Does aluminum rust? Short answer: no. But it does corrode. What, will you say? Technically, rust only refers to iron and steel corrosion. But, aluminum does corrode. It’s actually quite prone to corrosion.
The product of aluminum corrosion is aluminum oxide—a very hard material acting as a barrier against further corrosion. So the corrosion resistance of aluminum and its alloys relies heavily on this protective oxide film. Aluminum oxide is stable in fluids with a pH between approximately 4.0 and 8.5, and is naturally self-renewing, so scratching the surface film is rapidly repaired.
Conditions promoting aluminum corrosion are therefore found in environments:
- Continuously abrading the oxide film mechanically or;
- Where the protective film is degraded and the availability of oxygen to rebuild it minimized.
The acidity (pH lower than 4.0) or alkalinity (pH higher than 8.5) of the environment significantly affects the corrosion behavior of aluminum. At these pH, aluminum is more likely to corrode, but not always. For example, aluminum is quite resistant to concentrated nitric acid. But, when the oxide film is perforated locally, aluminum is attacked more rapidly than it oxidizes under alkaline conditions—the result is pitting. In acidic conditions, the oxide is attacked more rapidly than the aluminum, resulting in more widespread aluminum “rust”.
Assessing Corrosion in Aluminum without Direct Access to the Surface under Test
In many situations, it’s crucial to detect and assess the extent of corrosion damage without direct access to the surface under test (far side). Applications such as in storage tanks of nuclear power plants or multilayered aluminum structures of aircrafts are some examples.
When performing this kind of inspection to prolong the life of assets and preventing costly, dangerous leaks, for example, single-channel eddy current testing (ECT) is powerful enough to detect defects. However, it’s time-consuming and inspection results are rather difficult to analyze. Ultrasonic testing (UT) also has several drawbacks—namely, being time-consuming because of the small active surface of the transducer.
Eddy Current Array to the Rescue
ECA technology regroups several individual coils into a single probe. The coils are multiplexed—activated in specific sequences to eliminate the interference between them and tailor a magnetic field suited to target defects—achieving a wider inspection area than conventional ECT probes. This drastically cuts down on the time required to inspect an entire tank floor, for example, and makes ECA considerably less dependent on its users than ECT.
Less Surface Preparation
The absence of couplant—inherent in ultrasonic testing—is a natural advantage of ECA. The way an ECA probe’s active surface can also be ruggedized means it can be used on surfaces with only minimal pre and post-inspection surface preparation such as cleaning and stripping.
ECA can be made flexible and shaped to specifications, making it capable of scanning surfaces inaccessible to conventional ECT and UT.
The added number of channels provided by an ECA probe requires a test instrument such as Eddyfi’s Reddy® offering the necessary power to handle the channels. ECA data can also easily be encoded, making positioning defects much simpler than with conventional ECT probes.
Powerful Acquisition and Analysis Software
The increased amount of data generated by an ECA probe also requires software capable of processing all the channels simultaneously—such as Magnifi® —which enables analysts to display data as C-scans, making defect discrimination all the more simple.
Case Study—Aluminum Storage Tank Floor
Storage tanks come in various shapes, sizes, and materials. As a case study, the aluminum tank floor under test is slightly concave, approximately 5.2 m (17 ft) in diameter and, as is usually the case in in-service inspections, its far side isn’t accessible for direct inspection.
The coils of the ECA probe used in this inspection are 6 mm (0.236 in) in diameter and matched to cover the 0.6–20 kHz low-frequency range (central frequency of 5 kHz) necessary to penetrate through and reach the far side of the 6.35–7.94 mm (0.250–0.313 in) tank floor.
The ECA probe used a wide, flexible active surface that can adapt to the slightly concave tank floor, but it can also adapt to similarly convex geometries.
This semi-flexible ECA solution was originally tested on a calibration plate to validate the probe’s performance. The phase angle was used in the analysis software to assess the extent of corrosion, discriminating between near-surface and farther defects. In addition to the traditional impedance plane, ECA technology offers advanced imaging capabilities. C-scans prove extremely useful when interpreting signals. Scanning the calibration plate probe yielded the following results:
Scan Results on Calibration Plate
The ECA probe can clearly detect the pitting-like indications in the calibration plate. Each column contains flat-bottom holes (FBH) 80%, 60%, 40%, 20% and 10% in thickness (respectively, from the left). The FBH have the following diameters (from the left):
- First row: 1.6 mm (1/16 in )
- Second row: 3.2 mm (1/8 in)
- Third row: 6.4 mm (1/4 in)
- Fourth row: 12.7 mm (1/2 in)
In the field, this led to discovering much degraded tank floor plates. The entire tank floor inspection with the ECA solution was completed in about a tenth the time it usually takes with UT, and carried out by a single technician instead of several.
Scan Results on Tank Floor Showing Clear Signs of Corrosion
So, ECA rises to the challenge of detecting corrosion without direct access to the surface under test, offering several considerable benefits:
- Rapidly scanning large regions of interest cuts down on the time it takes to perform inspections.
- The capacity of ECA technology to adapt to various geometries make it considerably more versatile than others.
- ECA enables operators to perform high-precision assessments of localized indications (e.g., pitting) and general degradation (e.g., thinning).
- The C-scan imaging possible with ECA technology are much easier to analyze than conventional imaging, giving operators confidence with little ECA knowledge about their assessments.
- The full data recording and archiving capabilities that come with ECA technology are great for in-depth post-inspection analysis, defect evolution monitoring, and more.