corrosion review

Corros Rev 2014; aop

Ahmad Abdolahi , Esah Hamzah * , Zaharah Ibrahim and Shahrir Hashim

Microbially influenced corrosion of steels by Pseudomonas aeruginosa Abstract: Microbially influenced corrosion (MIC) is a destructive type of corrosion that is initiated, facilitated, or accelerated by the presence and metabolic activity of bacteria. MIC of steels is a great issue in many industries such as marine, freshwater systems, and gas/oil pipelines. Pseudomonas aeruginosa is one of the aerobic slime-form-ing bacteria that are ubiquitous in marine environment that corrode steel structures. This article aims to provide a review on MIC of steels caused by bacteria, mostly in the case of P. aeruginosa . The mechanisms of MIC will be dis-cussed based on bacteria-metal reactions and emphasize the role of P. aeruginosa on corrosion of steels.

Keywords: bacteria; biofilm; microbially influenced corro-sion; pitting; Pseudomonas aeruginosa ; steels.

DOI 10.1515/corrrev-2013-0047 Received October 16 , 2013 ; accepted April 23 , 2014

1 Introduction Microbially influenced corrosion (MIC) is a destructive type of corrosion that is initiated, facilitated, or acceler-ated by the presence and activity of bacteria ( Coetser & Cloete, 2005 ; Javaherdashti, 2008 ) and mostly appears in the form of localized pits and crevices on metal surfaces ( Xu, Zhang, Cheng, & Zhu, 2007 ). Bacteria tend to attach to a substrate and form a biofilm layer where it creates a condition that accelerates corrosion. In other words, MIC has two parts: corrosion and bacteria. The bacteria in the biofilm state tend to accelerate and facilitate the corrosion and cause severe damage to the metal ( Beech, 2003, 2004 ;

Beech & Sunner, 2004 ; Liao, Fukui, Urakami, & Morisaki, 2010 ).

One group of metal alloys that is less resistant to MIC are steels, which include carbon steels ( Herrera & Videla, 2009 ; Javaherdashti, 2009 ; Lee, Lewandowski, Nielsen, & Hamilton, 1995 ; Rao, Sairam, Viswanathan, & Nair, 2000 ; Videla, Borgne, Panter, & Singh Raman, 2008 ) and stain-less steel ( Cheng etal., 2009 ; Sheng, Ting, & Pehkonen, 2007 ; Xu et al., 2007 ). These metals are usually used in marine industries because of their good mechani-cal properties and relatively lower cost. However, their common limitation is that they are not immune to MIC. Generally, steels are susceptible to MIC, as shown by their chemical reaction with different types of bacteria such as iron-reducing bacteria (IRBs; Videla etal., 2008 ), sulfate-reducing bacteria (SRBs), iron-oxidizing bacteria (IOBs; Xu et al., 2007 ), manganese-oxidizing bacteria ( Lin-hardt, 2004 ), and slime-forming bacteria (SFBs; Yuan & Pehkonen, 2007 ). These bacteria cause localized pitting or crevice corrosion on the steel surface through the forma-tion of biofilms and further colonization by other bacterial types.

Pseudomonas aeruginosa is a dominant bacterium in marine environments and one of the aerobic SFBs, which form a biofilm layer on steel surface. The chemical reac-tion of the biofilm layer with steel and the formation of differential aeration cells create conditions on steel that initiate and accelerate the corrosion process ( Hamzah, Hussain, Ibrahim, & Abdolahi, 2013 ). The generation of these concentration cells is detrimental to the integrity of the oxide layer and enhances the susceptibility of steels to corrosion ( Hamzah etal., 2013 ). This article aims to review the MIC behavior of steels caused by bacteria, especially in the case of P. aeruginosa . This review will discuss cor-rosion, MIC, bacteria, biofilm, mechanisms of MIC, MIC caused by P. aeruginosa , and finally, the available litera-ture on MIC of steels caused by P. aeruginosa .

2 Corrosion The corrosion process is the deterioration of metal substrate due to its electrochemical reaction with its

*Corresponding author: Esah Hamzah, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia, e-mail: [email protected] Ahmad Abdolahi: Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia Zaharah Ibrahim: Faculty of Bioscience and Medical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia Shahrir Hashim: Faculty of Chemical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia

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2A. Abdolahi etal.: Microbially influenced corrosion

environment ( Fontana, 2005 ). From the thermodynamic point of view, corrosion occurs because pure metals tend to convert to their oxidized state, which is what happens in ore. This is the reason why corrosion is called extrac-tive metallurgy in reverse. In the case of iron, the most available natural ore is hematite, -FeOOH. The corrosion product of iron mostly appears in the form of rust, FeOOH. The corrosion process is normally an electrochemical reac-tion, i.e., a combination of cathodic and anodic reactions with transfer of electrons. The anodic sites are mostly oxi-dized with loss of the electron, and at the cathodic site, the electrons are consumed. In the case of iron, the overall corrosion process consists of anodic and cathodic reac-tions and are presented as follows:

