Corrosion Mapping

Corrosion mapping (ultrasonic corrosion mapping / C-scan) is an advanced non-destructive testing technique that images wall thickness loss in metallic components through a scan covering the whole surface rather than through individual spot readings. The method is based on the fundamental physical principle of ultrasonic thickness measurement: a sound pulse sent into the material by the probe is reflected from the back surface (inner wall) and returns to the probe. Multiplying the round-trip time (time-of-flight) by the sound velocity in the material and dividing by two gives the remaining wall thickness at that point. What distinguishes it from conventional spot readings is that this measurement is linked to a position encoder and taken continuously at close intervals across the surface, with the results presented as a two-dimensional, colour-coded thickness map (C-scan). In this way an isolated pit, a local gouge or general thinning is documented together with its location and severity, minimising the possibility of it being missed.

In practice the inspection surface is first cleaned mechanically; loose rust, paint, coating and scale are removed since they disrupt sound transmission, and the surface roughness is brought to a suitable level. A couplant (gel or water) is used to provide acoustic coupling between the probe and the material. Scanning is performed either with a hand-guided encoded probe assembly or with a semi-automatic/automatic scanner; the encoder continuously tracks the probe position and links every thickness value to an x-y coordinate. The acquired data is visualised in the instrument software with a colour palette in which the minimum thickness is shown in red and full thickness in shades of green, so that thinned areas are immediately distinguishable on the map.

Technically, the measurement is carried out with single-crystal, dual-crystal or phased array probes working on the pulse-echo principle. Dual-crystal probes are preferred where internal surface corrosion and near-field resolution are required, while phased array corrosion mapping probes scan a wide band electronically in a single pass and deliver high-resolution, fast and repeatable results. Measurement accuracy is directly related to the frequency and diameter of the probe, the scan step (index), the actual sound velocity in the material and the surface condition. For this reason the instrument is calibrated before every inspection on step-wedge reference blocks of known thickness having the same sound velocity as the material under test; the effect of temperature and the presence of coating are also taken into account.

In equipment such as pressure vessels, atmospheric and pressurised storage tanks, pipelines and heat exchanger shells, corrosion is usually not evenly distributed; elbows with high flow turbulence, tank floors, shell-to-floor joints and condensation zones thin selectively. In addition to general thinning, corrosion mapping also reveals the extent of mechanisms such as pitting, erosion-corrosion, corrosion under insulation (CUI) and hydrogen-induced damage/blistering (HIC/blistering). The measured minimum remaining thickness is compared with the calculated required minimum thickness (t-min) to assess the remaining life and the next inspection interval; by overlaying the maps from successive inspections, the corrosion rate can be monitored.

In reporting, the acceptance criteria are determined according to the relevant structural integrity or inspection code (for example API 510, API 570, API 653); the result is documented together with the C-scan images, minimum/nominal thickness values, the scan plan and the calibration records. The value of the method lies in the absence of ionising radiation compared with radiography, its applicability under service conditions without shutting down the plant, and its production of numerical, repeatable and archivable data instead of subjective interpretation. TÜV AUSTRIA SILA KALİTE performs corrosion mapping as a TÜRKAK-accredited inspection body under TS EN ISO/IEC 17020, with authorised personnel and approved procedures.