International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 03, March 2019, pp. 404-411. Article ID: IJMET_10_03_041
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=3
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
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COMPARISON OF BIOCORROSION BEHAVIOR
OF STAINLESS STEEL 316 L AND MILD STEEL
INDUCED BY SLIME PRODUCING BACTERIA
Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara
Syazrinni Rooshde
School of Ocean Engineering,
Universiti Malaysia Terengganu, Terengganu, Malaysia
Mohd Sabri Mohd Ghazali
School of Fundamental Science,
Universiti Malaysia Terengganu, Terengganu, Malaysia
ABSTRACT
The present study investigated the biocorrosion behaviour of Stainless steel 316L
(SS 316L) and mild steel coupons exposed to a medium containing slime producing
bacteria, Escherichia coli (E.coli). Corrosion analysis was performed to observe the
progression of corrosion process on tested steels induced by E.coli and its metabolite
activity for 7 days. The findings revealed that both SS 316L and mild steel experienced
severe pitting corrosion. The colonization of E.coli and secretion of exopolymeric
substances or slime have accelerated corrosion process on mild steel surface and
damaged the passivation layer on SS 316L. As a result, the tested mild steel specimen
exhibited 25% higher corrosion rate compared to that of the SS 316L.
Keywords: Biocorrosion, SS 316L, Mild steel and E.coli.
Cite this Article: Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin
Ibrahim, Maishara Syazrinni Rooshde, Mohd Sabri Mohd Ghazali, Comparison of
Biocorrosion Behavior of Stainless Steel 316 L and Mild Steel Induced by Slime
Producing Bacteria, International Journal of Mechanical Engineering and
Technology, 10(3), 2019, pp. 404-411.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=2
1. INTRODUCTION
Steels are one of the major class of engineering materials and they are widely applied in
multiple sectors including construction, transportation, energy as well as the manufacturing
industry. A steel is generally an iron-based alloy containing some amount of alloying
elements such as Ni, Cr, Mo, Mg, Co, Nb, B, C and many more. Different grades of steel
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Comparison of Biocorrosion Behavior of Stainless Steel 316 L and Mild Steel Induced by Slime
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ranging from stainless steel to carbon steel are made with different selections of mechanical
strength, malleability and corrosion vulnerability.
Stainless steel 316L (SS 316L) is a Mo-bearing austenitic steel with excellent corrosion
resistant. Due to the presence of Cr in its alloying formulation, SS 316L is self-passivated by
layer of Cr oxides whenever it is exposed to corrosive and oxygen rich environment. Mild
steel on the other hand is a steel with approximately 0.05 – 0.25% carbon. It is easy to form
due to its high malleability and ductility. The mild steel has relatively lower mechanical
strength and lower tolerance to corrosion. Both types of steels have significant roles in the
development of marine structures. The SS 316 L is mostly selected for obtaining structures
with high tensile strength and must be used under direct exposure to chloride-containing
condition. Meanwhile, the mild steel is needed where the structure has to be deformed or
welded.
Biocorrosion of steels is a great challenge for numerous engineering applications. In this
process, the general metal corrosion process is accelerated by attachment and colonization of
microorganism on metal surfaces [1-3]. Metabolic activity of microorganisms adhering to the
surface of metal particularly the steels can damage the passivation layer, inhibit its
development or alter the rate of redox reaction at cathode or anode terminal [4-6]. Eventually,
the unintended event promotes the occurrence of localized corrosions such as pitting, crevice
or grain boundary corrosions [6,7]. For that reason, the biocorrosion can lead to costly
economic loss and give rise to many environmental issues [8,9].
Recent studies reported that slime producing bacteria is one type of corrosive bacterial
community that has great potential to influence the rate of steel corrosion [10-12]. This
bacteria adheres to steel walls and secretes gel-like exopolymeric substances which contain
polysaccharides, proteins, lipids, and nucleic acids [11]. The roles of this slimy layer of EPS
on biocorrosion behaviour of steels are still unclear. In certain condition, EPS layer increases
the corrosion rate of metals while in other conditions, the layer sometimes serves as additional
protective layer on top of metal surfaces, thus reducing the corrosion rate [4].
In order to identify the best strategy for corrosion prevention and control, an
understanding on the biocorrosion behavior of different types of steels is required. The
present study investigated the biocorrosion behaviour of Stainless steel 316L (SS 316L) and
mild steel coupons upon immersion in medium containing slime producing bacteria,
Escherichia coli (E.coli).
2. MATERIALS & EXPERIMENTAL PROCEDURES
2.1. Materials
Two types of metallic specimens studied are made of mild steel and stainless steel 316L.
Table 1 indicates the composition of specimens used in this work.
