Anal. Chem. 1997, 69, 1635-1641
In Situ Fluorescence Imaging of Localized Corrosion with a pH-Sensitive Imaging Fiber Anna A. Panova, Paul Pantano,† and David R. Walt*
The Max Tishler Laboratory for Organic Chemistry, Department of Chemistry, Tufts University, Medford, Massachusetts 02155
A fiber-optic pH-imaging sensor array capable of both visualizing remote corrosion sites and measuring local chemical concentrations at these sites was applied to realtime corrosion monitoring. The imaging fiber’s distal face, containing an immobilized pH-sensitive fluorescent dye, was brought into contact with metal surfaces submerged in aqueous buffers and fluorescence images were acquired as a function of time. Heterogeneous fluorescence signals were observed due to both pH increases at cathodic surface sites and pH decreases at anodic surface sites. These fluorescence signals showed both localization and rates of corrosion activity. Three corrosion processes were investigated, galvanic corrosion at a copper/aluminum interface and crevice corrosion and pitting at a stainless steel surface. The spatial resolution of the technique was limited by proton/hydroxide diffusion and the diameter of the individually clad optical fibers comprising the imaging bundle. Corrosion causes significant damage to our economic infrastructure, threatening bridges, ships, planes, automobiles, and roads and posing a safety issue when catastrophic failure occurs. Macroscopic manifestations of corrosion (e.g., observable rust) are the latter stages of a complex, dynamic process that begins at the microscopic level. The need to assess the initial stages of corrosion has spurred the development of a multitude of analytical techniques.1-4 The ability to both visualize heterogeneous corrosion sites and measure local corrosion rates simultaneously is of particular importance. For example, fluorescence microscopy has been used to image heterogeneous electrochemical processes on metal surfaces submerged in an aqueous solution containing a pH-sensitive fluorescent dye.5,6 The technique’s spatial resolution was diffusion limited, required dye to be added to the sample environment and the sample to be brought to the microscope stage. Recently, we described a new technique for concurrently viewing a sample and measuring surface chemical concentrations † Present address: Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083. * Author to whom correspondence should be addressed. Electronic mail: . (1) Sukamto, J. P. H.; Smyrl, W. H.; Casillas, N.; Al-Odan, M.; James, P.; Jin, W.; Douglas, L. Mater. Sci. Eng. 1995, A198, 177-196. (2) Wipf, D. O. Colloids Surf., A 1994, 93, 251-261. (3) Williams, K. P. J.; Wilcock, I. C.; Hayward, I. P.; Whitley, A. Spectroscopy 1996, 11, 45-50. (4) Aldykewicz, A. J., Jr.; Isaak, H. S.; Davenport, A. J. J. Electrochem. Soc. 1995, 142, 3342-3350. (5) Engstrom, R. C.; Ghaffari, S.; Qu, H. Anal. Chem. 1992, 64, 2525-2529. (6) Bowyer, W. J.; Xie, J.; Engstrom, R. C. Anal. Chem. 1996, 68, 2005-2009.
S0003-2700(96)01025-6 CCC: $14.00
© 1997 American Chemical Society
using a coherent imaging fiber.7,8 Imaging fibers are densely packed fiber-optic bundles in which the relative spatial coordinates of the individually clad optical fibers are retained in a fixed arrangement (i.e., the position of each optical fiber is the same at both the input and output ends). In the “combined imaging and chemical sensing” approach, an analyte-sensitive fluorescent indicator (e.g., a pH-sensitive fluorescent dye) is immobilized in a thin polymer layer on an imaging fiber’s distal face such that it does not hinder the fiber’s imaging capabilities. In association with an epifluorescence microscope and a charge-coupled device (CCD), these modified imaging fibers can display both visual and chemical information of a remote sample with ∼4-µm spatial resolution and with an areal coverage of tens of thousands of square micrometers.8 In this paper, we report the use of pH-sensitive imaging fibers to visualize and monitor three types of localized corrosion: galvanic corrosion, crevice corrosion, and pitting. Galvanic corrosion was investigated by employing a copper/ aluminum galvanic pair. The reactions expected at the copper/ aluminum interface in slightly acidic medium are the reduction of water (eq 1) and/or the reduction of oxygen (eq 2), both
2H2O + 2e- ) 2OH- + H2
(1)
O2 + 2H2O + 4e- ) 4OH-
(2)
producing hydroxide at the cathodic sites, and dissolution of aluminum (eq 3) followed by solvolysis of its ions and corresponding evolution of protons (eq 4) at the anodic sites.
Al ) Al3+ + 3e-
(3)
Al3+ + 3H2O ) Al(OH)3 + 3H+
(4)
Crevice corrosion and pitting were investigated on a stainless steel surface in the presence of chloride ions. The reactions expected to occur in the areas of crevice and pit initiation and growth are anodic dissolution of Fe, Cr, and Ni, followed by proton evolution from solvolysis of these ions.9 The corresponding cathodic reactions expected are the hydroxide-producing reductions of oxygen and water on the metal surface adjacent to crevices and pits.9,10 (7) Pantano, P.; Walt, D. R. Anal. Chem. 1995, 67, 481A-487A. (8) Bronk, K. S.; Michael, K. L.; Pantano, P.; Walt, D. R. Anal. Chem. 1995, 67, 2750-2757. (9) Walton, J. C.; Cragnolino, G.; Kalandros, S. K. Corros. Sci. 1996, 38, 1-18.
Analytical Chemistry, Vol. 69, No. 8, April 15, 1997 1635
In this work, a fiber-optic pH sensor capable of both visualizing corrosion sites and measuring local chemical concentrations was applied to real-time corrosion monitoring. The distal face of a pH-sensitive imaging fiber was brought into contact with metal surfaces submerged in aqueous buffers, and fluorescence images were acquired as a function of time. At both cathodic and anodic sites, the fluorescence of the pH-sensitive dye was modulated by protons and/or hydroxide ions that diffused into the immobilized pH-sensing layer. The observed changes in fluorescence due to pH increases at cathodic sites and pH decreases at anodic sites were indicative of localized corrosion and corrosion rates. Structural surface changes monitored after the reactions using atomic force and optical microscopies corroborated the observed fluorescence changes. EXPERIMENTAL SECTION Materials. Fluorescein isothiocyanate isomer I (FITC), triethylenetetramine, (3-aminopropyl)triethoxysilane, acrylamide, THF, and N-acryloxysuccinimide were purchased from Aldrich Chemical Co. (Milwaukee, WI); 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) was obtained from Sigma Chemical Co. (St. Louis, MO); sodium phosphate (dibasic heptahydrate and monobasic dihydrate), potassium chloride, and acetone (optima grade) were purchased from Fisher Scientific (Fair Lawn, NJ); azobis(isobutyronitrile (AIBN) was obtained from Pfaltz & Bauer (Waterbury, CT); and epoxy (Epo-Tek 353ND) was obtained from Epoxy Technology Inc. (Billerica, MA). Aluminum-clad copper wire (o.d. ∼250 µm, diameter of copper ∼100 µm), 500-µm-diameter aluminum wire, 500-µm-diameter copper wire, and 500-µm-diameter stainless steel 304 wire were purchased from Goodfellow (Berwyn, PA). The 350-µm-diameter imaging fibers, comprising ∼6000 optical fibers, each with a diameter of 2-3 µm were obtained from Sumitomo Electric Industries (Torrance, CA). 30-, 15-, 3-, and 0.3-µm lapping films were purchased from General Fiber Optic (Fairfield, NJ). pH Sensor. The covalent attachment of a thin (
