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Corrosion of Steel and Corrosion Protection

• satyendra
• March 6, 2014
• 1 Comment
• corrosion, Corrosion mechanism, Corrosion products, corrosion resistance, Corrosivity, electro chemical, protection, Rust,

Corrosion of Steel and Corrosion Protection

Some metals, such as gold, silver, and platinum, occur naturally in their pure form. Several other metals, including iron, are found in their natural state as ores, natural oxides, sulphides, and other reaction products. These metals are to be derived from their ores by smelting, from which the metal absorbs and retains the energy needed to free it from the ore. This metallic state is unstable, however, since the metal tends to recombine with elements in the environment and return to its natural state, losing the extra energy in the process. The process of a metal reverting to its natural state is called oxidation, or corrosion.

Metals tend to transform into much more thermodynamically stable substances (like oxides, hydroxides, salts, or carbonates) during corrosion. Metals have a tendency to shift to their lower energy, relatively natural ore state (e.g., iron ore) after production and shaping. The Law of Entropy controls this tendency. Naturally, it is very difficult to find metals in their pure state. They combine with other elements to form ores.

The corrosion subject has witnessed an inevitable transition from the state of isolation and obscurity to an established engineering discipline. There have already been considerable advances over time in the field of corrosion and corrosion prevention. There are, however, still several problems which the  corrosion scientists / engineers need to resolve.

In the 1960s, the significance of corrosion was recognized when it was understood that damage was caused by the corrosion to the economies of developed nations, a useful life of manufactured goods was being reduced and resources are being wasted by the anti-metallurgical processes. In addition, corrosion also involved issues pertaining to human life and safety, huge environmental impact, and conservation of materials.

Corrosion can be a horrific experience and can lead to some unpleasant effects when it applies to industry which relies on non-corroded metal to operate. The unpleasant effects are not always as recognizable and include lack of production, injuries, and major financial losses. Annually, industries are leaching a large quantity of funds out of their operations, primarily because of corrosion. The overall cost of corrosion around the globe is huge, however, through obtaining and using highly qualified corrosion practitioners, harmonizing standards, along with continuing education, and training, all underpinned through fostering higher corrosion knowledge, there exists a big scope to reduce the cost.

The high cost of corrosion affects several industries which highlights the need for improved corrosion preventing measures. Effective corrosion inhibition has a high economic value. In the oil, gas, and chemical industries alone, corrosion is one of the most challenging tasks. It is not only the high cost of corrosion, but also the health and environmental risks associated with potential failure of the oil and gas equipment which drives the developments of corrosion resistant materials and improved corrosion mitigation strategies.

The fundamentals of corrosion include the mechanisms of the different forms of corrosion, applicable thermodynamic conditions and kinetic laws, and the effects of the major variables. Even with all of the available generalized knowledge of the principles, corrosion in majority of the cases is a very complex process in which the interactions among several different reactions, conditions, and synergistic effects are to be carefully considered.

The corrosion rate can be expressed as an increase per unit in the depth of corrosion time micrometers per year (penetration rate, micrometers /year) or weight loss per unit area per unit area time, normally mdd (milligrams per square decimeter a day) or the current of corrosion. Although the preferred SI unit for expressing the corrosion rate is mm per year.

All corrosion processes show some common features. Thermodynamic principles can be applied to determine which processes can occur and how strong the tendency is for the changes to take place. Kinetic laws then describe the rates of the reactions. There are, however, substantial differences in the fundamentals of corrosion in such environments as aqueous solutions, non-aqueous liquids, and gases which warrant a separate treatment.

The three main reasons for the importance of corrosion are economics, safety, and conservation. To reduce the economic impact of corrosion, corrosion engineers, with the support of corrosion scientists, aim to reduce material losses, as well as the accompanying economic losses which result from the corrosion. Corrosion can compromise the safety of operating equipment by causing failure (with catastrophic consequences). Safety is a critical consideration in the design of equipment. Loss of metal by corrosion is a waste not only of the metal, but also of the energy, the water, and the human effort which has been used to produce and fabricate the metal structures in the first place. In addition, rebuilding corroded equipment needs further investment in all these resources namely metal, energy, water, and human. Economic losses because of corrosion are direct losses as well as indirect losses.

When unalloyed or alloyed steel without corrosion protection is exposed to the atmosphere, the surface takes on a reddish-brown colour after a short time. This reddish-brown colour indicates rust is forming and the steel is corroding. Rust is reaction product of iron and steel corrosion. Rusting applies to the corrosion of iron or iron-base alloys with the formation of corrosion products consisting largely of hydrous ferric oxides. Non-ferrous metals, hence, corrode, but do not rust.

Majority of the materials decay over time. All the products, plants, constructions, and buildings made of ferrous materials are subject to physical deterioration during use. The physical deterioration of materials is because of the mechanical, thermal, chemical, electro-chemical, micro-biological, electric, and radiation related actions on the materials. This has an adverse impact on the life of the products, plants, constructions, and buildings. The technical and economic overcoming of physical deterioration of the materials is difficult, since there are several causes which are intertwined and mutually influence each other. Fig 1 shows some types of the material deterioration processes.

