Metal Corrosion and the Efficiency of Corrosion Inhibitors in Less Conductive Media

Summary

The testing of processes associated with material corrosion can often be difficult especially in non-aqueous environments. Here, we present different methods for short-term and long-term testing of corrosion behavior of non-aqueous environments such as biofuels, especially those containing bioethanol.

Abstract

Material corrosion can be a limiting factor for different materials in many applications. Thus, it is necessary to better understand corrosion processes, prevent them and minimize the damages associated with them. One of the most important characteristics of corrosion processes is the corrosion rate. The measurement of corrosion rates is often very difficult or even impossible especially in less conductive, non-aqueous environments such as biofuels. Here, we present five different methods for the determination of corrosion rates and the efficiency of anti-corrosion protection in biofuels: (i) a static test, (ii) a dynamic test, (iii) a static test with a reflux cooler and electrochemical measurements (iv) in a two-electrode arrangement and (v) in a three-electrode arrangement. The static test is advantageous due to its low demands on material and instrumental equipment. The dynamic test allows for the testing of corrosion rates of metallic materials at more severe conditions. The static test with a reflux cooler allows for the testing in environments with higher viscosity (e.g., engine oils) at higher temperatures in the presence of oxidation or an inert atmosphere. The electrochemical measurements provide a more comprehensive view on corrosion processes. The presented cell geometries and arrangements (the two-electrode and three-electrode systems) make it possible to perform measurements in biofuel environments without base electrolytes that could have a negative impact on the results and load them with measurement errors. The presented methods make it possible to study the corrosion aggressiveness of an environment, the corrosion resistance of metallic materials, and the efficiency of corrosion inhibitors with representative and reproducible results. The results obtained using these methods can help to understand corrosion processes in more detail to minimize the damages caused by corrosion.

Introduction

Corrosion causes great material and economic damage around the world. It causes considerable material losses due to partial or complete material disintegration. The released particles can be understood as impurities; they can negatively change the composition of the surrounding environment or the functionality of various devices. Also, corrosion can cause negative visual changes of materials. Thus, there is a need to understand corrosion processes in more detail to develop measures to prevent corrosion and minimize its potential risks¹.

Considering environmental issues and the limited fossil fuel reserves, there is an increasing interest in alternative fuels, among which biofuels from renewable sources play an important role. There are a number of different potentially available biofuels, but bioethanol produced from biomass currently is the most suitable alternative for substituting (or blending with) gasolines. The use of bioethanol is regulated by the Directive 2009/28/EC in the European Union^2,3.

Ethanol (bioethanol) has substantially different properties in comparison with gasolines. It is highly polar, conductive, completely miscible with water, etc. These properties make ethanol (and fuel blends containing ethanol also) aggressive in terms of corrosion⁴. For fuels with low ethanol content, contamination by small amounts of water can cause separation of the water-ethanol phase from the hydrocarbon phase and this can be highly corrosive. Anhydrous ethanol itself can be aggressive for some less noble metals and cause "dry corrosion"⁵. With existing cars, corrosion can occur in some metallic parts (especially from copper, brass, aluminum or carbon steel) that come into contact with the fuel. Furthermore, polar contaminants (especially chlorides) may contribute to the corrosion as a source of contamination; oxygen solubility and oxidation reactions (that can occur in ethanol-gasoline blends (EGBs) and be a source of acidic substances) can also play an important role^6,7.

One of the possibilities on how to protect metals from corrosion is the use of so-called corrosion inhibitors that make it possible to substantially slow down (inhibit) corrosion processes⁸. The selection of corrosion inhibitors depends on the type of corrosive environment, the presence of corrosion stimulators, and particularly on the mechanism of a given inhibitor. Currently, there is no versatile database or classification available that would enable simple orientation in corrosion inhibitors.

Corrosion environments can be divided into aqueous or non-aqueous, as the intensity and the nature of corrosion processes in these environments differ significantly. For non-aqueous environments, electrochemical corrosion connected with different chemical reactions is typical, whereas only electrochemical corrosion (without other chemical reactions) occurs in aqueous environments. Moreover, electrochemical corrosion is much more intensive in aqueous environments⁹.

In non-aqueous, liquid organic environments, corrosion processes depend on the degree of polarity of the organic compounds. This is associated with the substitution of hydrogen in some functional groups by metals, which is connected with the change of the characteristics of the corrosion processes from electrochemical to chemical, for which lower corrosion rates are typical in comparison with electrochemical processes. Non-aqueous environments typically have low values of electrical conductivity⁹. To increase conductivity in organic environments, it is possible to add so called supporting electrolytes such as tetraalkylammonium tetrafluoroborates or perchlorates. Unfortunately, these substances can have inhibitive properties, or, on the contrary, increase corrosion rates¹⁰.

