Electrochemical and anticorrosion properties of 5-hydroxytryptophan on mild steel in a simulated well-acidizing fluid

Peer review under responsibility of Taibah University.

Figures & data

Table 3 Effect of Intensifiers an inhibition efficiency (%) of 10 × 10⁻⁵ M 5-HTP on mild steel in a 1 M HCl solution.

T (°C)	5-HTP Only	5-HTP + KI	5-HTP + PEG	5-HTP + NaG	5-HTP + GLU
30	96.0	99.3	99.9	97.2	99.9
45	91.1	99.1	99.0	96.2	99.6
60	84.2	98.8	98.7	94.1	98.9
75	78.0	94.4	92.5	88.3	96.3
90	66.0	89.0	88.5	73.1	90.2

Table 4 Effect of Intensifiers on inhibition efficiency (%) of 10 × 10⁻⁵ M 5-HTP on mild steel in a 15% HCl solution.

T (°C)	5-HTP Only	5-HTP + KI	5-HTP + PEG	5-HTP + NaG	5-HTP + GLU
30	78.1	93.9	95.1	88.4	96.3
45	74.3	92.1	91.0	85.2	95.0
60	67.1	90.0	90.9	80.0	90.4
75	52.4	85.0	85.7	74.2	85.6
90	39.9	80.7	78.4	65.2	82.5

Fig. 1 Chemical structure of 5-HTP.

Table 1 Corrosion rate, inhibition efficiency and fractional surface coverage data for the inhibition of mild steel corrosion in 1 M and 15% HCl using different concentrations of 5-HTP at 30 °C.

Concentration (M)	In 1 M HCl			In 15% HCl
	R (mmpy)	εWL (%)	θ	R (mmpy)	εWL (%)	Θ
Blank solution	26.72	–	–	153.94	–	–
1 × 10^−5	4.26	84.07	0.84	54.16	64.82	0.65
3 × 10^−5	3.39	87.32	0.87	49.35	67.94	0.68
5 × 10^−5	2.59	90.32	0.90	41.49	73.05	0.73
8 × 10^−5	2.25	91.58	0.92	37.84	75.42	0.75
10 × 10^−5	1.07	96.01	0.96	33.65	78.14	0.78

Table 2 Effect of temperature on the inhibition efficiency of 10 × 10⁻⁵ M 5-HTP as a corrosion inhibitor for mild steel in acidic solutions.

T (°C)	1 M HCl	15% HCl
30	96.01	78.14
45	91.05	74.36
60	84.24	67.14
75	78.08	52.38
90	66.95	39.88

Fig. 2 Langmuir adsorption isotherms for the inhibition of mild steel corrosion in (i) 1 M and (ii) 15% HCl by different concentrations of 5-HTP.

Table 5 Parameters deduced from the Langmuir adsorption isotherm.

T (°C)	Slope	Kads (M^−1)	ΔGads (kJmol^−1)
30	1.43	3.879	−13.53
45	1.23	2.943	−13.47
60	1.15	2.226	−13.34
75	1.06	1.435	−12.67
90	1.02	1.417	−13.17

Fig. 3 Arrhenius plot for the inhibition of corrosion of mild steel in (i) 1 M and (ii) 15% HCl using different concentrations of 5-HTP.

Table 6 Activation parameters for the corrosion of mild steel in both 1 M and 15% HCl containing different concentrations of 5-HTP .

Conc (×10^−5 M)	1 M HCl			15% HCl
	Ea (kJmol^−1)	ΔH* (kJmol^−1)	ΔS* (kJmol^−1)	Ea (kJmol^−1)	ΔH* (kJmol^−1)	ΔS* (kJmol^−1)
0	17.04	−14.36	−0.19	11.25	−22.43	−0.12
1	35.42	−32.74	−0.16	18.32	−37.88	−0.13
3	36.34	−33.09	−0.15	23.92	−44.01	−0.11
5	38.29	−35.81	−0.14	27.45	−53.55	−0.09
5	42.03	−39.07	−0.12	31.17	−60.18	−0.90
10	48.44	−45.74	−0.11	34.33	−72.18	−0.07

Fig. 4 Transition state plot for the corrosion of mild steel in (i) 1 M and (ii) 15 % HCl in the absence and presence of different concentrations of 5-HTP.

Fig. 5 (a) Plot of OCP against time for the inhibition of MS corrosion in 1 M HCl. (b) Nyquist and Bode modulus/phase angle plots for inhibition of mild steel corrosion in 1 M HCl using different concentrations of 5-HTP.

Fig. 6 Electrochemical equivalent circuit ($R_s\left(R_{ct}{\varphi }_{CPE}\right)$) model used for data fitting/analyses.

Table 7 Some parameters obtained from the EIS technique used to monitor the inhibition of mild steel corrosion in 1 M HCl containing different concentrations of 5-HTP.

EIS Parameters	Test solutions
	HCl	1 × 10^−5	5 × 10^−5	10 × 10^−5
Rct (Ωcm^2)	102.3	732.1	878.5	1013.7
Rs (Ωcm^2)	1.035	0.902	0.907	0.947
Yo (μΩ^−1s^n)	157.7	191.1	155.9	146.4
Fit (×10^−6)	351.8	218.2	143.8	124.8
n	0.872	0.857	0.835	0.828
Cdl (μFcm^−2)	127.7	53.1	10.4	6.0
εwl (%)	–	86.03	88.36	89.91

Fig. 7 Tafel plots for the inhibition of mild steel corrosion in 1 M HCl by 5-HTP at 30 °C.

Table 8 Some parameters obtained from the PDP technique used to monitor the inhibition of mild steel corrosion by 5-HTP at 30 °C.

PDP parameters	HCl	1 × 10^−5	5 × 10^−5	10 × 10 × ^−5
βa (mV/decade)	95.4	54.8	57.6	57.8
βc (mV/decade)	64.7	91.2	97.2	100.9
Icorr (μAcm^−2)	969.3	128.8	49.7	26.5
Ecorr (mV)	−499	−452	−454	−453
εPDP (%)	–	86.7	94.75	97.25

Fig. 8 UV–vis spectra of 1 M HCl containing 10 × 10⁻⁵ M HCl before and after immersion of mild steel.

Fig. 9 FTIR spectra of 5-HTP in 1 M HCl before and after immersion of mild steel.

Fig. 10 SEM/EDAX profiles of an abraded mild steel surface (left) and MS immersed in 1 M HCl without (middle) and with 10 × 10⁻⁵ M 5-HTP.

Table 9 Summary of atoms obtained on a pure MS surface (case I), corroded MS surface (case II), and MS corrosion inhibited by 5-HTP surface (case IV) investigated by EDAX.

Element	Case I		Case II		Case IV
	Wt%	At%	Wt%	At%	Wt%	At%
Fe	89.85	64.17	80.24	49.28	69.74	42.10
C	09.07	29.90	09.79	30.69	10.78	30.17
O	01.04	04.91	07.96	17.07	12.31	14.88
N	–	–	–	–	05.73	11.03
Cl	–	–	02.01	02.96	01.44	01.82