Author Contributions
Conceptualization, J.H.P.; methodology, O.J.L., J.H.P., D.J.W. and C.B.; validation, O.J.L., J.H.P., D.J.W. and C.B.; formal analysis, O.J.L.; investigation, O.J.L.; resources, J.H.P., D.J.W. and C.B.; data curation, O.J.L.; writing—original draft preparation, O.J.L.; writing—review and editing, O.J.L., J.H.P., D.J.W. and C.B.; supervision, J.H.P., D.J.W. and C.B.; project administration, J.H.P., D.J.W. and C.B. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Chemical structures of (
a) metal acetylacetonate and (
b) metal hexafluoroacetylacetone, where M = Ce or La, modified from [
24].
Figure 1.
Chemical structures of (
a) metal acetylacetonate and (
b) metal hexafluoroacetylacetone, where M = Ce or La, modified from [
24].
Figure 2.
EDX spectra of (a) and (b) MS coupon exposed to 3.5% NaCl solution containing and , (c,d) 304 SS inhibited with and .
Figure 2.
EDX spectra of (a) and (b) MS coupon exposed to 3.5% NaCl solution containing and , (c,d) 304 SS inhibited with and .
Figure 3.
Potentiodynamic polarisation curves of mild steel in 3.5% NaCl solution with (a) MS without inhibitor, (b) , (c) , (d) , and (e) inhibitor.
Figure 3.
Potentiodynamic polarisation curves of mild steel in 3.5% NaCl solution with (a) MS without inhibitor, (b) , (c) , (d) , and (e) inhibitor.
Figure 4.
Potentiodynamic polarisation curves of 304 SS in 3.5% NaCl solution with (a) 304 SS without inhibitor, (b) , (c) , (d) , and (e) inhibitor.
Figure 4.
Potentiodynamic polarisation curves of 304 SS in 3.5% NaCl solution with (a) 304 SS without inhibitor, (b) , (c) , (d) , and (e) inhibitor.
Figure 5.
Optical micrographs of inhibited mild steel in 3.5% NaCl solution doped with at (a) 20 °C, (b) 40 °C, and (c) 60 °C.
Figure 5.
Optical micrographs of inhibited mild steel in 3.5% NaCl solution doped with at (a) 20 °C, (b) 40 °C, and (c) 60 °C.
Figure 6.
Optical micrographs of inhibited 304 SS in 3.5% NaCl solution doped with at (a) 20 °C, (b) 40 °C, and (c) 60 °C.
Figure 6.
Optical micrographs of inhibited 304 SS in 3.5% NaCl solution doped with at (a) 20 °C, (b) 40 °C, and (c) 60 °C.
Figure 7.
SEM micrographs of (a) MS and (b) 304 SS electrode surfaces before exposure in NaCl solution.
Figure 7.
SEM micrographs of (a) MS and (b) 304 SS electrode surfaces before exposure in NaCl solution.
Figure 8.
SEM micrographs of uninhibited (a–c) MS and (d–f) 304 SS sample surfaces obtained after exposure to 3.5% NaCl solution at 20 °C, 40 °C, and 60 °C.
Figure 8.
SEM micrographs of uninhibited (a–c) MS and (d–f) 304 SS sample surfaces obtained after exposure to 3.5% NaCl solution at 20 °C, 40 °C, and 60 °C.
Figure 9.
SEM micrographs of inhibited mild steel sample surfaces with (a–c) and (d–f) obtained after exposure to 3.5% NaCl solutions at 20 °C, 40 °C, and 60 °C.
Figure 9.
SEM micrographs of inhibited mild steel sample surfaces with (a–c) and (d–f) obtained after exposure to 3.5% NaCl solutions at 20 °C, 40 °C, and 60 °C.
Figure 10.
SEM micrographs of 304 SS sample surfaces inhibited with (a–c) and (d–f) obtained after exposure to 3.5% NaCl solutions at 20 °C, 40 °C, and 60 °C.
Figure 10.
SEM micrographs of 304 SS sample surfaces inhibited with (a–c) and (d–f) obtained after exposure to 3.5% NaCl solutions at 20 °C, 40 °C, and 60 °C.
