Impedance of anodic coatings

Method for measurement of impedance of anodic coatings on aluminium Recommended practice for rating of electroplated panels subjected to atmospheric exposure... 

The method described by ASTM B 457, Test Method for Measurement of Impedance of Anodic Coatings on Aluminum, is used to study the seal performance of anodized aluminum 141,1421. This method uses a 1-V rms 1-kHz signal source from an impedance bridge to determine the sealed anodized aluminum impedance. The test area is again defined with a portable cell, and a platinum or stainless steel auxiliary electrode is typically used. The sample is immersed in 3.5 % NaCl (see ASTM B 457). 

ASTM Standard B 457, Test Method for Measurement of Impedance of Anodic Coatings on Aluminum. 

Tests for quality of sealing of anodic coatings have become internationally standardised. They include dye spot tests with prior acid treatment of the surface (ISO 2143 1981 and BS 6161 Part 5 1982), measurement of admittance or impedance (ISO 2931 1983 and BS 6161 Part 6 1984), or measurement of weight loss after acid immersion (ISO 3210 1983 and BS 6161 Part 3 1984, and ISO 2932 1981 and BS 6161 Part 4 1981). Of these the chromic-phosphoric acid immersion test (ISO 3210) has become the generally accepted reference test. 

The cathodic reaction is the reduction of iodine to form lithium iodide at the carbon collector sites as lithium ions diffuse to the reaction site. The anode reaction is lithium ion formation and diffusion through the thin lithium iodide electrolyte layer. If the anode is cormgated and coated with PVP prior to adding the cathode fluid, the impedance of the cell is lower and remains at a low level until late in the discharge. The cell eventually fails because of high resistance, even though the drain rate is low. 

In order to study Y2C coating on anodized aluminum, the following electrochemical cell configuration is used to study the overall impedance and interface model of the coated samples or parts as shown in Fig. 27 [28, 78]. 

Where Z is the total impedance, Rb is the barrier layer resistance of anodization, Rp is the porous layer resistance of porous layer of anodized aluminum, CPE is the constant phase element of the porous layer, Rc is the coating resistance, and Rs is the solution resistance. 

Figure 22 Different stages in behavior of a coated system with progressing exposnre time to water. (Left to right) System, impedance response, and eqnivalent circnit Stages (A) Water permeation (B) corrosion initiation (C) qnasi-stationary corrosion. (R = resistance, C = capacitance, Z = diffusion impedance, pf = paint film, cat = cathodic, an = anodic, dl = double layer, u = electrolyte.)...

Given the ability of incorporating nanoparticles into porous anodic films and conducting polymer matrices, with the latter forming within the pores, the incorporation of both TiOi nanoparticles and polyaniline in single step anodising is considered here. The corrosion performance of the coated aluminium alloy has been examined by salt spray testing and by electrochemical impedance spectroscopy. 

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