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Nanoscale Investigation of the Impact of pH and Orthophosphate on the Corrosion of Copper Surfaces in Water Brian R. Lewandowski,† Darren A. Lytle,‡ and Jayne C. Garno*,† †
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, and ‡ U.S. Environmental Protection Agency, NRMRL, WSWRD, Cincinnati, Ohio 45268 Received June 29, 2010. Revised Manuscript Received August 1, 2010
Advanced surface characterization techniques were used to systematically investigate either the corrosion or passivation of copper after immersion in water as impacted by pH and orthophosphate water chemistries. Atomic force microscopy, depth profiling with time-of-flight secondary ion mass spectrometry, and X-ray diffraction were used to evaluate changes in surface chemistry of copper surfaces resulting from various chemical treatments. Nanoscale differences in surface morphology are clearly evident after 6 and 24 h immersion in water samples. Orthophosphate and pH dramatically influence the evolution and progression of changes during surface corrosion. For example, in the absence of orthophosphate the surface of copper exposed to water at pH 6 had formed relatively large cubic crystals on the surface up to 400 nm in height. In the presence of orthophosphate, the morphology and growth rate of corrosion byproduct changed dramatically, and the formation of identifiable crystals diminished. These investigations provide insight into the mechanisms of surface passivation and the evolution of nanoscale mineral deposits on surfaces at very early stages of the corrosion of copper surfaces in water.
Introduction Corrosion of household plumbing is a major concern for copper contamination of drinking water due to the deleterious health effects of copper toxicity as well as the expense incurred from damage to distribution pipelines attributable to corrosion.1,2 The nature (e.g., structural integrity, porosity, etc.) and solubility of corrosion byproducts that have formed on the interior walls of copper pipes determine the level of copper found at the consumer’s tap and the type of corrosion (e.g., uniform and pitting corrosion). In 1991, the United States Environmental Protection Agency (USEPA) issued the Lead and Copper Rule which specifies an action limit of 1.3 mg/L for copper.3 Elevated copper levels can be reduced using corrosion control strategies such as adjustment of pH or alkalinity or through the addition of corrosion inhibitor chemicals. Specifically, cupric phosphate is considered to have a major role in reducing copper release for copper surfaces treated with polyphosphates by forming an insoluble protective layer.4 Phosphate compounds deposited on the surface of copper pipes have been shown to slow the rate of corrosion and reduce copper release. Though widely used for drinking water distribution systems, copper piping does not last forever. Copper in oxygenated water *Corresponding author: Ph 225-578-8942; Fax 225-578-3458; e-mail
[email protected]. (1) Goh, K.-H.; Lim, T.-T.; Chui, P.-C. Evaluation of the effect of dosage, pH and contact time on high-dose phosphate inhibition for copper corrosion control using response surface methodology. Corros. Sci. 2008, 50, 918–927. (2) Isaac, R. A.; Gil, L.; Cooperman, A. N.; Hulme, K.; Eddy, B.; Ruiz, M.; Jacobson, K.; Larson, C.; Pancorbo, O. C. Corrosion in Drinking Water Distribution Systems: A Major Contributor of Copper and Lead to Wastewaters and Effluents. Environ. Sci. Technol. 1997, 31, 3198–3203. (3) USEPA Lead and Copper Rule: A Quick Reference Guide: http://www.epa. gov/ogwdw/lcrmr/pdfs/qrg_lcmr_2004.pdf (February 2, 2009). (4) Vargas, I. T.; Alsina, M. A.; Pasten, P. A.; Pizarro, G. E. Influence of solid corrosion by-products on the consumption of dissolved oxygen in copper pipes. Corros. Sci. 2009, 51, 1030–1037. (5) Ives, D. J. G.; Rawson, A. E. Copper Corrosion III. Electrochemical Theory of General Corrosion. J. Electrochem. Soc. 1962, 109, 458–462.