Figure 11.
EDS of (a,b) MS in 3.5% NaCl solution containing 0.5% wt. (m/v) of and and (c,d) 304 SS with and .
Figure 11.
EDS of (a,b) MS in 3.5% NaCl solution containing 0.5% wt. (m/v) of and and (c,d) 304 SS with and .
Figure 12.
IR spectra of REE(β-diketone)3 inhibitors and the corrosion products formed on mild steel and 304 stainless steel: (a) REE β-diketone inhibitor; (b) steel REE β-diketone inhibitor film.
Figure 12.
IR spectra of REE(β-diketone)3 inhibitors and the corrosion products formed on mild steel and 304 stainless steel: (a) REE β-diketone inhibitor; (b) steel REE β-diketone inhibitor film.
Figure 13.
Raman spectra of , , , and inhibitors and the corrosion products formed on mild steel and 303 SS: (a) REE β-diketone inhibitor; (b) steel REE β-diketone inhibitor film.
Figure 13.
Raman spectra of , , , and inhibitors and the corrosion products formed on mild steel and 303 SS: (a) REE β-diketone inhibitor; (b) steel REE β-diketone inhibitor film.
Table 1.
Nominal compositions of mild steel and 304 stainless steel.
Table 1.
Nominal compositions of mild steel and 304 stainless steel.
Nominal Composition (wt.%) | C | S | Si | Mn | Cr | Ni | Mo | Fe |
---|
Mild steel | 0.12 | 0.040 | 0.015 | 0.8 | - | - | | Balance |
Stainless steel (type 304) | 0.05 | 0.019 | 0.320 | 1.5 | 18.5 | 9.0 | 0.46 | Balance |
Table 2.
Comparison of corrosion rate of mild steel and 304 SS and inhibition efficiency in the absence and presence of , , , and inhibitors in 3.5% NaCl solution.
Table 2.
Comparison of corrosion rate of mild steel and 304 SS and inhibition efficiency in the absence and presence of , , , and inhibitors in 3.5% NaCl solution.
| REE Inhibitor | CR with Inhibitor (μm/y) | CR without Inhibitor (μm/y) | IE (%) |
---|
Mild steel | Ce(acac)3 | 42 | 156 | 74 |
Ce(hfac)3 | 41 | 166 | 75 |
La(acac)3 | 43 | 180 | 76 |
La(hfac)3 | 45 | 159 | 72 |
304 SS | Ce(acac)3 | 0.0 | 0.7498 | 100 |
Ce(hfac)3 | 0.0 | 0.6722 | 100 |
La(acac)3 | 0.0 | 0.5430 | 100 |
La(hfac)3 | 0.0 | 0.6378 | 100 |
Table 3.
Electrochemical parametersof MS obtained from the potentiodynamic polarisation curves.
Table 3.
Electrochemical parametersof MS obtained from the potentiodynamic polarisation curves.
3.5% NaCl solution without inhibitor |
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.552 | 1.62 × 10−5 | 54 | 360 | 191 | |
40 | −0.626 | 2.41 × 10−5 | 42 | 314 | 284 | |
60 | −0.591 | 2.89 × 10−5 | 64 | 284 | 340 | |
|
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.434 | 1.01 × 10−5 | 109 | 186 | 118 | 38.0 |
40 | −0.604 | 1.54 × 10−5 | 69 | 207 | 181 | 36.0 |
60 | −0.630 | 1.60 × 10−5 | 47 | 215 | 188 | 33.5 |
|
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.554 | 1.00 × 10−5 | 135 | 81 | 104 | 46.0 |
40 | −0.643 | 1.57 × 10−5 | 54 | 120 | 163 | 43.0 |
60 | −0.597 | 1.94 × 10−5 | 46 | 301 | 200 | 41.0 |
|
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.575 | 1.04 × 10−5 | 126 | 211 | 109 | 36.0 |
40 | −0.592 | 1.61 × 10−5 | 125 | 182 | 169 | 33.0 |
60 | −0.584 | 2.09 × 10−5 | 66 | 151 | 219 | 28.0 |
|
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.650 | 1.01 × 10−5 | 212 | 116 | 119 | 38.0 |
40 | −0.637 | 1.58 × 10−5 | 69 | 164 | 186 | 34.0 |
60 | −0.691 | 2.04 × 10−5 | 92 | 127 | 240 | 29.0 |
Table 4.
