Physical derusting or transformation methods mainly include mechanical derusting and ultrasonic method [5]. Mechanical derusting is the direct derusting with hand tools such as steel needle and scalpel. The method is simple and the effect is good. Ultrasonic method uses the energy generated by ultrasonic vibration to produce the effect of rapid cleaning. There are other methods, but they all need further development.
2. Chemical removal or conversion methods
Chemical removal or conversion methods mainly include sodium sesquicarbonate method, silver oxide method, PbO method, zinc powder conversion method [5-7] and so on. Sodium sesquicarbonate method is to soak bronze wares in Na2CO3 NaHCO3 solution, and replace Cl- in CuCl with CO32- to form stable copper salt. This method takes a long time in practical application. The silver oxide method converts CuCl into Cu2O through the following reactions:
Ag2O+2CuCl=2AgCl+Cu2O
This method has a certain effect. Ag2O can transform some harmful rust, but not completely. The silver chloride produced by the reaction has strong conductivity. From the point of view of electrochemical reaction, its protective effect on bronzes decreased with the passage of time, and then evolved into a promoting factor for bronze corrosion. The principle of PbO method is the same as Ag2O method. Zinc powder conversion method utilizes the activity of zinc, and its reaction formula is as follows:
Copper chloride+zinc → cuprous oxide+zinc hydroxide
Copper chloride+zinc → cuprous oxide+zinc hydroxide
Zn(OH)2 produced in the reaction is a colloidal substance, which can play a sealing role. The weakness of chemical treatment is that it will do harm to the cultural relics themselves, so its application is limited.
3. Corrosion inhibitor protection method
There are BTA method, BTA derivative method [8- 12], 2- amino -5- mercapto-1, 3,4-thiadiazole (AMT) method [1, 13- 14], 2- mercaptobenzoxazole (MBO) method [15- 18], 2- mercaptobenzothiazole (MBT) method and 2- mercaptobenzimidazole (MBI) method BTA. BTA derivatives are obtained by replacing hydrogen atoms on BTA benzene rings with other groups such as alkyl, alkoxy, amino, hydroxyl and carboxyl. The corrosion inhibition effect of some BTA derivatives on copper is better than BTA, but its application in cultural relics protection needs to be developed. AMT method uses AMT to form a complex with copper ions in copper rust to achieve the purpose of protection. According to literature reports, AMT can completely remove powdery rust, but the surface of bronzes treated by AMT is slightly discolored. 2- mercaptobenzoxazole (MBO) has obvious corrosion inhibition effect on copper. MBO can interact with monovalent copper ions to form a three-dimensional corrosion inhibition film at the electrode/solution interface, thus inhibiting the corrosion process of Cu. It has high corrosion inhibition efficiency and basically does not affect the appearance of bronzes. It is an excellent corrosion inhibitor. MBT and MBI have low corrosion inhibition efficiency on copper, and have a serious impact on the appearance of bronzes, so they are not suitable for bronze protection. Zhang Daquan [20-22] and others studied the synergistic corrosion inhibition of imidazole (IM) and benzotriazole, 8- hydroxyquinoline (HQ) and benzotriazole on copper, and found that IM and BTA can be adsorbed on copper surface, and the presence of IM is beneficial to the adsorption of BTA, which has a good synergistic effect. After BTA and HQ are used together, the membrane resistance of copper electrode increases and the membrane capacitance of electrode decreases. They have synergistic corrosion inhibition.
4. Protection method of sealing coating
The protection method of sealing coating is generally the last step in the process of bronze cultural relics protection. At present, most of the coatings used in the protection of bronze cultural relics at home and abroad are acrylic coatings. Protection of corroded bronzes by Wang Huizhen of Northwest University [23] and others; Ma Zhuo Liu Yue [24] of Xi Cultural Relics Protection and Restoration Center and others have protected and restored the bronzes of the Western Zhou Dynasty; Liu Linxi [25] restored and protected the Han Dynasty bronze cymbals; Yang Xiaolin [26] of the Chinese History Museum and others' protection of the gilded bronze statue of the heavenly king in the Tang Dynasty; Gansu Provincial Museum Cultural Relics Protection Office [7] protected and restored 80 pieces of Lingtai bronzes; Song Man, Museum of Chinese History [27] The bells, rhinoceros statues, He Zun and other cultural relics unearthed from Caihou Tomb in Shouxian County, Anhui Province are all protected with acrylic resin as sealing material. Other sealing materials are also used, such as Lu Shoulin of the Palace Museum and Li Huayuan of the China Institute of Cultural Relics Protection [28]. They use polyvinyl butyral as a sealing coating to protect corroded bronzes, and add a proper amount of BTA to the sealing agent formula as a supplement to anticorrosion; Zhao Maozhen of the Palace Museum and others used wax to protect bronzes, but wax is easy to absorb dust, which is its weakness. Tian et al. [30] conducted experiments on acrylic varnish, nitro varnish, polyurethane varnish, silicone varnish and other resins produced by Tianjin Paint North Factory, Shanghai Paint Factory, Beijing Hongshi Paint Research Institute and Beijing Paint Factory according to the standard inspection method of paint industry. The results show that silicone coating products have corrosion resistance, moisture resistance and mildew resistance, but the coating temperature must be heated to 50℃. Polyurethane resin has good mechanical properties, but it is not suitable for cultural relics protection because of its high viscosity and dark color. Nitro varnish has the advantages of quick drying, good luster, high hardness and wear resistance, but its acid and alkali resistance is low. Acrylic resin is colorless and transparent, easy to use, fast curing at room temperature, high strength, light resistance, aging resistance, high heat resistance, good thermoplasticity, strong corrosion resistance and good outdoor durability. Therefore, among the selected coatings, acrylic resin has the best effect and can be used for surface protection of metal cultural relics. But not all acrylic products can be used for surface protection of metal cultural relics, and only some can meet the requirements of cultural relics protection. Beijing university of chemical technology Xu Chunchun and others have developed a waterborne composite sealant for bronze cultural relics, which consists of a bottom sealant and a top sealant. The bottom sealant is obtained by modifying acrylate polymer emulsion with excellent performance with corrosion inhibitor, and the top sealant is obtained by modifying acrylate polymer emulsion with nano-materials. The corrosion resistance of bronze ware can be significantly improved by compounding corrosion inhibitor into waterborne acrylate polymer emulsion, and the nano-composite sealant obtained by modifying nano-materials has achieved a functional leap. The properties of composite sealant were studied by various testing methods. The results show that the composite sealant has a strong protection ability for copper and copper alloys, and the nano-materials give full play to their ultraviolet absorption characteristics, effectively improve the anti-ultraviolet aging ability of the coating, and have certain superhydrophobic characteristics, so that it has a self-cleaning function. At the same time, due to the addition of corrosion inhibitor, the acid resistance, alkali resistance, salt water resistance and salt spray resistance of sealant are obviously improved. The developed composite sealant contains no organic solvent, is nontoxic and tasteless, and is beneficial to environmental protection. Moreover, it is colorless, transparent and dull, which can keep the original appearance of cultural relics and conform to the principle of cultural relics protection. It is a sealing agent for bronze cultural relics with excellent comprehensive performance. Papers on this sealant have been accepted by Materials Protection, Corrosion Science and Protection Technology, Journal of beijing university of chemical technology and other publications.
Generally speaking, people often use the above methods to protect bronzes.