Why Does MFL Struggle to Detect Cracks

Magnetic Flux Leakage (MFL) is a staple in pipeline inspection – fast, efficient, low-fuss, and highly effective for detecting metal loss such as corrosion or pitting. 

But when it comes to cracks, especially axial or stress corrosion cracking (SCC), MFL falls short.  

Read on to learn why. 

How Does MFL Work? 

MFL magnetizes a pipeline and detects where the magnetic field leaks out, indicating spots where metal may be missing. 

The method works a bit like someone running a magnet over a sheet of metal. If the amount of metal is uniform, the pull on the magnet will be consistent. If the magnet passes an area where there is severe corrosion, the pull on the magnet will reduce. MFL uses this same principle – turbocharged with sensitive sensors – to measure metal loss. 

1. Cracks Are Not Metal Loss 
When there is a reduction in wall thickness, the magnetic field "leaks" from the steel, and sensors pick up that distortion. 

But cracks typically do not remove material. They create tiny spaces within the material. In fact, some tight axial cracks may only be a few microns wide, causing minimal flux leakage. 

With no metal missing, MFL struggles to pick-up on the change in the material. 

2. Crack Orientation Matters 
Most MFLs are sensitive to circumferential anomalies, since MFL tools typically magnetize the pipe axially. 

Circumferential defects cut across the magnetic field, interrupting it more sharply, while axial defects, parallel to the field, have a more subtle effect. 

Think of it like water flowing in a river. A long rock – perpendicular to the river – will cause a major disruption to the flow; that same rock – parallel to the river – will be less disruptive. 

Most cracks, including stress corrosion cracks (SCC), are axial, running parallel to the magnetic field.   

This makes axial cracks much harder to detect and even harder to size. 

3. Noise & False Positives 
Because cracks cause only subtle magnetic disturbances, they can be easily masked by noise or confused with benign features, like welds or laminations.  

This leads to poor reliability and a high rate of false positives or missed detections. 

Bottom line: MFL is excellent at detecting metal loss but struggles with cracks. 

For high-resolution crack detection, especially in aging or high-risk pipelines, operators turn to ultrasonic technologies, which directly measure cracks based on the wave’s interaction with the defect. 

Dexon offers both MFL & ultrasonic ILI tools, able to deliver regardless of the solution required.