Industrial systems such as chemical plants use extensive piping systems for transporting hazardous materials and pressure. Their integrity must be maintained stringently against degradations such as corrosion. According to Ref. [1], “chemical process system components are often made of carbon steel or steel alloys, and are covered with insulation.” Insulation may trap moisture underneath, causing “corrosion under insulation (CUI). This refers only to damage occurring between the inside layer of the insulation and the outside surface of the metal components.” Generally, piping may develop flaws such as cracks, pitting, local thin areas (LTAs), and blisters, among which CUI tend to form pits and LTAs on the outside surface [2]. Thus, characteristically, damages of CUI are local, the size ranging from much smaller than the pipe diameter to similar to the diameter.

It is also asserted [1] that “attempts to detect this corrosion have, in most cases, required the removal of insulation prior to inspection... It has been estimated that these efforts cost many hundreds of thousands of dollars.” There is thus a strong demand for appropriate NDE techniques for CUI without insulation removal/restoration. The challenge is significant because of the stand-off distance and the damage locality. Weatherproof coating over the insulation sometimes adds difficulty.

This presentation first reviews the lists of NDE methods applicable to CUI in the existing reports and papers [1-5], highlighting the three presently common implementations, i.e., real-time digital radiography, long-range (guided wave) ultrasonics, and pulsed eddy current (PEC). The second part of the paper is devoted to an examination of several electromagnetic methods, particularly PEC [4, 6], based on theoretical models and laboratory data. Novel modalities such as T-ray [7], with their plausibility, may be discussed.