Corrosion in Carbon Steels

Corrosion in Carbon Steels

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’. Corrosion is seen when there is a change in the metal’s or system’s properties which can lead to an undesirable outcome. This can range simply from visual impairment to complete failure of technical systems which cause big economic damage and even present a hazard to the people.

Corrosion can be defined broadly as the destruction or deterioration of metal by direct chemical and electro-chemical reaction with its environment. Most simply stated, metallic corrosion is the reverse of electroplating. The metal being corroded forms the anode while the cathode is that being electroplated. Metallic corrosion occurs since in many environments, the majority metals are not inherently stable and tend to revert to some more stable combination of which the metallic ores as found in nature are familiar examples

Carbon steel is the most widely used engineering material. It has relatively limited corrosion resistance. The cost of corrosion of carbon steel to the total economy is very high. Since the carbon steels represent the largest single class of alloys in use, both in terms of tonnage and total cost, the corrosion of carbon steels is a problem of enormous practical importance. In carbon steel, the typical corrosion process can be regarded as the thermodynamically favoured reverse reaction of the metal-winning (extraction) process as shown in Fig 1.

Fig 1 Chemical reaction of iron during corrosion and metal extraction reaction

Carbon steel (which include mild steels) is by its nature has limited alloy content, usually less than 2 % by weight for the total of all additions. Unfortunately, these levels of addition do not generally produce any remarkable changes in general corrosion behaviour. One possible exception to this statement is the weathering steels, in which small additions of copper, chromium, nickel, and / or phosphorus produce significant reductions in corrosion rate in certain environments. At the levels of various elements in which they are present in the carbon steel, the elements have no significant effect on corrosion rate in the atmosphere, neutral waters, or soils. Only in the case of acid attack, an effect observed. In this case, the presence of phosphorus and sulphur markedly increase the rate of attack. Indeed, in acid systems, the pure irons appear to show the best resistance to attack.

Corrosion reactions take place when conditions are thermodynamically in favour of the chemical reactions. When this happens, then potential other factors drive the speed of the reaction (kinetics of the reaction). The rate of corrosion is highly dependent on the environment, in which the carbon steels are used. In solving a particular corrosion problem, a dramatic change in attack rate can often be attained by altering the corrosive environment. Since corrosion is such a multifaceted phenomenon, it is generally useful to attempt to categorize the various types based upon the environmental basis, such as atmospheric corrosion, aqueous corrosion, corrosion in soils, concrete, and boilers etc.

Types of corrosion reactions

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.

The overall reaction can be separated in two partial reactions namely (i) metal dissolution, also known as oxidation or anodic reaction (Fe = Fe2+ + 2 e-), and (ii) reduction or cathodic reaction, a reaction mainly involving the oxygen present in the air with water (O2 + 2 H2O + 4 e- = 4 OH). These two partial reactions can take place on the metal surface in a fairly homogenous distribution leading to uniform attack or can occur locally and separately, leading to localized forms of corrosion such as pitting corrosion.

The mechanism and the electrochemical nature of the corrosion reaction define the necessary requirements for corrosion to take place (Fig 2). These requirements are (i) a conducting metal, (ii) an electrolyte (a thin moisture film on the surface is already sufficient), and (iii) oxygen for the cathodic reaction.

Fig 2 Corrosion reactions in carbon steel

Fig 2 also shows the basic corrosion mechanism of iron under a drop of water. Both iron dissolution and oxygen reduction reactions take place with slight separation on the surface, and their products (Fe ions and OH ions) react in the water drop to form red rust (corrosion product). The simple model of the corrosion reaction of Fig 2 explains many forms of corrosion and also to deduct measures to reduce corrosion. By preventing or slowing down one of the partial reactions, the overall corrosion rate can be reduced.

Forms of corrosion

There are six main forms of corrosion (Fig 3). These are (i) uniform corrosion / shallow pitting corrosion, (ii) pitting corrosion, (iii) environmental induced cracking, (iv) crevice corrosion, (v) inter-crystalline (inter-granular) corrosion, and (vi) galvanic corrosion.

Fig 3 Forms of corrosion 

Uniform corrosion/ shallow pitting corrosion – Uniform corrosion 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). 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.

Pitting corrosion – Pitting corrosion is a localized form of corrosion which leads to 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. 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 often appears only as small pinholes on the surface. The amount of material removed below the pinholes is generally 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 4 Pitting corrosion phases and factors needed for environmental induced cracking

Environmental induced cracking – There are two types of environmental induced cracking. These are (i) stress corrosion cracking, and (ii) hydrogen-assisted cracking (Fig 4).

Stress corrosion cracking 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.

Stress corrosion cracking 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.

