The use of Robinia pseudoacania L fruit extract as a natural corrosion 4 inhibitor in the protection of historical bronze objects

34 Background: The phenomenon of bronze disease is considered as the most important factor in 35 the destruction of bronze objects. Different methods have been proposed to deal with it. The 36 most important inhibitors used in this regard are BTA and AMT. While these inhibitors control 37 the corrosion, they are toxic and cancerous. In ideal conditions, these inhibitors are able to slow 38 down the activity of chlorine ion, but leave some side effects after a period of treatment. Today, 39 plant extracts are used for this purpose. In this study, Robinia pseudoacania L extract was 40 selected for this purpose. 41 Material and methods: The acacia fruit extract (200ppm to 1800 ppm) was used to the 42 prevention of corrosion inhibition of bronze alloy in corrosive sodium chloride solution 0.5 M. 43 The Bronze alloy used in this research, was made based on the same percentage as the ancient 44 alloys (Cu-10Sn). This alloy was used for the effect of corrosion inhibitors in the potentiostat 45 device, the weight loss method. The experiment was conducted in split plot desgin in time based 46 on the randomized complete design in four replications. 47 Results: Mean comparison showed that the highest rate of corrosion inhibition (93.5%) was 48 obtained at a concentration of 1800 ppm and with increasing the concentration of the extract, 49 corrosion inhibition also increased. EDX analysis of the control sample matrix showed that the 50 amount of chlorine, which is the most important factor in the destruction of bronze disease, was 51 8.47%wt while in the presence of corrosive sodium chloride solution, after 4 weeks, the amount 52 of chlorine detected was 3.20%wt. According to the morphology (needle and rhombus) of these 53 corrosion products based on the SEM image, it can be said that they are of the type of atacamite 54 and paratacamite that have caused bronze disease in historical bronze works. At higher 55 concentrations of acacia extract, the presence and growth of fungi has been observed, which 56 can cause poor performance of acacia extract in the long run. 57 Conclusion: the natural inhibitor of acacia fruit aqueous extract can play an effective role in 58 inhibiting corrosion of bronze, but at higher concentrations, it became fungal, which can reduce 59 the role of acacia fruit aqueous extract and even ineffective. Because natural inhibitors can play 60 an effective role in preventing the corrosion of bronze works. But to get better performance of 61 these inhibitors, more tests need to be done to improve and optimize. 62 63

provide better stability for metal corrosion. Using corrosion inhibitors is very common in 93 protecting metal works. By forming a thin impermeable layer of the work, inhibitor compounds 94 slow down the anodic and cathodic activities (Zucchi et al. 1996). This protection method can 95 be used as the last and most common solution to fight bronze disease and get rid of this problem. 96 The inhibitory effect of BTA (Benzotriazole) and AMT (5-ami-no-2-mercapto-1, 3, 4-97 thiadiazole) on historical bronze art works has been proved previously (Artesani et   These elements act as a corrosion inhibitor. The use of organic compounds containing oxygen, 106 sulfur and especially nitrogen to reduce the corrosion attack on steel has been studied in detail.  Corrosion control of metals is of technical, economic, environmental and aesthetic importance.

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The use of corrosion inhibitors is one of the best options for protecting metals and alloys.

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Organic corrosion inhibitors are somewhat toxic, so green corrosion inhibitors have been shown 114 to be biodegradable because they are biodegradable and do not contain heavy metals or other 115 toxic compounds. In addition to being environmentally friendly, plant products have acceptable 116 environmental effects, are cheap, and are readily available and renewable. As a result, the 117 corrosion inhibitory abilities of tannins, alkaloids, amino acids and organic dyes of plant origin 118 are considered. Although significant research has been devoted to the inhibition of corrosion 119 by plant extracts, reports on the exact mechanisms of the adsorption and identification process 120 of the active substance are still scarce(Rani and Basu 2012).

121
Rosemary leaves were investigated as corrosion inhibitor for the Al + 2.5Mg alloy in a 3% 122 NaCl solution at 25°C (Kliškić et al. 2000), and El-Etre studied natural honey as a corrosion 123 inhibitor for copper (El-Etre 1998) and studied opuntia extract on aluminum (El-Etre 2003).

124
The inhibitive influence of the extract of khillah (Ammi visnaga) seeds on the corrosion of SX 125 316 steel in HCl solution was determined utilizing weight loss amounts as well as the 126 potentiostatic method. Delonix regia extracts inhibited the corrosion of aluminum in 127 hydrochloric acid extracts (Abiola et al. 2007).

