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Corrosion is a natural process that deteriorates materials, commonly metals, due to chemical or electrochemical reactions with their environment. It's a significant concern across various industries, including infrastructure, manufacturing, and transportation. The effects of corrosion can range from minor aesthetic damage to catastrophic structural failure, leading to enormous economic costs and safety hazards.

Several factors influence corrosion, including environmental conditions such as moisture, temperature, pH levels, and the presence of corrosive agents like oxygen, sulfur compounds, and salts. Additionally, the material's composition and microstructure play crucial roles in its susceptibility to corrosion.

To mitigate corrosion and prolong the lifespan of materials, various protection methods are employed:

Barrier Protection: This involves applying coatings or barriers to physically isolate the material from its environment. Common barrier materials include paints, polymer coatings, and enamels. These coatings create a protective layer that prevents corrosive agents from reaching the underlying material.
Cathodic Protection: This method involves making the metal to be protected the cathode of an electrochemical cell, thus reducing its corrosion rate. Cathodic protection can be achieved through sacrificial anodes, where a more reactive metal (such as zinc or magnesium) is connected to the metal to be protected, sacrificing itself to protect the base metal.
Anodic Protection: Conversely, anodic protection works by polarizing the metal to be protected to make it the anode in an electrochemical cell. This method is suitable for metals that exhibit passivity, such as stainless steel. By maintaining the metal in its passive state, its corrosion rate is significantly reduced.
Inhibitors: Corrosion inhibitors are chemicals that are added to the environment surrounding the metal to reduce its corrosion rate. Inhibitors work by adsorbing onto the metal surface, forming a protective layer that blocks corrosive agents from reaching the metal. Common inhibitors include organic compounds, chromates, and phosphates.
Alloying: Alloying involves mixing the base metal with other elements to improve its corrosion resistance. For example, stainless steel contains chromium, which forms a passive oxide layer on the surface, protecting the underlying metal from corrosion.
Design Modification: Sometimes, corrosion can be mitigated through design modifications that minimize exposure to corrosive environments or improve drainage to prevent the accumulation of moisture.
Each protection method has its advantages and limitations, and the choice of method depends on factors such as the material, the environment, cost considerations, and the required durability. In many cases, a combination of protection methods may be employed to provide optimal corrosion resistance.

1. Corrosion requires a comprehensive multidisciplinary and interdisciplinary outlook with core knowledge from the fields of metallurgy & materials science together with chemistry, electrochemistry and surface science. Text Book 1- Corrosion Engineering by Mars G. Fontana Reference Books 1- Corrosion and corrosion control by Uhlig 2- Corrosion- understanding the basics by ASM international 3- Corrosion mechanisms in theory and practice by Philippe Marcus Corrosion and its Protection

2. Historical Background The word corrosion is as old as the earth, but it has been known by different names. Corrosion is known commonly as rust, an undesirable phenomenon which destroys the luster and beauty of objects and shorten their life. Origin of word corrosion is from F rench and Latin origin means “gnaw away”

3. There is a historical record of observation of corrosion by several writers, philosophers and scientists, but there was little curiosity regarding the c auses and mechanism of corrosion until Robert Boyle wrote his ‘Mechanical Origin of Corrosiveness’

4. M ost important contributions were later made by Faraday (1791-1867) Who established a quantitative relationship between chemical action a nd electric current. Faraday’s first and second laws are the basis for calculation of corrosion r ates of metals. Schonbein (1836) showed that iron could be made passive. Whitney (1903) provided a scientific basis for corrosion control based on electrochemical observation ( Iron corrodes rapidly in dilute nitric Acid but remains unattacked in concentrated nitric acid ) U.R. Evans (1923) to provide a modern understanding of the causes and control of corrosion based on his classical electrochemical theory. The poineers of modern understanding of corrosion have also been identified by Uhlig and Fontana .

