Eddy Current Testing of Galvanized Steel: Best Practices

Galvanized steel is widely used across industries as a strong, corrosion-resistant building material. Applications range from children’s playgrounds to industrial-grade ventilation systems, to critical structural components of bridges and naval ships. What separates galvanized steel from other types is a coat of zinc applied to the external surfaces of the metal in a hot-dip process—this is galvanization. 

While galvanized steel is corrosion-resistant, it is not entirely corrosion proof. Over time, water or humidity can find holes in the zinc’s protective coating and begin to eat away at the steel beneath. At the same time, galvanized steel can also develop small or large cracks and other internal defects. Especially when used in critical applications, such as the hulls of ships, galvanized steel must be inspected regularly to detect any evidence of corrosion. The best way to inspect galvanized steel is by using dedicated eddy current imaging technology. 

Inspecting Galvanized Steel with Eddy Current Technology

Eddy current technology uses magnetic fields to detect corrosion, surface cracks, and subsurface defects in conductive metals. Eddy current probes create the magnetic fields and channel them through sheets of metal. Defects and irregularities in the metal impede the magnetic field just as a rock or a hole impedes the even flow of a river. These impediments cause the magnetic field to swirl around them, dancing into their shadows as eddies.

The eddy current probes collect information on the flow of the magnetic field, and send this information to a connected instrument. The eddy current instrument uses powerful software to create a visual representation of the physical characteristics of the galvanized steel or other material being scanned. Technicians read this chart to identify defects, reporting what they find. Armed with this information, company, agency, or military managers can determine whether the steel is sound for continued use, or whether it must be repaired or replaced. 

The importance of performing eddy current inspections on galvanized steel varies directly with the importance of the steel’s function. In applications where materials failure is harmless, there’s no urgency behind inspections. But in applications where materials failure can have dire repercussions, inspections can make the difference between mission success or mission failure and loss of life.

Difficulties Inherent in Eddy Current Testing of Galvanized Steel 

While eddy current technology can effectively create internal images of galvanized steel, the metal’s properties do create some difficulties. Overcoming these difficulties requires using specialized equipment and powerful analysis software. There are four primary issues that eddy current equipment must overcome to successfully image galvanized steel, all related to magnetism. 

• First, the zinc coating of galvanized steel prevents the magnetic signal’s full strength from reaching the steel underneath. Likewise, it diminishes the strength of the return signal.
• Second, the magnetic properties of the metal increase the eddy current probe’s area of effect. This can cause underlying defects to appear smaller than they actually are, leading technicians to miscategorize the extent of corrosion or other damage. 
• Third, when the magnetic pulse enters the conductive steel, it creates a strong irrelevant signal the eddy current probe detects for the first few milliseconds of scanning. This effect diminishes rapidly. 
• Fourth, the magnetic field applied by the eddy current probe causes the ferromagnetic steel to physically move in response to magnetic attraction. This vibration can mask or distort the signal the eddy current instrument is attempting to decipher, causing false positives or missed negatives. 

While these phenomena do increase the difficulty of performing accurate eddy current scans of galvanized steel, they can be mitigated using specialized equipment and proper technique. 

Effective Eddy Current Equipment and Techniques for Galvanized Steel

To compensate for these effects, eddy current technicians must use the right equipment. Inspection program managers should seek out eddy current instruments with powerful, modern software that can account for the signal distortions caused by the magnetism of galvanized steel. Advanced software should be able to filter out noise, and algorithmically adjust for the increased area of effect. Eddy current software should also detect and filter the extra signal returned during the first few milliseconds of each scan. 

Multiple-array eddy current probes are also important, as they can help mitigate the reduction in signal strength that occurs as a result of the galvanization. Single array probes, working hard to deal with this signal reduction, take significantly longer to return useful results over an inspection area than probes with multiple arrays. 

Lastly, vibration dampening should be applied to the steel, to prevent bad signals arising from the physical movement of the metal in response to the magnetic field. Vibration dampening can be physical, or it can be produced by saturating the metal with a powerful magnetic field. Once the metal is saturated, it no longer moves in response to the probe’s field, and can be more easily studied. 

Magnetic saturation of galvanized steel prior to and during eddy current testing has been posited to also reduce signal diminishment, normalize the probe’s signal footprint, and reduce the noise created in the first milliseconds of scanning. Some eddy current equipment manufacturers have created specialized eddy current probes with permanent magnets attached, for the specific purpose of testing galvanized steel. 

While testing galvanized steel poses unique difficulties for inspectors, the use of eddy current instrumentation remains the most effective way to detect surface cracks, corrosion, and subsurface defects. 

Zetec supplies high-quality eddy current equipment for companies and agencies testing galvanized steel and other critical materials. Contact Zetec today to learn more about how our inspection solutions can meet your testing needs.