Fig 1 Types of material deterioration processes

While mechanical actions lead to wear, chemical and electro-chemical reactions cause corrosion. Such processes originate from the materials’ surfaces and lead to modifications of the material properties or to their destruction.

Corrosion is the destructive attack of a metal by chemical or electro-chemical reaction with its environment. Deterioration by physical causes is not called corrosion. In some instances, chemical attack accompanies physical deterioration, as described by the terms as corrosion-erosion, corrosive wear, or fretting corrosion.

Steel has a natural tendency to corrode and to return to its natural state as iron ore, typically ferric oxide (Fe2O3). The rate of steel corrosion depends on the availability of water, oxygen, and aggressive ions, as well as the pH and temperature of the surrounding environment, and on the internal properties of the steel, such as composition, grain structure, and entrained fabrication stresses.

The interaction with certain media of the environment results in undesired reactions of the materials which trigger corrosion, weathering, decaying, embrittlement, and fouling. Corrosion is a multi-faceted phenomenon which adversely affects and causes deterioration of properties in metals through oxidation.

As per ISO 8044:2020, corrosion is defined as ‘Physico-chemical interaction between a metallic material and its environment that results in changes in the properties of metal, environment or the technical system of which these form a part’. It is to be noted that the interaction is frequently of an electro-chemical nature. Some of the definitions related to corrosion as included in this standard are given below.

Corrosive agent – It is ‘a substance that will initiate or promote corrosion when in contact with a given metal’.

Corrosive environment – It is the ‘environment that contains one or more corrosive agents’.

Corrosion system – It is the ‘system consisting of one or more metals and those parts of the environment that influence corrosion.’ It is to be noted that ‘Parts of the environment may be, for example, coatings, surface layers or additional electrodes’.

Corrosion effect – It is the ‘change in any part of the corrosion system caused by corrosion’.

Corrosion damage – It is the ‘corrosion effect that causes impairment of the function of the metal, the environment or the technical system, of which these form a part’.

Corrosion failure – It is the ‘corrosion damage characterized by the total loss of function of the technical system’.

Corrosion product – It is the ‘substance formed as a result of corrosion’.

Corrosion depth – It is the ‘distance between a point on the surface of a metal affected by corrosion and the original surface of the metal’.

Corrosion rate – It is the ‘corrosion effect on a metal per unit time. It is to be noted that ‘The unit used to express the corrosion rate depends on the technical system and on the type of corrosion effect. Thus, corrosion rate is typically expressed as an increase in corrosion depth per unit time, or the mass of metal turned into corrosion products per area of surface and per unit time, etc. The corrosion effect may vary with time and may not be the same at all points of the corroding surface. Therefore, reports of corrosion rates are typically accompanied by information on the type, time dependency and location of the corrosion effect’.

Corrosion resistance – It is the ‘ability of a metal to maintain serviceability in a given corrosion system’.

Corrosivity – It is the ‘ability of an environment to cause corrosion of a metal in a given corrosion system’.

The chemical or electro-chemical reaction between a material and its environments produces a deterioration of the material and its properties. Corrosion is a physico-chemical reaction occurring when a metal is exposed to its environment, which changes the properties of the metal and, in several cases, results in degradation of the metal, adjacent environment, or technical system.

Corrosion damage is any change in the corrosion system because of corrosion and which is considered to cause degradation of the metal, the adjacent environment, or the technical system they constitute together. Corrosion system comprises one or multiple metals and all of the parameters of the environment, which contribute to corrosion. Such parameters of the environment can also be the coating surface layer, electrode, and so forth.

Anti-corrosion or corrosion protection refers to the modification of the corrosion system in a way which retards or inhibits the formation of corrosion damage. Corrosion protection painting refers to the coating of metal surfaces with corrosion protection paint.

Corrosion mechanism in metals

Metal corrosion is presently explained by the formation of local electro-chemical electrode pairs on the metal surface. The electrode pair is called an anode-cathode pair. Positive metal ions are dissolved from the anode to the solution, and produce negative electrons in the metal lattice, which migrate in the metal to the cathode. In the cathode, the electrons are consumed in multiple cathode reactions.

In acidic solutions, hydrogen gas is produced, while in pH neutral solutions, oxygen reduction produces hydroxide ions. The electrically conductive electrolyte between the anode and cathode closes the circuit. The anode and cathode sites can be next to one another, resulting in the formation of uniform corrosion or separated from one another resulting in localised corrosion. The anode site is the metal surface’s less noble site or a site with a higher surface energy. Combining the anodic and cathodic reactions provides the total reaction given by equations (i) Fe2+ + 2OH- = Fe(OH)2, and (ii) 4Fe(OH)2 + O2 = 2H2O + 2(Fe2O3.H2O). Fe2O3.H2O is hydrous iron oxide, i.e., rust. Fig 2 shows schematic representation of corrosion in steel along with anodic and cathodic reactions.

