The 8 Forms of Corrosion
- 8 October 2018
- Posted by: Stm Coatech
- Category: Educational Articles
Corrosion is a costly and potentially life threatening problem in any industry. Wherever there is metal the potential for a corrosive reaction exists. It is a natural part of nature, there is no escaping it. That doesnt mean that we are doomed though. Through a careful analysis of the process and the design we can mitigate most of the costs of corrosion, both actual and potential, before the first pipe is in place. The science of corrosion engineering is viewed by many as magic, or art, but once the basics of how a corrosion cell works and a little understanding of chemistry almost anyone can learn to decipher the clues to the causes of a corrosion problem. Most corrosion problems encountered fall into five basic categories:
• Uniform or general corrosion: this is the most common form of corrosion encountered.
• Localized corrosion: as the name implies this form of corrosion occurs in discreet areas on the surface of the metal.
• Metallurgically induced corrosion: this form of corrosion attacks the metals physical and chemical makeup.
• Mechanically assisted corrosion: the physical parameters of the system play an important part in this form of corrosion.
• Stress corrosion cracking: this form of corrosion results in cracks in the metal that are induced by both stresses and corrosive elements.
With this information we can hopefully reduce the costs of replacing expensive assets before they have fully depreciated, but our ultimate goal is to reduce the chances of a catastrophic failure or loss of life due to a corroded system.
Thermodynamic and Electrochemical Processes
The corrosion process of metals is a natural result of the inherent tendency to revert to a more stable compound, such as an oxide. Metal ore that is mined must be refined and then alloyed for use. Energy is required to refine these ores into useable metals. Entropy, a thermodynamic property, drives these metals to corrode. Every system which is left to itself will, on the average, change toward a condition of maximum probability. (G. N. Lewis) Energy is required to keep these metals in the refined state and when left alone they will, over time, revert back to the more stable compounds in which they occur naturally. An example of this is iron. Hematite is the principle ore of iron. Hematite is a form of iron oxide, its chemical composition is Fe2O3. Processed iron ore, coke, and limestone are added to the top of a blast furnace (Figure 1-1). The coke is the source of the chemical energy in the blast furnace. When it is burnt by the hot air it releases both heat energy and the main reducing agent, CO.
2C + O2 -> 2CO ∆H = -394 kJ/mol
The limestone is decomposed by the heat into calcium oxide, CaO, and additional carbon monoxide.
2CaCO3 -> 2CaO + 2CO + O2
The main chemical reaction takes place, and the calcium oxide combines with the impurities in the ore to form slag.
Fe2O3 + 3CO -> 3CO2 + 2Fe
The refined iron then runs down and out of the blast furnace to form pig iron. The pig iron is then combined with other alloys to get different kinds of steels.
Figure 1-1. The blast furnace is used to turn iron ore into pig iron for the steel making process
At this point, the second law of thermodynamics and free energy come into play. The second law of thermodynamics quite simply states: The stable, equilibrium state of a system is the state of minimum free energy. Iron in the processed form, Fe, has a greater amount of free energy than iron oxide. That energy was added when the ore was heated in the blast furnace. The system, in this case the processed iron, will always be headed towards its most stable state, which is the state of minimum free energy. When moisture and oxygen are present the iron will undergo a chemical reaction to reduce the amount of free energy available. This redox, reduction oxidation, reaction produces hydrated iron oxide:
4Fe+2(aq) + O2(g) + [(4 + 2x)H2O](l) -> [2Fe2O3 • xH2O](s) + 8H+(aq)
The resulting iron oxide, Fe2O3, is one of the most stable states for iron to be found in. This simple example of an electrochemical reaction demonstrates one of the most important principles of metallic corrosion: the rate of oxidation equals the rate of reduction. It is important to note that when alloys are corroded more than one oxidation and one reduction reaction can occur.
Another way of looking at the corrosion process is as a series of anodic and cathodic reactions. In this situation four elements are necessary for corrosion to occur:
• An anode.
