Microbiologically influenced corrosion is one type of corrosion that could be harmful to about all technology stuffs. 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 procedure whereby microorganisms may be able to originate, ease or corrosion reactions through the interaction of the three constituents that make up this system: metal, solution and micro-organisms Mistake: Reference beginning non found.
MIC refers to the influence of microorganism on the dynamicss of corrosion procedures of metals, caused by micro-organisms adhering to the interfaces. A requirement for MIC is the presence of microorganisms. If the corrosion is influenced by their activity, farther demands are: 1. an energy beginning 2. a C beginning 3. an negatron donator 4. an negatron acceptor 5. H2O.
MIC is taking topographic point whenever the reactants or merchandises of the microbic metabolic reactions interact with the reactants or merchandises of electrochemical reactions happening between the metal surface and the environment in such manner that these interventions affect the thermodynamics and/or dynamicss of anodal disintegration of metal Mistake: Reference beginning non found.
Bacterial bugs associated with MIC are omnipresent. In the environment, it can be found in the signifier of metal cut downing bacteriums ( MRB ) , metal-depositing bacteriums ( MDB ) , slime-producing bacteriums, acid-producing bacteriums ( APB ) , Fe oxidising bacteriums ( IOB ) and sulphate reduction bacteriums ( SRB ) .
From those types of bacteriums, SRB have been recognized as the major perpetrator in MIC. It is because of their characteristic which can boom easy, live in anarerobic and sulphate environment and produced H sulfide ( H2S ) which is known as a toxic and caustic gas. A brief information of SRB is given below.
Sulphate cut downing bacteriums ( SRB )
SRB are a diverse group of obligate anaerobic, heterotrophic and mixotrophic bacteriums, typified by Desulfobacter and Desulfovibrio Error: Reference beginning non found. SRB are bacterial species that can do unsimilarity decrease of S compounds, such as sulphate, thiosulfate, sulfite and even sulphur to sulfide, utilizing sulphate as the terminal negatron acceptor Mistake: Reference beginning non found. Javaherdasty Mistake: Reference beginning non found narrowed the SRB definition to include any being that metabolically capable of cut downing sulphate to sulfides.
The most common cell morphologies of SRB are curved and egg-shaped to bacillar. Their diameters normally range from 0.5 to 2 Aµm. Many SRB are actively motile by scourge. Other signifiers are domains and long multicellular fibrils. Several types of SRB tend to turn in bunchs or cell sums and stick to surfaces.
SRB can be found everyplace. They are widespread in dirt, saltwater, fresh H2O and muddy deposits. The most common genera of SRB is Desulfovibrio, belonging to the the Desulfovibrionaceae household in the large group Gram-negative mesophilic bacteriums. Desulfovibrio is besides the most often found species in anaerobiotic parts of dirt, saltwater, fresh H2O and muddy deposits. It can turn good within the temperature scope between 5A°C and 50A°C, and the pH scope from 5 to 10.
SRB used H or a few simple organic compounds such as lactate or pyruvate as negatron givers for sulphate decrease. However, species of SRB are now known that oxidize C compounds, runing from ethanoate to long-chain fatty acids. A list of Desulfovibrio genera of SRB is presented in Table 2. Mistake: Reference beginning non found.
Table 2.. Described Desulfovibrio genera of Sulphate cut downing bacteriums.
Electron giver and acceptors ( metamorphosis merchandises )
Lactate and pyruvate oxidized ; Sulphate, sulphite and thiosulphate reduced
Lactate, pyruvate, malate oxidized ; Sulphate, sulphite and thiosulphate reduced.
Butyrate, 2-methylbutyrate, higher fatty acids to 18 Cs, lactate, pyruvate oxidized to acetate ; sulfate, sulphite reduced
Formate, ethanoate, propionate, butyrate, isobutyrate, 2-methylbutyrate, higher fatty acids to 18 Cs oxidized to CO2 ; sulfate, sulphite, thiosulphate reduced.
Table 2. shows that Desulfovibrio thermophilus has least metamorphosis merchandises than other genera. Therefore, the metamorphosis merchandises of Desulfovibrio thermophilus genera will be investigated in this survey. The mean concentration of the metamorphosis merchandises is listed in Table 2..
Table 2.. Average SRB metamorphosis merchandises concentrations.
Optimum value of sulfide produced by SRB. Number of SRB detected is 1 Ten 106/mL Mistake: Reference beginning non found.
Typical sum of sulfate found in H2O chilling system Error: Reference beginning non found. That sum of sulfate could originate from SRB activity. It is noted that SRB are besides found in H2O chilling system.
Number of SO32- found in urban environment Mistake: Reference beginning non found.
It is a maximal value of pyruvate produced by SRB Error: Reference beginning non found.
200 – 3200 ppm
Range sum of lactate produced by SRB after 23 yearss vaccination. The Number of SRB detected is 1.1 ten 107 /ml Mistake: Reference beginning non found.
