D.H. van der Weijde

Electrochemical techniques for delamination studies

Electrochemical impedance spectroscopy (EIS) has been described by several authors as a technique to measure delamination of organic coatings. The interpretation of EIS data can be performed in several ways. In this paper two methods are critically compared. These two methods are the breakpoint frequency method (BF) as described and used by Hirayama and Mansfeld and the most probable impedance equivalent circuit method (MPI) as described by van Westing and Geenen. Experiments are presented which were designed for a proper evaluation of these two methods.

A filiform corrosion and potentiodynamic polarisation study of some aluminium alloys

The aim of the present investigation is a combined study of filiform corrosion of aluminium alloys by accelerated exposure tests and potentiodynamic polarisation measurements. The accelerated exposure tests are performed on binary Al-Cu, Al-Mg, Al-Si and Al-Zn model alloys, a ternary Al-MgSi alloy and on the two commercial alloys, AA2024-T351 and AA7075-T651, with variations of composition and surface treatments. The surface treatments cover simple degreasing, chromate and cerium based treatments. A trend of a higher filiform corrosion susceptibility with increasing alloying elements concentrations was observed for all model systems. Furthermore, the filiform corrosion susceptibility varies with the solute atom, in particular Cu was found to have a detrimental effect on the filiform corrosion properties. Both chromating and cerating improve the filiform corrosion resistance of the alloys significantly. To explain the trends observed in the exposure tests, polarisation measurements were performed on the untreated Al-Cu and Al-Zn alloys in bulk anolyte and catholyte solutions which are characteristic for the local anodic and cathodic sites in the filaments on the aluminium substrates. From these measurements a filiform corrosion current, defined as the intercept of the anodic and cathodic curves, can be determined. The present set of experiments shows a correlation between the filiform corrosion properties during accelerated exposure tests and the potentiodynamic polarisation measurements for the Al-Cu alloys. When comparing the results for the Al-Cu and Al-Zn binary alloys it can be concluded that the correlation factor differs significantly with the solute atom and the filiform corrosion current proves to be a non-uniquely discriminating parameter for the filiform corrosion susceptibility of the model alloys. The difference in correlation factor for the Al-Cu and Al-Zn alloys is attributed to differences in the electrochemical behaviour of these alloys with local variations in substrate composition. For the Al-Cu and Al-Zn model alloys the filiform corrosion initiation characteristics are related to the passive range and thus implicitly to the ease of pitting of the alloy. A smaller passive range corresponds to a higher filiform site density for both the Al-Cu and Al-Zn alloys.

EIS measurements on artificial blisters in organic coatings

When blisters are formed underneath organic coatings the anodic and cathodic areas will become separated, which further stimulates the growth of the anodic and cathodic areas. In the case of a blister with a defect in the coating the contribution of these areas theoretically becomes visible in the impedance spectrum. Due to the lack of knowledge on the processes and especially the geometry of the anodic and cathodic regions it is not known whether these contributions can be quantitatively analysed. From experiments it became clear that the corrosion products in the defect are dominating the impedance characteristics, thus masking other contributions. In order to control the geometry we designed several types of artificial blisters. In one type the anodes and cathodes were fixed by using substrates that were partially covered with gold. With two other types we produced the film of corrosion products that normally is present in blisters in an artificial way without a coating. The impedance measurements on these artificial blisters led to an extension of the mechanistic model for cathodic delamination. From the experiments the conclusion can be drawn that characterisation of the anodic and cathodic regions in blisters is not possible with EIS. However, the impedance characteristics of the corrosion product in the defect can be investigated.

Anodising as pretreatment of durable aluminium : The behaviour of several aluminium alloys to filiform corrosion

Aluminium alloys AA 3005, AA 3103 and AA 6063 were anodised in sulphuric acid under different conditions whereby a set of porous layers with varying thicknesses was produced. The influence of porous layer thickness on the filiform corrosion behaviour was examined by means of the Lockheed test. The filiform corrosion behaviour as a function of time was investigated. It appeared that the alloys showed similar filiform corrosion behaviour since for all alloys filiform corrosion was reduced considerably by increasing the porous layer thickness. The number of filaments N and the average filiform corroded surface R increase in time until a saturation level has been reached. However, the saturation level appeared to be different for each combination of alloy and porous layer thickness. The results indicate that the mechanism for filiform corrosion is the same for all three aluminium alloys. Anodische Oxidation als Vorbehandlung fur bestandiges Aluminium: Das Verhalten verschiedener Aluminiumlegierungen gegenuber Filiform-Korrosion Die Aluminiumlegierungen AA 3005, AA 3103 und AA 6063 wurden in Schwefelsaure bei unterschiedlichen Bedingungen anodisch oxidiert. Hierdurch wurde eine Vielzahl an porosen Oxidschichten mit jeweils variierender Schichtdicke auf den Aluminiumlegierungen hergestellt. Der Einflus der Dicke dieser porosen Schichten auf das Filiform-Korrosionsverhaltens wurde mittels des sogenannten „Lockheed-Tests“ untersucht. Zusatzlich wurde dabei die Filiform-Korrosion als Funktion der Zeit verfolgt. Die Verminderung der Filiform-Korrosion bei zunehmender Schichtdicke der porosen Oxidschicht bei allen untersuchten Legierungen legt den Schlus nahe, das die Legierungen ein vergleichbares Filiform-Korrosionsverhalten zeigen. Die Zahl der Filamente N und die mittlere, durch Filiform-Korrosion korrodierte Oberflache R, nehmen als Funktion der Zeit bis zu einem Sattigungsniveau zu. Dies Sattigungsniveau ist fur jede Kombination Legierung/Schichtdicke unterschiedlich. Die Resultate fuhren zu der Annahme, das der Mechanismus, der die Filiform-Korrosion verursacht, fur alle drei Legierungen gleich ist.

Dielectric Properties Reflecting Coating Performance

The use of Electrochemical Impedance Spectroscopy (EIS) in coatings research is widespread. The information obtained by this technique is often limited, which is the result of too high expectations and inadequate analysis and interpretation. Analysis of the dielectric properties of organic coatings using a Constant Phase Element (CPE) in the equivalent circuit reveals detailed information on coating properties as curing stage, water uptake and electrolyte sensitivity, swelling and onset of delamination, but also on the weathering of the coating. The determination of weathering using EIS is developed further and a quantitative parameter the Weathering Index W is introduced. Examples of results on coatings exposed to several types of accelerated weathering are given.