Corrosion refers to the process in which a metal reacts with the environment thus leading to its destructiveness. The electrochemical and chemical reactions that occur because of the instabilities thermodynamics in the environment are what cause corrosion. (1) Typical corrosive environments include water (distilled, fresh, salt), steam, air and humidity, atmospheres (urban, industrial, natural, marine), and gases. Other materials like ceramic and plastics also undergo degradation just like metals. Nonetheless, it is only where metals are involved where the term corrosion is used. There are various technically significant attributes depicted by metals including temperature resistance, tensile strength, and thermal conductivity, among many others which make them distinct from other materials.
All the important components of cars, machines, and instruments are usually made of metal. As such, resistance to corrosion greatly affects their durability. The corrosion in storage (contamination, leaking), construction (bridges- steel reinforcements in concrete, buildings), industry (loss of product, efficiency, and money because of repairs), and transport experience a huge problem since corrosion is not usually visible up to the point when huge damage is already inflicted. (2) There is an extremely high cost associated with corrosion due to the extent of the damage. The average corrosion cost for most industrialized countries is between 3.5 and 4.5% GNP. (1) When adequate preventive measures are utilized, the damage and cost can be minimized. The rate of corrosion can be kept minimal sufficiently when materials and surface treatment are correctly chosen.
A lot of metals naturally exist in the formation of oxides and sulfides. In order to convert these into the pure metal form, a lot of energy will be needed. This implies that they are naturally thermodynamically unstable such that they can be spontaneously converted back to their original form. As such it is extremely difficult to avoid corrosion of metals. The corrosion process can be explained in the form of a galvanic cell.
A closed electric circle is represented by the cell and this includes an aqueous solution with two metal electrodes of cathode and anode. In the event that a conductor links the electrodes, the difference in potential-nobility will cause currents to flow from the cathode to anode. Electropotential series including all semimetals and metals are the ones that determine nobility and these are paced in the order of their potential’s value. (3) Metals that are resistant to oxidation are noble metals and these include Ag, Au, and Pt. Oxidation occurs on the anodes and this includes loss of material (1) and reduction occurs on the cathode (2)-(5). In the event that the metals are exposed to varying environments, then the reaction becomes different. The popularly known reactions include hydrogen formation (2), oxygen reduction in an acidic environment (3), oxygen reduction in an alkaline environment (4), and metal ion reduction (5). These reactions are written as follows:
Such a process will not always involve two varying metals. Similar metals can also be subjected to both oxidation and reduction. The surface of the metal is not completely homogenous most of the time. Various defects that can be experienced include kink sites, adsorption of ions from solution, edges, or a surface or steps contaminants such as an impurity metal’s presence. Redox reactions are caused to occur in a coupled manner because of potential differences in the event that the metal comes into contact with the electrolyte. Corrosion begins when micro galvanic cells are formed on the metal’s surface- the anodic and cathodic sites.
Thermodynamics of Corrosion Process
It is important to note that one cannot directly measure Icorr. Most of the time, this is estimated using the data of current versus voltage. The logarithmic current can be measured versus a potential curve over a range of like a half volt. The scan of the voltage can be centered on Eoc. The measured data is then fit on the corrosion process’s theoretical model. (4) The rates of both cathodic processes and the anodic processes which are controlled by the reaction at the surface of metal of kinetics are assumed by the corrosion process model that we utilize. The Tafel equation, Eq. 1 is obeyed by the electrochemical reaction.
In this equation:
I is the current resulting from the reaction
I0 is a reaction-dependent constant called the exchange current
E is the electrode potential
E0 is the equilibrium potential (constant for a given reaction)
β is the reaction’s Tafel constant (constant for a given reaction, with units of volts/decade.
The behavior of one isolated reaction is described by the Tafel equation. There are two opposing reactions in a corrosion system. These are cathodic and anodic. The Butler-Volmer equation is developed following a combination of the Tafel equations for the corrosion system’s cathodic and anodic reactions. See (Eq. 2).
