DEGREE PROJECT IN MATERIALS SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS
STOCKHOLM, SWEDEN 2020
Sustainable corrosion protection
for metallic materials by Mussel
adhesive pr
o
tein modified lignin
film
DI WANG
KTH ROYAL INSTITUTE OF TECHNOLOGY
Abstract
Lignin has the potential to be used as green material to inhibit the corrosion of metallic substrates. Mussel adhesive protein is used to modify lignin due to its great adhesive and film forming abilities. Electrochemical impedance spectroscopy (EIS) has been applied to in-situ measure the corrosion resistance of the formed surface composite films in the corrosive environment. The equivalent circuit is used to fit the EIS data to obtain the quantitative results of the surface films. The results show that MAP modified lignin composite film can provide enhanced corrosion protection to the carbon steel substrate and presents self-healing property.
Sammanfattning
Lignin har potential att användas som grönt material för att hämma korrosion av metalliska underlag. Mussellhäftande protein används för att modifiera lignin på grund av dess stora lim- och filmbildningsförmåga. Elektrokemisk impedansspektroskopi (EIS) har applicerats för att mäta korrosionsbeständigheten hos de bildade ytkompositfilmerna i den korrosiva miljön. Ekvivalentkretsen används för att passa EIS-data för att erhålla kvantitativa resultat från ytfilmerna. Resultaten visar att MAP-modifierad ligninkompositfilm kan ge förbättrat korrosionsskydd för kolfastsubstratet och uppvisar självhelande egendom.
Table of Contents
Abstract ... 1
Sammanfattning ... 2
1.Introduction... 1
1.1.Green corrosion inhibitors ... 1
1.2.Mussel adhesive protein ... 2
1.3.Lignin... 2
1.4.Methodology ... 4
1.4.1.Electrochemical impedance spectroscopy ... 4
1.4.2.Equivalent circuit ... 6
1.4.3.Time constant of EIS ... 7
1.4.4.Data processing and analysing of EIS ... 7
2.Experimental ... 9
2.1.Materials ... 9
2.1.1.Metal substrate ... 9
2.1.2.Mussel adhesive protein ... 9
2.1.3.Lignin ... 9
2.1.4.Solution ... 9
2.2.Film formation... 10
2.3.EIS measurement ... 11
3.Result and discussion ... 12
3.1.Optical observation ... 12
3.2.Corrosion tests ... 13
3.2.1.The MAP/lignin composite films formed with one-step immersion. 13 3.2.2. The multilayer MAP/lignin composite films formed with alternative immersion ... 15
3.2.3.The self-crosslinked MAP/lignin composite films ... 19
3.2.4.Social and ethical reflection ... 23
4.Conclusion ... 24
1 1. Introduction
Corrosion is widely occurred and results in huge losses of materials. Steel corrosion is a major type of corrosion and causes mass loss of steel products, which leads to security risks and economic costs.
Steel corrosion is typically due to the corrosive liquid layer adsorbed on the metal surface. The electrochemical reactions occur between the metal and oxidant that is usually acid or the oxygen dissolved in a liquid layer. Metal atoms lose electrons and oxygen gains electrons, eventually form corrosion products on the metal surface. To reduce the losses caused by metal corrosion, many different approaches have been taken, including improving the composition of metallic materials, adding corrosion inhibitors, using electrochemical methods, etc.
Corrosion inhibitor is defined as the chemical compounds that could decrease the corrosion rate by reducing the reaction rate between metal and oxidant or forming the film to isolate the metal with the oxidant. The effect of traditional corrosion inhibitors is significant. However, the ingredients in traditional inhibitors are sometimes toxic and harmful to human or environment, for example, chromates, phosphate [1]. Thus, it is important to find more environmentally friendly and effective corrosion inhibitors. This study was carried out in order to verify whether lignin has the potential to work as corrosion inhibitor and test its inhibition efficiency, and to explore the improvement of film forming by introducing mussel adhesive protein into lignin.
1.1. Green corrosion inhibitors
Biological inhibitors, vegetable inhibitors and rare-earth as water-soluble inhibitors are promising green corrosion inhibitors.