Fig 2 Schematic representation of corrosion in steel

In a simplified way, the corrosion process of steel progresses and is chemically based on the equations (i) Fe + SO2 + O2 = FeSO4, and (ii) 4Fe + 2 H2O+ 3O2 = 4FeOOH. The corrosion processes begins when a corrosive medium acts on a material. Since base metals (energy-rich), which are recovered from naturally occurring ores (low-energy) by means of metallurgical processes, tend to transform to their original form, chemical and electro-chemical reactions occur on the material’s surface. Two kinds of corrosion reactions are distinguished namely (i) chemical corrosion which is the corrosion that exclude electro-chemical reaction, and (ii) electro-chemical corrosion which is the corrosion that includes at least one anodic and one cathodic reaction.

Types of corrosion

Corrosion does not only occur as linear abrasion, but in versatile forms of appearance. The standard ISO 8044:2020 describes altogether 56 types of corrosion. These 56 types of corrosion are given below.

Electro-chemical corrosion – It is the ‘corrosion involving at least one anodic reaction and one cathodic reaction’.

Chemical corrosion – It is the ‘corrosion not involving electro-chemical reaction’.

Gaseous corrosion – It is the ‘corrosion with dry gas as the only corrosive environment and without any liquid phase on the surface of the metal’.

Atmospheric corrosion – It is the ‘corrosion with the earth’s atmosphere at ambient temperature as the corrosive environment’.

Marine corrosion – It is the ‘corrosion with sea water as the main agent of the corrosive environment’. It is to be noted that ‘This definition includes both immersed and splash zone conditions’.

Underground corrosion – It is the ‘corrosion of buried metals, soil being the corrosive environment’. It is to be noted that ‘The term soil includes not only the naturally occurring material but also any other material, such as ballast and backfill, used to cover a structure’.

Microbial corrosion – It is the corrosion associated with the action of micro-organisms present in the corrosion system’.

Bacterial corrosion – It is the ‘microbial corrosion due to the action of bacteria’.

General corrosion – It is the ‘corrosion proceeding over the whole surface of the metal exposed to the corrosive environment’.

Localized corrosion – It is the ‘corrosion preferentially concentrated on discrete sites of the metal surface exposed to the corrosive environment’. It is to be noted that ‘Localized corrosion can result in, for example, pits, cracks, or grooves’.

Uniform corrosion – It is the ‘general corrosion proceeding at almost the same rate over the whole surface’.

Galvanic corrosion – It is the ‘corrosion due to the action of a corrosion cell’. It is to be noted that ‘The term has often been restricted to; the action of bimetallic corrosion cells, i.e., to bimetallic corrosion’.

Bimetallic corrosion DEPRECATED: contact corrosion – It is the ‘galvanic corrosion, where the electrodes are formed by dissimilar metals’.

Impressed current corrosion – It is the ‘electro-chemical corrosion due to the action of an external source of electric current’.

Stray-current corrosion – It is the ‘impressed current corrosion caused by current flowing through paths other than the intended circuits’.

Pitting corrosion – It is the ‘localized corrosion resulting in pits, i.e., cavities extending from the surface into the metal’.

Crevice corrosion – It is the ‘localized corrosion associated with, and taking place in, or immediately around, a narrow aperture or clearance formed between the metal surface and another surface (metallic or non-metallic)’.

Deposit corrosion – It is the ‘localized corrosion associated with, and taking place under, or immediately around, a deposit of corrosion products or other substance’.

Water-line corrosion – It is the ‘corrosion along, and as a consequence of the presence of, a gas / liquid boundary’.

Selective corrosion – It is the ‘corrosion of an alloy whereby the components react in proportions that differ from their proportions in the alloy’.

Dezincification of brass – It is the ‘selective corrosion of brass resulting in preferential removal of zinc’.

Graphitic corrosion – It is the ‘selective corrosion of grey cast iron, resulting in partial removal of metallic constituents, leaving graphite’.

Intergranular corrosion – It is the ‘corrosion in or adjacent to the grain boundaries of a metal’.

Weld corrosion – It is the ‘corrosion associated with the presence of a welded joint and taking place in the weld or its vicinity’.

Knife-line corrosion – It is the ‘corrosion resulting in a narrow slit in or adjacent to the filler / parent boundary of a welded or brazed joint’.

Layer corrosion – It is the ‘corrosion of internal layers of wrought metal, occasionally resulting in exfoliation, i.e., detachment of unattacked layers’. It is to be noted that the ‘Exfoliation is generally oriented in the direction of rolling, extrusion, or principal deformation’.

Erosion corrosion – It is the ‘process involving conjoint corrosion and erosion. It is to be noted that the ‘Erosion corrosion can occur in, for example, pipes with high fluid flow velocity and pumps and pipe lines carrying fluid containing abrasive particles in suspension or entrained in a gas flow’.

Cavitation corrosion – It is the ‘process involving conjoint corrosion and cavitation’. It is to be noted that the ‘Cavitation corrosion can occur, for example, in rotary pumps and on ships’ propellers’.

Fretting corrosion – It is the ‘process involving conjoint corrosion and oscillatory slip between two vibrating surfaces in contact’. It is to be noted that the ‘Fretting corrosion can occur, for example, at mechanical joints in vibrating structures’.

