Michael A. Martinez

Characterizing corrosion behavior under atmospheric conditions using electrochemical techniques

Abstract AC and DC electrochemical experiments were performed as a function of humidity and contaminant concentration in an effort to identify the range of atmospheric environments where corrosion processes could be detected and possibly quantified. AC measurements exhibited two time constants at 25% relative humidity (RH), possibly indicating the ability to resolve both electrolyte resistance and interfacial impedance. Galvanic current measurements were sensitive to the presence of Cl 2(g) at 30% RH and electrochemical transients were detected at both 30% and 50% RH levels, also indicating sensitivity to interfacial processes. Higher humidity levels allowed better quantification due to decreasing electrolyte and interfacial impedances.

The Effects of Chloride Implantation on Pit Initiation in Aluminum

High-purity aluminum samples were implanted with 35 keV Cl + then polarized in both Cl - -containing and Cl - -free electrolytes in order to ascertain corrosion behavior as a function of Cl - content in the oxide. Implant fluence between 5 × 10 15 and 2 X 10 16 Cl + cm -2 resulted in little or no localized attack. Implant fluences of 3 X 10 16 and 5 X 10 16 Cl + cm -2 resulted in significant pitting in a Cl - -free electrolyte with the severity scaling as a function of implant fluence. The low variability in the pitting behavior of the 5 X 10 16 Cl + cm -2 sample suggests that this implant dosage results in a critical Cl - concentration in the oxide for pit nucleation. The passive current density (i pass ) decreased with increasing implant fluence. A space-charge effect is proposed to account for this phenomenon, although effects from defect interactions and possible oxide thickening are still under consideration.

Relationship Between Induction Time for Pitting and Pitting Potential for High-Purity Aluminum

The objective of this study was to determine if a distribution of pit induction times (from potentiostatic experiments) could be used to predict a distribution of pitting potentials (from potentiodynamic experiments) for high-purity aluminum. Pit induction times were measured for 99.99 Al in 50 mM NaCl at potentials of -0.35, -0.3, -0.25, and -0.2 V vs. saturated calomel electrode. Analysis of the data showed that the pit germination rate generally was an exponential function of the applied potential; however, a subset of the germination rate data appeared to be mostly potential insensitive. The germination rate behavior was used as an input into a mathematical relationship that provided a prediction of pitting potential distribution. Good general agreement was found between the predicted distribution and an experimentally determined pitting potential distribution, suggesting that the relationships presented here provide a suitable means for quantitatively describing pit germination rate.

Comparison of the Effects of Implanted and Aqueous Cl on Aluminum Pitting Behavior

A statistics-based mathematical treatment was used to compare the pitting behavior of Cl-implanted Al to the behavior of nonimplanted Al. Although pitting potentials for both types of samples fall within a similar potential range, the pit germination rates are much higher for the implanted samples. Differences in germination rates are attributed to the Cl adsorption/absorption and/or Cl migration processes that are bypassed by implanting Cl directly into the aluminum oxide.

The effects of chloride implantation on pit initiation in aluminum.

High-purity aluminum samples were implanted with 35 keV Cl{sup +} then polarized in both Cl{sup -}-containing and Cl{sup -}-free electrolytes in order to ascertain corrosion behavior as a function of Cl{sup -} content in the oxide. Implant fluence between 5 x 10{sup 15} and 2 x 10{sup 16} Cl{sup +} cm{sup -2} resulted in little or no localized attack. Implant fluences of 3 x 10{sup 16} and 5 x 10{sup 16} Cl{sup +} cm{sup -2} resulted in significant pitting in a Cl{sup -}-free electrolyte with the severity scaling as a function of implant fluence. The low variability in the pitting behavior of the 5 x 10{sup 16} Cl{sup +} cm{sup -2} sample suggests that this implant dosage results in a critical Cl{sup -} concentration in the oxide for pit nucleation. The passive current density (i{sub pass}) decreased with increasing implant fluence. A space-charge effect is proposed to account for this phenomenon, although effects from defect interactions and possible oxide thickening are still under consideration.A statistical description of pit initiation in high-purity aluminum (99.99) is developed based on the statistical frameworks established by Baroux [Corros. Sci., 28, 969 (1988)] and Shibata and Takeyama [Corrosion, 33, 243 (1977) and 52, 813 (1996)]. The statistical treatment is used to examine the dependency of pitting behavior on electrode area, scan rate, and chloride concentration. It is shown that the average pitting potential is proportional to the log of the scan rate and the log of one over electrode area, implying an exponential dependence of pit germination rate on applied potential. The constants that describe pit germination rate are independent of area for electrodes of diameter 125 μm and smaller. However, the constants are area-dependent for larger electrodes. Based on these results a homogeneous pit nucleation process is proposed to account for pit initiation on 125 μm diam and smaller electrodes, while a heterogeneous process is proposed to control the behavior of 500 μm diam electrodes. The results from these studies also demonstrate the importance of correctly choosing experimental conditions when studying pitting behavior. The dependencies of pitting potential on scan rate and area are found to be sensitive to electrode size, scan rate, and aggressiveness of the electrolyte.

Quantifying Atmospheric Corrosion Processes Using Small Length-Scale Electrochemical Measurements and 3-D Electric Field Modeling

Small length scale atmospheric probes were coupled with highly sensitive electrochemical instrumentation in order to assess the range of applicability of electrochemical measurements to quantifying the corrosion behavior of metals exposed to atmospheric conditions. Additionally, the influence of electrolyte conductance on the galvanic corrosion behavior in heterogeneous systems was studied by developing a 3-D electric field model. Atmospheric probes were constructed using either thin-film lithographic techniques or using off-the-shelf materials uch as thin foils, wires and insulating coatings. A range of electrochemical techniques was evaluated including AC electrochemical impedance spectroscopy, galvanic current measurements and DC polarization techniques. Thin-film electrodes had much higher parasitic impedance than the foil electrodes and could be used with AC techniques to resolve electrochemical behavior over a much larger range of frequency. The foil-based electrodes suffered from low parasitic impedance but were ideal for making DC measurements. Thin film gold electrodes allowed quantification of the conductivity of the thin adsorbed electrolyte that forms on the metal surfaces under atmospheric conditions. Aluminum and copper foilbased electrodes demonstrated sensitivity to galvanic corrosion at all humidity levels tested (down to 0% relative humidity). Furthermore, the aluminum foil-based electrodes were capable of resolving to extremely small, localized corrosion events, and it was