Due to the severe and rapid corrosion of metallic equipment by strong acids at high temperatures, a high concentration of acidizing corrosion inhibitors (ACIs) is required during acidizing processes. There is always a need to develop more effective and environmentally friendly ACIs than current products. In this work, a highly effective ACI obtained from a novel main component and its synergistic effect with paraformaldehyde (PFA) and potassium iodide (KI) is presented. The ACI was prepared from the crude product of benzyl quinolinium chloride derivative (BQD) synthesized from benzyl chloride and quinoline in a simple way. The new ACI formulation, named “synergistic indolizine derivative mixture” (SIDM), which consists of BQD, PFA, and KI, showed superior corrosion inhibition effectiveness (IE) and temperature stability compared with commercially available ACI. More importantly, the SIDM formulation eliminates the need for commonly used highly toxic synergists (e.g., propargyl alcohol and As2O3). In a 20 wt% hydrochloric acid (HCl) solution, the addition of 0.5 wt% SIDM mitigates the corrosion rate of N80 steel down to less than 0.00564 lbm·ft−2 at 194°F, while the corrosion rate at 320 °F is 0.0546 lbm·ft−2·when 4.0 wt% SIDM is added.

In the oilfield acidizing process of rock formation, ACIs have an important role in mitigating severe steel corrosion and are of great value in industrial applications. Increasing acid concentrations, rising downhole temperatures, and growing environmental concerns and expenses are presenting new and difficult challenges to conventional ACIs. Severe corrosion caused by concentrated acid during acid treatment necessitates the discovery and development of effective corrosion inhibitors. This helps reduce the cost of metallic piping or equipment and meet stringent health, safety, and environmental requirements.

In recent decades, various heterocyclic quaternary ammonium salts (e.g., N-alkylpyridinium salts, N-alkylquinolinium salts, diquaternary ammonium salts, etc.) have been widely used and confirmed to exhibit high corrosion IE for steel under simulated acidizing conditions (Zhang et al. 2014; Wylde et al. 2017).

Despite the known fact that IE for such quaternary ammonium salts is high, several researchers pointed out that the corrosion inhibition of benzyl quinolinium chloride (BQC) for carbon steel in concentrated HCl has not been adequately characterized due to the influence of impurities (Yang et al. 2015; Zhan et al. 2015). It was reported that the IE of purified BQC (e.g., by recrystallization) is only 70–80% at 194°F in 15 wt% HCl, while the crude product of BQC (by quaternization of quinoline and halide) has an IE of more than 90–95%. This unexpected decrease in the IE of purified BQC, which has been widely accepted as the main component of ACI since the 1960s, has raised questions because the main component of an ACI formulation is expected to provide high IE (Finšgar and Jackson 2014; Sitz et al. 2012). For comparison, another commonly used main component of current ACI products, the Mannich base, can have excellent IE performance after adequate purification (Zhang et al. 2022).

Because the purified BQC has lower IE than the crude product of BQC, this suggests that, in previous ACI formulations derived from BQC, the presence of synergists (e.g., inorganic salts, small organic molecules, organic macromolecules, etc.) was critical for these formulations to achieve high IE (Chiang 1994; Crawford et al. 2016). After the limited IE of purified BQC was reported, researchers started to investigate the significant difference between the crude product of BQC and the purified BQC. In 2015, a highly effective BQD was separated from the byproduct of BQC (yield < 1%). Its chemical structure was confirmed to be an intermolecular dimer of BQC (Fig. 1). Subsequently, a series of similar dimer derivatives of quinolinium salts were successfully synthesized (with 40–60% yield) by a 1,3-dipolar cycloaddition reaction, and these molecules were classified as indolizine derivatives because they have an indolizine structure within the molecule (Yang et al. 2020, 2021).

Fig. 1

Formation scheme of the conversion of BQC to BQD (with a highlighted indolizine structure within the molecule).

Fig. 1

Formation scheme of the conversion of BQC to BQD (with a highlighted indolizine structure within the molecule).

Close modal

Moreover, based on the corrosion inhibition behavior of these dimer derivatives, the corrosion inhibition was found to be improved for the dimer derivatives compared with quinolinium salt precursors. The dimers showed significantly higher IE than the original quinolinium salts, even at much lower concentrations (Wang et al. 2018, 2022; Yang et al. 2019).