Biological inhibitors are obtained from biological tissues. In addition to mussel adhesive protein, there are other biological molecules that can be used as corrosion inhibitors. Chitosan, a low cost, renewable marine polymer, which mean source is the shells of crustaceans. Chitosan can be used as anode inhibitor in acid medium to inhibit corrosion of stainless steel and work as mixed inhibitor to inhibit the corrosion of copper in acid solution [2, 3].
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inhibitor to inhibit the corrosion of aluminum alloy in seawater, while caffeic acid can inhibit the corrosion of iron, copper and low carbon steel in acid medium [6-9].
The development of rare earth as corrosion inhibitor is relatively slow. Lanthanide salts (Ce3+, Pr3+, Nd3+) have been used as corrosion inhibitors of aluminum alloy, zinc, galvanized steel and iron-based alloy in aqueous environment due to their good effect and low toxicity [10-13].
1.2. Mussel adhesive protein
Researchers are finding new inhibitors which are friendly to the environment and human, Mytilus Edulis foot protein 1 (mefp-1) is one of the mussel adhesive protein (MAP) and has potential to work as a new generation corrosion inhibitor. MAP is the protein extracted from blue mussels, by which mussels attach themselves onto surfaces. At least 6 mefps have been extracted and identified so far, from mefp-1 to mefp-6 [14]. The reason for using mefp-1 is that it was the first protein to be purified and the most studied protein in mefps. It has a large molecular weight (108 kDa) and contains a high level (up to 15mol%) of di-hydroxyphenylalanine (Dopa) functional group [15, 16].
Figure 1 Dopa functional group [17]
Dopa is the primary functional group in mefp-1, which has unique adhesive and cohesive properties. Previous research has developed mefp-1 into a film-forming corrosion inhibitor with the cooperation of nanoparticles or 2D materials. The composite films present enhanced corrosion resistance and anti-wear properties to the carbon steel[18]. The formed Mefp-1 composite film can interact with the corrosion products of Fe3+ or iron oxides, forming complexes via functional groups of Dopa, and thus, presents the improved corrosion resistance and the self-healing property [15, 19]. 1.3. Lignin
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cellulose. Lignin accounts for 25–30% of non-fossil organic molecules on earth [20]. Lignin is important to vascular plants, the mean function of lignin in plants is to cement the cellulose fibers together: The plant cell wall consists of four hollow tubes with different layers [20, 21]. In each layer of cell wall, cellulose is embedded in lignin and hemicellulose matrix [22]. The production of lignin is more than 50 million tons/year from International Lignin Institute. Coniferyl, sinapyl and p-coumaryl alcohol are three types of phenylpropane units which form the lignin as biosynthesis [23-25]. These units are depicted in Figure 2.
Figure 2 Three basic units of lignin [26]
According to the structural unit composition of lignin, lignin can be divided into three categories: soft wood, hard wood and grass lignin [27]. Typical soft wood lignin consists of coniferyl alcohol units. Hard wood lignin consists of coniferyl and sinapyl alcohol units. Grass lignin consists of coniferyl, sinapyl and p-coumaryl units [21, 27].
Figure 3 chemical bonds between lignin units [28]
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lignin units. Due to the exist of these carbon-carbon bonds and carbon-oxygen bond, it is possible to crosslink lignin, and the rigidity of lignin structure will change with the degree of substitution. The dominant units of different lignin are different. Therefore, soft wood lignin is easy to branch and cross link, while hard wood lignin is more linear [21].
In nature, lignin is hydrophobic, so the cell wall is impermeable. This property of lignin is also conducive to its application in corrosion resistance. Lignin is an environmentally friendly biopolymer, which does not contain any toxic components. Recently, reports show that alkali lignin could be used as a corrosion inhibitor in acid solution, and the higher lignin concentration leads to higher inhibition efficiency [29, 30]. Alkali lignin is isolated by alkali and precipitated with mineral acids and has potential complexation sites and large surface area, which can interact with metal ions, for example, iron [31, 32]. The molar mass of alkali lignin is very close to the molecule of soft wood lignin [21]. Lignin could absorb on the metal surface with metal hydroxide film, and lignin film could protect against water and oxygen to protect metal [32]. Lignin contains phenolic hydroxyl functional groups and has the potential to react with Dopa, so it is used to crosslink with mefp-1 to improve corrosion resistance.