128
One of the most important causes of damage to historical monuments is bronze disease, one 129 of the most important causes of this damage is chlorine ion (Scott 2011). Therefore, in the 130 present study, following various other researches, chloride medium has been used to study the

168
But one of the most important drawbacks of these inhibitors is that they are toxic. Although 169 natural inhibitors have less inhibitory efficiency than organic inhibitors, their performance can 170 also be optimized by re-treating these environmentally friendly inhibitors and making these 171 inhibitors available.    The acacia fruit extract was separately mixed and treated with a corrosive solution of sodium 211 chloride 0.5 M with pH=5.5, so that its corrosion power could be examined by the potentiostat 212 device (Table 1)  indicates that the type of inhibitor is combinatorial (mixed-type inhibitor (Some anodic and 216 some cathodic)) ( Fig. 2). In addition to change in potential of corrosion, a slight flow (Slight 217 current drop) is also seen in the anodic branch.

218
If the potential increases continuously, the curve will be anodic polarization and also if the 219 potential decreases continuously, the curve will be cathodic polarization. If polarization causes 220 a slight change to positive or negative, the curve will be of the combined (mixed-type inhibitor)  Calculating the corrosion efficiency using potentiostat device calculations 226 IE% was used to obtain the inhibitory efficiency percentage (formula 1). In this formula, 227 Icorr is density of the corrosion flow with inhibitory and I0 is corrosion flow without inhibitory.

228
(1) The corrosion current density (The corrosion current density is determined at a constant pH     The Tafel polarization of Robinia pseudoacania L fruit extract at 1400 ppm was shown 342 showed that the inhibitory solution corrosion potential is -216 mV, which compared to control 343 solution, it has a shift of direction to positive values (Fig. 3). The corrosion has also had slight 344 drop (Slight current drop) in the anodic and cathodic branches. The corrosion potential of the Robinia pseudoacania L fruit extract was -213mV at 1600 ppm 351 that it is showed displacement of 30mV, compared to the corrosion solution (Fig. 4). A slight 352 drop (Slight current drop) is also observed in the anodic branch. The corrosion potential of the Robinia pseudoacania L fruit extract was -213mV at 1800 ppm 359 that it is showed displacement of 30mV, compared to the corrosion solution (Fig. 5). A slight 360 drop (Slight current drop) is also observed in the anodic branch.  (Table 1). Mean comparison showed 370 that the highest rate of corrosion inhibition (93.5%) was obtained at a concentration of 1800 371 ppm and with increasing the concentration of the extract, corrosion inhibition also increased, ie 372 more bronze was prevented from burning (Fig. 6). Also, the highest corrosion inhibitory activity 373 of acacia extract (79.66) was in the second week and with increasing duration, this effect has 374 decreased (Fig. 7).    (Table 2). As the result, the relationship between potential, 397 current intensity and corrosion diagram was obtained based on the using of the anodic and 398 cathodic hypertrophy, measuring the potential difference between this electrode and the 399 reference electrode, as well as measuring the anodic and cathodic current intensity. Using the 400 data derived from the Potentiostat device (Table 2), the Weight reduction rates of Robinia 401 pseudoacania L fruit extract was calculated.

402
The Weight-reduction rate method is the simplest in the study of corrosion inhibition due to the 403 lack of need for a device. This experiment takes a long time, but because the results of this 404 method are more realistic than the electrochemical method, it is still used.

405
The results of the analysis of variance showed that the effect of different concentrations of 406 acacia aqueous extract on bronze weight reduction rate was significant (P <0.01). In the study 407 of the Weight reduction rates method, the results showed that the least Weight reduction rate 408 occurred at a concentration of 1800 ppm of acacia extract ( fig. 8), in general, the alloy weight 409 loss was least with increasing the concentration of acacia extract (fig 9). These results are the 410 same as results that were obtained by device  finally the device automatically calculated these slopes.

Acacia Concentrations
Week 1 Week 2 Week 3 Week 4 corrosion products (Fig. 10). However, it was observed that the surface of the same coupons 437 were covered after being placed in corrosive solution and acacia extract (Fig. 11), and no trace 438 of corrosion products were observed on the surface of the alloy (Fig. 12).

439
Scanning electron microscopy was used to evaluate and accurately perform this inhibitor on the 440 surface of these coupons. In the SEM images of the control coupon (Fig. 13), the corrosion 441 products in the grain boundaries were identified and showed the high impact of the corrosive 442 environment on this alloy. According to the morphology (needle and rhombus) of these 443 corrosion products, it can be said that they are of the type of atacamite and paratacamite that 444 have caused bronze disease in historical bronze works. These corrosion products are mostly 445 concentrated in the grain boundaries.