Wear corrosion – It is the ‘process involving conjoint corrosion and friction between two sliding surfaces in contact’.

Corrosion fatigue – It is the ‘process involving conjoint corrosion and alternating straining of the metal, often leading to cracking’. It is to be noted that the ‘Corrosion fatigue can occur when a metal is subjected to cyclic straining in a corrosive environment’.

Stress corrosion – It is the ‘process involving conjoint corrosion and straining of the metal due to applied or residual stress’.

Stress corrosion cracking – It is the ‘cracking due to stress corrosion’.

Hydrogen embrittlement – It is the ‘process resulting in a decrease of the toughness or ductility of a metal due to absorption of hydrogen. It is to be noted that the ‘Hydrogen embrittlement often accompanies hydrogen formation, for example by corrosion or electrolysis, and can lead to cracking’.

Blistering – It is the ‘process resulting in dome-shaped defect visible on the surface of an object and arising from localized loss of cohesion below the surface’. It is to be noted that ‘For example, blistering can occur on coated metal due to loss of adhesion between coating and substrate, caused by accumulation of products from localized corrosion. On uncoated metal, blistering can occur due to excessive internal hydrogen pressure’.

Spalling – It is the ‘fragmentation and detachment of portions of the surface layer or scale’.

Tarnishing – It is the ‘dulling, staining or discoloration of a metal surface, due to the formation of a thin layer of corrosion products’.

Aqueous corrosion – It is the ‘corrosion with water or a water-based solution as the corrosive environment’.

Microbiologically influenced corrosion, MIC – It is the ‘corrosion influenced by the action of microorganisms’.

Dealloying – It is covered under see selective corrosion.

Environmentally assisted cracking– It is the ‘cracking of a susceptible metal or alloy due to the conjoint action of an environment and mechanical stress’.

Hydrogen induced cracking, HIC – It is the ‘planar cracking that occurs in metals due to induced stresses when atomic hydrogen diffuses into the metal and then combines to form molecular hydrogen at trap sites’.

Hydrogen stress cracking, HSC – It is the ‘cracking that results from the presence of hydrogen in a metal and tensile stress (residual or applied or both)’. It is to noted that the ‘HSC describes cracking in metals that are not sensitive to sulphide stress corrosion cracking (SSCC) but which may be embrittled by hydrogen when galvanically coupled, as the cathode, to another metal that is corroding actively as an anode. The term “galvanically induced HSC” has been used for this mechanism of cracking’.

Irradiation assisted stress corrosion cracking – It is the ‘intergranular cracking of austenitic stainless steels resulting from a reduction in the chromium concentration in a very narrow band at the grain boundaries following exposure to high neutron irradiation doses exceeding one displacement per atom (which causes the migration of point defects to the grain boundaries)’.

Stepwise cracking, SWC – It is the ‘cracking that connects hydrogen induced cracks (HICs) on adjacent planes in a metal’. It is to be noted that ‘This term describes the crack appearance. The linking of hydrogen induced cracks to produce stepwise cracking is dependent upon local strain between the cracks and embrittlement of the surrounding steel by dissolved hydrogen. HIC / SWC is usually associated with low-strength plate steels used in the production of pipes and vessels’.

Sulphide stress corrosion cracking, SSCC – It is the ‘cracking of metal involving corrosion and tensile stress (residual and / or applied) in the presence of water and hydrogen sulphide’. It is to be noted that ‘SSCC is a form of hydrogen stress cracking (HSC) and involves embrittlement of the metal by atomic hydrogen that is produced by acid corrosion on the metal surface. Hydrogen uptake is promoted in the presence of sulphides. The atomic hydrogen can diffuse into the metal, reduce ductility and increase susceptibility to cracking. High strength metallic materials and hard weld zones are prone to SSCC’.

Stress oriented hydrogen induced cracking, SOHIC – It is the ‘staggered small cracks formed approximately perpendicular to the principal stress (residual or applied) resulting in a “ladder-like” crack array linking (sometimes small) pre-existing HIC cracks.’ It is to be noted that the ‘The mode of cracking can be categorised as SSCC caused by a combination of external stress and the local strain around hydrogen induced cracks. SOHIC is related to SSCC and HIC / SWC. It has been observed in parent material of longitudinally welded pipe and in the heat affected zone of welds in pressure vessels. SOHIC is a relatively uncommon phenomenon usually associated with low-strength ferritic pipe and pressure vessel steels. It is to be compared with hydrogen embrittlement’.

Exfoliation corrosion – It is the ‘stratified form of subsurface stress corrosion of susceptible primary wrought alloy mill products materials having a highly directional grain structure, accompanied by detachment of separate layers from the body of the material, formation of cracks and finally usually complete layer-by-layer disintegration of metal’. It is to be noted that ‘Exfoliation generally proceeds along grain boundaries, but with certain alloys and tempering it may develop along trans-granular paths or a mixed inter-granular / trans-granular path. Layer corrosion can be developed on the first stage’.

