**3. Results and discussions**

178 Corrosion Resistance

b. 0.2 g of Na4TFP Ac porphyrin (C44H26N4Na4O12S4 × H2O) dissolved in 40 ml 10% H2SO4,

c. 0.2 g of H2TPP porphyrin (5,10,15,20 tetrakis 4 phenyl-21H,23H) dissolved in

a. 0.2 g of 5,10,15,20 tetrakis(1-methyl-4pyridyl)21H,23H-porphine,tetra-p-fosylate salt dissolved in 40 mL benzonitrile, mentioned from this point forward as **system I** b. 0.2 g of 4,4',4',4'''(porphine-5,10,15,20-tetrayl)-tetrakis (benzeric sulfonic acid) dissolved in the same solvent namely benzonitrile (40mL) mentioned from this point forward as

Various apparatuses were used, like the DCTC 600 salt spray chamber or the Dynamic EIS Voltalab. The results are presented as mm/year corrosion speed, thus evaluating the

The electrochemical studies namely cyclic voltammetry and Tafel curves carried out to test the protective layer were conducted using the PGZ 402 Dynamic EIS Voltalab. For the data acquisition the Voltamaster 4, version 7.08, was used. This specialized software can determine, based on references, from the Tafel test's values, the exact corrosion speed,

The voltammetry measurements, the Tafel tests, were conducted between -1000 and 1000 mV potentials at a sweep rate of 100 mV/s. Before each experiment, the working electrodes were polished with a series of wet sandpapers of different grit sizes (320, 400, 600, 800, 1000 and 1200). After polishing, the carbon-steel electrode are washed with ultrapure water and dried at room temperature and then the active part was immersed in porphyrin solution.

The working electrode is the carbon-steel electrode, (prepared as mentioned earlier) with 0,28 cm2 active surface, (coated or uncoated); platinum counter electrode with 0.8 cm2 active surface and saturated calomel electrode, (SCE), as reference electrode; all of which

The thickness loss and weight loss tests were not conducted, due to the relatively small size

Three electrodes are uncoated/untreated, three electrodes are immersed for 5 minutes in **system I,** three electrodes are immersed for 60 minutes in **system I,** three electrodes are immersed for 5 minutes in **system II,** three electrodes are immersed for 60 minutes in **system II,** three electrodes are coated with anticorrosive paint. The porphyrin systems are dissolved and then applied on the electrodes; the electrodes are immersed in the solution for

To test the corrosion resistance of the porphyrin systems eighteen electrodes were used. For a good repeatability and accuracy the eighteen electrodes are pretreated as follows:

5, respectively for 60 minutes, thus simulating a shorter and a longer exposure time.

connected to the PGZ 402 Dynamic EIS Voltalab, from Radiometer Copenhagen.

20% Na2SO4 solution was used as base electrolyte.

mentioned as system B.

The **second set** consists of:

**system II.** 

different coating systems.

measured in mm/year.

of the electrodes.

benzonitrile, mentioned as system C.

System C presented the best anticorrosive properties.

Regarding the electrochemical studies of the corrosion resistance of the protective layers formed from the first set of porphyrin it has been demonstrated that the electrodes which have been treated with the system C (0.2 g of H2TPP porphyrin (5,10,15,20 tetrakis 4 phenyl-21H,23H) disolved in 40 mL benzonitrile) gave the best results. The immersion time was 5 minutes.


Table 1. Results of cyclic voltammograms.


Table 2. Results of Tafel test.

The notations from the table: i→peak – peak current density for anodic polarization; ε→peak – peak potential for anodic polarization; i←peak - peak current density for cathodic polarization; ε←peak - peak potential for cathodic polarization; εO2 - oxygen generation potential; ε pas passivation potential; ipas - passivation current.

Comparative Study of Porphyrin Systems Used as Corrosion Inhibitors 181

Fig. 5. Cyclic voltammogram of uncoated electrodes.

Fig. 6. Tafel tests of uncoated electrodes.

Fig. 7. Cyclic voltammogram of electrodes immersed for 5 minutes in system I.

