1. Introduction {#sec0005}
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The corrosion properties of brass, herein referred to as an alloy of copper and zinc, have attracted great attention over the years owing to the importance of brass in various technological and artistic applications [@bib0005; @bib0010; @bib0015; @bib0020]. Most corrosion-related studies have been performed in bulk solution, and the results largely explained on the basis of selective dissolution and dezincification mechanisms caused by the difference in electrochemical nobility between the main alloy constituents [@bib0025; @bib0030; @bib0035; @bib0040; @bib0045; @bib0050; @bib0055; @bib0060]. Some studies have been concerned with the passive behavior of brass in various solutions using electrochemical methods in combination with surface analytical techniques. The results suggest the passivation to be attributed to the formation of a complex passive layer on brass consisting of mixtures of ZnO and Cu~*x*~O~*y*~, and with a composition strongly influenced by the anions of the solution [@bib0065; @bib0070; @bib0075; @bib0080]. Other studies have focused on the oxidation behavior of brass in either pure oxygen or air at ambient or elevated temperature, and interpreted the results mainly through the semiconducting and electrochemical properties of the passive layer formed [@bib0085; @bib0090; @bib0095; @bib0100].

So far, the fundamental aspects of the atmospheric corrosion of Cu--Zn alloys have received relatively little attention, although a detailed understanding exists for their pure elements Cu and Zn [@bib0105; @bib0110; @bib0115; @bib0120; @bib0125; @bib0130]. Hence, this study is a first effort to explore the fundamental processes governing the atmospheric corrosion of a brass (Cu--20Zn) alloy through a multi-analytical in situ and ex situ approach. Results from the oxidation of brass will be presented based on exposure in air at 90% relative humidity. This paper will be followed by a second, in which the influence of carboxylic acids on the atmospheric corrosion of brass will be explored in some detail.

2. Materials and methods {#sec0010}
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2.1. Sample preparation {#sec0015}
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Commercially available brass sheets (Cu--Zn, with 20% Zn by weight, size 20 mm × 20 mm × 1 mm) were abraded with SiC paper and then polished with diamond paste down to 0.25 μm until an even mirror-like surface was obtained. After sonication in 99.5% ethanol each sample was immediately flushed in dry nitrogen gas.

In order to reveal the grain boundaries of the brass surface, some of the polished samples were etched in a solution containing 2 M of hydrochloric acid (HCl) and 0.2 M of iron chloride (FeCl~3~).

2.2. Exposure conditions {#sec0020}
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The experimental setup for preparation of humid air has been presented elsewhere [@bib0135]. The corrosive air was generated by mixing streams of synthetic dry air, purified from gases and particles through filters, and humidified air. The resulting laminar air flow velocity was 3.5 cm/s along the sample surface. The desired exposure conditions throughout all exposures were a relative humidity (RH) of 90 ± 3% and a temperature of 19.5 ± 0.5 °C. This relative humidity represents oxidation conditions which are fast enough to follow the oxide growth under current in situ measurement conditions.

2.3. In situ infrared reflection absorption spectroscopy (IRAS) {#sec0025}
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The IRAS setup has been described in more detail in earlier publications [@bib0130]. IRAS spectra were recorded by using a commercial Digilab 4.0 Pro FTIR spectrometer with 1024 scans at a resolution of 4 cm^−1^. P-polarized light impinged the sample with a grazing angle of around 78° from surface normal and passed through CdTe-windows when entering and leaving the exposure cell. The IRAS spectra were recorded in absorbance units (−log(*R*/*R*~0~)), where *R* is the reflectance of the exposed sample and *R*~0~ is the reflectance of the sample before exposure (background spectrum).

The sample was kept in humidified air for 30 min to collect a background spectrum. This was subtracted from all later in situ spectra to eliminate overlap from physisorbed water [@bib0140]. Thereafter in situ IRAS spectra were collected continuously. Post spectra analysis was performed by using the Digilab Resolutions Pro 4.0 software. Under current controlled exposure conditions, the temperature (±0.5 °C) and relative humidity (±3%) variations correspond to variations in the determination of cuprite thickness of around 0.1 nm, which is below the detection limit of the IRAS system (one monolayer of cuprite). Hence, such variations have only a minor influence on the quantification procedure presented in this study.

