Study the Corrosion and Corrosion Protection of Brass Sculpture by Atmospheric Pollutants in Winter Season

Brass is an important metalloid which is used in construction of sculptures. It is noticed that sculpture of brass is corroding due to interaction of pollutants. The pollutants develop chemical and electrochemical reaction on the surface of base material. Their concentrations of corrosive pollutants are increased in winter season. The air quality becomes very poor in winter season. Inside sculpture different forms of corrosion are observed like galvanic, pitting, stress, crevice etc. The major components of pollutants are oxides of carbon, oxides of nitrogen, oxides of sulphur, ammonia, ozone and particulates. Among these pollutants oxides of sulphur and ammonia are major corroder of brass. Ammonia is observed moist air to form ammonium hydroxide. It produces chemical reaction with brass metal and form complex compounds like [Zn(NH4)4](OH)2, [Zn(NH4)4]SO4, [Zn(NH4)]CO3, [Cu(NH4)4](OH)2, [Cu(NH4)4]SO4, [Cu(NH4)]CO3 etc. Oxides of sulphur react with moist air to exhibit sulphurous and sulphuric acids. They interact with brass to develop corrosion cell zinc metal and it is oxidized into Zn2+ ions and these ions are active to humidity and carbon dioxide to yield Zn(OH)2.ZnCO3.2H2O. Copper is converted into Cu2+ and it reacts with moist air and carbon dioxide to produce Cu(OH)2.Cu(CO3)2 and these complex compound detached on the surface of brass metal by rain water. These pollutants change their physical, chemical and mechanical properties and they also tarnish their facial appearance. Brass’ sculpture is affected by uniform corrosion. This type of corrosion can be control by nanocoating and electrospray techniques. For this work (6Z)-5,8-dihydrazono5,8-dibenzo[a,c][8]annulene and TiO2 are used as nanocoating and electrospray materials. The corrosion rate of material was determined by gravimetric and potentiostat technique. The nanocoating and electrospray compounds are formed a composite layer on surface of base metal. The formation of composite layer is analyzed by thermal parameters like activation energy, heat of adsorption, free energy, enthalpy and entropy. These thermal parameters were calculated by Arrhenius, Langmuir isotherm and transition state equations. Thermal parameters results are depicted that both materials are adhered with sculpture through chemical bonding. The surface coverage area and coating efficiency indicates that nanocoating and electrospray are produced a protective barrier in ammonia and sulphur dioxide atmosphere.


Introduction
The sculpture of brass comes in contact of contaminated air thus its deterioration starts for protection various types methods can be applied [1]. Brass [2] has major components is copper and zinc. Zn reacts the hot air to produce ZnO which is active in humidity and [Zn(NH 3 ) 4 ]SO 4 that complex layer is eroded in rain water. In acidic medium brass outer face has developed CuSO 4 and ZnSO 4 when dust particulates [13] are deposited on their surface which contains Fe to remove Cu and Zn from outer surface. Dust particulates are possessed oxides of alkali metal in presence of moisture, it produces NaOH or KOH [14] that is create hostile environment for Zn and it forms complex compound [15] Na 2 [Zn(OH) 4 ]or Na[Zn(OH) 3 .H 2 O] or Na[Zn(OH) 3 .(H 2 O) 3 ]. The oxides of NO 2 reacts with moist air to give HNO 3 that acid produces chemical reaction with Cu and it converted into Cu(NO 3 ) 2 . Some organic acids [16] available in air like acetic acid which develop corrosive environment for Cu and Zn which converts Cu into Cu 2 (CH 3 COO) 4 .H 2 O and Zn into (CH 3 COO) 6 .
Zn 4 O complex compounds [17]. They are eroded by rain water on the surface of brass. Organic compounds [18] like amnio and sulpur increased day by day in atmosphere. They develop hostile environment for brass and corroding it. Corrosive pollutants [19] concentrations like oxides of carbon, oxides of nitrogen, oxides of sulphur, hydride of sulphur and nitrogen, ozone and particulates are enhanced due to industrials wastes, effluents, flues and other factors are like burning of coals, woods and cow dung cakes.
Harmful pollutants [20] come into atmosphere through agricultural wastes, human wastes, pharmaceutical wastes, household wastes, food wastes and decomposition of living things. Various types of transports like road, water and air are evolving CO, NO 2 and SO 2 gases which produce acidic environments for brass. Several types of techniques are used to control the corrosion of brass like metallic coating; polymeric coating, paint coating, organic and inorganic coating of materials but these didn't give satisfactory results in corrosive medium. Some organic and inorganic inhibitors are applied to protect the corrosion of materials in acidic but they provide good results. Hot dipping, electroplating and galvanization techniques is used as protective tools for brass corrosion in acidic medium but these methods don't shave base metals. In this work it is to mitigate corrosion of brass corrosion by nanocoating and filler techniques. These materials form composite barrier on the surface base metal and blocked porosities and stop diffusion or osmosis process of pollutants.

Experimental
Brass coupons 15sqcm were taken for experimental analysis.

Results and Discussion
Brass metal was exposed in moist SO 2 (Table 1). The corrosion rate of brass metal was recorded in the months of November, December, January and February, the results ( Table   1) was shown that corrosion rate of metal increased in January to  (Table   1). The addition of nanocoating and electrospray were reduced the corrosion rates as temperatures variation, it noticed in K versus T in (Figure 7).      (1-K/Ko) X100 (where Ko is corrosion rate without coating and K is corrosion rate with coating) and their values were given (Table 1).
( Figure 9) show plot between %C (percentage coating efficiency) versus T (temperature in K). This figure indicated that percentage coating efficiency enhanced as temperatures varies in Nov to Feb months and their values were recorded in (Table 1). Figure 6 plotted between θ (surface coverage area) versus C (concentration in mM) and covered areas were produced by (6Z)-5, 8-dihydrazone- (Table 1).

5,8-dibenzo[a,c][8]annulene and TiO 2 were mentioned in
The results were shown that nanocoating compound occupied less surface areas with respect of electrospray. The surface coverage area developed by nanocoating and electrospray compound was calculated by formula θ = (1-K/Ko). (Figure 10) plotted between θ (surface coverage area) versus T (temperature) noticed that temperatures were varies from Nov to Dec but surface coverage area and electrospray values were increased and their values were written in (Table 1).

Figure 9
: V s C nanocoating and electrospray.    (Table 2). The plot between logK versus 1/T was found to be straight line as shown in (Figure 11). The plot between log K and 1/T found to be straight line. It observed that activation before coating activation energy high but decreased after coating.

Month C(mM) Temp(K) SO 2 PPM Ea o (kJ/mol) Ea(kJ/Mol) q (kJ/mol) ΔG (kJ/Mol) ΔH (kJ/mol) ΔS(kJ/K)
These trends indicated that nanocoating compound adhered on the surface of base metal. Heat of adsorption was calculated by Langmuir isotherm log(θ/1-θ) = log(AC) -q/2.303R T and their values were mentioned in (Table 2). Its values were found to negative, it indicated nanocoating compound formed chemical bond with base metal. (Figure 12 and their values were recorded in (Table 2) (Table 2). These values were found to be negative which indicated these compounds adhered on the surface of metals.
All thermal parameters versus T (temperature) plotted in ( Figure   13) which indicated composite barrier formed on surface of base metal. Thermal parameters Values of TiO 2 eleectrospray activation energy, heat of adsorption, free energy, enthalpy and entropy were written in (Table 3) and their plot against T (temperature) in ( Figure   14). (Table 3) (Table 4). (Figure 15) was plotted ΔE(corrosion potential versus I(corrosion current density).
The results of (