As a result, numerous researchers are trying to verify and explain the reason for the improved IE of these dimer derivatives. In addition, the simplification of the cycloaddition reaction process and the successful conversion of the quinolinium salts into their dimers pose another challenge in the field of corrosion inhibition.

Recently, a relatively straightforward strategy for the preparation of BQD has been found that offers economic advantages, and the entirely new and simplified route brings a lower environmental impact compared with the previous method (Yang et al. 2015). By using solid sodium nitrite (NaNO2) as oxidant/base, the yield of BQD could be significantly increased up to 60%. Herein, the “one-pot” method is presented to obtain the crude product BQD directly from benzyl chloride and quinoline for the first time. This method offers promising practical prospects for the study and application of ACI. Based on the newly established method for the preparation of BQD, several experiments were conducted to develop highly effective formulations using BQD and synergists exhibiting high IE.

Materials

The alloy samples used in this study were N80 steel. The composition of the N80 samples was as follows: (wt%) 0.20 Cr; 0.92 Mn; 0.008 S; 0.01 P; 0.19 Si; 0.31 C; and the rest Fe. The dimensions of the N80 steel samples for weight loss analysis were 1.968×0.394×0.118 in. Samples were polished with 60-grit SiC grinding paper, followed by 400-grit SiC grinding paper. A 20 wt% HCl solution was used as the corrosive medium for the weight loss measurements at both 194°F (90°C) and 320°F (160°C). The acid solution was prepared by diluting 37 wt% HCl with ultrapure water. The chemical structure of the product was confirmed by nuclear magnetic resonance (NMR, Avance NEO 400, Bruker, Germany) in CDCl3 and high-resolution mass spectrometry (Thermo Scientific, Exploris 480) using an electrospray ionization source.

“One Pot” Synthesis

Step 1: Quinoline and benzyl chloride were mixed in a 1:1 molar ratio (for quinoline 10.33 g, which is approximately 80 mmol, and for benzyl chloride 10.13 g, which is approximately 80 mmol) in a thick-walled pressure bottle (Fig. 2). About 20 mL of ethanol was used as a solvent for the quaternization reaction. The mixture was heated under magnetic stirring in a silicone oil bath at 90°C for 24 hours. After cooling, a thick, dark red-black solution (with 90–95% yield of BQC) was obtained at room temperature.

Fig. 2

Photo of the thick-walled pressure bottle used for the synthesis in Steps 1 and 2.

Fig. 2

Photo of the thick-walled pressure bottle used for the synthesis in Steps 1 and 2.

Close modal

Step 2: In the same container, 3.0 molar equivalents of NaNO2 powder [16.56 g (0.24 mol)] and 180 mL of ethanol were added to the solution obtained in Step 1. Then, the mixture was heated under magnetic stirring in a silicone oil bath at 130°C for 8 hours until a dark orange-brown sticky mixture was obtained. The mixture was filtered, and the undissolved residue (inorganic salts) was separated. The filtrate containing BQD and other byproducts was collected. After filtration:

  1. When the solvent was not removed, the content of BQD in the mixture can be quantified by 1H NMR (using CH2Br2 as an internal standard) (Wang et al. 2023).

  2. When the solvent was concentrated using a rotary evaporator, a sticky precipitate was observed on the bottom of the flask. Then, a solvent mixture [methanol/acetone (1:1, V/V)] was used to recrystallize (two to three times) the solid to obtain an orange crystal as purified BQD (Fig. 3 ). When 80 mmol of quinoline and benzyl chloride were added in Step 1, 9.81 g of BQD was obtained in Step 2 after purification. The conversion ratio of the starting materials quinoline and benzyl chloride to BQD was 52.1%. All the above chemicals were purchased from Adamas-beta® (No. 15 Xinfei Rd. Songjiang District, Shanghai, P.R. China), with a purity of 99% and used as obtained.

Fig. 3

Crude product of BQD obtained in Step 2 (before it was filtered) and the purified solid of BQC and BQD by recrystallization.