200 – 2400 ppm
Range sum of ethanoate produced by SRB after 23 yearss vaccination. Number of SRB detected is1.1 x 107/ml Mistake: Reference beginning non found.
& gt ; 0.15 millimeter
Number of thiosulphate produced by SRB Error: Reference beginning non found.
Corrosion mechanism by SRB
In rule, MIC occurs at the stuff interface where sessile cells influence the corrosion dynamicss of anodal and/or cathodic reactions. MIC does non raise any new electrochemical reactions, but the engagement of micro-organism does alter the physiochemical environment at the interface. Example of this includes concentration of nutrition, pH, redox potency and H2O chemical science. A figure 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 Mistake: Reference beginning non found and confirmed by Bryant et Al. Mistake: Reference beginning non found. The early work of von Wolzogen Kuhr and van der Vlught suggested the undermentioned electrochemical reactions:
( anodal reaction ) 2.
( H2O dissociation ) 2.
( cathodic reaction ) 2.
( bacterial ingestion ) 2.
( corrosion merchandises ) 2.
( corrosion merchandises ) 2.
( overall reaction ) 2.
The overall procedure was described as “ depolarisation ” based on theory that these bacteriums remove H that accumulates on the Fe surface. The negatron remotion as a consequence of H use consequences in cathodic depolarisation coercing more Fe to be dissolved at the anode. The direct remotion of H from the surface is tantamount to take downing the activation energy for H remotion by supplying a “ depolarisation ” reaction as shown in Figure 2. Proposed reaction of anaerobiotic corrosion in the presence of SRB on an Fe surface Error: Reference beginning non found. The enzyme, hydrogenase, synthesized by many species of Desulfovibrio, may be involved in this specific depolarisation procedure.
Figure 2. Proposed reaction of anaerobiotic corrosion in the presence of SRB on an Fe surface Error: Reference beginning non found
King and Miller Error: Reference beginning non found concluded that accelerated corrosion of mild steel in the presence of SRB was due chiefly to the formation of Fe sulfide. Because Fe sulfide is non a lasting cathodic depolarizer, sustained corrosion rates were found to be dependent on the remotion of the edge H by the action of bacterial hydrogenase. On contrast, Costello Error: Reference beginning non found proposed that dissolved H2S produced by SRB is responsible for the cathodic depolarisation.
Lee Error: Reference beginning non found concluded that corrosion of mild steel in the SRB environment was chiefly determined by the nature of metal and environmental conditions such as dissolved Fe. When formation of Fe sulfide movie on mild steel was prevented before biofilm accumulated, the metal surface retained its abrasion lines. However, when Fe sulfide was formed before the accretion of biofilm, seeable localised corrosion appeared after 14 yearss and increased up to 21 yearss. Intergranular and opposing onslaughts were found in the localised corrosion country. The hypothesized localised corrosion procedure is illustrated schematically in Figure 2. Anaerobic corrosion procedure of mild steel on a precoated Fe sulfide movie followed by biofilm accretion up to 21 yearss [ 23 ] ..
Figure 2. Anaerobic corrosion procedure of mild steel on a precoated Fe sulfide movie followed by biofilm accretion up to 21 yearss Mistake: Reference beginning non found.
Silva Mistake: Reference beginning non found proposed that hydrogenase play a cardinal function in the induction of corrosion caused by SRB. Its engagement in cathodic depolarisation should be considered as the catalys of a decrease reaction, alternatively of the ingestion of a decrease merchandise.
Romero Mistake: Reference beginning non found-Error: Reference beginning non found built a corrosion mechanism by SRB. He correlated the corrosiove species with clip and unfastened circuit potency, corrosion merchandises, sessile bacterial growing and onslaught morphology. He divided the mechanism of SRB corrosion into three phases. The first was controlled by the surface assimilation of bacterial cells and press sulfide merchandises, chiefly mackinawite and pyrite, over the metallic surface, triping it through the formation of micro voltaic corrosion cells which generated a H pervasion extremum. The 2nd phase showed bacterial and inorganic equilibrium, in which the metal was somewhat ennobled by the formation of a more compact Fe sulfide movie assorted with polymers generated planktonically by the bacteriums. The 3rd phase was controlled by a terrible, localised caustic procedure configured into groups of deep, rounded holes, produced chiefly by local decrease of fool’s gold to mackinawite, due to the sourness generated by bacterial corrosion, and its subsequent withdrawal, go forthing the base metal active confronting a really big cathode made up of different Fe sulfide merchandises adhering to the metal: mackinawite, fool’s gold, esmitite, marcasite, troilite and pyrrotite. The corrosion procedure is illustrated schematically in Figure 2. Anaerobic corrosion procedure of mild steel on a precoated Fe sulfide movie followed by biofilm accretion up to 21 yearss [ 23 ] . and the reactions are shown in reactions 2.8 to 2.23.
Phase 1: 2.
Phase 2: 2.
Phase 3: 2.