1.4. Methodology
1.4.1. Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) is a non-destructive technique that is useful for corrosion monitoring. For a stable electrochemical linear system, if a sine wave is used as the input disturbance signal (current or potential), the system will output a sine wave signal (potential or current) accordingly, both input and output signals have the same angular frequency ω. The relationship between input (X) and output (Y) is:
𝑌𝑌 = 𝐺𝐺(𝜔𝜔)𝑋𝑋 �1�
G is a function of frequency, which reflects the frequency response of the system and is determined by the internal structure of the system. Information on the internal structure of the linear system can be obtained from G.
Three basic conditions must be met to ensure that the disturbance and the response of the system are sine wave signals with the same angular frequency: causality, linearity and stability.
Causality
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the response signal, ensuring that the relationship between disturbance and response is the only cause and effect relationship.
Linearity
When the amplitude of the sine wave of the potential signal is small, the current density and potential of the electrode process can be approximated as a linear relationship. Therefore, in order to meet the linear condition, the amplitude of the sine wave of the potential during the EIS measurement should not exceed 25 mV.
Stability
The stability requires that the disturbance does not cause the internal structure change of the system. When the disturbance stops, the system should return to its previous state. G is a vector varies with frequency and is represented by a complex function with variable f or angular frequency ω:
𝐺𝐺(𝜔𝜔) = 𝐺𝐺′(𝜔𝜔) + 𝑗𝑗𝐺𝐺′′(𝜔𝜔) �2�
Where 𝑗𝑗 = √−1, 𝐺𝐺′ is the real part of immittance and 𝐺𝐺′′ is the imaginary part of
immittance.
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Figure 4 A typical Nyquist plot (a) and corresponding Bode plot (b) 1.4.2. Equivalent circuit
For an electrode process, using electrical components to form a circuit that has same impedance spectrum with measured electrochemical impedance spectroscopy, then this circuit is the equivalent circuit of the electrode process. There are four main components in the equivalent circuit: resistance (R), capacitance (C), inductance (L), and constant phase element (CPE) (Q). These components are composed of series and parallel connections to simulate the processes that occur in the electrochemical system. The impedance behaviour is similar to that of the real electrochemical system and help to analyse real electrochemical phenomena.
The characteristics of resistance, capacitance and inductance in electrochemical impedance spectroscopy are the same as normal electrical components. The electric double layer between the electrode and the solution is generally expressed by an equivalent capacitance, but due to the dispersion effect, there are deviates between the impedance behaviour of the electric double layer and the capacitance. In order to
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describe this phenomenon a CPE was applied. The impedance of CPE is
𝑍𝑍 = 𝑌𝑌1
0∙ (𝑗𝑗𝜔𝜔)
−𝑛𝑛 �3�
CPE has two parameters, Y0 and n. The unit of parameter Y0 is Ω-1∙cm-2∙s-1, which is used to represent the capacitance part in CPE. Another parameter n is a dimensionless constant, sometimes called dispersion coefficient. n is generally in the range of 0 to 1. If n is 0, CPE is a pure resistance; if n is 0, CPE is a pure capacitance; and if n is -1, CPE is pure inductance.
1.4.3. Time constant of EIS
The process that the state variable deviates from the steady-state value after receiving the disturbance and returns to the original steady-state value after the disturbance disappears is called the relaxation process. The relaxation process of a state variable can be characterised by a time characteristic quantity τ (s), which is called the time constant of the relaxation process of the state variable. The larger the value of time constant means slower relaxation process. According to the definition of the time constant, the number of the time constant corresponds to the number of state variables. Observing the φ-lgf Bode plot of EIS is a common method to determine the number of time constants. In general, time constants can be directly distinguished on the impedance spectrum. In addition to the peak or half peak caused by solution resistance (Rs) at high frequency, the number of remaining peaks and troughs is the number of time constants in the impedance spectroscopy.