Fig. 3

Crude product of BQD obtained in Step 2 (before it was filtered) and the purified solid of BQC and BQD by recrystallization.

Close modal

The structure of the molecule BQD was characterized and identified with 1H NMR (500 MHz, CDCl3): δ12.26(s, 1 H), 9.35 (d, J = 13.7 Hz, 1 H), 8.01 (m, 2 H), 7.90 (d, J = 8.6 Hz, 1 H), 7.75 (m, 2 H), 7.65 (m, 2 H), 7.60-7.20 (m, 12 H), 6.35 (s, 2 H). Moreover, high-resolution mass spectrometry was performed: [C32H23N2]+ Calculated: 435.1854; Found: 435.1851 (Fig. 4).

Fig. 4

Mass spectrum of BQD (positive polarity).

Fig. 4

Mass spectrum of BQD (positive polarity).

Close modal

Weight Loss Tests

After weighing, the N80 steel samples were immersed in 20 wt% HCl solutions with and without various concentrations of ACI for 4–6 hours at different temperatures (Kalfayan 2008; de Wolf et al. 2017). The experiments were performed in triplicate for each test, and 800 mL of acid was used as the corrosive medium. After the immersion test, the samples were taken out of the corrosive medium, cooled to room temperature, and rinsed with distilled water and ethanol. The samples were then dried under a stream of compressed air and weighed. The average value of the corrosion rate (CR) was calculated in lbm·ft-2 using Eq. 1 (Yang et al. 2019; Wang et al. 2022):

CR=W1 W2S 144,
(1)

where W1 (lbm) and W2 (lbm) are the masses of the samples before and after immersion, respectively. S (ft2) is the surface area of the specimens. The IE was determined by using Eq. 2:

θ×100%=IE[%]=CR0CRCR0×100,
(2)

where CR0 and CR are the average corrosion rate of steel samples in the absence and presence of certain concentrations of the ACI, respectively.

The weight loss test at 194°F was performed in a three-neck flask without stirring and heated in a water bath, while the experiment at 320°F was performed in a pressurized autoclave of Hastelloy B3 material (Fig. 5). After the weight loss test, the steel samples were examined visually for localized corrosion, observed as pits or edge attacks, and the pitting corrosion was qualitatively described by assigning a visual pitting index (PI) (Hughes et al. 2014; Barmatov et al. 2015). Evaluation of corrosion rates is based on the petroleum industry-acceptable level of less than 0.05 lbm·ft−2 and a PI ≤ 2 (Al-Mutairi et al. 2005; Kalfayan 2008). The visual PIs ranging from PI = 0 (no pitting) to PI = 8 (severe and extensive pitting) are shown in Table 1 .

Table 1

Definition of different PIs for steel samples after weight loss test.

Description of PittingPI
None 
Minor cut-edge corrosion 
No pits on major surfaces. Small shallow pits on cut edge 
Pinpoint pits on surface < 25 
Pinpoint pits on surface > 25 
< 10 pits 16–31 mils diameter, 8–16 mils deep 
11–25 pits with PI of 5 
> 25 pits with PI of 5 
Large pits 63–126 mils diameter, > 31 mils deep 
Description of PittingPI
None 
Minor cut-edge corrosion 
No pits on major surfaces. Small shallow pits on cut edge 
Pinpoint pits on surface < 25 
Pinpoint pits on surface > 25 
< 10 pits 16–31 mils diameter, 8–16 mils deep 
11–25 pits with PI of 5 
> 25 pits with PI of 5 
Large pits 63–126 mils diameter, > 31 mils deep 
Fig. 5

High-temperature and high-pressure autoclave for weight loss test.

Fig. 5

High-temperature and high-pressure autoclave for weight loss test.

Close modal

Contact Angle Measurements

The contact angles (CAs) of the metal surfaces were determined by the sessile drop method using a CA analyzer (JC2000D3M, POWEREACH, Shanghai, China) at 77°F. First, the N80 samples were immersed in the 20 wt% HCl solution with or without ACI for 1 hour at 194°F. Then, the specimens were removed from the solution, washed with ultrapure water, and dried with compressed air. The CA of the surface was determined by dropping a small drop of ultrapure water on the metal surface.