2 2Fe 3 / 4O 1/ 2H O FeOOH ( Overall reaction)+ + (1)

The reaction is composed of the anodic (2) and cathodic reactions (3):

- -

2Fe 3OH FeOOH+H O+3e (anodic reaction)+ (2)

- -

2 23 / 4O 3 / 2H O 3e 3OH (cathodic reaction).+ + (3)

Generally, any factor that influences anodic reaction, cathodic reaction, or both can affect the corrosion rate of the overall process. The factors that change the corrosion rate include temperature, pH, and presence of microor-ganisms such as bacteria. One of the important factors that accelerates the corrosion rate of a metal is bacteria in biofilm mode, which can facilitate electron transfer and induce corrosion ( Heyer etal., 2013 ).

3 MIC process

MIC is an electrochemical type of corrosion that is initi-ated, facilitated, or accelerated by the presence of bacte-ria and mostly appears in the form of localized pitting or crevice corrosion ( Javaherdashti, 2008 ). MIC is detrimental to many industries, including marine, water systems, and gas/oil pipelines, as approximately 20% 30% of all corro-sion deteriorations are related to MIC, with a direct finan-cial cost of $ 30 50 billion per year ( Javaherdashti, 2008 ).

Put simply, MIC is an interaction between the bac-teria and the metal substrate resulting in corrosion fail-ures. The following section is a brief review on bacteria and its cell wall structure, which has an important role on its tendency to attach to metal substrates and further colonization.

3.1 Bacteria

Bacteria are small microorganisms that are ubiquitous in the environment, especially in marine environments. Bacteria are generally classified by their morphology and Gram staining. Based on their shapes, bacteria mostly exist in the shape of spheres, rods, and spirals. Based on their Gram staining, bacteria are divided into Gram positive and Gram negative ( Lichter, Van Vliet, & Rubner, 2009 ).

3.1.1 Gram-positive bacteria

In Gram-positive bacteria, the cell wall consists of three layers: (1) cell wall protein, (2) cell membrane, and (3) cytoplasmic membrane ( Frankel & Bazylinski, 2003 ). Cell membrane consists mainly of peptidoglycan (15 25 nm thick), which comprises multiple layers of repeating units of two sugar derivatives, N -acetylglucosamine and N -acetylmuramic acid, and a small group of amino acids that is rich in carboxylate groups, which are responsi-ble for the net negative charge on the bacterial cell wall ( Frankel & Bazylinski, 2003 ).

3.1.2 Gram-negative bacteria

Gram-negative bacteria have structurally more complex cell wall than the Gram-positive type. It has three different layers, which are (i) the outer membrane, (ii) the periplasm (contains peptidoglycan layer), and (iii) the inner mem-brane. The peptidoglycan layer is about 3 nm thick and does not contain secondary polymers ( Frankel & Bazylin-ski, 2003 ). The outer membrane is composed of phospho-lipid and lipopolysaccharide (LPS), which is highly anionic. LPS consists of O -polysaccharide, the core polysaccharide, and lipid A. In Gram-negative cells, LPS has the major role in metal binding because of its high concentration of phos-phate and carboxyl groups ( Frankel & Bazylinski, 2003 ).

Generally, the cell wall of bacteria (either Gram posi-tive or Gram negative) is electrically negative ( Frankel & Bazylinski, 2003 ). This negative charge contributes to their tendency to adhere to the metallic substrate. The attach-ment of the bacteria to the metal substrate is the initial stage of biofilm formation ( Palmer, Flint, & Brooks, 2007 ).

3.2 Biofilm formation

Bacteria in the environment exist in the planktonic state, which tends to attach to solid substrate and form a slimy



A. Abdolahi etal.: Microbially influenced corrosion3

layer, which consists of a community of bacteria, water, and exo polymeric substances (EPS) ( Donlan, 2002 ). This slimy layer called EPS is resistant to undesirable conditions such as biocide penetration or diffusion. The biofilm layer is formed through the following stages: (1) formation of a conditioning layer, (2) transportation of planktonic cells to the metal surface, (3) irreversible adhesion of bacterial cells through formation of EPS, (4) formation of a steady-state biofilm layer, and (5) detachment of bacterial cells from the biofilm outer surface ( Stoodley, Sauer, Davies, & Costerton, 2002 ). Generally, the biofilm layer is formed for two main reasons, which are summarized in Table 1 .

The first reason is that the biofilm layer is formed to protect the bacteria communities against undesir-able conditions and toxins such as biocides. The second reason is the limitation of nutrient in the water. Thus, biofilm formation depends on nutrient complexation by the EPS matrix ( Sim es, Sim es, & Vieira, 2010 ). Gener-ally, biofilm formation is known as the initial stage of MIC.

