Effect of Acid Hydrolysis on Inhibition Performance
Figure 3: Effect of Acid Hydrolysis on Inhibiting Activity for Diamine-A at a Concentration of 200ppm and 60ºC (Linear Polarisation)
100 200 300 400 500 600 700
0 0 mpy = mils per year.
Table 1: Effect of Inhibitor Concentration in 1M HCl at 60ºC for Six Hours (Weight Loss)
Inhibitor 50
1, 12-dodacandiamine 13.07 Diamine-Ab Diamine-Aa Diamine-Bb Diamine-Ba Diamine-Cb Diamine-Ca
61.60 53.42 66.92 88.38 85.66 78.93
Per Cent Protection
Inhibitor Concentration (ppm) 100
200
16.33 88.17 81.73 90.61 95.29 91.16 90.66
29.62 92.38 90.97 96.80 95.60 96.04 96.42
a = after acid hydrolysis; b = before acid hydrolysis; ppm = parts per million.
Table 2: Effect of Acid Concentration on Inhibition Efficiency of Diamine Corrosion Inhibitors at 60ºC Compound
% IE at 400 ppm Diamine-A
1M HCl 6h
96.8 IE = inhibition efficiency; ppm = parts per million.
Table 3: Corrosion Rate and Percent Inhibition Obtained for Mild Steel in 1M HCl with 200ppm at 60ºC (Tafel Plot)
Compound Name Blank
Diamine-Ab Diamine-Aa Diamine-Bb Diamine-Ba Diamine-Cb Diamine-Ca
Corrosion Rate (mpy) Per Cent Inhibition 1,557 25.17 31.19 22.98 19.73 43.84 35.20
0
98.4 98
98.5 98.7 97.2 97.7
a = after acid hydrolysis; b = before acid hydrolysis; mpy = mils per year.
The electrochemical techniques used in this study included Tafel plot and linear polarisation.
Tafel Plot Measurements
Tafel plot measurements were carried out on the low-carbon steel coupons with an exposure area of 1cm2. The low-carbon steel coupons acted as the working electrode, where a saturated calomel electrode and a graphite electrode were used as the reference and the counter electrode, respectively.
78 % IE
4.1M HCl 2h
96.4 % IE
7.7M HCl 2h
78.2 400
38.73 96.80 95.10 97.05 96.89 95.74 95.48
Corrosion Inhibitors Tested The corrosion inhibitors tested were synthesised from
1,12-dodecanediamine (H2N-[CH2]12-NH2). The reaction products were purified using different methods, such as:
• solvent extraction; • column chromatography; and • vacuum distillation.
Then, the purified products were characterised by 1H nuclear magnetic resonance, 13C nuclear magnetic resonance and infrared spectroscopies.
The acid hydrolysis process was carried out for a number of diamine compounds (see Figure 1). HCl (5cm3) at a concentration of 10% was added to a solution of nearly 2–5g of diamine compound and stirred in a closed vessel at 50°C for 48 hours. The reaction mixture was cooled to room temperature and the hydrolysed quaternary salt was separated
from the mixture by purging the reaction mixture with N2 to remove H2O and adding 15cm3 of ether.
After decanting the upper ether layer, the oily residue was dissolved in minimum quantity of methanol (~2cm3) and again separated from the solvent by adding 15cm3 of ether. This procedure was repeated five times to ensure the removal of any unreacted starting material and formaldehyde. The oily liquid was dried under vacuum at 50°C until reaching constant weight (99%).
The structures of these diamine inhibitors before and after acid hydrolysis are shown in Figures 1 and 2, respectively. The starting material (1,12-dodecanediamine) has a molecular weight of 200.37g/M.
Results and Discussion
The effects of inhibitor and acid types and concentrations on the performance of inhibitors before and after acid hydrolysis were
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 1
This test is used mainly for screening new corrosion inhibitors. The cell used in linear polarisation tests was 100ml full with 1M HCl and contained three low-carbon steel electrodes; among them was the working electrode. The tests were conducted at 300rpm at 60°C for six hours.
50 100 150 200 250 Exposure time (min) 300 350 400 C1 N- + (CH2)12
NH3C1- +
The instrument used to conduct Tafel plot experiments was the EG&G Princeton Applied Research 352 SoftCorrTMIII. The specimen was scanned anodically (250mV) and cathodically (250mV) at a scan rate of 0.166mV/s from the open circuit potential. The logarithm of the generated current between the working electrode (low-carbon steel specimen) and the counter electrode was plotted as a function of potential.
Tafel plot tests were conducted to evaluate the influence of synthesised corrosion inhibitors on the reduction of corrosion rate of low-carbon steel in 1M HCl at 60°C. They were also used to investigate the type of mechanism that controls inhibition.
+ C1 N-
(CH2)12 NHCHO
Linear Polarisation
Linear polarisation was conducted using online monitoring software developed by Saudi Aramco. This programme provides the necessary software to control the potentiostat model 273A from a host computer. The programme calculates the corrosion rate versus time using assumed values for beta cathodic (βc) and beta anodic (βa) of 170mV/decade.
Corrosion rate (mpy)
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