Microbiologically Influenced Corrosion Mic Biology Essay

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Microbiologically influenced corrosion is one type of corrosion that could be harmful to almost all engineering materials. The term of MIC has been defined in many ways that are more or less similar. Some of the definitions for MIC are as follows:

MIC is an electrochemical process whereby micro-organisms may be able to initiate, facilitate or corrosion reactions through the interaction of the three components that make up this system: metal, solution and micro-organisms Error: Reference source not found.

MIC refers to the influence of micro-organisms on the kinetics of corrosion processes of metals, caused by micro-organisms adhering to the interfaces. A prerequisite for MIC is the presence of micro-organisms. If the corrosion is influenced by their activity, further requirements are: 1. an energy source 2. a carbon source 3. an electron donator 4. an electron acceptor 5. water.

MIC is taking place whenever the reactants or products of the microbial metabolic reactions interact with the reactants or products of electrochemical reactions occurring between the metal surface and the environment in such way that these interferences affect the thermodynamics and/or kinetics of anodic dissolution of metal Error: Reference source not found.

Bacterial microbes associated with MIC are ubiquitous. In the environment, it can be found in the form of metal reducing bacteria (MRB), metal-depositing bacteria (MDB), slime-producing bacteria, acid-producing bacteria (APB), iron oxidizing bacteria (IOB) and sulphate reducing bacteria (SRB).

From those types of bacteria, SRB have been recognized as the major culprit in MIC. It is because of their characteristic which can thrive easily, live in anarerobic and sulphate environment and produced hydrogen sulphide (H2S) which is known as a toxic and corrosive gas. A brief information of SRB is given below.

Sulphate reducing bacteria (SRB)

SRB are a diverse group of obligate anaerobic, heterotrophic and mixotrophic bacteria, typified by Desulfobacter and Desulfovibrio Error: Reference source not found. SRB are bacterial species that can cause dissimilarity reduction of sulfur compounds, such as sulfate, thiosulfate, sulfite and even sulfur to sulfide, using sulfate as the terminal electron acceptor Error: Reference source not found. Javaherdasty Error: Reference source not found narrowed the SRB definition to include any organism that metabolically capable of reducing sulfate to sulphides.

The most common cell morphologies of SRB are curved and oval to rod-shaped. Their diameters usually range from 0.5 to 2 µm. Many SRB are actively motile by flagella. Other forms are spheres and long multicellular filaments. Several types of SRB tend to grow in clumps or cell aggregates and stick to surfaces.

SRB can be found everywhere. They are widespread in soil, seawater, fresh water and muddy sediments. The most common genera of SRB is Desulfovibrio, belonging to the the Desulfovibrionaceae family in the big group gram-negative mesophilic bacteria. Desulfovibrio is also the most frequently found species in anaerobic regions of soil, seawater, fresh water and muddy sediments. It can grow well within the temperature range between 5°C and 50°C, and the pH range from 5 to 10.

SRB used hydrogen or a few simple organic compounds such as lactate or pyruvate as electron donors for sulphate reduction. However, species of SRB are now known that oxidize carbon compounds, ranging from acetate to long-chain fatty acids. A list of Desulfovibrio genera of SRB is presented in Table 2. Error: Reference source not found.

Table 2.. Described Desulfovibrio genera of Sulphate reducing bacteria.

Organism

Shape

Thermophilic

Salt requirement

Electron donor and acceptors (metabolism products)

Desulfovibrio thermophilus

Rod

Yes

No

Lactate and pyruvate oxidized; Sulphate, sulphite and thiosulphate reduced

Desulfovibrio aculatus

Rod

No

No

Lactate, pyruvate, malate oxidized; Sulphate, sulphite and thiosulphate reduced.

Desulfovibrio sapovorans

Curved rod

No

No

Butyrate, 2-methylbutyrate, higher fatty acids to 18 carbons, lactate, pyruvate oxidized to acetate; sulphate, sulphite reduced

Desulfovibrio baarsif

Curved

No

No

Formate, acetate, propionate, butyrate, isobutyrate, 2-methylbutyrate, higher fatty acids to 18 carbons oxidized to CO2; sulphate, sulphite, thiosulphate reduced.

Table 2. shows that Desulfovibrio thermophilus has least metabolism products than other genera. Therefore, the metabolism products of Desulfovibrio thermophilus genera will be investigated in this study. The average concentration of the metabolism products is listed in Table 2..

Table 2.. Average SRB metabolism products concentrations.

No

Metabolic products

Concentrations

Remarks

1.

Sulphide

60 ppm

Optimum value of sulphide produced by SRB. Number of SRB detected is 1 X 106/mL Error: Reference source not found.

2.

Sulphate

50 ppm

Typical amount of sulphate found in water cooling system Error: Reference source not found. That amount of sulphate could arise from SRB activity. It is noted that SRB are also found in water cooling system.

3.

