Corrosion Inhibition of Two Brass Alloys by ... - Corrosion Journal

CORROSION ENGINEERING SECTION

Corrosion Inhibition of Two Brass Alloys by Octadecanethiol in Humidified Air with Formic Acid Mattias Forslund,* Jinshan Pan,* Saman Hosseinpour,** Fan Zhang,* Magnus Johnson,* Per Claesson,* and Christofer Leygraf ‡,*

ABSTRACT Self-assembled monolayers of octadecanethiol (ODT) have previously shown to provide excellent corrosion inhibition on copper exposed to humidified air containing formic acid, mimicking indoor atmospheric corrosion. ODT layers are, however, much less efficient corrosion inhibitors for zinc. In this work, we elucidate the possibility of using ODT monolayers to inhibit corrosion of brass. Based on a quantitative analysis of corrosion products, we found that ODT provides equally good corrosion inhibition of single-phase Cu20Zn as of pure copper, retarding the transportation of corrosion stimulators to the brass surface. On double-phase Cu40Zn, however, local galvanic effects led to less efficient corrosion inhibition and more corrosion products than on Cu20Zn. KEY WORDS: atmospheric environments, atomic force microscopy, copper, corrosion inhibitor, interfacial effects, selfassembled, zinc

INTRODUCTION Copper has been widely used throughout human history because of several favorable chemical and physical properties.1 Various types of copper alloys, Submitted for publication: January 27, 2015. Revised and accepted: March 9, 2015. Preprint available online: March 9, 2015, http://dx.doi.org/10.5006/1648. ‡ Corresponding author. E-mail: [email protected]. * KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden. ** Max Planck Institute for Polymer Research, Department of Molecular Spectroscopy, Ackermannweg 10, 55128, Mainz, Germany.

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including brass, have been developed to improve mechanical and other properties desirable for different engineering applications. Brass, i.e., copper alloyed with zinc as major alloying element, may exhibit lower corrosion resistance than pure copper because of the enhanced tendency of zinc to corrode and concomitant dezincification problems. For zinc content up to approximately 35 wt%, zinc substitutes copper in the structure of the material, resulting in a single-phase (α) brass structure. However, when the zinc content is in the range of around 35 wt% to 45 wt%, brass has a duplex structure consisting of α- and β-phases with different zinc contents. This commonly results in more complex corrosion behavior of the brass alloy.2-3 Self-assembled monolayers of alkanethiols have long been recognized for their surface functionalizing properties, in particular on noble metals, because of the relatively strong chemical bond between sulfur and the noble metal.4 The formed monomolecular thin film is relatively inert and stable under many conditions. It can easily be deposited on objects with different shapes and has, therefore, found applications in, e.g., microelectronics, micromechanics, and nanoelectromechanical systems.5-6 Such films have also been investigated for their potential role as corrosion inhibitors, in particular on copper.7-10 Investigations of the corrosion inhibition efficiency of alkanethiols on electrochemically more active metals are much less common. Recent studies explored the corrosion inhibiting efficiency of octadecanethiol (CH3[CH2]17SH, herein ODT) on zinc exposed to humidified air with additions of formic acid,11 an atmosphere known to mimic

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TABLE 1 Chemical Composition (wt%) of Cu20Zn and Cu40Zn Cu20Zn Cu40Zn

Cu

Zn

79.7 20.2 59-63(A) 37-41(A)

Pb 0.001 0.1-0.3

Fe 0.006 Cu20Zn≈Cu.

