Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 914-918
Published Online September 2012 (
Atmospheric Corrosion Studies of Ductile Iron and
Austenitic Stainless Steel in an
Extreme Marine Environment
Olasupo Ogundare1*, Babaniyi Babatope2, Adelana Razaq Adetunji3,4,
Samuel Olugbenga Oloruntoba Olusunle5
1Engineering Materials Development Institute, Akure, Nigeria
2Department of Physics, Obafemi Awolowo University, Ile-Ife, Nigeria
3Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria
4Prototype Engineering Development Institute, Ilesa, Nigeria
5National Agency for Science and Engineering Infrastructure, Abuja, Nigeria
Email: *
Received June 11, 2012; revised July 23, 2012; accepted August 17, 2012
This paper presents the corrosion characteristics and the accompanying changes in the microstructure of unalloyed duc-
tile iron (DI) and austenitic stainless steel (ASS) in table salt medium representing an upper limit in an extreme marine
environment. The individual corrosion rates of DI and ASS was evaluated for the maximum time period of 1200 hr.
Using the immersion test technique, the corrosion rate of DI was evaluated and found to be four-orders of magnitude
greater than that of ASS. The corrosion product morphologies of the DI showed that the nodular matrix was gradually
covered up as immersion time prog ressed while the corrosion channels and vo lume of pits that initially formed in ASS
respectively deepened and increased with increased exposure time. This work is important as a reference point for the
quantification of the corrosion effectiveness of alloying DI. Th e microstructur es of the corroded samples showed corr o-
sion initiation and gradual accumulation of corrosion products.
Keywords: Atmospheric Corrosion; Ductile Iron; Austenitic Stainless Steel; Sodium Chloride and Microstructure
1. Introduction
The importance of austenitic stainless steel (ASS) in in-
dustrial applications and development cannot be over-
emphasized. Its excellent properties which range from
high tensile strength, good impact, corrosion and wear
resistances have found various applications in many in-
dustries. This material is used in almost all environments
that require an optimization of these properties, some of
which are low and high pressure boilers and v essels, fos-
sil-fired power plant, flue gas desulphurization equip-
ment, evaporator tubing, super heater reheating tubing
and steam headers and pipes to mention but a few [1-3].
ASS is known for its corrosion resistance principally due
to the presence of chro mium which is soluble in the aus-
tenitic matrix. Chromium adds to the overall corrosion
resistance through a passivation process by forming a
complex spinel-type {(Fe,Ni)O(Fe,Cr)2O3} passive film
[4-6]. This produces a coherent, adherent insulating and
regenerating chromium oxide protective film on the
metal surface. The corrosion behavior of 18/8 stainless
steel and nickel-plated low carbon steel in cassava fluid
has been investigated [7]. Very large amounts of carbon
steels are generally used in marine applications, such as
construction, nuclear and fossil fuel power plants, che-
mical processing, mining and transportation. Carbon ste-
els are primarily affected by general corrosion and they
are prone to deleterious corrosion by seawater [8,9].
Ductile Iron (DI) consists of graphite in the form of
nodules or spheroids in a matrixof either ferrite or pear-
lite [10,11]. DI is not a single material bu t part of a group
of materials which can be produced to have a wide range
of properties through control of the microstructure. The
common defining characteristic of this group of materials
is the morphological dominance of the graphite structure.
In DIs, the graphite is in the form of spherical nodules
rather than flakes (as in grey iron), thus inhibiting the
creation of cracks and providing the enhanced ductility
that gives the alloy its name. The formation of nodules is
achieved by addition of nodularizing elements into the
melt especially magnesium and less often, cerium [12].
Yttrium has also been studied as a possible nodularizer
*Corresponding author.
Copyright © 2012 SciRes. JMMCE
[12]. In contrast, the effect of chloride salts on the me-
chanical properties of gray cast iron has been recently
reported [13]. The high temperature corrosion of two
ductile cast irons (Si-Mo and Ni-Resist Alloys) in syn-
thetic diesel and gasoline exhaust gases has also previ-
ously been reported [14]. Cast iron deterioration with time
in various aqueous salt solutions has also been studied [15].
