Corrosion Testing Using Electrochemical Impedance Spectroscopy (EIS) Research Paper

The fight against corrosion of metals is one of the most important problems of contemporary civil engineering. In connection with this fact, the last decade is characterized by a rapid development of approaches to improve the corrosion resistance of metals and different designs and constructions of various purposes. Thus, the problem of further improving the corrosion research methods is of great importance in civil engineering. 

The current development of the chemical industry and the development of new technological processes, which occur in corrosive environments, impose high standards on structural materials. The most important structural materials are metals and their alloys. Corrosion occurs when products have electrical or physical contact with each other. Corrosion processes are irreversible and often lead to the failure of various machines and devices. So they need to be detected at an early stage to quantify the corrosion damage and predict the risk of corrosion.  Establishing the causes of corrosion allows to choose the right protection methods.

The purpose of corrosion studies is not only to determine the durability of the metal in certain circumstances, but the disclosure of corrosion process and mechanism which can be electrochemical, chemical or mixed. In addition, it is essential to determine the mechanism and process of corrosion: kinetic, diffusive or mixed in the case of chemical attack or the types of control of electrochemical corrosion processes, for example, the cathode. Aggressive components of the environment are also installed such as O2, CO2, water vapor that causes the chemical corrosion of metal or depolarizers (O2, H +, H2 O, HSO3-, etc.) that promote the electrochemical corrosion of metals. The effect of internal (structure, alloy composition, surface condition, the presence of internal stresses) and external (the composition of the corrosive environment, its velocity, temperature, pressure, etc.) is being observed and researched.

Classification of corrosion research methods

In fact, corrosion “…attacks mainly austenitic stainless steels and it is a kind of relatively rapid and local corrosion associated with a failure of microstructure known as the precipitation of carbides” (Nerádová 152). According to Bell, “Corrosion is caused through chemical interactions between metal and gases in the surrounding environment. By removing the metal from, or changing, the type of environment, metal deterioration can be immediately reduced” (par. 4).  Hence, a variety of environmental conditions requires the use of various means to determine the corrosion resistance in various cases as laboratory studies and practical tests. Each method of corrosion testing should proceed from this fundamental view of the mechanism of corrosion of metals.

In the most common forms, the practical purpose of corrosion testing may be expressed as determining the durability of the metal material under given conditions; the results of corrosion tests should allow to draw a conclusion about the corrosion behavior of a metal or alloy in an operational environment.

In a special laboratory for high-grade corrosion tests, scientists reproduce real operating conditions to get maximum visibility and approximation to reality picture material changes (if they occur). To speed up the testing conditions may tighten. During the tests different types of corrosion are being tested. It can be as uniform and pitting or can be chemical or electrochemical in nature.

Sometimes a special corrosion chamber can be used to conduct testing where we can see an estimated impact on the salt material, condensate and hot air (or combinations thereof). Anticorrosive coatings are tested no less carefully – any significant damage to the material or coating (for some types of impacts using solutions). It can recognize test results unsatisfactory. The test results are necessarily logged for subsequent formation of opinion. In the report you can see the entire test program with the description of progress and all the factors. There will also be listed different methods of evaluation.

The purposes of corrosion problems and researches can be summarized as follows:

  1. Disclosure of the mechanism of the corrosion process, which may be chemical, electrochemical or mixed (chemical and electrochemical);
  2. Establishment of aggressive components (O2, CO2, H2O vapors, etc.) that cause the chemical destruction of the metal, or a depolarizer (O 2, H +, H 2 O, HSO3, etc.) that is responsible for the electrochemical corrosion of metal;
  3. Establishment of the influence of the main internal (composition, structure, condition of the surface of the alloy, the presence of stress, etc.) and external (the composition of the corrosive environment, its movement, temperature, etc.) factors on the corrosion resistance of a metal or alloy;
  4. Selection of the best metal or alloy for use in certain operational conditions. Thus, it is necessary to consider not only the corrosion resistance of the material, but also its strength, processing properties, cost and so forth.
  5. Comparing the corrosiveness media with respect to one or more of the metallic materials;
  6. Verification methods of protection of metals from corrosion: to determine the effectiveness of anti-doping, the use of corrosion inhibitors, the reliability of protective coatings, etc.;
  7. Checking the quality of products in terms of corrosion resistance, for example, control of chrome-nickel stainless steel no tendency to intergranular corrosion test for protective coatings and so on.