5. Definitions of Corrosion

6. Corrosive Environment Corrosion can not be defined without a reference to environment. All environments are corrosive to some degree. Following is the list of typical corrosive environments:

7. Corrosion Corrosion is the spontaneous destruction of metals and alloys caused by chemical, bio-chemical, and electrochemical interaction between metals and alloys and the environment. Deterioration by physical causes is not called corrosion , but is described as erosion, galling, or wear. Chemical attack accompanies physical deterioration, as described by the corrosion – erosion, corrosive wear, or fretting corrosion.

8. Corrosive environments include moisture, oxygen, inorganic and organic acids, high pressure, temperature, and chlorides. “ Rusting ” applies to the corrosion of iron or iron - base alloys with formation of corrosion products consisting largely of hydrous ferric oxides. Nonferrous metals , therefore, corrode, but do not rust. During corrosion, metals tend to convert to more thermodynamically stable compounds such as oxides, hydroxides, salts, or carbonates. Recovering the original compounds (minerals and ores) from metals by spontaneous corrosion as the result of a decrease in free energy. Hence, the energy used for metal winning from ore or alloying is emitted during corrosion reactions Corrosion processes are classified as chemical, biochemical, and electrochemical corrosion .

9. Chemical Corrosion In order for corrosion to proceed as a chemical reaction, the reacting particles must come into contact to transfer the electrons. Thermodynamically, the reaction is governed by the ratio of internal energy to activation energy. The laws of heterogeneous chemical reactions control spontaneous metal destruction. Examples of chemical corrosion are destructive metal interaction with nonconductive organic compounds and high temperature corrosion in the presence of aggressive gases. Microbial activities that produce sulfides, organic, or inorganic acids causing direct metal oxidation are major driving forces in bio-corrosion . Biochemical corrosion is enhanced by stagnant water, soil, and organic products. Biochemical Corrosion

10. Electrochemical Corrosion Electrochemical corrosion is governed by electrochemical kinetics. The rate of charge transfer reactions is determined by Faraday’s law. Corrosion may affect the entire metal surface (general corrosion) or locally, resulting in pitting or stain corrosion. It attacks metals exposed in electrolytes (liquid corrosion), soils (soil corrosion), and gas in the presence of moisture on the metal surface (atmospheric corrosion). Corrosion caused by an external electric current is a special case of electrochemical corrosion. This includes stray current corrosion of underground metal structures when current is applied from bare electric lines. The potential difference is established between two electrochemically active areas of a metal structure; a cathodic area that receives current from the external circuit, and an anodic area that flows current to the soil or any other conductive medium.

11. IMPORTANCE OF CORROSION The three main reasons for the importance of corrosion Economics Safety Conservation Economics 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, that result from the corrosion of piping , tanks, metal components of machines, ships, bridges, marine structures, and so on.

12. Safety Corrosion can compromise the safety of operating equipment by causing failure (with catastrophic consequences) of, for example, pressure vessels, boilers , metallic containers for toxic chemicals, turbine blades and rotors, bridges, airplane components, and automotive steering mechanisms. Safety is a critical consideration in the design of equipment for nuclear power plants and for disposal of nuclear wastes. Loss of metal by corrosion is a waste not only of the metal , but also of the energy, the water, and the human effort that was used to produce and fabricate the metal structures in the first place. In addition, rebuilding corroded equipment requires further investment of all these resources —metal , energy, water, and human. Direct Losses : costs of replacing corroded structures and machinery, repainting structures, capital costs plus maintenance of cathodic protection systems for underground pipelines, extra cost of using corrosion - resistant metals and alloys

13. Indirect Losses : Shutdown of plant, Loss of Product, Loss of Efficiency, Contamination of Product Studies of the cost of corrosion to USA, Australia , Great Britain, Japan, and other countries have also been carried out. In each country studied, the cost of corrosion is approximately 3 – 4 % of the Gross National Product