Surface Characteristics After Exposure The fact that ODT protects Cu20Zn more efficiently than Cu40Zn will be elucidated further in this section, mainly by analyzing the amount and location of corrosion products by means of LOM and AFM. We also report data for the structural order of the ODT layer before and after exposure on both alloys as determined by means of VSFS. Figure 7 displays LOM images of ODT-covered Cu20Zn and Cu40Zn before and after 168 h of exposure to humidified air containing formic acid. The images show Cu20Zn (Figure 7[a]) and Cu40Zn (Figure 7[c]) before exposure. The appearance of both α- and β-phase grains can be clearly seen on ODT-covered Cu40Zn. Figures 7(b) and (d) show the corresponding surfaces after 168 h exposure. Small areas with corrosion products can be seen on ODT-covered Cu20Zn (Figure 7[b]), which were a result of formation of zinc formate as monitored by IRAS (Figures 5 and 6). In agreement with the IRAS data, the abundance of corrosion products was significantly higher on ODTcovered Cu40Zn (Figure 7[d]) than on ODT-covered Cu20Zn (Figure 7[b]). An AFM image of ODT-covered Cu40Zn is displayed in Figure 8. Corrosion products with submicron size are seen both on the α-matrix (less abundant), and on the β-phase grains (more abundant). In addition, a few corrosion products, sized 10s of µm, are seen preferentially located close to the border between the β-phase grains and the α-phase matrix. They can be seen on ODT-covered Cu40Zn only, and their size and location suggest that galvanic effects played a significant role to explain the difference in zinc formate formation rate between Cu40Zn and Cu20Zn, seen in Figure 6. The fact that galvanic effects were seen not only on bare but also on ODT-protected Cu40Zn was explained by the penetration of corrosion stimulators through the ODT layer along the whole surface. When present in sufficient amounts, local corrosion products start to form on the β-phase closest to the α-phase (Figure 8). Their size suggest, furthermore, that the ODT layer on these spots locally have disap-



FIGURE 7. Optical microscopy images of ODT-covered Cu20Zn before (a) and after (b) 168 h exposure to humidified air containing 94 ppbv formic acid, and of ODT-covered Cu40Zn before (c) and after (d) the same exposure. The same samples have also been analyzed with VSFS (Figure 9). The two images to the far right are digital magnifications of the marked areas on the exposed samples.

peared, in accordance with the ODT-covered micropatterned Cu-Zn sample.11 In order to investigate the effect on the ODT layer upon formation of corrosion products, VSFS examinations were performed on unexposed and exposed brass samples. The VSFS spectra in Figure 9 were obtained from the surfaces shown in Figure 7. Because the diameter of the laser beams is approximately 5 mm, the VSFS spectra resulted from an average behavior of the ODT layer in a fairly large area.13 The characteristic peaks seen in the figure were all obtained in the CH-region and include the CH2 symmetric stretching vibration at approximately 2,850 cm–1, the CH3 symmetric stretching vibration at approximately 2,875 cm–1, the CH3 Fermi resonance at approximately 2,940 cm–1, and the out of plane CH3 antisymmetric stretch at approximately 2,965 cm–1.10 VSFS has previously been used to assess the quality of adsorbed ODT on pure copper.10 This was accomplished by analyzing the characteristic peaks of ODT in the CH stretching region, whereby the SF signal of the symmetric stretching vibration of the terminating CH3 groups in the alkyl chain was compared with the signal from CH2 groups in so-called gauche defects in the alkyl chain.10 The ratio between the CH2 and CH3 symmetric stretching vibrations is commonly

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used to establish the degree of ordering of n-alkanethiol monolayers.17,40 In systems where no gauche defects are present, no methylene signal is observed, as there is an inversion center between each pair of CH2 groups, thus rendering the SF inactive. In systems with gauche defects present, the inversion center is canceled, and the methylene signal is observed. The terminating CH3 group always gives rise to a signal, because no inversion center is present. Thus, when a large amount of gauche defects (high disorder) is present, the CH2/CH3 intensity ratio is high, and vice versa. The absence or the weak appearance of the CH2 peak at 2,850 cm–1 before exposure (Figures 9[a] and [c]) suggests a relatively well-ordered ODT layer on both Cu20Zn and Cu40Zn prior to exposure. After exposure (Figures 9[b] and [d]) the CH2 peak was still very weak for both Cu20Zn and Cu40Zn, thus indicating that the alkyl chains were well ordered despite the formation of corrosion products. It should be emphasized that the relatively large area analyzed by VSFS (diameter approximately 5 mm) compared to the extension and amounts of larger corrosion products (size in the order of 1 µm or smaller) makes the technique insensitive to local disordering or local removal of the ODT layer. It is, furthermore, worth noting that because of the different amounts of zinc in the two

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FIGURE 8. AFM-image of ODT-covered Cu40Zn sample exposed for 168 h in humidified air with addition of 94 ppbv formic acid. The AFM-image is sized 20×20 mm square. The corrosion products are seen as smaller or larger white areas, and the β-phase is indicated as darker regions, surrounded by the α-phase matrix.