3.3 Differential aeration cell

Generally, when the biofilm layer is formed on the steel sub-strate, it has a heterogeneous state and creates two differ-ent areas, the anode and the cathode. The anode, located beneath the biofilm layer, is anoxic due to lack of oxygen, and the cathode, located at the surface of steel, is enriched with oxygen. The anode and cathode areas activate differen-tial aeration cells, resulting in corrosion acceleration of the steel substrate ( Flemming, Murthy, & Venkatesan, 2009 ). Owing to the heterogeneous nature of the biofilm, the most probable type of corrosion is localized pitting or crevice.

3.4 Corrosion-causing bacteria

Generally, the bacteria within the biofilm are alive and can activate based on their oxygen requirement. In this regard, the bacteria are divided into (a) aerobic, such as P. aerugi-nosa , and (b) anaerobic, such as Desulfovibro sp. Aerobic bacteria can live and proliferate in oxygen-containing envi-ronment, and anaerobic bacteria can live without oxygen. It

Table 1 Benefits of biofilm formation for bacteria communities.

Reasons for biofilm formation Microbial survival strategy

Protection from harmful conditions (toxins)

Defense

Limitation in nutrients Food supply

is reported that both aerobic and anaerobic bacteria within their biofilm layer cause steel destruction ( Dinh etal., 2004 ; Pillay & Lin, 2013 ). Thus, based on their metabolism, cor-rosion-causing bacteria can be divided into aerobes and anaerobes. They are further subdivided into different types such as iron oxidizer, iron reducer, and slime former.

3.5 Mechanisms of MIC of steels

Generally, to understand the MIC mechanisms of steels caused by bacteria, two main issues have to be consid-ered: (a) the metabolism of bacteria during growth and (b) the EPS produced by the biofilm and its chemical reaction with steel ( Beech, Sunner, & Hiraoka, 2010 ). It is important to note that when referring to bacteria, it means the bacteria in the biofilm layer. In the first mechanism, based on their metabolic activity, bacteria are divided into aerobic and anaerobic groups and subdivided into IOBs, sulfur-oxidizing bacteria (SOBs), SFBs, SRBs, and IRBs. Owing to their metabolic activity, bacteria in the biofilm make some changes on the steel surface that induce cor-rosion damage ( Herrera & Videla, 2009 ; Sheng etal., 2007 ; Xu et al., 2007 ; Yuan, Pehkonen, Ting, Kang, & Neoh, 2008 ; Yuan, Liang, Zhao, & Pehkonen, 2013 ). In the second mechanism, EPS, which is secreted during biofilm forma-tion, can also influence the corrosion behavior of steel.

The chemical reaction of EPS and steel substrate can induce corrosion damages ( Annuk & Moran, 2009 ). In the following sections, the MIC mechanism caused by aerobic and anaerobic bacteria through the metabolism of bacte-ria and the EPS-metal interaction is discussed. It is impor-tant to note that these mechanisms might simultaneously induce the corrosion damage of steels. For example, the differential aeration cell is the general MIC mechanism for almost all corrosion-causing bacteria because they form a biofilm layer on the steel substrate. However, bacteria are divided into different groups to better investigate the mechanism of MIC and understand their individual roles.

3.5.1 Mechanisms of MIC through anaerobic bacteria

Generally, the main anaerobic bacteria that induce corro-sion damages on steels are SRBs ( Lee etal., 1995 ; Sheng etal., 2007 ) and IRBs ( Herrera & Videla, 2009 ).

3.5.1.1 Sulfate reducing bacteria In their biofilm state, SRBs are responsible for pitting damages of steels in aquatic environment under anoxic




conditions. SRBs produce hydrogen sulfide (H 2 S) during their metabolism. H 2 S can react with Fe and form FeS deposits on the steel substrate; if SRBs are growing on steel surfaces, their biofilms are likely to promote the formation of pits beneath sulfide deposits ( Javaherdashti, 2011 ). Gen-erally, the cathodic depolarization mechanism is believed to have a secondary role on the pitting corrosion of iron in the presence of SRBs. The main factor that causes pitting is the formation of galvanic cell between the corrosion prod-ucts (FeS) and the steel substrate ( Javaherdashti, 2011 ).

3.5.1.2 Iron reducing bacteria IRBs are bacteria that can reduce insoluble Fe(III) to soluble Fe(II), which is related to their growth and metab-olism. These bacteria have a major role on the availabil-ity of iron ions through solubilization of insoluble iron oxides. Thus, they are able to induce the corrosion of steels through the reduction and removal of passive oxide films from the metal surface ( Herrera & Videla, 2009 ). She-wanella oneidensis is known as one of the important IRBs that cause corrosion damage in steels ( Videla etal., 2008 ).

3.5.2 MIC mechanism caused by aerobic bacteria

Based on their metabolic activity, aerobic bacteria are divided into (a) metal-oxidizing bacteria and (b) SFBs. The metal-oxidizing bacteria generally lead to the production of aggressive compounds that induce corrosion damage ( Chamritski, Burns, Webster, & Laycock, 2004 ). The SFBs can also induce corrosion through the formation of differ-ential aeration cells ( Hamzah etal., 2013 ).