2.4. Confocal Raman microspectroscopy (CRM) {#sec0030}
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The confocal Raman measurements were performed with a WITec alpha 300 system. The scan range was 25 μm × 25 μm and a 532 nm laser was used as laser source. The measurement was performed ex situ and the integration time per Raman spectrum was in the order of 50 ms. A Nikon objective, Nikon NA0.9 NGC, was used for the measurements together with a pinhole with 50 μm diameter. The resulting stack Raman spectra were produced in the scanning area with a lateral resolution of around 300 nm and a vertical resolution of around 2 μm. WITec Project software was applied for spectra imaging.

2.5. Scanning Kelvin probe force microscopy (SKPFM) {#sec0035}
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A Nanoscope IV with facilities for Volta potential measurements of unexposed brass was used. The principals and details of the SKPFM technique have been described elsewhere [@bib0145]. The data were acquired in a two-pass mode: one for topography and the other for Volta potential mapping of the same surface area [@bib0150]. The probe, antimony-doped Si, supplied by Veeco, was lifted up to a constant distance from the surface (80 nm in this work) to collect Volta potential data. The lateral resolution under current conditions is of the order of 100 nm [@bib0145].

2.6. Atomic force microscopy (AFM) {#sec0040}
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The topography of the exposed sample surface was investigated by an AFM from Quesant Instrument Ltd. The measurements were performed in contact mode with a commercially obtained SiN cantilever.

2.7. Scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDS) {#sec0045}
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A Philips XL 30 SEM/EDS instrument was used for surface microstructure characterization of both polished and etched brass.

2.8. Electrochemical measurements {#sec0050}
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To aid in the identification of the oxides formed upon exposure and to deduce their average mass and overall polarization resistance, cathodic reduction (CR) and electrochemical impedance spectroscopy (EIS) measurements were performed using a standard three-electrode cell with a KCl saturated Ag/AgCl electrode as reference and a platinum mesh as counter electrode. Minor Cl^−^ leakage from the reference electrode into the cell cannot be excluded. For comparison, a saturated calomel reference electrode in a 25 mL electrochemical cell increased the Cl^−^ concentration from zero to about 1 μM after a ten times longer exposure time than was used here [@bib0155]. Exposed brass (Cu--20Zn), Cu and Zn sheets with 1 cm^2^ surface area acted as working electrodes. A 0.1 M sodium perchlorate (NaClO~4~) solution was used as electrolyte. The KCl saturated Ag/AgCl reference electrode in contact with the perchlorate solution may induce minor precipitation of KClO~4~, which has very low solubility in water. The solution was purged with N~2~ for 30 min before starting each reduction and throughout its duration. A potentiostat Solartron 1287 was applied for the CR measurements. The scan rate was 1.0 mV/s in the range from −0.4 to −1.3 V~Ag/AgCl~. The equivalent mass of ZnO and Cu~2~O was then determined from Faraday\'s law [@bib0160] by integrating the corresponding reduction peak using a Corrview software. Combining the potentiostat with a frequency response analyzer Solartron 1255, EIS was performed at the open circuit potential with a sinusoidal voltage signal of 10 mV applied over the frequency range 10^4^--10^−2^ Hz. A Zview (Scribner Associates Inc.) software was used for analysis of EIS spectra.

3. Results and discussion {#sec0055}
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3.1. Surface characterization of unexposed brass {#sec0060}
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### 3.1.1. SEM/EDS {#sec0065}

[Fig. 1](#fig0005){ref-type="fig"}(a) is a SEM image obtained from a newly diamond polished brass surface, whereas [Fig. 1](#fig0005){ref-type="fig"}(b) exhibits diamond polished brass after slight etching to reveal the microstructure, including grain boundaries. A common observation is that complementary EDS measurements show slightly higher Zn-content in some micrometer sized grains.