Sulphite

100 ppm

Number of SO32- found in urban environment Error: Reference source not found.

4.

Pyruvate

600 ppm

It is a maximum value of pyruvate produced by SRB Error: Reference source not found.

5.

Lactate

200 - 3200 ppm

Range amount of lactate produced by SRB after 23 days inoculation. The Number of SRB detected is 1.1 x 107 /ml Error: Reference source not found.

6.

Acetate

200 - 2400 ppm

Range amount of acetate produced by SRB after 23 days inoculation. Number of SRB detected is1.1 x 107/ml Error: Reference source not found.

7.

Thiosulphate

> 0.15 mM

Number of thiosulphate produced by SRB Error: Reference source not found.

Corrosion mechanism by SRB

In principle, MIC occurs at the material interface where sessile cells influence the corrosion kinetics of anodic and/or cathodic reactions. MIC does not invoke any new electrochemical reactions, but the involvement of microorganism does change the physiochemical environment at the interface. Example of this includes concentration of nutrition, pH, redox potential and water chemistry. A number of MIC mechanisms of metal corrosion by SRB has been proposed since the first cathodic depolarizaton theory (CDT) was suggested by von Wolzogen Kuhr and van der Vlught Error: Reference source not found and confirmed by Bryant et al. Error: Reference source not found. The early work of von Wolzogen Kuhr and van der Vlught suggested the following electrochemical reactions:

(anodic reaction) 2.

(water dissociation) 2.

(cathodic reaction) 2.

(bacterial consumption) 2.

(corrosion products) 2.

(corrosion products) 2.

(overall reaction) 2.

The overall process was described as "depolarization" based on theory that these bacteria remove hydrogen that accumulates on the iron surface. The electron removal as a result of hydrogen utilization results in cathodic depolarization forcing more iron to be dissolved at the anode. The direct removal of hydrogen from the surface is equivalent to lowering the activation energy for hydrogen removal by providing a "depolarization" reaction as shown in Figure 2. Proposed reaction of anaerobic corrosion in the presence of SRB on an iron surface Error: Reference source not found. The enzyme, hydrogenase, synthesized by many species of Desulfovibrio, may be involved in this specific depolarization process.

Figure 2. Proposed reaction of anaerobic corrosion in the presence of SRB on an iron surface Error: Reference source not found

King and Miller Error: Reference source not found concluded that accelerated corrosion of mild steel in the presence of SRB was due principally to the formation of iron sulphide. Because iron sulphide is not a permanent cathodic depolarizer, sustained corrosion rates were found to be dependent on the removal of the bound hydrogen by the action of bacterial hydrogenase. On contrast, Costello Error: Reference source not found proposed that dissolved H2S produced by SRB is responsible for the cathodic depolarization.

Lee Error: Reference source not found concluded that corrosion of mild steel in the SRB environment was mainly determined by the nature of metal and environmental conditions such as dissolved iron. When formation of iron sulphide film on mild steel was prevented before biofilm accumulated, the metal surface retained its scratch lines. However, when iron sulphide was formed before the accumulation of biofilm, visible localized corrosion appeared after 14 days and increased up to 21 days. Intergranular and pitting attacks were found in the localized corrosion area. The hypothesized localized corrosion process is illustrated schematically in Figure 2. Anaerobic corrosion process of mild steel on a precoated iron sulphide film followed by biofilm accumulation up to 21 days [23]..

Figure 2. Anaerobic corrosion process of mild steel on a precoated iron sulphide film followed by biofilm accumulation up to 21 days Error: Reference source not found.

Silva Error: Reference source not found proposed that hydrogenase play a key role in the initiation of corrosion caused by SRB. Its involvement in cathodic depolarization should be considered as the catalys of a reduction reaction, instead of the consumption of a reduction product.

Romero Error: Reference source not found-Error: Reference source not found built a corrosion mechanism by SRB. He correlated the corrosiove species with time and open circuit potential, corrosion products, sessile bacterial growth and attack morphology. He divided the mechanism of SRB corrosion into three stages. The first was controlled by the adsorption of bacterial cells and iron sulphide products, principally mackinawite and pyrite, over the metallic surface, activating it through the formation of micro galvanic corrosion cells which generated a hydrogen permeation peak. The second stage showed bacterial and inorganic equilibrium, in which the metal was slightly ennobled by the formation of a more compact iron sulphide film mixed with polymers generated planktonically by the bacteria. The third stage was controlled by a severe, localized corrosive process configured into groups of deep, rounded holes, produced mainly by local reduction of pyrite to mackinawite, due to the acidity generated by bacterial corrosion, and its subsequent detachment, leaving the base metal active facing a very large cathode made up of different iron sulphide products adhering to the metal: mackinawite, pyrite, esmitite, marcasite, troilite and pyrrotite. The corrosion process is illustrated schematically in Figure 2. Anaerobic corrosion process of mild steel on a precoated iron sulphide film followed by biofilm accumulation up to 21 days [23]. and the reactions are shown in reactions 2.8 to 2.23.

