Organic coatings potential measurement

In 1979, Leidheiser ( reviewed the use of corrosion potential measurements with regards to the prediction of corrosion at metal-organic coating interfaces. Wolstenholme had last reviewed this literature in 1970 (10). Work in the 1930-1940 s focused on the magnitude of the corrosion potential and how it changed with time (11-14). Negative potentials with respect to uncoated substrates were indicative of corrosion beneath the coating. Positive potentials with respect to uncoated substrates were indicative of the absence of corrosion. 

Wet adhesion phenomena represent a potentially fruitful area of research since so little is known. Some of the important questions are (1) How does one measure quantitatively the magnitude of the adhesion when the coating is wet (2) What is the governing principle that determines whether or not water collects at an organic coating/metal interface (3) What is the thickness of the water layer at the interface and what determines the thickness A recent paper (1.) correlates the wet adhesion properties of a phosphated surface with the crystalline nature of the zinc phosphate at the metal surface. 

Instrument limitations must be considered in the measurement of organic coatings because their resistivities are large, 1012 Q cm or more. The input resistance of common potentiostats is usually not more than 10n to 1012 ohms, and if the cell resistance approaches or exceeds the input resistance of the potenti-ostat, a significant fraction of the applied signal will pass across the input impedance and not the cell impedance. In these cases the collected data do not reflect just the impedance of the electrochemical interface, which is a fundamental assumption in almost all data analyses. In fact, the potentiostat input impedance in parallel with the stray capacitance associated with the potentiostat measuring leads will be obtained. 

Determination of R does not require that the current and potential signal be correlated. However, if measurements are made at different times, they should be made with similar working electrodes and similar exposure conditions. R values have been used to assess the degradation of organically coated metals. In these studies, R data appeared to correlate with the film resistance (impedance magnitude at low frequency) values determined from EIS (145), or with visual inspection of painted samples subject to various cabinet exposures (146). 

Fig. 31.5 HR-SKP measurement of a single filiform thread on an organically coated aluminum substrate (aluminum alloy AA5015 spin-coated with a commercial clear coat about 5 pm thick) under active FFC conditions (a) l-IR-SKP topography map (b) F-IR-SKP potential map.

Electrochemical impedance spectroscopy (EIS) is a convenient and effective method of assessing the properties and performance of organic-coated metal systems. The AC impedance of an electrochemical cell can be determined by applying a sine wave of potential (V) of a certain frequency (co) and measuring the corresponding current (I) flowing across the cell. The ratio of potential and current is the impedance of the cell (Z) at the chosen frequency, according to Ohm s law ... 

Fig. 22 Typical potential maps above a delaminating organic coating on iron as measured with SKP as a function of time in humid air. The defect (filled with 0.5 M KCl) is located on the left. The mapping covers the area from the defect border to the intact area.

By using the SKP (see Sect. 5.4.2.5), it was possible to measure the local potentials underneath an organic coating in situ without the deterioration of the corroding system [178]. The mechanism of FFC consisting of an anodic reaction at the corrosion front is reflected in rather different electrode potentials around the filament s head. Whereas, for cathodic undermining, the delamination front is positively polarized with respect to the already delaminated zone and the head of the filiform filament shows a negative potential with respect to the tail (Fig. 35) [178]. Therefore, the tip can be identified as the local anode and the local cathode is situated behind the anode within the tail. 

FT-EIS allows for measurements of nonstationary systems evolving slowly with time or during a potential sweep. In addition, it allows for detecting and quantifying the presence of time variance and nonlinear distortions in experimental data [123-127, 135]. In these experiments, a series of odd harmonics was applied from which every third or fourth frequency was removed. They were 1, 3, 9, 11, 15, 17, 21... or 3, 5, 7,11, 15, 17, 19, 21, 25... This method was applied to study organic coatings on A1 [125], The signal contained frequencies between 0.1 Hz and... 

The first situation is quite similar to the delamination of organic coatings from steel, as has been described before (Fiirbeth and Stratmann, 1999a). A typical potential profile as measured during delamination is shown in Fig. 7-63, and the interpretation of the potential profile is identical to the one discussed for iron earlier. However, as zinc is amphoteric subsequent reactions will start in the alkaline electrolyte at the interface which differ significantly from the one observed on iron, which is rather stable under alkaline conditions. The differences are obvious if the potential profiles along the interface are measured after a sudden drop of the... 

Other than for electrodes immersed in bulk electrolyte, on electrodes covered by ultrathin layers the electrode potential may differ significantly across the electrode surface. Hence, localised measurements are of interest, being performed by scanning the tip across the sample. This was first applied for organic coated metals where the coating was electrochemically delaminating, driven by corrosion [12-14, 29], Even on the submicron scale the Kelvin probe technique can be applied for such studies, and then based on a modified atomic force microscope, see [34, 35]. Recent developments are the combination of Kelvin probe and SECM [36] and the use of Kelvin probe for hydrogen detection [37]. 

Standard guide for testing filiform corrosion resistance of organic coatings on metal Standard practice for measurement of corrosion potentials of aluminium alloys... 

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