• A cathode.
• An electrolyte (water)
• A metallic path for electron flow.
When all four of these elements are present a corrosion cell is formed. It is important to remember that the anodic and cathodic reactions do not have to occur at the same spot on the material. As long as an electrolyte and a metallic path exist, these halfcells can exist and corrosion will occur. Figure 1-2 is an example of the corrosion cell with the four elements labeled.
Figure 1-2. Corrosion cell diagram depicting the current flow from the corrosion pit.
The same redox reactions that occurred in the afore mentioned example are occurring in the corrosion cell. The oxidation reaction takes place at the anode, and the reduction reaction takes place at the cathode. It is this series of oxidation and reduction reactions that make up the electrochemistry of the corrosion process.
It is important to note that there is a flow of electrons from one half-cell to the other during the corrosion process. The electric current flows from the anodes to the cathodes of the reaction through the electrolyte. The flow of electric current in the corrosion cell per unit area is referred to as the Corrosion Current Density (icorr). As each oxidation reaction occurs a transfer of electrons to the electrolyte occurs. Since icorr is a measure of current flow per unit area per unit time it can be seen that icorr can be related to units of metal loss per unit time. Corrosion rates are generally expressed in mils per year of metal thickness reduction (MPY), so icorr is a good representative of the corrosion rate in the cell. Armed with this information it is possible to slow down or even prevent the corrosion from occurring by increasing the resistance in the electrochemical corrosion circuit. Adding chemical inhibitors and coatings to the materials to reduce the current flow will help to increase the circuit resistance.
The free energy available in the metal and in the electrolyte creates a potential at the anode and at the cathode. It is the difference in potential between these two sites that causes the electron flow. Most corrosion reactions are considered wet and occur in the presence of a liquid. Marcel Pourbiax, a well-known Belgian scientist, developed a diagram which relates the stability of a metal in a given pH environment in 1938. By plotting the potential of a metal and the pH of the environment it is possible to determine if the metal is in a region where the possibility of corrosion is high, the possibility for corrosion is non-existent, or the possibility exists but there is a tendency for the development of a protective film. Figure 1-3 is the Pourbiax diagram for an iron-water system at 25 degrees Celsius. The gray zone represents the area where the base metal, iron, is stable. This is the region where the possibility of corrosion is non-existent. The orange zone is where rust, the non-protective form of corroded iron, is expected. The blue zone is where the most stable iron species, Fe(OH)2, is encountered. This species is called blue rust and is fairly rare and highly soluble. The white areas are where corrosion is most probable.
Figure 1-3. Pourbiax diagram for an iron-water system at 25ß C.
These Pourbiax diagrams are time consuming to create and a new diagram must be created for each metal or alloy that is being used and the environment it is in. There are software programs available that can produce these diagrams for the student of corrosion processes.
It is not enough to determine if the possibility for corrosion exists, a corrosion rate must also be determined. It is entirely possible for the probability of corrosion to be high and resultant corrosion rates to be very low. In these cases the corrosion might not be a problem.
Corrosion rates are determined through polarization curves. Polarization curves are produced through the application of a current to the metal surface. If the potential of the metal surface is polarized by the current in a positive sense it is referred to as being anodically polarized; a negative sense signifies that it is cathodically polarized. The degree of polarization is a measure of how the rates of the anodic and cathodic reactions are hindered by various environmental and surface process factors. The environmental factors (the concentrations of metal ions, dissolved oxygen in the solution, etc.) are referred to as the concentration polarization. The surface process factors (film formation, adsorption, etc.) are referred to as the activation polarization. The polarization curve is a graph of the variation of the potential as a function of the current, which allows the effects of the concentration and activation processes on the rate at which the anodic or cathodic reactions can give or receive electrons to be determined. This allows for a rate determination for the reactions that are involved in the corrosion process, in effect a corrosion rate.