Stage 1: 2.

2.

2.

2.

2.

2.

2.

Stage 2: 2.

2.

2.

2.

Stage 3: 2.

2.

2.

2.

2.

Figure 2. The mechanism of SRB action in MIC based on sulphide corrosion and iron sulphide corrosion products Error: Reference source not found.

A latest SRB mechanism was built by Gu et al. Error: Reference source not found-Error: Reference source not found based on biocatalytic cathodic sulphate reduction (BCSR) theory. This theory assumes that MIC occurs because the electrons released by iron dissolution at the anode are utilized in the sulphate reduction at the cathode. The actual cathodic reactions are more complex, but this theory considers only the overall effect as shown in reaction 2.25.

2.

2.

Reaction 2.25 occurs at a negligible rate without biocatalysis from biofilms. The reaction is catalyzed by the hydrogenase enzyme system of hydrogenase positive SRB cells that is responsible for accelerate sulphate reduction. Some hydrogen sulphide ion will convet to hydrogen sulphide, especially in acidic pH as shown in reaction 2.26.

2.

In the presence of carbon source e.g. lactate, the sulphate reduction uses electrons donated by oxidation of lactate as shown below:

2.

In summary, most of SRB mechanism gives consideration limited on the effect of sulphide. To our knowledge, there is no SRB mechanism that capture other effects of SRB metabolism products. Therefore, it become challenges to study the influence of other SRB metabolism products on the corrosion mechanism and kinetic of steel.

Failure cases caused by SRB

MIC failures due to SRB have been reported for piping and equipment exposed in the marine environment, oil refining industry, fossil fuel, nuclear power plants, and process industries. As such based on the open literature, some examples of the failures are summarized below:

A rotating cylinder board mould (stainless steel type 303 EN 58 M) used for the manufacture of paper and board failed in the creviced regions formed between the axial rods and the outer face of the external spirally wound stainless steel mesh. The failures occurred three years after the mould had been commissioned. Examination revealed pit depths of 3-4 mm occurred in grain boundaries rich in manganese sulphide. It is also found that most of the corrosion had penetrated longitudinally inside the rod creating a hollow section covered only by a thin skin of metal Error: Reference source not found.

Pitting, having an etched and granular morphology, had been found on the parts of vertical axial suction pumps e.g. impeller, wear ring and bell house. Sulphide was detected in both the pitted regions and the corrosion products taken from several locations. The corrosion products, the slime films present on the surface of the various components and the river water all contained a large population of bacteria with SRB as the predominant species Error: Reference source not found.

Severe internal corrosion, with over 50% thickness loss in many locations, was encountered in a 610 mm diameter, API 5L Grade-B Sch-20 carbon steel pipeline used for carrying light crude oil from a wet tank to a common header. The design life of such a pipeline is typically more than 30 years. However, the severe corrosion damage occurred after about 7 years of service. A high H2S content was detected and it was an indication of SRB activity Error: Reference source not found.

A transmission oil products API 5L X52 pipeline in northern part of Iran cracked in 2004. Failure occurred in a portion of the pipeline that was placed at the top of a forest zone hill. The cracked zone was at 9 o'clock position. Field observation showed loosening, overlap-opening and disbanding of the applied polyethylene tape coating on the external surface of the pipeline in corroded section. High intensity of sulphur component and the observation of black corrosion product on the external surface of the pipe indicate SRB activity. A number of NDE and microbial activity test confirmed that SRB have been created and intensified pitting corrosion and have had important roles in crack development Error: Reference source not found.

SRB experiments

MIC by SRB has been extensively studied to seek better understanding on its influence on the corrosion kinetic and mechanism which expected to improve prevention and mitigation techniques.

Ocando et al. Error: Reference source not found studied the effect of ferrous ions on the pH and H2S on biofilms generated by SRB. A SRB pure culture of Desulfovibrio desulfuricans, grown in modified ATCC 1249 medium, was used in this study. They concluded that in the absence of ferrous ions, the pH on the iron surface decreased sharply to very low values due to a complex biofilm formation, which protected the material and impeded the hydrogen ions consumption by the corrosion process. However, in the presence of ferrous ions, the pH at metal interface remained almost constant and near to neutral values, due to the severity of the corrosion process, where the HS- and H+ are consumed and massive sulphides precipitation occurred. In addition, they found that the bacteria and corrosion products were mixed and formed a complex biofilm structure that covered the iron surface, being in some cases protective depending mainly on the ferrous ions presence.