There are several methods for short-term and long-term testing of corrosion rates of metallic materials or the efficiency of corrosion inhibitors, namely with or without environment circulation, i.e., static and dynamic corrosion test, respectively^{11,12,13,14,15}. For both methods, the calculation of the corrosion rates of metallic materials is based on weight losses of the tested materials over a certain time period. Recently, electrochemical methods are becoming more important in corrosion studies due to their high efficiency and short measurement times. Moreover, they can often provide more information and a more comprehensive view on corrosion processes. The most commonly used methods are electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and the measurement of the stabilization of the corrosion potential in time (in a planar, two-electrode or in a three electrode arrangement)^{16,17,18,19,20,21,22,23}.

Here, we present five methods for the short-term and long-term testing of the corrosion aggressiveness of an environment, the corrosion resistance of metallic materials and the efficiency of corrosion inhibitors. All of the methods are optimized for measurements in non-aqueous environments and are demonstrated on EGBs. The methods allow for obtaining representative and reproducible results, which can help to understand corrosion processes in more detail to prevent and minimize corrosion damages.

For the static immersion corrosion test in metal-liquid systems, static corrosion tests in metal-liquid systems can be performed in a simple apparatus consisting of a 250 mL bottle equipped with a hook for hanging an analyzed sample, see Figure 1.

For the dynamic corrosion test with liquid circulation, metal corrosion inhibitors or the aggressiveness of liquids (fuels) can be tested in a flow apparatus with the circulation of the liquid medium presented in Figure 2. The flow apparatus consists of a tempered part and a reservoir of the tested liquid. In the tempered part, the tested liquid is in contact with a metallic sample in the presence of air oxygen or in an inert atmosphere. The gas (air) supply is ensured by a frit with the tube reaching the bottom of the flask. The reservoir of the tested liquid containing about 400-500 mL of the tested liquid is connected with a reflux cooler that allows for the connection of the apparatus with the atmosphere. In the cooler, the evaporated portion of the liquid is frozen at -40 °C. The peristaltic pump allows for the pumping of the liquid at a suitable rate of about 0.5 Lh^-1 via a closed circuit from chemically stable and inert materials (e.g., Teflon, Viton, Tygon) from the storage part into the tempered part, from which the liquid returns via the overflow into the storage part.

For the static immersion corrosion test with a reflux cooler in the presence of gaseous medium, corrosion inhibitors, the resistance of metallic materials or the aggressiveness of a liquid environment can be tested in the apparatus presented in Figure 3. The apparatus contains two parts. The first part consists of a two-necked, tempered 500 mL flask with a thermometer. The flask contains a sufficient amount of a liquid environment. The second part consists of (i) a reflux cooler with a ground glass joint to achieve a tight connection with the flask, (ii) a hanger for placing the metallic samples and (iii) a frit with a tube for gas (air) supply reaching the bottom of the flask. The apparatus is connected to the atmosphere via the cooler that avoids liquid evaporation.

The apparatus for the electrochemical measurements in the two-electrode arrangement is presented in Figure 4. The electrodes are made from metal sheets (3 x 4 cm, from mild steel), which are completely embedded in epoxide resin on one side to protect them from the surrounding corrosive environment. Both electrodes are screwed to the matrix so that the distance between them is about 1 mm²².

The electrochemical measurements in the three-electrode arrangement consist of working, reference and auxiliary electrodes placed in the measuring cell so that a small distance between the electrodes is ensured; see Figure 5. As the reference electrode, calomel or argent-chloride electrodes with a salt bridge containing either (i) a 3 molL^-1solution of potassium nitrate (KNO₃) or (ii) a 1 molL^-1solution of lithium chloride (LiCl) in ethanol can be used. A platinum wire, mesh or plate can be used as the auxiliary electrode. The working electrode consists of (i) a measuring part (tested material with a screw thread) and (ii) a screw attachment isolated from the corrosion environment, see Figure 6. The electrode must be sufficiently isolated by an anti-underflow seal.

