Investigation of delamination mechanisms in polymer coatings by scanning acoustic microscopy

Abstract

Scanning acoustic microscopy (SAM) was used to investigate disbonding, delamination and blister formation in polymer coatings for different layer structures and initial defect depths during exposure to a corrosive environment. The time evolution of disbonding and blister initiation and their growth was investigated by analysing the SAM images exposure to an electrolyte solution (Harrison solution). From investigations of a model system of coatings on steel substrates, it was possible to differentiate between water and/or ion transport (i) through the coating and (ii) along the coating-steel interface. For samples without clear coat randomly distributed blisters appeared at the coating-steel interface after exposure to the electrolyte solution, irrespective of the location of initial defects. The random distribution of growing blisters is related to diffusion of water and/or ions through the coating and "nucleation" at weak points of the substrate or within the polymer. For samples with clear coat a propagating migration front along the coating-steel interface of 3-4 µm height -starting from initial defects -was detected. The linear propagation of this front cannot be explained by Fickian diffusion. Therefore, it is discussed in terms of an accelerated diffusion or crack growth kinetics. Since blistering starts only at sites, where the migration front has passed, the presence of a thin water layer at the coating-steel interface, the loss of adhesion and the following corrosive processes are prerequisites for the nucleation of blisters. The blister growth shows a square-root time dependence which was related to a diffusion controlled transport mechanism.

Nondestructive measurement of mill-scale thickness on steel by terahertz time-of-flight tomography

“…To date, various nondestructive-evaluation (NDE) techniques have been explored to study the structure and condition of coatings on metals, such as electrochemical impedance spectroscopy (EIS) [1], scanning electrochemical microscopy [2], acoustic emission [3], thermography and eddycurrent testing [4]; however, not all of the techniques are capable of providing quantitative information in depth such as the layer thickness. Approaches such as scanning acoustic microscopy [5] and laser ultrasonic imaging [6] suffer from high attenuation at the requisite high frequencies which limits the penetration of ultrasound waves into the sample.…”

Effects of biochar nanoparticles on anticorrosive performance of zinc-rich epoxy coatings

“…The scanning acoustic microscopy is an equipment which can focus high frequency ultrasound on one spot and image the in-depth information of an object by surface scan. SAM is commonly used in the semiconductor and packaging industry as a nondestructive technique for failure detection, its application in coating degradation investigation has also been reported [43][44][45][46].…”

A model for cathodic blister growth in coating degradation using mesomechanics approach

“…It is, therefore, essential to understand the physical and metallurgical phenomenon of blistering from both scientific and engineering point of view. One of the most severe forms of blistering is known as cathodic blistering [14][15][16], which takes place when a metallic substrate with coating defects is exposed to saline environment, such as salt spray or immersion in salt solution [17][18][19][20][21].…”