In this equation:
I is the measured current from the cell in amperes
Icorr is the corrosion current in amperes
E is the electrode potential
Ecorr is the corrosion potential in volts
βa is the anodic β Tafel constant in volts/decade
βc is the cathodic β Tafel constant in volts/decade
Each exponential term equates to one at Ecorr. Just as we would expect, the cell current is zero. Both exponential terms adjacent to Ecorr contribute to the general current. Therefore, one exponential term predominates as the potential is taken far from Ecorr by the potentiostat while the other one can be ignored. A plot of logarithmic current versus potential becomes a straight line if this happens. The plot of log I versus E is referred to as a Tafel plot.
Image source: https://www.gamry.com/application-notes/corrosion-coatings/basics-of-electrochemical-corrosion-measurements/
In the figure above, it is directly from the Butler-Volmer equation where the Tafel plot was derived from.
Free energy of a corrosion reaction
The study of electrochemical and chemical equilibrium is needed in order to understand and quantify the corrosion phenomena. Most reactions happen at constant pressure 𝑝 and temperature 𝑇. This thus means that Gibbs free energy is the most convenient state function with which equilibrium is described. This is also referred to as free entalphy 𝐺=𝐻−𝑇𝑆. Entalphy here is represented by the symbol H, T represents temperature while entrophy is represented by S. (5) Gibbs free energy’s total derivative is:
This equation is related to a closed system. If 𝑛𝑖 moles of various species i, are contained in the system, this can be extended to:
Here, 𝜇𝑖 is the chemical potential where the molar free energy is pua re substance. It is given by:
The chemical reaction can be expressed as:
The stoichiometric coefficients here are given by 𝑣𝑖. These are positive for the products of the reaction and negative for the reactants. The participant species in the reaction is given by 𝐵𝑖. The following represents the change in Gibbs free energy:
Δ𝐺 = Σ𝑣𝑖 𝜇𝑖.
An alternate definition ideal solution will be obtained when the equation above is integrated by the use of chemical potential.
𝜇𝑖 = 𝜇𝑖0 + 𝑅𝑇𝑙𝑛𝑎𝑖.
The standard chemical potential is represented by 𝜇𝑖0 while the activity in the i th species is given by 𝑎𝑖. For liquid H2O and solids, a= 1 by convention. a is replaced by concentration c for ions activity and for other liquids while a is replaced by pressure p for gases activity. (6)
The Gibbs free energy has its minimum Δ𝐺 = 0 in chemical equilibrium. The corrosion reaction is spontaneous when there is a negative free energy Δ𝐺 < 0. When equilibrium constant and the above two equations are introduced, we obtain:
Because of the transport of electrons the conductor linking the two electrodes, electrical work can be supplied by the electrochemical cell.
The calculation of equilibrium potential as a function of temperature and concentration (activity) is made possible through the Nernst equation. (7) Electrodes at equilibrium are applied. Two half-cell reactions are included in the electrochemical cell- reduction and oxidation. The cell equation is
The process of oxidation is described by the left side of the equation while the process of reduction is explained by the right side. The stoichiometric coefficients are represented by 𝑣𝑜𝑥, and, 𝑣𝑟𝑒𝑑,𝑖. The “oxidized” and “reduced” species are 𝐵𝑜𝑥, and 𝐵𝑟𝑒𝑑,𝑖. ,,,A convention was r equired to scale the standard electrode potentials for equilibrium potentials of varying electrode reactions to be compared. The equilibrium potential of hydrogen electrodethe at standard conditions was allocated the zero value. Tto to to to he equation obtained is:
and it tends to have free energy:
The ‘oxidized’ and ‘reduced’ activities are the symbols 𝑎𝑟𝑒𝑑, and ,𝑎𝑜𝑥,𝑖. The general form of an electrochemical system thus is:
We can estimate the resistance to corrosion of varying metals using the standard potentials of electrode reactions 𝐸0 which are offered numerically as an electrochemical series. However, such series only consider pure metals in idealized conditions. The Nernst equation cannot predict the exact thermodynamic behavior of the much more complex realistic corrosion systems. (8) There is a condition for reversibility since the system is not always in equilibrium. However, the thermodynamic stability of a system is still universally assessed using the Nernst equation regardless of this effect.