Protocol

1. The Static Immersion Corrosion Test in Metal-Liquid Systems

1. Add 100–150 mL of the tested liquid corrosion environment for testing the resistance of metallic materials or the efficiency of corrosion inhibitors (i.e., aggressive EGB contaminated with water and trace amounts of chlorides, sulfates and acetic acid) into a 250 mL bottle equipped with a hook for hanging an analyzed sample (Figure 1).
2. Adjust the surface of the metallic samples by grinding using sandpaper (1200 mesh) and polishing under running water so that the surface is adjusted evenly. Then, degrease the sample surface thoroughly with about 25 mL of acetone and about 25 mL of ethanol, dry it freely or using pulp tissue, and weigh the sample on an analytical balance with an accuracy of four decimal places.
   NOTE: The sample treatment must always be performed in the same manner, otherwise measurements can be loaded by an error. It is crucial to always use sandpaper with the same grain size and the used sandpapers must be disposable, i.e., one piece of sandpaper for each sample and measurement. The surface must be adjusted evenly, it cannot contain any surface defects such as scratches, pits, etc.
3. After the surface treatment, hang the metallic sample into the liquid in the bottle so that it does not lie on the bottom of the bottle, see Figure 1. Close the bottle tightly enough to prevent liquid evaporation and air entry.
4. Choose the volume of the tested liquid so that the liquid/metal surface ratio is about 10 cm³/1 cm² minimally.
5. At regular intervals, remove the metallic sample from the bottle, rinse it with about 25 mL of acetone, and use pulp tissue to dry it and remove the surface layer of excess corrosion products. Then, weigh the sample on an analytical balance with an accuracy of four decimal places. After weighing, return the sample back into the bottle.
   NOTE: The intervals for removing and weighing the samples should be chosen individually for each tested sample based on a visual evaluation of the changes in the sample surface during the test. Shorter intervals (e.g., 8 h or less) should be applied when intensive surface changes are observed and the intervals can become longer (e.g., 24 h, 48 h) when less intensive or no surface changes are visible. When comparison between the samples is required, the test duration must be the same.
6. From the weight of the metallic sample, calculate the weight loss from the beginning of the experiment related to the sample surface for the given exposure time. After steady state in the metal-liquid system occurs (no increase in the weight loss over time has been observed), terminate the experiment.
7. Calculate the corrosion rate according to the procedure presented in Step 4 (before pickling) or in Step 5 (after pickling of the surface corrosion products).
   NOTE: Corrosion rates obtained after pickling of the surface corrosion products are used for the evaluation of the efficiencies of corrosion inhibitors, for more details, see Representative Results.

2. The Dynamic Corrosion Test with Liquid Circulation

1. Add 500 mL of the tested liquid corrosion environment into the four-necked flask of the storage part of the apparatus. Lubricate the ground glass joints of the flask with a silicone grease and fix (i) a reflux cooler, (ii) a thermometer, (iii) a suction capillary connected to a pump and (iv) the overflow connected to the tempered part on the necks of the flask according to Figure 2.
2. Turn on the cryostat connected to the cooler and set the temperature to -40 °C. Fill the closed cooling circuit with ethanol.
3. Use the capillary for fuel pumping to connect the pump to the preheating spiral of the tempered part, which brings a preheated fuel via the bottom of the measuring cell. Turn on the pump and set the desired fuel flow rate (500 mL×h^-1). Turn on the thermostat of the tempered part and set the temperature to the desired value (40 °C).
4. Once the tempered part is filled with fuel and the fuel starts to flow via the overflow part back into the storage flask, open the measuring cell that consists of two parts connected via a ground glass joint and hang the ground, polished, degreased and weighed sample (metal sheet with appropriate proportions) on the hanger.
   NOTE: The sample treatment is performed according to the procedure presented in Step 1.2.
5. Connect the frit to the tube for air supply with a pressure vessel via a pressure regulator and a flowmeter and set the desired gas flow rate on the flowmeter (20–30 mL×min^-1).
6. At regular intervals, remove the metallic sample from the tempered part and follow the instructions presented in Step 1.5.
7. Follow the instructions presented in Steps 1.6 and 1.7.

3. The Static Immersion Corrosion Test with a Reflux Cooler in the Presence of Gaseous Medium

1. Add 200–300 mL of the tested sample (e.g., tested engine oil containing an aggressive E100 fuel) into the tempered flask.
2. Hang a ground, polished, degreased and weighed sample on the hook of the cooler. Lubricate the ground glass joint of the cooler with a silicone grease and fix the cooler into the flask.
   NOTE: The sample treatment is performed according to the procedure presented in Step 1.2.
3. Connect the frit to the tube for the air supply with a pressure vessel via a pressure regulator and a flowmeter and set the desired gas flow rate (80 mL×min^-1) on the flowmeter.
4. Set the temperature to 80 °C on the thermostat for flask tempering and to -40 °C on the cryostat connected to the cooler.
5. After an appropriate period (e.g., 14 days), remove the metallic sample from the apparatus and follow the instructions presented in Step 1.5.
6. Follow the instructions presented in Steps 1.6 and 1.7.

