Application: Detection of stress corrosion cracking (SCC) in pressure containment equipment such as pipelines, using eddy current array testing.

Problem: Detection of anomalies in pipelines is essential to continued safe operation. Numerous catastrophic events have occurred due to anomalies created by manufacturing and environmental conditions. Stress corrosion cracking develops in carbon steel and stainless steel pipelines due to a combination of environmental factors including absolute hoop, tensile, and fluctuating stress. Other influential factors include material (steel) type, number of inclusions, surface roughness, coating integrity, and soil pH.

Two types of SCC are found in pipelines: high pH (9 to 11) and near-neutral pH (6 to 8). General characteristics of both forms of SCC are as follows:
- Cracks are usually oriented in longitudinal direction but may exist in other orientations depending on the direction of applied stress.
- Occurrence in clusters consisting of several cracks to hundreds of cracks.
- Cracks tend to interlink to form long shallow flaws which may grow to cause ruptures.
- Fracture faces are covered with magnetite and carbonate films.

Eddy current array testing offers a number of advantages over alternate NDT techniques such as magnetic particle inspection:
- Minimal surface preparation needed as compared to magnetic particle testing.
- Rapid scanning of large surface areas.
- Determine approximate depth of the cracks detected.
- C-Scan imaging of the area scanned can be used for reporting and subsequent inspection comparison
- Selection of the deepest crack(s) within a cluster of cracks.

The Importance of Weld Inspection

The quality of welds is becoming increasingly important as customer expectations rise. Products and components are expected to be of a high quality and not to fail unexpectedly.

Such failures have large financial and social consequences that can often be avoided with the proper inspection techniques.

Inspecting welds can also reduce costs by detecting defects in the early stages of manufacture, reducing the cost of customer returns and extending the life of components by detecting and correcting any defects.
Eddy Current Non Destructive Testing is a reliable, quick and inexpensive way to carry out preventative maintenance and ensure safety. Staveley's range of eddy current equipment has a world-wide reputation for its reliability and accuracy, while their service and support ensure that you make the most of your equipment.

General

Corrosion is the deterioration of a metallic material by chemical (or electrochemical) attack. This is normally caused by the environment (most often water), and sometimes by another material. The corrosion products generated are not electrically conductive, so we will be measuring the thinning of the material under test.
An eddy current instrument and probe can be used for detection, and, using a specific procedure, it is often possible to perform a quantitative measurement.

There are several types of corrosion:

• Uniform corrosion that extends evenly across the surface
• Pitting corrosion that is uneven has smaller deep areas (pits)
• Exfoliation corrosion that moves along layers of elongated grains
• Intergranular corrosion that grows along grain boundaries

In most situations, particularly in the aerospace industry, the material under inspection will be a type of aluminum alloy, so we will concentrate on this application. Corrosion in steel is not normally detectable with eddy currents, although there are some exceptions, such as the use of the reflection remote field technique, mostly for tubing inspection.

Application: Use of dual element transducers ("duals") for measurement of remaining metal thickness in corrosion applications.

Problem: Just about anything that is made of common structural metals can be subject to corrosion. A particularly important problem that faces many industries is measurement of remaining wall thickness in pipes, tubes, or tanks that may be corroded on the inside surface. Such corrosion is often not detectable by visual inspection without cutting or disassembling the pipe or tank. Structural steel beams, particularly bridge supports and steel pilings, are also subject to corrosion that reduces the original thickness of the metal. If undetected over a period of time, corrosion will weaken walls and possibly lead to dangerous structural failures. Both safety and economic considerations require that metal pipes, tanks, or structures that are subject to corrosion be inspected on a regular basis. Ultrasonic testing is a widely accepted nondestructive method for performing this inspection, and ultrasonic testing of corroded metal is usually done with dual element transducers.