2. Metallic corrosion and failures

Accidents arising from the metallic corrosion can produce injury or death of people by explosion, fire, and so on. The economic losses are classified into direct and indirect; the first includes the replacement of corroded materials, labor, periodic maintenance (coatings, cathodic protection, inhibitors in closed circuits, etc.) while the last involve aspects such as the discontinuity in the productive system, the loss and the contamination of raw materials and finished goods, and so on. The indirect losses are usually between 8 and 10 times the direct ones.

In industrialized countries, the total economic losses reach values between 3.5 and 4.5% of gross national product, despite applying all available technologies. It should also be mentioned

that the frequency of failure in metals by the various forms of corrosion reaches an average level of 60% (the remaining 40% is due to mechanical failures).

Metallic corrosion is usually defined as the destructive attack of a metal by chemical or electrochemical reaction with the environment [1]. Chemical corrosion involves the alteration of a metal in a non-ionic medium, such as gases or non-condensed vapors, high temperature, and so on. On the other hand, the electrochemical corrosion occurs with a simultaneous transport of electricity through the metal and the electrolyte (saline, atmosphere, seawater, etc.).

The most common metallic corrosion takes place electrochemically; it requires, as it is already put, electrical conductivity. Metals are electronic conductors of first specie while solutions and pure liquids are electrolytes of second specie.

Metallic or electronic conductors transport electricity through the electrons. The metals consist of a relatively rigid network of positive ions and of mobile electrons. When an electrical potential is applied, the electrons move in one direction while the positive ions remain static; the quoted electricity transport is produced without appreciable movement of matter. Since the electrons have a negative charge, the direction in which they move is the opposite at which is conventionally considered as positive current.

Meanwhile, the electrolytes carry the electric current through ions, that is, with a significant movement of matter. Ions are atoms or groups of atoms that have lost or gained electrons, reaching in this way positive charges (loss of electrons) or negative (gain of electrons), Figure 1. The positive ions (cations) move in the direction of current and the negative ions (anions) in the opposite one.

The determinant factors of metallic corrosion are the heterogeneity of metal (phases in alloy, remainder mechanical stresses, etc.) and/or of electrolyte (gradients of concentration, differential aeration, etc.). Meanwhile, the chemical nature of electrolyte (ion conductivity or equivalent) significantly influences the kinetics of the corrosive process and the geometry of the corrosion cell (higher conductivities usually favor the location of electrodes more distant from each other than solutions of high resistivity).

Figure 1. Corrosion mechanism.

A measure of the electrochemical kinetics (rate of reaction on electrode) is given by the equation i ¼ zFV, where i is the current density (current per unit area of electrode), z the number of equivalents per mole, F the Faraday constant (96,500 Coulomb/equivalent), and V the rate of reaction in moles per unit area and time.

that the frequency of failure in metals by the various forms of corrosion reaches an average

Metallic corrosion is usually defined as the destructive attack of a metal by chemical or electrochemical reaction with the environment [1]. Chemical corrosion involves the alteration of a metal in a non-ionic medium, such as gases or non-condensed vapors, high temperature, and so on. On the other hand, the electrochemical corrosion occurs with a simultaneous transport of

The most common metallic corrosion takes place electrochemically; it requires, as it is already put, electrical conductivity. Metals are electronic conductors of first specie while solutions and

Metallic or electronic conductors transport electricity through the electrons. The metals consist of a relatively rigid network of positive ions and of mobile electrons. When an electrical potential is applied, the electrons move in one direction while the positive ions remain static; the quoted electricity transport is produced without appreciable movement of matter. Since the electrons have a negative charge, the direction in which they move is the opposite at which is

Meanwhile, the electrolytes carry the electric current through ions, that is, with a significant movement of matter. Ions are atoms or groups of atoms that have lost or gained electrons, reaching in this way positive charges (loss of electrons) or negative (gain of electrons), Figure 1. The positive ions (cations) move in the direction of current and the negative ions (anions) in the

The determinant factors of metallic corrosion are the heterogeneity of metal (phases in alloy, remainder mechanical stresses, etc.) and/or of electrolyte (gradients of concentration, differential aeration, etc.). Meanwhile, the chemical nature of electrolyte (ion conductivity or equivalent) significantly influences the kinetics of the corrosive process and the geometry of the corrosion cell (higher conductivities usually favor the location of electrodes more distant from

electricity through the metal and the electrolyte (saline, atmosphere, seawater, etc.).

level of 60% (the remaining 40% is due to mechanical failures).

pure liquids are electrolytes of second specie.

