From Pipeline Mapping to Corrosion Detection: How CMI Evolved Beyond 3D Positioning
From Pipeline Mapping to Full Integrity Assessment
When EMPIT introduced Current Magnetometry Inspection (CMI), the technology set a new benchmark for above-ground 3D pipeline positioning. By feeding a multi-frequency AC current into the pipeline and measuring the resulting electromagnetic field with up to 70 tri-axial sensors at 20,000 data points per second, CMI delivered the most precise contactless pipeline mapping available — with measurement errors as low as 1 cm using RTK-FIX GNSS.
But precise mapping was only the beginning.
The Challenge: Conventional Methods Stop at the Coating
Standard above-ground survey methods — DCVG, ACVG, CIPS, and conventional CAM — can identify coating defects, but they share a fundamental limitation: they cannot determine what is happening at the steel surface beneath the coating holiday. Is the defect actively corroding? Or has a protective calcareous layer formed, rendering the defect operationally harmless?
Without this distinction, pipeline operators face a costly dilemma. Industry data shows that up to 95 % of excavations triggered by conventional methods are operationally unnecessary — the coating defects they flag turn out to be passivated and pose no immediate integrity risk. Each unnecessary dig means avoidable cost, traffic disruption, and environmental impact.
CMI Coating Assessment: Seeing What Others Miss
CMI advanced beyond pipeline geometry by integrating coating assessment directly into the measurement process. As the inspection current flows through the pipeline, every coating defect creates a low-resistance path between the pipe and the surrounding soil. This causes a measurable attenuation in the electromagnetic signal — detectable by CMI's sensor arrays.
The result: CMI identifies coating holidays as small as 1 mm² with a probability of detection exceeding 95 % for defects of 1 cm² — validated across thousands of kilometres of real-world inspections. For each detected defect, CMI calculates the spread resistance according to ISO 18086, providing a quantitative measure of the defect's electrical characteristics.
The Breakthrough: Corrosion State Classification
The decisive step beyond coating assessment came with CMI's patented frequency-dependent spread resistance analysis. By applying AC current at multiple frequencies (2 Hz – 2 kHz) and analysing the amplitude and phase response at each coating defect, CMI determines the electrochemical condition of the steel surface:
- Passivated defects: The spread resistance increases with decreasing frequency, revealing a capacitive barrier — a protective calcareous layer has formed. The corrosion cell is electrically blocked. These defects require monitoring only.
- Active corrosion: All frequencies show the same amplitude — no frequency spreading. This indicates bare steel in direct soil contact with an active corrosion cell. These defects demand excavation and repair.
This classification is expressed through the Corrosion Risk Index (CRI), which assigns each defect to one of three zones: passivation (CRI 0–90), transitional (CRI 90–98), or active corrosion (CRI >98). The DVGW research project NEMEK confirmed a method reliability of 99.96 % across 6,672 detected coating defects — active corrosion was observed at excavation in only 3 cases.
One Run, Full Integrity Picture
Today, a single CMI inspection run delivers 10+ services simultaneously: 3D pipeline positioning, depth of cover, coating assessment, corrosion state classification, E_IR-free potential calculation, bending strain assessment, AC corrosion risk evaluation, and more. All contactlessly from the surface — without shutdown, without direct pipeline access, and without disrupting cathodic protection systems.
This is the transition that defines CMI: from the most precise above-ground mapping system to the only above-ground method that combines coating defect detection with reliable corrosion state determination in a single inspection run.