C.
|
Conc.
|
Rp, Ω cm2
|
Cdl, μF cm−2
|
θ
|
% IE
|
|
Blank
|
77.7
|
105.25 ± 1.731
|
—
|
—
|
H4
|
1 × 10−6
|
123.2
|
85.05 ± 2.1254
|
0.369
|
36.92
|
2 × 10−6
|
148.2
|
79.47 ± 2.0215
|
0.476
|
47.56
|
5 × 10−6
|
286.8
|
72.30 ± 2.0924
|
0.729
|
72.90
|
10 × 10−6
|
298.9
|
54.40 ± 2.2364
|
0.740
|
74.00
|
15 × 10−6
|
338.5
|
42.99 ± 2.423
|
0.770
|
77.04
|
20 × 10−6
|
461.8
|
40.50 ± 2.0612
|
0.832
|
83.17
|
H5
|
1 × 10−6
|
147
|
94.88 ± 2.1423
|
0.471
|
47.13
|
2 × 10−6
|
164.5
|
89.95 ± 2.1932
|
0.528
|
52.75
|
5 × 10−6
|
215.7
|
76.53 ± 2.12764
|
0.640
|
63.97
|
10 × 10−6
|
393.4
|
63.60 ± 2.1135
|
0.802
|
80.24
|
15 × 10−6
|
411.7
|
58.20 ± 2.16349
|
0.811
|
81.12
|
20 × 10−6
|
545.9
|
42.00 ± 2.2218
|
0.858
|
85.76
|
H6
|
1 × 10−6
|
123.2
|
79.51 ± 2.3160
|
0.321
|
36.94
|
2 × 10−6
|
148.3
|
72.78 ± 2.1147
|
0.495
|
47.32
|
5 × 10−6
|
286.4
|
56.77 ± 2.0077
|
0.751
|
72.15
|
10 × 10−6
|
317.8
|
51.46 ± 2.16831
|
0.762
|
75.25
|
15 × 10−6
|
338.4
|
48.81 ± 2.1734
|
0.784
|
77.01
|
20 × 10−6
|
653.7
|
43.33 ± 2.1635
|
0.891
|
88.45
|
3.4 Atomic force microscope (AFM) examination
AFM gives microscopic photos for carbon steel surface topography perfectly, which assess the roughness of the examined metal. The 3D AFM morphologies for pure carbon steel outer surface and carbon steel in 1 M HCl in the absence and existence of (H4 & H5 & H6) for 24 hours have appeared in Fig. 10. The photograph of carbon steel outer surface in 1 M HCl has a larger roughness (993.8 nm) than the free carbon steel sample (17.5 nm), which clarifies that the carbon steel blank sample is severely corroded because of the corrosive attacks. The obtained roughness of inhibited carbon steel as shown in Table 8 and Fig. 10 was reduced to low values (160.3 nm in H4, 279.9 nm in H5 & 134.2 nm in H6) because of the effectiveness of the adsorbed layer of inhibitors on the outer surface, hence impeding the corrosion of carbon steel.41
|
|
Fig. 10 AFM 3d photos of: (a) carbon steel free surface, (b) carbon steel in 1 M HCl only, (c) carbon steel in 1 M HCl + 20 × 10−6 M of H4, (d) carbon steel in 1 M HCl + 20 × 10−6 M of H5, (e) carbon steel in 1 M HCl + 20 × 10−6 M of H6 (after 24 hours of immersion).
|
|
Table 8 Roughness of all samples that appeared through atomic force microscope (AFM) examinations
Sample
|
Roughness (nm)
|
Free
|
17.5
|
Blank
|
993.8
|
H4
|
160.34
|
H5
|
279.9
|
H6
|
134.2
|
3.5 FT-IR spectroscopy analysis
FT-IR spectroscopy shows the functional groups of the solutions and its behavior on the metal surface after adsorption, with high precision.42 From Fig. 11–13 which concern (H4) inhibitor, the FTIR charts could be interpreted as illustrated in Table 9. Fig. 11–13 illustrate FT-IR spectra of pure inhibitors liquid and the layer formed on carbon steel samples after putting in 1.0 M HCl for a day in the presence of 20 × 10−6 M of (H4) when comparing the spectra of inhibitor solution with the spectra of the carbon steel surface after immersion, the two spectra have the same properties, which mean that the compounds were adsorbed on the carbon steel surface.18 The obtained results illustrate the mechanism of interference between (H4 & H5 & H6) and carbon steel surface. The shifting and missing in the spectra after immersion showed that the interaction between (H4 & H5 & H6) and carbon steel surface was happened through functional groups mentioned in Table 9.
|
|
Fig. 11 IR spectra of 20 × 10−6 M of compound (H4) solution at 25 °C.
|
|
|
|
Fig. 12 IR spectra of carbon steel surface after 3 hours immersion in 20 × 10−6 M of compound (H4) at 25 °C.
|
|
|
|
Fig. 13 Combined IR chart of pure solution and carbon steel surface after 3 hours immersion in 20 × 10−6 M of compound (H4) at 25 °C.
