Basic Concepts of Corrosion of Iron and Steel

Basic Concepts of Corrosion of Iron and Steel

Iron and steel materials in practical use are normally exposed to corrosion in the atmospheric and aqueous environments. Corrosion is the deterioration of these materials by chemical interaction with their environment. It is one of the problems frequently encountered in the present day industrialized society. As per ISO 8044:2010, ‘corrosion is the physico-chemical interaction between a metal and its environment, which results in changes in the metal’s properties and which may lead to significant functional impairment of the metal, the environment, or the technical system of which they form a part’.

Modern corrosion science was set off in the early twentieth century with the local cell model and the corrosion potential model. The two models have joined into the modern electro-chemical theory of corrosion, which describes metallic corrosion as a coupled electro-chemical reaction consisting of anodic metal oxidation and cathodic oxidant reduction. The electro-chemical theory is applicable not only to wet corrosion of metals at normal temperature but also to dry oxidation of metals at high temperature.

Iron and steel materials corrode in a variety of gaseous and aqueous environments. The most common corrosion of iron and steel takes place in aqueous solution and in wet air in the atmosphere. In general, metallic corrosion of iron and steel produces in its initial stage soluble metal ions in water, and then, the metal ions develop into solid corrosion precipitates such as metal oxide and hydroxide.

Iron and steel is found in nature as ores. The manufacturing process of converting these ores into metals involves the input of energy. Iron and steel materials can hence be regarded as being in a metastable state and they tend to lose their energy by reverting to compounds more or less similar to their original state. The materials corrode since they are chemically unstable in natural environments- air, soil, and water. During the corrosion reaction the energy added in manufacturing is released, and the metal is returned to its oxide state. Since most metallic compounds, and especially corrosion products, have little mechanical strength, a severely corroded piece of steel material is quite useless for its original purpose.

Virtually all corrosion reactions are electro-chemical in nature, at anodic sites on the surface the steel goes into solution as ferrous ions, this constituting the anodic reaction. As iron atoms undergo oxidation to ions they release electrons whose negative charge quickly builds up in the metal and prevent further anodic reaction, or corrosion. Hence this dissolution only continues if the electrons released can pass to a site on the metal surface where a cathodic reaction is possible. At a cathodic site the electrons react with some reducible component of the electrolyte and are themselves removed from the metal. The rates of the anodic and cathodic reactions are to be equivalent according to Faraday’s laws, being determined by the total flow of electrons from anodes to cathodes which is called the ‘corrosion current’.

There is a distinction between the types of corrosion which normally describes the interaction between the metal and the environment, and forms of corrosion which describes the phenomenological appearance. 

Chemical reaction – It is a typical chemical corrosion reaction which occurs at high temperatures, where the metal reacts with hot gases and forms an oxide layer.

Metallo-physical reaction – The example of the metallo-physical reaction is the embrittlement caused by hydrogen which diffuses into the metal, possibly leading to failure of a component. Embrittlement can be the result of a careless manufacturing process. It can also be initiated by corrosion processes (metal dissolution) such as corrosion-induced hydrogen assisted cracking.

Electro-chemical reaction – It is the most common type of corrosion reaction. The reaction implies an electrical exchange by way of electrons in the metal and ions in a conducting electrolyte, such as a water film on its surface.

Since the corrosion current is also to flow through the electrolyte by ionic conduction, the conductivity of the electrolyte influences the way in which corrosion cells operate. The corroding piece of steel is described as a ‘mixed electrode’ since simultaneous anodic and cathodic reactions are proceeding on its surface. The mixed electrode is a complete electro-chemical cell on one steel surface.

In the environment, the corrosion process normally takes place in aqueous solutions and is therefore electro-chemical in nature. The four requirements for the formation of a corrosion cell are (i) anode (corrodes), (ii) cathode (protected), (iii) electrolyte (normally soil or water), and (iv) metallic return path (steel product)

Corrosion involves the ionization of metal atoms and the loss of these ions into solution or into a corrosion product. Since the ionization reaction means giving up electrons, a flow of electrons away from the site of this reaction is to occur to avoid a build-up of negative charge. Thus, corrosion is an electro-chemical reaction. The site where the loss of metal occurs is called the anode, or anodic region, and the electrons flow through the metal to a site, called a cathode, where they are consumed in a cathodic reaction.