The potential-pH diagrams are used to summarize the corrosion thermodynamic of a particular metal. The Nernst equation is represented by these as a function of pH and is usually referred to as Pourbaix diagrams. All possible reactions at 25℃ are considered when drawing the diagram for any given metal. These diagrams, since they represent metal as a function of potential and pH at a given combination of these two quantities, can be used to determine a stable phase of species. Calculations using the solubility of data for the metal and its species and Nernst equation in equilibrium are used to develop this diagram. The corrosion behavior of metal can be determined using the Pourbaix diagram. However, the following have to be known: active state (corrosion), a phase that forms a passive layer in which the corrosion process on the metal surface is inhibited, potential dependence of the three metal states in a corrosive medium, and thermodynamic stability (immunity of a metal).
Electrochemical Kinetics of corrosion
Exchange current density
The rate of either reduction or oxidation at the equilibrium electrode is what constitutes an exchange current. This is usually expressed in the current form. Technically it is a misnomer due to the fact that there is no flow of net current. The rates of both reduction and oxidation in any given electrode at equilibrium is represented by an exchange current. Exchange current density, on the other hand, is whereby in an equilibrium state, there is an equal flow in both directions of the current density. (9) The rate of reaction increases with the increase in the exchange current density with the opposite being true.
The electrode material experiences no gain or loss at the equilibrium. The rate of reaction at the electrode is similar. There are both forward and reverse reactions here thus leading to a zero net reaction rate and a zero net current. The exchange current in terms of the corrosion of a particular reaction includes the current coming from both of the two directions of the reaction that is reversible. For both the reversed anodic reaction and anodic reaction, the reactions can be given as:
The main variables used to express the exchange current density include:
• Metal composition: both the solution and composition of the metal or alloy are significant and tend to influence the exchange current density.
• The roughness of the metal surface.
• The concentration of soluble species
A material’s corrosion potential is a significant parameter that is used to predict both the future and current corrosion detection and damage. It is also used to monitor the electrochemical reaction that causes corrosion both in the laboratory and at the worksite. Therefore, this is a variable that is significant to the engineers of equipment, structures, pipelines, and systems. Maintenance engineers also utilize this factor to minimize the economic losses that are experienced because of the deterioration of materials and also detection the costly and high-risk failures, detection expenses, and also the costs of repair and replacement costs.
Corrosion potential basically refers to the attribute of the metal and nonmetal surfaces in the presence of an electrolyte to lose electrons. Two electrodes are formed spontaneously in the corrosion process and these are the anode and the cathode. The electrode potential acquired automatically by the material surface in its environment is what constitutes of corrosion potential. (10) The difference between the material’s surface and an appropriate reference electrode in on immersion of a metal surface in a particular electrode is what makes up the potential difference. A high impedance voltmeter that has high accuracy is used to do this measurement by observing and recording small voltages with the significant current flow not being needed.
Polarization is an activity that leads to a change in an electrode’s potential during electrolysis. During these mechanisms, the nobility if the anode’s potential is greater than that of the cathode. Depending on the conditions, this phenomenon usually reduces the batteries’ output voltage and increasing the voltage needed for the electrolysis cells or minimizing currents. (11) It when an electric current passes via a galvanic cell and contributes to the equilibrium deviating to kinetic. This can either happen at the anode (anodic polarization) or at the cathode (cathodic polarization).
Electrochemical polarization is very significant as far as the corrosion process is concerned. The corrosion rate is always reduced by cathodic polarization for each alloy or metal in an aqueous environment. The process of applying cathodic polarization to a corroding system is known as cathodic protection. There are three ways in which polarization occurs. These include resistance polarization, concentration polarization, and activation polarization.
Concentration polarization of an electrode occurs when a diffusion layer forms next to the electrode surface where the ion concentration gradient exists. The electrochemical reaction is controlled by the diffusion of the ions via the layers. This is significant for processes like corrosion and electroplating. Resistance polarization where the high resistivity of the electrolyte with which the electrode is surrounded leads to a potential drop. Ohm’s law is used to express this. Activation polarization when there are several successive steps in which the electrochemical reaction follows. The slowest step of the reaction determines the overall reaction.
This is where the voltage is lost because of the transportation of charge. A loss in cell voltage occurs thanks to the intrinsic resistance to charge flow seen in conductors. Electrical résistance in the cell components leads to ohmic polarization. (11) The electrolyte, the gas diffusion layer, interface contacts, the catalyst layer, bipolar plates, and terminal connection are the cell components that cause electrical resistance.