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corrodes to initially form a layer of cuprite (Cu2O) on the surface.5 In drinking water, oxidizing agents are predominantly dissolved oxygen and aqueous chlorine species.6,7 Further stages of corrosion proceed through the transfer of electrons through the cuprite layer rather than the metal, and the oxidant is reduced at the exterior side of the cuprite layer. Oxidation of cuprite results in the formation of a relatively porous cupric layer. Lattice defects in the cuprite layer can lead to the development of pits. Phosphorus compounds are commonly used as corrosion inhibitors for the protection of metal materials.8-12 Orthophosphate and hexametaphosphate have been shown to reduce the soluble copper release from corrosion products.13 Phosphorus compounds often have been used to inhibit corrosion and to protect metal surfaces.8-10,12,14,15 Aminophosphonic acid has (6) Reiber, S. H. Copper plumbing surfaces - An electrochemical study. J. Am. Water Works Assn. 1989, 81, 114–122. (7) Atlas, D.; Coombs, J.; Zajicek, O. T. The corrosion of copper by chlorinated drinking waters. Water Res. 1982, 16, 693–1982. (8) Truc, T. A.; Pebere, N.; Hang, T. T. X.; Hervaud, Y.; Boutevin, B. Study of the synergistic effect observed for the corrosion protection of a carbon steel by an association of phosphates. Corros. Sci. 2002, 44, 2055–2071. (9) Amar, H.; Benzakour, J.; Derja, A.; Villemin, D.; Moreau, B. A corrosion inhibition study of iron by phosphonic acids in sodium chloride solution. J. Electroanal. Chem. 2003, 558, 131–139. (10) Laamari, M. R.; Derja, A.; Benzakour, J.; Berraho, M. Calcium monofluorophosphate: a new class of corrosion inhibitors in NaCl medium. J. Electroanal. Chem. 2004, 569, 1–6. (11) Benzakour, J.; Derja, A. Electrochemical Passivation of Iron in Phosphate Medium. Electrochimica Acta, 1993, 38, 2547–2550. (12) Ramesh, S.; Rajeswari, S.; Maruthamuthu, S. Corrosion inhibition of copper by new triazole phosphonate derivatives. Appl. Surf. Sci. 2004, 229, 214– 225. (13) Edwards, M.; Hidmi, L.; Gladwell, D. Phosphate inhibition of soluble copper corrosion by-product release. Corros. Sci. 2002, 44, 1057–1071. (14) To, X. T.; Pebere, N.; Peleprat, N.; Boutevin, B.; Hervaud, Y. A corrosionprotective film formed on a carbon steel by an organic phosphonate. Corros. Sci. 1997, 39, 1925–1934. (15) Lebrini, M.; Bentiss, F.; Chihib, N. E.; Jama, C.; Hornez, J. P.; Lagrenee, M. Polyphosphate derivatives of guanidine and urea copolymer: Inhibiting corrosion effect of armco iron in acid solution and antibacterial activity. Corros. Sci. 2008, 50, 2914–2918.
Published on Web 08/27/2010
DOI: 10.1021/la102624n
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Figure 1. Surface of the copper control sample viewed with contact mode AFM. Successive images acquired at different magnifications: (A) 30 30 μm2; (B) 10 10 μm2; (C) 0.85 0.85 μm2; (D) cursor profile for the line in (C). Tall and shallow features are displayed as bright and dark areas, respectively.
been shown to inhibit corrosion of iron surfaces16 as well as certain surfactants.17 Strategies for corrosion passivation for copper distribution systems must be amenable to concerns of human health and the practicality of scale-up to large systems. Approximately 67% of the water utilities in the United States use polyphosphate or a blend of polyphosphate and orthophosphate as corrosion inhibitors, while the remaining use orthophosphate.13 Phosphates reduce corrosion by producing a passivating film on the pipe wall, which inhibits surface corrosion. The mechanisms of aqueous copper corrosion as influenced by changes in pH and orthophosphate levels have not been thoroughly investigated at the nanometer scale. Little is known about the effect of pH on polyphosphate performance with regard to copper corrosion control, especially in the typical pH range of drinking water (i.e., pH 7-10). Various in situ and ex situ characterizations have been applied for speciation of products of copper corrosion and to track the evolution of corrosion processes over time.18-24 Typical compounds that are produced during copper corrosion are deposits of calcite (CaCO3), calcium phosphate, cuprite (Cu2O), and copper hydroxide (Cu(OH)2). Corrosion events are influenced by a number of different factors, such as the exposure time, concentration (16) Telegdi, J.; Shaglouf, M. M.; Shaban, A.; Karman, F. H.; Betroti, I.; Mohai, M.; Kalman, E. Influence of cations on the corrosion inhibition efficiency of aminophosphonic acid. Electrochim. Acta 2001, 46, 3791–3799. (17) Murira, C. M.; Punckt, C.; Schniepp, H. C.; Khusid, B.; Aksay, I. A. Inhibition and Promotion of Copper Corrosion by CTAB in a Microreactor System. Langmuir 2008, 24, 14269–14275. (18) Scherer, J.; Vogt, M. R.; Magnussen, O. M.; Behm, R. J. Corrosion of Alkanethiol-Covered Cu(100) Surfaces in Hydrochloric Acid Solution Studied by in-Situ Scanning Tunneling Microscopy. Langmuir 1997, 13, 7045–7051. (19) Hayon, J.; Yarnitzky, C.; Yahalom, J.; Bettelheima, A. Surface Processes Characterization for the Corrosion of Copper in Borate Solutions. J. Electrochem. Soc. 2002, 149, B314–B320. (20) Persson, D.; Leygraf, C. Vibrational Spectroscopy and XPS for Atmospheric Corrosion Studies on Copper. J. Electrochem. Soc. 1990, 137, 3163–3169. (21) Calle, G. R.; Vargas, I. T.; Alsina, M. A.; Pasten, P. A.; Pizarro, G. E. Enhanced Copper Release from Pipes by Alternating Stagnation and Flow Events. Environ. Sci. Technol. 2007, 41, 7430–7436. (22) Dragowska, M.; Brossard, L.; Menard, H. Copper Dissolution in NaHCO3 and NaHCO3 þ NaCI Aqueous Solutions at pH 8. J. Electrochem. Soc. 1992, 139, 39–47. (23) Pyun, C.-H.; Park, S.-M. In Situ Spectroelectrochemical Studies on Anodic Oxidation of Copper in Alkaline Solution. J. Electrochem. Soc. 1986, 133, 2024– 2030. (24) Monticelli, C.; Fonsati, M.; Meszaros, G.; Trabanelli, G. Copper Corrosion in Industrial Waters A Multimethod Analysis. J. Electrochem. Soc. 1999, 146, 1386–1391. (25) Edwards, M.; Meyer, T.; Rehring, J. Effect of selected anions on copper corrosion rates. J. Am. Water Works Assn. 1994, 86, 73–81. (26) Edwards, M.; Rehring, J.; Meyer, T. Inorganic anions and copper pitting. Corrosion 1994, 50, 366–372. (27) Azzaroni, O.; Cipollone, M.; Vela, M. E.; Salvarezza, R. C. Protective Properties of Dodecanethiol Layers on Copper Surfaces: The Effect of Chloride Anions in Aqueous Environments. Langmuir 2001, 17, 1483–1487.