Figure 2. The mechanism of SRB action in MIC based on sulphide corrosion and Fe sulfide corrosion merchandises Error: Reference beginning non found.
A latest SRB mechanism was built by Gu et Al. Mistake: Reference beginning non found-Error: Reference beginning non found based on biocatalytic cathodic sulfate decrease ( BCSR ) theory. This theory assumes that MIC occurs because the negatrons released by Fe disintegration at the anode are utilized in the sulfate decrease at the cathode. The existent cathodic reactions are more complex, but this theory considers merely the overall consequence as shown in reaction 2.25.
Chemical 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 sulfate decrease. Some H sulphide ion will convet to hydrogen sulphide, particularly in acidic pH as shown in reaction 2.26.
In the presence of C beginning e.g. lactate, the sulfate decrease uses negatrons donated by oxidization of lactate as shown below:
In drumhead, most of SRB mechanism gives consideration limited on the consequence of sulfide. To our cognition, there is no SRB mechanism that gaining control other effects of SRB metamorphosis merchandises. Therefore, it become challenges to analyze the influence of other SRB metamorphosis merchandises on the corrosion mechanism and kinetic of steel.
Failure instances caused by SRB
MIC failures due to SRB have been reported for piping and equipment exposed in the Marine environment, oil refinement industry, fossil fuel, atomic power workss, and procedure industries. As such based on the unfastened literature, some illustrations of the failures are summarized below:
A revolving cylinder board mold ( unstained steel type 303 EN 58 M ) used for the industry of paper and board failed in the creviced parts formed between the axial rods and the outer face of the external spirally wound chromium steel steel mesh. The failures occurred three old ages after the mold had been commissioned. Examination revealed pit deepnesss of 3-4 millimeter occurred in grain boundaries rich in manganese sulfide. It is besides found that most of the corrosion had penetrated longitudinally inside the rod making a hollow subdivision covered merely by a thin tegument of metal Mistake: Reference beginning non found.
Piting, holding an engraved and farinaceous morphology, had been found on the parts of perpendicular axial suction pumps e.g. impeller, wear ring and bell house. Sulphide was detected in both the pitted parts and the corrosion merchandises taken from several locations. The corrosion merchandises, the sludge movies present on the surface of the assorted constituents and the river H2O all contained a big population of bacteriums with SRB as the prevailing species Mistake: Reference beginning non found.
Severe internal corrosion, with over 50 % thickness loss in many locations, was encountered in a 610 millimeter diameter, API 5L Grade-B Sch-20 C steel grapevine used for transporting light rough oil from a wet armored combat vehicle to a common heading. The design life of such a grapevine is typically more than 30 old ages. However, the terrible corrosion harm occurred after about 7 old ages of service. A high H2S content was detected and it was an indicant of SRB activity Mistake: Reference beginning non found.
A transmittal oil merchandises API 5L X52 grapevine in northern portion of Iran cracked in 2004. Failure occurred in a part of the grapevine that was placed at the top of a wood zone hill. The chapped zone was at 9 o’clock place. Field observation showed loosening, overlap-opening and disbanding of the applied polythene tape surfacing on the external surface of the grapevine in corroded subdivision. High strength of sulphur constituent and the observation of black corrosion merchandise on the external surface of the pipe indicate SRB activity. A figure of NDE and microbic activity trial confirmed that SRB have been created and intensified opposing corrosion and have had of import functions in cleft development Mistake: Reference beginning non found.
MIC by SRB has been extensively studied to seek better apprehension on its influence on the corrosion kinetic and mechanism which expected to better bar and extenuation techniques.
Ocando et Al. Mistake: Reference beginning non found studied the consequence of ferric ions on the pH and H2S on biofilms generated by SRB. A SRB pure civilization of Desulfovibrio desulfuricans, grown in modified ATCC 1249 medium, was used in this survey. They concluded that in the absence of ferric ions, the pH on the Fe surface decreased aggressively to really low values due to a complex biofilm formation, which protected the stuff and impeded the H ions ingestion by the corrosion procedure. However, in the presence of ferric ions, the pH at metal interface remained about changeless and near to impersonal values, due to the badness of the corrosion procedure, where the HS- and H+ are consumed and monolithic sulfides precipitation occurred. In add-on, they found that the bacteriums and corrosion merchandises were assorted and formed a complex biofilm construction that covered the Fe surface, being in some instances protective depending chiefly on the ferric ions presence.
Rainha et Al. Mistake: Reference beginning non found studied the influence of SRB, grown in a lactate/sulphate medium, on the anaerobiotic corrosion of mild steel. The bacteriums used was Desulfovibrio desulfuricans ATCC 27774. Higher corrosion rates every bit wall as the transpassive disintegration of Fe ( 0 ) or Fe ( II ) compounds to Fe ( III ) were observed. These effects are most likely due to high measures of sulphide and/or to other changes in the sulphate/lactate medium produced by the microbic activity of the SRB. In add-on, they confirmed that the presence of SRB induces alterations in the dynamicss and mechanism of the anodal disintegration of Fe in the lactate/sulphate media.
