Corrosion Types

Valeria Sanabria, winter semester 2023/2024

In order to avoid misunderstandings because of the many connotations of “corrosion”, in this article it will be defined as “… an irreversible interfacial reaction of a material (metal, ceramic, polymer) with its environment which results in consumption of the material or in dissolution into the material of a component of the environment”. ^[1] However, the focus of this article is specifically corrosion in metals.

The importance of corrosion is not always clear, but it is present in almost every sector of engineering, e.g., infrastructure, transport, manufacturing, etc., and it can lead to very expensive damages. ^[2-4] In the last years the direct cost^{[Notes 1]} of corrosion entails between 3% and 4% of each nation’s gross domestic product (GDP). In other words, corrosion has been causing US$2,2-2,5 trillion annual expenses in industrialized countries^{[3; 5; 6]} .These costs, as well as collateral damages of corrosion such as contamination, personal injuries, waste of resources and loss of production, can be minimized by the prevention or early detection of corrosion.^{[3; 5; 6]}  Therefore, this article will give an overview of the different types of corrosion and the methods used to identify it, together with recommendations on how to prevent it.

Classification of Corrosion

The classification of corrosion can be approached in different ways, depending on the purpose of the study. Examples of classification criteria can be: based on the corroded material (metallic and non-metallic corrosion); on the state of the mediums involved in the process (electrochemical/chemical); or according to its form (uniform/localized corrosion). 

For the purpose of this article, metallic corrosion will be classified according to the conditions in the environment, as this facilitates a general understanding about the processes behind corrosion itself (see Figure 1).^[7]  However, a classification based on geometry and appearance will also be used to specify the types of metallic corrosion that can be identified by inspection of materials (see Figure 2). ^{[7; 8]}

Corrosion of metals and alloys is classified into two main groups based on the presence of fluids: wet or dry corrosion. These groups can be subdivided according to the state of the mediums involved.  Wet corrosion is divided in aqueous, atmospheric and non-aqueous corrosions. Dry corrosion includes oxidation, molten-salt corrosion and hydrogen attack. As shown in Figure 1, metallic corrosion can be explained with (electro-)chemical reactions (refer to section “Physical Principles”).

Figure 2 shows an example of a common classification of metallic corrosion, as it is differentiated between uniform and localized corrosion. Depending on the size of the corrosion damage, the localized corrosion can be macroscopic or microscopic. In the section titled “Forms of Metallic Corrosion”, the examples listed under microscopic and macroscopic corrosion will be explained.

Physical Principles

As there are many chemical reactions that can happen by metallic corrosion according to the reactants and conditions involved and as most of the corrosion processes are of electrochemical nature, only the electrochemical corrosion will be further explained.^[2]  

Electrochemical Corrosion

Electrochemical corrosion involves the change of the chemical state of metal atoms to metal ions (chemical reaction) and the transfer of electrons to an aqueous electrolyte environment.^{[9; 10]}

To better understand this sort of degradation, it is necessary to know that metals can oxidize (anodic reaction) and reduce (cathodic reaction).
The anodic site is responsible for the dissolution of the metal as it gives valence electrons to the environment and builds soluble ions or insoluble compounds (i.e., oxides).^{[9; 10]} The cathodic site consumes electrons and builds other substances depending on the reducible species in the environment .^[9-11]

The result of the loss of electrons and the building of new species is corrosion. An example of electrochemical corrosion is the redox reaction of iron in the presence of an electrolyte like water.^{[9; 12-14]} Figure 3 illustrates this.

To conclude, it’s important to know, that reduction and oxidation can take place between two sites in the same metal, between two different metals or between a metal and the environment. Corrosion is then the result of the species built by the redox reaction (9).

Forms of Metallic Corrosion

Uniform Corrosion:

This is the simplest form of corrosion and refers to an homogeneous attack of the exposed metal’s surface.^{[2; 7; 8]} This form of corrosion is of electrochemical nature and can reduce the thickness of the metal´s original surface (see Figure 4). An example from daily life is rust, which can appear in steel pipelines, heat exchanger tubes, etc.^[7]

Uniform corrosion is not considered dangerous, as it can be easily identified by visual inspection and the material damage can be calculated. This allows the setting of tolerances of amount of corrosion (thickness or material loss) in an object.^[8] In addition, there are many prevention methods, e.g., coatings, cathodic coating protection or even change of environment/material.^[8]

Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals come in contact by the process of an electrolyte (see “Electrochemical corrosion”). Because of the potential difference the less noble one acts as an anode and the more noble one as a cathode. This phenomenon causes the dissolution of the less noble metal and the more noble could also be attacked by the hydrogen.^{[2; 8; 15]} An example of galvanic corrosion can be the couple of magnesium alloys and copper. As magnesium is less noble, it starts corroding. The damage of this corrosion is exemplified in Figure 5.