3.5.2.1 Metal oxidizing bacteria The main aerobic corrosion-causing bacteria are the SOBs and IOBs.

3.5.2.1.1Sulfur oxidizing bacteria (SOB) Generally, SOBs, owing to their metabolic activity, can release aggressive products such as organic (acetic acid, succinic acid) or inorganic acid (sulfuric acid), which dete-riorates steel integrity and causes severe corrosion failures. One of the important SOBs is Thiobacillus , which forms sulfuric acid, which is a strong corrosive agent that causes severe damage on steels and concrete ( Pillay & Lin, 2013 ).

3.5.2.1.2Iron oxidizing bacteria (IOB) IOBs are ubiquitous and have been identified in numerous environments. IOBs can oxidize Fe(II) to Fe(III) due to it

metabolic activity and conserve energy from this process and convert inorganic carbon, in the form of carbon dioxide (CO 2 ), in the biofilm ( Weber, Achenbach, & Coates, 2006 ).

The IOBs are able to oxidize ferrous iron to ferric hydroxide, which is precipitated on the steel surface. The tubercles can lead to crevice attack and can also provide a suitable environment for the anaerobic bacteria in the region beneath the tubercle. Different IOBs are involved in steel corrosion, such as Gallionella , Leptothrix , and Siderocapsa species ( Ray, Lee, & Little, 2010 ).

3.5.2.2 Slime forming bacteria Another group of bacteria that causes corrosion of steels are SFBs, which form a dense slime layer on the sub-strate. The slime growth on the steel surface can create differential aeration cells. The presence of a biofilm layer on the steel causes the formation of two areas: (a) cathode, which is on the steel surface and rich in oxygen, and (b) anode, which is beneath the biofilm layer and has less oxygen. The differential aeration cells thus formed cause the localized pitting or crevice corrosion ( Flemming et al., 2009 ). Moreover, the slime exudate from the bacteria is generally acidic, which also deterio-rates the steel substrate.

Pseudomonas aeruginosa is known as one of the important types of aerobic SFBs that accelerate the cor-rosion rate due to the formation of differential aeration cells in the presence of its biofilm layer. P. aeruginosa has the ability to reduce ferric to ferrous iron, exposing steel to further oxidation because ferrous iron compounds are more soluble and the protective ferric iron layer is solubi-lized by this process ( Pillay & Lin, 2013 ).

The general MIC mechanism caused by different types of bacteria was discussed based on their metabolism activ-ity. Another important mechanism of MIC is related to EPS secreted from the biofilm layer. The chemical reaction of the EPS-metal substrate also induces corrosion damage on the steel substrate ( Beech etal., 2010 ).

3.5.3 MIC mechanism through EPS-metal interaction

The chemical reaction of the EPS of the biofilm and steel substrate has an important role in MIC. The EPS com-prises macromolecules such as proteins, polysaccharides, uronic acid, nucleic acids (DNA), and lipids (fatty acids) ( Flemming & Wingender, 2010 ). These molecules have anionic functional groups (carboxyl, phosphate, sulfate, glycerate, pyruvate, and succinate) with metal-binding




capacity. Owing to the presence of anionic functional groups, EPS can also bind to metals.

The multivalent metal ions such as Mg 2 + and Fe 3 + have a good affinity for binding to anionic groups of EPS. The presence of metal ions in different oxidation states in EPS could result in considerable shifts in the standard redox potential. For example, Fe(III/II) redox potential varies significantly with different ligands (in EPS) (from 1.2 to -0.4 V). EPS-bound metal ions can therefore act as elec-tron shuttles and open up novel redox reaction path-ways in the EPS/metal system, such as direct electron transfer from the metal. In the presence of a suitable elec-tron acceptor (e.g., oxygen in oxic systems or nitrate under anaerobic conditions), such redox pathways would lead to cathode depolarization and thus enhance the corrosion process ( Beech & Sunner, 2004 ).

Although the presence of metal ions within the biofilm matrix is related to MIC, the role of EPS-bound metal ions in direct electron transfer from the base metal to a suitable electron acceptor is the major cause of corrosion ( Beech & Sunner, 2004 ).

4 MIC caused by P. aeruginosa

4.1 P. aeruginosa

Pseudomonas aeruginosa is an aerobic SFB that is domi-nant in marine environment. This bacterium has a rod shape, approximately 1.0 2.5 m long and 0.4 0.6 m in diameter ( Yuan & Pehkonen, 2007 ).

This type of bacterium in its biofilm state could be det-rimental to steels and cause severe corrosion damages. It forms a heterogeneous biofilm layer on the steel surface and causes the formation of differential aeration cells, which induce localized corrosion ( Morales, Esparza, Gon-zalez, Salvarezza, & Arevalo, 1993 ).

