Corrosion Inspection Using Pulsed Eddy Current - GPEND

11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic
Corrosion Inspection Using Pulsed Eddy Current
Ivan C. SILVA1, Ygor T. B. SANTOS1, Lurimar S. BATISTA1, Claudia T. FARIAS1
1
Nondestructive Testing Research Group, Federal Institute of Bahia; Salvador, Brazil
Phone: +55 71 21029423; e-mail: [email protected]
Abstract
Equipment and metal components undergo the action of corrosive processes, which causes the reduction of the
wall thickness, limiting operating conditions and reducing its useful lifetime. With high rate of corrosion,
corrosion under insulation (CUI) is a damaging mechanism, because it can occur without warning in
equipment’s and piping with insulation that apparently are in good condition. Therefore, non-destructive
inspection techniques and monitoring are necessary to ensure the health and safety of industrial systems. Among
the non-destructive testing, pulsed eddy current technique has been used for inspection of coated metal
components. This paper describes the operation of a pulsed eddy current inspection system with a probe based
on a solid state (GMR) sensor. The experiments were performed on test samples of carbon steel with machined
discontinuities. To simulate the insulation and cladding, acrylic plates and an aluminum sheet were used.
Keywords: Pulsed Eddy Currents, Corrosion Under Insulation, GMR sensor
1. Introduction
Corrosion under insulation occurs in isolated lines due to liquid infiltration or humidity
retained between the metal and insulating material. Local subject to corrosion under
insulation are: in insulated fuel lines (with stream tracing out of operation), lines under
concrete (fire proof), insulated lines without heating or dead stretches and drains or coatings
of piping systems.
The technique of pulsed eddy current (PEC, Pulsed Eddy Current) has proven to be able to
detect and measure discontinuities in metallic equipment in the aeronautical (1), Nuclear (2),
and Oil industries (3). Since it is a technique where the inspection can be performed without
direct contact of the sensor with the tested metal, it has been used in submerged (4) and
coated (5) components. The PEC technique has several advantages over the conventional
technique of single-frequency eddy currents, such as: greater depth of penetration; wealth of
information about defects and robustness against interference. This technique also requires a
less expensive instrumentation, compared with the multifrequency eddy currents, which
would also be another advantage (7).
However, despite many advantages, the technique has limiting factors compromising their
application. Changes in magnetic permeability of the materials affect the probe signal, as well
as corrosion products. In insulated materials, it's commonly used a metallic cladding that can
affect the probe response (8).
These limiting factors have provided many researches that each day optimize the equipment
and extend its use. The first probes used a coil-coil arrangement (9), but current-coil sensor
arrays has been employed for providing a direct reading of the magnetic field and simplifying
the instrumental (10, 11).
In this paper, is described the used experimental setup and the probe mounted at the
laboratory. Tests were performed in carbon steel plates with machined holes with different
values of depth and Lift-Off.
2. Theory
2.1. Corrosion Under Insulation (CUI)
Corrosion under insulation is an electrochemical process and usually occurs due to the
presence of humidity and oxygen dissolved in a range of -4 ° C to 175 ° C. This corrosive
environment remains retained between the metal and the insulating material due to absorption
characteristics of the last one. Contaminant such as chlorides and sulfides increases the
corrosivity of the water (12). The worst insulating materials in descending order are:
fiberglass, mineral wool, ceramic fiber and calcium silicate (13). Figure 1 shows the CUI in a
pipe carbon steel insulated.
Figure 1 – Corrosion under a thermal insulation in a carbon steel
pipe. (14).
2.2.Pulsed Eddy Current
The PEC technique uses repetitive pulses of short time duration instead the sinusoidal signal
with single frequency. The Fourier transform of a square pulse contains a series of different
frequency components. Since the penetration factor depends on the frequency, the diffusion
of the generated eddy currents covers a wide range of thickness. High frequency components
penetrate less and can be observed firstly, while lower frequency components reach deeper
thickness. This feature provides a time character that allows determination of depth
discontinuities (15).
The probe used for PEC testing utilizes a coil to generate eddy currents in the metal, while the
magnetic field generated by them is detected by a solid-state sensor, Figure 2. Since both the
field from the coil, and the small field generated by the eddy currents are detected by the
sensor, the detection of discontinuities becomes very difficult. To overcome this limitation,
Differential arrangements are employed. These arrangements may use a second sensor within
the coil, or between two coils (16). Alternatively, an arrangement with one sensor can be used
to subtract a reference signal acquired outside the inspection area (17).
Figure 2. – PEC principle and theoretical model.
For a cylindrical coil with rectangular cross section located over a metal specimen as shown
in Figure 2., the magnetic field in the air in the region of the metal is given by (18-19):
BZ = BZc + ∆BZ
(1)
Where BZc is the field produced by the coil and ∆BZ is the field change caused by the metal.
