5.0 Background Information and Eddy Current Theory

5.0 Background Information and Eddy Current Theory

5.1 Anodize and Alodine Coatings

The purpose of conversion coating aluminum prior to painting is to prime the surface for paint adhesion and protect against corrosion. Table 1 summarizes the properties of anodized and alodined coatings. Anodizing is an electrochemical process which converts surface aluminum to aluminum oxide [7]. The process results in a non-conductive coating. The anodic finishing on the 737 rivets uses coating per MIL-A-8625. All anodized rivets are Class 1, Type I, IB or II. Type I is a chromic acid anodize, Type IB is a low-voltage chromic anodize, and Type II is a sulfuric anodize [8]. Anodized coatings provide better corrosion resistance and harder surface finish; they are less expensive than alodined coatings. Anodized coatings consist of a solution that varies with the type. For example, Type I and IB are chromate solutions, and Type II is sulfuric. A cathode is connected to the negative terminal of a voltage source and placed in the solution. An aluminum component is connected to the positive terminal of the voltage source and placed in the solution. When the circuit is on, the oxygen in the anodizing solution separates from the water molecules and combines with the aluminum on the part forming an aluminum oxide coating [9].

Alodined coatings, often referred to as chromate conversion coatings, are electrically conductive and clear. Most alodined coatings contain 18-20% Chromium, 5% Aluminum, 15-17% Phosphate and up to 2% Fluoride [10]. The alodined finish on the 737 rivets use coating per MIL-C-5541E [3]. The application of alodined coatings is easier than that of anodized coatings. The application of alodined coatings involves immersing the material in strong acid chromate solutions [11]. Upon heating, the coatings lose 40% of their weight [10]. The result is favorable because dehydration creates desirable corrosion resistance.

Table 1. Properties of Conversion Coatings [7, 8, 9, 10, 11]

FINISH / ANODIZED RIVETS / ALODINED RIVETS
Coating Description
Conductivity
Military Specification
Corrosion Resistance
Bonding Properties
Cost
Application Time
Hardness
Coating Thickness / Electrolytic Coating
Non-Conductive
MIL-A-8625F (Class I, Type I, IB, or II)
Excellent
Excellent
Inexpensive
Time Consuming
Harder Surface Finish
0.002 in ± 20% / Chromate Conversion Coating
Electrically Conductive
MIL-C-5541E (Class 1A, Colorless)
Good
Excellent
More Inexpensive
Quick Application
Hard Surface Finish
0.006 – 0.009 in

From the table one can see that the properties of alodined and anodized coatings do not greatly vary. The greatest property difference is the electrical conductivity of alodined coatings.

5.2 Boeing’s Rivet Change

One of the most important factors in the design of an aircraft is proper grounding. Improper grounding may result in unreliable system operation--e.g., EMI, electrostatic discharge damage to sensitive electronics, personnel shock hazard, or damage from lightning strike [5]. Grounding is the process of electrically connecting conductive objects to either a conductive structure or some other conductive return path for the purpose of safely completing either a normal or fault circuit [5]. Changing from anodized rivets to alodined rivets enables the fuselage to act like a channel because alodined rivets are electrically conductive and anodized rivets are not. As a result, physical damage to the aircraft during lightning storms is minimal. For this reason, Boeing mandated that all new lap joint rivets be alodined instead of anodized.

5.3 How Eddy Currents Work

Eddy current inspections are the most common non-destructive testing method for detecting surface and near-surface defects in metals that are electrically conductive [12]. There are two types of eddy current testing: high-frequency eddy current (HFEC) and low-frequency eddy current (LFEC). HFEC detects surface cracks, porosity, and corrosion. LFEC detects corrosion but is best at detecting subsurface cracks. This method passes alternating current through a coil that produces a magnetic field. When the coil is near an electrically conductive surface, the changing magnetic field induces current flow in the surface material. The changing magnetic field induces its measurement current flow in the materials being inspected. After its measurement, the flow detects flaws and characterizes material properties [12]. If the rivet-to-skin interface is non-conductive (if the lap joints contain anodized rivets), the receiving signal shown on the inspection instrument’s display reveals defects. However, lap joints that contain alodined rivets, and therefore contain a conductive rivet-to-skin interface, result in degraded signals on the instrument display. Because alodined rivets are electrically conductive, the current induced in the lap joint is able to flow through the fastener. For this reason, the current density underneath and around the rivet head is reduced and therefore causes a degradation in eddy current signal. If a defect is present, the signal may not reach the required threshold for rejection.

5.4 Eddy Current Detection

As mentioned earlier, alodined rivets conduct electricity therefore allowing current to easily flow into the fastener, reducing the current density underneath the rivet head and therefore resulting in a degradation of the signal. Current is emitted into the inspection surface via a transmitting coil [13]. The quantity of emitted current is defined by flux, Φ, which is described by equation 2.