Various Pseudomonas isolates have also been impli-cated in the reduction of ferric to ferrous iron, exposing steel to further oxidation, as ferrous iron compounds are more soluble, and the protective ferric iron layer is solubi-lized by this process ( Pillay & Lin, 2013 ).

Generally, the main MIC mechanisms caused by P.aer-uginosa biofilm can be divided into three: (a) differential aeration cell due to biofilm formation, which induces cor-rosion damages; (b) the chemical reaction between EPS and steel substrate, wherein the EPS-metal biomineral acts as corrosion inducer; (c) the role of siderophore pro-duced by P. aeruginosa in iron reduction.

4.2 Differential aeration cell caused by the P. aeruginosa biofilm layer

Pseudomonas aeruginosa is an SFB, which tends to form a biofilm layer on the steel surface. Owing to the presence of heterogeneous biofilm layer on steel, different areas of aeration cells are created. The anode is located beneath the biofilm layer and lacks oxygen, whereas the cathode is placed on the surface of the steel substrate and is enriched with oxygen. The difference in oxygen concentration between the anode and the cathode leads to the activation of electrochemical cells, which induce corrosion damage in the form of pitting or crevices ( Flemming etal., 2009 ).

4.3 The interaction of EPS of P. aeruginosa with steel

Generally, during biofilm formation, P. aeruginosa excrete acidic EPS, which contains exopolysaccharide (alginate) ( Boyd & Chakrabarty, 1995 ). Alginate contains anionic carboxylic groups with metal-binding ability. Generally, EPS tends to react with Fe to gain energy for bacterial growth by releasing proton from the carboxylic groups in EPS. The dominant cation of Fe in the environment is important for EPS-Fe(III) interaction. For example, in sea-water environment, one of the dominant ions of Fe(III) is in the form of Fe(OH) 2 + ; this ion interacts with the carboxyl group present in EPS. By interaction of Fe(III) with EPS, the acidity in the environment would be increased. This can be an indication that protons from EPS are released into the environment through deprotonation of carboxylic groups ( Tapia, Munoz, Gonzalez, Blazquez, & Ballester, 2011 ).

Therefore, through the dissociation of the carboxyl groups in EPS, RCOOH converts to RCOO, and these anions are responsible for the interaction with the ferric ion. The proposed chemical reaction of EPS and Fe(III) is shown in reaction 4.

(4)

According to reaction (4), the interaction mechanism assumes that each iron atom is associated with two car-boxylate groups, COO - , that combine, forming an oxalate group, C 2 O 4 2- ; therefore, it is likely that the presence of iron oxalates in the EPS is loaded with Fe(III). Thus, the EPS of P. aeruginosa can interact with ferric ion on the steel substrate (Fe(OH) 2 + ) through their carboxylic




OH

HOHN

HN

HN

OH

OH

OH

OH

HO HO

OH

NH

HN

HN

HN

HN

H

H

H

N

N

N

NH

NH

NH OHNH

NH

O

O

O

O

O

O

O

O

OO

OO

O

H3C

H2N

COOHN

NH

H

PyochelinPyoverdine

SS

H

Figure 1 Chemical structure of pyochelin, the siderophore of P. aeruginosa (Saha etal., 2013). Reproduced with permission from Wiley.

400

nm

800

nm80

0 nm

1.50

0 m

0-40

050

0 nm

0-50

070

0 nm

0-70

0

0 2.5 5.0Horiz distance (L)Vert distance 149.86 nm

1.641 m7.5

Sectional analysis A

B

C

Sectional analysis

Sectional analysis

10.0 m

0 2.5 5.0Horiz distance (L)Vert distance 439.69 nm

3.672 m7.5 10.0 m

0 10.05.0Horiz distance (L)Vert distance 776.98 nm

6.164 m15.0 m

24

68

10 m

24

6

5

10

810 m

15 m

Figure 2 AFM images of the presence of pits on the corroded surfaces of the stainless steel 304 coupon after different exposure times: (A) 14, (B) 28, and (C) 49days ( Yuan & Pehkonen, 2007 ). Reproduced with permission from Elsevier.

groups (RCOOH), resulting in the formation of Fe(OH)-(C 2 O 4 ) 2(H 2 O) as the corrosion product. This mineral product can act as a catalyzer for cathodic depolari-zation, which induces corrosion damage on the steel substrate.

4.4 Role of siderophore produced by P. aer-uginosa in iron reduction

Generally, iron is an essential nutrient for bacteria such as P. aeruginosa . Iron is mostly oxidized from soluble Fe + 2




to Fe + 3 and insoluble ferric oxyhydroxide in presence of oxygen and neutral pH. Insoluble ferric oxyhydroxide is not useful for P. aeruginosa because the bioavailability of iron is approximately 10 -9 10 -10 m . In such a condition, i.e., the concentration of iron is too low, P. aeruginosa pro-duces a chelator known as siderophores. Siderophores are low-molecular-weight compounds with high affinity for iron, which solubilize ferric ions and transport these ions into the bacterial cell for growth and activity of bacteria. The siderophores of P. aeruginosa are pyochelin and pyo-verdin ( Brandel etal., 2012 ) ( Figure 1 ).