### 3.1.2. SKPFM {#sec0070}

[Fig. 2](#fig0010){ref-type="fig"} shows topography (a) and Volta potential (b) images obtained by using the AFM-based SKPFM to map a newly diamond polished brass surface. The images reveal the variations in surface morphology and corresponding relative nobility variations of the very same surface area. The Volta potential map displayed in [Fig. 2](#fig0010){ref-type="fig"}(b) clearly shows numerous darker rounded features, a few micrometers in size, of lower Volta potential. Their size is comparable to the size of some of the grains seen in [Fig. 1](#fig0005){ref-type="fig"}(b). Supported by separate EDS-analysis that reveals variations in Zn-content in different grains of the brass alloy (Section [3.1.1](#sec0065){ref-type="sec"}), the areas with lower Volta potential may be associated with grains of slightly higher Zn-content. Brass, in which Zn substitutes Cu in the alloy, consists of an α-phase with FCC crystal structure up to 35 wt% Zn [@bib0165]. The variations in Zn-content at different grains could be due to different grain orientations in combination with thermal treatments, abrasion or polishing procedures.

3.2. Surface characterization of brass exposed to humidified air {#sec0075}
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### 3.2.1. IRAS {#sec0080}

[Fig. 3](#fig0015){ref-type="fig"} displays the in situ IRAS spectrum collected during exposure of polished brass in synthetic air at 90% RH up to 72 h. For comparison the pure Cu and pure Zn spectra obtained under similar exposure condition are also presented. In order to visibly display the bands in the figure, all spectra are presented with some base-line correction.

Changes in surface chemistry of brass are clearly seen from the bands located at 660 and 570 cm^−1^. These bands correspond to the formation of cuprous oxide (most likely Cu~2~O or a Cu~2~O-like structure) and zinc oxide (most likely ZnO) [@bib0170; @bib0175; @bib0180]. The Cu~2~O band at 660 cm^−1^ is shifted to higher wavenumber and is broader compared to the spectrum for pure Cu, probably due to a more amorphous structure [@bib0180; @bib0185; @bib0190].

The in situ IRAS spectra are used to follow the growth kinetics of ZnO and of Cu~2~O on brass, see [Fig. 4](#fig0020){ref-type="fig"}, where the absorbance of Cu~2~O and of ZnO are plotted against exposure time. The absorbance is directly proportional to the corresponding amount of each oxide formed. By applying reported quantification procedures of in situ IRAS spectra of ZnO [@bib0175] and of Cu~2~O [@bib0180], the estimated masses of Cu~2~O and ZnO after 72 h are 0.6 μg/cm^2^ and 0.5 μg/cm^2^ respectively. When considering also the molar ratio of Cu and Zn in the alloy, the results clearly show an accelerated ZnO growth and a retarded Cu~2~O growth. As discussed later, this is attributed to galvanic effects caused by the brass surface composition heterogeneity.

### 3.2.2. Cathodic reduction {#sec0085}

Identification of the oxides formed on brass can also be performed by cathodic reduction, following similar procedures as previously reported for pure Zn and Cu [@bib0175; @bib0195]. [Fig. 5](#fig0025){ref-type="fig"} shows a representative curve recorded during cathodic reduction from −0.4 to −1.3 V~Ag/AgCl~ of the brass surface after exposure to air at 90% RH for 72 h. Two main reduction peaks are seen. The reduction peak around −0.62 V~Ag/AgCl~ originates most likely from Cu~2~O, whereas the reduction peak around −1.2 V~Ag/AgCl~ originates from ZnO. The estimated masses of Cu~2~O and ZnO on brass are 0.7 and 0.6 μg/cm^2^ respectively, higher than what was obtained by IRAS ([Fig. 4](#fig0020){ref-type="fig"}). A possible reason for this difference in results may be a later formed oxide during transport in ambient air or during parts of the cathodic reduction procedure, despite careful handling of the samples between exposure and cathodic reduction, and N~2~ purging during cathodic reduction. Consistent with the results based on IRAS ([Fig. 4](#fig0020){ref-type="fig"}), the absolute amount for ZnO is smaller than for Cu~2~O. The lack of any reduction peak around −0.8 V~Ag/AgCl~ suggests that no CuO has been formed.