4 New Technologies in Protective Coatings

conventionally considered as positive current.

each other than solutions of high resistivity).

Figure 1. Corrosion mechanism.

opposite one.

The abovementioned heterogeneity leads at the metal-solution interface to a gradient of electric potential between two adjacent areas. From a thermodynamic point of view, the quoted potential gradient is correlated with a difference of free energy ΔG. This is a thermodynamic function that is used as a criterion of spontaneity; it depends only on the initial and the final states, that is, that it is independent of the path: it decreases its value in a spontaneous transformation, either of physical or of chemical nature at constant temperature and pressure. Accordingly, it is possible to conclude that metal surfaces with high free energy are thermodynamically unstable and therefore tend to spontaneously evolve into a state of lower energy and greater stability.

The free energy is related to the electromotive force of a corrosion cell through the equation ΔG ¼ - zFE, where E is the reversible potential in volt, z the number of equivalent by mol, and F the Faraday constant.

Consequently, ΔG is the electrical work carried out by corrosion cell; it is observed that a reaction occurs spontaneously, at a constant temperature and pressure, when the value of E is positive.

Electrochemical corrosion is actually a network of shorted galvanic cells arranged on the metallic surface. Metal dissolves in the anode areas in equivalent relation to the reaction that takes place in the cathodic areas. In general, the anodic reaction is faster in almost all media, that is, that cathodic reaction is usually the decisive stage of the overall speed of corrosion process.

The cathodic reaction, in deaerated solutions, involves the reduction of protons (fast in acid and slow in neutral and alkaline media); instead, the quoted reaction, in aerated solutions, is accelerated by the reduction of dissolved oxygen:

$$2\text{ H}^+ + 2\text{ e} \leftrightarrow \text{H}\_2\tag{1}$$

$$\text{O}\_2 + 2\text{ H}\_2\text{O} + \text{ 4 e} \leftrightarrow 4\text{HO}^-\tag{2}$$

In both cases, there is an alkalization of the cathodic area, either by the decrease of the concentration of protons or directly by the generation of hydroxyl groups.

Meanwhile, the anodic reaction involves loss of electrons, from atoms of higher free energy, arranged on the metallic surface:

$$\mathbf{Fe}^0 \leftrightarrow \mathbf{Fe}^{+2} + \mathbf{2} \,\mathrm{e} \tag{3}$$

$$\mathbf{Fe}^0 \leftrightarrow \mathbf{Fe}^{+3} + \mathbf{3} \,\mathrm{e} \tag{4}$$

As a result of the ferrous and/or ferric ions reacting with ions hydroxyl of the medium for generating hydroxides, acidification of the anodic area occurs.

The sum of the anodic and cathodic hemi-reactions, in aerated media, is as follows:

$$\text{Fe}^{0} + \text{H}\_{2}\text{O} \uparrow + 1/2\,\text{O}\_{2} \leftrightarrow \text{Fe}(\text{OH})\_{2} \tag{5}$$

$$2\,\text{Fe}^{0} + 3\,\text{H}\_{2}\text{O} + 3/2\,\text{O}\_{2} \leftrightarrow 2\text{Fe}(\text{OH})\_{3} \tag{6}$$

The reaction in a corrosion cell involves the formation of hydrated ferrous oxide (ferrous hydroxide), which forms a first barrier for the diffusion of oxygen (polarization). This hydroxide is white in its pure state and has a pH of 9.5 in saturated solution. In a second sequential reaction, the hydrated ferrous-ferric oxide, which is the intermediate layer, is formed. This product, black in color, has magnetic properties. Subsequently, the reaction leads to the generation of hydrated ferric oxide, which makes up the third (external) layer of the oxidized system. This compound is orange/dark red in color and has a nearly neutral pH in saturated solution; exists as αFe2O3 (non-magnetic, with higher free negative energy of formation, i.e., more stable) and as γFe2O3 (magnetic).

On the other hand, the failures of metals take place by different causes due to the great amount of variables involved; as previously mentioned, the frequency of failure in the metals by corrosion reaches average levels of 60% in the different productive sectors. The types of corrosion failure and their frequency are given in Table 1.

Uniform corrosion. It is characterized in that the cathodic and anodic areas are modified alternately in space and time; as examples, it is possible to cite the case of a metal in direct contact with a solution of reduced electrical conductivity (the corrosion products, due to the reduced distance between the electrodes, are deposited simultaneously on the anodic and cathodic areas controlling the kinetics of process) and also the case in which the metal is exposed to


Table 1. Types of failure and frequency.

high temperature in a relatively dry atmosphere. Preventive measures generally include selecting suitable materials for each aggressive medium, changing or inhibiting the electrolyte (closed systems), specifying resistant coatings, and designing anodic protection (passivation).