14. CAUSES OF CORROSION Change in Gibbs Free Energy The change in Gibbs free energy, ΔG, for any chemical reaction indicates the tendency of that reaction to go. Reactions occur in the direction that lowers the Gibbs free energy. The more negative the value of ΔG , the greater the tendency for the reaction to go. Pilling– Bedworth Ratio Pilling – Bedworth ratio is a parameter that can be used to predict the extent to which oxidation may occur. The Pilling – Bedworth ratio is where M and D are the molecular weight and density, respectively, of the corrosion product scale that forms on the metal surface during oxidation ; m and d are the atomic weight and density, respectively, of the metal, and n is the number of metal atoms in a molecular formula of scale; for example, for Al 2 O 3 , n= 2.

15. The Pilling – Bedworth ratio indicates whether the volume of the corrosion product is greater or less than the volume of the metal from which the corrosion product formed. If Md / nmD <1 , the volume of the corrosion product is less than the volume of the metal from which the product formed. A film of such a corrosion product would be expected to contain cracks and pores and be relatively nonprotective . If Md / nmD >1 , the volume of the corrosion product scale is greater than the volume of the metal from which the scale formed , so that the scale is in compression, protective of the underlying metal. A Pilling – Bedworth ratio greater than 1 is not sufficient to predict corrosion resistance . If Md / nmD >> 1 , the scale that forms may buckle and detach from the surface because of the higher stresses that develop. For Al, which forms a protective oxide and corrodes very slowly, the Pilling – Bedworth ratio is 1.3, whereas for magnesium, which tends to form a nonprotective oxide, the ratio is 0.8.

16. TYPES OF CORROSION DAMAGE Corrosion is often thought of only in terms of rusting and tarnishing. However, corrosion damage occurs in other ways as well, resulting, for example, in failure by cracking or in loss of strength or ductility. The five main types of corrosion classified with respect to outward appearance or altered physical properties are as follows: 1. General Corrosion, or Uniform Attack 2. Pitting 3. Dealloying , Dezincification , and Parting 4. Intergranular Corrosion 5. Cracking 1. General Corrosion, or Uniform Attack This type of corrosion includes the commonly recognized rusting of iron or tarnishing of silver. “ Fogging ” of nickel and high - temperature oxidation of metals are also examples of this type.

17. Rates of uniform attack are reported in various units, with accepted terminologies being millimeters penetration per year (mm/y) and grams per square meter per day ( gmd ). Other units that are frequently used include inches penetration per year ( ipy ), mils (1 mil = 0.001 inch) per year ( mpy ), and milligrams per square decimeter per day ( mdd ). These units refer to metal penetration or to weight loss of metal, excluding any adherent or nonadherent corrosion products on the surface. Steel , for example, corrodes at a relatively uniform rate in sea water of about 0.13 mm/y, 2.5 gmd , 25 mdd , or 0.005 ipy . These represent time averaged values. Generally , for uniform attack, the initial corrosion rate is greater than subsequent rates

18. For handling chemical media whenever attack is uniform, metals are classified into three groups according to their corrosion rates and intended application. These classifications are as follows: <0.15 mm/y ( <0.005 ipy ) Metals in this category have good corrosion resistance to the extent that they are suitable for critical parts, for example, valve seats, pump shafts and impellors, springs. 0.15 to 1.5 mm/y (0.005 to 0.05 ipy ) Metals in this group are satisfactory if a higher rate of corrosion can be tolerated, for example, tanks, piping , valve bodies, and bolt heads. >1.5 mm/y ( >0.05 ipy ) Usually not satisfactory

19. 2. Pitting This is a localized type of attack, with the rate of corrosion being greater at some areas than at others. If appreciable attack is confined to a relatively small, fixed area of metal, acting as anode, the resultant pits are described as deep . If the area of attack is relatively larger and not so deep, the pits are called shallow . Depth of pitting is sometimes expressed by the pitting factor, the ratio of deepest metal penetration to average metal penetration as determined by the weight loss of the specimen. A pitting factor of unity represents uniform attack Sketch of deepest pit in relation to average metal penetration and the pitting factor

