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Volume 2 Extended Abstract 19

Submitted 26th August 1999

An Electrochemical Impedance Spectroscopy Analysis of Protective Behaviour of Final Coatings on Naval Steel.

E.C. Bucharsky#, E.B. Castro and S.G. Real 
Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Suc. 4, C.C.16 (1900) La Plata, Argentina. #Universidad Nacional de Quilmes, Roque Saenz Peña 180, (1876) Bernal, Argentina

Keywords: Impedance spectroscopy, film capacitance, water uptake, marine environments

Introduction

Coatings or linings with organic and inorganic compounds are a widespread method to provide corrosion protection in order to improve the durability of engineering structures. The corrosion protection properties are determined by complex mechanisms including factors such as the coating formulation, the transport of corrosive species, gradual defect formation which promotes the progressive adhesion loss and attack of the underlying metal. In the development of coatings it is therefore necessary to optimize the permeability of the coating for water and corrosive species. Water in organic coatings, becomes the major cause of swelling, loss of adhesion, deterioration of the mechanical properties and start of the corrosion process. Modern electrochemical techniques, such as electrochemical impedance spectroscopy, has been used for the investigation of the protective coating properties on metals [1]. In the present work, changes of coating properties have been monitored employing electrochemical impedance spectroscopy to evaluate the protective behaviour of the final coating system containing an alkyd-type-top coating and either sprayed zinc or zinc rich paint (ZRP) based on different organic binders such as epoxy, alkyd or ethyl-silicate. The whole impedance spectrum of each coating system, in the frequency range 0.01 Hz £ f £ 50 kHz., was strongly affected by the exposure time in the artificial sea water (ASTM Standard D-1141/90). The set of impedance diagrams recorded at different exposure time contain valuable information related to the characteristic coating parameters as well as to the kinetics of corrosion process. Impedance spectra were analyzed using a non linear fit routine according to the following transfer function: 

 

where Ctop is the top-paint film capacitance, Rp the pore resistance, RC is related to the charge transfer resistance of the oxygen reduction reaction and RA accounts for the charge transfer resistance of zinc dissolution reaction. A finite diffusion impedance was considered in order to account for the transport process involved in the cathodic partial reactions; being ZW=RD0 (jS)-1/2 tanh(jS)½, were S=l2w/D, l and D being the diffusion length and diffusion coefficient respectively.

Figure 1. Simulated (--) and experimental (· ) Bode diagrams for the alkyd- type-top and epoxy-ZRP coating after 9 days of exposure. 

Figure 1 shows a fairly good agreement between experimental and fitted results obtained according to the transfer function described by equations (1) and (2). Permeation of water in the coating is considered the first stage of deterioration of the total system. Water uptake can be monitored through changes in the top-paint film capacitance values (Ctop) with immersion time. The permeation of water in the top film determines an increase in Ctop. according to the relation of Brasher and Kinsbury[2]. In this work, the diffusion coefficient of water in the top paint was determined by a non-linear least squared fit (NLLS) of the Ctop vs. t curve[3]. Dynamic system analysis employing small signal perturbation allows the determination of specific parameters of the system, which characterize the gradual deterioration of the final paint scheme with increasing immersion time in sea water.

References

[1] E.C. Bucharsky, S.G. Real and J.R. Vilche, Corrosion Reviews,14, No1-2,(1996) 

[2] D.M. Brasher and A. H. Kinsbury., J. Appl. Chem. 4, 62 (1954). 

[3] E.P.M. van Westing, G. M. Ferrari and J.H. W. De Wit, Corros. Sci., 36, 957 (1994).


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