Filiform corrosion – It is the ‘type of corrosion proceeding under coating materials on metals in the form of threads, generally starting from bare edges or from local damage to the coating’. It is to be noted that ‘Usually the threads are irregular in length and direction of growth, but they may also be nearly parallel and of approximately equal length. It should be noted that filiform corrosion can occur under different protective coatings’.

Tribo-corrosion– It is ‘any form of corrosion that involves constant removal of the passive layer due to fluid or particles impact on the corroding surface or the friction between the corroding surface and another surface’. It is to be noted that the ‘Tribo-corrosion includes but is not restricted to: wear corrosion, fretting corrosion, and erosion corrosion). This process may result in an increase in friction of bearing surfaces in addition to causing material loss’.

Impingement attack – It is the ‘form of erosion corrosion in aqueous liquids under high velocity or turbulent flow conditions associated on the metal surface causing repetitive disruption of protective films leading to accelerated localised corrosion’.

High temperature corrosion – It is the ‘corrosion by gases or deposits or both gases and deposits occurring at elevated temperatures under conditions where aqueous electrolytes no longer exist’. It is to be noted that the ‘High temperature corrosion can become significant at temperatures above 170 deg C depending on material and environment’.

Hot corrosion – It is the ‘corrosion by gases or deposits or both gases and deposits forming a liquid phase during a high temperature corrosion reaction’. It is to be noted that the ‘Hot corrosion is a sub-term of high temperature corrosion’, and ‘The most common liquid phases in which hot corrosion occurs are metal sulphates, metal vanadates and metal chlorides’.

Sulphidation – It is the ‘reaction of a metal or alloy with a sulphur-containing species to produce metal sulphides on or beneath the surface of the metal or alloy’.

Metal dusting – It is the ‘carburization of metallic materials in process gases containing carbon oxides and hydrocarbons and with extremely low oxygen partial pressures leading to disintegration of the metal into dust of graphite, metal or carbides, or combinations’. It is to be noted that the ‘The temperature range for metal dusting lies between 400 deg C and 900 deg C. For the mechanism to happen, a carbon activity higher than 1 in the process gas is required’.

Rebar corrosion – It is the ‘corrosion of reinforcement bars in concrete’.

Atmospheric-corrosivity categories – As per ISO 9223, atmospheric environments are classified into six atmospheric-corrosivity categories (i) C1, very low corrosivity, (ii) C2, low corrosivity, (iii) C3, medium corrosivity, (iv) C4, high corrosivity, (v) C5, very high corrosivity, and (vi) CX, extreme corrosivity. It is to be noted that CX covers different extreme environments. One specific extreme environment is the offshore environment.

For the determination of corrosivity categories, the exposure of standard samples is strongly desired. Tab 1 defines the corrosivity categories in terms of the mass or thickness loss of such standard samples made of low-carbon steel and / or zinc after the first year of exposure. For longer times, the exposure does not give reliable results and is hence not permitted. The mass or thickness losses obtained for steel and zinc samples can sometimes give different categories. In such cases, the higher corrosivity category shall be taken.

If it is not possible to expose standard samples in the actual environment of interest, the corrosivity category can be estimated by simply considering the examples of typical environments given in Table 1 of ISO 12944-2:2017 which is given below.

 Methods of corrosion control

Corrosion management needs the implementation of engineering principles and techniques to, by the most economical method, mitigate corrosion to an appropriate degree. To mitigate or control corrosion, there are different practices which can be used. These practices are shown in Fig 3. The use of the method relies upon multiple parameters, for example, the type and the location of the corrosion, the practical usage of the surface / structure to be protected, and the local environment etc. The objective of the corrosion control techniques is to minimize corrosion of materials to an acceptable limit, so that they are able to attain their normal desired life-time. In a limited number of cases, corrosion control methods are designed to eliminate it completely.

Fig 3 An outline of different methods used in the corrosion control

Proper selection of materials – The methodology involves the selection and use of materials with high corrosion resistance to increase a structure’s longevity in a specific setting. While there are no materials which are resistant to all corrosive conditions, it is important to choose suitable materials to avoid certain forms of corrosion failure. Titanium, for example, is a highly corrosion-resistant material, but it is far more costly than steel. Also, it is not as ductile as steel. Carbon steel is the material of choice in oil production systems, especially for equipment such as wells, pipelines, vessels, and tanks, because of its good mechanical properties and low cost. However, options such as stainless steel can be used for situations where more corrosion-resistant material is needed.

Environmental modification – A chemical reaction between the corrosive state of the metal and the local atmosphere causes corrosion. Metal corrosion can be regulated instantly by excluding the metal from, or modifying, the state of the atmosphere. This can be as simple as reducing contact with rain or sea-water by indoor storage of metal materials or can be in the form of direct environmental manipulation affecting the metal. The rate of metal corrosion can be lowered by methods to minimize the sulphur, chloride, or oxygen content of the surrounding atmosphere. For example, in order to minimize corrosion in the interior of the unit, feed water for water boilers can be formulated with softeners or several other chemicals in order to alter the hardness, alkalinity, or oxygen content.