These sidophores can uptake the iron from the steel substrate for growth and activity of P. aeruginosa ( Saha, Saha, Donofrio, & Bestervelt, 2013 ). The sidophore-iron interaction and the uptake of iron from the steel substrate might enhance the corrosion process of steel.

5 Literature on MIC of steels in presence of bacterium P.aeruginosa

A number of research investigated the effect of P. aerugi-nosa biofilm on the MIC process of steels ( Hamzah etal., 2013 ; Morales etal., 1993 ; Yuan & Pehkonen, 2007 ; Yuan etal., 2008 ). In this article, we review some research work and highlight the significant role of P. aeruginosa biofilm on pitting corrosion of steels.

In 1993, Morales et al. have investigated localized pitting of 304 stainless steels in the presence of P. aerugi-nosa in neutral-buffered solutions. Their results showed that stainless steel is susceptible to pitting corrosion due to P. aeruginosa activity in the biofilm state through the (a) formation of differential aeration cells, which induce pitting corrosion under biofilm layer, and (b) reduction of thickness of the passive oxide layer due to metabolite production.

Yuan and Pehkonen (2007) have studied the MIC of 304 stainless steels by aerobic Pseudomonas NCIMB 2021 biofilm. Figure 2 shows the depth of pits on the steel sub-strate after different exposure times in bacteria-incubated medium.

As shown in Figure 2 A, the shallow pit with a depth of 150nm that appeared on the steel substrate after 14days of exposure to bacteria-incubated medium indicates the initiation of micro-pits. By extending the exposure time, the width and depth of the pits increased ( Figure 2 B and C). After 28days of exposure, the pits reached a depth of 440 nm on the steel substrates, and after 49 days, the depth reached 770 nm. The increase in the depth of pits

on the steel substrate due to prolonged exposure time in bacteria-incubated medium reveals the aggressive role of the bacterium in inducing corrosion damage ( Yuan & Pehkonen, 2007 ).

Yuan etal. (2008) have studied the corrosion reaction of stainless steel in artificial seawater in the presence of Pseudomonas bacteria. Through electrochemical analysis such as Tafel plots, they inferred that the corrosion rate of steel samples in bacteria-containing solution increased by extending the exposure time ( Figure 3 , Table 2 ). In con-trast, the corrosion rate of the steel samples in the sterile medium was constant due to the protective role of the chromium oxide layer naturally formed on the stainless steel substrate ( Yuan etal., 2008 ).

As shown in Table 2 and Figure 3 , by extending the exposure time over 49 days, i corr increases dramatically and reaches a high value of 7.38 A/cm 2 after 77days of expo-sure. The corrosion potential, E corr , undergoes a negative

E/V

(vs. Ag

/AgC

l)E/

V (vs

. Ag

/AgC

l)

1E-9 1E-8 1E-7 1E-6I/(Acm-2)

1E-5 1E-4

1E-9 1E-8 1E-7 1E-6I/(Acm-2)

1E-5 1E-4

-0.5

-0.4

-0.3

-0.2

-0.1

0

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.17 days

77 days63 days49 days

14 days21 days35 days

A

B

Figure 3 Tafel diagrams of 304 stainless steel substrates in the Pseudomonas -contained medium after (A) short-term exposure of 7, 14, 21, and 35days and (B) long-term exposure of 49, 63, and 77days ( Yuan etal., 2008 ). Reproduced with permission from the American Chemical Society.




OMn

Mn

Mn

Fe

Fe

Fe

Ni

NiNi

Cr

O

Mn

Mn

Mn

Fe

Fe

Fe

Ni

Ni

Ni

Cr

Cl

ClC

Cr

Cr

Si

Cr

CrCCl

Cl

Si

0

ElementAtomic%

C26.06

Si0.57

Cl2.43

Mn1.07

Cr10.44

Ni4.07

Fe36.63

O18.73

1 2 3 4 5 6 7 8 9 10keV

0

ElementAtomic%

C30.58

Si0.47

Cl3.36

Mn0.68

Cr8.47

Ni3.03

Fe28.49

O24.92

1 2 3 4 5 6 7 8 9 10keV

A

B

Figure 4 The scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy (EDX) spectra of the pit as formed on the 304S coupon surface in presence of Pseudomonas bacteria after (A) 14 and (B) 35days ( Yuan etal., 2008 ). Reproduced with permission from the American Chemical Society.

Table 2 Analysis parameters of Tafel plots of 304 SS coupons in the Pseudomonas -inoculated medium after different exposure periods ( Yuan etal., 2008 ).