 Galvanic corrosion is difficult to prevent, as it is to predict the reactions between the two metals.^[8] Nevertheless, there are several prevention methods. The main one is to use metals which are close in the galvanic series, in order to minimize the potential. Other factor to consider is the area effect. In the worst case the less noble metal has very small surface area and the more noble very large one.^[2] This will lead to a corrosion rate 100 to 1000 times greater than by two metals with the same surface area.^[7] Other methods are to insulate the metals or to design an access to change the less noble metal.

Pitting Corrosion

Pitting is one of the most aggressive types of corrosion, as it attacks a localized part of the material causing sudden failures. It usually occurs on materials with a protective film (against corrosion in normal conditions).^[7] When this film starts breaking, the chemical and physical properties of the material are irregular and the pitting starts on the weak areas of the surfaces. Pits can grow over several months until the failure of the component, and they cannot be identified easily, because of their size and the corrosion product which fills the holes.^[15] The form of pits is shown in Figure 6.

The best method of prevention is the adequate selection of material (e.g., aluminum alloys with magnesium for seawater purposes) and using cathodic protection.^[8]

Crevice Corrosion

This form of corrosion appears in narrow gaps or openings between metals or nonmetal-metals connections, where a small amount of liquid can stagnate or where a local difference of oxygen exists, such as lap joints.^[2;7] The lower concentration of oxygen or stagnate liquid causes an anodic effect in the crevice, which leads to the localized penetration of the material in this part. Figure 7 exemplifies this corrosion.

Crevice corrosion can occur in every metallic material, but metals which tend to build an oxide film (e.g., stainless steel) are more susceptible to corrode this way.^[2;8]  Salt solutions, specially environment with chlorides promote crevice corrosion.

This corrosion form can be prevented by using adequate materials like high-alloy steels with high Molybdenum (Mo) content. Preventions can be taken on the design by avoiding the depositions and crevices or by cleaning the components between service periods. Cathodic protection can be also applied on the materials.

Stress Corrosion

Stress corrosion is very dangerous because it causes sudden failures as the surface seems undamaged but fine cracks are building trough the metal.

The attack of the metal is caused by the presence of simultaneous tensile stresses (e.g., microscopic cracks) in the material and a corrosive medium. The cracks in the material core are related to total stresses (applied and residual) in the metal, solution and metal properties and temperature. They grow when electrochemical corrosion starts, e.g., protective film breaks or a pit has been made. This makes the tip of the crack act as an anode and the surface with the corrosion as a cathode. While the surface starts corroding, the stresses concentrate in the tip of the crack until this relieves by building a new surface (crack grows). So, the electrochemical mechanism begins again and this happens until the material fails.^[2;7] 

There are two types of stress corrosion: intergranular and transgranular. Intergranular refers to the cracks that go along the metallic grains and the other one refers to the cracks which go through these grains. Both types are exemplified in Figure 8.

Methods of prevention are the monitoring of cracks when the component has suffered another form of electromechanical corrosion, or the general monitoring of cracks in new materials, materials under high stress or temperatures.

Selective Leaching or Dealloying

This form of corrosion attacks alloys, when one of its metallic component is considerably less noble than the other one. ^[7:8;15] The less noble metal starts to be consumed (see electrochemical corrosion). At the end the material becomes porous with low strength and ductility.^[8]

It is difficult to detect selective leaching as the material doesn’t change its shape and the corrosion products cover the corroded areas. An example of selective dealloying is dezincification of brass (Cu-Zn alloys).^[7;8] Brass is yellow and cooper is red. When zinc is consumed the surface starts becoming red. This color difference can be seen, when the material has been cleaned.^[7;8] Figure 9 represents this example.

In this case the corrosion can be uniform or local, depending on the zinc amount in brass and if the environment is alkaline, neutral or acidic.

Erosion Corrosion

Erosion corrosion occurs on the contact between corrosive fluids at relative high velocities and the surface of metals. The contact at high velocities increases the metallic wear, abrasion and the supply of substances needed by the corrosion (e.g., O₂, CO₂, H₂S) in metals, breaking at the end the protective film of metals. When this film breaks, the corrosion accelerates in the unprotected parts of the surface. ^[7]  The conditions, where erosion corrosion appears, are ideal for cavitation, fretting and fatigue corrosion.^[2;15] Thus, the damages come often from a mixed form of these corrosion types.

