Assessment of Some Corrosion Inhibitors and protective coatings for protection of bronze artifacts

Ahmed. E. Hemaid1*, Mohamed El-Gohary2 and Fatma M. Helmi3

1Department of applied science and technology, Politecnico di Torino, 10126, Turin, Italy, ahmed.elsayed@polito.it

2 Conservation dept., Sohag Univ., 82524, Sohag, Egypt, m_1968_algohary@yahoo.com 3 Conservation dept., Cairo Univ., 12613, Cairo, Egypt, dr.fatma25@outlock.com

Abstract Preventive conservation is one of the most important processes used for saving artifacts. The

corrosion inhibitors and protective coatings are highly recommended for protecting metals from degradation and corrosion process.

This research focuses on laboratory tests of five corrosion inhibitors and protective coatings on Bronze samples in order to measure the efficiency of these materials to be applied on Bronze artifacts. Two derivatives of acrylics, (Paraloid B72 and B82), two Amino acids (Cysteine and Valine), and Benzoteriazol (BTA) with concentration 3% were compared as a protective coating. Two laboratory assessment methods are used, weight loss and polarization tests. Bronze samples are divided into six groups, the first group is a blank group and the others apply one material for each group and each group contains three samples.

The tested samples are exposed to 1M copper chloride solution for 24 hours and measures are record. After this step we apply the materials on samples and expose these samples to temperature 40ᵒ and humidity 100% for 24 hours in a controlled chamber.

Weight loss test proved that the best Protection and inhibition efficiency of Martials is Cysteine efficiency record 99.71%. Paraloid B72 efficiency is 99.42% compared to other materials.

The electrochemical test illustrated that the best protection efficiency of these Martials are Paraloid B82 which is 99.74% and Paraloid B72 which is 99.64 %.

Key words: Bronze, Coatings, BTA, Amino acids, Polarization.

Introduction In the humid and aggressive environments Metallic artifacts is effects and attacks by different

corrosion factors [1-2]. Copper and its alloys suffering from a lot of deterioration factors in uncontrolled environment. Oxygen interact with copper and bronze to form copper oxide as a first step in the atmospheric corrosion, then other cycles start to completely damage the artifacts by other factors for example, chlorine attack to form (Atacamite), a copper hydroxide chloride Cu2Cl(OH)3 [3-

4]. The research aimed to use good protective materials for metallic artifacts especially bronze artifacts [5]. The applying of protective coatings or corrosion inhibitors are necessary processes for metal artifacts conservation to safe the metals after treatment from corrosion factors in indoor and outdoor environments [6]. There are different composition types of corrosion inhibitors and coatings, organic inhibitors as Benzoteriazol (BTA), and inorganic inhibitors such as silicates group [5] like SiO2, phosphates, molybdates and tungstates [7,8].

Related to the inhibitors mechanisms there are three types, anodic inhibitors, Cathodic Inhibitors, and mix of them. There mechanisms work to slow the electrochemical corrosion processes. The materials which can be used should be reversible, dependable, transparent and friendly for environment, also the efficiency of these materials should work in low concentration to protect metals in corrosion environments [1,6, 10,11]. The research focused on test five materials as a corrosion inhibitors and protective coatings to use for bronze artifacts in museum environment. The bronze alloy was casted in the same composition of ancient bronze artifacts and in the same casting way in the Islamic ages in Egypt. The uses of weight loss and Polarization test in NaCl media to measuring the efficiency of these materials to protect bronze artifacts in the microclimate. [12-14]

Materials and methods:

The composition of the bronze alloy which used was 90% copper with 10% tin. It was prepared by traditional casting methods (fig 1a&b). The alloy divided to six groups each group contains three bronze disks in. the disks size was 5.5 diameter, and 2.5 mm thickness. All samples were polished with 1000 grade paper. The groups were one for each material and the last was standard group (fig 1c&d).