1.4.4. Data processing and analysing of EIS
The purpose of EIS measurement is to fit the equivalent circuit according to the measurement results. Combined with other methods, the dynamic process and mechanism contained in the electrode system could be speculated. The other purpose is to determine the parameters of the related components through the established equivalent circuit, to estimate the dynamic parameters of the related process.
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the measured value, it is necessary to reexamine the equivalent circuit.
9 2. Experimental
2.1. Materials
2.1.1. Metal substrate
The metal sample used in the experiment is cold rolled steel (CRS) (DC 01, 1.0330, supplied by IVF, Sweden). The chemical composition of the CRS is listed in Table 1. CRS substrate was cut to1.5cm*1.5cm pieces and polish to 800# by silicon carbide abrasive paper.
Table 1 chemical composition of CRS
C Mn P S
Composition/wt.% 0.12 0.60 0.045 0.045
2.1.2. Mussel adhesive protein
Mefp-1 is used as the mussel adhesive protein for the experiment, and the MAP is purchase from Biopolymer AB for Sweden. Mefp-1 is prepared as 2mg/ml solution and refrigerated 4 ℃ for long time storage.
2.1.3. Lignin
Lignin used for this experiment is lignin alkali and supplied by Sigma Aldrich. 2.1.4. Solution
Water used for preparing solution is Milli-Q quality. Before each experiment, mefp-1 and lignin solution are sonicated for 5 minutes to keep the particles in good dispersion.
Mixture of MAP/lignin with different ratio
Dilute mefp-1 solution from 2 mg/ml to 0.05 mg/ml with Milli-Q water and sonicate for 5 minutes. Prepare lignin solution with the concentration of 0.05 mg/ml, 5 mg/ml and 50 mg/ml, respectively with water. Sonicate for 5 minutes each. Adjust pH of mefp-1 solution to 6 with HCl solution. Mix mefp-mefp-1 and lignin solutions in a volume ratio of 1:1 and sonicate 5 minutes. For Self-crosslinking lignin film, diluting mefp-1 solution from 2 mg/ml to 0.1 mg/ml with water, sonicate for 5 minutes. Prepare the lignin solutions with a concentration of 50 mg/ml and 100 mg/ml and sonicate 5 minutes. Mix mefp-1 and 100 mg/ml lignin solutions in a volume ratio of 1:1 and sonicate 5 minutes to get lignin-MAP mixture solution.
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1:1 solution is slightly lower than other due to its dilute concentration. The pH is adjusted to 6 to reduce self-crosslinking of mefp-1 molecules.
Table 2 Measured pH value of solution with different MAP/Lignin ratio for one-step Map/lignin composite film
MAP/Lignin rate pH
1:1 5.4
1:100 6.8
1:1000 6.9
Table 3 Measured pH value of solution with different MAP/Lignin rate in multilayer absorption
MAP/Lignin rate pH
1:1 5.6
1:100 6.9
1:1000 6.3
2.2. Film formation
Formation of MAP/lignin composite films via one-step immersion:
Immerse the carbon steel sample in the lignin-MAP mixture solutions at the ratio of 1:1, 100:1, and 1000:1, as well as the lignin solution with a concentration of 50 mg/ml, respectively, for 60 minutes to deposit films on the steel surface. The immersion procedures were carried out at room temperature. After the deposition, the sample is gently rinsed with Milli-Q water and dried with N2 gas. The samples were kept in air overnight at room temperature before the EIS test.
Formation of MAP/lignin composite films via alternative immersion:
The carbon steel samples were first immersed for 20 minutes in 0.05 mg/ml mefp-1 solution and then 20 minutes in 0.05 mg/ml, 5 mg/ml and 50 mg/ml lignin solution, respectively. This procedure was repeated four times to ensure sufficient adsorption of the composite film on the sample surface. Between each step, the samples were gently rinsed with Milli-Q water without drying. The immersion procedures were carried out at room temperature. After the adsorption, the samples were rinsed with Milli-Q water, dried with N2 gas, and kept in air overnight at room temperature before the EIS test.