The complexity of the research problem, the complexity and diversity of the corrosive environment and the corrosion conditions force to develop a system of research and testing methods, which would allow to answer all the questions concerning this particular issue.
In this case, the following requirements to a system of methods can be formulated:

  1. The method shall conform to the goal. The method itself should use the simplest and shortest path to solve the given task. If this method proves too difficult, it is necessary to differentiate the task into a series of simple and appropriate methods to create the system.
  2. The method must conform to modern notions about the corrosion mechanism, i.e., to be at the present level of science.
  3. The method should provide for the strict control of all the factors. Only under this condition, it is possible to get reproducible results. Here we can speak about the demand for the comparative simplicity of the method.
  4. The method shall reproduce exactly the type of corrosion, which is practically important for this material. For example, stainless steels under practical conditions when operating in humid air and neutral or slightly acidic salt solutions are often found either pitting or intercrystal – destruction. If the test method is designed so that the stainless steel corrosion will wear uniform character, then this method will not be practically useful.
  5. The method should provide for a relatively simple decoding test results, even if they were not straight, e.g., it does not represent the number of the directly destroyed metal by corrosion per unit time. This option only appears if the method is built on a solid scientific basis, i.e., is in accordance with the theory of corrosion processes.

As a result, “there are several competing materials that can meet the corrosion requirements, and the material selection process becomes one of determining which of the candidate materials provides the most economical solution for the particular service. Consideration of corrosion resistance is often as important in the selection process as the mechanical properties of the alloy. A common solution to a corrosion problem is to substitute and alloy with greater corrosion resistance for the alloy that has corroded” (2000 ASM International7).

According to the general nature of the research, these methods are divided into three groups:

  1. laboratory studies – studies of metal corrosion of samples in the laboratory, artificially created conditions;
  2. non-laboratory studies – metal corrosion studies of natural specimens, operating conditions (including research in natural conditions: in the atmosphere, in the sea, in a ground, etc.).
  3. operational researches – testing machines, devices, structures and remedies under operating conditions. Thus, the object of general research laboratory and non-laboratory researches is common, as the terms are different studies, while common to the non-laboratory studies are operational and their conditions and they are subject to different studies.

Typically, scientists conduct laboratory, then non-laboratory and performance tests. Various types of research are complementary.

According to the duration of research, methods are divided into two groups:

  1. long methods – correspond to operating conditions;
  2. accelerated methods – carried out in artificial conditions, accelerating corrosion processes in operating conditions; the acceleration test is usually achieved by facilitating the flow of controlled processes, but without changing the nature of the corrosion process.

The results of the accelerated corrosion tests in an environment where not only reproduce the mechanism of aggressive action, but also it is possible to predict the durability and susceptibility of certain types of corrosion test materials and coatings. Selecting the corrosive environment, corrosion accelerating manifestation of the results allows short-term tests only to compare the corrosion resistance of different materials and coatings. Selecting accelerated corrosion allows to control, detect defects and weaknesses in the corrosion protection of components and structures. Acceleration and long-term methods of research supplement and monitor each other. Comparing the results in these studies allows you to get conversion rates, which eliminate the need to conduct long-term tests.

Corrosion Performance

Corrosion indicators can be qualitative and quantitative. Qualitative corrosion performance:

1) monitoring the appearance of samples with photographing, sketches or a brief description and monitoring of changes in the corrosion solution;

2) micro-studies to determine the type of corrosion, the presence or absence of intergranular corrosion, and so on;

3) the use of colored indicators for detecting the anodic and cathodic sites corroding the metal surface.

Quantitative indicators of corrosion are the following:

Quantitative corrosion indicators include an indicator of the corrosion propensity, focal component, depth measure corrosion, rate of mass change, volumetric rate of corrosion, current rate of corrosion (the density of corrosion current), mechanical component, rate of change of electrical resistance, reflective (or optical) corrosion – expressed as a percentage change in reflectance of the metal surface over the corrosion process, and so forth.

Methods for corrosion testing

Evaluation of the depth of corrosion damage is done by qualitative and quantitative methods.

Qualitative methods in corrosion research. Visual inspection. Determination of corrosion to change the appearance of the sample (or any other object of study) is the simplest, yet most rough method. However, when properly set observations and this method can provide valuable qualitative and quantitative data.