The anodic reaction is normally Fe = Fe2+ + 2e- and the cathodic reaction, in the presence of water and sufficient oxygen, is normally 2H2O + O2 + 4e- = 4OH-. The corrosion product, rust, forms from Fe2+ + 2OH- = Fe(OH)2.

The actual electro-chemical mechanism can be appreciated if one considers how a rust pit forms. A pit begins at some inhomogeneity on the surface, such as an impurity particle, and the above reactions occur. The pit-type geometry forms because the anodic reaction continues to occur underneath the rust cover.  Fig 1 shows the corrosion mechanism.

Fig1 Corrosion mechanism

The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are frequently more serious than the simple loss of a mass of metal. Failures of various kinds and the need for expensive replacements can occur even though the amount of metal destroyed is quite small.

Types of corrosion

There are several forms of corrosion (Fig 2). These include (i) inter granular corrosion, (ii) environmentally induced corrosion, (iii) pitting corrosion, (iv) dealloying or selective leaching, (v) erosion corrosion, (vi) corrosion in reinforced concrete, (vii) galvanic corrosion, (viii) crevice corrosion, (ix) microbially induced corrosion, (x)  uniform or general corrosion. These types of corrosions are described below.

Fig 2 Forms of corrosion

Inter granular corrosion

It is also known as inter-crystalline corrosion. It is a special form of localized corrosion, where the corrosive attack takes place in a quite narrow path preferentially along the grain boundaries in the metal structure. The most common effect of this form of corrosion is a rapid mechanical disintegration (loss of ductility) of the material. Normally it can be prevented by using the right material and the production process.

On a microscopic level, Iron and steels have small, distinguishable regions called grains. Within an individual grain, the orientation of the atomic arrangement (called a lattice) is the same. Individual grains have different orientations and the boundary between the grains is called the grain boundary. Normally, grain boundaries are no more reactive in corrosion than the grain itself. However, under certain conditions the grain boundaries are altered from the grain itself by impurities and / or enrichment (or depletion) of one of the alloying elements. Heat treatment and welding can lead to changes in the composition which can cause inter granular corrosion. In severe cases, inter granular corrosion can lead to a marked decrease in mechanical properties and can, in extreme cases, turn the steel into a pile of individual grains.

One of the most common examples of inter granular corrosion is its occurring in stainless steels. During welding of the stainless steel, or heating in the temperature range of 500 deg C to 800 deg C, the stainless steel becomes sensitized or susceptible to inter granular corrosion as shown in Fig 3. The chromium carbide (Cr23C6) is not soluble in this temperature range and precipitates out of the grain into the grain boundary. As a result, the area of the grain adjacent to the grain boundary is depleted of the chromium and becomes anodic to the rest of the grain and to the grain boundary. Consequently, the grain boundaries possess a lower degree of corrosion resistance than the residual material, leading to localized corrosive attack. The simplest solution to the stainless steel inter-granular corrosion problem is to have a composition with a carbon content of less than 0.03 %. Low carbon content prevents the formation of the chromium carbide and the chromium stays in solution. The corrosion of the depleted grain boundary area is very severe and occurs in the acidic environments.

Fig 3 Inter granular corrosion

Environmentally induced corrosion

Environmentally induced corrosion is also called environmental induced cracking.  There are two types of environmental induced cracking. These are (i) stress corrosion cracking, and (ii) hydrogen assisted cracking. Environmentally induced cracking also takes place because of corrosion fatigue. Fig 4 shows environmentally induced corrosion.

Fig 4 Environmentally induced corrosion

Stress-corrosion-cracking (SCC) occurs in steels under the several threshold conditions such as (i) specific corrosive environment solution composition, (ii) minimum tensile stress levels, (iii) temperature, (iv) steel composition, and (v) steel structure. An example is the stainless steels. Stainless steels crack in chloride solutions. The interplay of the conditions leading to SCC is not well understood. It is believed that the corrosion causes a pit or surface discontinuity to form on the metal which then functions to act as a stress concentrator. The presence of a minimum threshold tensile stress, coupled with the corrosion, causes the crack to propagate. Additionally, during the initial corrosion, the tensile stresses can cause the protective films on the surface to rupture, thereby exposing the metal to the corrosive environment.

This is a dangerous corrosion type which can be the result of environmental factors or cyclic stresses. The major types of cracking attack are due to the (i) corrosion fatigue which is the accelerated failure of the steel which undergoes cyclic loading due to its presence in a corrosive environment, and (ii) SCC which is the corrosion induced cracking that occurs in steels under high tensile stress. The cracks start on the surface and go inward. It is to be noted that the stress can be the result of cold working, forming, or external loading.