Cathodic protection – Cathodic protection involves polarization of the structure, to be protected, to potentials more negative than the corrosion potential, hence thermodynamically preventing the occurrence of anodic reaction. For this purpose, an external power source is used. Another common way is to use ‘sacrificial anodes’. In this method the anode is made of a more active metal (e.g., magnesium) than the structure to be protected (e.g., iron or carbon steel). Hence, the structure which needs to protected acts as the cathode of a new corrosion cell. This type of protection is widely used to protect some underground structures, for example, water storage tanks, buried pipelines, ship hulls, and marine facilities. There are two methods of cathodic protection namely (i) impressed current method, and (ii) sacrificial anode method.

Impressed current method – The impressed current cathodic protection (ICCP) technique is widely used for the protection of buried pipelines and the hulls of ships immersed in seawater. A DC (direct current) electrical circuit is used to apply an electric current to the metallic structure. The negative terminal of the current source is connected to the metal needing protection. The positive terminal is connected to an auxiliary anode immersed in the same medium to complete the circuit. The electric current charges the structure with excess electrons and hence changes the electrode potential in the negative direction until the immunity region is reached. The layout for a typical ICCP system is shown in Fig 4.

Fig 4 A general representation of impressed current cathodic protection method

ICCP is a specialized technology and can be very effective if correctly designed and operated. Typical materials used for anodes are graphite, silicon, titanium, and niobium plated with platinum. Coatings are frequently used in conjunction with ICCP systems to minimize the effect of corrosion on marine structures. One of the difficulties in designing a combined coating and ICCP system is that coatings deteriorate with time. Precious metals are used for impressed current anodes since they are highly efficient electrodes and can handle much higher currents. Precious metal anodes are platinized titanium or tantalum anodes. The platinum is either clad to or electroplated on the substrate. Impressed current systems are more complex than sacrificial anode systems and mostly used to protect pipelines.

Sacrificial anode method – The principle of this technique is to use a more reactive metal in contact with steel structure to drive the potential in the negative direction until it reaches the immunity region. Fig 5 shows the principle, in which sacrificial metals used for cathodic protection. It consists of magnesium-base and aluminum-base alloys and, to a lesser extent, zinc. No external power source is needed with this type of protection system and much less maintenance is needed. These metals are alloyed for improving the long-term performance and dissolution characteristics. Fig 5 shows a general representation of sacrificial anode cathodic protection method.

Fig 5 A general representation of sacrificial anode cathodic protection method

Sacrificial anodes serve essentially as sources of portable electrical energy. For cathodic protection of off-shore platforms, aluminum anodes, made from aluminum-zinc alloys, are the preferred material. Majority of the off-shore petroleum production platforms use sacrificial anodes because of their simplicity and reliability, even though the capital costs are lower with the impressed-current systems.

Magnesium anodes have been used off-shore in recent years to polarize the structures to a protected potential faster than zinc or aluminum alloy anodes. Magnesium tends to corrode quite readily in salt water, and majority of the designers avoid the use of magnesium for permanent long-term marine cathodic protection applications. Zinc anodes are also used to protect ballast tanks, heat exchangers, and several mechanical components on ships, coastal power plants, and similar structures. In sea-water, passivity can be avoided by alloying additions, such as tin, indium, antimony, or mercury.

The three most common types of sacrificial anodes are activated aluminum, zinc, and magnesium. Aluminum is the most widely used material for anodes, as it has a higher current capacity in comparison to the other metals. Magnesium is to be considered when the chloride content is less than 10,000 ppm (parts per million).

Anodic protection – Anodic protection, which is an electro-chemical method of controlling corrosion based on the phenomenon of passivity, is a comparatively new method suggested by Edeleanu. For building a protective oxidized layer on the protected base material, also known as the substrate, an electrical current is used in the process. The passive potential is automatically maintained, normally electronically, by an instrument called the potentiostat. The method is applicable to metals having active passive transitions like nickel, titanium, iron, chromium, and their alloys. Its usefulness and its low current demands in highly corrosive environments are the significant benefits of anodic protection.

Coating, linings and nonmetallic piping – A coating is a thin material applied as a liquid or powder, which, on solidification, is firmly and continuously attached to the material to be protected. For an internal use, this method can be called lining. Coatings need to be flexible, to be resistant to chemical fluid attacks, to have strong adhesion, to have low or no porosity, and to be stable at working temperatures. The coatings are frequently applied in conjunction with cathodic protection systems to take care of any damage caused to the coating material.

Application of non-metallic piping is also good in several applications as they do not corrode, but the limitation is that they can deteriorate or be weakened by attack from the environment. Poly-vinyl chloride (PVC) is one example of this type of piping. This material is normally repeatedly reheated, softened, and reshaped without destruction. However, where mechanical / structural integrity is important, such as in cases of high internal or external pressures and loads, PVC and other polymer-based materials cannot be used, and the use is then normally confined to metals.

Metal coatings can also be incorporated in this category. As an example, corrosion susceptible metals (e.g., carbon steels) can be coated with a thin chromium coating. Chromium is a highly corrosion-resistant material, and as long as the chromium coating is compact, it protects the underlying structure from corrosion.