In all the tests it is necessary to carefully register the state of the sample before the experiment. It is necessary to carefully register all kinds of surface flaws such as scratches, flaws, captivity, inclusion and so. Observations are made at intervals, the duration of which depends on the speed of the corrosion process. They should be chosen so that:

a) register the beginning of the appearance of visible corrosion products,

b) changes between two observations were quite sharp. If the sample for observation to be extracted from the fluid (in vivo tests, for example, in the sea), the intervals must be chosen long as each such observation would involve a violation of the natural course of the corrosion process. It is better to first make more frequent observations, and then lengthen the intervals between them.

The result of monitoring should be:

  1. Registration of changes of the appearance of the metal surface, such as darkening of the coating spots (such and such a color, hue), was brilliant, and became dull, and so on.
  2. Registration of corrosion products, their nature and distribution. First of all, there is the color of corrosion products, such as white, brown, then the character, such as flakes, coating, film, etc. Also, if possible, mark density and strength products solicitation to the metal surface: easily separated is not separated from the metal under moderate pressure.

Visual observation of the appearance of the samples is applicable in all cases and is valuable aid in evaluating and expressing corrosion. Visual observation of the changes in the solution. This method of qualitative assessment can be only conducted in laboratory conditions, i.e., with a limited amount of electrolyte. If the metal passes into solution in whole or in part as a colored ion, the solution color change may serve as a rough measure of a quality of corrosion. For example, in case of corrosion of copper in different electrolytes blue solution indicates the transition to a solution of copper in the form of Ca + ion. If the result of the corrosion produced insoluble corrosion products that are poorly retained on the sample surface and the greater part sink to the bottom of the vessel in the form of floc, then the amount of sediment is a rough measure of the corrosion process.

Microscopic studies

Further development of the visual method of investigating corrosion is a microscopic examination. As in previous cases, microscopic examination may be carried out after and during the corrosion test. Microscopic examination allows first of all to study in detail the election and the local character of corrosion.

Microscopic observation of corrosion processes over time provides valuable data about the beginning and the nature of the corrosion damage. To monitor the corrosion process under the microscope sample surface – a thin section or prepared in another way – is placed in a tray so that the working surface is rotated towards the lens of the microscope.

Indicator method. The method is based on the fact that with the appropriate reagents it is possible to determine the location of the anodic and cathodic sites on the surface of the corrosive metal. This method can be used to identify clearly heterogeneous corrosion i.e. with a clear delineation of the anode and cathode sections, which may occur in the presence of non-uniform deformation of the metal in contact with other metals and non-metals.

Electrochemical Impedance Spectroscopy

Impedance spectroscopy is a method that plays a significant role in the fundamental and applied aspects of materials science. Impedance spectroscopy is a powerful tool for obtaining information about the processes of transport of charge carriers and allows to characterize the electrochemical behavior due to several intrinsically related processes, all of which can be run at different speeds or in different channels. As Lasia states “initially applied to the determination of the double-layer capacitance1-4 and in ac polarography, 5-7 they are now applied to the characterization of electrode processes and complex interfaces” (4). Impedance spectroscopy is suitable for the study of all types of solid and liquid materials: ionic, mixed, semiconductor and even insulators. Especially it is essential to study the charge transfer in heterogeneous systems, including the phase boundary, electrode boundary, microstructure elements, such as crystallites and their boundaries. It is impossible to obtain information about the complex processes of charge transport by using other methods. What is more, “Electrochemical Impedance Spectroscopy (EIS) is a traditional method, central to electrochemical science and technology. Electrochemistry usually investigates interfacial charge transfer between a solid conductor (the working electrode, WE) and an electrolyte” (Bisquert and Fabregat-Santiago 1).

By using impedance spectroscopy it is possible to study the behavior of chemical sensors, fuel cells, corrosion processes. In recent years, the value of this method is increased due to the increased availability of high-quality commercial instruments that allow to conduct automated measurements in the frequency range from millihertz to megahertz.

Impedance spectroscopy is based on the analysis of the electrical response of the system (of the material) to an applied electric field to the system that causes the transfer of the charge carriers. It is important that the applied electric field is a function of a certain time (e.g., periodic). Electrical response of the system is due to the superposition of the microscopic processes of charge transfer, resulting in the measurements to the measured macroscopic response of the material (the current flowing through the system) to the applied potential difference.

The processes of intervalence charge transfer include the transport of electrons or ions through the volume of the conductors, the charge transport through the interface, processes the charging interfaces, etc.

Furthermore, electrochemical reactions at the interface or in the bulk should also be taken into consideration. The flux of charged particles (current) is dependent on the ohmic resistance of bulk parts of the conductors, the resistance (generally nonohmic) of boundaries recharging speed sections with different type of carriers (e.g., electrolyte-electrode).