Stress corrosion cracking 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 stress corrosion cracking 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 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 stress corrosion cracking, it can lead to sudden failure of steel parts without any detectable warning signs. In common applications, hydrogen damage is usually only relevant for high-strength steel with a tensile strength of around 1 MPa or higher.

As for the stress corrosion 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.

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). 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 can be 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 steels involved, the corroding environment and wet / dry cycles.

Inter-crystalline (inter-granular) corrosion – Inter-crystalline corrosion 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.

A well-known example is the so called sensitization of stainless steel. When certain grades of this material are kept at a temperature within the range of 500 deg C to 800 deg C for a considerable time, e.g. during a welding process, chromium-rich carbides are formed, resulting in chromium depletion at the grain boundaries. Consequently, the grain boundaries possess a lower degree of corrosion resistance than the residual material, leading to localized corrosive attack.

Galvanic (contact) corrosion – 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.

Atmospheric corrosion of carbon steel

The corrosion of carbon steel in the atmosphere and in many aqueous environments is best understood from a film formation and breakdown standpoint. It is an inescapable fact that iron in the presence of oxygen and / or water is thermodynamically unstable with respect to its oxides. Thus, the question is never whether the steel is going to corrode, but rather at what rate. In the absence of film formation and with a constant environment, one is to expect the oxidation rate to be constant. On the other hand, if the corrosion product film which forms isolates the steel from the corrosive environment, then a zero corrosion rate is expected after the initial film formation period. A tightly adherent film which permits only diffusion transfer of the reactants is characterized by a corrosion rate which decreases with the square root of the exposure time. Since the above idealizations are rarely encountered in the corrosion of carbon steels, it is obvious that other factors which tend to disrupt stable film formation are operative. These factors can be external, such as erosion by wind or rain, or they can be internal to the film itself, such as stresses caused by the different specific volumes of metal and oxide.

The corrosion of iron in the atmosphere proceeds by the formation of hydrated oxides. The half-cell reactions can be expressed by the equations (i) 1/2O2 + H2O + 2e = 2(OH)- (cathodic), and (ii) Fe = (Fe)2+ + 2e (anodic). Further reactions can then occur, such as (i) (Fe)2+ + 2(OH)- = Fe(OH)2, and (ii) 2Fe(OH)2 + H2O + 1/2O2 = 2Fe(OH)3.

The hydrated oxides can lose water during dry periods and revert to the anhydrous ferrous and ferric oxides. In addition, a layer of magnetite (Fe3O4) or FeO·Fe2O3 often forms between the layers of iron oxide (FeO) and hematite (Fe2O3). Actually, the various oxides and hydroxides of iron form a rather complicated system of compounds. The compound FeOOH has been found to exist in three different crystal forms plus an amorphous form. The occurrence of the various oxide types is dependent on pH, availability of oxygen, various atmospheric pollutants, and the composition of the carbon steel, as in weathering steels containing copper and phosphorus. The actual nature of the corrosion film is important since FeO and FeOOH seem to be more adherent than Fe3O4 and Fe2O3, and hence more likely to slow the corrosive attack, but the higher oxides and oxy-hydroxides are more prone to spallation.

Since there is a substantial variation in the corrosion rates of carbon steels in different atmospheric environments (rural, urban, industrial, and marine etc.), it is only logical to determine which of the factors that contribute to these differences. Although the prediction of corrosivity is not possible, it appears that humidity, temperature, and the levels of chloride, sulphate, and probably other atmospheric pollutants present each exert an influence on the corrosion rate of carbon steels.

Effects of humidity and atmospheric pollutants – Since atmospheric corrosion is an electrolytic process, the presence of an electrolyte is needed. This does not mean that the steel surface is to be awash with water. In fact, a very thin absorbed film of water is all that is needed. During an actual exposure, the steel spends some portion of the time awash with water because of rain or splashing and a portion of the time covered with a thin adsorbed water film. The portion of time spent covered with the thin water film depends quite strongly or relative humidity at the exposure locations. Various studies have shown that time of wetness, although an important factor cannot be considered in isolation when estimating corrosion rates. An example of this fact is shown in Fig 5, in which the weight gain of iron is plotted as a function of relative humidity for an exposure of 55 days in an atmosphere containing 0.01 % sulphur dioxide. In the lower right-hand corner of Fig 5 is the measured corrosion rate for iron exposed for the same time in a sulphur dioxide free environment at 99 % relative humidity.

