Using Eddy Current Testing for Corrosion Detection in Welded Structures

In many industrial facilities, corrosion rarely appears randomly across a structure. Instead, inspectors often notice that degradation tends to develop near welded joints. Weld toes, attachment welds, and heat-affected zones (HAZs) are frequently the first locations where corrosion initiates.

Anyone who has inspected pipelines, storage tanks, or offshore structures has likely observed this phenomenon. While coatings may remain intact across large surfaces, localized deterioration may begin around welds where geometry, residual stresses, and metallurgical transitions interact in complex ways.

Traditionally, inspectors evaluate these areas using techniques such as ultrasonic testing (UT) or magnetic particle testing (MT). These methods remain indispensable for weld inspection and structural integrity assessment. However, they are not always optimized for detecting early-stage corrosion or shallow material loss near the surface.

In these situations, eddy current testing (ECT) provides a complementary inspection approach based on electromagnetic interaction with conductive materials.

Electromagnetic Interaction with Welded Materials

Eddy current inspection operates on the principle of electromagnetic induction. When alternating current flows through a probe coil, it generates a changing magnetic field. If a conductive material is placed within this field, circulating electrical currents, known as eddy currents, are induced within the metal.

These induced currents interact with the probe’s magnetic field, thereby influencing the electrical impedance of the inspection coil. Any discontinuity that disrupts the flow of these currents produces measurable variations in the signal.

Corrosion near welds can significantly alter the distribution of these currents. Even small material losses, localized pitting, or surface degradation can disturb the electromagnetic field enough to generate detectable signals.

However, welded structures introduce additional complexity. The weld metal, HAZ, and base metal may each possess slightly different electrical conductivity and magnetic permeability due to metallurgical transformations during welding. These variations produce a characteristic signal response that must be understood before corrosion indications can be correctly interpreted.

Frequency Selection and the Skin Effect

One of the most important variables in ECT is test frequency. Eddy currents do not penetrate uniformly into a material. Instead, they follow the phenomenon known as the skin effect, in which the current density decreases exponentially with depth below the surface.

Higher frequencies concentrate the induced currents near the surface, increasing sensitivity to small discontinuities such as pitting corrosion or shallow material loss at weld toes. Lower frequencies allow deeper penetration and may reveal corrosion that has progressed slightly below the surface or beneath thin coatings.

Selecting the appropriate frequency, therefore, becomes a critical part of the inspection strategy. The choice depends on several factors, including expected corrosion depth, coating thickness, and material electromagnetic properties.

Carbon Steel vs. Aluminum

Material behavior significantly influences ECT performance. Carbon steels are ferromagnetic materials with high magnetic permeability. This property alters the magnetic field distribution and reduces the effective eddy current penetration depth. Additionally, variations in permeability near welds can produce signal changes that resemble corrosion indications if not properly interpreted.

For this reason, ECT of carbon steel welds requires careful calibration and experienced signal interpretation.

Aluminum alloys behave quite differently. Aluminum is nonferromagnetic and has magnetic permeability close to that of free space. Combined with relatively high electrical conductivity, this allows eddy currents to flow more uniformly, producing cleaner and more predictable signals.

These characteristics are among the reasons ECT has long been widely used for inspecting aluminum structures in the aerospace industry.

Complementary Role in Weld Inspection

In welded structures, ECT is rarely used as a standalone method. Instead, it serves as a complementary technique alongside other nondestructive testing methods.

Typical applications include corrosion detection at weld toes in pipelines, inspection of attachment welds in storage tanks, and evaluation of localized degradation in offshore structures.

Because the technique requires no coupling and can often be performed through thin coatings, it offers practical advantages for rapid field inspections.

Conclusion

Corrosion near welded joints remains a persistent challenge in maintaining the integrity of industrial structures. Detecting early-stage degradation in these regions requires inspection methods capable of detecting subtle changes in material condition.

ECT provides a sensitive electromagnetic approach for identifying corrosion in complex welded geometries. When applied with proper understanding of material behavior, frequency selection, and weld metallurgy, it becomes a powerful complementary tool for corrosion assessment.

For inspectors and engineers responsible for the long-term reliability of welded systems, mastering this technique provides another means to assess material condition by observing the invisible flow of electromagnetic currents within the metal.

JORGE T. REYNA (contacto.jrsa@gmail.com) is the owner of JRSA Engineering in Monterrey, Mexico. He is an AMPP Master Coatings Inspector, AMPP Senior Certified Coating Inspector, AMPP Protective Coating Specialist, ANST Level III, and AWS Senior Certified Welding Inspector.