Influence of Formic Acid on Corrosion Behavior of Bronze under Thin Electrolyte Layer

TANG Yijing ,CAI Lankun ,WANG Ying ,WANG Min ,ZHOU Hao ,WU Laiming ,YAN Ying＊

(1.School of Resources and Environmental Engineering,East China University of Science and Technology,Shanghai 200237,China;2.Shanghai Museum,Shanghai 200050,China)

Abstract: The corrosion behavior of formic acid towards bronze under thin electrolyte layer (TEL) was investigated by means of electrochemical measurement.Bare bronze had smaller self-corrosion current density than Cu2O patina bronze and CuCl patina bronze,while it had higher polarization resistance.The corrosion behavior of bronze materials had differences in various TELs and bulk solutions.The critical thickness of TEL for Bare Bronze was 200 μm,which normly occurred in the transformation from anode control to cathode control.The thickness of TEL had a negligible effect on the corrosion rate of Cu2O patina bronze when it was greater than 150 μm.For CuCl patina bronze,the corrosion process accelerated with thinner TEL.SEM was used to analyze the morphology and composition of corrosion products.Cu2O,Cu(OH)(HCOO) and Cu(HCOO)2 were formed on the surface of Bare Bronze and Cu2O patina bronze,nevertheless,the main corrosion product of CuCl patina bronze was Cu2Cl(OH)3.

Key words: EIS;bronze;formic acid;atmospheric corrosion

1 Introduction

Bronze is a copper-tin alloy that has been used diffusely in sculptures and artefacts by its excellent corrosion resistance,ease of fabrication,and low cost.However,the performance or artistic value of many bronze artifacts were affected and even destroyed due to the increasing air pollution.

The atmospheric corrosion behavior of bron-ze have been comprehensive studied,which suggested that organic acids played a critical role in the corrosion process of bronze.Unlike outdoor pollutants such as oxides,sulfur oxides,nitric oxides and ozone,organic acids are mainly emitted from the indoor wood furniture.In addition to the fact that the hydrolysis of acetyl esters produces organic acids in wood itself,the binders,glues and paints used in the furniture manufacturing process also release carboxylic acids.Moreover,in a relatively sealed environment of furniture,cabinets and storage facilities,the concentration of organic acids inside the space will reach a higher level.Studies by Allen and Miguel suggested that exposure to organic acids in indoor environments has increased by 2-15 times in comparison with that in outdoor.Cultural heritage institutions have shown that in a closed environment with limited air exchange,the concentration of organic acids can reach hundreds or even thousands of ppb.

Use of wooden storage and display units indoors can gradually result defect on the artifacts when increasing concentrations of organic acid.Low concentrations of formic acid greatly accelerate the atmospheric corrosion of lead.The main corrosion products on lead artifacts surface are the lead formate and basic lead carbonate.Meanwhile,formic acid can also induce“ant-nest corrosion”in the copper tube,causing serious damages to the equipment.Zinc-copper alloy formed hemispherical corrosion products (zinc formate dihydrate) at the presence of low concentration formic acid (100-120 ppb) and high relative humidity (RH,80%-90%).George and Nesic investigated the role of acetic acid in atmospheric corrosion of aqueous X65 mild steel,and reported that the corrosion process of carbon steel accelerated obviously after adding 100 ppm acetic acid.Bronze materials undergo serious corrosive damage exposed to low concentration formic acid.Tomas

et al

reported that organic acids were more aggressive to bronze materials when the concentration in air was about 300 ppb.Although bronze was vulnerable in organic acid,attention focus on the corrosion behavior between the bronze matrix and rust layer is still relatively rare.

In this study,the role of formic acid in affecting the corrosion behavior of bronze was investigated by electrochemical measurement under thin electrolyte layer (TEL).The bare bronze,CuO patina and CuCl patina bronze were analysed to find out the relationship between surface rust layer and the corrosion behavior of bronze materials since most of the bronze handicrafts undergo atmospheric corrosion with the rust layers on surface.XRD (X-ray diffraction),SEM (scanning electron microscope) and EDS (energy dispersive spectrometer) were used to characterize the surface morphology and composition of the bronze materials.Initial corrosion mechanism was proposed to describe the corrosion process of bronze materials in formic acid.