The sum of the anodic and cathodic hemi-reactions, in aerated media, is as follows:

more stable) and as γFe2O3 (magnetic).

6 New Technologies in Protective Coatings

Table 1. Types of failure and frequency.

corrosion failure and their frequency are given in Table 1.

The reaction in a corrosion cell involves the formation of hydrated ferrous oxide (ferrous hydroxide), which forms a first barrier for the diffusion of oxygen (polarization). This hydroxide is white in its pure state and has a pH of 9.5 in saturated solution. In a second sequential reaction, the hydrated ferrous-ferric oxide, which is the intermediate layer, is formed. This product, black in color, has magnetic properties. Subsequently, the reaction leads to the generation of hydrated ferric oxide, which makes up the third (external) layer of the oxidized system. This compound is orange/dark red in color and has a nearly neutral pH in saturated solution; exists as αFe2O3 (non-magnetic, with higher free negative energy of formation, i.e.,

On the other hand, the failures of metals take place by different causes due to the great amount of variables involved; as previously mentioned, the frequency of failure in the metals by corrosion reaches average levels of 60% in the different productive sectors. The types of

Uniform corrosion. It is characterized in that the cathodic and anodic areas are modified alternately in space and time; as examples, it is possible to cite the case of a metal in direct contact with a solution of reduced electrical conductivity (the corrosion products, due to the reduced distance between the electrodes, are deposited simultaneously on the anodic and cathodic areas controlling the kinetics of process) and also the case in which the metal is exposed to

Type of failure Failure frequency, %

Uniform corrosion 31.2 Corrosion-fatigue and corrosion under tension 23.4 Corrosion by pitting 15.7 Inter-granular corrosion 10.2 Corrosion-erosion, corrosion-wear and corrosion-cavitation 8.4 High-temperature corrosion 2.3 Corrosion by welding 2.1 Thermo-galvanic corrosion 2.0 Galvanic corrosion and corrosion in concentration cells 1.4 Corrosion by electrolysis 1.1 Corrosion by selective attack 1.0 Microbial corrosion 0.7 Corrosion by hydrogenation 0.5

Fe0 <sup>þ</sup> H2O <sup>þ</sup> <sup>1</sup>=2 O2 \$ Fe OH ð Þ<sup>2</sup> (5)

2 Fe<sup>0</sup> <sup>þ</sup> 3 H2O <sup>þ</sup> <sup>3</sup>=2 O2 \$ 2Fe OH ð Þ<sup>3</sup> (6)

Corrosion fatigue. It is characterized by the action of alternating tensions in the presence of a corrosive medium. The causes are basically the same that can be attributed to static fatigue but adding cyclic loads. The deteriorating effect of combined fatigue and corrosion is much greater than the sum of individual damages. The most suitable measures to avoid this type of corrosion are to eliminate the cyclic tensions, increase the size or thickness in critical sections, reduce the concentration of stresses or redistribute them, provide sufficient flexibility to diminish over-fatigue by thermal expansion, control the vibration or shocks, eliminate the sudden changes in loads, temperature, or pressure, specify the right surface finishing, and select the appropriate protective system.

Corrosion under tension. It consists of premature breakage caused by the combined action of corrosive medium and residual or applied stress on the piece of metal, that is, that it takes place by combining high efforts and the presence of an electrolyte. Efforts by static charges in the metal surface and corrosive action that diminishes the section of the piece may exceed the elastic limit and even the breaking load. The forms of controlling this failure are to reduce mechanical tensions, ensure a sufficient flexibility, increase the size of the critical sections, select materials in the joints with a similar expansion coefficient, design adequate protection, and use a medium of suitable nature and composition.

Corrosion by pitting. It is a localized phenomenon that produces an appreciable penetration in the metal, generating either cavities or a discontinuity of the protective coating that lead to the formation of a concentration cell. To avoid this pathology, it is convenient to control the properties and the main characteristics of protective film (dry and wet adhesion, thickness, permeability, etc.), select a good geometry to prevent attacks, and specify properly the electrolytic medium.

Inter-granular corrosion. It is the preferred attack on grain boundaries of a metal or an alloy; it is characterized by a selective deterioration and an inter-crystalline cracking along inter-granular streaks (e.g., in stainless steels in chrome-deprived areas). Frequently, the specifications contemplate to select materials with a suitable thermal treatment for each particular case and realize weldings that do not generate temperatures superior to those used in the pretreatment of material.