Amaya and Perez Error: Reference beginning non found studied SRB influence on the corrosion behavior of API-XL70 steel. They indicated that the presence of micro-organism is controlled through the diffusion of the reaction at the cathode. Their studied besides showed that SRB induced localised corrosion.
Benetton and Castaneda Error: Reference beginning non found observed SRB biofilm growing and its influence in corrosion monitoring. The bacteriums used was Desulfovibrio gabonensis ( DSM 10636 ) and Desulfovibrio capillatus ( DSM 14982 ) grown in supplemented unreal saltwater. The consequences showed that bio movie formation induced diffusion controlled corrosion, where biofilm combined with corrosion merchandises is moving as an infinite diffusion bed. Furthermore, they stated that cathodic depolarisation mechanism is limited to the activation controlled ( no biofilm ) . Once biofilm is established, the rate confining measure is diffusion controlled.
Miranda et Al. Mistake: Reference beginning non found studied the function of Desulfovibrio capillatus on the corrosion behavior of C steels under anaerobiotic conditions. Different concentrations of thiosulphate as negatron acceptor for bacterial growing were employed. Their survey showed that the corrosion activity of C steel notably increased, due to high concentration of bacterial metabolites. It is besides noted that thiosulphate is used by SRB as the chief factor in the corrosion procedure.
Duan et Al. Mistake: Reference beginning non found studied corrosion behavior of C steel influenced by anaerobiotic biofilm in natural saltwater. The bacteriums used were sulphate cut downing bacteriums, Desulfovibrio caledoniensis and Fe oxidizing bacteriums Clostridium sp. They found that individual species ( SRB merely ) produced iron sulfide and accelerated corrosion, but assorted species ( SRB and IOB ) produced sulphate green rust and inhibited corrosion. In add-on, they stated that the biotic sulfide produced by SRB, could merely temporarily accelerated C steel corrosion. The continued being of SRB was the key to the accelerated corrosion, connoting that steel and bacteriums should do direct or indirect contact through carry oning FeS or perchance through negatron birds.
Dzierzewicz et Al. Mistake: Reference beginning non found investigated the relationship between microbic metabolic activity ( expressed by coevals clip, rate of H2S production and the activity of hydroogenase and adenosine phosphosulphate ( APS ) reductase enzymes ) and biocorrosion of C steel. The bacteriums used was Desulfovibrio desulfuricans, isolated from dirt and clay samples. The bacteriums were incubated for 6 yearss in the lactate/sulphate liquid medium under anaerobiotic conditions. It is noted that the rate of H2S production was about straight relative to the specific activities of the investigated enzymes. These activities were reciprocally relative to the coevals clip. The C steel MIC rate was strongly affected by bacterial opposition to metal ions. On contrast, it is observed weaker correlativity between the MIC rate and the activity of enzymes.
Kuang et Al. Mistake: Reference beginning non found studied the effects of SRB on the corrosion behavior of C steel. Their consequences showed that SRB turning procedure consisted of three different phases, viz. : exponential, decease and residuary stages. The corrosion behavior of C steel in the system incorporating SRB barely related on the active SRB figure. But it depends on the accretion of the metamorphosis merchandises of SRB. Furthermore, the anode procedure and the corrosion rate are accelerated during the exponential stage and stable during the decease and residuary stage.
Gayosso et Al. Mistake: Reference beginning non found-Error: Reference beginning non found evaluated the corrosion rate of X52 steel, induced by a microbic pool, isolated from the Atasta Nohoch gas transporting grapevine 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 survey besides indicated that the harm observed on the metal surface depends upon the sessile micro-organism ‘s population.
Frank et Al. Mistake: Reference beginning non found investigated the consequence of CO2 debut on the corrosion behavior of C steel in bacteria environment. It was observed that SRB growing was stimulated likely due to the creative activity of an anaerobiotic environment, giving a extremely caustic environment.
Mendoza et Al. Mistake: Reference beginning non found observed the corrosion dynamicss X52 steel caused by SRB. The bacterium was isolated from the interior sedimentations of a grapevine that transports rancid gas in the marine part of Mexico. The bacterium 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. Mistake: Reference beginning non found studied the corrosion behavior of C steel influenced by SRB in dirt environments. They concluded that the being of SRB greatly influences the corrosion behavior of C steel. The possible in control instance ( biocide added ) was around -600 millivolt and ever more positive than that in SRB instances. However, in the presence of SRB, the potency increased somewhat for the first 6 yearss and so maintained around -740 mV/SCE, but the possible fluctuated -600 millivolt to -800 mV/SCE after 50 yearss until the experiment ended. In control instance, 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 maximal value of 0.4 mm year-1. Furthermore, they concluded that the corrosion behavior of C steel in anaerobiotic conditions was divided into three classs, i.e. , ( 1 ) anaerobic inorganic corrosion which depends on the ability to use the cathodic reactants, e.g. H2O or H ion. ( 2 ) the precipitation of protective movie caused no lessening of electrical opposition ( no start of corrosion ) . ( 3 ) MIC induced by SRB ; this corrosion starts after the protective movie ruptured, caused development of localised corrosion.
