Terahertz Quantitative Nondestructive Evaluation of Failure Modes in Polymer-Coated Steel

Terahertz reflective imaging is applied to characterize the failure modes in a polymer coating on a steel plate. The coating was initially scratched, then after accelerated aging, several types of failure have occurred. In order to resolve the thin coating (~50 μm), terahertz frequency-wavelet domain deconvolution is implemented. With the deconvolved signals, the temporally overlapping echoes of the incident, roughly single-cycle terahertz pulse are clearly resolved, and three important failure modes, viz. corrosion, delamination, and blistering, are characterized quantitatively. Terahertz images in three dimensions clearly exhibit the coating thickness distribution across the entire damaged coating, highlighting the terahertz features associated with different failure modes, thus demonstrating that terahertz imaging can be considered as an effective modality for characterizing damage mechanisms in polymer coatings on metals.

I. Introduction

Nondestructive evaluation (NDE) techniques for monitoring and characterizing coatings on metals are essential to verify protection of the metal substrate from corrosion during service. Coating failure can have many causes and manifestations. Especially, when a region of a coating system becomes detached from its substrate, the term “adhesion failure” is commonly used. Delamination and blistering are two important types of coating failure in which compromised adhesion is strongly implicated [1]. Since the exact cause and nature of coating failure is still in dispute [2], various NDE techniques have been explored to study damage mechanisms in coatings, such as electrochemical impedance spectroscopy (EIS) [3], scanning electrochemical microscopy [4], acoustic emission [5], and thermography [6], [7], to name a few. However, not all of the NDE techniques mentioned above can provide quantitative information in depth, nor may they all be capable of monitoring the condition of adhesion. Scanning acoustic microscopy (SAM) [2], [8], [9] and laser-ultrasonics [10] can provide depth-specific information with enough resolution for characterizing coating systems; however, high attenuation in polymer materials limits the penetration depth of the ultrasonic waves [8]. Therefore, alternative NDE techniques with relatively high resolution are still sought for quantitative evaluation of polymer coatings.