Page 198 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

176 CORROSION CAUSES

Isolated deep pits in partially passivated areas and/or impingement attack in
essentially passivation-free areas are typical of NAC. The damage is in the form
of unexpected high corrosion rates on alloys that would normally be expected to
resist sulfidic corrosion. In many cases, even very highly alloyed steels (i.e., 12 Cr,
AISI types 316 and 317) have been found to exhibit sensitivity to corrosion under
these conditions. NAC differs from sulfidic corrosion by the nature of pitting and
impingement and the severe attack at high velocities in crude distillation units. Crude
feedstock heaters, furnaces, transfer lines, feed and reflux sections of columns,
atmospheric and vacuum column heat exchangers, and condensers are the types of
equipment subject to pitting and impingement attack.
Sulfidic attack occurs because of the presence of various forms such as elemental
sulfur, hydrogen sulfide, mercaptans, sulfides, and polysulfides. Sulfur is the second
most abundant element in petroleum. Sulfur at a level of 0.2% and greater is known
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to be corrosive to carbon and low-alloy steels at temperatures from 230 to 455 C
∘
(446–851 F).
At high temperatures, especially in furnaces and transfer lines, the presence of
naphthenic acids may increase the severity of sulfidic corrosion. The presence of the
organic acids may disrupt the sulfide film and thereby promote sulfidic corrosion
of the alloys that are normally expected to resist this form of attack. The alloys in
question are 12 Cr and higher. In some cases, such as in side-cut piping, the sulfide
film formed because of hydrogen sulfide is thought to offer some degree of protection
from NAC.
In general, the corrosion rates of all alloys in the distillation units increase with
an increase in temperature. NAC occurs primarily in high-velocity areas of crude
∘
∘
distillation units in the 220–400 C (430–750 F) temperature range. No observable
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∘
corrosion damage is usually found at temperatures greater than 400 C (750 F) prob-
ably because of the decomposition of naphthenic acids or protection from the coke
formed at the metal surface.
Velocity, and more importantly, wall shear stress are the important parameters
affecting NAC.
Fluid flow velocity lacks predictive capabilities. Data related to fluid flow param-
eters, such as wall shear stress, and the Reynolds Number are more accurate because
the density and viscosity of liquid and vapor in the pipe, the degree of vaporization
in the pipe, and the pipe diameter are also taken into account. Corrosion rates are
directly proportional to sheer stress. Typically, the higher the acid content, the greater
the sensitivity to velocity. When combined with high temperature and high velocity,
even very low levels of naphthenic acid may result in very high corrosion rates.

3.24.5 Corrosion-Related Failure in Refineries
Corrosion-related failure (32) that occurred in refineries of the Union Oil Company of
California resulted in a disastrous explosion and fire. An amine absorber pressure ves-
sel ruptured and released large amounts of flammable gases and vapors. The accident
resulted in 17 deaths; 17 individuals were hospitalized; and more than $100 million
in damages resulted.