Corrosion-erosion. The failure generated by the relative movement of the electrolytic medium (generally accelerated by abrasion due to the presence of solid particles in suspension) releases the corrosion products adhered to metal (depolarization) and also causes surface wear. For satisfactory corrosion-erosion control, it is appropriate to decrease the fluid velocity to achieve laminar movement, suppress the localized turbulence and the discontinuous flows, eliminate the abrupt changes in the direction of flow (aligning sections of ducts), avoid the obstructions, increase the material thickness in critical areas, design anodic parts so they can be changed quickly, specify the surface roughness, select the suitable coatings, and carry out cathodic protection.

Corrosion wear. It is defined as the deterioration located at the interface between two surfaces in contact, accelerated by a relative movement of sufficient amplitude to produce slippage. Generally, it occurs under heavy loads and instantaneous movements produced by highfrequency vibrations; the wear of surface-protective film (inorganic primers, organic coatings, etc.) can initiate a corrosion process. The main prevention methods to avoid corrosion wear are to eliminate the transmission of vibrations, introduce barriers between metals that slip, increase the load to slow the movement, provide protective layers to porous materials or use suitable lubricants, isolate those moving parts of the static ones, and finally increase the abrasion resistance.

Corrosion cavitation. It is associated with vapor bubbles arranged inside the liquid that collapse on the surface of the solid. Repeated collapses on a metal surface can deteriorate the protective film and severely deform the surface, fracturing it or generating fatigue. Low-pressure areas are created by divergent flows, vibrations, and so on. To control these damages, it is very important to select conditions that diminish absolute pressure, reduce hydrodynamic pressure differences, control the vibration, design the system to avoid formation or accumulation of bubbles, prevent the entry of dispersed air, select resistant materials or coatings, specify the finishing polishing, use cathodic protection, and so on.

Corrosion by high temperature. It is associated to the effect of atmospheric conditions and the presence of gases, metals, and/or molten salts at high temperature; the kinetics depends on the nature of the metals, the composition of the medium, and the time of exposure. The reduced dimensional stability of the corrosion products (hydration/dehydration by thermal changes) produces tangential cutting stress to the surface leading to the partial detachment of the different oxide layers, generating heterogeneities that favor corrosive processes. The most recommended therapies are to select materials stable to the thermal action, adjust the nature and/or composition of the medium, and regulate, if possible, the contact time.

Corrosion by welding. A weld can have low corrosion resistance due to the chemical nature of the electrode (e.g., it should be used with those having a low hydrogen content), to the residual stress and to the metallurgical structure of the weld zone. Corrosion in welding joints can be avoided by careful selection of materials, of the technique used, and of the type of finishing.

Thermo-galvanic corrosion. It is the result of the operation of a galvanic cell generated from a temperature gradient; the heating and the heat dissipation in heterogeneous form are the responsible factors for the formation of this cell. The most efficient actions are to avoid point heating and/or unequal cooling, use a continuous and adherent coating, and introduce thermostated components from the outside to the system.

Galvanic corrosion. It involves the corrosion associated with the current resulting from the contact of different electrodes (metals of dissimilar chemical nature) arranged in a conducting electrolyte that closes the circuit of the cell. The most important preventive measures are to eliminate interaction of diverse metals or to produce a complete dielectric insulation, avoid contact of a small anode and a large cathode, extend the distance between dissimilar metals in conductive media, design the anodic parts that can be easily replaced or apply thicker protective films, use suitable protective systems and regulate the degree of aeration, temperature, composition, or movement of the medium that is suitable for the metal coupling.

Corrosion wear. It is defined as the deterioration located at the interface between two surfaces in contact, accelerated by a relative movement of sufficient amplitude to produce slippage. Generally, it occurs under heavy loads and instantaneous movements produced by highfrequency vibrations; the wear of surface-protective film (inorganic primers, organic coatings, etc.) can initiate a corrosion process. The main prevention methods to avoid corrosion wear are to eliminate the transmission of vibrations, introduce barriers between metals that slip, increase the load to slow the movement, provide protective layers to porous materials or use suitable lubricants, isolate those moving parts of the static ones, and finally increase the

Corrosion cavitation. It is associated with vapor bubbles arranged inside the liquid that collapse on the surface of the solid. Repeated collapses on a metal surface can deteriorate the protective film and severely deform the surface, fracturing it or generating fatigue. Low-pressure areas are created by divergent flows, vibrations, and so on. To control these damages, it is very important to select conditions that diminish absolute pressure, reduce hydrodynamic pressure differences, control the vibration, design the system to avoid formation or accumulation of bubbles, prevent the entry of dispersed air, select resistant materials or coatings, specify the