Organic coatings – Such coatings afford protection by providing a physical barrier between the metal and the environment. The most widely used protective coatings are used to protect aluminum, zinc, and carbon steel from corrosion. In these coatings, they can also contain corrosion inhibitors. Natural coatings are found in paints, resins, lacquers, and varnishes. Heavy organic coatings are frequently used, like mastics and coal tars, to coat aluminum structures embedded in soils and concrete.

Inorganic coatings – Inorganic coatings are frequently used for providing a barrier between the atmosphere and the metal. Enamel, glass linings, and conversion coatings are all inorganic coatings. The treatment transforms the metal surface into a metallic oxide film or a compound which is more resistant to corrosion than the natural oxide film and provides an effective base or additional safety key, such as paints.

Metallic coatings – Metallic coatings are another type of coatings which provide a barrier in between metal substrate and the atmosphere. Also, when the coating is damaged, metallic coatings can frequently provide cathodic protection. Using a variety of techniques, including hot dipping, electro-plating, cladding, thermal spraying, and chemical vapour deposition (CVD), metallic coatings and other inorganic coatings are developed.

Use of corrosion inhibitors – In different industries, among the most efficient methods of effectively reducing corrosion is by using corrosion inhibitors. Corrosion inhibitors are chemical substances which stop or retard corrosion of the metallic surface when added to the corrosive media in small quantities. Majority of the inhibitors actively used are organic molecules. Through adsorption on the metallic surface, these inhibitors work to protect the surface by forming a film which minimizes the contact between the metal surface and the aggressive environment. The physical blockage effect or the effects of the inhibitor on the mechanisms and kinetics of the corresponding corrosion reactions can be related to this minimization. Inhibitors are typically delivered from a solution or dispersion, but some are used in preparations for protective coating.

Factors affecting rate of corrosion – There are several internal and external factors which directly or indirectly affect the rate of corrosion. Here the focus is given on external influences, since other influences cannot be changed, and their impact on the corrosion of metals, namely the effect of electrolyte solutions on the chemical material are considered. In order to predict the corrosion behaviour of metals under particular conditions, this effect is to be understood. Some of the factors which contribute to the rate of corrosion are outlined below. Fig 6 gives a general illustration of different factors affecting the corrosion.

Fig 6 A general illustration of different factors affecting the corrosion

Effect of the electrical conductance of media – Because of the acceleration of the corrosion current, if the electrical conductance is strong, the electrical conductance of media is a very significant factor in corrosion processes. The lower the medium’s electric resistance R, the higher the corrosion current Icorr.  Icorr = V/R = (Ek – Ea)/R.

All electro-chemical (cathodic) safety parameters (shape selection, density of electrical current, etc.) depend on the electrical resistance of the media and the anode shape. The electrical resistance of the soil frequently depends on the distribution of stray electric currents. Of course, electro-chemical measurements depend, for example, on the electrical resistance of the media to determine the electrical current as a function of the electrical potential (V). The ohmic potential drop (known as IR drop) occurs during an electrical current flow between the working electrode and a reference electrode in solution. This is a possible decrease due to the electrical resistance of the solution layer. The higher is an electrical resistance of the solution, the higher is the IR decrease. This error in potential measurements needs to be accounted for. An IR decrease can involve polarization measurements in pure water and non-aqueous solvents, as well as metal electrodes with coatings.

Influence of the pH on metallic corrosion – As per their relationship (corrosion resistance) to pH values, all metals can be split into five groups.

In the first group metals, corrosion resistance of gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), mercury (Hg), tantalum (Ta), niobium (Nb), osmium (Os), and iridium (Ir) is not affected by the pH value. At pH between 0 to 14, they are resistant toward the corrosion.

In the second group metals, iron (Fe), chromium (Cr), and manganese (Mn) are strongly corroded at temperatures above 80 deg C in acidic solutions (pH less than 4) and in very hot alkaline solutions (pH higher than 13.5). They frequently corrode, but at neutral pH (6 to 8) at a low rate, and are immune at pH 9 to 13.

In the third group metals, magnesium (Mg), titanium (Ti), hafnium (Hf), vanadium (V), and bismuth (Bi) are extremely resistant to neutral and alkaline solutions (strong pH) and corrode at a high rate at low pH (in acid solutions).

In the fourth group metals, molybdenum (Mo), tungsten (W), and rhenium (Re) are resistant to corrosion in acid and neutral solutions but corrode in alkaline solutions.

The fifth group metals are beryllium (Be), aluminum (Al), copper (Cu), zinc (Zn), cadmium (Cd), tin (Sn), lead (Pb), cobalt (Co), nickel (Ni), zirconium (Zr), gallium (Ga), and indium (In). They corrode in both acidic and alkaline liquids, and are hence called amphoteric metals. Only neutral solutions are resistant to these metals. It is evident that for various amphoteric metals, an area of pH where high resistance exists is distinct. Aluminum is resistant at pH 4.5 to 8.3, and zinc is resistant at pH 6.5 to 12. The pH region of the high resistance of amphoteric metals significantly alters the temperature and presence of different ions in a solution. Strictly speaking, the amphoteric metals are frequently associated with iron and chromium as they dissolve at low and very high pH (above 13.5), the higher is the temperature, the lower is the pH when iron is dissolved in alkaline solutions.