446
Although the aqueous extract of acacia fruit creates a uniform layer, which covers the surface 447 of the alloy and prevents the formation of corrosion products on the surface of these coupons 448 (Fig. 11) the SEM images showed that grain corrosion was induced in this alloy in the presence 449 of acacia extract (Fig. 13). At higher concentrations of this extract, i.e. from 1000 to 1800 ppm,  (Fig. 15).

459
In the present study, it was found that the natural inhibitor of acacia fruit aqueous extract can

495
In the present study, the Potentiostat device method and also the classic method of weight 496 loss used to determine the efficiency of corrosion inhibitory. The results shown that this 497 efficiency in the Potentiostat device method was 92% but in the classical method was equal to 498 55% that more has been mentioned than research (Vatankhah et al. 2011). Because acacia 499 contained flavonoids and phenolic compounds with a complex structure and high molecular 500 weight, it was expected to prevent corrosion, but it did not.

501
The inhibitory effect of two natural honeys (oak and acacia) with a mixture of black 502 horseradish juice on corrosion of tin in aqueous media and sodium chloride solution by weight 503 loss methods and polarization techniques has been studied. The results showed that the yield of 504 acacia honey was lower than oak honey and by adding black horseradish juice to both honeys, 505 their yield increased. The inhibition efficiency (IE) of all the inhibitors examined obtained by 506 both methods used decrease in order: chestnut honey with black radish juice > acacia honey 507 with black radish juice > chestnut honey > acacia honey (Radojčić et al. 2008). It has been 508 found that acacia extract has less effect than black horseradish extract. In the present study, 509 acacia extract had a positive effect at 1000 ppm, but with increasing acacia extract, the corrosion 510 inhibitory effect decreased. 511 Natural honey has been studied as a corrosion inhibitor of carbon steel in high-salt 512 environments. Inhibitory efficiency has been calculated through weight loss and static potential 513 polarization technique and the results have introduced natural honey as a suitable inhibitor for 514 corrosion of steel in high-salt environments. However, this beneficial effect has been limited to 515 a certain level and after a while, due to the growth and development of fungi, its inhibitory 516 efficiency has been reduced (El-Etre 1998). In the present study, acacia extract had a fungal 517 growth of 1800 ppm, which over time reduced the inhibitory effect.

518
Inhibition of organic compounds such as honey and rosemary (Salvia rosmarinus L.) on four 519 metals, aluminum, copper, iron and zinc in sodium chloride and sodium sulfate solution has 520 been investigated. The results showed that the inhibitors had no effect on aluminum in sodium 521 chloride and sodium sulfate solutions. The reason why honey is not inhibited is that honey may 522 play a small cathodic inhibitory role on aluminum when placed in polarized sodium chloride 523 solution (Yee 2004). Perhaps the reason for not increasing the inhibition of acacia extract by 524 increasing the concentration of the extract to 1800 ppm and the inhibition oscillation between 525 cathode and anodic is also due to the fact that the role of cathodic and anodic inhibition after 526 oscillation in polarized sodium chloride solution has decreased. However, it can have another 527 reason, such as less absorption power on the bronze (Cu-10Sn).

528
The Corrosion inhibition of steel in hydrochloric acid conducted by betanin as a green 529 inhibitor. The results obtained showed that betanin is a good "green" inhibitor for mild steel in Zn 2+ had about 98% inhibition efficiency to carbon steel immersed in well water(the results 535 shown that a mixed-type effect existed between BR extract and Zn 2+ ) (Selvi et al. 2009). In the 536 present study, the results indicate that the type of inhibitor is mixed-type inhibitor. Given the investigations on Robinia pseudoacania L fruit using potentiostat device, it was 540 revealed that the data derived from this device showed that Robinia pseudoacania L fruit 541 inhibitory efficiency at 1800 ppm with a corrosion rate of 12.78% is 55% for bronze alloy with 542 percentage of (Cu-10Sn) and has a mixed inhibitory effect. In the classic weight reduction rate, compounds to this inhibitor for better efficiency so that appropriate and optimal conditions for 549 this type of inhibitor can be defined.

550
One of the points that should be considered during the restoration process is not to change 551 the structure and appearance of the historical monument. Since corrosion in a metal monument, 552 especially copper and bronze, has a special place and importance from a historical, structural 553 and sometimes aesthetic point of view. As the result it seems necessary to pay more attention 554 to the rate of color changes after the application of new materials.   571 All authors read and approved the final manuscript for publication.

572
Availability of data and material 573 All the data is embedded in the manuscript.