**2.6 Hard-coatings as protection; borided treatment**

Other attractive uses of the *EIS* technique are its application to evaluate the integrity and coating performance during its exposure in corrosive environments as a function of time. Actually, *EIS* is used as a quality control to evaluate the process

#### **Figure 18.**

*EIS spectra in bode plots obtained from the pipeline steel API-5 L-X52 samples immersed in H2SO4 1 M as a function of the different concentration of natural molecules.*

of surface finishing treatments in many industries. In this sense, the results of **Figure 19** show the characteristic impedance spectra that indicate the quality properties and corrosion resistance of a Fe2B/FeB hard coating formed by boron atomic diffusion on the steel surface of a 1045 and 304 stainless steel during the boriding thermochemical treatment. Boriding is recognized as a thermochemical surface treatment in which boron diffuses into the ferrous substrate and reacts with Fe atoms of the bulk material to form a single (Fe2B) or double-phase (Fe2B/FeB) layer with a well-define thickness and composition [14]. The thickness of each layer has considerable effects on the mechanical behavior and corrosion behavior of the borided steels. However, the quality of the hard boride coatings depends essentially on the boriding temperature, treatment time, chemical composition of the steel substrate and the amount of boron atoms available around the sample surface to be coated.

formed on the 1045 steel surface, its morphology consisting a deep saw-tooth derived from the existence of diffusion paths (porosity and micro-cracks) in the surface of the steel matrix, in which the boron atoms are interstitial inserted to the surface forming a stable phase. For the borided stainless steel SS304 at the same conditions forms two-well defined layers on the surface, the columnar phase that was growth on the 1045 steel is less intense for SS304, this is due to the high concentration of chromium and nickel on the substrate surface, so the diffusion of boron stops by reacting immediately to form interstitial compounds of CrB, Cr2B or Ni3B in combination with FeB and Fe2B. EIS for the borided 1045 steel were recorded over 72 days of exposure to HCl 1 M solution, which the hard coating degrades slowly due to the defects on the coating structure that allow Cl ions infiltrate, this is denote by changing the *EIS* spectra shape from one time constant to two time constant with a clearly phase-angle shifted and loss of impedance value, that means pitting corrosion initiation. No-corrosion damage was observed for the borided SS304 during its exposure in HCl 1 M solution for at least 170 days. Three times constants were observed after 44 days that's reveal the presence of the FeB

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion…*

Steel-can containers are manufactured from thin metal plates and are commonly used for the distribution or storage of food or beverages. Most conventional steel beverage cans have bent to form a tube and then welding both sides leaving a firm seam, then joining the bottom end to the tube, finally, the steel can is filling-out with the content. However, it is necessary to mention that the steels cans have an internal polymer coating or have been treated by electroplating to coated internally with a thin layer of tin in order to prevent any oxidizing or electrochemical corrosion during the steel exposure to the liquid product that it contains, which could be carbonated soft drinks, alcoholic drinks, fruit juices, teas, herbal teas, energy drinks and others [64, 65]. Despite of this internal coating having the good quality, it may fracture during storage or dissolve in small amounts in the liquid product, which depends on certain factors such as temperature, stowage load and handling of the products during their storage, as well as the chemical composition of the liquid and steel. Due to this, efforts have been managed to replace tin-based coatings by chemical compounds derived from epoxy resins or polymers. Nevertheless, the set-up of the factors mention above may situate the metal container (e.g. steel cans) at a potential risk to develop internal corrosion. On the other hand, the sale of beverages storage in steel cans are committed to their handling in warehouse, in this way, there is a predisposition of the people who buy drink-cans, they think if cans are struck or bent the coating has been damaged and could be associated that the liquid product is contaminated with Fe<sup>+</sup> ions. The impedance diagrams of **Figure 20** show that the *EIS* technique can be applied to assess the corrosion resistance of the internal coating in a specific beverage can. In this case experimental corrosion tests on laboratory conditions were performed in a metal container used for the distribution of orange juice in Mexico. This can is made of steel with internally coated by a higher density polymer. Three particular cases are studied as denoted in the scheme of **Figure 20**; *EIS* spectra shown the behavior for a) with the coating, b) when the coating is mechanically damaged by a scratch and c) absence of coating, measured in HCl 1 M as a function of *AC* amplitude signal from 5 to 1000 mV. The bode diagrams indicate the presence of two welldefined time constants in the entire frequency domain for 5 and 10 mV of signal, the first one is related to the polymer coating with a resistance of electron -ion transfer of

Ω -cm2 with a micro-porous net (conducting paths) inside the coating as indicated by the second time constant. However as increasing the amplitude of signal

layer, after Fe2B layer and the diffusion layer.