Rainha et al. Error: Reference source not found studied the influence of SRB, grown in a lactate/sulphate medium, on the anaerobic corrosion of mild steel. The bacteria used was Desulfovibrio desulfuricans ATCC 27774. Higher corrosion rates as wall as the transpassive dissolution of Fe(0) or Fe(II) compounds to Fe(III) were observed. These effects are most probably due to high quantities of sulphide and/or to other alterations in the sulphate/lactate medium produced by the microbial activity of the SRB. In addition, they confirmed that the presence of SRB induces changes in the kinetics and mechanism of the anodic dissolution of iron in the lactate/sulphate media.

Amaya and Perez Error: Reference source not found studied SRB influence on the corrosion behaviour of API-XL70 steel. They indicated that the presence of microorganisms is controlled through the diffusion of the reaction at the cathode. Their studied also showed that SRB induced localized corrosion.

Benetton and Castaneda Error: Reference source not found observed SRB biofilm growth and its influence in corrosion monitoring. The bacteria used was Desulfovibrio gabonensis (DSM 10636) and Desulfovibrio capillatus (DSM 14982) grown in supplemented artificial seawater. The results showed that bio film formation induced diffusion controlled corrosion, where biofilm combined with corrosion products is acting as an infinite diffusion layer. Furthermore, they stated that cathodic depolarization mechanism is limited to the activation controlled (no biofilm). Once biofilm is established, the rate limiting step is diffusion controlled.

Miranda et al. Error: Reference source not found studied the role of Desulfovibrio capillatus on the corrosion behaviour of carbon steels under anaerobic conditions. Different concentrations of thiosulphate as electron acceptor for bacterial growth were employed. Their study showed that the corrosion activity of carbon steel notably increased, due to high concentration of bacterial metabolites. It is also noted that thiosulphate is used by SRB as the principal factor in the corrosion process.

Duan et al. Error: Reference source not found studied corrosion behaviour of carbon steel influenced by anaerobic biofilm in natural seawater. The bacteria used were sulphate reducing bacteria, Desulfovibrio caledoniensis and iron oxidising bacteria Clostridium sp. They found that single species (SRB only) produced iron sulphide and accelerated corrosion, but mixed species (SRB and IOB) produced sulphate green rust and inhibited corrosion. In addition, they stated that the biotic sulphide produced by SRB, could only temporarily accelerated carbon steel corrosion. The continued existence of SRB was the key to the accelerated corrosion, implying that steel and bacteria should make direct or indirect contact through conducting FeS or possibly through electron shuttles.

Dzierzewicz et al. Error: Reference source not found investigated the relationship between microbial metabolic activity (expressed by generation time, rate of H2S production and the activity of hydroogenase and adenosine phosphosulphate (APS) reductase enzymes) and biocorrosion of carbon steel. The bacteria used was Desulfovibrio desulfuricans, isolated from soil and mud samples. The bacteria were incubated for 6 days in the lactate/sulphate liquid medium under anaerobic conditions. It is noted that the rate of H2S production was approximately directly proportional to the specific activities of the investigated enzymes. These activities were inversely proportional to the generation time. The carbon steel MIC rate was strongly affected by bacterial resistance to metal ions. On contrast, it is observed weaker correlation between the MIC rate and the activity of enzymes.

Kuang et al. Error: Reference source not found studied the effects of SRB on the corrosion behaviour of carbon steel. Their results showed that SRB growing process consisted of three different stages, namely: exponential, death and residual phases. The corrosion behaviour of carbon steel in the system containing SRB hardly related on the active SRB number. But it depends on the accumulation of the metabolism products of SRB. Moreover, the anode process and the corrosion rate are accelerated during the exponential phase and stable during the death and residual phase.

Gayosso et al. Error: Reference source not found-Error: Reference source not found evaluated the corrosion rate of X52 steel, induced by a microbial consortium, isolated from the Atasta Nohoch gas transporting pipeline in Mexico. The major species identified was Desulfovibrio viatnamensis. They recorded the corrosion rate of X52 steel was about 0.3 mm year-1. Their study also indicated that the damage observed on the metal surface depends upon the sessile microorganism's population.

Frank et al. Error: Reference source not found investigated the effect of CO2 introduction on the corrosion behaviour of carbon steel in bacteria environment. It was observed that SRB growth was stimulated probably due to the creation of an anaerobic environment, yielding a highly corrosive environment.

Mendoza et al. Error: Reference source not found observed the corrosion kinetics X52 steel caused by SRB. The bacteria was isolated from the inner deposits of a pipeline that transports sour gas in the marine region of Mexico. The bacteria was identified as Desulfovibrio sp. By weight loss method, they recorded that the corrosion rate of X52 steel was 0.15 mm year-1.