Only when the potential of an electrode is pushed away from its value at a circuit that is open or corrosion potential will an electrode be polarized. Because of the electrochemical reaction induced at the electrode surface by an electrode’s polarization, the current is made to flow. The following equation gives the true picture of resistance in polarization:
The resulting polarization current is the variation of the applied potential around the corrosion potential
this is a reaction that is controlled by the reaction. The rate of electrochemical charge transfer process controls the reaction rate leading to an activation-controlled process. (12) Kinetics are formed because of this. The following equation tends to explain such kinetics:
The current exchange density is given by io, the overpotential is given by η, the anodic transfer coefficient is given by αA while the cathodic transfer coefficient is given by αC.
Polarization behavior on the corrosion rate
As far as corrosion is concerned, polarization is the potential shift the open circuit potential of a system that is corroding. The open-circuit potential is also termed as free corroding potential. Anodic polarization occurs when there is a potential shift in the “positive” direction (above Ecorr). Cathodic polarization occurs when the potential shifts in the “negative” direction (below Ecorr).
The rate of corrosion is always reduced by cathodic polarization for all metals and alloys in an aqueous environment. The application of a cathodic polarization to a corroding system is what is referred to as cathodic protection. (13) The corrosion rate is increased by an anodic polarization for any non-passive system such as steel in seawater. The rate of corrosion will be initially increased by anodic polarization for systems that depict active-to-passive transition which then later causes a huge reduction in the rate of corrosion. The application of an anodic polarization to a corroding system is referred to as anodic protection.
Basics of corrosion measurement
Polarization resistance refers to the resistance between the electrolyte and electrodes. The chemical reactions at the electrodes tend to cause increased resistance to the flow of current in a voltaic cell. Reduction in the electric potential is caused by polarization. It is the event that an electrode is pushed away from its value at corrosion potential or open circuit when an electrode is polarized. (14)Because of the electrochemical reactions induced by polarization at the surface of an electrode, the currents is caused to flow.
The deviation of the electrochemical process due to current passing via the galvanic cell from equilibrium is what makes up polarization. It can occur either at the anode (anodic polarization) or at the cathode (cathodic polarization). There are three types of polarization with resistance polarization being one of them. This refers to the drop of potential that is caused either by an insulation effect of the film on the surface of the electrode caused by the reactants or by the high resistivity of the electrolyte that surrounds the electrode. Polarization resistance is given by:
refers to the differences of the potential applied around the corrosion potential while the resulting polarization current is given by . The high corrosion resistance occurs when the metals have a high Rp while low corrosion resistance happens when there is a low Rp. Therefore, the ratio of the applied potential resistance and the resulting current response is what makes up the polarization resistance. There is an inverse relationship between resistance and the uniform corrosion rate.
The corrosion rate can be estimated when the polarization resistance has been tested under steady-state conditions. Tafel slopes on the potentiodynamic scan are used to do this. This is also utilized to explain the procedure at which slope is used to measure corrosion rates. Behaving like a resistor, one can calculate the polarization resistance by using the inverse of corrosion potential or the slope of the current potential curve at an open circuit. The reaction kinetics and diffusion of reactants both away from and towards the electrode controls the magnitude of the current during an electrode’s polarization. (15) Several factors to be considered during the measurement of polarization include the scan rate effects, surface conditions, solution resistance, and pitting potential. Misinterpretation can be caused by not properly accounting for these factors thus the results are significantly affected.
Calculation of corrosion rates
From polarization data-stern and Geary equation
Polarization resistance of a material is the slope of a potential-current density curve at free corrosion potential. This leads to the polarization resistance with which the Stern-Geary equation relates to the corrosion current:
Polarization resistance is represented by Rp, icorr represents the corrosion current. (16) One can empirically determine the proportionality constant B for a certain system. Separate weight loss measurements are used to make the calibrations as depicted by the Stern and Geary. The slopes of anodic and cathodic Tafel, ba and bc can be used in the calculation.