Formation of self-crosslinked MAP/lignin composite film with MAP/lignin
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steel samples until the droplets completely covering the samples. Heat the samples to 220℃ for 6 hours and cool down in the furnace.
Instead of dripping the solution onto the samples, the samples were immersed in lignin-MAP mixture solution. The depth of the solution is approximately 5 mm. The immersed samples were heated at the same condition.
2.3. EIS measurement
12 3. Result and discussion
The macromorphology of lignin and MAP modified lignin films were shown by photos, and then the electrochemical data of different groups were listed and analyzed by equivalent circuit fitting.
3.1. Optical observation
As shown in Figure 5 and 6, the surface of the films getting darker with the increase of lignin content. Moreover, the multilayer film is darker than the single-layer film, which means the adsorption lignin of multilayer is more than single-layer. Figure 7 shows the crosslinked lignin film, dripped operation causes the nonuniform film, while the improved operation could produce a uniform film.
Figure 5 Surface photograph of lignin film with (a)1:1, (b)1:100 and (c)1:1000 MAP/lignin rate
Figure 6 Surface photograph of multilayer adsorption lignin film with (a)1:1, (b)1:100 and (c)1:1000 MAP/lignin rate
(a)
(b)
(c)
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Figure 7 Surface photograph of self-crosslinking lignin film with (a)pure lignin, (b)original operation and (c)improved operation
3.2. Corrosion tests
3.2.1. The MAP/lignin composite films formed with one-step immersion
Figures 8 and 9 show the Nyquist plot and Bode plot comparing the representative EIS spectra of carbon steel with different MAP/lignin composite films formed at 1:1, 1:100, 1:1000 ratio with one-step immersion after 1 hour, 2 days, 4 days and 6 days expose in NaCl solution.
Figure 8 Bode plots for the carbon steel surface coated with MAP/lignin composite film after (a)1h, (b)2d, (c)4d and (d)6d exposure in 0.1 mol/L NaCl solution
(a)
(b)
(c)
0.01 0.1 1 10 100 1000 10000 100 1000 Z (Ω ) Frequency (Hz) 1:1 6d (modulus) 1:100 6d (modulus) 1:1000 6d (modulus) 1:1 6d (phase) 1:100 6d (phase) 1:1000 6d (phase) 0 20 40 60 80 -P ha se (° ) 0.01 0.1 1 10 100 1000 10000 100 1000 Z (Ω ) Frequency (Hz) 1:1 4d (modulus) 1:100 4d (modulus) 1:1000 4d (modulus) 1:1 4d (phase) 1:100 4d (phase) 1:1000 4d (phase) 0 20 40 60 80 -P ha se (° ) 0.01 0.1 1 10 100 1000 10000 100 1000 Z (Ω ) Frequency (Hz) 1:1 2d (modulus) 1:100 2d (modulus) 1:1000 2d (modulus) pure lignin 2d (modulus) 1:1 2d (phase) 1:100 2d (phase) 1:1000 2d (phase) pure lignin 2d (phase)0 20 40 60 80 -P ha se (° ) 0.01 0.1 1 10 100 1000 10000 100 1000 Z (Ω ) Frequency (Hz) 1:1 1h (modulus) 1:100 1h (modulus) 1:1000 1h (modulus) pure lignin 1h (modulus) 1:1 1h (phase) 1:100 1h (phase) 1:1000 1h (phase) pure lignin 1h (phase)
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Figure 9 Nyquist plots for carbon steel with MAP/lignin composite film after (a)1h, (b)2d, (c)4d and (d)6d exposure in 0.1 mol/L NaCl solution
Both Nyquist and Bode plots implied that only one time constant is included in the EIS spectra with lignin film. To quantitatively assess the corrosion resistance with different MAP/lignin rates, electrochemical impedance is analysed by fitting the data with equivalent circuit. The equivalent circuit describes the interface between metal and electrolyte. The simplest circuit, which consists of polarisation resistance (Rp), solution resistance (Rs) and interfacial capacitance (C) in parallel, is used to fit the EIS spectra. The constant phase element (CPE) is used to replace the capacitance because the interfacial capacitance is not ideal. The equivalent circuit is shown in Figure 10 below [33].