**2.7 Steels used as beverages container**

*DOI: http://dx.doi.org/10.5772/intechopen.94470*

about 108

**27**

In this study, in particular a powder-pack boriding was used on AISI-SAE 1045 steel and SS316 stainless steel as surface thermochemical treatment to improve hardness and wear resistance to the steel samples, due to its low cost of hard coating processing. Boriding can also enhance the corrosion resistance of ferrous materials as shown in **Figure 19**. The results indicate that a single boride layer of Fe2B is

#### **Figure 19.**

EIS *spectra for borided samples immersed in HCl 1 M as a function on exposure time. Boriding treatment was performed on AISI 1045 steel or AISI SS304 stainless steel treated at 950°C for 6 h [14]. a) Phase angle response for borided 1045 steel and b) Phase angle dependence for borided SS304 steel.*

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion… DOI: http://dx.doi.org/10.5772/intechopen.94470*

formed on the 1045 steel surface, its morphology consisting a deep saw-tooth derived from the existence of diffusion paths (porosity and micro-cracks) in the surface of the steel matrix, in which the boron atoms are interstitial inserted to the surface forming a stable phase. For the borided stainless steel SS304 at the same conditions forms two-well defined layers on the surface, the columnar phase that was growth on the 1045 steel is less intense for SS304, this is due to the high concentration of chromium and nickel on the substrate surface, so the diffusion of boron stops by reacting immediately to form interstitial compounds of CrB, Cr2B or Ni3B in combination with FeB and Fe2B. EIS for the borided 1045 steel were recorded over 72 days of exposure to HCl 1 M solution, which the hard coating degrades slowly due to the defects on the coating structure that allow Cl ions infiltrate, this is denote by changing the *EIS* spectra shape from one time constant to two time constant with a clearly phase-angle shifted and loss of impedance value, that means pitting corrosion initiation. No-corrosion damage was observed for the borided SS304 during its exposure in HCl 1 M solution for at least 170 days. Three times constants were observed after 44 days that's reveal the presence of the FeB layer, after Fe2B layer and the diffusion layer.

#### **2.7 Steels used as beverages container**

Steel-can containers are manufactured from thin metal plates and are commonly used for the distribution or storage of food or beverages. Most conventional steel beverage cans have bent to form a tube and then welding both sides leaving a firm seam, then joining the bottom end to the tube, finally, the steel can is filling-out with the content. However, it is necessary to mention that the steels cans have an internal polymer coating or have been treated by electroplating to coated internally with a thin layer of tin in order to prevent any oxidizing or electrochemical corrosion during the steel exposure to the liquid product that it contains, which could be carbonated soft drinks, alcoholic drinks, fruit juices, teas, herbal teas, energy drinks and others [64, 65]. Despite of this internal coating having the good quality, it may fracture during storage or dissolve in small amounts in the liquid product, which depends on certain factors such as temperature, stowage load and handling of the products during their storage, as well as the chemical composition of the liquid and steel. Due to this, efforts have been managed to replace tin-based coatings by chemical compounds derived from epoxy resins or polymers. Nevertheless, the set-up of the factors mention above may situate the metal container (e.g. steel cans) at a potential risk to develop internal corrosion.

On the other hand, the sale of beverages storage in steel cans are committed to their handling in warehouse, in this way, there is a predisposition of the people who buy drink-cans, they think if cans are struck or bent the coating has been damaged and could be associated that the liquid product is contaminated with Fe<sup>+</sup> ions. The impedance diagrams of **Figure 20** show that the *EIS* technique can be applied to assess the corrosion resistance of the internal coating in a specific beverage can.

In this case experimental corrosion tests on laboratory conditions were performed in a metal container used for the distribution of orange juice in Mexico. This can is made of steel with internally coated by a higher density polymer. Three particular cases are studied as denoted in the scheme of **Figure 20**; *EIS* spectra shown the behavior for a) with the coating, b) when the coating is mechanically damaged by a scratch and c) absence of coating, measured in HCl 1 M as a function of *AC* amplitude signal from 5 to 1000 mV. The bode diagrams indicate the presence of two welldefined time constants in the entire frequency domain for 5 and 10 mV of signal, the first one is related to the polymer coating with a resistance of electron -ion transfer of about 108 Ω -cm2 with a micro-porous net (conducting paths) inside the coating as indicated by the second time constant. However as increasing the amplitude of signal