Exposure time (days)

E corr (V) i corr ( A/cm 2 ) Corrosion rate (mm/year)

7 -0.224 2.58 0.026714 -0.236 3.24 0.033521 -0.239 3.27 0.033935 -0.248 3.62 0.037549 -0.244 5.45 0.056463 -0.254 6.92 0.071677 -0.262 7.38 0.0765

Reproduced with permission from the American Chemical Society.

shift in the Pseudomonas -inoculated medium with time. This behavior is related to the breakdown of the protective chromium oxide layer naturally formed on the steel due to the aggressive role of Pseudomonas bacteria and chloride ions ( Yuan etal., 2008 ).

Another interesting observation was the synergistic effect of the biofilm layer and the chloride ions on the occurrence of severe pitting corrosion of the steel speci-mens. Figure 4 shows the EDS analysis of the pit areas on the sample immersed in bacteria-containing simulated seawater.

The EDS showed that the pits are enriched with carbon, oxygen, and chlorine because of the presence of bacterial cells, their EPS, and chloride ions (Cl - ) in the pitted areas. In addition, the amount of metallic ele-ments such as iron, chromium, nickel, and manganese has decreased in the pitted areas, and this can be related to the occurrence of localized corrosion in the presence of aggressive chloride ions (Cl - ) and colonizing bacteria. The formation of heterogeneous biofilm layer on steel and the aggressive chloride ion in the medium have an important role in the initiation of pitting. There are three prerequisites for the occurrence of pits on the metal sub-strate through Cl - ions: (i) Cl - ion must be present in the




medium; (ii) there must be potential difference between the anodic area (pit area) and cathodic area (rest surface on the metal); (iii) the reaction temperature must exceed a critical temperature. The heterogeneous P. aeruginosa biofilm layer creates a potential difference between the anodic (pit) area and the cathodic (metal surface) areas through the formation of differential aeration cells, which leads to the weakening and breakdown of the oxide layer. Thus, both Pseudomonas biofilms and chloride (Cl - ) ions could be inferred to have an aggressive role in the occur-rence of pitting corrosion on steel ( Yuan etal., 2008 ).

Yuan and Pehkonen (2009) investigated the role of Pseudomonas NCIMB 2021 biofilm layer on the pitting cor-rosion of the 304 stainless steels. Figure 5 shows the for-mation and development of the biofilm layer on the steel substrate after different exposure times.

As shown in Figure 5 A, some bacterial cells are distributed on the steel substrate after 3 days of expo-sure to bacteria-incubated medium, which indicates the initial attachment of Pseudomonas on the coupon surface, which is important in further biofilm formation. By extending the exposure time to 14 days, the bacte-rial cells tend to secrete polymeric substances (EPS) to improve their binding to steel and form a biofilm layer ( Figure 5 B). By extending the exposure time to 42 days, the more heterogeneous and patchy biofilm layer is formed on the steel substrate. By extending the expo-sure time from 3 days to 42 days, the arithmetic mean roughness, R a , noticeably increases from 48 nm to 246nm, which indicates the increase in heterogeneity of the biofilm on the steel coupon surface ( Figure 5 A C). The increase in thickness and the heterogeneity of the

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Figure 5 AFM images of biofilm layer formed on 304 SS substrates after (A) 3, (B) 14, and (C) 42days exposed in Pseudomonas -containing medium ( Yuan & Pehkonen, 2009 ). Reproduced with permission from Elsevier.




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Figure 7 (A) SEM image of Pseudomonas aeruginosa biofilm layer formed on 304 stainless steel substrate after 21days of exposure in bacteria-inoculated nutrient rich simulated seawater (NRSS) media. (B) AFM image of pitting damage after 49days of exposure in bacteria-inoculated NRSS medium ( Hamzah etal., 2013 ). Reproduced with permission from Maney.

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Figure 6 AFM images of pits occurred on 304 SS substrates after (A) 21 and (B) 42days of exposure in Pseudomonas -incubated medium ( Yuan & Pehkonen, 2009 ). Reproduced with permission from Elsevier.

biofilm layer with time are detrimental to the oxide layer on the steel surface because they can give rise to local differences in metabolic products, pH, or dissolved oxygen (i.e., differential aeration cells), all resulting to its deterioration and formation of pitting corrosion ( Yuan & Pehkonen, 2009 ).

Figure 6 shows the pitting corrosion of two stainless steel substrate exposed to bacteria-containing solution with different exposure times.

The two-dimensional images, together with the sectional analysis graphs, are shown in each group of atomic force microscopy (AFM) images. By increasing the exposure time from 21days to 42 days, the depth of pits

increased from 320 nm to 500 nm. By extending the exposure time, the pits become deeper and wider. This is due to the formation of a more heterogeneous biofilm layer and the aggressive role of the chloride ions ( Yuan & Pehkonen, 2009 ).

Hamzah etal. (2013) investigated the role of P. aerugi-nosa on the corrosion behavior of 304 stainless steels in nutrient-rich simulated seawater ( Hamzah etal., 2013 ). As shown in Figure 7 , their results depict the formation of the biofilm layer on the stainless steel substrate and severe pitting damages under the biofilm layer.

