Modeling of Electrochemical Impedance Data of a Magnesium-Rich Primer

Abstract

The application of Mg-rich primers ͑MRPs͒ for the protection of aluminum structures represents an attractive alternate to the environmentally unfriendly Cr-rich primers that are presently used. The protective modes of MRPs are similar to those of Zn-rich primers ͑ZRPs͒ on steel and include cathodic protection driven by the more active Mg particles compared to the Al substrate and a barrier-type protection due to the insulation of the substrate from the environment. Interpretation of ZRP electrochemical impedance spectroscopy ͑EIS͒ data has been accomplished using a transmission-line model that accounted for the contact impedance between the zinc particles, the impedance associated with the zinc dissolution, and the percolation resistance of the coating. EIS experiments results are presented here for an MRP on a gold substrate under immersion in dilute Harrison's solution. The data were analyzed using the measurement model technique to determine the consistency with respect to Kramer-Kronig relationships. The transmission-line model was used to analyze the data to demonstrate their applicability for analyzing the protection afforded by the MRP. Gold was used as a noble substrate and to promote that the electromotive degradation of the MRP as the mixed potential of an MPR/gold system was more positive than that of an MRP/Al system. Available commercial, nonchromate inhibitor pigments have been shown to be much less effective in corrosion protection as compared to the industrial standard of SrCrO 4 . A nonchromate pigment has been reported to perform as effectively as SrCrO 4 on Alodine 1200-treated AA 2024-T3. 1 The use of Alodine does expose the substrate to chromates and as such the influence of chromate cannot be discounted. The application of Mg-rich primers ͑MRPs͒ for the protection of aluminum structures has been demonstrated by Bierwagen and co-workers and represents an attractive alternate to the environmentally unfriendly Cr-rich primers that are presently used for such protection. 2-6 This system does not require an Alodine pretreatment and represents a Cr-free system.The protective modes of MRPs include cathodic protection driven by the more active Mg particles compared to the Al substrate and a barrier-type protection due to the insulation of the substrate from the environment by-products of the hydroxide and hydroxy carbonate species of Mg that fill in pores and voids. 3,7,8 The behavior of the MRPs on Al substrate has been characterized using opencircuit potential evolution, potentiodynamic polarization, electrochemical impedance spectroscopy ͑EIS͒, scanning vibrating electrode technique, and scanning electrochemical microscopy. [2][3][4][5][6] Interpretation of the EIS data has been limited to two and three time-constant equivalent circuit models where parameters attributed to coating resistance and charge-transfer resistance were observed to reduce with time immersed in 0.1 wt % NaCl. 3 The protective modes of MRPs are similar to those that have been associated with Zn-rich primers ͑ZRPs͒. 3,4 The c...

Transmission Line Modeling of EIS Data for a Mg-Rich Primer on AA 2024-T3

“…The technique has been described in detail for the analysis of data associated with an MRP on a gold substrate. 29 The experimental data associated with the full frequency range from 0.003 Hz to 100 kHz for several immersion times were consistent with the Kramers-Kronig relations. The results are not included here.…”

“…23 The regression of EIS data associated with a coating to a model is performed under the assumptions that the data are free of instrument artifacts due to nonstationary behavior, that the noise level in the data is acceptable, and that the weighting strategy is appropriate. 29 The measurement model technique was used to qualify these assumptions by analyzing the residual error between the measurement model impedance fit and the measured impedance data. The technique has been applied in literature to identify the contributions of systematic, bias, and stochastic errors to the residual error.…”

Magnesium-Based Sacrificial Anode Cathodic Protection Coatings (Mg-Rich Primers) for Aluminum Alloys

“…Allahar et al modeled EIS data of a Mg-rich primer on a gold substrate under immersion in DHS and analyzed it for consistency with Kramers-Kronig relations and applicability for use with a transmission-line model [48]. The data in the frequency range of 1 mHz to 100 kHz were Kramers-Kronig consistent, while the transmission-line model was shown to be applicable for data in the 1 mHz to 10 kHz range.…”

Transmission Line Modeling of EIS Data for a Mg-Rich Primer on AA 2024-T3

“…4 with the inclusion of the contact impedance between the particles being attributed to Gabrielli who applied it to analyze the EIS data associated with electroactive fluidised beds (37,38). The transmission line model was shown to be applicable to MRPs on a gold substrate (29).…”

“…The technique involves using a weighting strategy for the complex nonlinear least squares regression that is based on the measured error structure associated with replicated EIS data. The technique has been described in detail for the analysis of data associated with a MRP on a gold substrate (29).…”