The expression for the component of the field depends on the characteristics of the region
with respect to the axial distance and is given by
BZc =
µ0i0 ∞
2
∫J
0
(ar )
χ (ar1 , ar2 )
0
a
2
(e a ( z − z1 ) − e a ( z − z2 ) )da , for z1 ≥ z
(2)
NI
is the coil current density with N turns, r1 the inner
(r2 − r1 )( z2 − z1 )
radius, r2 the outer radius, z1 the lift-off distance and z2 – z1 the height of the coil. The
magnetic field component of the coil in z direction due the presence of a conductor material is
And the current i0 =
∆BZ =
µ0i0
2
∞
∫J
0
(ar )e −az
0
χ (ar1 , ar2 )
a
2
 aµ − a 
(e a ( z − z1 ) − e a ( z − z2 ) ) r 1 da
 aµ r + a1 
(3)
x2
where χ ( x1 , x 2 ) = ∫ xJ 1 ( x)dx is the Bessel function of first order and a1 = a 2 + jωµ 0 µ rσ
x1
(4)
where a is the integration variable, ω is the angular frequency, µ 0 is the magnetic
permeability of free space, µ r is the relative magnetic permeability and σ the conductivity
3. Experimental set-up and specimen
The experimental set-up used in this research consists of a pulse generator, an oscilloscope, a
power supply, a computer and a probe, as shown in Figure 3. The pulse generator is used to
generate the exciting pulse with 6V amplitude and 0.1ms. The probe coil is excited by this
pulse, generating a primary field which generates eddy currents inside the test piece. The
eddy currents generate a secondary field and the resultant magnetic field is detected by the
GMR sensor inside the coil. The oscilloscope acquires the output signal from GMR sensor.
This signal is subtracted from the reference one acquired outside of the defect. The signal
processing is performed in a P computer with Matlab. Table 1 shows the coil dimensions and
parameters.
Figure 3. PEC experimental set-up.
Table 1. Coil parameters.
Ø - Diameter
(mm)
(Øext) (Øint)
24
14
Height
(mm)
Ø- Wire
(mm)
Lift-Off
(mm)
Inductance
(mH)
Wire Resistance
(Ohm)
10
0,27
2
3.53
11
The specimen used was a 1020 carbon steel plate with 150x250x12.5 mm. Drilled holes of 2
mm diameters and depths of 2, 4 and 6 mm were inserted. To simulate the insulation, lift-off
distances of 10mm, 20mm and 30mm were adopted. Additionally, an aluminum plate with
1mm thick was used covering the insulation.
Measurements were made with the reference signal acquired in a region without holes.
Differential measurements were performed by subtracting the signals from the region without
defect (reference) minus those from the hole.
4. Results
Figure 5 shows signal response from the GMR sensor for three values of lift-off. The lift-off
increase the signal time constant, so for 10mm of lift-off, the response is closer to the driver
pulse format. As the lift-off reduces, the response rises slowly, because the effect of generated
eddy currents, due metal proximity, becomes stronger. The magnetic field for 0mm lift-off
rises slowly and has smaller amplitude.
Figure 5. GMR sensor response for different values of lift-off.
The effect of the cladding was investigated, Figure 6. An aluminum sheet of one millimeter
thickness superimposed to the insulation turned the signal slower, especially after the drive
pulse is switched off.
Figure 7, shows the differential signals from the drilled holes of 2, 4 and 6mm depth, 30mm
lift-off (insulation) and 1mm of aluminum cladding. Although the 2mm depth almost can´t be
seen, the others are well distinguished. Table 2, shows the main parameters from these
signals, the amplitude of the differential signal decreased with the lift-off (insulation plus
cladding). The Time Zero Crossing (TZC) increased with the hole depth, but decreased with
the lift-off. Since the amount of magnetic field reduces with the distance to metal and it´s
dissipation becomes faster, the overall amplitudes and times must decrease.
Table 2. Extracted features for two holes measurements.
Lift-Off
(mm)
11
21
31
4mm
Time Peak
ZCT
25.6
46.4
23.6
43.2
20.4
42.8
PA(mV)
39.0
21.0
13.6
6mm
Time Peak ZCT PA(mV)
30.4
52
94.2
28.4
47.6
50.6
20.4
46.4
30.4
Figure 6. Effect of cladding in PEC signal for 20mm lift-off.
Figure 7. PEC signal for three holes of 2mm diameter with 30mm of
insulation and 1mm of aluminum cladding.
5. Conclusions
This paper investigated the effect of lift-off and cladding in PEC signals. Different lift-off
values were used to simulate insulation. The signal time constant increases with lift-off. The
addition of one millimeter cladding slowed the rising and falling rates of the signal. The
drilled holes were detected with lift-off distance up to 30mm plus 1mm cladding.
Acknowledgements
The authors thank IFBA and FAPESB for funding this study.
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