A. Zagórski1, H. Matysiak1, Z. Słobodian2, O. Zvirko2, H. Nykyforchyn2,
CORROSION DEGRADATION OF OIL STORAGE TANK
1 Faculty of Materials Science and Engineering, Warsaw University of Technology
141, Woloska St., 02-507 Warszawa, Poland
2 Department of Corrosion-Hydrogen Degradation and Material Protection, Karpenko Physico-Mechanical
Institute of NASU
5, Naukova St., 79601 Lviv, Ukraine
State of the inner surface of the oil storage tank after its long term service was investigated and
numerous local corrosion damages were revealed, especially on the tank head due a condensation of a moisture
and light crude corrosion active components and on low parts, which were in contact with water deposited on the
bottom. Existence of the aggressive bottom water is the main reason of corrosion damages of the storage tank
steel in oil environment. Corrosion resistance of the different parts of storage tank after its long-term service is
investigated. Differences between corrosion resistance of the different parts of tank after its long-term service
was revealed and it are result of the different preliminary steel degradation. It was established that the tank
material contacted only with oil during service, is characterized by the highest corrosion resistance in the bottom
water and oil-water emulsion. On another hand, the material from tank wall closer to bottom, which was in
contact with bottom water for a long time, has the highest corrosion rate in this environment. Corrosion rate
levels of the different parts of the tank are graded in oil-water emulsion. It was shown that the stationary
potential of the different steel tank parts is sensitive to corrosion degradation of metal and in generally correlates
with corrosion resistance of these parts. Thus, the stationary potential can be taken as parameter, sensitive to the
degradation of exploited steel and it can be used as nondestructive method to control the metal state.
Key words: oil storage tank, steel, corrosion, stationary potential, degradation, oil environment,
In recent years, a significant problem connected with exploitation of oil storage tanks has been
stated. Numerous corrosion damages on the inner surfaces of the tanks fragments were observed .
The highest density of corrosion pits was revealed in the material of: (a) tank covers exposed to
moisture and volatile aggressive components of oil, and (b) tank bottom part that is in contact with so-
called deposited water. (Such water is present as a separate phase in liophobic water-oil emulsion).
Aggressiveness of deposited water is caused by acids and salts dissolved in it and soluble in
oil. The highest concentration of the substances dissolved in water is present under the oil layer, what
speeds up corrosion in the elements exposed to “oil-water” phase boundary . Also the degradation
phenomena taking place on inner surfaces of oil pipelines were explained by the aggressive effect of
the deposited water . Corrosion rates measured for the samples cut out from the bottom fragments
of pipes exposed to long-time service conditions were significantly higher than for the same fragments
of “virgin” pipes. Hypothesis has been put forward  that this phenomenon is caused by the
sustained combined effect of mechanical loading and the hydrogen absorbed in the metal. The
increased concentration of hydrogen in the lower fragment of the pipe has been confirmed
experimentally . Results presented in the study  indicate that the mechanical loading and
aggressive environment of the deposited water may lead to serious degradation during operation of oil
The goal of examinations undertaken within the present study was to recognize phenomena
connected with corrosion degradation of material of the oil storage tank.
MATERIAL AND METHODS
Plain carbon steel (designated as St3S in the Polish code) has been used for manufacturing an
oil storage tank decommissioned after over 25 years in service. Samples of the material in question
have been cut out from the following zones of the tank:
Zone I – upper part of the wall periodically exposed to condensed vapours of oil and water
Zone II – wall of the tank in constant contact with oil,
Zone III – lower part of wall exposed to deposited water,
Zone IV – tank bottom in constant contact with deposited water.
Corrosion resistance tests were undertaken in the environment of the deposited water (pH =
6,5 – 6,6) taken from two oil tanks being in service (diphase systems “oil–water” and “oil–water
emulsion in 1:1 proportion) using gravimetric method. The time of exposure of the samples was 7
days. Results of experiments were averaged and presented in the form of the relationship to the density
of corrosion defects (K) and corrosion pits (P) per surface units. Electrochemical measurements of
stationary potential of the material of samples exposed to different corrosion environments were also
In addition to corrosion resistance test, mechanical properties of the specimens have been
evaluated by measurements of hardness, impact strength and tensile strength.
RESULTS AND DISCUSSION
Surfaces of all examined samples of the tank were covered with a layer of corrosion products
loosely bound with the substrate metal. The corrosion of the tank wall is uniform. On the other hand
tank cover, bottom and walls in the vicinity reveal pitting corrosion. With the depth of pits reaching
several millimetres, it is evident that the material of zones I, III and IV is exposed too much more
aggressive in-service environment.
Despite differences in the corrosion damage, no significant differences in the values of yield
point for the different tank sections were observed. However, other mechanical parameters show
systematic variations listed in Table 1. Specimens representative of the material of Zone II, is
characterized by the lowest hardness and the highest impact strength. On the other hand, the material
of Zone III is characterized by the highest crack sensitivity.
Mechanical properties measured for the specimens representative for the four sections of the tank
distinguished in the present study
Yield point, MPa
Impact strength, J/cm2
In the course of the corrosion examinations, significant differences in the corrosion rates of the
material of different zones in the deposited water environment were found (see Table 2). Zone II, tank
wall being in constant contact with oil, is characterized by the highest corrosion resistance.
Measurements of stationary potential of the specimens representative for different tank zones,
carried out in the deposited water environment (with respect to silver-silver chloride electrode), show
that there is a correlation between potential value and the material degradation degree. Values of the
potential of material in particular zones are as follows: Zone II (-360 mV) < Zone I (-445mV) < Zone
II (-460mV) < Zone IV (-490 mV). It can be concluded that the potential values correspond to
corrosion resistance of material. In view of this, the stationary potential seems to be a useful factor
describing a degree of metal degradation during operation of the tank.
It should be also noted that no significant differences between corrosion rates of material of
different tank zones in diphase system “oil-water” (without mixing of environment) and in deposited
water have been observed. Table 2
Corrosion rate indicators (K) and (P)
Material of Zone II was characterized by the lowest corrosion rate, whereas the highest has
been revealed for the material of Zone III. It can be explained partly as the effect of mechanical
loading which generates higher stresses in the Zone III.
Cyclic filling and emptying of oil storage tanks causes formation of the oil-water emulsion.
Hence measurements of corrosion rates of the material of different zones of the tank in the oil-water
emulsion were also carried out. The magnetic mixer was used to produce the emulsion. As indicated
by the values listed in Table 2, the corrosion rates in such conditions are insignificant and differences
in the corrosion resistance of the material of particular zones decay. It is probably caused by the
corrosion inhibition due to oil micelles in the dynamic environment.
In the course of examinations, differences in the corrosion resistance of the material from
different zones of oil storage tank were revealed. It was established that the material of tank being in
contact with oil is characterized by the lowest corrosion rates during later exposition in deposited
water environment and in the oil-water emulsion. The lowest corrosion resistance is characteristic for
the material of the tank bottom, where the deposited water is present in in-service environment. The
corrosion rate of material fragments is the lowest in the oil-water emulsion, and in such case there are
no significant differences of corrosion rates of various sections of the tank.
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