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Non‐destructive indication of delaminations under thick organic coatings using active IR‐thermography

The integrity of organic coatings can be impaired by disbondment due to permeation of water vapor in temperature and osmotic gradients between the outer environment and the base metal surface, due to cathodic protection at locally destructed coatings or simply by poor adhesion. For thin coatings the degree of delamination is easily determined by the intensity of blisters which generally arise already after short exposure periods. However, thick coatings with thicknesses above 2 mm take much longer for blistering and larger delaminations, and develop visible blisters only in an advanced state. The degree of delamination is generally detected by mechanical destruction of the coating. The present paper presents a new non‐destructive test (NDT) method for detecting delaminations and blisters in or under organic coatings with thicknesses up to 6 mm already at stages when the defects are not yet visible. This is accomplished by active IR thermographic methods with induction and/or optically excited lock‐in thermography. The potential of the method is exemplified in this paper for blister determination formed under ΔT‐test conditions and for non‐destructive quantification of the length of delamination after standardized cathodic disbondment laboratory tests.