Effect of substances forming chemical complexes with metals on corrosion – Substances of metals forming chemical complexes intensify their corrosion. Neutral molecules (ammonia NH3) or ions (cyanides CN-) can be these compounds. Ammonia is highly soluble in water, producing an ammonium hydroxide alkaline solution. In the presence of ammonia and dissolved oxygen in water, copper and copper alloys are not immune because of the formation of a complex as given in equation 4Cu(s) + 8NH3(aq) + O2(g) + 2H2O(l) = 4[Cu(NH3)2]+(aq) + 4OH-(aq).

Using copper condensers with water from bays near agricultural fields where ammonia fertilizers are used is hence very risky. Because of their high corrosion resistance, gold and silver are ‘noble’ metals. They are normally immune to all media sources. But if one places them in an aqueous solution of sodium or potassium cyanide (NaCN or KCN), with these ‘noble’ metals, anions of cyanide form chemical complexes. The equations are (i) 4Au(s) + 8CN-(aq) + 2H2O(l) + O2(g) = 4[Au(CN2)]-(aq) + 4OH- (aq), and (ii) 4Ag(s) + 8CN- (aq) + 2H2O(l) + O2(g) = 4[Ag(CN2)]-(aq) + 4OH-(aq).  Hence, in solutions containing cyanide anions, gold and silver are not resistant.

Effect of cations participating in cathodic reactions – In addition to the presence of dissolved oxygen (O2) in neutral and alkaline solutions and H3O+ cations in acidic solutions, Fe3+ and Cu2+ cations can be involved in cathodic processes on the surface of the metal and accelerate corrosion. It is good to say that according to the reaction, Fe2+ cations formed in the cathodic reaction can be oxidized to Fe3+ as given by the equation 4Fe2+(aq) + O2(g) + 4H3O+(aq) = 4Fe3+(aq) + 6H2O(l). So, Fe3+ cation regeneration continues all the time, and corrosion is intensified. It is hence very important to recognize and track such cations in a medium which can take part in cathodic reactions and accelerate corrosion.

Effect of temperature – In chemistry, temperature is the big parameter for adjusting the rate of chemical reactions. Under large variations in environmental conditions, metals and alloys are used. The thermodynamics and kinetics of metallic corrosion can be influenced by temperature. Temperature rises typically accelerate anodic and cathodic reactions, decrease gaseous oxygen dissolution in the media, and accelerate the diffusion of cathodic agents (O2, H3O+, and Fe3+ etc.). The dissolution and transformation of corrosion products forming on metal surfaces are also changed by temperature. The effect of temperature is complex, and it is not easy to predict the behaviour of metals with changes in temperature. Temperature hence influences corrosion by two factors namely (i) it accelerates both anodic and cathodic reactions and (ii) it decreases the concentration of dissolved oxygen (cathodic reaction as a result).

These two variables are identical at a certain temperature, and corrosion reaches the maximum value. Then the decrease in oxygen concentration in water prevails over the rise in the anodic and cathodic reaction rates at higher temperatures. Dissolved oxygen disappears from the ‘scene’ as a cathodic partner, and corrosion decreases. In the corrosion of metals, oxygen is so critical that, in specific cases, it can result in acceleration of corrosion and, in other cases, decrease and even stop it. In certain cases, the dissolved oxygen concentration in the media is non-uniform and induces the development of differential aeration cells.

Effect of the dissolved oxygen – In the corrosion of metals, dissolved oxygen plays a very crucial and complex role. In neutral, alkaline, and acidic media, oxygen takes part in cathodic procedures on the metal surface. Hence, for corrosion to occur, its existence is needed. If dissolved oxygen is absent in water, corrosion in neutral and alkaline solutions decrease to almost zero. If the concentration of dissolved oxygen increases, as a consequence of oxygen involvement in the cathodic processes, corrosion accelerates. What is going to happen if more and more water with oxygen is injected.  It is defined that oxygen under some particular conditions (in water of high purity) and high temperature can result in the formation of a passive protective dense film composed of metal oxides on the metal surface, and corrosion then decreases. One of the corrosion prevention strategies at power stations is the introduction of oxygen into water. If water contains ions (Cl2-, SO2-), passive films can be damaged and the passivation effect is not obtained.

Effect of dissolved salts in water on corrosion – The chemical content of drinking water, cooling water, seas, oceans, rivers, lakes, and subterranean waters varies. For all these types of water, their almost neutral pH (normally between 5.5 and 8.3), the presence of inorganic and organic compounds, and dissolved gases are the common denominators. Slight changes in the chemical content of water species can result in drastic changes in metal corrosion rates. pH values from 5.5 to 8.3 have no effect on the corrosion of metals. Metal cations formed in anodic metal dissolution react with hydroxide anions OH- formed in aqueous neutral solutions in a cathodic reduction of dissolved oxygen, and corrosion products are formed in the form of hydroxides. The rate of corrosion depends on the form of salt and its concentration.