Using real evaluation plots, the Tafel slopes can be experimentally evaluated. A generic conversion chart or Faraday’s law can be used to convert the corrosion currents estimated using such techniques into penetration rates. The most widespread use of electrochemical measurements both in the field and in the laboratory could be the study of uniform corrosion or studies where uniformity of corrosion is assumed. However, it does not mean that the electrochemical techniques do not have complications by them being utilized on al larger scope. For the results of both Tafel exploration and linear polarization to be valid, they need special precautions. The various obstacles in conducting polarization measurements include:
• Effect of solution resistance
• Determination of Pitting Potential
• Effect of Scan rate
• Changing surface conditions
From the corrosion current
According to Faraday’s Law,
Where Coulombs is represented by Q, a number of electrons in the electrochemical reactions is represented by n, the faraday constant is F, the weight of the electroactive species is W, the molecular weight is M.
From the above equation, W = QM/nF,
Since Q= it from Faraday’s law:
The corrosion rate in grams per second is given by W/t. expressing the corrosion rate in milli-inches per year is a convenient and traditional way. An indication of penetration is provided by these units.
When the above equation is divided by the electrode area and the density, we get
C.R. (cm/ sec) = i (E.W.)/ dFA. This is a method of converting seconds to years and centimeters to milli-inches.
To convert the Faraday (amp-sec/ eq) to microamps, we will use:
All the constants can be combined to get:
Electrochemical techniques to measure polarization resistance
Liner polarization technique
Linear polarization resistance is the only corrosion monitoring method that allows corrosion rates to be directly measured in real-time. This technique is made clearly superior to other corrosion monitoring methods by the response time and data despite it being limited to electrolytically conducting liquids. This method is utilized to quickly identify corrosion upsets and develop corrective measures and therefore reducing unscheduled downtime and prolonging plant life. This method is highly effective in the event that it is installed as a monitoring system that is continuous. For over thirty years, this technique has been effective in all types of corrosive, water-based surroundings.
Corrosion potential measurements as a function of time (ocp vs time)
The in situ corrosion potential has to be measured as a means of ascertaining the availability of corrosion products on a metal surface. This needs to be more positive for a metal surface that has corrosion products for bare metal by relying on the morphology, thickness, and composition of the corrosion products. (17) At the beginning of the exposure, it has been found that a bare metal’s corrosion potential can reach a value of -1000 ± 20 mV vs. SCE, which moves with time towards more positive values up to reaching – 600 ± 25 mV vs. SCE following exposure of 15 months.
Tafel extrapolation method
This was a method utilized by Traud and Wagner so as to verify the mixed potential theory. The region of extrapolation of the Tafel will give corrosion density either in anodic polarization or cathodic polarization and corrosion current rate can, therefore, be calculated using this. Tafel equations are used to describe the anodic and cathodic Tafel plots:
r is the difference between the potential of the specimen and corrosion potential, the Tafel constant is represented by β, while icorr represents the corrosion current density in μA/cm.
Potentiodynamic polarization measurements
In lab corrosion testing, the polarization methods used include potentiostaircase, potentiodynamic polarization, and cyclic voltammetry. Enough important information as far as corrosion mechanisms, the susceptibility of various materials to corrosion and corrosion rate can be given by such techniques. In potentiodynamic polarization, there is a variation of the electrode through the electrolyte at a particular rate of application of current.
Advantages and Limitations
Just like every other phenomenon, corrosion also has its benefits and limitations. In order to examine, the benefits and limitations of thermodynamics processes, the corrosion coupons will be used as a point of reference since most trial studies have been done based on them. Both the steel and its related components are likely to undergo a virtually unstoppable and continuous corrosion process. The electrochemical reaction produces rust as the end product.
An excellent source of information to any building owner or plant operator is given by the corrosion coupons. This is highly likely in the event that there is an accumulated history of the coupon test results and if the monitoring is continuously maintained. (18) An indication of the corrosion status is still given by the coupons despite them having a number of limitations. They also offer an overview of how an existing piping system would be in the event of the type and conditions of deposits. Through the application of the thermodynamics of corrosion in the field and various laboratory tests, corrosion coupons were developed as highly useful tools as far as the prediction and maintenance process is concerned. This is usually done by comparing the outcomes to that of wall loss information that is confirmed. This is usually given via spool piece measurement, metallurgical analysis, ultrasonic thickness, or actual pipe removal. Corrosion coupons tend to offer an efficient indication of whether the corrosion potential will be reducing or increasing after regular tests are done in the existing rigorously controlled conditions.