Corrosion Inhibition of Purified BQC and Purified BQD

Figs. 6 and 7  show the effect of ACI dosage on CR and PI in 20 wt% HCl at 194°F (4-hour immersion). In Fig. 6 , when the purified BQC dosage was increased from 0.05 wt% to 1.0 wt%, the CR gradually decreased from 0.828 lbm·ft-2 to 0.180 lbm·ft−2, and the PI dropped from 8 to 5. On the other hand, Fig. 7  shows that, when purified BQD was used as ACI, the CR was significantly lower, even though the concentration of purified BQD was much lower than that of purified BQC. The CR was 0.0024 (PI = 2) and 0.002 lbm·ft−2 (PI = 0) at 0.1 wt% and 0.2 wt% of purified BQD, respectively. For comparison, based on the determined CR, it was found that the purified BQD has a higher IE than the novel ACI molecules such as QBBD and O-16, as reported previously (Gu et al. 2015; Wang et al. 2020).

Fig. 6

Influence of purified BQC dosage on the CR and the PI in 20 wt% HCl at 194°F (4-hour immersion).

Fig. 6

Influence of purified BQC dosage on the CR and the PI in 20 wt% HCl at 194°F (4-hour immersion).

Close modal
Fig. 7

Influence of purified BQD dosage on the CR and the PI in 20 wt% HCl at 194°F (4-hour immersion).

Fig. 7

Influence of purified BQD dosage on the CR and the PI in 20 wt% HCl at 194°F (4-hour immersion).

Close modal

Different Synergism of PFA and KI with ACI

Initially, a screening of several potential synergists was performed. Of these potential synergists, PFA and KI showed reasonable synergistic effects with purified BQD. Fig. 8  shows that, when the different molar ratio of PFA was combined with purified BQD, the CR gradually decreased as the PFA content increased from 0.025 wt% to 0.2 wt%. However, a similar trend was not observed for KI. When the mass ratio of KI/BQD is greater than 1 and the dosage of purified BQD is 0.05 wt%, the presence of KI hinders the corrosion inhibition effect of purified BQD, causing CR to increase. KI is frequently considered a useful and commonly used synergist in many corrosion inhibitor formulations, especially for conventional quaternary quinoline salt inhibitors (Shen et al. 2014; Feng et al. 2019; Mao et al. 2021). On the other hand, the disadvantage of excessive dosing of KI for purified BQD has never been reported. This unexpected phenomenon suggests that the percentage of KI in the ACI formulation needs to be further investigated to determine the corrosion inhibition mechanism. It is crucial to recognize that KI must be treated as an important component in the formulation design and added only in limited dosage.

Fig. 8

Influence of PFA and KI dosage (ratio) on the CR in 20 wt% HCl at 194°F when 0.05 wt% purified BQD served as the ACI (4-hour immersion).

Fig. 8

Influence of PFA and KI dosage (ratio) on the CR in 20 wt% HCl at 194°F when 0.05 wt% purified BQD served as the ACI (4-hour immersion).

Close modal

Synergistic Study of BQD

Considering the presence of KI, which can show a positive synergistic effect at a relatively low dosage (see Fig. 8 ), KI was selected as an additional synergist besides PFA to develop the formulation based on BQD. Table 2  shows the results of CR in 20 wt% HCl at 194 °F, and the CA and PI values for different combinations after the 4-hour immersion test. Examples of the CA measurements and the photographs of the coupons before and after immersion are given in Fig. 9  (other values are reported in Table 2 ). The lowest CR of 0.00256 lbm·ft−2 was obtained when a combination of 0.05 wt% BQD, 0.10 wt% PFA, and 0.05 wt% KI was used (Experiment 10 in Table 2 ). Experiment 10 shows that this combination of BQD, PFA, and KI leads to the most hydrophobic surface (CA = 106.8°) and the lowest PI of 0, among all the combinations tested. On the other hand, for the experiment where KI was not present in the solution, the CA dropped to 92.8° and the PI increased to 1 (Experiment 4). Furthermore, for the experiment where PFA was not present in the solution, the CA decreased to 70.7° and the PI increased to 4 (Experiment 6). The sample immersed in noninhibited solution had a CA of 15.3°, thus being with the most hydrophilic surface compared with the samples immersed in inhibited solutions (Table 2). The noninhibited sample also had the highest PI of 8, showing a high corrosion attack. However, when 0.05 wt% BQD was present in the solution, the CA increased significantly (83.2°) compared with the noninhibited solution (Experiment 2). Experiments 3–5 showed the positive synergistic corrosion inhibition effect and the increase in hydrophobicity when PFA or KI was present in solution in combination with BQD because the CR decreased and CA increased compared with Experiment 2. On the other hand, when KI was used at higher dosage (0.10 wt%, Experiment 6), CR increased compared to the experiment with lower KI dosage (Experiment 5).