The quantification procedure from a pure metal and an alloy may be different, in the same way as the properties of oxides formed on brass most likely differ from those formed on the pure metals. Earlier studies in our group [@bib0190] have shown that when independently quantifying the amount of cuprite on pure copper by QCM, IRAS and cathodic reduction, there was an agreement between the results from all methods with a relative accuracy of 12% or better. Looking at the absolute amount of cuprite on brass in the present study obtained by cathodic reduction (0.7 μg/cm^2^) and by IRAS (0.6 μg/cm^2^), the difference seems to be of the same order, in particularly when considering the circumstance that cathodic reduction may result in slightly higher mass than IRAS due to the short exposure in air. This forms evidence that the quantification procedures for cuprite on brass and on Cu are the same within a relative accuracy of approximately 15%.

In all, cathodic reduction supports the conclusions from IRAS that two main oxides have been formed on brass during exposure in humidified air: Cu~2~O and ZnO, with a total mass of 1.3 μg/cm^2^.

### 3.2.3. EIS {#sec0090}

EIS measurements were also performed, in order to explore the overall corrosion protective properties of the oxides formed on brass. [Fig. 6](#fig0030){ref-type="fig"} displays the Bode plots obtained in 0.1 M NaClO~4~ for brass exposed to humidified air at 90% RH for 72 h, and also for pure Cu and pure Zn after the same exposures. The spectra all reveal only one time constant, in which the higher frequency region represents the solution resistance (*R*~s~), the middle frequency region\'s slope exhibits the capacity between the passive layer and the substrate, while the lower frequency region is a measure of the protective ability and the electrolyte property by means of the polarization resistance (*R*~p~) [@bib0200]. By comparing the value difference between *R*~p~ and *R*~s~, which is a direct indication of the surface film protective ability, is concluded that the surface film resistance is highest for brass and decreases in the order brass \>Cu \> Zn. Hence, the oxide formed on brass results in the best protective ability.

### 3.2.4. AFM {#sec0095}

Whereas in situ IRAS and ex situ cathodic reduction only provide data on the total amount of Cu~2~O and ZnO formed, we consider next their lateral distribution along the brass surface. [Fig. 7](#fig0035){ref-type="fig"} displays the topography of the brass surface after exposure in humidified air (90% RH) for 72 h. Rounded features appear randomly on the brass surface, which protrude from the matrix. These features were not seen prior to exposure, and hence, have resulted from the oxidation of the brass surface.

### 3.2.5. Confocal Raman microspectroscopy {#sec0100}

Confocal Raman microspectroscopy was used to map the heterogeneous formation of both oxides by integrating over the ZnO band (490--600 cm^−1^) and Cu~2~O band (600--680 cm^−1^) in the stack Raman spectra. The Raman spectrum is very similar to the corresponding IRAS spectrum in the range from 500 to 1000 cm^−1^, see [Fig. 3](#fig0015){ref-type="fig"} for brass. A main ZnO Raman band at 570 cm^−1^ is attributed to an amorphous structure [@bib0205], while a dominating Cu~2~O band at 660 cm^−1^ is attributed to a resonance Raman enhancement [@bib0210], in agreement with earlier Raman studies of initial atmospheric corrosion of similar systems [@bib0215]. Combining all intensity variations for each band into one image result in the colored images displayed in [Fig. 8](#fig0040){ref-type="fig"}(a) and (b) respectively. The brighter color corresponds to the higher intensity of the corresponding band. [Fig. 8](#fig0040){ref-type="fig"}(a) provides evidence that ZnO is formed more locally, while Cu~2~O formation is more evenly distributed along the exposed brass surface ([Fig. 8](#fig0040){ref-type="fig"}(b)).