Romero and Urdaneta Error: Reference beginning non found studied the correlativity between Desulfovibrio sessile growing and OCP, H pervasion, corrosion merchandises and morphological onslaught on Fe. The bacteriums used was Desulfovibrio desulfuricans. Some decisions have been made from their survey:
H2S generated by SRB is the precursor for bacterial corrosion of steel.
In the presence of ferric ions, the genus Desulfovibrio badly corrodes Fe about 0.43 mm year-1 in the signifier of groups of deep holes.
In the presence of SRB and ferric ions, the Fe sulfide merchandises formed get downing with mackinawite, could be: fool’s gold, esmitite, marcasite, greigite, pyrrotite and troilite. However, fool’s gold is the most protective chiefly when it is assorted with extracellular polymeric membrane generated by the bacteriums.
Bacterial corrosion diminishes pH locally prefering the decrease of fool’s gold to mackinawite and severe localized steel corrosion where the bacteriums are formed in settlements.
The mackinawite formed does non hold protective features due to its hydrophilic character and its ample volume which causes it to detach go forthing the base metal bare and exposed to the caustic fluid.
Gramp et Al. Mistake: Reference beginning non found observed the formation of Fe sulfides in civilizations of SRB and in abiotic sulfide. Their consequences showed that makckinawite and greigite were dominant Fe sulfide phases found in SRB civilizations. Meanwhile, mackinawite, greigite and fool’s gold were found in abiotic sulfide with greigite as the more prevailing one.
Herbert et Al. Mistake: Reference beginning non found characterized the surface chemical science and morphology of crystalline Fe sulfides precipated in media incorporating SRB. Their survey showed that the Fe sulfide produced were composed of both ferrous and ferric Fe co-ordinate with monosulphide, with lesser sums of disulphide and polysulphides besides present. In add-on, they concluded that the precipitates possessed a surface composing similar to greigite, with the staying composed of broken mackinawite.
Zhao et Al. Mistake: Reference beginning non found studied the consequence of SRB on C steel corrosion in sea clay. It is observed that the presence of SRB increased the C steel corrosion rate by 182 % compared with that in unfertile sea clay. Wiith the surplus of dissolved H2S, they observed the transmutation of protective FeS movie to FeS2 or other non stoichimetric polysulphide. Such transmutation alterations the province of former bed and accelerated the corrosion procedure.
The growing behavior of SRB was investigated by Hu Error: Reference beginning non found. Her survey showed that both SRB growing rate and the protective Fe sulfide movie were affected by the ferric Fe concentration. Increasing ferric ion concentrations increased the SRB growing rate and corrosion rate. In add-on, it is observed that the addition of SO42- concentration within the scope of 1.93 g/l to 6.5 g/l decreased the planktonic growing and the corrosion rate of mild steel.
Jhobalia Error: Reference beginning non found studied the function of a biofilm and its features in MIC. His survey showed that the corrosion by SRB is besides influenced by temperature. At lower temperature ( 5 A°C and 25 A°C ) , the corrosion rate observed is lower than those at 37 A°C. This is due to the corrosion by SRB is influenced by the figure of SRB cell, and the cell groth rate is strongly affected by temperature. He besides found that the presence of Fe concentrations influenced the corrosion type. With the presence of 5 ppm and 50 ppm Fe concentrations, there was no localised onslaught observed. However, with the presence of 25 ppm, where the ace impregnation occurred, localized onslaught was observed.
In drumhead, H2S produced by SRB and FeS movie formed, have important function in corrosion caused by SRB. However, the similar function of biotic sulfide and abiotic sulfide in the presence of other SRB metamorphosis merchandises on the corrosion mechanism and kinetic of C steel is still ill-defined and need farther probe. Therefore, it becomes a challenge to qualify and compared the abiotic and biotic sulfide function on C steel.
Abiotic sulfide experiments compare to SRB experiments
As based on the reappraisal above, the corrosion caused by SRB is related to the sulfide produced and FeS movie formed. A figure of experiments have been conducted to look into the behavior of abiotic sulfide compared to biotic sulfide.
Newman et Al. Mistake: Reference beginning non found studied the consequence of abiotic sulfide on the corrosion rate of steel in impersonal solution relevancy to MIC. The corrosion rate measured in abiotic sulfide is a few times lower than those accomplishable in SRB experiment, nevertheless the similarity is striking. They underlined that the difference is likely related to the facet of SRB corrosion which has non been simulated, viz. the monolithic deposition of FeS that occurs when SRB grow in civilization media incorporating Fe2+ . In the abiotic experiments, FeS could merely organize as a consequence 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 movie formed can be delicate and may make crevice status on the metal surface.