Figure 1-4. General Polarization curve for a material with both anodic and cathodic polarization curves.
The polarization curve above shows the polarization curves for both anodic and cathodic reactions. The potential, E, is plotted as a function of the logarithm of icorr. Ecorr, the corrosion potential, is determined by the intersection of the extrapolation of the linear portions of the anodic and cathodic polarization curves. The value of the current at the intersection is the rate of corrosion expressed in current density, icorr.
From this polarization curve the Evans diagram can be extrapolated. Evans diagrams are useful in determining if the anodic, cathodic, or both reactions control the rate of the corrosion. Figure 1-5 shows the three types of Evans diagrams that can be extrapolated from the polarization curve, (a) shows that the rate is controlled by the anodic reaction, (b) is controlled by the cathodic reaction, and (c) is controlled by a combination of both reactions. This information is used by corrosion engineers to allow the evaluation of the effects of measurable factors, which can be controlled, on the rates of corrosion.
Figure 1-5. Evans diagram extrapolated from the polarization curve (a) shows that the rate is controlled by the anodic reaction, (b) is controlled by the cathodic reaction, and (c) is controlled by a combination of both reactions.
Another determining factor in the corrosion rate is the phenomenon of passivity and passive films. Passivity generally refers to materials that form insoluble films, which inhibit the anodic reactions and cause a polarization of the anode. These films can markedly reduce the rate of corrosion as long as they are not breached. These films can experience breakdowns from both electrochemical reactions and mechanical mechanisms. Those discrete sites where a breach in the film occurs can experience an accelerated rate of corrosion. One of the major causes of the breaching of the passive films is the chloride ion, which is readily available in nature.
An interesting process that some alloys undergo is repassivation. This process is very effective for combating localized corrosion. These alloys form a passive film that resists the breakdown process in a fairly effective manor, and are capable of repassivating at a rate high enough that once a breach has occurred the exposure to the corrosive environment is reduced to a minimum.
Forms of Corrosion
From the engineers standpoint the forms that corrosion takes can generally be identified by a visual examination. It is usually possible to determine a probable mechanism for the corrosion by corresponding the corrosion in question to one or more of general forms.
• Uniform or General corrosion: a regular loss of metal from the surface cases a uniform thinning.
• Localized corrosion: the majority of metal loss is experienced in discrete areas.
• Metallurgically Induced corrosion
• Mechanically Assisted corrosion: physical environmental factors play a significant role in metal loss.
• Stress corrosion: generally cracking that is induced by environmental factors.
These five basic types of corrosion can be broken down into eight visually identifiable forms, which are significantly different from one another. This allows an engineer to examine a specimen and make a reasonable assumption on the general cause of the corrosion and possible corrective measures that can be taken.
General corrosion, or uniform attack, is the most common form of corrosion. General corrosion occurs uniformly over the entire surface that is exposed. The metal gets thinner evenly over the entire surface, leading this form of corrosion to account for the largest tonnage of metal loss. This form of corrosion is generally the cheapest form of corrosion to deal with in industry. It is fairly consistent and rates can quickly be found by exposing a sample to the environment. Making the material thicker than necessary to accommodate the corrosion throughout the life of the product, or applying a coating to the surface, such as paint, are the standard methods of prevention for this form of corrosion.
Photo 2-1. Depicts general, or uniform, corrosion on a steel fitting
Galvanic, or two-metal, corrosion occurs because of the potential difference that exists between two dissimilar metals. When these two metals can be connected electrically, either through direct contact or an electrolyte, the difference in potential causes a flow of electrons, or current, between them. The further apart on the Galvanic Scale (figure 2-1) that these two metals are increases the amount of corrosion experienced by anodic member of the galvanic couple. The ensuing corrosion reaction is a perfect example of the corrosion cell in figure 1-2. The corrosion resistant, or noble, metal becomes the cathode and experiences very little, if any metal loss. While the less corrosion resistant metal becomes the anode.