The synergistic aggressive role of the chloride ions (Cl - ) and the P. aeruginosa biofilm was found to be responsible




for severe pitting corrosion of stainless steel ( Hamzah etal., 2013 ).

The aggressive role of Cl - ions within the medium in the breakdown of the oxide layer on steel substrates and further corrosion acceleration could be described by the following reactions:

2+ -Fe Fe 2e + (5)

2+ -

2 2Fe 2H O 2Cl Fe( OH ) 2HCl+ + + (6)

- -

2 3Fe(OH) + 3Cl FeCl +2OH (7)

3 2 3FeCl + 3H O Fe(OH) + 3HCl (8)

The interaction of Cl - ions with the hydroxide layer leads to the formation of a soluble FeCl 3 product, and FeCl 3 is further hydrolyzed to produce a very porous Fe(OH) 3 . The Fe(OH) 3 product is not stable and could not protect the steel against corrosion ( Hamzah et al., 2013 ). The aggressive role of the biofilm layer in inducing corrosion damage can be summarized as follows: the biofilm formed on the steel becomes larger, thicker, and more heteroge-neous with longer exposure time. Patchiness and the het-erogeneous nature of the biofilm generate a condition on the steel surface promoting local differences in pH, corro-sion products, and differential aeration cells all of which induce corrosion damage on steel. Thus, the synergistic role of aggressive Cl - ions and biofilm layer could induce pitting corrosion on steel ( Hamzah etal., 2013 ).

6 Conclusion

MIC of steels caused by different aerobic and anaerobic bacteria is a great issue in many industries such as marine, water distribution systems, and gas/oil pipelines.

Generally, biofilm formation is the initial stage of MIC, and the corrosion damages mostly appeared in the form of localized pitting or crevice corrosion.

Different types of corrosion-causing bacteria such as SRBs, IRBs, SOBs, IOBs, and SFBs in their biofilm state could induce corrosion damage of steel.

P. aeruginosa is an SFB that forms an aerobic heterogeneous biofilm layer on the steel substrate. Due to the presence of the biofilm layer, differential aeration cells are created, which induce the localized pitting corrosion on the steel substrate. Moreover, due

to the presence of acidic functional groups in their EPS, the EPS-metal chemical reaction could induce more corrosion damages.

The depth of pits on the steel substrate was observed to increase with longer exposure time to bacteria-containing solution.

The synergistically aggressive roles of biofilm and chloride ions are responsible for severe pitting corrosion of steel substrate in seawater.

Acknowledgments: The authors thank the Minis-try of Higher Education of Malaysia (MOHE) and Universiti Teknologi Malaysia (UTM) for providing financial support under Research University Grant No. Q.J130000.2524.04H87.

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Ahmad Abdolahi is a PhD student in Materials Engineering at Uni-versiti Teknologi Malaysia. He studies the use of conductive poly-mers to inhibit microbially influenced corrosion of steels. Abdolahi earned his BSc in Materials Science and Engineering from Azad University, Iran, in 2009 and his MSc in Engineering from Universiti Teknologi Malaysia in 2011.

Esah Hamzah graduated with a BSc in Materials Science and Technology from Swansea University, UK, and an MSc and a PhD in Metallurgy from the University of Manchester, UK. She is cur-rently working at the Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, as a Professor of Metallurgy. Her main research interests are in the areas of materials characterization, creep, fatigue, oxidation, corrosion, and coating.

Shahrir Hashim graduated with a BSc in Chemical Engineering from Colorado State University, USA, in 1989, an MSc in Polymer Science and Technology from the University of Manchester, UK, in 1991, and a PhD in Chemical Engineering from the University of Loughbor-ough, UK, in 2001. He is currently working at the Faculty of Chemical Engineering, Universiti Teknologi Malaysia. His main research interests are polymerization, polymer hydrogel, polymer coating, conductive polymer, and biodegradable polymer.

Zaharah Ibrahim graduated with a BSc in Biochemistry and an MSc in Chemistry from Northern Illinois University, USA, and a PhD in Chemistry from Universiti Teknologi Malaysia. Her main research interests are in the areas of biochemistry, environmental microbiol-ogy, and microbial bioremediation.



Corros Rev 2013 | Volume xx | Issue x1

Graphical abstract

Ahmad Abdolahi, Esah Hamzah, Zaharah Ibrahim and Shahrir HashimMicrobially influenced corrosion of steels by Pseudomonas aeruginosa

DOI 10.1515/corrrev-2013-0047Corros Rev 2014; xx(x): xxxx

Review: Microbially influenced corro-sion of steels caused by Pseudomonas aeruginosa has been reviewed.

Keywords: bacteria; biofilm; micro-bially influenced corrosion; pitting; Pseudomonas aeruginosa ; steels.

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corrosion review

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