O. O. AJIDE, K. W. AGARA 837
revealed that in both aerobic and anaerobic exposures,
corrosion was more aggressive on horizontally oriented
coupons compared to vertically oriented samples. The
corrosion behaviour of low carbon steel was investigated
in natural seawater and various synthetic seawaters as
reported by [5] in 2006. It was found that the steel cor-
roded nearly four times faster in a 3.5% NaCl solution
than in natural seawater for an exposure time of 21 days.
The corrosion rate after immersion in synthetic seawaters
(ASTM D1141 and Marine Biological Laboratory sea-
water) is similar to the corrosion rate after immersion in
natural seawater. Calcium carbonate (aragonite) deposits
were found on the surface of the steel after immersion in
natural seawater and the synthetic seawaters. Some
magnesium-containing deposits were also found after
immersion in the natural seawater. These deposits act as
a barrier against oxygen diffusion and thereby lower the
corrosion rate. The morphology of the calcium carbonate
deposits that formed during immersion in the natural
seawater is different from those formed during immer-
sion in the solution. [6] studied the electrochemical cor-
rosion behaviour of carbon steel X60 using the electro-
chemical impedance spectroscopy and potentiodynamic
polarization methods. 0.5 M test solutions of sodium
chloride, sodium sulphate and sulphuric acid were used
in a three electrode open cell. The findings of the authors
showed that the rate of corrosion penetration is higher for
carbon steel X60 in 0.5 M sulphuric acid and smaller in
0.5 M sodium sulphate. [7] studied the corrosion behav-
iour of steel for snow and rockfall barriers using electro-
chemical techniques to quantify corrosion rate. His re-
sults generally showed that steels have high corrosion
rate in these media. Synthetic seawaters. This may ex-
plain the slightly lower corrosion rates obtained in the
natural seawater. X-ray diffraction also showed that the
oxy-hydroxides formed in the 3.5% NaCl solution dif-
fered from those formed in the other solutions. [8] inves-
tigated the corrosion behaviour of carbon steel in alkaline
medium in the presence of very low concentration of
polymeric nanoaggregates (0.0024 wt% polyethylene
oxide-PEO-113-b-PS70 micelles). The steel electrodes
were investigated in chloride-free and chloride-contain-
ing cement extracts. The electrochemical measurements,
electrochemical impedance spectroscopy and potentio-
dynamic polarization indicate that the presence of mi-
celles alters the composition of the surface layer and in-
fluences the electrochemical behaviour of the steel. Au-
thors’ observation shows that micelles initially improved
the corrosion resistance of the steel whereas no signifi-
cant improvement was observed within longer immersion
periods. Surface analysis, performed by environmental
scanning electronic microscopy, energy-dispersive X-ray
analysis and X-ray photoelectron spectroscopy supports
and elucidates the corrosion performance characteristics
of carbon steel in simulated pore solution in the presence
of Micelles. [9] in 2011 did a comparative study of cor-
rosion resistance between 316 and different duplex
stainless steel grades. He examined and compared the
corrosion properties of 316L austenitic stainless steel and
duplex grades of LDX2101, SAF2304, AL2003, LDX-
2404, 2505 and 2507.The stainless steels were given heat
treatment at temperature of 800˚C for 30 minutes. Influence
of heat treatment on pitting susceptibility of stainless
steels was estimated using cyclic polarization scan which
is based on ASTM standard G150. Metallurgical analysis
was conducted to find a correlation between microstruc-
ture and pitting resistance. Light microscope was used
for the examination of stainless steel microstructure. In
addition, test samples were examined virtually after pit-
ting tests and critical pitting test to determine the corro-
sion form which was present. The results of his research
showed that pitting corrosion resistance and critical pit-
ting temperature (CPT) values of heat treated highly al-
loyed steels were affected adversely compared with the
results from non-heat treated materials. The author af-
firmed that for stainless steel alloys, the results can be
attributed to metallurgical aspects such as sigma, chro-
mium nitrides, secondary austenite and etc. The author
concluded that the precipitations have significant effects
on corrosion behaviour in stainless steel alloys. [10] in-
vestigated the influence of CO2 on the corrosion behav-
iour of 13Cr martensitic stainless steel AISI 420 and
low-alloyed steel AISI 4140 exposed to saline aquifer
water environment. In order to guarantee the safety of the
site, CO2-corrosion of the injection pipe steels has to be
given special attention. To get to know the corrosion
behaviour samples of the heat treated steel AISI 4140,
42CrMo4, used for casing, and the martensitic stainless
injection pipe steel AISI 420, X46Cr13 were kept at T =
60˚C and p = 1 - 60 bar for 700 h - 8000 h in a CO2-satu-
rated synthetic aquifer environment similar to the geo-
logical CCS-site at Ketzin, Germany .The isothermal
corrosion behaviour obtained by mass gain of the steels
in the gas phase, the liquid phase and the intermediate
phase gives surface corrosion rates around 0.1 to 0.8
mm/year. This implies that Severe pit corrosion with pit
heights around 4.5 mm are only located on the AISI 420
steel. Main phase of the continuous complicated multi-
layered carbonate/oxide structure is siderite FeCO3 in
both types of steel. The corrosion of 18-8 stainless steel
in sodium chloride solutions was studied by [11]. It was
established that under certain conditions, 18-8 stainless
steel is likely to fail in contact with sodium chloride so-
lutions through formation of deep pits. 200 series
stainless steel is currently of great interest to material
researchers, engineers and steel vendors due to its dis-
tinctive mechanical characteristics and acceptable corro-
sion behaviour. According to [12], new grades of the 200
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