Proper material selection can eliminate most of the problems associated with galvanic corrosion. Combinations of metals and alloys used together should not be widely separated on the galvanic scale. Dissimilar metal crevices, such as threaded connections, should be avoided, and fasteners of the same material or a more noble material should be used.
Crevice corrosion is an intensive, localized form of corrosion. It generally occurs in cracks, holes, crevices, lap joints, under gaskets, rivet and bolt heads, and anywhere else that small amounts of solution can lay stagnate. Crevice corrosion begins with the typical corrosion cell (figure 1-2), an oxygen concentration cell. As the oxygen is depleted the growth of the corrosion cell is driven by the accumulation of acidic hydrolyzed salts in the crevice area.
Proper material selection and design features that limit the number of crevices and allow for drainage are the most effective way of mitigating the effects of crevice corrosion.
Pitting is a troublesome form of corrosion. It can cause a failure with very little metal loss. Pitting is an extremely localized form of attack that often causes rapid penetration of the wall thickness. The holes that pitting causes can be very small or quite large in diameter. The fact that failure can occur with very little metal loss and with just one little area affected is what makes this form of corrosion so difficult to detect and prevent.
Intergranular corrosion is the corrosion of the edges of the individual grains that make up the metal. The preferential attack along the grain boundaries can lead to a decrease in effective cross section of material and eventual mechanical failure. Metals experiencing intergranular corrosion that should otherwise exhibit excellent corrosion resistant tendencies have been sensitized. Sensitizing usually occurs through damaging thermal exposures, such as welding. Material selection, heat-treating, quenching, and proper welding techniques are effective methods for avoiding intergranular corrosion.
Selective leaching, or dealloying, is the removal of one element from an alloy through a corrosion process. The most common form of dealloying is the removal of zinc from brass alloys. Common yellow brass is 30% zinc and 70% copper. The brass turns a red or copper color that contrasts with the original yellow brass, or zinc oxide builds up on the outside of the brass piece.
Erosion corrosion is the accelerated increase in deterioration of a metal due to the velocity of the fluid. The fluid does not have to contain any abrasive particles, but the metal will be worn away faster if there are abrasives present.
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is the cracking from a combined presence of tensile stresses and a corrosive medium. The tensile stresses can come from applied stresses or residual stresses. Residual stresses are introduced into a material through forming, welding, machining, heat treatments, and grinding. The metal remains unattacked over its surface, while fine cracks progress through it.
Görsel bir inceleme ile, korozyon problemini mevcut olan korozyon formlarıyla sınıflandırmak genellikle mümkündür. Mevcut korozyon türü, ilgili korozyon mekanizmaları ve korozyon sorununu kontrol etmek veya hafifletmek için olası yöntemler hakkında çok şey anlatır. Korozyon genellikle aşağıdaki görsel formlardan birine veya daha fazlasına yerleştirilebilir:
Through a visual examination it is often possible to classify a corrosion problem by the forms of corrosion that are present. The type of corrosion present tells a lot about the corrosion mechanisms involved and possible methods to control or alleviate the corrosion problem. Corrosion can usually be placed in to one or more of the following visual forms:• General or Uniform Corrosion.
• Galvanic Corrosion.
• Crevice Corrosion.
• Intergranular Corrosion.
• Selective Leeching.
• Erosion Corrosion.
• Stress Corrosion Cracking.
Through a careful analysis of the problem a method to avoid any similar problems in the future can be found. The most important part of any design from the corrosion standpoint is: Are the materials chosen the right materials for the job, and will they work together?
Sorunun dikkatli bir analizi ile gelecekte oluşabilecek benzer problemlerden kaçınmak için bir yöntem bulunabilir. Korozyon açısından herhangi bir tasarımın en önemli parçası şudur: İş için doğru malzemeler seçildi mi ve bu malzemeler birlikte mi çalışacaklar?
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