Li et al. Error: Reference source not found studied the corrosion behaviour of carbon steel influenced by SRB in soil environments. They concluded that the existence of SRB greatly influences the corrosion behaviour of carbon steel. The potential in control case (biocide added) was around -600 mV and always more positive than that in SRB cases. However, in the presence of SRB, the potential increased slightly for the first 6 days and then maintained around -740 mV/SCE, but the potential fluctuated -600 mV to -800 mV/SCE after 50 days until the experiment ended. In control case, the corrosion rate observed was stable around 0.02 mm year-1. However, with the presence of SRB, the corrosion rate was fluctuating with the maximum value of 0.4 mm year-1. Moreover, they concluded that the corrosion behaviour of carbon steel in anaerobic conditions was divided into three categories, i.e., (1) anaerobic inorganic corrosion which depends on the ability to utilize the cathodic reactants, e.g. water or hydrogen ion. (2) the precipitation of protective film caused no decrease of electrical resistance (no start of corrosion). (3) MIC induced by SRB; this corrosion starts after the protective film ruptured, caused developing of localized corrosion.

Romero and Urdaneta Error: Reference source not found studied the correlation between Desulfovibrio sessile growth and OCP, hydrogen permeation, corrosion products and morphological attack on iron. The bacteria used was Desulfovibrio desulfuricans. Some conclusions have been made from their study:

H2S generated by SRB is the precursor for bacterial corrosion of steel.

In the presence of ferrous ions, the genus Desulfovibrio severely corrodes iron approximately 0.43 mm year-1 in the form of groups of deep holes.

In the presence of SRB and ferrous ions, the iron sulphide products formed starting with mackinawite, could be: pyrite, esmitite, marcasite, greigite, pyrrotite and troilite. However, pyrite is the most protective principally when it is mixed with extracellular polymeric membrane generated by the bacteria.

Bacterial corrosion diminishes pH locally favoring the reduction of pyrite to mackinawite and severe localized steel corrosion where the bacteria are formed in colonies.

The mackinawite formed does not have protective characteristics due to its hydrophilic character and its sizeable volume which causes it to detach leaving the base metal bare and exposed to the corrosive fluid.

Gramp et al. Error: Reference source not found observed the formation of Fe sulphides in cultures of SRB and in abiotic sulphide. Their results showed that makckinawite and greigite were dominant iron sulphide phases found in SRB cultures. Meanwhile, mackinawite, greigite and pyrite were found in abiotic sulphide with greigite as the more prevalent one.

Herbert et al. Error: Reference source not found characterized the surface chemistry and morphology of crystalline iron sulphides precipated in media containing SRB. Their study showed that the iron sulphide produced were composed of both ferric and ferrous iron coordinate with monosulphide, with lesser amounts of disulphide and polysulphides also present. In addition, they concluded that the precipitates possessed a surface composition similar to greigite, with the remaining composed of disordered mackinawite.

Zhao et al. Error: Reference source not found studied the effect of SRB on carbon steel corrosion in sea mud. It is observed that the presence of SRB increased the carbon steel corrosion rate by 182% compared with that in sterile sea mud. Wiith the excess of dissolved H2S, they observed the transformation of protective FeS film to FeS2 or other non stoichimetric polysulphide. Such transformation changes the state of former layer and accelerated the corrosion process.

The growth behaviour of SRB was investigated by Hu Error: Reference source not found. Her study showed that both SRB growth rate and the protective iron sulphide film were affected by the ferrous iron concentration. Increasing ferrous ion concentrations increased the SRB growth rate and corrosion rate. In addition, it is observed that the increase of SO42- concentration within the range of 1.93 g/l to 6.5 g/l decreased the planktonic growth and the corrosion rate of mild steel.

Jhobalia Error: Reference source not found studied the role of a biofilm and its characteristics in MIC. His study showed that the corrosion by SRB is also influenced by temperature. At lower temperature (5 °C and 25 °C), the corrosion rate observed is lower than those at 37 °C. This is due to the corrosion by SRB is influenced by the number of SRB cell, and the cell groth rate is strongly affected by temperature. He also found that the presence of iron concentrations influenced the corrosion type. With the presence of 5 ppm and 50 ppm iron concentrations, there was no localized attack observed. However, with the presence of 25 ppm, where the super saturation occurred, localized attack was observed.

In summary, H2S produced by SRB and FeS film formed, have significant role in corrosion caused by SRB. However, the similar role of biotic sulphide and abiotic sulphide in the presence of other SRB metabolism products on the corrosion mechanism and kinetic of carbon steel is still unclear and need further investigation. Therefore, it becomes a challenge to characterize and compared the abiotic and biotic sulphide role on carbon steel.

Abiotic sulphide experiments compare to SRB experiments

As based on the review above, the corrosion caused by SRB is related to the sulphide produced and FeS film formed. A number of experiments have been conducted to investigate the behaviour of abiotic sulphide compared to biotic sulphide.

Newman et al. Error: Reference source not found studied the effect of abiotic sulphide on the corrosion rate of steel in neutral solution relevance to MIC. The corrosion rate measured in abiotic sulphide is a few times lower than those achievable in SRB experiment, however the similarity is striking. They underlined that the difference is probably related to the aspect of SRB corrosion which has not been simulated, namely the massive deposition of FeS that occurs when SRB grow in culture media containing Fe2+. In the abiotic experiments, FeS could only form as a result of corrosion. Furthermore, they highlighted the importance of biofilm formation including extracellular protein produced by SRB which help to cement the particulate FeS together. In an abiotic experiment, the FeS film formed can be fragile and may create crevice condition on the metal surface.