Figure 10 Equivalent circuit consists of polarisation resistance (Rp), solution resistance (Rs) and constant phase element (CPE)
The fitting results are summarised in Table 4. Y0 and n are the parameters for CPE, Y0 is a constant corresponding to capacitance, while n is the factor region from 0 to 1, the
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CPE is a pure capacitance when n is 1. Rp, the impedance is used to compare the corrosion resistance. As Table 4 shows, film with MAP/lignin rate 1:1 has highest polarisation resistance after an hour expose. The Rp of the composite film with 1:1000 ratio keeps increasing and after four days expose it has the highest resistance. The lignin film should take some time to become protective. The film with pure lignin has a high resistance at 1 hour, but it decreased rapidly probably because of the desorption of lignin. Table 4 Polarisation resistance and constant phase element data obtained from EIS spectra fitting for different lignin film
MAP/Lignin ratio Time Rp/Ω·cm2 Y0/ Ω-1·cm-2sn n
Pure lignin 1h 1.62 × 103 8.76 × 10-4 0.8 2d 1.02 × 103 1.99 × 10-3 0.7 1:1 1h 1.62 × 103 1.31 × 10-3 0.8 2d 1.74 × 103 1.75 × 10-3 0.7 4d 1.58 × 103 1.78 × 10-3 0.7 1:100 1h 1.33 × 103 1.24 × 10-3 0.8 2d 1.28 × 103 1.51 × 10-3 0.6 4d 1.38 × 103 1.57 × 10-3 0.7 1:1000 1h 1.30 × 103 1.26 × 10-3 0.8 2d 1.37 × 103 2.08 × 10-3 0.7 4d 1.75 × 103 2.03 × 10-3 0.7
3.2.2. The multilayer MAP/lignin composite films formed with alternative immersion
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Figure 11 Bode plots for the carbon steel coated with MAP/lignin composite films after (a)1h, (b)2d, (c)4d, (d)6d, (e)9d and (f)12d exposure in 0.1 mol/L NaCl solution
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Figure 12 Nyquist plots for the carbon steel coated with MAP/lignin composite films after (a)1h, (b)2d, (c)4d, (d)6d, (e)9d and (f)12d exposure in 0.1 mol/L NaCl solution
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Figure 13 The fitted results of the (a) resistance (Rp) and (b) capacitance (Y0 ) values of the
composite films with different MAP/lignin ratio, plotted as a function of time.
Figure 13 is plotted based on Table 5, presenting the change of Rp and Y0 as function of time, respectively. It shows that the lignin film provides a certain inhibition effect during the initial exposure period, and the inhibition increases with time for about one week reaching to a pronounced level. The resistance for the MAP/lignin composite film at 1:1000 ratio provided the lowest corrosion resistance at 1 hour, but it keeps increasing with time during the exposure process. It demonstrated again that the protection of the film takes time. The slight increasing of Y0, which represents the capacitance of the film, indicates that the film becomes more compact during the expose.