|
|
Table 9 IR spectra of (H4 & H5 & H6) pure solutions and the spectra of the metal surface after inhibitors adsorption
Compound
|
Pure solution beaks & frequencies (cm−1)
|
Frequencies refer to
|
Shifting and missing of beaks & frequencies (cm−1) after adsorption
|
H4
|
3359
|
OH, N–H stretching
|
3636
|
2974, 2928 and 2884
|
(CH3) and (C–H) extending
|
One beak at 2965
|
1666
|
(C 0) attached to NH
|
1720
|
1047
|
(C–O) stretch
|
1169
|
1387
|
(C–H)
|
1511
|
881
|
( CH2, C–H)
|
832
|
H5
|
3349
|
OH, N–H stretching
|
Missed
|
2974, 2928 and 2883
|
(CH3) and (C–H) extending
|
(3037, 2969, 2882)
|
1668
|
(C 0) attached to NH
|
1720
|
1047
|
(C–O) stretch
|
1169
|
1386
|
(C–H)
|
1298
|
881
|
( CH2, C–H)
|
831
|
H6
|
3344
|
OH, N–H stretching
|
Missed
|
(2974, 2928 and 2883)
|
(CH3) and (C–H) extending
|
(3036, 2965, 2876)
|
1668
|
(C 0) attached to NH
|
1720
|
1047
|
(C–O) stretch
|
1169
|
1385
|
(C–H) holding
|
1320
|
881
|
( CH2, C–H)
|
831
|
3.6 X-ray photoelectron spectroscopy (XPS) examination
It is a perfect system that can predict the adsorbed atoms on the metal surface. The XPS examination of H4 was mainly prospected for definite atoms such as (C, O, N and Fe), the obtained results are shown in Fig. 14 for carbon steel after immersion in 1 M HCl with 20 × 10−6 M of (H4) at 25 °C for 24 hours. Analysis of the obtained data43,44 for the three inhibitors were summarized in Table 10.
|
|
Fig. 14 XPS graphs of (a) XPS device, (b) general survey, (c) C 1s scan (d) O 1s scan (e) N 1s scan (f) Fe 2p scan of carbon steel after immersion in 1 M HCl + 20 × 10−6 M of (H4) inhibitor for 24 h.
|
|
Table 10 Binding energies of different surveys and its expected bonds
C.
|
Scan type
|
Binding energies peaks (eV)
|
Peak refers to
|
H4
|
C 1s
|
284.5
|
C–C
|
287.1
|
–C O
|
285
|
C–N
|
O 1s
|
530.1
|
O2− (Fe2O3 mainly)
|
531.5
|
OH− of FeOOH
|
532.45
|
O2 of adsorbed Water.
|
N 1s
|
398.2
|
N–Fe
|
403
|
Protonated nitrogen atoms of hydrazine group
|
Fe 2p
|
712.8
|
FeCl3
|
710.6
|
Fe2O3/Fe3O4/FeOOH
|
720
|
Ferric compounds satellites
|
H5
|
C 1s
|
284
|
C–C
|
288.5
|
–C O
|
286.4
|
C–N
|
O 1s
|
530.1
|
O2− (Fe2O3 mainly)
|
531.88
|
OH− of FeOOH
|
532.5
|
O2 of adsorbed water
|
N 1s
|
398.2
|
N–Fe
|
401
|
Protonated nitrogen atoms of hydrazine group
|
Fe 2p
|
720
|
Ferric compounds satellites
|
710
|
Fe2O3/Fe3O4/FeOOH
|
713
|
FeCl3
|
H6
|
C 1s
|
284.9
|
C–C
|
288.2
|
–C O
|
286.7
|
C–N
|
O 1s
|
529.8
|
O2− (Fe2O3 mainly)
|
531.1
|
OH− of FeOOH
|
533
|
O2 of adsorbed water
|
N 1s
|
399
|
N–Fe
|
403.3
|
Protonated nitrogen atoms of hydrazine group
|
Fe 2p
|
706.2
|
Fe0
|
710.1
|
Fe2O3/Fe3O4/FeOOH
|
713.2
|
FeCl3
|
720
|
Ferric compounds satellites
|
XPS technique was used to investigate the composition of the organic adsorbed layer on the carbon steel surface in normal hydrochloric medium by investigated inhibitors. In this way, the high-resolution peaks for C 1s, O 1s, N 1s and Fe 2p for carbon steel surface after 24 h of immersion in 1 M HCl solution containing 20 × 10−6 M of inhibitor could be measured. All XPS spectra contained complex forms, which were assigned to the corresponding species through a deconvolution fitting procedure (a non-linear least squares algorithm with a Shirley base line and a Gaussian–Lorentzian combination). All mentioned groups and bonds are found in the investigated inhibitors, so the experiment elucidated the adsorption of the investigated inhibitors on the metal surface. To illustrate data that collected in Table 10 we will take an example of Fe 2p3/2 of inhibitor H6. The deconvolution of the high-resolution Fe 2p3/2 XPS spectrum divided to four peaks. These peaks referred to iron in environments associated with iron oxide and hydroxide. Indeed, the first peak located at 706.2 was assigned to metallic iron (Fe0). The second peak at a BE ∼710.1 eV assigned to Fe3+ was attributed to ferric compounds such as Fe2O3 (i.e., Fe3+ oxide) and/or Fe3O4 (i.e., Fe2+/Fe3+ mixed oxide) and FeOOH (i.e., oxyhydroxide), while that located at around 713.2 eV is attributed to the presence of a small concentration of FeCl3 on the metal surface. The last peak, observed at 720 eV is probably ascribed to the satellites of the ferric compounds.45
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