Videla et al. Error: Reference source not found-Error: Reference source not found compared the corrosion products formed in biotic and abiotic media. From their study, the chemical and structural analyses of sulphide films formed under abiotic and biotic solutions presented the following characteristics:

In biotic and abiotic sulphide films, the outer layers are formed by both FeS and FeS2. However, in a biogenic sulphide film, FeS is the major specie whereas in an abiotic sulphide film FeS2 is predominant.

The chemical composition of tubercles formed in abiotic and biotic solutions is different. The main contrast is that the corroded metal surface underneath a biogenic film is made up of iron sulphide whereas in a non biogenic film corresponds to an iron hydroxide or oxide.

The films formed under biogenic conditions are more adherent to the surface of the metal than those formed in abiotic media, which are flaky and loosely adherent.

The inner shell contained more sulphur in biotic films than those formed in abiotic media.

Biogenic sulphide solution is less aggressive compared to abiotic sulphide.

The previous history of the sulphide film may play a relevant role in the corrosion behaviour of carbon steel. According to sulphide concentration, and to the presence or absence a biofilm, the protective characteristics of the sulphide corrosion product layer may change. During the different stages of the biofilm growth, biogenic layers of corrosion products can offer some protection to the metal by improving the adherence of the sulphide film but can also enhance corrosion by inducing the presence of heterogeneities at the metal surface.

The type of FeS formed (either as a compact film, or as a soft precipitate, or in suspension) conditions the sulphide effect on iron dissolution.

Kuang et al. Error: Reference source not found concluded that the corrosion rate caused by SRB is hardly relates on the active SRB number, but it depends on the accumulation of the metabolism SRB products, i.e. sulphide. Their results also showed that the potentiodynamic polarization curves in the presence of SRB showed consistency results with potentiodynamic polarization curves in the medium containing different concentrations of Na2S.

Sherar et al. Error: Reference source not found characterized the corrosion morphology of carbon steel induced by abiotic sulphide and biotic sulphide. It is concluded that biofilm formation and corrosion product morphology are highly nutrient dependent. Reducing the carbon content in solution appears to favour abiotic corrosion leading the formation of crystalline FeS. It is also confirmed that the dominant iron phase formed was mackinawite under both abiotic and biotic conditions. In addition, they claimed that the use of abiotic sulphide is sufficient enough to develop steel rate prediction. However, this simplistic approach does not account for the heterogeneity that exists in bacterial system.

From the review above, it is seen that the use of abiotic sulphide could be used to simulate the SRB experiments. However, in the real SRB experiments, the corrosive species in not only limited to the sulphide. In their metabolic activities, SRB also produced other species that could harmful the steel, e.g. CO2, acetate, sulphite, pyruvate, sulphate and lactate. The presence of these species could alter the role of sulphide on the corrosion kinetic and mechanism. Therefore, it is quite challenging to investigate the effect of sulphide in the presence of other species as relevance to MIC caused by SRB.

Abiotic H2S Corrosion

The role of corrosion by SRB is related to the sulphide produced. The sulphide will react to the available hydrogen forming H2S. Therefore, a brief review of hydrogen sulphide is given below.

The dissociation of hydrogen sulphide in water involves a series of chemical reactions as described from Equations 2.28 to 2.32. The proposed chemical reactions steps are Error: Reference source not found:

H2S dissolution

H2S(g) ↔ H2S(aq) 2.

H2S dissociation

H2S(aq) ↔ HS- (aq) + H+(aq) 2.

HS- dissociation

HS-(aq) ↔ H+(aq) + S2-(aq) 2.

H2S Reduction

2H2S(aq) + 2e- → H2(g) + 2HS-(aq) 2.

FeS formation by precipitation

Fe(s) + S2-(aq) ↔ FeS(s) 2.

The reactions of H2S in aqueous vary with pH. At acidic solutions, the dominant sulphide species is molecular H2S. At pH of about 6, the solutions will contain bisulphide ions. The higher pH will result in the formation of bisulphide. At pH of around 7, the amount of H2S molecular and bisulphide forms is similar Error: Reference source not found.