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Table 5 Polarisation resistance and constant phase element data obtained from EIS spectra fitting for multilayer absorption lignin film
MAP/Lignin rate Time Rp/Ω·cm2 Y0/ Ω-1·cm-2sn n
1:1 1h (3.08 ± 0.11) × 103 (1.90 ± 0.52) × 10-3 0.7 2d (1.50 ± 0.07) × 103 (1.41 ± 0.19) × 10-3 0.8 4d (1.46 ± 0.14) × 103 (1.39 ± 0.09) × 10-3 0.7 6d (2.06 ± 0.30) × 103 (1.45 ± 0.07) × 10-3 0.7 9d (2.65 ± 0.51) × 103 (1.69 ± 0.01) × 10-3 0.7 12d (2.44 ± 0.38) × 103 (1.98 ± 0.15) × 10-3 0.7 1:100 1h (3.09 ± 0.08) × 103 (1.60 ± 0.72) × 10-3 0.7 2d (1.37 ± 0.12) × 103 (1.70 ± 0.44) × 10-3 0.7 4d (1.50 ± 0.13) × 103 (1.73 ± 0.35) × 10-3 0.7 6d (2.03 ± 0.27) × 103 (1.78 ± 0.35) × 10-3 0.7 9d (2.58 ± 0.08) × 103 (2.02 ± 0.36) × 10-3 0.7 12d (2.70 ± 0.23) × 103 (2.23 ± 0.41) × 10-3 0.7 1:1000 1h (1.46 ± 0.11) × 103 (4.40 ± 1.54) × 10-4 0.8 2d (1.34 ± 0.10) × 103 (1.36 ± 0.14) × 10-3 0.8 4d (1.66 ± 0.08) × 103 (1.31 ± 0.09) × 10-3 0.7 6d (2.09 ± 0.19) × 103 (1.35 ± 0.07) × 10-3 0.7 9d (2.88 ± 0.30) × 103 (1.46 ± 0.07) × 10-3 0.7 12d (2.86 ± 0.31) × 103 (1.61 ± 0.08) × 10-3 0.7 3.2.3. The self-crosslinked MAP/lignin composite films
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Figure 14 Bode plots for the carbon steel coated with MAP/lignin composite films after (a)1h, (b)2d, (c)4d, (d)6d, (e)8d and (f)11d exposure in 0.1 mol/L NaCl solution
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Figure 15 Nyquist plots for the carbon steel coated with MAP/lignin composite films after (a)1h, (b)2d, (c)4d, (d)6d, (e)8d and (f)11d exposure in 0.1 mol/L NaCl solution
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Table 6 Polarisation resistance and constant phase element data obtained from EIS spectra fitting for self-crosslinked MAP/lignin composite film
MAP/lignin rate Time Rp/Ω·cm2 Y0/ Ω-1·cm-2sn n
1:1000 Dripped operation 1h (1.25 ± 0.14) × 103 (1.53 ± 0.12) × 10-3 0.8 2d (2.07 ± 0.27) × 103 (1.52 ± 0.35) × 10-3 0.7 4d (2.03 ± 0.01) × 103 (1.64 ± 0.31) × 10-3 0.7 6d (2.18 ± 0.25) × 103 (1.78 ± 0.39) × 10-3 0.6 8d (2.00 ± 0.23) × 103 (2.04 ± 0.29) × 10-3 0.6 11d (2.06 ± 0.32) × 103 (2.17 ± 0.35) × 10-3 0.6 1:1000 Immersed operation 1h (7.39 ± 0.88) × 102 (2.10 ± 0.34) × 10-3 0.8 2d (1.64 ± 0.06) × 103 (1.65 ± 0.68) × 10-3 0.7 4d (1.89 ± 0.01) × 103 (2.18 ± 0.33) × 10-3 0.7 6d (2.26 ± 0.06) × 103 (2.34 ± 0.41) × 10-3 0.6 8d (1.84 ± 0.04) × 103 (2.47 ± 0.43) × 10-3 0.6 11d (2.01 ± 0.14) × 103 (2.89 ± 0.68) × 10-3 0.6
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Figure 16 (a)Rp and (b)Y0 as a function of time with pure lignin film, original and improved film
3.2.4. Social and ethical reflection
The application of lignin and MAP as corrosion protective films or coatings has no visible negative effect so far. Lignin and MAP are abundant renewable resources, and friendly to the environment and safe for users. And there are great economic prospects of lignin and MAP. As introduced above, the pulp industry is a key element of Sweden’s bioeconomy, and lignin is enriched in the waste of pulp mills. The targeted lignin-based coatings bring added value to the paper industry. Mussel aquaculture is an efficient way to reduce eutrophication of water. The large-scale utilization of MAP will contribute to cleaning of nutrition-rich water.
24 4. Conclusion
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