14672 DOI: 10.1021/la102624n
Table 1. Water Chemistry for Immersion of Copper Samples DIC sulfate chloride chlorine orthophosphate sample pH (mg C/L) (mg/L) (mg/L) (mg/L) (mg PO4/L) 1 2 3 4
9 9 6.5 6.5
10 10 10 10
111 104 107 110
72 65 80 75
2.9 3.2 2.4 2.5
0 6.1 0 7.4
of ions,19,22,25-27 pH,23-25 and the presence of dissolved oxygen.28 Techniques that have specifically been used to characterize surfaces of copper with nanoscale sensitivity include atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), and X-ray diffraction (XRD) analysis. High-resolution AFM furnishes an experimental approach to directly view the initial events and formation of corrosion deposits on a surface. Topographic information provided by AFM can sensitively reveal surface changes resulting from corrosion down to the nanoscale; however, it does not provide information for identifying the chemical nature of the surface species. Increasingly, AFM is being applied for corrosion studies to gain insight on local changes of metal surfaces, providing three-dimensional (3D) information for a wide range of surface materials with micrometer to nanometer resolution. High-resolution AFM provides highly sensitive measurements for systematically investigating changes, enabling one to control a wide range of experimental parameters for surface treatments. Unlike macroscopic corrosion studies that take weeks or months for visible changes to progress, AFM enables visualization of surface changes at the early onset of corrosion. Previously, AFM has been applied for studies of the corrosion of materials such as steel, silver, and copper.29-35 The effectiveness (28) Ives, D. J. G.; Rawson, A. E. Copper Corrosion. J. Electrochem. Soc. 1962, 109, 452–457. (29) Sanchez, J.; Fullea, J.; Andrade, C.; Gaitero, J. J.; Porro, A. AFM study of the early corrosion of a high strength steel in a diluted sodium chloride solution. Corros. Sci. 2008, 50, 1820–1824. (30) Martin, F. A.; Bataillon, C.; Cousty, J. In situ AFM detection of pit onset location on a 304L stainless steel. Corros. Sci. 2008, 50, 84–92. (31) Wang, R. An AFM and XPS study of corrosion caused by micro-liquid of dilute sulfuric acid on stainless steel. Appl. Surf. Sci. 2004, 227, 399–409. (32) Kleber, C.; Hilfrich, U.; Schreiner, M. In situ QCM and TM-AFM investigations of the early stages of degradation of silver and copper surfaces. Appl. Surf. Sci. 2007, 253, 3712–3721. (33) Aastrup, T.; Wadsak, M.; Schreiner, M.; Leygraf, C. Experimental in situ studies of copper exposed to humidified air. Corros. Sci. 2000, 42, 957–967. (34) Li, J.; Lampner, D. In-situ AFM study of pitting corrosion of Cu thin films. Colloids Surf., A 1999, 154, 227–237. (35) Wadsak, M.; Schreiner, M.; Aastrup, T.; Leygraf, C. A comparison of preparation methods of copper surfaces for in situ scanning force microscopy investigations. Appl. Surf. Sci. 2000, 157, 39–46.
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Figure 2. Copper surfaces after immersion in water sample 1 at pH 9. Zoom-in AFM views after 6 h for (A) 20 20 μm2, (B) 10 10 μm2, and
(C) 5 5 μm2. (D) Line profile for (C). Changes after 24 h immersion for areas of (E) 20 20 μm2, (F) 10 10 μm2, and (G) 5 5 μm2. (H) Line profile for (G).
of corrosion inhibitors has also been evaluated using AFM characterizations.36-38 Certain surface spectroscopy techniques such as SIMS and XRD can provide detailed information about the chemical nature of surface adsorbates and furnish complementary information for identifying the byproduct of corrosion. Both spectroscopies provide sensitive characterizations of the outermost surface layer, with ion or X-ray beams that penetrate to depths of a few nanometers. In recent developments, SIMS provides new capabilities for obtaining depth profiles for complex species. In this report, we investigate the early stages of the water corrosion (