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Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara Syazrinni
Rooshde, Mohd Sabri Mohd Ghazali
Table 1 Compositions of the mild steel and SS 316L specimens
Elements
C
Mn
P
S
Si
Cr
Ni
Mo
N
Cu
Fe
Stainless Steel 316L
Composition (%)
0.03
2.00
0.045
0.03
0.75
18.00
14.00
2.00
0.10
Balance
Mild Steel Composition
(%)
0.33
0.76
0.05
0.05
0.19
0.12
0.11
0.003
Balance
Escherichia coli (E.coli), the Gram negative bacteria was selected and cultured as the
model for slime producing bacteria. Bacteria culture was obtained from frozen bacteria stock
stored in 15% glycerol at -20 oC. The bacteria was grown in 100 ml of nutrient broth before
further use.
2.2. Methods
2.2.1. Preparation of Specimens
The metal specimens were prepared according to the procedures suggested in ASTM G31-72.
The SS 316L and the mild steel sheets were were cut into smaller rectangular coupons having
25 mm width x 25 mm length x 1.5 mm thick. The specimens were polished with abrasive
paper sized P320, P600, P800, P1000, P1200, P1500 and P2000 using a polishing machine
(Mecapol P225U) until a mirror surface was obtained.
2.2.2. Preparation of Medium and Bacteria Culture
The nutrient agar was prepared by mixing 20 g nutrient agar powder for every 1 L of distilled
water. The nutrient agar was autoclaved at 121oC for 15 minutes and cooled down to
approximately 35 oC. It was then poured into a plate and left to solidify in a chiller for 24
hours. By using the streak plate technique as suggested in [13], the pure culture of E.coli was
inoculated onto the plate containing nutrient agar.
2.2.3. Bacterial Attachment Test
Determination of bacterial attachment on substrate surface can provide useful information
regarding the affinity of the bacteria species to settle on a certain metal substrate before
analyzing the subsequent corrosion process. This study adopted the procedures suggested in
[6,14,15] with some modifications. The metal specimen was placed on nutrient agar which
has been streaked with bacteria and incubated for 24 hours. The progress of corrosion process
on tested specimen was recorded for 7 consecutive days. The inspection was completed using
metallurgical microscopy and scanning electron microscopy techniques. The observations
were also tabulated in term of scoring damage as suggested by Smirnov et al., [16].
2.2.4. Determination of Weight Loss and Calculation of Corrosion Rate
Determination of weight loss is a procedure to measure the metal loss after metal substrates
immersion test at specific intervals. The technique involves weighing of the mass of metal
substrates before and after the immersion test and calculation of the net metal loss. Equation
(1) asserts the calculation to find percentage of metal loss.
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Comparison of Biocorrosion Behavior of Stainless Steel 316 L and Mild Steel Induced by Slime
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% Weight loss 
Initial mass of metal specimen
 100%
Mass loss of metal specimen after immersion
(1)
Initial mass and mass loss both are calculated in gram and multiply by hundred percent to
get percentage of mass loss.
The standard corrosion rate calculation was done according to the ASTM G31-72. The
average corrosion rate in millimetres per year (mm/y) can be calculated by the following
Equation (2).
Corrosion rate per year (mm/year) 
(K W )
( A  T  D)
(2)
where K is a constant, T is the time exposure in hours, A is the area in cm2, W is the mass
loss (g) and D is the density (g/cm3). The K constant value used in this experimental work is
8.76 x 104 while the density for stainless steel and mild steel is 7.98 g/cm3 and is 7.86 g/cm3,
respectively.
3. RESULTS & DISCUSSION
The evolution of corrosion on SS 316L and mild steel specimens exposed to medium
containing E.coli was monitored for 7 consecutive days and the observations are tabulated in
Table 2 whereas the scoring damages on the specimen surface are listed in Table 3.
Tarnishing of both SS 316L and mild steel specimens was observed after Day 1 and it was
more rapid on the SS 316L. At this stage, the passivation layer has started to develop on SS
316L due to oxidation of Cr with dissolved oxygen in the medium. Early signs of corrosion
product deposition was observed as early as on the Day 3 for both SS 316L and mild steel.
After the 4th day, the mild steel indicated many signs of accelerated corrosion including
tarnishing, appearance of black spot and rusting at the back of surface and corroded slimy
surface. The deposition of red-brown corrosion product on the surface of corroded mild steel
could be associated to formation of -hematite or Fe2O3.H2O or Fe(OH)3 whereas the
blackish corrosion product was probably Fe2O3 or FeO. Further inspection was performed
using a metallurgical microscope and the scanning electron microscopy technique in order to
analyze the corrosion effects at microscopic scale.