2 Experimental

2.1 Materials

The chemical composition of bronze used in this study was Pb 4 wt%,Sn 5 wt% and Cu 91 wt%.The bronze alloy was embedded into nylon,leaving a size of 1.0 × 1.0 cmas working surface.The working electrodes were abraded gradually from 240 to 1 200 grit with water sandpaper,cleaned with acetone and ultrapure water and dried in nitrogen gas lastly.

The experiment was conducted in the mixture solution containing 0.01 M NaSO,0.028 M NaCl and 500 mg/L formic acid solution.The solution including 0.01 M NaSOand 0.028 M NaCl was used as the background value of the environmental influence,because large amount of sulfate and chloride corrosion products were found on the surface of the bronze material.The CuO and CuCl patina layers were made by electrochemical method according to previous studies.

2.2 TEL setup

The experimental set-up was based on our previous work.The whole TEL setup is shown in Fig.1.A three-electrode electrochemical system with saturated calomel electrode as reference electrode,bronze as the working electrode and platinum wire as counter electrode was installed around the working electrode.The electrochemical cell was placed on the horizontal platform to ensure the stability of the setup and the ohmmeter.Platinum wire and micrometer were used to measure the thickness of TEL on the working electrode surface.The ohmmeter had a sudden change in the number while the Pt needle touched the working electrode surface leading to the connection of the circuit.The two values of micrometer could be measured while the tip of Pt needle moved down slowly and contacted the working electrode surface and electrolyte surface,respectively.The thickness of the TEL could be calculated according to the two recorded micrometer values.

Fig.1 Thin electrolyte layer (TEL) schematic description

2.3 Experimental measurements

EIS tests were measured by advanced electrochemical system (PARSTAT 2273) at open circuit potential (OCP) disturbed with a sinusoidal potential perturbation of 10 mV and the frequency range of measurement was from 10 kHz to 5 MHz.The cathodic polarization curves measurements was conducted with the sweep rate of 0.5 mV/s.All measurements were studied at room temperature (25 ±1 ℃).

The X-ray diffraction (XRD,D/MAX 2550VB/PC),scanning electron microscope (SEM,S-3400N)and energy dispersive spectrometer (EDS,APOLLO X)were used to characterize the surface morphology and composition of corrosion products on bronze surface.

3 Results and discussion

3.1 1EIS measurement

The effects of formic acid on rust layer bronze and bare bronze at various TELs were conducted by EIS measurements.The Nyquist and Bode plots are illustrated in Fig.2,respectively.The impedance spectrum in the bulk solution is shown in the upper right.

Fig.2 Nyquist and Bode diagrams of bronze materials in various TELs in atmospheric background solution with 500 mg/L formic acid:(a)bare bronze;(b) Cu2O bronze;(c) CuCl bronze

The Nyquist plots of the bare bronze in different TELs show similar shape as shown in Fig.2(a),which consists of a capacitive arc at high and intermediate frequencies(HFs and MFs) and a straight diffusion tail(Warburg impedance) at lower frequency (LFs).The HF loop is due to the response of corrosion product film resistance and solution resistance,while MF capacitive arc corresponds to the charge transfer resistance in material surface.The straight diffusion tail (Warburg impedance) may be related to the diffusion process of oxygen and metal-iron.This indicates that the corrosion process of bronze is affected by both the diffusion process and charge transfer.

Warburg impedance can be clearly observed in the Nyquist plots of bare bronze because the rapid interfacial charge transfer leading to the diffusion of redox reactants becomes more prominent in the initial stage of corrosion process.

The radius of the capacitive-reactance arc of bare bronze is the smallest when the TELs thickness are 197 and 298 μm.When the thickness of the TEL is decreased to 100 and 148 μm,the radius of the capacitive-reactance arc becomes larger.It suggests a weakness of the corrosion resistance through the thickness of TEL increase.The capacitive-reactance arc radius of bare bronze was the largest in bulk solution,indicating that with the increase of TEL thickness,diffusion of oxygen became difficult and the control of cathode was more prominent when the TEL thickness exceeds 298 μm.