In H2S corrosion system, there are different possibilities of iron sulphide formation in aqueous solution Error: Reference source not found. The formation of solid film on the surface is due to anodic dissolution of iron. Ferrous ions dissolve into solution and react with sulphide ions in the solution, hence no film of corrosion product on the surface. The formation of sulphide can also by mixing reaction between ferrous ions that react on the surface and in solution. Those film formations bring different film porosities of iron sulphides. The porous surface facilitates the cathodic reaction and creates anodic dissolution of iron that affects to the corrosion rate Error: Reference source not found. The types of FeS are influenced by temperature and H2S activity Error: Reference source not found. Based on kinetics theories, several types of FeS are commonly found in oil field corrosion are pyrite (FeS2), pyrrhotite, troilite, amorphous iron sulphide, cubic iron sulphide and mackinawite. Figure 2. Corrosion sequence for carbon steel in aqueous H2S solution shows corrosion sequence for carbon steel in aqueous H2S solution Error: Reference source not found-Error: Reference source not found. Table 2. Properties of the iron sulphide shows properties of the iron sulphide Error: Reference source not found.

Table 2. Properties of the iron sulphide

Compound

Mackinawite

Pyrhotite

Greigite

Marcaisite

Pyrite

Smythit

Formula

Fe(1-x)S

Fe(1-x)S

Fe(1-x)S

Fe(1-x)S

Fe(1-x)S

Fe(1-x)S

Value of x

0.057-0.064

0.00-0.14

0.25

0.5

0.5

0.00-0.18

Crystal type

Tetragonal

NiAs type

Cubic

Orthorhombic

Cubic

Rhombohedral

Smith et al. Error: Reference source not found-Error: Reference source not found proposed two mechanisms of H2S as shown in Figure 2. Two mechanisms for H2S corrosion. After the initial adsorption of H2S on the steel surface, mackinawite can be formed from amorphous FeS either by path 1 or path 2 [57]-[58].. The second path is more preferable and could be described as follows:

H2S diffuses to the steel surface.

H2S reacts with the steel to form mackinawite scale on the surface.

Mackinawite scale dissolves to Fe(HS)+ and HS-.

Fe(HS)+ diffuses away from the steel surface, and

More H2S diffuses to react with the exposed steel.

Carbon steel + H2S/H2O

Solid state growth

Mackinawite

Film rupture and precipitation

O2

Solid state

O2 Mackinawite Cubic FeS Troilite

Solid state

Dissolution & precipitation

H2SGreigite

H2S Pyrhotite

Dissolution & electrodeposition

Pyrite Marcasite

Figure 2. Corrosion sequence for carbon steel in aqueous H2S solution.

Path 1:

Path 2

Figure 2. Two mechanisms for H2S corrosion. After the initial adsorption of H2S on the steel surface, mackinawite can be formed from amorphous FeS either by path 1 or path 2 Error: Reference source not found-Error: Reference source not found.

Beside accelerated corrosion, the presence of H2S could inhibit the rate of corrosion. Ma et al. Error: Reference source not found proposed a probable mechanism of the inhibitive effect of H2S as follows:

2.

2.

2.

The species FeSH+ may be incorporated directly into a growing layer of mackinawite via Eq. (2.36).

2.

Or it may be hydrolyzed to yield Fe2+ via Eq. (2.37)

2.

Ma et al. Error: Reference source not found stated, if reaction (2.36) dominated the electrode surface, then the nucleation and growth of one or more of the iron sulphides, i.e. mackinawite, cubic ferrous sulphide or troilite occurs. However, the role of H2S, accelerates or inhibits the rate of corrosion, depending on the pH value. At lower pH values (<2), ferrous ion will dissolve through reaction (2.37). As a result, there will be less iron sulphide film due to its high solubility at low pH. Meanwhile, at the pH values of 3-5, H2S begins to exhibit its inhibiting effect as FeSH+ species may form partially mackinawite through reaction (2.36). The mackinawite can convert into troilite which is more stable and protective. At a pH of more than 5, mackinawite was the only observed product of corrosion. As mackinawite has less protective ability that troilite, the inhibiting effect of H2S decreases.

Tang et al. Error: Reference source not found studied the effect of H2S concentration (59 - 409 ppm) on the corrosion behaviour of carbon steel at 90°C. The results showed that the corrosion rate increased with the increase of H2S concentrations. H2S showed dtrong acceleration effect on the cathodic hydrogen evolution of carbon steel, causing carbon steel to be seriously corroded. The corrosion products formed on carbon steel surface were composed of mackinawite, which was loose and did not show any protective properties. Severe localized corrosion on the steel surface was also observed, which may attributed to cemetites stripped off from the grain boundary.

When H2S gas presents with CO2 gas, there will be a growth competition between FeCO3 and FeS films which affects to the corrosion rate. Nesic et al. Error: Reference source not found constructed a model that identified the growth of film formation containing H2S/CO2 gas. The initial film formed is started by FeS film formation. Then, the FeCO3 film becomes thicker and denser at the metal/film interface due to an increase in pH and Fe2+ concentration.

Brown Error: Reference source not found found that the corrosion rate in CO2 environment increased in the presence of small H2S concentration of less than 30 ppm. However, he also observed a decreasing of corrosion rate in the presence of 100 ppm H2S. The scale produced was adherent and protective enough to retard corrosion attack. The scale was more protective when temperature was increased to 80oC.