Videla et Al. Mistake: Reference beginning non found-Error: Reference beginning non found compared the corrosion merchandises formed in biotic and abiotic media. From their survey, the chemical and structural analyses of sulphide movies formed under abiotic and biotic solutions presented the undermentioned features:
In biotic and abiotic sulfide movies, the outer beds are formed by both FeS and FeS2. However, in a biogenic sulfide movie, FeS is the major coinage whereas in an abiotic sulfide movie FeS2 is prevailing.
The chemical composing of tubercles formed in abiotic and biotic solutions is different. The chief contrast is that the corroded metal surface underneath a biogenic movie is made up of Fe sulfide whereas in a non biogenic movie corresponds to an Fe hydrated oxide or oxide.
The movies formed under biogenic conditions are more adherent to the surface of the metal than those formed in abiotic media, which are flakey and loosely disciple.
The inner shell contained more sulfur in biotic movies than those formed in abiotic media.
Biogenic sulphide solution is less aggressive compared to abiotic sulfide.
The old history of the sulfide movie may play a relevant function in the corrosion behavior of C steel. Harmonizing to sulphide concentration, and to the presence or absence a biofilm, the protective features of the sulphide corrosion merchandise bed may alter. During the different phases of the biofilm growing, biogenic beds of corrosion merchandises can offer some protection to the metal by bettering the attachment of the sulfide movie but can besides heighten corrosion by bring oning the presence of heterogeneousnesss at the metal surface.
The type of FeS formed ( either as a compact movie, or as a soft precipitate, or in suspension ) conditions the sulphide consequence on Fe disintegration.
Kuang et Al. Mistake: Reference beginning non found concluded that the corrosion rate caused by SRB is barely relates on the active SRB figure, but it depends on the accretion of the metamorphosis SRB merchandises, i.e. sulfide. Their consequences besides showed that the potentiodynamic polarisation curves in the presence of SRB showed consistence consequences with potentiodynamic polarisation curves in the medium incorporating different concentrations of Na2S.
Sherar et Al. Mistake: Reference beginning non found characterized the corrosion morphology of C steel induced by abiotic sulfide and biotic sulfide. It is concluded that biofilm formation and corrosion merchandise morphology are extremely alimentary dependant. Reducing the C content in solution appears to favor abiotic corrosion taking the formation of crystalline FeS. It is besides confirmed that the dominant Fe stage formed was mackinawite under both abiotic and biotic conditions. In add-on, they claimed that the usage of abiotic sulfide is sufficient plenty to develop steel rate anticipation. However, this simplistic attack does non account for the heterogeneousness that exists in bacterial system.
From the reappraisal above, it is seen that the usage of abiotic sulfide could be used to imitate the SRB experiments. However, in the existent SRB experiments, the caustic species in non merely limited to the sulfide. In their metabolic activities, SRB besides produced other species that could harmful the steel, e.g. CO2, ethanoate, sulphite, pyruvate, sulfate and lactate. The presence of these species could change the function of sulfide on the corrosion kinetic and mechanism. Therefore, it is rather ambitious to look into the consequence of sulfide in the presence of other species as relevancy to MIC caused by SRB.
Abiotic H2S Corrosion
The function of corrosion by SRB is related to the sulfide produced. The sulfide will respond to the available H organizing H2S. Therefore, a brief reappraisal of H sulfide is given below.
The dissociation of H sulfide in H2O involves a series of chemical reactions as described from Equations 2.28 to 2.32. The proposed chemical reactions stairss are Error: Reference beginning non found:
H2S ( g ) a†” H2S ( aq ) 2.
H2S ( aq ) a†” HS- ( aq ) + H+ ( aq ) 2.
HS- ( aq ) a†” H+ ( aq ) + S2- ( aq ) 2.
2H2S ( aq ) + 2e- a†’ H2 ( g ) + 2HS- ( aq ) 2.
FeS formation by precipitation
Fe ( s ) + S2- ( aq ) a†” FeS ( s ) 2.
The reactions of H2S in aqueous vary with pH. At acidic solutions, the dominant sulfide species is molecular H2S. At pH of about 6, the solutions will incorporate bisulphide ions. The higher pH will ensue in the formation of bisulphide. At pH of around 7, the sum of H2S molecular and bisulphide signifiers is similar Mistake: Reference beginning non found.