In Fig.2(b) and Fig.2(c),Warburg impedance of CuO and CuCl patina bronze can not be identified because of the rusty layer on the surface which leads to a slow interfacial charge transfer.The capacitivereactance arc radius of bare bronze is larger than that of CuO patina and CuCl patina bronze,indicating that formic acid has higher aggression to rusty layer than that of bare bronze.The capacitive-reactance arc radius of CuO patina bronze at the TEL thickness of 99 μm is the largest,indicating the highest corrosion resistance at this time.The capacitive-reactance arc radius of CuCl patina bronze increases with the increase of the thickness of TEL,showing a weakness of the corrosion resistance through the thickness of TEL decrease.The corrosion reaction of CuCl patina bronze is cathode control due to the loose rusty layer.

Fig.3 Equivalent circuit for EIS results fitting

The EIS data is fitted by the equivalent circuits as shown in Fig.3,which can quantify the effect of formic acid on the corrosion behavior of bronze.The circuit elements are defined as:

R

is the charge transfer resistance,

R

represents the solution resistance,

R

(the film resistance) is the film resistance of the corrosion layer on material surface,

Z

is the Warburg diffusion impedance and

R

(the polarization resistance) can be expressed as the sum of

R

and

R

,which is used to evaluate the corrosion resistance of materials generally.In Fig.3,

Q

and

Q

can be explained by dispersion effects that caused by microscopic roughness of a solid surface,while

n

and

n

represent respectively the diffusion coefficients corresponding to

Q

and

Q

.The impedance data is fitted by two different equivalent electrical circuits.Fig.3(a) is applied to fit the EIS data corresponding to rusty bronze(CuO and CuCl patina bronze) and Fig.3(b) is for bare bronze due to the Warburg diffusion impedance.The fitted impedance spectrum parameters for bronze materials are shown in Tables 1-3,and the fitting effect is ideal (

χ

about 10-10).It is clearly shown in Table 1,the polarization resistance (

R

)of bare bronze under different thickness of TELs issequentially decreased as follows:bulk solution >148 μm >100 μm >298 μm >197 μm.The corrosion resistance gradually increases with the increasing TEL thickness due to lower Oconcentration,suggesting that cathodic reaction dominates corrosion process of bare bronze.When the TEL thickness becomes thinner and less than 200 μm,the diffusion of corrosion products became harder,and the anode reaction control plays a prominent role in corrosion process.The minimum corrosion resistance was observed when the cathode and anode reaction control switched,and a critical thickness corresponded to 200 μm.

Table 2 Fitting parameters of EIS for CuO patina bronze in various TELs in atmospheric background solution with 500 mg/L formic acid

Both the film resistance (

R

) and charge transfer resistance (

R

) of bare bronze increases significantly compared with that of rusty bronze,reflecting that formic acid had higher aggression to rust layer.The film resistance (

R

) value of rusted bronze increases with increasing TEL thickness,reflecting the enhancement of corrosion resistance,which is the opposite of bare bronze.The polarization resistance (

R

) CuO patina bronze as shown in Table 2 at a TEL thickness of 99 μm is much larger than that of other TELs which is caused by higher

R

values under thinner TEL thickness.The corrosion resistance of CuO patina bronze is similar in formic acid when the thickness exceeds 150 μm.The formic acid may exhibit a higher aggression toward CuO patina bronze which leads the influence of TEL thickness is small in corrosion process.Table 3 shows that the

R

of CuCl patina bronze decreased in the sequence of bulk solution >297 μm >200 μm >150 μm >102 μm.The thinner TELs,the easier diffusion of oxygen and the smaller corrosion resistance.

3.2 Tafel polarization curve

The Tafel polarization curve and the fitting date were used to study the corrosion behavior of bronze materials in formic acid as shown in Fig.4 and Tables 4-6.

As shown in Fig.4 (a),the Tafel polarization curve of bare bronze is asymmetric,and its anode branch is steeper than the cathode branch,indicating that the cathode process plays a crucial role in the corrosion process.Oxygen diffusion may be the main factor cause the fluctuation of cathode polarization curve in bare bronze.As shown in Table 4,the self-corrosion current can reflect the corrosion rate of the bronze material,meaning that the corrosion rate is the largest at 200 μm.And the bare bronze reaches a critical thickness at about 200 μm.The result is consistent with the impedance spectrum.