Another feature of interest is the apparent existence of a critical humidity level below which the corrosion rate is small. The critical humidity in a sulphur dioxide containing environment is around 60 %. This behaviour contrasts with that of steel in contact with particles of sea salt, as shown in Fig 5. In Fig 5, the corrosion rate shows a steady increase with increasing humidity. Although there is a scarcity of data, it seems reasonable that oxides of nitrogen in the environment also show an accelerating effect on the corrosion of carbon steel. Indeed, any gaseous atmospheric constituent capable of strong electrolytic activity is to be suspected as being capable of increasing the corrosion rate of carbon steel. In short, it can be seen that there is an accelerating effect of chloride ions on the atmospheric corrosion,

Fig 5 Effect of relative humidity on the corrosion of C steels

Corrosion protection of carbon steels

Corrosion protection (Fig 6) is often a necessary consideration in selecting carbon steel for a given structural application. Corrosion can reduce the load-carrying capacity of a component either by generally reducing its size (cross section) or by pitting, which not only reduces the effective cross section in the pitted region but also introduces stress raisers which can initiate cracks. Obviously, any measure which reduces or eliminates corrosion extends the life of a component and increases its reliability. The economics, environmental conditions, degree of protection needed for the projected life of the part, consequences of unexpected service failure, and importance of appearance are the main factors which determine not only whether a steel part needs to be protected against corrosion but also the most effective and economic method of achieving that protection.

Fig 6 Corrosion protection of steel

There are two methods of minimizing the corrosion of steels. The first is to separate the reacting phases, and the second is to reduce the reactivity of the reacting phases. The separation of the reacting phases can be accomplished by metallic, inorganic or organic coatings, and film-forming inhibitors. Reactivity can be reduced by alloying, anodic or cathodic protection, and chemical treatment of the environment. Some methods of protection combine two or more forms.

In most environments, the corrosion rate of carbon steel is typically around 20 micrometers per year in a rural outdoor atmosphere and rising to more than 100 micrometers per year in coastal environments. It is normally too high for a satisfactory application. The product design does not generally account for a base material loss. Hence, cost-efficient corrosion protection solutions are necessary for carbon steel. In alkaline surroundings, however, steel normally remain stable. This explains why, for example, reinforcement bars made of carbon steel are already very well protected against corrosion in the alkaline environment of the surrounding concrete.

Coating protection of carbon steel

Several types of coatings are applied to enhance the corrosion resistance of carbon steels. Coating practices range from oiling for low-cost, temporary protection to vapour deposition for long-term corrosion, heat, and wear resistance. For economic reasons, the desired degree of protection is to be determined before a coating is selected.

Effective temporary protection during shipment or storage can be obtained by coating the carbon steel with mineral oil, solvents combined with inhibitors, emulsions of petroleum-base coatings, or waxes. These types of coatings are applied after acid pickling or between coating sequences. These coatings are not expected to provide long-term corrosion protection.

Surface preparation is important for all coating processes. Any oxide on the steel surface is to be removed by pickling or blasting. Degreasing is necessary after oxide removal or when the steel has been given a temporary coating, and it can be accomplished by several means. Ideally, the first step in the coating process is to be started immediately after cleaning.

Coating processes – These are used to apply coatings of zinc, aluminum, lead, tin, and some alloys of these metals to carbon steels. The hot-dip process consists of immersing the steel in a molten bath of the coating metal.

Zinc coating or galvanizing is a good choice for the corrosion protection of carbon steel. Several suitable processes are available for the application of zinc coatings on steel. The corrosion rate of zinc is more than ten times lower than that of steel, at around 0.5 micrometers per year in rural/urban atmospheres and rising to up to around 5 micrometers per year in coastal environments. The low corrosion rates are the result of the formation of stable layers of corrosion products containing carbonates (from CO2 in the air) and chlorides (if they are present in the atmosphere). Conditions where the formation of such insoluble corrosion products is not possible leads to much higher corrosion rates, limit the suitability of zinc as a protective coating. These include permanently wet conditions or exposure to high concentrations of industrial pollutants such as sulphur dioxide. In these environments, soluble corrosion products are formed preferentially and they can be washed off by rainfall. In addition to decreased corrosion rates, zinc also provides cathodic or sacrificial protection to the underlying steel. Where scratching, chipping or any other damage to the zinc coating exposes the steel, a special form of galvanic corrosion takes place. Zinc, being a less noble metal than steel, corrodes preferentially, thereby helping to keep the exposed steel surface protected.

Zinc coatings are consumed quite homogeneously during atmospheric corrosion. Accordingly, in a given application, doubling of the coating thickness normally also doubles the time until the zinc is consumed and red rust on the steel substrate occurs. Zinc is not stable in alkaline environments and is readily attacked in solutions with a pH-value of 10 or higher.

The method of zinc coatings are electro galvanizing, hot dip galvanizing, and Sherardizing. Sherardizing is a method of zinc coating utilizing a thermal diffusion process.