SCC is a combined mechanical and electro-chemical corrosion process which results in cracking of certain materials. It can lead to unexpected sudden brittle failure of normally ductile metals subjected to stress levels well below their yield strength. Internal stresses in a material can be sufficient to initiate an attack of stress corrosion cracking.

SCC is not simply an overlapping of corrosion and mechanical stresses, but can be understood as an auto-catalytic, self-accelerating process leading to high metal dissolution rates (anodic reaction). Initially, a small pit is formed and develops into a crack due to the applied or residual stress in the material. The crack formation opens up a new active (non-passive) metal surface, which again corrodes very easily. This leads to further crack propagation and again to the exposure of new highly active metal surfaces in the crack. Metal dissolution in the crack advances rapidly until mechanical failure occurs.

SCC is a highly specific form of corrosion which occurs only when the following three different requirements are fulfilled at the same time (Fig 4) namely (i) mechanical (load, stress), (ii) material (susceptible alloy, e.g. steel), and (iii) environment (highly corrosive, chlorides). It is well known that certain grades of steel can suffer SCC in harsh environments such as indoor swimming pools. In most of these cases, corrosion is initiated by chlorides attacking the passive layer.

Hydrogen-assisted cracking is also known as hydrogen embrittlement. It is due to the loss in ductility of steel because of the saturation of atomic hydrogen in the grain boundaries. It occurs at local cathodic sites and is aggravated by stress and compounds such a hydrogen sulphide.

Hydrogen-assisted cracking is caused by the diffusion of hydrogen atoms into the steel. The presence of hydrogen in the lattice weakens the mechanical integrity of the metal and leads to crack growth and brittle fracture at stress levels below the yield strength. Like SCC, it can lead to sudden failure of steel parts without any detectable warning signs. In common applications, hydrogen damage is normally only relevant for high-strength steel with a tensile strength of around 1 MPa or higher. As for the hydrogen assisted cracking, three different conditions (Fig 4) are to be present at the same time. These are (i) mechanical (load, stress), (ii) material (hardness structure), (iii) environmental (external hydrogen, internal hydrogen).

The source of hydrogen can be the production process such as steelmaking, pickling and electro-galvanizing (primary hydrogen). A secondary source can be the hydrogen formed during a corrosion process. During the corrosion process, hydrogen is formed and diffuses into the material. This hydrogen intake leads to a decrease in the toughness or ductility of the steel.

Pitting corrosion

Pitting corrosion is extremely localized attack which eventually results in the creation of small holes or pits in the steel. This form of corrosion is mainly found on passive metals mainly which owe their corrosion resistance to a thin oxide layer on the surface with a thickness of only a few nanometers.

Pitting corrosion is one of the most destructive and dangerous forms of corrosion. Basically, the steel object to pitting are those which rely on an oxide film for protection, such as stainless steels. The initiation of a pit can be the result of any of the reasons namely (i) chemical attack, such as ferrous chloride or aerated sea-water on stainless steel, (ii) mechanical attack such as an impact or scratching which removes small areas of the protective film, and (iii) crevice corrosion resulting from tiny deposits on the surface, especially in stagnant sea-water. Some theories state that pitting is just a special case of crevice corrosion.

The corrosion initiating process starts with a local break-down of the passive layer. Local corrosive attack can be initiated on steels, for example, by chloride ions. Pitting corrosion can be quite problematic. Whereas uniform corrosion can be seen clearly on the surface, pitting corrosion frequently appears only as small pinholes on the surface. The amount of material removed below the pinholes is normally unknown, as hidden cavities can form, making pitting corrosion more difficult to detect and predict. Technically, there is no reasonable way to control pitting corrosion. This form of corrosion must be excluded right from the start through design considerations and use of the right material. Fig 5 shows the pitting corrosion.

Fig 5 Pitting corrosion

Dealloying or selective leaching

Dealloying or selective leaching corrosion results from areas of a metal surface being different metallurgically from other, adjacent areas. This type of corrosion is more prevalent in non ferrous metals and alloys. For example, brass is an alloy with zinc and copper in a ‘solid solution’. It can corrode with the zinc being selectively removed from the alloy, leaving behind the copper. It makes the alloy porous and compromises its mechanical properties. In brass it can be identified when its yellow natural colour turns reddish or coppery in appearance. It is helpful to add a small amount of tin to the alloy to prevent dealloying. Such selective leaching is known as ‘dezincification’.