alloys, as well as the formation of corrosion products, the backgrounds in the spectra shown appear slightly dissimilar. An example is the apparent dip at the lower wave numbers in the spectrum of Cu20Zn exposed for 168 h (Figure 9[c]). To conclude, the ODT layer appears to be well ordered on both Cu20Zn and Cu40Zn before, as well

as after, exposure as based on VSFS and also on the observation of similar dipole contributions from ODT on the α- and β-phases before exposure (see section Surface characteristics before exposure). The ODT layer retarded the transport of the corrosion promoters H2O, HCOOH, and O2 to the brass surface. When local corrosion product formation occurred, no disordering of the alkane chain could be observed with VSFS. Galvanic effects on primarily Cu40Zn stimulated the corrosion process and resulted in enhancement of corrosion product formation at the border between α- and β-phase grains. Although not shown herein, the abundance of local corrosion products on Cu40Zn presumably resulted in local removal and in overall reduction in corrosion inhibition of the ODT layer, whereby larger areas became covered by corrosion products. Based on previously performed comparisons between laboratory-exposed and field-exposed zinc and copper, it should be added that the exposure conditions used herein may correspond to an accelerated corrosion rate in the order of 100 when compared with the exposure to representative indoor exposure conditions.13 Hence, the corrosion effects observed after 1 week (168 h) of laboratory exposure may correspond to those after approximately 2 y to 3 y of indoor exposure characterized by relatively benign corrosion conditions.

SF Intensity (arbitrary unit)

(b)


(a)

2,800

2,850 2,900 2,950 3,000 Wave Number (cm–1)

3,050

2,800

2,800

3,050

2,850 2,900 2,950 3,000 Wave Number (cm–1)

3,050


(d)


(c)

2,850 2,900 2,950 3,000 Wave Number (cm–1)

2,850 2,900 2,950 3,000 Wave Number (cm–1)

3,050

2,800

FIGURE 9. VSFS spectra of the CH-region of ODT-covered Cu20Zn ([a] and [b]) and Cu40Zn ([c] and [d]) surfaces before (left) and after (right) 168 h exposure to humidified air containing 94 ppbv formic acid. The arrows show the CH2 vibration band in the alkane chain that appears as the result of gauche defects, i.e., a disordering of the alkane chains.

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CONCLUSIONS v ODT as a corrosion inhibitor for brass has been explored during accelerated exposure of up to 1 week to humidified air with addition of formic acid, corresponding to an estimated atmospheric indoor exposure of a few years. v ODT-covered Cu20Zn exhibits a formation rate of zinc formate, similar to the formation rate of copper formate on ODT-covered pure copper, despite the earlier observation that ODT is a much less efficient corrosion inhibitor on pure zinc than on pure copper under current exposure conditions. v The formation rate of zinc formate is significantly higher on ODT-covered Cu40Zn than on ODT-covered Cu20Zn. This is attributed to a higher extent of galvanic corrosion as a result of larger local compositional and Volta potential variations along the surface of the two-phase Cu40Zn alloy compared to the single-phase Cu20Zn alloy, which results in a lowering of the overall corrosion inhibition efficiency. v Prior to exposure, the ODT layer is as well ordered on Cu20Zn and Cu40Zn as previously observed on pure copper. The inhibiting action of the ODT layer is a result of hindrance of the transport of corrosion promoters to the brass surface. The ODT layer seems unaffected by the corrosion promoters and retains its ordered molecular structure throughout the exposure for both Cu20Zn and Cu40Zn.

ACKNOWLEDGMENTS We acknowledge valuable discussions with Dr. Jonas Hedberg, KTH, Sweden, and Prof. Steven Baldelli, University of Houston, United States. Financial support from the Swedish Science Foundation (VR) is greatly acknowledged. REFERENCES 1. L. Landner, L. Lindeström, Copper in Society and in the Environment (Västerås, Sweden: Swedish Environmental Research Group, 1999), p. 329. 2. R.W. Revie, H.H. Uhlig, Uhlig’s Corrosion Handbook (Hoboken, NJ: Wiley, 2011), p. 1253. 3. L.L. Shreir, R.A. Jarman, G.T. Burstein, Corrosion (Oxford, England: Butterworth-Heinemann, 1994), p. 3184. 4. J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chemical Reviews 105, 4 (2005): p. 1103-1169. 5. J.G. Vos, R.J. Forster, T.E. Keyes, Interfacial Supramolecular Assemblies (Chichester, England: Wiley, 2003), p. 317.

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