Aluminum hot-dip coatings (aluminizing) provide carbon steels with resistance to both corrosion and heat. In many environments, aluminum protects steel galvanically in much same way as zinc. Zinc-aluminum and aluminum-zinc alloys are also applied to steel by hot dipping. Heating aluminized steel results in the formation of an iron-aluminum inter-metallic compound which resists oxidation at temperatures upto around 800 deg C. Aluminized steel is often used where heat resistance is needed, for example, in automotive exhaust systems.

Hot-dip tin coatings provide a decorative and non-toxic barrier coating. Tin does not galvanically protect the steel substrate. For this reason, lacquers or other organic coatings are often used to fill pores in the tin coating and provide enhanced barrier protection.

Hot-dip lead coatings are sometimes used on steel which is exposed to sulphuric acid fumes or other aggressive chemical environments. Terne plate, a lead-tin alloy coating, gives more protection than pure lead coatings and is solderable.

Electroplated coatings are applied to steel for corrosion resistance, appearance, solderability, or other special requirements. A wide variety of materials are electroplated on steel, including zinc, aluminum, chromium, copper, cadmium, tin, and nickel. Multi-layer coatings can also be applied by electroplating. An example is the copper-nickel-chromium plating system used for bright automotive trim.

Clad metals – Carbon steels can be bonded to more corrosion-resistant materials, such as copper and stainless steels, by cold roll bonding, hot roll bonding, hot pressing, explosion bonding, and extrusion bonding. The resulting lamellar composite material has specific properties not obtainable in a single material.

Thermal spray coatings – These coatings provide effective long-term corrosion protection for steels in a wide range of corrosive environments. They are applied by one of several processes, including wire flame spraying, powder flame spraying, and electric arc spraying. Zinc, aluminum, and zinc-aluminum alloys are the most common coating materials applied by thermal spray techniques. Austenitic stainless steels, aluminum bronzes, and MCrAlY (where M = Co, Ni or Co/Ni) coating materials have also been used for specific applications. For maximum corrosion resistance, thermal sprayed coatings are sealed with an organic top coat. Thermal spray coatings are frequently used for corrosion protection in marine applications.

Vapour-deposited coatings – These are sometimes used for the protection of carbon steel, although the cost of such coatings can be very high. In vapour deposition, whether it is physical vapour deposition or chemical vapour deposition, the coating material is transported to the substrate in the form of individual atoms or molecules. A wide range of coating materials can be applied by vapour deposition. If applied to a sufficient thickness, the coating is essentially pore-free and dense, thus providing excellent barrier protection. A well-known application for vapour-deposited coatings on steel is ion vapour deposited aluminum coatings on steel aircraft and aerospace components.

Phosphate or chromate conversion coatings – These are used to enhance the corrosion resistance of steels. By themselves, they provide slightly better corrosion resistance than bare steel. More often, they are used in conjunction with another coating system. Conversion coatings are applied after hot-dip galvanizing and provide good corrosion protection when top coated with an organic coating system.

In the phosphating, steel is dipped into an acidic solution containing metal (Zn, Fe) phosphate salts. The solution reacts with the steel surface forming a micro-crystalline layer of phosphates on the surface. This results in a rough surface with excellent oil-retaining properties.

Organic coatings – These coating consists of paints, which are used more often for corrosion protection of steels than any other type of coating. Properly applied, the paints provide excellent protection at a relatively low cost. A wide variety of coating materials and application methods are available for the applications of the paints.

Ceramic coatings – Ceramic coatings used to protect steel include silicate cements and porcelain enamels. Monolithic cement linings provide good resistance to chemicals and thermal insulation. They can be applied by casting or spraying. Porcelain enamels are glass coatings which are fused onto the steel surface at or above 425 deg C to provide a glassy coating with good corrosion resistance and high hardness. The composition of the enamel can be varied to provide desired properties, such as improved resistance to alkalis.

Other nonmetallic materials coatings – These coatings are sometimes used as coatings or linings for steel in corrosion applications. These include rubbers (both natural and synthetic) and other elastomers and such plastic materials as epoxies, phenolics, and vinyls. A wide variety of properties and resistances to specific environments are available. Rubber linings have been used for many years in steel storage tanks for hydrochloric acid and sulphuric acid. Plastic linings are employed for plating tanks and similar applications.

Multi-layer coatings – When the corrosion protection provided by the metallic coating is not sufficient, then the steel can be further protected by additional coatings, mainly organic paint with or without metallic flakes. An example of this is the multi-layer coating on fasteners consisting of an electro-plated Zn alloy coating with an additional organic top coat.

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