**2. Experimental**

The initial data consisted of 2 types of porphyrins dissolved KOH, H2SO4 and benzonitrile, as presented bellow, as the first set:

a. 0.2 g of Na4TFP Ac porphyrin (C44H26N4Na4O12S4 × H2O) dissolved in 40 ml 10% KOH, mentioned as system A.

Comparative Study of Porphyrin Systems Used as Corrosion Inhibitors 179

After testing the porphyrin systems by electrochemical studies, the DCTC 600 dry salt spray chamber, (Figure 2) and the ASTM B 117 method (dry salt spray corrosion test), were used. The method establishes the spraying and drying times respectively the spraying and drying frequencies; in our case, the 5% NaCl solution is sprayed for 5 minutes at 35°C, afterwards is dried at 50°C for 55 minutes. The salt solution was prepared using 1 kg of pure NaCl dissolved in 20 liters of distilled water, resulting the 5%

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

i→peak [mA/cm2] 290 190 90 280 ε→pic [mV] 900 750 600 1300 i← peak [mA/cm2] - 60 100 85 ε← peak [mV] - 50 100 50 ε O2 [mV] 1500 1500 1500 1500 ε pas[mV] 1350 900 850 1600 ipas [mA/cm2] 25 8 0 50

i cor [mA/cm2] 0.7666 0.9792 0.6506 0.0718 v cor [mm/year] 8.99 11.48 7.63 0.842

Rp 50.91 129.51 59.57 147.67 C 0.9962 0.9996 0.9997 1.000

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 -

Electrodes Uncoated System A System B System C

Electrodes Uncoated System A System B System C

The exposure time in the salt spray chamber of the probes was 336 hours.

NaCl solution, mentioned earlier.

**3. Results and discussions** 

Parameters

Parameters

Table 2. Results of Tafel test.

Table 1. Results of cyclic voltammograms.

passivation potential; ipas - passivation current.

minutes.


System C presented the best anticorrosive properties.

The **second set** consists of:


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 different coating systems.

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, measured in mm/year.

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

20% Na2SO4 solution was used as base electrolyte.

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

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:

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

After testing the porphyrin systems by electrochemical studies, the DCTC 600 dry salt spray chamber, (Figure 2) and the ASTM B 117 method (dry salt spray corrosion test), were used. The method establishes the spraying and drying times respectively the spraying and drying frequencies; in our case, the 5% NaCl solution is sprayed for 5 minutes at 35°C, afterwards is dried at 50°C for 55 minutes. The salt solution was prepared using 1 kg of pure NaCl dissolved in 20 liters of distilled water, resulting the 5% NaCl solution, mentioned earlier.

The exposure time in the salt spray chamber of the probes was 336 hours.