4. Calculation of the Corrosion Rate from Weight Losses

1. From the corrosion losses obtained according to the methods presented in Steps 1-3, calculate the value of the corrosion rate according to Equations 1 and 2.
               (1)
                   (2)
   where n_Pm is the corrosion rate in g·m⁻²·h⁻¹, ρ is the density of the metallic material in g·cm⁻³, Δm is the average weight loss in g, S is the surface area of the metallic material in m², and T is the time (in hours) from the beginning of the test to the removal of the metal plate for measurement.

5. Pickling of the Corrosion Products on the Metal Surface

1. Pickle the corroded samples of mild steel in a 10 wt. % solution of chelaton III at 50 °C for 5 min. Then, remove the sample from the solution, clean it using a brush under running water, rinse it with acetone, dry and weigh it. After that, put the sample back into the chelaton solution and repeat the procedure until a constant weight is obtained.
2. Pickle the corroded samples from brass, bronze or copper in a 10 vol. % solution of sulfuric acid under nitrogen bubbling (to remove dissolved air oxygen) for 1 min. Then, remove the sample from the solution, clean it using a brush under running water, rinse it with acetone, dry and weigh it. After that, put the sample back into the acid solution and repeat the procedure until a constant weight is obtained.

6. Electrochemical Measurements in the Two-Electrode Arrangement

1. Remove the electrode system from the measuring cell, unscrew it, adjust the surface of the electrodes according to the procedure presented in Step 1.2 (without weighing) and then complete the electrode system again.
2. Fill the measuring cell with 80 mL of the tested liquid corrosion environment and close it through the electrode system. Put the whole cell into a grounded Faraday cage. Connect the galvanostat and potentiostat to the electrode system so that one electrode of the system acts as a reference electrode and the second electrode acts as a working and an auxiliary electrode at the same time.
3. In the instrument software, set the sequence containing the open circuit potential measurements (OCP, stabilization of corrosion potential in an open circuit) and the electrochemical impedance spectroscopy (EIS) measurement. The stabilization perform for at least 30 min to minimize the potential change.
4. Undertake the EIS measurements at sufficiently high amplitude according to the conductivity of corrosion environment (fuel).
   NOTE: The lower the fuel conductivity is, the higher amplitude values are needed. For fuels containing more than 80 vol. % of ethanol, choose the amplitude values in the range of 5–10 mV. For fuels containing ethanol in the range of 10–80 vol. %, choose the amplitude values in the range of 10–50 mV. For fuels containing less than 10 vol. % of ethanol, choose the amplitude values in the range of 50–80 mV.
5. Undertake the impedance measurements in a sufficient range of frequencies (1–5 mHz) to be able to evaluate the low- and also high-frequency parts of the spectra.
6. Determine the cell constant K_s for each electrode by measurement in n-heptane, which has a permittivity of about 1.92 according to the following equation:
                   (3)
   where C is the capacitance obtained from the high-frequency part of the impedance spectrum measured in a planar electrode arrangement in the n-heptane-metal system, ε_r is the relative permittivity of n-heptane, and ε₀ is the relative permittivity of the vacuum.
7. Use the obtained cell constant for the calculation of the fuel permittivity ε and for the recalculation of the resistivity R according to the following equations:
                   (4)
                   (5)