In H2S corrosion system, there are different possibilities of Fe sulfide formation in aqueous solution Mistake: Reference beginning non found. The formation of solid movie on the surface is due to anodal disintegration of Fe. Ferric ions dissolve into solution and react with sulphide ions in the solution, therefore no movie of corrosion merchandise on the surface. The formation of sulfide can besides by blending reaction between ferric ions that react on the surface and in solution. Those movie formations bring different movie porousnesss of Fe sulfides. The porous surface facilitates the cathodic reaction and creates anodal disintegration of Fe that affects to the corrosion rate Mistake: Reference beginning non found. The types of FeS are influenced by temperature and H2S activity Mistake: Reference beginning non found. Based on dynamicss theories, several types of FeS are normally found in oil field corrosion are pyrite ( FeS2 ) , pyrrhotite, troilite, formless Fe sulfide, three-dimensional Fe sulfide and mackinawite. Figure 2. Corrosion sequence for C steel in aqueous H2S solution shows corrosion sequence for C steel in aqueous H2S solution Mistake: Reference beginning non found-Error: Reference beginning non found. Table 2. Properties of the Fe sulfide shows belongingss of the Fe sulfide Mistake: Reference beginning non found.
Table 2. Properties of the Fe sulfide
Fe ( 1-x ) Second
Fe ( 1-x ) Second
Fe ( 1-x ) Second
Fe ( 1-x ) Second
Fe ( 1-x ) Second
Fe ( 1-x ) Second
Value of x
Smith et Al. Mistake: Reference beginning non found-Error: Reference beginning non found proposed two mechanisms of H2S as shown in Figure 2. Two mechanisms for H2S corrosion. After the initial surface assimilation of H2S on the steel surface, mackinawite can be formed from formless FeS either by way 1 or path 2 [ 57 ] – [ 58 ] .. The 2nd way is more preferred and could be described as follows:
H2S diffuses to the steel surface.
H2S reacts with the steel to organize mackinawite graduated table on the surface.
Mackinawite graduated table dissolves to Fe ( HS ) + and HS- .
Fe ( HS ) + diffuses off from the steel surface, and
More H2S diffuses to respond with the exposed steel.
Carbon steel + H2S/H2O
Solid province growing
Film rupture and precipitation
O2 Mackinawite Cubic FeS Troilite
Dissolution & A ; precipitation
Dissolution & A ; electrodeposition
Figure 2. Corrosion sequence for C steel in aqueous H2S solution.
Figure 2. Two mechanisms for H2S corrosion. After the initial surface assimilation of H2S on the steel surface, mackinawite can be formed from formless FeS either by way 1 or path 2 Mistake: Reference beginning non found-Error: Reference beginning non found.
Beside accelerated corrosion, the presence of H2S could suppress the rate of corrosion. Ma et Al. Mistake: Reference beginning non found proposed a likely mechanism of the inhibitive consequence of H2S as follows:
The species FeSH+ may be incorporated straight into a turning bed of mackinawite via Eq. ( 2.36 ) .
Or it may be hydrolyzed to give Fe2+ via Eq. ( 2.37 )
Ma et Al. Mistake: Reference beginning non found stated, if reaction ( 2.36 ) dominated the electrode surface, so the nucleation and growing of one or more of the Fe sulfides, i.e. mackinawite, cubic ferric sulfide or troilite occurs. However, the function of H2S, accelerates or inhibits the rate of corrosion, depending on the pH value. At lower pH values ( & lt ; 2 ) , ferric ion will fade out through reaction ( 2.37 ) . As a consequence, there will be less iron sulfide movie due to its high solubility at low pH. Meanwhile, at the pH values of 3-5, H2S begins to exhibit its suppressing consequence as FeSH+ species may organize partly mackinawite through reaction ( 2.36 ) . The mackinawite can change over into troilite which is more stable and protective. At a pH of more than 5, mackinawite was the lone ascertained merchandise of corrosion. As mackinawite has less protective ability that troilite, the suppressing consequence of H2S lessenings.
Tang et Al. Mistake: Reference beginning non found studied the consequence of H2S concentration ( 59 – 409 ppm ) on the corrosion behavior of C steel at 90A°C. The consequences showed that the corrosion rate increased with the addition of H2S concentrations. H2S showed dtrong acceleration consequence on the cathodic H development of C steel, doing C steel to be earnestly corroded. The corrosion merchandises formed on C steel surface were composed of mackinawite, which was loose and did non demo any protective belongingss. Severe localised corrosion on the steel surface was besides observed, which may impute to cemetites stripped off from the grain boundary.
When H2S gas nowadayss with CO2 gas, there will be a growing competition between FeCO3 and FeS movies which affects to the corrosion rate. Nesic et Al. Mistake: Reference beginning non found constructed a theoretical account that identified the growing of movie formation incorporating H2S/CO2 gas. The initial movie formed is started by FeS movie formation. Then, the FeCO3 movie becomes thicker and denser at the metal/film interface due to an addition in pH and Fe2+ concentration.
Brown Error: Reference beginning non found found that the corrosion rate in CO2 environment increased in the presence of little H2S concentration of less than 30 ppm. However, he besides observed a decreasing of corrosion rate in the presence of 100 ppm H2S. The graduated table produced was adherent and protective plenty to retard corrosion onslaught. The graduated table was more protective when temperature was increased to 80oC.