Fig. 9

CA measurements for the N80 steel samples (experiments were performed using the dosages reported in Table 2 ).

Fig. 9

CA measurements for the N80 steel samples (experiments were performed using the dosages reported in Table 2 ).

Close modal
Table 2

Variation of CR, CA, and PI with different doses of PFA and KI as cosynergists for BQD in 20 wt% HCl at 194°F (4-hour immersion).

ExperimentDosage of BQD (wt%)Dosage of PFA (wt%)Dosage of KI (wt%)CR×103
(lbm·ft−2)
CA
(degrees)
PI
Before immersion – – – – 93.1 
0.00 0.00 0.00 827.64 15.3 
0.05 0.00 0.00 7.64 83.2 
0.05 0.05 0.00 4.64 89.4 
0.05 0.10 0.00 3.96 92.8 
0.05 0.00 0.05 4.36 88.8 
0.05 0.00 0.10 14.56 70.7 
0.05 0.05 0.025 4.72 84.5 
0.05 0.10 0.025 3.96 87.1 
0.05 0.05 0.05 3.08 99.5 
10 0.05 0.10 0.05 2.56 106.8 
11 0.05 0.05 0.10 8.00 101.2 
12 0.05 0.10 0.10 7.28 98.7 
13 0.05 0.05 0.20 15.00 84.4 
14 0.05 0.10 0.20 15.76 72.8 
ExperimentDosage of BQD (wt%)Dosage of PFA (wt%)Dosage of KI (wt%)CR×103
(lbm·ft−2)
CA
(degrees)
PI
Before immersion – – – – 93.1 
0.00 0.00 0.00 827.64 15.3 
0.05 0.00 0.00 7.64 83.2 
0.05 0.05 0.00 4.64 89.4 
0.05 0.10 0.00 3.96 92.8 
0.05 0.00 0.05 4.36 88.8 
0.05 0.00 0.10 14.56 70.7 
0.05 0.05 0.025 4.72 84.5 
0.05 0.10 0.025 3.96 87.1 
0.05 0.05 0.05 3.08 99.5 
10 0.05 0.10 0.05 2.56 106.8 
11 0.05 0.05 0.10 8.00 101.2 
12 0.05 0.10 0.10 7.28 98.7 
13 0.05 0.05 0.20 15.00 84.4 
14 0.05 0.10 0.20 15.76 72.8 

By comparing Experiments 11 and 13 and Experiments 12 and 14, the higher amount of KI results in an increase in CR and PI and a decrease in CA. The latter indicates that the amount of KI significantly impacts the final performance of this formulation, where the right ratio of the amount of BQD and KI is crucial for the high IE.

The synergistic effect presented above was performed using purified BQD. However, it is more practical and useful for industrial purposes to use the primary “one-pot” products of BQD directly without any purification or pretreatment. Therefore, to evaluate the corrosion inhibition of the new formulation, the mixture must be prepared from the crude product of BQD (after filtration).

The filtered crude mixture of BQD consists of the solvent (ethanol 200 mL), several undefined byproducts of the 1,3-cycloaddition reaction, and some of the benzyl chloride and quinoline (see Step 2 in the “one-pot” synthesis section).