which has a great interest on giving an educational orientation and practical teaching guide of how to use the outstanding Electrochemical Impedance Spectroscopy (EIS) technique in metal corrosion technology. Therefore, this review considers a wide variety of practical electrochemical impedance cases based on the fundamental and qualities aspects of EIS theory and its experimental interpretation. This book chapter also serves as a support for postgraduate students to have a criterion in deciding through their own experiences when using the electrochemical impedance technique. The practical cases discussed here are part of the research experienced of Dr. Héctor Herrera Hernández (DR.3H) and his students & research group. It is worth to mention that EIS has been extended to various disciplines of science and technology, thus demonstrating great efficiency in evaluating the performance and integrity of metallic materials as can be seen in detail in the practical examples presented in this review work. So, EIS is not only applied to stationary conditions, but also more complex variables can be monitored such as: flow parameters, variable that undoubtedly represents the real conditions and could be an interesting challenge for analyzing and interpreting these phenomena by means of EIS data. The fitting EIS data using a mathematical model such as an equivalent electrical circuit is a critical process in the analysis and validation of EIS data for the acquisition of the system's electrical parameters that can be related to the corrosion rate of the material under study and also gives information of its capacity of electrons charge. Finally, EIS seeks to obtain information on the system and its evolution with time by applying a sinusoidal voltage as a function of frequency range, in order to determine the properties and feasibility of materials that serve under severe service conditions, such as industrial steels as is this case of the reviewed book chapter.

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion…*

*DOI: http://dx.doi.org/10.5772/intechopen.94470*

The authors dedicate this chapter to the memory of Professor Florian B.

remembered forever for his outstanding knowledge and contibutions.

The authors would like to acknowledge and express their gratitude to CONACyT for the SNI distinction as research membership and the monthly stipend received. Héctor Herrera Hernández (DR.3H) also would like to thanks to CIDETEQ and Secretaria de Investigación y Estudios Avanzados SIyEA/UAEM for their financial support through research project (4602/2018E). This project was conducted in the (Laboratory of Electrochemical and Corrosion of Industrial Materials at UAEM). Finally, DR.3H dedicates this work in memory to Professor **Florian B. Mansfeld**, for his teaching and guidance in the way of science (EIS technique), FBM will be

**Acknowledgements**

Mansfeld, USC.

**29**

#### **Figure 20.**

*Phase angle EIS response obtained for metal beverage containers at different surface condition after immersed in NaCl 0.5 M as a function on AC amplitude signal. a) Uniform polymer coating, b) scratch defect on coating, c) polished surface no-coating.*

voltage the |*Z*| value drops below 104 Ω-cm2 , this response is associated with local stain-spots on the coating, which is indicted by a third time constant a low frequency. In the condition for the coating damaged by a localized defect such as a scratch or fracture, the impedance value decreases severely to 105 to 102 Ω-cm2 as increased the *AC* signal, one time constant indicates the electron charge transfer processes through the defect that cause ions to be diffused below the coating until its failure. Finally, for the condition in the absence of the coating on the steel plate, the impedance diagrams show the corrosion process of the steel at different *AC* signal amplitudes, which shows severe corrosion after 200 mV showing 101 Ω-cm2 of |*Z*| value.

## **3. Conclusions**

This review study is related to the basic aspects of EIS to understand the corrosion mechanism of industrial steels that serve at different corrosive conditions,

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion… DOI: http://dx.doi.org/10.5772/intechopen.94470*

which has a great interest on giving an educational orientation and practical teaching guide of how to use the outstanding Electrochemical Impedance Spectroscopy (EIS) technique in metal corrosion technology. Therefore, this review considers a wide variety of practical electrochemical impedance cases based on the fundamental and qualities aspects of EIS theory and its experimental interpretation. This book chapter also serves as a support for postgraduate students to have a criterion in deciding through their own experiences when using the electrochemical impedance technique. The practical cases discussed here are part of the research experienced of Dr. Héctor Herrera Hernández (DR.3H) and his students & research group. It is worth to mention that EIS has been extended to various disciplines of science and technology, thus demonstrating great efficiency in evaluating the performance and integrity of metallic materials as can be seen in detail in the practical examples presented in this review work. So, EIS is not only applied to stationary conditions, but also more complex variables can be monitored such as: flow parameters, variable that undoubtedly represents the real conditions and could be an interesting challenge for analyzing and interpreting these phenomena by means of EIS data. The fitting EIS data using a mathematical model such as an equivalent electrical circuit is a critical process in the analysis and validation of EIS data for the acquisition of the system's electrical parameters that can be related to the corrosion rate of the material under study and also gives information of its capacity of electrons charge. Finally, EIS seeks to obtain information on the system and its evolution with time by applying a sinusoidal voltage as a function of frequency range, in order to determine the properties and feasibility of materials that serve under severe service conditions, such as industrial steels as is this case of the reviewed book chapter.