The finding by Brown was supported by Lee Error: Reference source not found. Lee concluded that small of amount of H2S (10 ppm) lead to rapid reduction of the corrosion rate. Based on the SEM observation, they found that the scale formed on the surface that inhibited corrosion rate have a mackinawite structure.

Agrawal et al. Error: Reference source not found observed that the phenomena of accelerated corrosion in a CO2 and H2S environment occurs at low H2S concentration. They found that there was a strong correlation between the corrosion rates and the temperature. In the range of H2S concentration studied, the corrosion rate showed a polynomial curve with increasing the temperature.

Kvarekval et al. Error: Reference source not found studied the effect of H2S concentrations ranging from 150 - 450 ppm in a CO2 environment. The results showed that higher corrosion rates were obtained in the presence of H2S compared to experiments without H2S. The corrosion rates were in the range of 0.1-2 mm year-1.

Singer et al. Error: Reference source not found observed that trace amounts of H2S greatly retards the CO2 corrosion with general corrosion rates usually 10 to 100 times lower than their pure CO2 equivalent. The most protective conditions were observed at the lowest partial pressure of H2S. However, corrosion rate increased when more H2S was added. The presence of trace amounts of H2S (0.004 bar) in the CO2 environment sharply decreases the corrosion rate by two orders of magnitude. As the partial pressure of H2S is increased to 0.13 bar, the tendency is reversed and the general corrosion rate increased by an order of magnitude.

Carew et al. Error: Reference source not found observed a rapid and significant reduction in the CO2 corrosion rate both in single and multiphase flow in the presence of 10 ppm H2S. At higher H2S concentrations (up to 250 ppm) the trend was reversed and a mild increase of the corrosion rate was observed.

Schmitt et al. Error: Reference source not found stated that a change in pH from 4 to 6 had only little effect on the corrosion rate, and at pH 6, 60 °C and 25 ppm H2S, protective corrosion films were formed and no localized corrosion were observed Error: Reference source not found. The effect seems to vanish at higher pH values (5.5-7) and higher temperatures (>80°C), when a protective film is formed. They concluded that an increase of the CO2 partial pressure in the same flow system from 3.8 to 10.6 bar reduces the maximum corrosion rates from about 15 to 0.2 mm/y under conditions when semi-protective films are formed, e.g. in the pH range below 5.2 Error: Reference source not found.

In combination with CO2, corrosion rate of H2S showed different phenomena compared to without CO2 as reported by Makarenko et al. Error: Reference source not found. With CO2, the corrosion process is accelerated by cathodic reaction of hydrogen ion reduction. It has been proven that CO2 corrosion of carbon steel increases by 1.5-2 times with increase of H2S content in the mixture (p H2S<0.5 MPa) in the temperature range 20-80°C. Further increasing in H2S content (p H2S≥0.5-1.5 MPa), the corrosion rate will decrease, especially in the temperature range 100-250°C, because of the influence of FeS and FeCO3 on corrosion. It may relate to formation of protective film Error: Reference source not found.

Choi et al. Error: Reference source not found studied the effect of H2S on the CO2 corrosion of carbon steel in acidic solutions. The results showed that the addition of 100 ppm H2S to CO2 induced rapid reduction in the corrosion rate at both pHs 3 and 4. The inhibition effect is attributed to the formation of thin FeS film on the steel surface that suppressed the anodic dissolution reaction.

Abelev et al. Error: Reference source not found examined the effect of H2S on iron corrosion in 3 wt% NaCl solution saturated with CO2 in temperature range of 25-85 °C. Small H2S concentrations (5 ppm) have an inhibiting effect on corrosion in the presence of CO2 at temperatures from 25 to 55 °C. However, 50 ppm H2S is needed to provide significant corrosion inhibition. At higher H2S concentrations, the corrosion rate increases rapidly, while still remaining below the rate for the H2S free solution. Corrosion protection in the temperature range 25 to 55 °C is attributed to adsorption of sulphur on the native iron oxide, and this layer provides significant corrosion inhibition. The main species responsible for inhibition included Fe(II) bonded to S and O. Meanwhile, at higher H2S concentrations a thicker layer of iron corrosion products forms on the surface by a dissolution precipitation mechanism. However, this layer is porous and inhomogeneous, having voids and irregularities yielding less protective characteristic to the steel.

Sun Error: Reference source not found showed that mackinawite is the dominant scale formed on the steel surface, which protects the steel from corroding in CO2/H2S corrosion. She also highlighted that the makeup of the surface scale not only depends on the water chemistry and the respective solubility of iron carbonate and iron sulphides, but also on the competitiveness of the two scale formation mechanisms. Only at very high supersaturation of iron carbonate are both iron carbonate and mackinawite scale are found on the steel surface, with iron carbonate in the outer portion of the mackinawite scale.

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