Seawater is a complex electrolytic solution, which has
contributed to the corrosion of ocean oilrigs, water trans-
port vessels (i.e. ships), tools, chemical plants, etc. It is a
complex solution because it contains about 92 different
chemical elements, the most common of which is NaCl.
The salinity of water is 35 part per thousand in which 35
parts of salt is found in 1000 parts of seawater [16].
Corrosion studies of metallic structur es remain a major
area of interest for scientific investigation. Recent effort
in the study of mild steel and AISI 304L stainless steel in
the presence of dissolved ions in seawater was recently
reported [17] in which the Copper ions were found to
have pronounced effect on corrosion rate.
In this work, we compare the corrosion rates of DI and
ASS as a measure of the suitability or otherwise of duc-
tile iron for marine environment applications with refer-
ence to ASS, an established corrosion resistant material.
It is also to develop necessary experimental data to serve
as a reference point for the corrosion of alloyed DIs, a
major ongoing research program. The medium used in
this work is a replica of extreme and “upper limit” “sea-
side” conditions. This is to allow us generate the highest
possible corrosion response of the metallic structures
under investigation in this type of high chloride profile
2. Materials and Method
The plain DI used for this research work was produced
with a uniform, well-graded and clean scrap following
processing procedure already optimized at the Engineer-
ing Materials Development Institute, Akure, Nig eria with
EMR-100 Rotary Furnace developed at the institute and
already in commercial production. An effort in this di-
rection has been used for smelting white cast-iron and
low alloy cast-carbon steels [18]. The DI test specimens
were fabricated by sand casting while the ASS is the
classic 18/8 type referred to as “A2” in accordance to
ISO 3506. The materials were spectrometrically analyzed
with a XRF Spectrometer. With the spectrometer, analy-
sis of up to 41 elements isobtainable within 2 minutes
except for light elements such as Al, Mg and Si. The
comparative composition by weight percent of the two
materials under investigation is as given in Table 1.
The DI and ASS were cut into 20 mm × 20 mm × 10
mm and 20 mm × 20 mm × 1 mm coupons respectively
(Figures 1(a) and (b)). The coupons surfaces polished to
600 grit, thoroughly washed under tap, swabbed with
acetone and then dried. Subsequently, the initial weight
of the coupons was measured on a metler balance to
0.001 g accuracy. Each coupon was weighed and the
dimensions carefully taken and recorded before exposure
to the test media for a period of 1200 hr. Ten coupons
each of both materials was fully immersed in table salt
placed in separate containers. On the completion of each
exposure test, th e coupons were cleaned with wir e brush,
rinsed und er tap and air dried pr ior to second we igh ing of
samples to determine weight losses due to corrosion. This
procedure is in consonance with ASTM G1-90 [19]. The
corrosion rate of each coupon was calculated and the cou-
pon examined under a software-driven opti cal microscope.
A small piece was cut from each coupon before the
corrosion test to serve as a control specimen. It was
mounted and mechanically ground progressively on
grades of SiC impregnated emery paper (120 - 1200 grits)
with water as the coolant. The ground coupon was then
Table 1. Comparative chemical composition of DI and ASS.
% composition of a l l oying elem e n ts
Element Fe C Cr Ni Mo Si Mn P S Nb Mg
DI (wt%) 82.242 3.600
0.014 2.000 0.300 0.007 0.010 0.050
ASS (wt%) 68.495 0.080 18.000 8.000 2.500 0.750 2.000 0.045 0.030 0.100
(a) (b)
Figure 1. (a) DI coupons; (b) ASS coupons after the experiment.
Copyright © 2012 SciRes. JMMCE
polished with 1.0 microns diamond polishing paste fol-
lowed by 0.5 microns paste. The micrograph of each
polished coupon showing the respective microstructural
feature was also obtained.
3. Results and Discussion
The corrosion rate was computed in mils per year (mpy)
with the standard expression [20-22].
where W is weight loss in mg, ρ is the density of the
coupons in g/cm3, A is the exposed area in square inch
and T is the exposure time in hours [20]. This shows that
corrosion rate is linearly proportional to the weight loss.