Cast irons can corrode in such a matter that the iron is selectively corroded away, leaving behind a soft graphite layer. This is referred to as ‘graphitization’. Other examples are referred to as dealuminification, denickelification, and decobaltification, etc. where the terms refer to the metallic element which is selectively corroded away. Dealloying or selective leaching is shown in Fig 6.

 Fig 6 Dealloying or selective leaching

The mechanism of selective leaching has been explained with an example of a brass alloy. In this case first the brass corrodes. Then the zinc ions stay in solution, and the copper plates back on as a solid layer. The problem with this theory is that the corrosion occurs even under high electrolyte flow velocities when one surmises that the copper ions are swept away before they can plate out.

A second theory, again for brass, is that the zinc corrodes preferentially, leaving behind copper in a lattice structure. It is a corrosion process in which the less noble metal in an alloy is attacked preferentially and replaced in the matrix by cathodic products. The most common example of this occurs with brass and is termed dezincification. In the dezincification of brass, the zinc in the alloy’s matrix is attacked and copper remains.

Erosion corrosion

Erosion corrosion results from a high velocity electrolyte flow whose abrasive action accelerates the corrosion. This corrosion is especially severe when the electrolyte contains solids in suspension. The effect is to remove a protective oxide from the film surface, thus exposing fresh alloy to corrode. Erosion corrosion can be thought of as pitting on a much larger scale. There is, in fact, a limit to what electrolyte velocities can be tolerated by specific metals.

Copper-nickel alloys are selected for seawater service based on their resistance to erosion-corrosion (amongst other requirements). In addition to erosion, other forms of attack related to velocity effects are as follow.

Cavitation – It is the deterioration of a surface caused by the sudden formation and collapse of bubbles and voids due to the turbulence in the liquid. It is normally marked by a pitted or rough metal surface.

Impingement attack – It is the localized corrosion caused by turbulence or impinging flow. Normally there is a critical velocity below which no impingement occurs and above which attack increases rapidly. In many cases, the three corrosion processes can occur simultaneously. Fig 7 gives erosion corrosion.

Fig 7 Erosion corrosion

Corrosion in reinforced concrete

Reinforced concrete (RC) is a composite material comprising steel reinforcement bars (rebars) embedded in a concrete mass.  Rebars carries the bulk of the tensile load and imparts a degree of cracking resistance to the concrete which itself is compressively loaded. Steel in concrete is normally in a non-corroding, passive condition. However, non-corroding, passive conditions are not always achieved in practice due to which corrosion of rebars takes place. Corrosion of rebars has physical consequences such as decreased ultimate strength and serviceability of the concrete structures. The steel rebar corrosion in concrete is a big universal problem. The damage which happens from corrosion can to a large extent reduce the serviceability and structural integrity of the RC. Corrosion induced damage to the concrete structure necessitates an early repair or in some cases complete replacement of the concrete structure.

Whenever the reinforcement bar embedded in steel corrodes, the corrosion products increase its volume. All forms of iron oxide and hydro-oxide have specific volumes which is greater than that of the steel. The expansive forces generated by the steel corrosion leads to tensile cracking and rust staining of the concrete. This, in turn, causes reduction in the serviceability and structural integrity of concrete besides affecting its aesthetics. Once the corrosion starts, it is only a matter of time before a cumulative amount of damage occurs to the concrete structure and it fails well before its design life.

The two main causes of the corrosion of the rebars in concrete are (i) localized failure of the passive film on the steel by the chloride ions and (ii) general failure of the passivity by neutralization of the concrete alkalinity due to the reaction with atmospheric CO2 (carbon dioxide).  The main factors affecting the corrosion of rebars in RC are (i) loss of alkalinity due to carbonation, (ii) loss of alkalinity due to chlorides, (iii) cracks in the concrete because of mechanical loading, (iv) stray currents, (v) atmospheric pollution, (vi) moisture pathways, (vii) water-cement ratio, (viii) low tensile strength of the concrete, (ix) electrical contact with dissimilar metals, and (x) corrosion due to difference in environments.