Figure (1) shows a. & b. the bronze alloy casting, and c. & d. shows the samples cutting and the selected groups

The Weight loss test used to study the efficiency of the chosen materials by take the samples

weight before and after apply the protective coating and inhibitors, then expose the samples to accelerating corrosion by Immerge samples for 24 hours in 1M NaCl, in room temperature (fig 2A).

a b

c d

Group 1 was a standers group without any coating or inhibitors. Group 2 was coated by Paraloid B72 (copolymer of 70 % of ethyl methacrylate and Methyl acrylate 30%) in 3% concentration in toluene it was applied by brush [15,16]. Group 3 was coated with Paraloid B82 (Methyl Methacrylate) in 3% concentration in toluene and also applied by brush. Group 4 the samples was immerged in Benzoteriazol (BTA) (C6H5N3) [17,18] 3 % in Ethyl alcohol for 24 for hours. Group 5 and 6 was coated by two amino acids, Cysteine {HO2CCH(NH2)CH2SH} 3% in water for group 5, and Valine {HO2CCH(NH2)CH(CH3)2

} 3% in water for group 6, and applied by brush (fig 2B) [19-21]. After applied the materials the samples exposed 100% RH in 40o C for 24 hours (fig 2C)

Figure (2) shows a. samples after 24 hours of immerged in 1M NaCl, b. samples after applied the materials, c. expose samples in 100% RH in 40o

Polarization test its one of the electrochemical measurements which determines different

parameters such as corrosion current density (icorr), corrosion potential (Ecorr), and finally and Tafel slopes. It used to test the corrosion resistance of the protective coatings efficiency against the electrochemical corrosion [22-24]. The samples also divided for six groups like the previous test but one sample only for each group, and exposed it in the cell with 3.5% NaCl (Figure 3). The test conditions were as descried in table (1). The Auxiliary Electrode was Platinum; the Reference Electrode was Calomel (Hg2Cl2).

Table (1) shows the conditions of the polarization test Apparatus Volta Lab (PGZ100) Corrosion Rate µm / Year

Polarization Range (- 800 : + 200) Corrosion Current µm / cm2

Scan Rate 0.5 mv/ Sec. Corrosion Potential mv

Solution NaCl 3.5% PH value 3

Sample Type Bronze Exposed Area .78 cm2

Figure 3 shows sample in the polarization cell

a b c

I - I1 IE % = Χ 100 I

Result and Discussion The result of weight loss measurements is described in the table (2) and figure (4) . By using

the following equation was calculated the efficiency of the materials Where, W and W1 are the corrosion rate of samples with and without the inhibitors [25-26]

Table 2 shows the weight loss test values for each group and materials Group

(Samples n.) Protective coatings

Weight average

W. average after NaCl

W. average after coatings

W. after ageing

W. Changes

Protective efficiency

Group1 (1,2,3) standard 42.984 42.275 - 42.286 0.698 -

Group2 (4,5,6)

Paraloid B72 3% 35.654 34.995 35.016 35.020 0.004 99.42%

Group3 (7,8,9)

Paraloid B82 3% 38.377 37.671 37.684 37.691 0.007 98.99%

Group4 (10,11,12) BTA 3% 31.108 30.377 30.442 30.419 0.023 96.70%

Group5 (13,14,15)

Cysteine 3% 36.481 35.809 35.820 35.818 0.002 99.71%

Geoup6 (16,17,18)

Valine 3% 31.408 30.663 30.672 30.666 0.006 99.14%

Figure 4 shows the efficiency of the protective coatings in weight loss

Polarization test results described in table (3) and figure (5-10) shows the polarization curves and slops. Inhibition degree was calculated by the following equation

Where, I and I1 are the density of corrosion current before and after applying the inhibitors [26].

Table (3) shows polarization test results

Efficiency %

)2(µA/cmI.corr Corr. Rate (µm/ Year)

(mv) corrE materials

- 5.3706 62.205 -325 Standard 99.64 0.0190 0.221 -245 Palraloid B72 99.74 0.0138 0.160 -230.8 Paraloid B82 96.03 0.2127 2.464 -252.7 Benzotriazol BTA 51.43 2.6086 30.210 -290.8 Cysteine 71.61 1.5240 17.654 -298.4 Valine

W - W1 IE % = Χ 100 W

Figure 5 polarization curve of the standard sample Figure 6 polarization curve of B72 sample

Figure 7 polarization curve of B82 sample Figure 8 polarization curve of BTA sample

Figure 9 polarization curve of Cysteine sample Figure 10 polarization curve of Valine sample

The results of the polarization test were different than the weight loss. It was calculated related to the equation: In this test the efficiency of the acrylic products were the height values. Paraloid B 82 was the first one with 99.74%, the second was Paraloid B72 with 99.64%. The third efficiency was Benzotriazol BTA with 96.03% and this efficiency was almost like weight loss test. The lowest efficiency were amino acids, Valine was 71.61%, and Cysteine was 51.43% Figure (11).

Figure 11 shows the inhibition efficiency of materials in polarization test

Conclusion

The research still under evaluation and will characterize the materials by optical microscopy and SEM to see how these materials affected by the previous tests.

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