Is there a model or other technique to estimate the useful lifespan of zinc-rich paint based on EIS data? I will not be able to undertake long-term testing of actual corrosion rates for comparison.
If a comparative Time To Failure (TTF) in a salt fog environment can be obtained following Kendig's method Article Rapid electrochemical assessment of paint
can this be extrapolated to a TTF in atmospheric conditions? Feliu has done long-term assessment of ZRPsArticle Deterioration of Cathodic Protection Action of Zinc-Rich Pai...
but does not correlate these results to short-term or rapid assessment techniques.I am aware REAP may not be the ideal method. My colleagues found no disbondment at the scribed edge after 24h immersion and so defaulted to an estimated value for the needed dx/dt term. If a longer-term method is necessary, what is the minimum time period for a reliable estimate of actual period of protection? And what other analysis is advised in either case?
I am looking at a 96wt% Zn (dry film) paint applied over weathered hot-dip galvanising.
A nice and complex question.
For instance, what kind of binder has been used in the formulation of the ZRP paint. Is it inorganic one, such as ethyl-silicate, or an organic one, such as epoxy-polyamide?.
Thanks for the response. The manufacturer describes it as a "reversible organic polystyrene binder". They do not provide a lot of technical data beyond what is attached here.
Is the binder type still a significant factor when the proportion is so low?
I'm really not very familiar with the REAP method.
In our studies we use the evolution of the polarization resistance, Rp, defined as the arc diameter that appears at high or medium frequencies in the impedance diagrams, with immersion time in NaCl0.6M.
At short test times, we have observed that the incorporation of insoluble zinc corrosion products to the coating produces a rapid increase in Rp values with time. At longer test times this tendency is reversed due to perforation of the coating and the appearance of red rust points. In the case of a coating formulated with an epoxy-polyamide vehicle and 97% zinc, we observed that this period was approximately 80 days.
You may find more details in
https://www.researchgate.net/publication/283797601_Study_of_protection_mechanisms_of_zinc-rich_paints_by_electrochemical_impedance_spectroscopy
I believe the lifespan of the paint will vary depending on material it is applied to. Corrosion current rate can be calculated from any viable Electrochemical Impedance Spectroscopy (EIS) test. In an EIS test, very small excitations of potential cause AC current in the system, mimicking a electrical circuit. The impedance of the circuit can be calculated from Bode and Nyquist plots for the system. When potential and impedance are known, corrosion current rate can easily be determined. You would need to perform a variety of tests for different materials and accuracy reasons.
Thanks again S. Feliu Jr. I have reviewed that paper before but not in detail. The REAP process is conducted in 24 hours and relies on measurements of paint disbondment after 24h immersion in NaCl solution. However the zinc-rich paint does not disbond in this timeframe so a rate must be estimated. I may be able to adapt this method and yours using a longer period of immersion, possibly up to 80d.
Probably the most interesting part of the system I am studying is the galvanised base. There is, in general, no exposed steel so the system is quite different from that usually studied. Consequently the Nyquist plots do not resemble the typical plots, to my inexperienced eyes anyway. My professor is becoming interested in me studying the effects of damage to the coating system exposing small areas of steel.
Thank you Shane Kelliher
As I said above, the paint is applied on a galvanised surface which results in no exposed steel. Corrosion of zinc-rich paint is generally thought to be only the first stage of protection, with a (longer) period of barrier protection following after the Zn corrosion products seal the pores of the paint. This makes accelerated testing an interesting problem.
Dear Toby Snaire
Thank you for your answer. Next you may find some references to papers from our group including Nyquist plots for bare or painted galvanized steels.
Kind Regards
Article An interpretation of electrical impedance diagrams for paint...
Article Reproducibility of electrical impedance data for a metal/paint system
Article EIS study of the corrosion behaviour of zinc-based coatings ...
Article EIS study of the corrosion behaviour of zinc-based coatings ...
Zn rich paints are quite different and REAP parameters would not correlate with Zn rich performance. Ray Taylor has done some work in this area. He has a test specifically for Zn Rich paints.
Thanks Martin Kendig I have read some of Taylor's work. Do you have the specific reference for ZRP? I have the Damage Tolerance test but it seems more general.
Thank you again S. Feliu Jr, those early papers are particularly helpful in understanding the Nyquist plots.
I now plan to use the ISO 11474 'scab test' to correlate lab data to an accelerated field test. This test is recommended in ASTM publications as the most applicable.
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