Continuing with the tests, after we have demonstrated that porphyrin (5,10,15,20 tetrakis 4 phenyl-21H,23H) had the best results, we have realized a study to compare this porphyrin with other two types of porphyrins, namely: 5,10,15,20 – tetrakis(1-methyl-4pyridyl)21H,23H-porphine,tetra-p-fosylate salt and 4,4',4'',4'''(porphine-5,10,15,20-tetrayl) tetrakis(benzeric sulfonic acid); also used as organic inhibitors.

From the obtained voltammograms were determined the anodic ipeak and ε peak and from the Tafel curves were determined the corrosion current, polarisation resistance (Rp), corrosion rate and the correlation coefficient.

We continued the studies, cyclic voltammetry and Tafel method for carbon steel electrodes treated in different ways.

Fig. 3. Cyclic voltammograms of coated electrode with system C.

Fig. 4. Tafel tests of coated electrode with system C.

Continuing with the tests, after we have demonstrated that porphyrin (5,10,15,20 tetrakis 4 phenyl-21H,23H) had the best results, we have realized a study to compare this porphyrin with other two types of porphyrins, namely: 5,10,15,20 – tetrakis(1-methyl-4pyridyl)21H,23H-porphine,tetra-p-fosylate salt and 4,4',4'',4'''(porphine-5,10,15,20-tetrayl)-

From the obtained voltammograms were determined the anodic ipeak and ε peak and from the Tafel curves were determined the corrosion current, polarisation resistance (Rp), corrosion

We continued the studies, cyclic voltammetry and Tafel method for carbon steel electrodes

tetrakis(benzeric sulfonic acid); also used as organic inhibitors.

Fig. 3. Cyclic voltammograms of coated electrode with system C.

Fig. 4. Tafel tests of coated electrode with system C.

rate and the correlation coefficient.

treated in different ways.

Fig. 5. Cyclic voltammogram of uncoated electrodes.

Fig. 6. Tafel tests of uncoated electrodes.

Fig. 7. Cyclic voltammogram of electrodes immersed for 5 minutes in system I.

Comparative Study of Porphyrin Systems Used as Corrosion Inhibitors 183

Fig. 11. Cyclic voltammogram of electrodes immersed for 60 minutes in system I.

Fig. 12. Tafel tests of electrodes immersed for 60 minutes in system I.

Fig. 13. Cyclic voltammogram of electrodes immersed for 60 minutes in system II.

Fig. 8. Tafel tests of electrodes immersed for 5 minutes in system I

Fig. 9. Cyclic voltammogram of electrodes immersed for 5 minutes in system II

Fig. 10. Tafel tests of electrodes immersed for 5 minutes in system II

Fig. 8. Tafel tests of electrodes immersed for 5 minutes in system I

Fig. 9. Cyclic voltammogram of electrodes immersed for 5 minutes in system II

Fig. 10. Tafel tests of electrodes immersed for 5 minutes in system II

Fig. 11. Cyclic voltammogram of electrodes immersed for 60 minutes in system I.

Fig. 12. Tafel tests of electrodes immersed for 60 minutes in system I.

Fig. 13. Cyclic voltammogram of electrodes immersed for 60 minutes in system II.

Comparative Study of Porphyrin Systems Used as Corrosion Inhibitors 185

Fig. 17. Cyclic voltammograms of the corrosion process for various electrodes: 1- coated with paint; 2- system I (immersion time 5 minutes); 3- system II (immersion time 5 minute);

Fig. 18. Cyclic voltammograms of the corrosion process for various electrodes: 1- coated with paint; 2- system I (immersion time 60 minutes); 3- system II (immersion time 60

4- uncoated.

minute); 4- uncoated.

Fig. 14. Tafel tests of electrodes immersed for 60 minutes in system II.

Fig. 15. Cyclic voltammogram of electrodes coated with paint.

Fig. 16. Tafel tests of electrodes coated with paint.

Fig. 14. Tafel tests of electrodes immersed for 60 minutes in system II.