Based on the 1H NMR analysis, the content of BQD in the filtrate was 5.2 wt%. To develop a formulation, 19.6 g PFA and 9.8 g KI were added to 180 g of the crude mixture of BQD. Finally, a mixture of formic acid/water (3:1, V/V), which served as a solubilizer, was added to obtain 250 g of solution. This final formulation was named SIDM (Fig. 10). The IE of SIDM for N80 steel was evaluated in 20 wt% HCl at 194°F and 320°F for 6 hours, and the results are shown in Figs. 11 and 12 , respectively. At 194°F, the CR was lower than 0.00564 lbm·ft−2 (PI = 1) when the dosage of the formulation was higher than 0.5 wt%, while, at 320°F, the CR was 0.0546 lbm·ft−2 (PI = 2) at a dosage of 4.0 wt%. Therefore, the corrosion inhibition of the formulation shown in Fig. 10  meets the requirements (up to 0.05 lbm·ft−2) of NACE SP21469 (2021).

Fig. 10

Appearance of the SIDM formulation.

Fig. 10

Appearance of the SIDM formulation.

Close modal
Fig. 11

Corrosion rate impact from SIDM-20% HCl, 6 hours at 194°F.

Fig. 11

Corrosion rate impact from SIDM-20% HCl, 6 hours at 194°F.

Close modal
Fig. 12

Corrosion rate impact from SIDM-20% HCl, 6 hours at 320°F.

Fig. 12

Corrosion rate impact from SIDM-20% HCl, 6 hours at 320°F.

Close modal

Moreover, attempts have been made to verify whether the synergistic effect of KI is essential for the SIDM formulation, as the dosage of KI is important and will only represent a limited improvement in the corrosion inhibition (Table 2), which might cause potential problems in the inhibitor mixture. In Figs. 11 and 12 , corrosion inhibition of the ACI formulation without KI was also evaluated, and the results showed that the presence of KI is important for the good performance of SIDM. During the weight loss test at 320°F (Fig. 12), the CR increased to 0.0732 lbm·ft−2 (PI = 3) when 4.0 wt% of the formulation (SIDM without KI) was used as the ACI. Compared with the original formulation of SIDM, an approximately 40% increase in CR occurred at 320°F in the absence of KI, and a higher PI was observed, showing that the presence of KI is crucial for SIDM.

In this work, the new method for the synthesis of the highly effective ACI by the intermolecular cycloaddition of BQC to a BQD is presented. Corrosion IE of BQD and BQD is determined, and the synergistic effect of BQD with potential compounds is tested to develop a novel ACI formulation.

Based on the presented results, the following conclusions can be made:

  1. The “one-pot” synthesis of BQD using NaNO2 as an oxidant/base achieves a yield of 52.1%. This method offers economic advantages as the cost increase is not significant compared with the current BQC production technique, considering that NaNO2 is a relatively inexpensive and readily available chemical.

  2. Compared to BQC, which is the main component of conventional ACI, the BQD provides significantly higher corrosion IE.

  3. BQD showed a synergistic effect with PFA. Even when the content of PFA is several times higher than that of BQD, positive synergistic effects were observed. However, for KI, a relatively high dosage has a detrimental effect on the corrosion inhibition of BQD. Positive synergistic effects can only be observed within a limited dosage range of KI.

  4. The formulation SIDM was developed from the crude mixture of BQD, PFA, and KI. The final formulation consisted of 180 g filtrate of the “one-pot” product of BQD, 19.6 g PFA, and 9.8 g KI. Finally, to the latter solution, a mixture of formic acid/water (3:1, V/V) was added to obtain 250 g of the final mixture. The SIDM formulation showed a significant corrosion inhibition effect for N80 steel in 20 wt% HCl at 194°F and 320°F.

This work is supported by the Key Projects of China National Key Research and Development Plan (2021YFE0107000), the National Natural Science Foundation in China (General Program, Grant No. 52074339), and the Slovenian Research Agency (Grant Nos. NK-0001 and P2-0118). The project is cofinanced by the Republic of Slovenia, the Ministry of Education, Science, and Sport, and the EU under the European Regional Development Fund. This work was also supported by the Qingdao Postdoctoral Application Research Project (QDBSH 20230101022).

Original SPE manuscript received for review 17 July 2023. Revised manuscript received for review 1 October 2023. Paper (SPE 218374) peer approved 24 October 2023.

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