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Novel electrochemical technique measures degradation rate of polymer coatings on iron

Think of a metal with paint on it, like an automobile or a pipeline carrying natural gas. The paint or polymer coating protects the metal underneath from degrading.

Although the coating should ideally protect the metal for a long time, a scratch or a defect in the coating can lead to early onset of metal degradation. This happens by a well-established mechanism known as cathodic disbondment, where quick diffusion of oxygen and water from the ambient atmosphere allows for the oxygen reduction reaction (ORR) to accelerate the degradation of the coating by virtue of the generated free radicals.

Of crucial significance is the rate of this ORR as it determines the lifetime of the metal underneath. Measuring this rate of corrosion of a painted metal is challenging as the interface between the coating and metal is buried or inaccessible.

Conventional techniques to measure this rate, such as potentiodynamic polarization, rely on polarizing this interface by means of using an auxiliary electrode, i.e., a counter electrode. The idea behind this approach is to enable an ionic current to flow in the electrolyte between the coated metal and the counter electrode so that an electronic current can be measured in the external circuit.

Knowing this current can give the rate of degradation of the coated metal. But here comes the challenge—an organic coating being ionically impermeable does not allow the coated metal to be polarized.

Whatever current is still being measured through this technique is only a result of pinholes, which are pores or defects introduced during the coating application process, and not the true corrosion rate. Therefore, new electrochemical techniques are needed to quantify the degradation rate of such coated metals.

Recently, we introduced a novel approach using hydrogen permeation-based potentiometry (HPP) and electrochemical impedance spectroscopy (EIS) to measure this rate. The research is in the journal Corrosion Science.

Firstly, the principle behind HPP is to make use of the electrochemically reducing nature of atomic hydrogen as a means to polarize the coated metal interface from the back side of a double electrochemical cell. Typically, the atomic hydrogen acts in the same way as the auxiliary electrode in conventional polarization, i.e., it polarizes the coated metal, but without the limitation of needing ion transport.

Hence, we first generated defined amounts of hydrogen on one side of the double electrochemical cell using a model electrocatalytic palladium (Pd) membrane. This hydrogen permeates through the Pd membrane and reaches the other cell, where it reacts with the already present oxygen and establishes an electrochemical equilibrium potential.

We then increased the amount of hydrogen generated in one cell in a stepwise fashion, which resulted in more hydrogen to permeate and, correspondingly, more oxygen to be reduced in the other cell, which was reflected as a decrease in the electrochemical potential.

By knowing how much hydrogen was generated in one cell and proving that almost all of that hydrogen could quantitatively be made to react with the oxygen in the other cell, the rate of ORR could first be measured on Pd.

We then used this method to measure ORR on a Pd membrane coated on one side with an acrylate polymer. We found that this approach could elegantly measure the kinetics of ORR underneath this coating, which vastly differed from the little or zero current that could be measured with conventional polarization. But there was still the question of whether this measured ORR rate is indeed the true kinetics of coating degradation.

That's when the idea of using a complementary technique such as classical EIS struck us. The principle behind using EIS was to primarily measure the charge transfer resistance for ORR and the barrier properties; i.e., pore resistance of the polymer coating.

If the HPP approach could indeed measure the true rate of coating degradation, then this must be associated with both a decreasing charge transfer resistance and coating pore resistance with progress of ORR.

This was exactly what we measured! Further, we could successfully extend this combined approach of using HPP-EIS to measure the rate of degradation of a polymer coating on a thin layer of industrial metal such as iron deposited on the Pd membrane.

This technique will be useful in determining the rate at which a polymer coating de-adheres from a pipeline carrying hydrogen blended with natural gas. But the implications of this novel HPP-EIS technique, in our opinion, extend beyond corrosion underneath coatings to the field of sensors, fuel cells and fundamental investigations relying on exploiting interfacial electrochemical phenomena.

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