3.3. General discussion {#sec0105}
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SEM/EDS ([Fig. 1](#fig0005){ref-type="fig"}) combined with SKPFM ([Fig. 2](#fig0010){ref-type="fig"}) reveals that there are slightly more Zn-rich grains with lower Volta potential than the surrounding more Cu-rich areas. These variations create conditions for spatially separated anodic and cathodic processes from the very early stages of atmospheric corrosion.

When exposed to ambient air the brass surface is covered by a naturally formed oxide layer of thickness in the range from 0.5 to 2.0 nm ([Fig. 9](#fig0045){ref-type="fig"}(a)) [@bib0180]. The chemical composition of this layer has previously been analyzed by XPS, and shown to be a mixed oxide with an average Cu/Zn-ratio equal to that of the bulk brass alloy [@bib0220]. Upon exposure of this brass surface to air with 90% RH, an aqueous adlayer spontaneously forms. The thickness of this reversibly adsorbed adlayer was not possible to measure in this study. However, under similar exposure conditions, the thickness of the aqueous adlayer on Zn was around 15 nm, sufficiently thick to obtain bulk water properties [@bib0175]. Once in contact with water, electrochemical reactions take place on brass, resulting in Zn and Cu dissolution as the anodic reaction and hydroxyl ion generation as the main cathodic reaction ([Fig. 9](#fig0045){ref-type="fig"}(b)). Dissolved metal ions will react with hydroxyl ions to produce aqueous metal-(OH) species which subsequently dehydrate into ZnO and Cu~2~O ([Fig. 9](#fig0045){ref-type="fig"}(c)).

IRAS ([Fig. 3](#fig0015){ref-type="fig"}) and cathodic reduction ([Fig. 5](#fig0025){ref-type="fig"}) both show Cu~2~O and ZnO to be the main oxide phases detected, whereby a selective oxidation of Zn to ZnO and a retarded growth of Cu~2~O on brass are revealed. AFM ([Fig. 7](#fig0035){ref-type="fig"}) and confocal Raman images ([Fig. 8](#fig0040){ref-type="fig"}) demonstrate the local formation of ZnO that protrudes from the more uniform Cu~2~O-like layer.

The Cu~2~O band detected with IRAS ([Fig. 3](#fig0015){ref-type="fig"}) is broader on brass than on pure Cu under similar exposure conditions [@bib0170], which suggests a more amorphous structure. The band also undergoes a shift indicating a compositional and/or structural change of the Cu~2~O-like oxide during growth on brass. The ZnO band on brass, on the other hand, is similar to ZnO on pure Zn [@bib0175], and suggests no major differences in nature of the two oxides formed. The fact that more amorphous structures in oxides result in higher corrosion resistance is well-known from the literature [@bib0225; @bib0230]. Zn seems to enhance the stability of the Cu~2~O layer, a phenomenon also shown in previous studies [@bib0235; @bib0240].

The measured mass of Cu~2~O and of ZnO can be transformed into an average oxide thickness. Assuming bulk density of Cu~2~O (6.0 μg/cm^3^) and of ZnO (5.2 μg/cm^3^), respectively, the resulting average oxide thickness is around 4 nm in all. This is significantly thinner than the corresponding average oxide thicknesses on pure Cu (14 nm) [@bib0170] and on pure Zn (47.5 nm) [@bib0175].