In a medium of perfect uniformity, corrosion is very unlikely to occur. However, reinforced concrete is by no means a homogeneous material and corrosion cells are set up when certain conditions exist. There are numerous reasons for the corrosion enhancing non-uniformity of concrete. Concrete can be honeycombed, porous, and unevenly wet and dry. Cracking causes differences in steel stress, differential aeration, and depositions of salt. There are always inherent non-uniformities in the rebar itself due to initial locked-in residual stresses and the manufacturing processes. As a result, regions of lower potential become anodic and regions of higher potential become cathodic. Moist concrete acts as the electrolyte, the action of which is further accelerated if salt ions exist. Corrosion in reinforced concrete normally falls under two general groups namely (i) cracked concrete, and (ii) uncracked concrete.

In case of fresh uncracked concrete normally there is ample resistance to corrosive attack. The concrete cover over the rebar is very effective in inhibiting the penetration of corrosive agents to the level of the steel. It is obvious that the thicker and denser the concrete cover the more effective it becomes in resisting corrosion. Also, fresh concrete has a very high pH value which usually inhibits corrosion reactions. The pH number is an index of the acidity or alkalinity of a medium. Numbers from 0 to 7 indicate acidity of a solution (in which corrosion is promoted), and numbers from 7 to 14 indicate solution alkalinity (in which corrosion is retarded). Fresh concrete has a high Ca(OH)2 (calcium hydroxide) content which gives it a pH of around 13. The last defense against corrosion is offered by the blue oxide film (mill scale) around the surface of the rebar. This oxide film prevents corrosive agents from coming into direct contact with the bare metal. Thus mill scale provides localized corrosion protection.

However, as time passes the above conditions tend to alter. Water, salt, oxygen, CO2, and industrial gases (if present) slowly begin penetrating the concrete, the rate of which depends on the permeability of the concrete cover. CO2, which penetrates into concrete through pores and cracks, reacts with Ca(OH)2 and produces calcium carbonate. Thus, both the pH value and the protective quality of concrete are reduced. The general mechanism by which corrosion occurs in reinforced concrete is shown in Fig 8.

Fig 8 Corrosion in reinforced concrete

Galvanic corrosion

When two different metals are exposed to a corrosive environment, an electrical potential difference exists. If the two metals are electrically connected, the more active metal becomes the anode in the resulting galvanic cell and its corrosion is increased. An example of such a corrosion cell is the use of steel bolts to hold copper plates together.

Galvanic corrosion refers to corrosion damage where two dissimilar metals have an electrically conducting connection and are in contact with a common corrosive electrolyte. In the electro-chemical model of corrosion, one of the two partial reactions (anodic metal dissolution and cathodic oxygen reduction) takes place almost exclusively on one metal. Normally, the less noble metal is dissolved (anodic metal dissolution), whereas the more noble part is not attacked by corrosion (serves only as the cathode for oxygen reduction). Where galvanic corrosion takes place, the rate of corrosion of the less noble metal is higher than it is in a free corroding environment without contact to another metal.

Using thermodynamic data and taking common experience gained in typical applications into account, it is possible to predict which material combinations are affected by galvanic corrosion. A positive example of the galvanic corrosion phenomenon is the way zinc protects carbon steels and low-alloyed steels. Zinc is the less noble metal which actively protects steel by being corroded itself.

Not all galvanic corrosion is detrimental. Zinc coated steel, or galvanizing, is used to protect steel, not because the steel is resistant to corrosion, but since the zinc, being anodic to the steel, corrodes preferentially. Hence, the steel is protected cathodically by making any exposed areas of steel into cathodes.

It is normally good practice not to use dissimilar metals unless it is necessary, but if it is to be used then the precautions which are necessary are (i) to attempt to electrically isolate the metals, (ii) to use protective coatings on the metal surface(s), normally the cathode, (iii) to cathodically protect the less noble metal, (iv) to put corrosion inhibitors into the system, (v) to use design in which anodic part can be replaced easily, (vi) to keep out moisture, (vi) to use metals which are close to one another in the galvanic series, (vii) to keep the anode / cathode area ratio is high in the design, and (viii) to use design allowances to account for the corrosion. Fig 9 shows the galvanic corrosion.

Fig 9 Galvanic corrosion

Crevice corrosion

Crevice corrosion refers to corrosion occurring in cracks or crevices formed between the two surfaces (made from the same metal, different metals or even a metal and a non-metal). This type of corrosion is initiated by the restricted entrance of oxygen from the air by diffusion into the crevice area leading to different concentrations of dissolved oxygen in the common electrolyte (the so-called aeration cell).

