Which measurement technique is used to assess coating integrity and barrier properties by applying a small AC signal across frequencies?

• A. Electrochemical Impedance Spectroscopy (EIS)
• B. Linear Polarization Resistance (LPR)
• C. Visual inspection
• D. Salt spray test only

Answer: (A)

Assessing coating integrity and barrier performance across frequencies is best done with Electrochemical Impedance Spectroscopy. In this method you apply a small-amplitude AC voltage to the coated metal immersed in an electrolyte and sweep across a range of frequencies, measuring the resulting current to obtain impedance as a function of frequency. This frequency-dependent response reveals how the coating blocks ions and how it stores charge.

At high frequencies, the coating’s dielectric properties dominate the response, while at low frequencies the signal reflects processes such as water ingress, pore pathways, and possible delamination or charge-transfer activity at the substrate. By fitting the data to an equivalent circuit, you can extract parameters like coating resistance, pore resistance, and capacitance, giving a quantitative picture of barrier quality and how it changes over time. This non-destructive, in-situ technique is specifically designed to characterize both the integrity of the coating and its barrier properties.

Other methods don’t provide this frequency-domain insight. Linear Polarization Resistance estimates corrosion rate around the corrosion potential using a DC-to-DC-sweep approach, not the full impedance spectrum. Visual inspection is qualitative and cannot quantify barrier performance. Salt spray tests accelerate corrosion exposure but do not deliver impedance-based metrics of coating integrity.