Impedance spectroscopy just allows you to select the elementary processes of charge transport in contrast to, for example, total current is calculated and averaged over the entire sample conductivity.

Parameters derived from the impedance spectra, generally divided into two categories: those that belong to the material, such as conductivity, dielectric constant, and the mobility of the charge, the equilibrium concentration of the charged particles and the volumetric generation rate – charge recombination; as well as those that characterize the interface – the rate of absorption capacity of the interface, the diffusion coefficients.

Advantages and disadvantages of Electrochemical Impedance Spectroscopy

This method is very beneficial in the field of contemporary civil engineering. It is used to explore the transfer of charges in heterogeneous systems, including the phase boundaries, microstructure elements such as crystallites. It is possible to investigate the behavior of chemical sensors, fuel cells, corrosion processes through impedance spectroscopy.

In recent years, the significance of this method has increased due to the increased availability of commercial high-quality instruments that allow to conduct different automated measurements.

Hence, EIS is one of the most accessible methods of investigation of electrochemical and electrical processes in the ion-conductive material due to the relatively low cost of equipment and relatively high sensitivity of the method. However, there is a problem of interpretation of the results. This is due to the complexity of processes in materials with ionic or mixed electronic-ionic conductivity.

The development of this theory is aimed at building the electrical equivalent circuit of the samples, which was quite a challenge. Accumulated experimental experience shows that the electrical properties of the samples often do not correspond to the resistor-capacitor model. To improve the accuracy, it is important to introduce inductance or negative capacitance. Thus, there are processes that result in a lag phase current, the nature of which is uncertain.

The second problem is related to the presence of unstable electrochemical processes. Such systems cannot be modeled passive two-terminal networks. However, when constructing ES active two-terminal network having internal energy sources are currently not used. An example is the instability of corrosion of the electrodes that is shown in registration with the negative capacitance measuring the electrical properties of the IP method.

The third problem relates to the fact that the electrochemical cells are distributed parameter systems. In the theoretical description of such objects having a function of coordinates and time, which is required for a finding of integrating the differential equation in partial derivatives. Thus, the construction of ES adequately describing the electrical properties of the sample requires serious theoretical and experimental studies.

Conclusion

Thus, taking the above-mentioned information into consideration, it is possible to draw a conclusion that corrosion of metals is called chemical degradation under the influence of various liquids and gases. The tests consist in immersion for a certain time in the corrosive environment (usually solutions of acids, salts, etc.) Sample cut from the welded joints, and further assess the nature and degree of destruction of the metal joints.

The current development of the chemical industry and new technological processes, which occur in corrosive environments, impose high standards on structural materials. In this case, the most important structural materials are metals and their alloys. Corrosion processes are irreversible and often lead to the failure of various machines and devices. As a result, they need to detect at the very early stage. Establishing the causes of corrosion damage allows to choose the right protection methods. The purpose of corrosion studies is not only to determine the durability of the metal in certain circumstances, but also to explore the corrosion process.

Corrosion studies include testing which is subjected to the determination of its durability and resistance to corrosive conditions of the environment. The purpose of corrosion studies is:

– Determining the mechanism of occurrence and type of corrosion damage;

– Studying the most active components of the environment, which provoke and accelerate the process of electrochemical and chemical corrosion;

– Establishing the ability to control the destructive processes. When we speak about chemical corrosion, it is possible to mention about kinetic, mixed, diffusive and the electrochemical mechanism of damage.

With the development of metal corrosion protection, methods of protection of metal constructions and structures from corrosion became significant among all the industries including civil engineering. There are methods of protection, which make it possible to successfully exploit the metals and alloys in the increasingly complex conditions of contemporary engineering. Undoubtedly, the success in the discovery and improvement of corrosion protection is only possible with a simultaneous in-depth development of the scientific basis of corrosion phenomena and the establishment of functional quantitative dependence of the kinetics on various factors from both metal and corrosive environment. Currently, the technology of anticorrosion protection is being rapidly improved and scientific methods of investigation of corrosion processes are being rapidly developed in a variety of conditions.

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Nerádová, Martina, et. all. “The Effect Of The Annealing Temperature On The Corrosion Resistance Of Weld Joint Of Aisi 310 Steel – Short Communication”. Materials Engineering, 18, 2011.

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R. Zuo. “Inhibiting mild steel corrosion from sulfate-reducing bacteria using antimicrobial-producing biofilms in Three-Mile-Island process water”. Appl. Microbiol. Biotechnol. 64, 2004.

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