Crevice corrosion is a localized attack which occurs when crevices, formed by lapped joints, or areas of partial shielding, are exposed to corrosive environments. Such resulting cells are referred to as concentration cells. Two common cases are oxygen cells and metal-ion cells. Oxygen concentration cells occur when the shielded area becomes depleted in oxygen and the area acts as an anode relative to the oxide region. As shown in Fig 10, the corrosion becomes quite rapid because of the small shielded area as compared to the unshielded area. In the case of an oxygen cell, there is an oxygen ‘gradient’ which forces the formation of the anode and cathodes with respect to the oxygen levels.

Fig 10 Crevice corrosion

Crevice corrosion refers to the corrosion occurring in cracks or crevices formed between two surfaces (made from the same metal, different metals or even a metal and a non-metal). This type of corrosion is initiated by the restricted entrance of oxygen from the air by diffusion into the crevice area leading to different concentrations of dissolved oxygen in the common electrolyte (the so-called aeration cell). Again, the two partial reactions take place on different parts of the surface. Oxygen reduction takes place in the outer areas with higher oxygen concentrations easily accessible by the surrounding air, whereas the anodic metal dissolution occurs in the crevice area resulting in localized attack (e.g. pitting). It can also occur under washers or gaskets, when the entry of water underneath is not prevented). There are lower and upper limits to the size of a crevice in which corrosion is induced. If the crevice is too tight, no electrolyte for corrosion is introduced. If the crevice is too wide to reduce oxygen entrance, the aeration cell and consequently different concentrations of oxygen cannot develop. However, the critical crevice width depends on several factors such as the type of metals involved, the corroding environment and wet / dry cycles.

The initial driving force of such corrosion is the oxygen cell. The continued growth is fostered by the accumulation (frequently caused by the same factors which produce the low oxygen levels of acidic, hydrolyzed salts within the crevice. Alloys, such as 18-8 stainless steels, are subject to oxygen cell crevice corrosion.

Metal-ion cells are formed mainly with copper alloys. The shielded area accumulates corrosion products and becomes cathodic to the regions outside of the crevice where corrosion products are kept washed away. The figure 10 shows this type of concentration cell.

Another example of metal-ion cell corrosion occurs when relative speeds of electrolyte over the metal surface are greater at one point than at another, thus resulting in metal-ion crevice corrosion. A good example is where a disc of metal is rotating at high speed in seawater. Corrosion occurs near the edge where linear velocities are the highest and the metal-ion concentration is low (since the ions are repeatedly swept away). The high velocity, higher than in regions closer to the hub of the disc, sweeps away the metal-ions, thus forming anode regions. At the centre of the disc, where velocities are lower, the metal acts as a cathode and is protected.

However, the two concentration cells corrode at different regions of the crevice. The oxygen cell corrodes under the shielded area while the metal-ion cell corrodes outside of the area. The initial driving force behind the corrosion is either the oxygen or the metal-ion cell. Its continued growth is governed by the accumulation of corrosion products, calcareous deposits, and salts within the crevice.

Again, the two partial reactions take place on different parts of the surface. Oxygen reduction takes place in the outer areas with higher oxygen concentrations easily accessible by the surrounding air, whereas the anodic metal dissolution occurs in the crevice area resulting in localized attack (e.g. pitting). It can also occur under washers or gaskets, when the entry of water underneath is not prevented.

Microbially induced corrosion

Biological organisms can play a major role in metal attack. Corrosion is caused or accelerated by micro-organisms. It can take place with or without presence of oxygen. Biological action can increase the severity of corrosion (i) as a result of the bio-deposits on the material surface, (ii) production of corrosive chemical species (i.e. hydrogen sulphide from sulphate reducing bacteria – SRBs), and (iii) disruption of normal electrochemical reactions and film formation.

Microbially (microbiologically) induced corrosion (MIC) is corrosion which is caused by the presence and activities of micro-organisms. Micro-organisms are the organisms which cannot be seen individually with the unaided human eye, including microalgae, bacteria, and fungi. Microbially induced corrosion can cause various forms of localized corrosion, including pitting, dealloying, enhanced erosion corrosion, enhanced galvanic corrosion, stress corrosion cracking, and hydrogen embrittlement. As a result of MIC, corrosion can occur at locations where it is not predicted, and it can occur at very high rates. The iron and steel materials undergo MIC. Furthermore, MIC can also take place in seawater, fresh water, distilled / demineralized water, hydrocarbon fuels, process chemicals, foods, soils, human plasma, saliva, and sewage.