A previously reported XPS-study of brass oxidized in air at room temperature has shown that preferential oxidation of Zn to ZnO takes place initially with a concomitant zinc depletion layer in the surface region of brass [@bib0090]. This promotes an overlayer of Cu~2~O to form which has been reported to be thinner than on pure Cu, in agreement with the present work. Possible reasons for the thinner oxide on brass may be due to changes in the semiconducting and transport properties of the Cu~2~O-layer caused by the adjacent ZnO-layer. The defect structures of Cu~2~O and of ZnO have a different nature, with Cu~2~O predominantly a p-type semiconductor and ZnO an n-type semiconductor [@bib0010; @bib0245; @bib0250]. Due to the importance of Cu~2~O/ZnO heterojunctions as candidates in solar cell devices [@bib0255], there are considerable interests in Zn-doping of Cu~2~O, which turns out to have a profound influence on, e.g., the electrical and structural properties of Cu~2~O [@bib0260]. Evidence of different properties of the Cu~2~O-layer formed on brass compared to pure Cu in the present study are the shift to higher wavenumber and the broadening of the Cu~2~O-band in the IRAS spectra ([Fig. 3](#fig0015){ref-type="fig"}), also the higher polarization resistance of the oxidized brass surface compared to pure Cu or Zn ([Fig. 6](#fig0030){ref-type="fig"}).

4. Conclusions {#sec0110}
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The duplex oxide growth on a diamond polished brass (Cu--20Zn) surface exposed to humidified air at 90% RH has been followed by means of a multi-analytical approach. The coherent results provide new details on the initial oxidation of brass in humidified air.

Prior to exposure the brass surface consists of slightly Zn-enriched areas, a few micrometers in size, characterized by lower Volta potential than the surrounding matrix.

The variations in nobility along the brass surface create conditions for galvanic effects which result in an accelerated local growth of ZnO and a retarded and more uniform growth of a Cu~2~O-like oxide.

The total mass of the duplex oxide after 72 h in humidified air at 90% RH is around 1.3 μg/cm^2^. This is significantly less than the corresponding oxide mass on the pure metals after the same exposures. The improved corrosion protective properties of the duplex oxides formed on brass may be attributed to a more amorphous Cu~2~O-oxide layer with altered semiconducting properties, possibly due to Zn-intermixing.

Financial support from the Chinese Scholarship Council (CSC) and from the Swedish Research Council (VR) is gratefully acknowledged. We are grateful to Fan Zhang and Eleonora Bettini (KTH) for performing SKPFM measurements, to Majid Sababi for metallography assistance (KTH), and to Prof. Inger Odnevall Wallinder (KTH), Prof. Jinshan Pan (KTH), and Dr. Harveth Gil (University of Antioquia, Colombia) for valuable suggestions.

![SEM images of diamond polished (a), and subsequently, etched (b) brass surface.](gr1){#fig0005}

![(a) Topography and (b) Volta potential images of a polished brass surface. Image size 10 μm × 10 μm.](gr2){#fig0010}

![In situ IRAS spectra in the region from 500 to 1000 cm^−1^ of brass, pure Zn and pure Cu after 72 h exposure in synthetic air at 90% RH.](gr3){#fig0015}

![Absorbance of the Cu~2~O and ZnO band at 660 cm^−1^ and 570 cm^−1^ versus exposure time during exposure of brass to synthetic air at 90% RH. The error bars are based on triplicates.](gr4){#fig0020}

![Cathodic reduction curve obtained in 0.1 M NaClO~4~ solution of the oxides formed on brass exposed to humidified air at 90% RH for 72 h.](gr5){#fig0025}

![Bode plots measured in 0.1 M NaClO~4~ solution for brass, pure Zn, and pure Cu after exposure in humidified air at 90% RH for 72 h.](gr6){#fig0030}

![AFM image of the brass surface after exposure in humidified air (90% RH) for 72 h. Image size 10 μm × 10 μm.](gr7){#fig0035}

![Raman images of brass exposed to humidified air (90% RH) for 72 h. (a) Distribution of ZnO (integration of band intensity from 490 to 600 cm^−1^). (b) Distribution of Cu~2~O (600--680 cm^−1^). Image size 25 μm × 25 μm.](gr8){#fig0040}

![Schematic illustration of important steps identified during initial oxidation of brass in humidified air. (a) Before exposure in humid synthetic air; (b) during exposure in humid synthetic air; (c) formation of corrosion products.](gr9){#fig0045}