This corrosion form is usually found in piping systems, propellers, turbine blades and in other elements where corrosive fluids with high velocities contact metals.^[7]  It is characterized for building grooves and pit patterns in flow direction as shown in Figure 10.

Cavitation Corrosion

High flow velocities and fluid dynamics lead to pressure variations. At low pressure zones/moment bubbles are built and they collapse when reaching a higher pressure zone/moment suddenly.^[8]  The collapse has an intense impact on the surface of the components in its proximity. If this phenomenon happens repetitively, it can cause fatigue and cracks in the material, starting the process of corrosion.^[8] 

The abrasion or pits are different as in erosion corrosion, because they appear perpendicular to the surface and are deeper. In Figure 11, this damage is exemplified.

Detection of Corrosion

Many corrosion forms have been explained in the previous sections, but now it is important to learn how corrosion can be detected at an early stage, so that catastrophic damages can be avoided.

There are two types of methods that can be used to identify corrosion: destructive and non-destructive. These can directly detect corrosion or they can detect factors that influence corrosion (indirect detection).^[2] As in many industrial sectors non-destructive testing and direct detection are mostly used. These methods will be explained below.

The diagram in Figure 12 shows examples of categories of non-destructive techniques used for the detection of different corrosion forms.

For testing purposes, corrosion can be divided in two groups depending on the ease to be detected.^[8] The first group is usually detected by simple visual inspections. This group includes uniform corrosion and some macroscopic corrosion forms such as pitting, crevice and galvanic corrosion. The second group is characterized by making microscopic examinations and using special inspection tools or testing methods such as ultrasonic or radiographic testing. Erosion, cavitation, stress corrosion are some examples of this group.

As the diagram of Figure 12 shows, visual inspection can also be applied for some macroscopic corrosion forms, like pitting. However, pitting can be at the beginning very small and covered with corrosion products. Even if the surface is completely clean, there are some cases in which the pits cannot be detected because of their size. Therefore, it is recommendable to use an additional testing method that warranties the intactness of the object.^[2] This second testing applies as well for components where corrosion has already been detected, as corrosion can cause other corrosion types in the material, as it is the case by pitting and stress corrosion.

A widely used non-destructive technique to inspect corrosion of the second and third group are ultrasonic and radiographic testing, as they tend to be very accurate when finding defects in materials.

Ultrasonic testing techniques have proven to be a reliable way to find the thickness and position of the corrosion inside and outside metallic objects, as they measure the time a sound wave takes to travel through the inspected object, which can be compared to a reference known to be undamaged.^[2;16] These methods can be used for the detection of stress corrosion, as cracks can be found relatively easily. The successful use of these techniques depends on many factors, for instance, on the transducer that is used. Although the limitations of ultrasonic testing are outside of the scope of this article, when investigating corrosion, they are strongly related to the fact that the size of the damage might be difficult to determine.

Radiographic testing uses radiographic images to examine the internal structure of objects, and when investigating corrosion it can be used to determine the thickness of material loss or/and to detect related damages inside the material, as well as its form.^[17] These methods are commonly used to inspect discontinuities in welding, castings or forging, as well as to monitor the thickness of existing corrosion in pipelines. Every type of corrosion can be analyzed with these methods. Although the limitations of radiographic testing are beyond the scope of this article, these methods are  difficult to automatize, as well as expensive and dangerous for the operator.^[16;18]  

Electromagnetic testing techniques are other commonly used methods (e.g., in aviation) for the detection of cracks (fatigue or stress corrosion) on or under the surface of metals. These methods identify changes in the material’s electromagnetic properties or/and geometry, leading to cracks detection.^[16] It has to be considered that these methods generally need a comparison element (ideal material), which makes it difficult to detect the actual form of the cracks, but it gives an overview of the state of the tested object, and can suggest which parts should be more thoroughly inspected.

Other non-destructive methods to test corrosion are thermographic testing, liquid penetrant testing and magnetic particle testing, to name a few.

Conclusion

To sum up, corrosion is a factor that causes substantial losses in industries and can result in disastrous damages if it is not treated on time. Therefore, it is important to take measures to monitor/detect corrosion as well as designing materials with correct coating against corrosion, or using them in the right environment.

Notes

1. Direct costs include: “essentially materials, equipment, and services involved with repair, maintenance, and replacement”.^[3]

Literature

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18. Ekinci, Sinasi; Bas, N.; Aksu, M.; Yildirim, A.; Bingöldag, M.; Kurtcebe, T. et al.: Corrosion and deposit measurements in pipes by radiògraphic techniques. In Insight - Non-Destructive Testing and Condition Monitoring 40. Ankara (1998), pp. 628–631.