7. Electrochemical Measurements in the Three-Electrode Arrangement

1. Adjust the measuring part of the working electrode from the tested metallic material according to the procedure presented in Step 1.2 (without weighing) and screw it onto the electrode extension.
2. Fill the measuring cell with 100 mL of the tested liquid corrosion environment and close it with a cap through which the working electrode from the tested material and the auxiliary electrode from the platinum wire are led. Twist the wire, i.e., auxiliary electrode, evenly around the working electrode. Through the side entry of the cell, insert the reference electrode with a bridge so that it is as close to the working electrode as possible.
   NOTE: Electrodes cannot touch each other.
3. Insert the cell into a grounded Faraday cell and connect the electrodes via a cable system to the galvanostat and potentiostat equipped with the appropriate software.
4. In the software of the used measuring devices, set the measuring sequence containing the measurement of (i) the OCP for a sufficiently long time period (at least 60 min), (ii) the EIS in the range of about 1 MHz–1 mHz at an amplitude value of 5–20 mV and (iii) the polarization characteristics (Tafel scan) in the range of 200–500 mV to the corrosion potential.
5. Calculate the current density j_corr according to the Stern-Geary equation:
                   (6)
                   (7)
   where j_corr is the corrosion current density, b_a and b_k are Tafel constants, and R_p is the polarization resistance estimated from the EIS measurements. Furthermore, calculate the instantaneous corrosion rate from the material weight losses. Determine the material weight losses from the current density from Faraday´s law as follows:
                   (8)
                   (9)
   where m is the mass of the substance in g; I is the current; t is the time; A is the proportionality constant designated as the electrochemical equivalent of the substance, measured in kg·C⁻¹; F is the Faraday constant (9.6485 × 10⁴ C·mol⁻¹); and z is the number of electrons needed to exclude one molecule.²²

8. Calculation of the Efficiency of Corrosion Inhibitors

1. Use the obtained values of polarization resistance or corrosion rate to calculate the efficiency of the corrosion inhibitors according to the following equations:
                   (10)
   or
                   (11)
   where E_f is the efficiency of corrosion inhibitors in %; R_i is the polarization resistance of material; n_I is the corrosion rate of material in a metal-fuel system containing the corrosion inhibitor; R₀ is the polarization resistance; n₀ is the corrosion rate in the metal-fuel system without the corrosion inhibitor.

Results

The above mentioned methods were used to measure the corrosion data of mild steel (consisting of 0.16 wt. % of C, 0.032 wt. % of P, 0.028 wt. % of S and balance F)²² in the environment of ethanol-gasoline blends (EGBs) containing 10 and 85 vol. % of ethanol (E10 and E85), respectively. For the preparation of these EGBs, gasoline in compliance with the requirements of the EN 228 containing 57.4 vol. % of saturated hydrocarbons, 13.9 vol. % of olefins, 28.7 vol. % of...

Discussion

The basic principle of the dynamic test and both static tests is the evaluation of weight losses of metallic samples in metal-corrosion environment (fuel) systems depending on time until steady state is achieved (i.e., no further weight loss occurs). The corrosion rate of the metal in the corrosion environment is calculated from the weight loss and time. The advantage of the long-term static corrosion test (Step 1) is the reliability of the obtained results, the simplicity and l...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
sulfuric acid	Penta s.r.o., Czech Republic	20450-11000	p.a. 96 % CAS: 7664-93-9 http://www.pentachemicals.eu/
acetic acid	Penta s.r.o., Czech Republic	20000-11000	p.a. 99 % CAS: 64-19-7 http://www.pentachemicals.eu/
sodium sulphate anhydrous	Penta s.r.o., Czech Republic	25770-31000	p.a. 99,9 % CAS: 7757-82-6 http://www.pentachemicals.eu/
sodium chlorate	Penta s.r.o., Czech Republic		p.a. 99,9 % CAS: 7681-52-9 http://www.pentachemicals.eu/
demineralized water	-
ethanol	Penta s.r.o., Czech Republic	71250-11000	p.a. 99 %  CAS: 64-17-5 http://www.pentachemicals.eu/
gasoline fractions	Ceská rafinerská a.s., Kralupy nad Vltavou, Czech Republic		in compliance with EN 228 (57.4 vol. % of saturated hydrocarbons, 13.9 vol. % of olefins, 28.7 vol. % of aromatic hydrocarbons, and 1 mg/kg of sulfur)
Aceton	Penta s.r.o., Czech Republic		pure 99 %
Toluen	Penta s.r.o., Czech Republic		pure 99 %
Name	Company	Catalog Number	Comments
Potenciostat/Galvanostat/ZRA
Reference 600	Gamry Instruments, USA		https://www.gamry.com/
1250 Frequency Response Analyser	Solarthrone
SI 1287 Elecrtochemical Interference	Solarthrone
Name	Company	Catalog Number	Comments
Software
Framework 5.68	Gamry Instruments, USA		https://www.gamry.com/
Echem Analyst 5.68	Gamry Instruments, USA		https://www.gamry.com/
Corrware 2.5b	Scribner		http://www.scribner.com/
CView 2.5b	Scribner		http://www.scribner.com/
Zview 3.2c	Scribner		http://www.scribner.com/
MS Excel 365	Microsoft
Name	Company	Catalog Number	Comments
Grinder
Kompak 1031	MTH (Materials Testing Hrazdil)