Fig. 15. Cyclic voltammogram of electrodes coated with paint.

Fig. 16. Tafel tests of electrodes coated with paint.

Fig. 17. Cyclic voltammograms of the corrosion process for various electrodes: 1- coated with paint; 2- system I (immersion time 5 minutes); 3- system II (immersion time 5 minute); 4- uncoated.

Fig. 18. Cyclic voltammograms of the corrosion process for various electrodes: 1- coated with paint; 2- system I (immersion time 60 minutes); 3- system II (immersion time 60 minute); 4- uncoated.

Comparative Study of Porphyrin Systems Used as Corrosion Inhibitors 187

*E i*

where, ΔE variation of the applied potential around the corrosion potential and Δi is the

Polarization resistance, RP, behaves like a resistor and can be calculated by taking the inverse of the slope of the current potential curve at open circuit or corrosion potential. High RP of a metal implies high corrosion resistance and low RP implies low corrosion resistance. From electrochemical studies it was observed that the electrodes treated with system I in a

For salt spray chamber tests the electrodes were investigated visually; special attention was given to the appearance of the first corrosion signs and a similar or identical evolution of

The electrodes were divided into three categories namely, 3 untreated electrodes, 3 electrodes painted with anti-corrosion paint and 12 electrodes treated according with system



The brown coloration, after 48 hours, intensifies, the electrode's surface becomes more

Specific symptoms appear after 120 hours, that is uniform corrosion throughout the

Corrosion progresses, symptoms are increasing after 192 hours. Rust formed is still

After 192 hours localized corrosion can be seen as brown spots on the surface of the

After 24 hours there is a brown coloration on the entire surface of the electrode

After 24 and 48 hours respectively, there are no reported signs of corrosion

other three electrodes with an immersion time of 60 minutes.

other three electrodes with an immersion time of 60 minutes.

After 264 hours, the rust layer becomes more voluminous

Only after 120 hours, there is loss of the initial gloss paint

 

*E* 0

These can be obtained from a *Tafel* plot or estimated from the experimental data.

Rp=

The polarization resistance or RP is defined by the following equation:

resulting polarization current.

corrosion rates has resulted.

I and system II.

Visual observations: Untreated electrodes:

rough

adherent

Painted electrodes:

electrodes

surface of the electrode

After 336 no major changes occur.

immersion time of 5 minutes gives better results.

These 12 fall into four subcategories, namely:

Fig. 19. Cyclic voltammograms of the corrosion process for various electrodes: 1- coated with paint; 2- system I (immersion time 5 minutes); 3- system I (immersion time 60 minute); 4- system II (immersion time 5 minute); 5- system II (immersion time 60 minute); 6 uncoated

Cyclic voltammograms and Tafel tests for 5 minutes and 60 minutes immersion time are presented in Table 3.


Table 3. Results obtained from cyclic voltammograms and Tafel tests.

The notations from the table:ipeak – peak current density; εpeak – peak potential; i cor – corrosion current density; v cor – corrosion rate; C – correlation coefficient; Rp – polarization resistance

Polarization resistance can be related to the rate of general corrosion for metals at or near their corrosion potential, Ecorr.

These can be obtained from a *Tafel* plot or estimated from the experimental data.

The polarization resistance or RP is defined by the following equation:

$$\mathbf{R}\_{\mathbf{P}} = \left(\frac{\Delta E}{\Delta i}\right)\_{\Delta E \to 0}$$

where, ΔE variation of the applied potential around the corrosion potential and Δi is the resulting polarization current.

Polarization resistance, RP, behaves like a resistor and can be calculated by taking the inverse of the slope of the current potential curve at open circuit or corrosion potential. High RP of a metal implies high corrosion resistance and low RP implies low corrosion resistance.

From electrochemical studies it was observed that the electrodes treated with system I in a immersion time of 5 minutes gives better results.

For salt spray chamber tests the electrodes were investigated visually; special attention was given to the appearance of the first corrosion signs and a similar or identical evolution of corrosion rates has resulted.

