5.0 Background Information and Eddy Current Theory5.1 Anodize and Alodine CoatingsTable 1. Properties of Conversion Coatings [7, 8, 9, 10, 11]5.2 Boeing’s Rivet Change5.3 How Eddy Currents Work5.5 Cause of Error in InspectionsFigure 3. Current Problem Associated With Alodined Rivets [15]5.0 Background Information and Eddy Current Theory5.1 Anodize and Alodine CoatingsThe purpose of conversion coating aluminum prior to painting is to prime the surface for paint adhesion and protect against corrosion. Table 1 summarizes theproperties 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 placedin 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 partforming 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 RIVETSCoating DescriptionConductivityMilitary SpecificationCorrosion ResistanceBonding PropertiesCostApplication TimeHardnessCoating ThicknessElectrolytic CoatingNon-ConductiveMIL-A-8625F (Class I,Type I, IB, or II)ExcellentExcellentInexpensiveTime ConsumingHarder Surface Finish0.002 in ± 20%Chromate ConversionCoatingElectrically ConductiveMIL-C-5541E (Class 1A,Colorless)GoodExcellentMore InexpensiveQuick ApplicationHard Surface Finish0.006 – 0.009 inFrom the table one can see that the properties of alodined and anodized coatingsdo not greatly vary. The greatest property difference is the electrical conductivity of alodined coatings.5.2 Boeing’s Rivet ChangeOne 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 WorkEddy 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 eddycurrent (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 areelectrically conductive, the current induced in the lap joint is able to flow throughthe 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 DetectionAs mentioned earlier, alodined rivets conduct electricity therefore allowing currentto 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. danBˆ(2)where B = flux densityanddan ˆ= inspection area over which current is transmittedBecause the magnetizing coil within an eddy current probe is generally held close to the inspection area, a flux,p, is generated and is a function of the coiler parameters and the primary excitation currentpI as stated in equation 3.)sin( tIIop(3)where oI = maximum currentω = angular frequencyand t= time [13]The angular frequency is calculated in equation 4.f2(4)where f = frequencyEquations 2 and 3 are related in equation 5. pppIN(5)Flux flowing through the inducting coil is oscillatory in nature and therefore induces a current in the