Figure 2 shows the plots of weight loss of DI and ASS
with time after immersion in table salt for 1200 hr.
However, it can be observed that the weight loss in duc-
tile iron increased progressively whereas the opposite
effect is observed for ASS as it progressively decreased.
This response can be attributed to increased anodic dis-
solution of the DI coupon in NaCl. This is in contrast to
the anodic passivation in ASS due to the presence of
chromium in the austenitic matrix [4-6]. From the che-
mical composition of DI, (Table 1), molybdenum is in
traces and chromium which could have enhanced corro-
sion resistance is conspicuously absent in the matrix of
plain DI. When the ASS and DI coupons were physically
inspected, no significant change in appearance was ob-
served in ASS coupons (Figure 1(b)). However, the DI
coupons (Figure 1(a) were observed to be substantially
corroded during the respective immersion time periods.
Figure 3 shows the comparative plo ts of the corrosion
rates of DI and ASS in sodium chloride as time changes.
A decrease in corrosion rate of for ASS with increasing
immersion time was also observed following similar pat-
tern when compared to the loss in weight. This effect
could be attributed to the adherent thin passive film of
chromium oxide formed on the metal surface [4-6]. This
prevents the solution from contacting the metal itself.
The observations made in respect of the weight loss
(Figure 2) is in agreement with that made for the corro-
sion rate (Figure 3) which is consistent with previously
reported works [4-6]. It has previously been observed
that that the presence of chromium alloying element adds
to the overall resistance throug h a passivation process by
forming a complex spinel-type {(Fe,Ni)O(Fe,Cr)2O3} pas-
sive film. This complex produces a coherent, adherent
insulating and regenerating chromium oxide protective
film on the metal surface; while molybdenum increases
the ability of stainless steel to resist the localized corro-
sion in aggressive ion environments [7].
Figure 4 shows the microstructures of DI before and
after immersion in NaCl at immersion times of 120 hr,
720 hr and 1200 hr depicting the corrosion product mor-
phologies at the onset, middle and end of the experiment.
Figure 2. Weight loss of DI and ASS in NaCl. Figure 3. Corrosion rate of DI and ASS in NaCl.
Before exposure After 120 Hr After 720 Hr After 1200 Hr
Figure4. Microstructures of DI before and after immersion in sodium chloride (400×).
Copyright © 2012 SciRes. JMMCE
Before exposure After 120 Hrs After 720 Hrs After 1200 Hrs
Figure 5. Microstructures of ASS before and after immersion in sodium chloride (400×).
The microstructures of DI (Figure 5) showed that the
corrosion produ cts gradually covered up the nodular ma-
trix as the immersion time increased. Initially, the cou-
pons revealed evenly distributed nodules in the pear-
lite/ferrite matrix of ductile iron.
Figure 5 shows the microstructures of ASS before and
after immersion in NaCl at times 0 hr, 120 hr, 720 hr and
1200 hr. It shows the morphologies of the surface corro-
sion product in which points of possible initial pitting
corrosion are revealed. The volume of pits formed on
each ASS coupon also increased progressively with time.
4. Conclusion
This paper has reported the study of corrosion behavior
of DI and ASS in table salt representing an extreme ma-
rine environment as the media. After 1200 hr, the corro-
sion rate of DI was 1.1 × 10–4 mpy compared to that of
ASS which was 6.8 × 10–7 mpy. The fact that ASS out-
performed DI has been established but what this work
has established is an almost 4 orders of magnitude dif-
ference in corrosion rate between the two materials. This
is not unconnected with the presence of chromium in
ASS coupon s which greatly adds to the overall cor rosion
resistance through a passivation process by forming a
complex spinel-type {(Fe,Ni)O(Fe,Cr)2O3} passive film.
While corrosion rate of DI increased progressively with
immersion time that of ASS decreased at comparable
immersion time periods.
5. Acknowledgements
The authors appreciate the support provided by the man-
agement of Engineering Materials Dev elopment Institu te,
Akure, Nigeria where the entire bench work was carried
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