Corrosion Resistant Alloys – Inspected

The use of corrosion resistant alloys (CRA) is increasing in the pipeline industry as more and more pipelines are operated under highly corrosive conditions especially in offshore areas. Compared to solid CRA, clad and lined pipes are less expensive and they provide a good balance between good mechanical properties of the carbon-steel carrier pipe and the corrosion resistance of the internal CRA layer. Clad pipes consist of an internal CRA layer which is metallurgically bonded to the backing carbon-steel. Different manufacturing processes (e.g. hot rolling or weld overlay) are used to bond the CRA layer to the carbon steel. In contrast to clad pipes, lined pipes have a CRA lining that is mechanically bonded to the carrier pipe.

Corrosion in CRA Pipes

Highly corrosive environments increase pipeline vulnerability to corrosion attack. This has considerably increased with the exploration of new oil and gas fields, higher levels of water cut and concentration of CO2 and H2S and the higher temperatures and pressures of deep sea exploration. CRA materials are used to prevent corrosion, especially in offshore applications. Under certain conditions, nevertheless, severe corrosion processes may develop in such pipes. The effectiveness of the corrosion resistance depends on the selection of the proper alloy for the expected operational conditions. Manufacturing related anomalies and imperfections of the CRA pipes can also lead to corrosion development. Furthermore, unfavorable handling during transportation, storage and installation can significantly impair the corrosion resistance characteristics.

In addition to a stationary non-destructive testing (NDT) for quality assurance during the manufacturing process, inline inspection (ILI) of pipelines is essential to ensure the integrity of the CRA material and thus the pipeline after commissioning (baseline survey) and later in operational life.

Improved Corrosion Resistance

CRA pipes offer improved corrosion resistance in pipelines while introducing new challenges for ILI. For a wide range of CRAs, pitting dimensions are smaller compared to carbon steel pipe and are well below detection limits of low-resolution inspection tools.

Examples of CRA Pipe Corrosion
  • Crevice Corrosion: Intensive localized electrochemical corrosion occurs within crevices when in contact with a corrosive fluid
  • Pitting Corrosion: Highly localized attack that eventually may result in leaks in the metal
  • Galvanic Corrosion: Potential difference between dissimilar metals in contact creates a current flow, causing corrosion
  • Stress Corrosion: Occurs in metal that is subject to both stress and a corrosive environment often starting at “stress risers”
Examples of CRA Pipes
  • Solid CRA pipes: The ultrasonic propagation and attenuation in solid CRA (e.g. duplex SS or 13% Cr steel) is similar to those in carbon steel. Therefore, the UT tool performance (detection and sizing capabilities) valid for carbon steel is also valid for most solid CRA pipes.
  • Metallurgically bonded pipes: As CRA cladded pipes are made of metallurgically bonded plates with a seam weld, the internal surface is typically quite smooth, and poses no limitations with regard to UT inspection. Disbonding between both layers can be detected and sized with high accuracy. Pipes constructed with internal cladding, produced by orbital or longitudinal welding can also be inspected by UT tools. Depending on the waviness of the weld overlay, the data quality can be affected.
  • Mechanically bonded pipes: In this case the pipe has a CRA inliner that is mechanically bonded to the carrier pipe e.g. by hydraulic expansion. UT tools can only inspect the inner layer as ultrasonic pulses cannot propagate through this type of bonding.

EVO Series UMP

NDT Global offers high-resolution pinhole and pitting (UMp) metal loss inspection service as standard.

The UMp service reliably detects defects and sizes as small as 5 mm (0.2 in) diameter, which represents a two-fold improvement in the minimum sizing threshold, from the previous entry level ultrasonic service of 10 mm (0.4 in).

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EVO Series UC

Ultrasonic crack inspection enables early detection and sizing of crack-like irregularities. This allows the pipeline operator to take appropriate measures to avoid pipeline failures caused by cracks.

The principle of the ultrasonic crack inspection tool is based on the 45° angle beam technique using shear wave. Due to the so-called corner reflection, even minor cracks from approximately 1 mm onwards give quite strong reflections.

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EVO Series UCC

Circumferential cracking defects can be addressed with ILI tools utilizing ultrasonic shear wave technology.

To accurately identify circumferential cracking, the diffracted shear wave signal travels in the pipe wall by an angle of 45° and is reflected either by an external or internal flaw. Based on the individual pattern of the reflected signal algorithms, experienced data analysts can read and translate the information into flaw properties, e.g. length, width, type and depth.

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Our Technology

NDT Global recently introduced the latest generation of UT tools - Evo Series 1.0. This generation offers inspection velocities of up to 4 m/s (9 mph). These high speed tools overcome reduction of flow rates for intelligent inspection runs.