Table 2 Evolution of corrosion process on SS 316L and mild steel specimens induced by E.coli
Day
Metal Specimens
SS 316L
Mild Steel
Day 1
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Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara Syazrinni
Rooshde, Mohd Sabri Mohd Ghazali
Metal Specimens
Day
SS 316L
Mild Steel
Day 3
Day 5
Day 7
Table 3 Scoring damage observed in SS 316L and mild steel specimens exposed to E.coli
Exposure day
0
1
2
3
4
5
6
7
Scoring Damage
SS 316L
A
D
D
D, E
G
G
G
G, I
Mild steel
A
B
B
F
E, F
E, F
G, H
I, J
Scoring damage: A: no changes, B: Minor tarnish on surface, C: A transparent layer easily
removed on the surface, D: Tarnish cover 25% of surface, E: Surface at the back (adhere on
agar) become brownish rust, F: Black spot at the back of surface, G: Tarnish 50% on surface,
H: Black spot on surface become bigger, I: Corroded slimy surface, J: Back surface of metal
100% rusting.
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Comparison of Biocorrosion Behavior of Stainless Steel 316 L and Mild Steel Induced by Slime
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Based on Figure 1, the surface of the immersed SS316L specimen was obviously affected
and the corrosion produced attacks in the form of spots or pits. The occurrence of pitting
corrosion implies that the passivation layer has weakened or damaged at specific points on the
SS 316L surface due to metabolite activity by E.coli. The corrosion impacts on the mild steel
surface were even more severe. After the 7th day of immersion in E.coli containing medium, a
combination of corrosion processes took place on the surface of the specimen. Figure 2
depicts that larger area are covered with corrosion products and the corrosion attacks at grain
boundary region and pits have been detected. SEM images in Figure 3 confirm that the
surfaces of both SS 316L and mild steel specimen were covered with dense layer of E.coli
cells. The appearance of exo-polymeric substances (EPS) can be clearly observed on the
surface of mild steel. In accordance to the data in Table 4, the SS 316L and mild steel
specimens lost 2.83 and 3.63% or its initial weight, respectively in just 7 days. In terms of
corrosion rate value, the mild steel specimen suffered slightly 25% more severe corrosion as
compared to the SS 316L.
Figure 1 The surface morphology of SS 316L (a) before exposure and (b) after 7 days of exposure to
E.coli containing medium
Figure 2 The surface morphology of mild steel (a) before exposure and (b) after 7 days of exposure to
E.coli containing medium
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Wan Rafizah Wan Abdullah, Nur Alia Johari, Noradhiha Farahin Ibrahim, Maishara Syazrinni
Rooshde, Mohd Sabri Mohd Ghazali
Figure 3 SEM images of E.coli adhesion on (a) stainless steel and (b) mild steel surfaces after 7 days
Table 4 Percentage of weight loss and the corrosion rate for stainless steel and mild steel after
exposure to E.coli containing medium
Specimen
% weight loss
Corrosion Rate (mm/year)
SS 316 L
2.83+ 0.01
2.20 + 0.01
Mild steel
3.63+ 0.01
2.74 + 0.01
The findings from this study indicates that slime producing bacteria like E.coli has great
potential to cause harm on both SS 316L and mild steel even in a short period of time. A
Gram negative bacteria, E.coli adheres to the steel surface to obtain carbon nutrient sources
for supporting its growth [12]. In addition, colonization of dense E.coli cells on steel surface
produces biofilm layer that may capture moisture and inhibit oxygen at the same time. In the
case of SS 316L, both conditions may have inhibited formation of passivation layer or
weakened it at certain spots. As a result, pitting corrosion progressed rapidly on its surface.
The corrosion induced by E.coli has relatively greater impacts on mild steel as it is not
protected by self-passivation mechanism. Exposure to moisture and colonization by E.coli
initiate corrosion process on its surface. Detection of abundant EPS on mild steel may suggest
the preferential colonization of bacteria on mild steel over the SS 316L. As supported by [4],
excretion of EPS promoted the cathodic reaction, thus encouraged the dissolution of iron from
steel. Deposition of corrosion products, the formation of pits and grain boundary cleavage
increased the surface roughness on mild steel, thus encouraging further attachment of E.coli
and its settlement. As the network of EPS grew, the mild steel surface was constantly in
contact with slimy and wet layer or biofilm. For that reason, corrosion process on its surface
became accelerated and more deterioration can be observed.
4. CONCLUSION
This study investigated the biocorrosion behavior of Stainless steel 316L (SS 316L) and mild
steel induced by slime producing bacteria, E.coli. The findings revealed that the corrosion
resistance of SS 316L deteriorated by E.coli and its metabolite activity. The SS 316L
subjected to medium containing E.coli has suffered from severe pitting corrosion due to
weakening or damaging of passivation layer at localized sites. Meanwhile, the results showed
the preferential colonization of E.coli on the mild steel surface. The subsequent corrosion
process has caused attacks on pits, grain boundary and larger surface area on mild steel. In
conclusion, microbially induced corrosion can occur rapidly on steels and self-passivation
alone could not fully protect steel from getting affected by the slime producing bacteria.
ACKNOWLEDGEMENTS
The authors would like to thank for the financial support from research grant provided by
Ministry of Higher Education under Fundamental Research Grant Scheme (FRGS), Grant No:
59436 and Universiti Malaysia Terengganu for providing the research facilities.
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