Although SRBs, active only in anaerobic (oxygen free) environments, are a very common cause of corrosion, MIC can also be caused by other types of micro-organisms. As an example, ‘thiobacilli’ which is sulphur oxidizing bacteria (SOB), oxidizes sulphur compounds to sulphuric acid. Other acid producing micro-organisms Include both bacteria and fungi. Microbes can adhere to metal surfaces forming a bio-film, consisting of a community of micro-organisms, leading to corrosion. When the acidic products of bacterial action are trapped at the bio-film – metal interface, their impact on corrosion is intensified.

Although iron does not corrode appreciably in deaerated water, the corrosion rate in some natural deaerated environments is found to be abnormally high. These high rates have been traced to the presence of SRBs (e.g., desulfovibrio desulfuricans). Their relation to an observed accelerated corrosion rate in soils low in dissolved oxygen was first observed in Holland. The bacteria are curved, measuring about 1 × 4 micrometers, and are found in many waters and soils. They thrive only under anaerobic conditions in the pH range of around 5.5 to 8.5. Certain varieties multiply in fresh waters and in soils containing sulphates, while others flourish in brackish waters and seawater, and still others are stated to exist in deep soils at temperatures as high as 60 deg C to 80 deg C.

SRBs easily reduce inorganic sulphates to sulphides in the presence of hydrogen or organic matter, and they are aided in this process by the presence of an iron surface. The aid which iron provides in this reduction is probably to supply hydrogen, which is normally adsorbed on the metal surface and which the bacteria use in reducing SO4. For each equivalent of hydrogen atoms they consume, one equivalent of Fe 2+ enters solution to form rust and FeS. Hence, the bacteria probably act essentially as depolarizers.

Ferrous hydroxide and ferrous sulphide are formed in the proportion of 3 moles to 1 mole. Analysis of rust in which SRBs were active shows this approximate ratio of oxide to sulphide. Qualitatively, the action of SRB as the cause of corrosion in water initially free of sulphides can be detected by adding a few drops of hydrochloric acid to the rust and noting the smell of hydrogen sulphide.

Severe damage by SRBs has occurred particularly in oil well casing, buried pipelines, water cooled rolling mills, and pipe from deep water wells. Within 2 years, well water can cause failure of a galvanized water pipe 50 mm in diameter by the action of SRB.

A combination of low temperature and low humidity is one approach for controlling the growth of bacteria, but fungi can be capable of growing under such conditions. Regular cleaning is a good practice to prevent bio-film formation and subsequent corrosion. Chlorination is used to eliminate bacteria which cause corrosion, but this treatment can produce by-products which are environmentally unacceptable. Aeration of water reduces activity of anaerobic bacteria since they are unable to thrive in the presence of dissolved oxygen. Addition of certain biocides can be beneficial, but micro-organisms are capable of becoming resistant to specific chemicals after long term use. Eradication of microbial populations can be achieved by combining several chemicals or by increasing the concentration of a biocide. Fig 11 shows example of microbially induced corrosion.

Fig 11 microbially induced corrosion

Uniform or general corrosion

Uniform corrosion (Fig 12) is a form of corrosion where the surface is removed almost evenly. The partial reactions (metal dissolution and oxygen reduction) are statistically distributed over the surface, leading to more or less homogenous dissolution of the metal and uniform formation of corrosion products (e.g. red rust on carbon steel). This corrosion results from the continual shifting of anode and cathode regions of the surface of a metal in contact with the electrolyte and leads to a nearly uniform corrosive attack on the entire surface. An example of such corrosion is the rusting of steel plate in seawater. Although it is termed uniform corrosion, it is characterized by the average surface loss.

Fig 12 Uniform corrosion

If the rate of metal loss is known, allowances can be made in design and maintenance to accommodate the corrosion. The extent of this form of corrosion can normally be well estimated on the basis of previous experience. The rate of corrosion is normally given in micrometers per year. Using these average values, it is possible to calculate the life expectancy of a component, and thus to enhance its life expectancy by increasing its thickness.

Uniform corrosion takes place on unprotected carbon steel and on zinc-coated steel under atmospheric conditions. In reality, purely homogenous corrosion attack is unlikely to take place. There are always areas, especially on complex steel parts, which corrode faster than others leading to a more or less rough surface with an irregular covering of corrosion products.

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