20. Iron buried in the soil corrodes with formation of shallow pits, whereas stainless steels immersed in seawater characteristically corrode with formation of deep pits. Many metals, when subjected to high velocity liquids, undergo a pitting type of corrosion called impingement attack , or sometimes corrosion- erosion . Copper and brass condenser tubes, for example, are subject to this type of attack . Fretting corrosion , which results from slight relative motion (as in vibration) of two substances in contact, one or both being metals, usually leads to a series of pits at the metal interface. Metal oxide debris usually fills the pits so that only after the corrosion products are removed do the pits become visible. Cavitation – erosion is the loss of material caused by exposure to cavitation, which is the formation and collapse of vapor bubbles at a dynamic metal/liquid interface for example, in rotors of pumps or on trailing faces of propellers. This type of corrosion causes a sequence of pits , as a honeycomb of small relatively deep fissures

21. 3. Dealloying , Dezincification , and Parting Plug-type dezincification in brass pipe (actual size) Dealloying is the selective removal of an element from an alloy by corrosion. One form of dealloying , dezincification , is a type of attack occurring with zinc alloys (e.g., yellow brass) in which zinc corrodes preferentially, leaving a porous residue of copper and corrosion products. Dezincified brass pipe may retain sufficient strength to resist internal water pressures until an attempt is made to uncouple the pipe

22. Parting is similar to dezincification in that one or more reactive components of the alloy corrode preferentially, leaving a porous residue that may retain the original shape of the alloy. Parting is usually restricted to such noble metal alloys as gold – copper or gold – silver used in gold refining. For example, an alloy of Au – Ag containing more than 65% gold resists concentrated nitric acid as well as does gold itself. However, on addition of silver to form an alloy of approximately 25% Au – 75% Ag, reaction with concentrated HNO 3 forms silver nitrate and a porous residue or powder of pure gold. Copper - base alloys that contain aluminum are subject to a form of corrosion resembling dezincification , with aluminum corroding preferentially.

23. 4. Intergranular Corrosion This is a localized type of attack at the grain boundaries of a metal, resulting in loss of strength and ductility. Grain boundary material of limited area, acting as anode, is in contact with large areas of grain acting as cathode. The attack is often rapid, penetrating deeply into the metal and sometimes causing catastrophic failures. Improperly heat treated 18-8 stain less steels or Duralumin - type alloys (4% Cu – Al) are among the alloys subject to intergranular corrosion. At elevated temperatures, intergranular corrosion can occur because, under some conditions, phases of low melting point form and penetrate along grain boundaries; for example, when nickel base alloys are exposed to sulfur bearing gaseous environments, nickel sulfide can form and cause catastrophic failures

24. 5. Cracking If a metal cracks when subjected to repeated or alternate tensile stresses in a corrosive environment, it is said to fail by corrosion fatigue . In the absence of a corrosive environment, the metal stressed similarly , but at values below a critical stress, called the fatigue limit or endurance limit, will not fail by fatigue even after a very large, or infinite , number of cycles. A true endurance limit does not commonly exist in a corrosive environment: The metal fails after a prescribed number of stress cycles no matter how low the stress. The types of environment causing corrosion fatigue are many and are not specific . If a metal, subject to a constant tensile stress and exposed simultaneously to a specific corrosive environment, cracks immediately or after a given time, the failure is called stress corrosion cracking .

25. The stress may be residual in the metal, as from cold working or heat treatment, or it may be externally applied. The observed cracks are intergranular or transgranular , depending on the metal and the damaging environment. Almost all structural metals (e.g., carbon - and low - alloy steels, brass, stainless steels , Duralumin, magnesium alloys, titanium alloys, nickel alloys, and many others ) are subject to stress - corrosion cracking in some environments.