The determination by Brown was supported by Lee Error: Reference beginning non found. Lee concluded that little of sum of H2S ( 10 ppm ) lead to rapid decrease of the corrosion rate. Based on the SEM observation, they found that the graduated table formed on the surface that inhibited corrosion rate have a mackinawite construction.
Agrawal et Al. Mistake: Reference beginning non 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 correlativity between the corrosion rates and the temperature. In the scope of H2S concentration studied, the corrosion rate showed a multinomial curve with increasing the temperature.
Kvarekval et Al. Mistake: Reference beginning non found studied the consequence of H2S concentrations runing from 150 – 450 ppm in a CO2 environment. The consequences showed that higher corrosion rates were obtained in the presence of H2S compared to experiments without H2S. The corrosion rates were in the scope of 0.1-2 mm year-1.
Singer et Al. Mistake: Reference beginning non found observed that hint sums of H2S greatly retards the CO2 corrosion with general corrosion rates normally 10 to 100 times lower than their pure CO2 equivalent. The most protective conditions were observed at the lowest partial force per unit area of H2S. However, corrosion rate increased when more H2S was added. The presence of hint sums of H2S ( 0.004 saloon ) in the CO2 environment aggressively decreases the corrosion rate by two orders of magnitude. As the partial force per unit area of H2S is increased to 0.13 saloon, the inclination is reversed and the general corrosion rate increased by an order of magnitude.
Carew et Al. Mistake: Reference beginning non found observed a rapid and important decrease in the CO2 corrosion rate both in individual and multiphase flow in the presence of 10 ppm H2S. At higher H2S concentrations ( up to 250 ppm ) the tendency was reversed and a mild addition of the corrosion rate was observed.
Schmitt et Al. Mistake: Reference beginning non found stated that a alteration in pH from 4 to 6 had merely small consequence on the corrosion rate, and at pH 6, 60 A°C and 25 ppm H2S, protective corrosion movies were formed and no localised corrosion were observed Mistake: Reference beginning non found. The consequence seems to disappear at higher pH values ( 5.5-7 ) and higher temperatures ( & gt ; 80A°C ) , when a protective movie is formed. They concluded that an addition of the CO2 partial force per unit area in the same flow system from 3.8 to 10.6 saloon reduces the maximal corrosion rates from about 15 to 0.2 mm/y under conditions when semi-protective movies are formed, e.g. in the pH scope below 5.2 Mistake: Reference beginning non found.
In combination with CO2, corrosion rate of H2S showed different phenomena compared to without CO2 as reported by Makarenko et Al. Mistake: Reference beginning non found. With CO2, the corrosion procedure is accelerated by cathodic reaction of H ion decrease. It has been proven that CO2 corrosion of C steel additions by 1.5-2 times with addition of H2S content in the mixture ( p H2S & lt ; 0.5 MPa ) in the temperature scope 20-80A°C. Further increasing in H2S content ( p H2Sa‰?0.5-1.5 MPa ) , the corrosion rate will diminish, particularly in the temperature scope 100-250A°C, because of the influence of FeS and FeCO3 on corrosion. It may associate to formation of protective movie Mistake: Reference beginning non found.
Choi et Al. Mistake: Reference beginning non found studied the consequence of H2S on the CO2 corrosion of C steel in acidic solutions. The consequences showed that the add-on of 100 ppm H2S to CO2 induced rapid decrease in the corrosion rate at both pHs 3 and 4. The suppression consequence is attributed to the formation of thin FeS movie on the steel surface that suppressed the anodal disintegration reaction.
Abelev et Al. Mistake: Reference beginning non found examined the consequence of H2S on Fe corrosion in 3 wt % NaCl solution saturated with CO2 in temperature scope of 25-85 A°C. Small H2S concentrations ( 5 ppm ) have an inhibiting consequence on corrosion in the presence of CO2 at temperatures from 25 to 55 A°C. However, 50 ppm H2S is needed to supply important corrosion suppression. At higher H2S concentrations, the corrosion rate additions quickly, while still staying below the rate for the H2S free solution. Corrosion protection in the temperature scope 25 to 55 A°C is attributed to surface assimilation of sulfur on the native Fe oxide, and this bed provides important corrosion suppression. The chief species responsible for suppression included Fe ( II ) bonded to S and O. Meanwhile, at higher H2S concentrations a thicker bed of Fe corrosion merchandises signifiers on the surface by a disintegration precipitation mechanism. However, this bed is porous and nonuniform, holding nothingnesss and abnormalities giving less protective feature to the steel.
Sun Mistake: Reference beginning non found showed that mackinawite is the dominant graduated table formed on the steel surface, which protects the steel from eating in CO2/H2S corrosion. She besides highlighted that the make-up of the surface graduated table non merely depends on the H2O chemical science and the several solubility of Fe carbonate and Fe sulfides, but besides on the fight of the two graduated table formation mechanisms. Merely at really high supersaturation of Fe carbonate are both Fe carbonate and mackinawite graduated table are found on the steel surface, with Fe carbonate in the outer part of the mackinawite graduated table.