Detection of Corrosion Under Paint Using Magneto-Optic Imaging Methods

Conventional NDI methods for surface corrosion are difficult. For example, because magnetic field- gradients associated with surface corrosion in steel are far smaller than those associated with cracks, magnetic-particle methods are ineffective in this context, even on smooth painted-surfaces. Ultrasonic methods requiring area-scans are too time-consuming. Similarly, NDI for aluminum surface-corrosion using scanned eddy-current coils is too time-consuming. Moreover, variable paint- thickness can easily mimic eddy-current corrosion signals. Real-time NDI techniques that are unaffected by standard thickness' of paint would be a major advance. Magneto-optic imaging is such a technique. The magneto-optic/eddy current imager (MOI) is widely used in NDI of (painted) aluminum airframes. Hence, MOI detection of aluminum surface-corrosion should be possible. Recent developments also permit MOI-imaging of surface corrosion on (painted) steel. This was accomplished by combining rotating in-plane magnetization with magneto-optic imaging. Hence, a single "hybrid" instrument, which is capable of performing surface-corrosion NDI on steel and aluminum should be possible. This proposal describes how "hybridization" will be accomplished, and presents experimental results confirming technical feasibility. The end-product of Phases I, II and III will be a portable, battery-operated, field-instrument which is designed to be used by relatively- inexperienced shipboard-personnel to accomplished surface-corrosion NDI on both steel and aluminum.