If a chemical inhibitor is present, this will be rapidly indicated by the corrosion coupons. The lack of too much wall loss can cause this. This will imply the corrosion coupon is efficient in offering a certain metal’s protection. The corrosion rate data also tends to be provided within a short period. This is one of the major advantages of this system. (19) This may be urgently required in a harsh chemical program or chemical cleaning evaluation. However, there are a number of reasons that make the corrosion coupons to produce inaccurate corrosion rate values based on the actual pipe wall loss. Therefore, the results produced by these elements are mainly just utilized as estimates of the fluid’s corrosivity instead of it being used as a measurement of the trues value of the metal that the pipe loses.
For instance, the number of factors acting against any circulating water system is limited by the when the coupon rack itself is placed externally. The corrosion estimates can be influenced by variation in the flow of water by up to ten folds. The corrosion rate estimates can also be significantly altered by the addition of rack layout, filtering of the coupon rack assembly, materials of construction or pipe size. Possible galvanic activity will be hugely eliminated by test layouts of constructed PVC. Significant differences in the measured corrosion can also be caused by the physical location of the coupon rack.
The always higher corrosion activity cannot be measured by the corrosion coupons when there is no availability of the flow of water. This usually happens in a periodic drain down or winter lay-up. If drained over 20 years, only 0.125 inches will be shown at the roof on there if a wall thickness of 0.335 inches filled. (20) This potential for failure can always remain hidden if the testing is only based on the areas filled by water.
The type of piping system involved also affects the degree of corrosion activity. The lowest corrosion activity is shown by the closed system while the highest pitting and corrosion activity is visible for the cooling tower and open condenser. The greatest fluctuation in the test result is also visible in the open circulating system. Therefore variation in the thickness of the wall is likely to be seen at the large and small pipe, other extremes of the system, from top to bottom, and at supply and return. Most corrosion coupons are totally unaffected by the numerous cathode/ anode electrochemical reaction due to the fact that they are isolated from any metal to metal contact via a galvanic insulator or the utilization of a center located plastic. A common example of galvanic forces is the well-recognized copper pipe or steel pipe effect. Most piping systems tend to possess this phenomenon to some degree. Because of this, the main corrosion mechanism that leads to a huge amount of material loss is always not measured.
It is within the areas of no flow where some of the most severe corrosion and pitting conditions are located. This is a common feature at the future lines, out-of-service equipment, bypass lines, and lead and lag equipment. A settlement basement for settlement of deposits is most notoriously offered by cooling tower bypass lines which are open at the supply side and closed at the downstream end. It is practically not possible to do test the corrosion coupon with there being no flow. Therefore, most of the vulnerable areas in the whole piping system are not tackled.
The adhesion of dirt, iron oxide and microorganism is minimized by polished, mirror-smooth corrosion coupons. Therefore, since they have irregularly worn and pitted interior surfaces, it is rare for these to be attacked in the same way as the aged piping system. New corrosion coupons bear no resemblance cyclic voltammetry to the surface of the pipe unlike pitted and worn out old piping systems. The reporting error is even increased by this phenomenon.
Corrosion coupons are mostly tested in a period between 30 and 90 days. Practically, one is unlikely to find any passivating layer of rust protection as early as 30 days. Because of this, falsely high corrosion rates would be recorded. 90 days is also a short time for there to be an accumulation of any deposit or microbiological buildup at the smooth surface of the coupon that is typically found in the actual piping system. Since the low corrosion rate is rarely questioned, all these scenarios are generally recognized factors that lead to incorrect figures as far as the corrosion activity data is concerned.
In conclusion, this literature review has tackled the thermodynamics of corrosion from a different point of view. For instance, the thermodynamics of the processes of corrosion have been discussed from the scope of the reaction measurements, the Nernst equation, and the free energy of a corrosion reaction. The electrochemical kinetics of the equation has also been tackled where previous journal articles that tackled the polarization curve, exchange current density, corrosion potential, and electrochemical polarization density have been looked at. Past studies have also been reviewed to look at the benefits and limitations as far as the thermodynamics of corrosion is concerned and it is clear that most studied have majored on the corrosion coupons and how the corrosion concepts affect it in terms of advantages and disadvantages.
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Gamry. Getting Started with Electrochemical Corrosion Measurement. [Internet]. 2020. Retrieved April 7, 2020, from https://www.gamry.com/application-notes/corrosion-coatings/basics-of-electrochemical-corrosion-measurements/