Fig.4 Polarization curve of bronze in various TELs in atmospheric background solution containing 500 mg/L formic acid:(a)bare bronze;(b) Cu2O patina bronze;(c) CuCl patina bronze

Fig.4 (b) and Fig.4 (c) indicate that the corrosion process of rusty layer bronze in bulk solution is different from that under TELs.Table 5 shows that the

I

of CuO patina bronze is the smallest at about 100 μm.The

I

of CuO patina bronze is close when the thickness of TEL exceeds 150 μm.The

I

of CuCl patina bronze increases with the decreasing TEL thickness as shown in Table 6,indicating that the diffusion of oxygen is the main factor restricting the electrochemical reaction,and the cathode reaction dominates the corrosion process.The

I

of bare bronze is significantly smaller than that of CuO and CuCl patina bronze,indicating that rusty bronze in formic acid is more vulnerable than bare bronze.

Table 5 Fitting parameters of polarization curve for CuO patina bronze in atmospheric background solution containing 500 mg/L formic solution

3.3 Characterization

The bronze materials were exposed in 100 mg/L formic acid for 15 days to investigate the changes in corrosion morphology and analyze the corrosion products in atmospheric environment.

The SEM images of three bronze samples are shown in Fig.5.After exposure experiments,the uniform polishing marks on the bare bronze surface were covered by bulk and particles.The dense granular rust layer covered on CuO patina bronze surface was destroyed,forming uneven corrosion particles after exposed for 15 days.The surface of CuCl patina bronze was covered with a layer of rugged flaky crystals that are disorderly arranged and loose in structure.The original large flaky crystals were destroyed and fine particles were formed after exposed for 15 days.

Tables 7-9 show the EDS results of three bronze samples.The difference between the three bronze samples before the exposure in formic acid vapor is mainly reflected in the O and Cl elements,which are related to the rust layer carried by the sample.The content of C and O elements increased significantly after exposure experiments,indicating that corrosion product may contain the formic acid compounds.

Table 7 EDS analysis of bare bronze exposed to 100 mg/m formic acid/wt%

Table 8 EDS analysis of CuO patina bronze exposed to 100 mg/m formic acid/wt%

Table 9 EDS analysis of CuCl patina bronze exposed to 100 mg/m formic acid/wt%

Fig.5 SEM micrographs before and after corrosion of three bronze materials in 100 mg/L formic acid.Before corrosion:(a) bare bronze;(b)Cu2O patina bronze;(c) CuCl patina bronze.After corrosion:(d) bare bronze;(e) Cu2O patina bronze;(f) CuCl patina bronze

Fig.6 XRD result of three rusty bronze materials in 100 mg/L formic acid:(a) bare bronze;(b) Cu2O patina bronze;(c) CuCl patina bronze(a=Cu;b=Cu2O;c=CuO;d=CuCl;e=Cu2Cl(OH)3;f=Cu(OH)(HCOO);g=Cu(HCOO)2,(1) after exposure;(2) before exposure

To combine with the XRD result shown in Fig.6,the corrosion products in bronze surface were determined before and after exposed in formic acid for 15 days.The main corrosion products on bare bronze surface are CuO,Cu(OH)(HCOO) and small traces of Cu(HCOO).CuO patina bronze is similar to that of bare bronze in formic environment,but the corrosion influence is more severe than bare bronze owing to the amount of corrosion products.There are no corrosion products of formate found in CuCl patina bronze surface and the main corrosion product is CuCl(OH),indicating that formic acid only provides an acidic environment during corrosion.There is a significant increase in copper content due to the exposure of the bronze matrix in formic acid atmosphere.

4 Conclusions

The results of electrochemical measurements were investigated in this paper,and the results show that formic acid not only had a corrosive damage to bronze matrix,but also had a stronger corrosive effect on CuO and CuCl rust layers than that of bare bronze.The corrosion process of bronze materials in various TELs and bulk solution are different.For bare bronze,the critical thickness of TEL is about 200 μm,which commonly occurs during the transition from anode control to cathode control.The corrosion rate of CuO patina bronze is higher,and is less affected by the thickness of TEL when the TEL thickness exceeds 150 μm.The corrosion rate of CuCl patina bronze decreases with thinner TEL.In addition,the diffusion of oxygen is the main factor restricting the electrochemical reaction.The corrosion products form on bare bronze and CuO patina bronze surface are CuO,Cu(OH)(HCOO) and Cu(HCOO),however the main corrosion product of CuCl patina bronze is CuCl(OH).