Electrochemical parameters of 304 SS obtained from the potentiodynamic polarization curves.
Table 4.
Electrochemical parameters of 304 SS obtained from the potentiodynamic polarization curves.
3.5% NaCl solution without inhibitor |
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.287 | 3.66 × 10−7 | 351 | 67 | 3.79 | |
40 | −0.359 | 1.40 × 10−6 | 76 | 49 | 14.50 | |
60 | −0.370 | 4.47 × 10−6 | 123 | 57 | 56.66 | |
|
Temp.(°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.134 | 5.84 × 10−8 | 243 | 79 | 0.60 | 84 |
40 | −0.157 | 3.5 × 10−7 | 206 | 86 | 3.63 | 75 |
60 | −0.190 | 2.14 × 10−6 | 391 | 100 | 22.17 | 52 |
|
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.122 | 4.07 × 10−8 | 482 | 70 | 0.42 | 89 |
40 | −0.132 | 3.73 × 10−7 | 345 | 93 | 3.49 | 76 |
60 | −0.135 | 2.11 × 10−6 | 323 | 102 | 11.50 | 53 |
|
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.243 | 1.02 × 10−7 | 379 | 72 | 1.07 | 72 |
40 | −0.198 | 1.01 × 10−6 | 344 | 77 | 10.46 | 25 |
60 | −0.177 | 3.53 × 10−6 | 185 | 80 | 36.56 | 21 |
|
Temp. (°C) | (V) | (A/cm2) | βa (mV/dec) | βc (mV/dec) | CR (µm/y) | IE (%) |
20 | −0.126 | 8.13 × 10−8 | 301 | 78 | 0.84 | 78 |
40 | −0.156 | 3.85 × 10−7 | 369 | 105 | 3.99 | 73 |
60 | −0.162 | 2.05 × 10−6 | 306 | 164 | 21.23 | 54 |
Table 5.
IR frequency bands of REE(β-diketone)3 inhibitors and the corrosion products formed on mild steel and 304 stainless steel.
Table 5.
IR frequency bands of REE(β-diketone)3 inhibitors and the corrosion products formed on mild steel and 304 stainless steel.
Inhibitor Used | Inhibitor Only | Corrosion Product |
---|
| Frequency (cm−1) | Assignment | Frequency (cm−1) | Assignment |
---|
| 1584 | C=O | 1576 | C=O |
| 3134 | O-H | 2967 | OH |
| 1595 | C=O | 1408 | C=O |
| 3335 | O-H | 2967 | OH |
| 1644 | C=O | 1632 | C=O |
| 3041 | O-H | 2921 | OH |
| 1648 | C=O | 1615 | C=O |
| 3041 | O-H | 2920 | OH |
Table 6.
Raman frequency band of inhibitor and the corrosion products on mild steel and 304 stainless steel.
Table 6.
Raman frequency band of inhibitor and the corrosion products on mild steel and 304 stainless steel.
Inhibitor Used | Inhibitor Only | Corrosion Product |
---|
| Frequency (cm−1) | Assignment | Frequency (cm−1) | Assignment |
---|
| 1583 | C=O | 1311, 248 | Fe–O |
| 455 | REE–O | 524 | REE–O |
2931 | O-H | 377 | γ-FeOOH |
| 1598 | C=O | 1316, 244 | Fe–O |
| 492 | REE–O | 524 | REE–O |
3069 | O-H | 377 | γ-FeOOH |
| 1654 | C=O | 1306, 250 | Fe–O |
| 445 | REE–O | 530 | REE–O |
3157 | O-H | 377 | γ-FeOOH |
| 1681 | C=O | 1308, 246 | Fe–O |
| 455 | REE–O | 526 | REE–O |
3152 | O-H | 377 | γ-FeOOH |