The electrodes were divided into three categories namely, 3 untreated electrodes, 3 electrodes painted with anti-corrosion paint and 12 electrodes treated according with system I and system II.

These 12 fall into four subcategories, namely:


Visual observations:

186 Corrosion Resistance

Fig. 19. Cyclic voltammograms of the corrosion process for various electrodes: 1- coated with paint; 2- system I (immersion time 5 minutes); 3- system I (immersion time 60 minute);

Cyclic voltammograms and Tafel tests for 5 minutes and 60 minutes immersion time are

Immersion time (60 minutes)

[mA/cm2] 337.9 30.68 160.2 279.5 51.67 1.541 εpeak [mV] 0.5178 0.2219 0.2172 0.3747 0.2824 0.21

[mA/cm2] 0.7791 0.6062 0.5516 0.3983 0.4394 0.7689

[mm/year] 91.37 7.109 6.468 4.670 5.153 9.18 Rp 34.62 68.15 59.81 76.07 93.97 46.42 C 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000

The notations from the table:ipeak – peak current density; εpeak – peak potential; i cor – corrosion current density; v cor – corrosion rate; C – correlation coefficient; Rp – polarization resistance Polarization resistance can be related to the rate of general corrosion for metals at or near

Electrodes

System I System II

Immersion time (5 minutes)

Coated with paint

Immersion time (60 minutes)

4- system II (immersion time 5 minute); 5- system II (immersion time 60 minute); 6-

uncoated

presented in Table 3.

Uncoated

Immersion time (5 minutes)

Table 3. Results obtained from cyclic voltammograms and Tafel tests.

Parameters

ipeak

i cor

v cor

their corrosion potential, Ecorr.

Untreated electrodes:


Painted electrodes:


**9** 

*Poland* 

**Properties of Graphite Sinters** 

**for Bipolar Plates in Fuel Cells** 

*1Department of Energy Engineering, Czestochowa University of Technology, 2Institute of Materials Engineering, Czestochowa University of Technology,* 

Fuel cell is an electrochemical device which transforms chemical energy stored in fuel directly into electrical energy. The only by-products of this conversion are water and heat. The factors which affect the intensity of electrochemical processes include properties of the materials used for fuel cell components and its working environment. Due to insignificant emissions of pollutants during energy production combined with high efficiency of these generators, and silent operation, fuel cells are an alternative to technologies of energy

 Studies on fuel cells today focus on extending their life, limitation of weight and size, and reduction of costs of manufacturing generators. Individual cell is composed of membrane/electrolyte and electrodes at both sides of MEA (membrane electrode assembly) (Fig. 1). The whole component is closed at both sides with bipolar or monopolar plates/interconnectors. Bipolar plates are the key components of generators since they take 80% of weight and 45% of costs of the cell [1].The task of the plates is to evenly distribute the fuel and air, conduct electricity between adjacent cells, transfer heat from the cell and

According to DOE (the U.S. Department of Energy), basic requirements for materials for bipolar plates in fuel cells include in particular **corrosion resistance under fuel cell's operating conditions, low contact resistance, suitable mechanical properties, high thermal and electrical conductivity, low costs of manufacturing** [2]. Due to high material and functional requirements, few materials can meet these conditions. Bipolar plates in fuel cells are typically made of non-porous graphite because of its high corrosion resistance [3]. However, low mechanical strength of graphite and high costs connected with processing of graphite elevate the costs of manufacturing of fuel cells. Obtaining graphite-based composites modified with steel will allow for obtaining the material with improved mechanical properties, ensuring suitable corrosion resistance and high thermal and electrical conductivity at the same time. The method of powder metallurgy, which allows for obtaining even complicated shape of components, eliminates the problem of mechanical

**1. Introduction** 

production from fossil fuels.

processing of graphite [4].

prevent from gas leakage and excessive cooling.

Renata Wlodarczyk1, Agata Dudek2, Rafal Kobylecki1 and Zbigniew Bis1


Electrodes treated as system I:


Electrodes treated as system II:

