Stainless steel tubing is usually made of 300 series (austenitic) stainless steel alloys. The use of these materials is increasing in several industries because of their material properties, such as resistance to corrosion. Three technologies are generally used to test for corrosion in austenitic stainless steel alloys: radiographic testing, ultrasonic testing, and eddy current array testing.

Radiographic Testing (RT) — For decades, has been the primary method used to detect corrosion in stainless steel pipes used on off-shore platforms. However, there are complicating factors that add to the expense and difficulty of using RT:

• When inspectors use radiography, there are strict rules about how close people can come to the X-ray equipment, and these regulations are costly for the industry.
• When radiography is used in environments where inspectors use ropes to access an area, something common in the oil & gas industry, they cannot remain close to the radiography tube, so inspectors are required to travel back and forth between a safe area and the inspection areas. This makes the entire inspection process slow and cumbersome.
• There is additional cost associated with the extra time required for inspectors to take multiple X-rays of a pipe to get an accurate corrosion measurement.
• As regulations governing the use of X-rays continue to tighten, RT is no longer permitted in some countries.

Close-up of an ECA corrosion probe

Ultrasonic testing (UT) — Using ultrasonic flaw detectors to test austenitic stainless steel can be difficult. Inherent material properties cause beam skewing and scattering, signal attenuation and a poor signal-to-noise ratio, lack of skip at interfaces, and other complications to the ultrasonic inspection process. These issues make this inspection method impractical.

Eddy Current Array (ECA) — The best option for corrosion detection and depth sizing is eddy current array (ECA) technology. Surface preparation is kept to a minimum and pipe inspections can be quickly completed. The inspector needs to travel to the pipe only once, and then make four passes with the instrument to scan each pipe section. It is easy to calibrate for the detection of flaws, and the data acquisition speed is very fast. Setup files can be saved and easily recalled for the next job. Data is recorded to enable seamless post-processing analysis and archiving, and reports can be easily created and exported directly from the instrument.

Using an Eddy Current Array Flaw Detector to Test Stainless Steel Pipes for Corrosion

Some portable eddy current array flaw detectors, like the OmniScan MX1 with the ECA module, can be used to perform inspections using rope access because it incorporates a four-point anchor system, and the probe includes a wrist strap.

When choosing an ECA flaw detector, look for one with the following features: C-scan view; Battery operated; Encoded or time-based scan; and Ability to record setups and data.

An inspection of the entire pipe circumference can be performed in four passes. (The inspection speed should not exceed 4 inches per second [100 mm/s]). Each pass overlaps about 10 mm with the next pass. Analyzing indications is easy using the flaw detector and a USB mouse back in the office. Most ECA flaw detectors enable you to save data to a compact flash card that can be easily removed and transferred from a rig to an office as needed.

Choosing the right probe is just as important

Choosing the right probe is just as important as using the right flaw detector. In this case, we used an Olympus ECT probe with the following characteristics:

• The probe has 37 coils that generate 32 usable channels. It was configured for reflection (driver-pickup) with a coverage of 64 mm and a pitch of 2 mm. The 5 rows of coils help ensure stable sensitivity, which we evaluated and found that it did not exceed a total of 12 per cent of the amplitude.
• The probe is 2 11/16 inches (68 mm) wide. It’s important to note that this configuration prevents the probe from fully obtaining readings until 67 mm after the start of the scan. This leads to a “dead zone” in the scan where corrosion is not detected. This “dead zone” can be rescanned and analyzed using a custom ECT probe.
• The left side of the probe is the origin (0 mm axial) for the setup.
• The total clearance needed around the pipe is 2.75 inches (60 mm) at the handle location. The clearance needed to pass the cable is 4.5 inches (114 mm).
• The probe’s wear surface is protected with wide, durable tape, and two thicker contact tape strips provide good stability and wear resistance.
• There is a ‘mini-wheel’ type encoder included in the probe assembly.

Using the proper reference standard is important for acquiring good data during your inspection. The standard you use should have the same wall thickness as the pipe that you’re inspecting. It should include at least one 1.2 mm diameter through-wall hole. Allow 4 inches (100 mm) before and after the hole to minimise any difficulty with scanning. Based on these requirements, the reference standard should be a minimum of 8-inches long (200 mm). If you want to see additional defects, allow 1.25 inches (38 mm) on the axial distance and 1 inch (25 mm) for the circumferential distance.

Calibrating sensitivity

To properly calibrate the ECA flaw detector’s sensitivity, use the following steps:

1. Set the probe firmly at one end of the reference standard. Make sure the probe’s left side is positioned exactly at the beginning of the standard.
2. Scan the standard moving no faster than 4 inches per second (100 mm per second).
3. Freeze the signals.
4. Use the cursors to locate the strongest signal resulting from the 1.2 mm diameter through-wall hole.
5. Make sure that the signal reaches 70 per cent of the full-screen height (7 divisions).
6. Adjust the gain as needed.
7. Place the red cursor in the middle of the through-wall hole signal and read the axial distance (scan start). It should match the actual distance to the nearest 1 mm.

Your sensitivity is now set.

Depth of Corrosion

The depth of corrosion can be measured using the data you recorded during the inspection. The software provides two important readings—the vertical amplitude peak-peak (V PP) and the phase angle, peak-peak (θ PP).  In some cases, amplitude peak-to-peak (A PP) can also be used when the signal has hardly any horizontal profile. The reading is a correlation between the depth and the defect. The bigger the number, the deeper the defect. 

Analysis and Reporting

Analysis can easily be performed using recorded data, and the results reported using your flaw detector’s reporting tools.

Conclusion

ECA is an effective technology for detecting and sizing corrosion when inspecting austenitic stainless steel pipes. An inspector only needs to travel to the pipe being inspected once and then make four passes with the instrument to scan each pipe section. If the pipe is painted, the inspection can be done directly through it without having to strip the paint off, reducing the need for chemicals and saving time. No radiation means that the job can be completed more quickly while the instrument’s intuitive C-scan images and analysis tools provide robust data that can be easily archived.