/ (2)

where = flux density

and

= inspection area over which current is transmitted

Because the magnetizing coil within an eddy current probe is generally held close to the inspection area, a flux,, is generated and is a function of the coiler parameters and the primary excitation current as stated in equation 3.

/ (3)

where = maximum current

ω = angular frequency

and t= time [13]

The angular frequency is calculated in equation 4.

/ (4)

where f = frequency

Equations 2 and 3 are related in equation 5.

/ (5)

Flux flowing through the inducting coil is oscillatory in nature and therefore induces a current in the object undergoing inspections [14]. The induced eddy currents are distributed into the rivet or other inspection media. Because eddy currents are circulatory, they also produce a secondary flux, which opposes the primary flux, . When the eddy current probe is not near a ferromagnetic material, the flux in the coil is the primary flux only. As the probe is moved toward the inspection area, the secondary flux is induced, causing the net flux within the coil to change[14].

According to Faraday’s Law, electrical current is generated by placing a conductor in a changing magnetic field. This process is referred to as induction because current is “induced in the conductor by the magnetic field” [13]. The amount of current induced in a conductor is dependent on the rate at which the magnetic field changes. In an eddy current probe, AC current is passed through an inducting coil into a conductive material. It should be noted that since it is the changing magnetic field that is responsible for inductance, it is only present in AC circuits and that high frequency AC will result in greater inductive reactance since the magnetic field is changing more rapidly. As the current in the coil changes, the properties of the magnetic field also change in both magnitude and direction [14]. Voltage is also applied as the magnetic field changes and is proportional to the change in current with time. Voltage is defined in equation 6.

/ (6)

where = induced voltage

L = impedance

and

= rate of change of current

The induced voltage, , opposes further changes in the applied voltage. If AC current is driven through an inductor, the voltage across the inductor will be maximum when the rate of change of current is highest. For a sinusoid wave, this occurs where the actual current is zero. Therefore the voltage applied to an inductor is at it’s maximum value a quarter-cycle (90°) before the current reached it’s maximum value [13]. In other words, the voltage leads the current by 90 degrees. The voltage and current values are calculated as follows

/ (7)

where = inductive reactance

Inductive reactance is defined in equation 8.

/ (8)

where L = inductance

f = frequency in hertz

Impedance is directly related to resistance and inductance as follows

/ (9)

is the relationship between phase angle and current as described in equation 10.

/ (10)

Material conductivity and eddy current flow have a direct relationship. Materials with larger conductivities have a greater flow of eddy currents on the surface. Eddy current density is greatest on the surface of a material and decreases with depth. Therefore a “standard depth of penetration” is defined and is the depth at which the eddy current is 1/e (37%) of it’s surface value [14]. The standard depth of penetration is

/ (10)

(9)

where = resistivity

f = frequency in hertz

Several trends are able to be noted with measurement of delta. Depth of penetration decreases as frequency increases and also as conductivity increase. “Effective depth of penetration” is defined as three times the standard depth. At this depth, eddy current density is reduced to approximately 3% of its surface value [14]. When a crack is present in the inspection media, the eddy current flow will decrease, causing a decrease in loading on the coil and therefore increasing the effective impedance. By monitoring the voltage changes across the coil, defects in the inspection material are noted [14].

5.5 Cause of Error in Inspections

After the rivet process change, degradation in eddy current signals occurred during routine lap joint inspections. Later, mechanics discovered that alodined rivets caused the degraded signal. Because alodined rivets conduct electricity, current flows easily into the fastener; thus, the current density underneath the rivet head diminishes and results in a degradation of the signal. Figure 3 illustrates the current path around a small crack of both an anodized and an alodined rivet head. From the figure, one can visualize the deviation of the current’s path through the alodined fastener and the degrading effect it has on the received signal. Even though a crack exists, the strength of the alodined rivet’s signal is not strong enough to reach threshold level. Instead, the instrument screen will display a false “accept indication.” Signal degradation causes false readings to occur and impairs the detection of premature cracks.

Figure 3. Current Problem Associated With Alodined Rivets [15]

It is important to note that when eddy current testing occurs, anodized rivets consistently create long rivet signals, and alodined rivets produce inconsistent, short rivet signals that have a phase change in the position versus impedance display [15].

5.6 Current Process for Rivet Finish Detection

Boeing’s current solution for determining the lap joint rivet finish uses low-frequency eddy current. The procedure uses a sliding probe and a measuring instrument with an impedance display. The method is inefficient because it is essential that the probe be aligned with the fasteners’ centerline. This requires the use of a probe guide or a second member of maintenance to monitor the probe’s alignment. The instrument calibration is both time-consuming and complicated. Furthermore, this eddy current method is not able to detect all alodined rivets. Boeing’s 737 NDT Manual states that alodined rivets having poor contact with the countersink may cause large signals and are not identifiable by the procedure [3]. The procedure requires that an additional internal inspection be conducted if the first inspection reveals alodined rivets at four or more tear strap locations or if it reveals twenty or more alodined rivets in one lap joint section [5].