Corrosion by high temperature. It is associated to the effect of atmospheric conditions and the presence of gases, metals, and/or molten salts at high temperature; the kinetics depends on the nature of the metals, the composition of the medium, and the time of exposure. The reduced dimensional stability of the corrosion products (hydration/dehydration by thermal changes) produces tangential cutting stress to the surface leading to the partial detachment of the different oxide layers, generating heterogeneities that favor corrosive processes. The most recommended therapies are to select materials stable to the thermal action, adjust the nature

Corrosion by welding. A weld can have low corrosion resistance due to the chemical nature of the electrode (e.g., it should be used with those having a low hydrogen content), to the residual stress and to the metallurgical structure of the weld zone. Corrosion in welding joints can be avoided by careful selection of materials, of the technique used, and of the type of finishing.

Thermo-galvanic corrosion. It is the result of the operation of a galvanic cell generated from a temperature gradient; the heating and the heat dissipation in heterogeneous form are the responsible factors for the formation of this cell. The most efficient actions are to avoid point heating and/or unequal cooling, use a continuous and adherent coating, and introduce

Galvanic corrosion. It involves the corrosion associated with the current resulting from the contact of different electrodes (metals of dissimilar chemical nature) arranged in a conducting electrolyte that closes the circuit of the cell. The most important preventive measures are to eliminate interaction of diverse metals or to produce a complete dielectric insulation, avoid contact of a small anode and a large cathode, extend the distance between dissimilar metals in conductive media, design the anodic parts that can be easily replaced or apply thicker

and/or composition of the medium, and regulate, if possible, the contact time.

abrasion resistance.

8 New Technologies in Protective Coatings

finishing polishing, use cathodic protection, and so on.

thermostated components from the outside to the system.

Corrosion by concentration cells. It is made up of a galvanic cell in which the electromotive force is due to the concentration difference of one or more reagents. The main causes are given either by differential aeration (different partial pressure of oxygen) generated in cracks, adherent deposits, and deep depressions that influence the diffusional process of oxygen and the existence of gradients of concentrations in the electrolyte generated by different causes. The most effective measures are to reduce surface irregularities especially in areas of heat transfer or where chemical reagents or oxygen are introduced, design drainage and a uniform environment, select forms that allow easy cleaning and application of protective layers, remove solids in suspension by filtration, use continuous welds, suppress porosity and cracking, and eliminate fibrous and/or absorbent packings.

Corrosion by electrolysis. It is generated by a current flow, that is, electric currents generally of an alternating nature, which cannot be controlled; they are often originated by sources external to the structure (e.g., bad ground connections, etc.), which enter through a conducting medium. It is convenient to connect properly the equipment to ground, isolate the apparatus from structures, use non-conducting fluids, eliminate errant or vagabond current sources, and incorporate sacrificial (cathodic protection) plates in the anodic areas near insulation joints.

Corrosion by selective attack. It is based on a process of extracting a soluble component from an alloy; generally, the percolation of the alloy occurs by the action of a solvent on an element of the metal (e.g., zinc, aluminum, etc.), which separates and consequently generates a corrosive action. The most appropriate measures involve selecting materials suitable for performing efficiently in the electrolytic medium in which the part or structure is inserted, reducing the aggressiveness of the medium if feasible (e.g., in closed systems), and using suitable protection methods.

Microbial corrosion. Bacteria and fungi, individually or together, and the subproducts of the biological activity attack the metal and/or the coating. The mentioned products (e.g., organic and inorganic acids and alkalis) display a significant aggressiveness to materials. Consequently, considering the causes described, it is convenient to avoid contamination, use specific biocides, control chemically the environment, select properly the protective coatings, and clean the surfaces as often as necessary.

Corrosion by hydrogenation. It is manifested by the reduction of the mechanical resistance produced by the inclusion of hydrogen gas in the crystal structure of the metal. The most common causes are linked to an inadequate de-oxidation and, fundamentally, to an oversizing of the cathodic protection. The most suitable therapies are to perform a suitable surface preparation, select properly coating systems, induce compressive stresses, heat the metallic substrate to 90–150C, and systematically control the electrical potential of the metal substrate modified by the cathodic protection.

It is worth mentioning that coating systems are the most convenient methods for controlling the kinetics of metallic corrosion from a technical-economical viewpoint [2–6].
