable phase in the subsurface, leading
to corrosion most of the time.

1.5.1.8 Dealloying or Selective Dissolution Dealloying is a corrosion process
involving the selective dissolution of one or more elements, leaving behind a porous
residue of the remaining elements. This phenomenon is also known as selective
leaching or parting corrosion and is a corrosion process in which the more active
metal is selectively removed from the alloy, leaving behind a porous weak deposit
of the more noble metal. For example, selective leaching of zinc from brass is
known as dezincification. In the case of gray iron, dealloying is known as graphitic
corrosion (9). Dealloying can occur in nearly any system in which a large difference
in equilibrium potential between the alloying components and the fraction of the
less noble component(s) exists and is significantly high (4).

1.5.1.9 Dezincification Copper–zinc alloys containing more than 15% of zinc are
susceptible to dezincification. The dezincification of brass involves selective removal
of zinc leaving behind a porous and weak layer of copper and copper oxide. Corro-
sion of a similar form leads to replacement of sound brass by weak porous copper.
The uniform dealloying in admiralty brass (dezincification) is shown in Figure 1.12
(8, 4, 43, 44).
Brass is only one strong phase of dissolved copper and zinc. Under certain
conditions, preferential dissolution of brass occurs. This dezincification can be
localized (plug dezincification) (Fig. 1.11) or more uniformly distributed (layer
dezincification).
Dezincification of -brass can be minimized by adding 1% tin and further inhibited
by the addition of less than 0.1% of arsenic, antimony, or phosphorus. When dezin-
cification is a problem, red brass, commercial bronze, inhibited admiralty metal, and
inhibited brass can be successfully used.

1.5.1.10 Graphitic Corrosion This form of corrosion is observed on buried
pipelines after many years of service. Gray cast iron consists of continuous graphite
network in its microstructure that is cathodic to iron and remains behind as a weak
porous mass as the iron is selectively removed from the alloy (4). This form of
corrosion (graphitic corrosion) can be reduced by the use of ductile or alloyed iron
in place of gray iron, changing the environment such as raising the pH to greater
than 7, the use of corrosion inhibitors, avoiding stagnant conditions, and use of
cathodic protection.