Dr. Bernhard Wessling
Ormecon Chemie GmbH & Co. KG, Ammersbek
(a subsidiary of Zipperling Kessler & Co.)
The Volta potential measured with a Kelvin sensor is suitable for non-contact measurements of surface potentials even under undamaged surface coatings[37],[38]..The function principle and experimental set-up is shown in Fig. 5. The measurement object, the working electrode, and the reference electrode of the Kelvin probe form, due to the small gap between them, a capacitor. Between them a potential is developed, the amplitude of which gives a measure of the chemical nature of the material on the surface of the metal. A periodic variation in separation by means of an actuator built into the sensor changes the capacitance of the set-up. The resulting signal is converted to a measurement signal by means of a lock-in amplifier[39].
The resulting data is a scan of the surface Volta potential of the metal under the coating, which can - under certain conditions - be related with the corrosion potential. But for the purpose we are looking for, this is not a necessary relation[40]. In this part of our method, we only need to know the differences of potentials (a) between various sites, i.e. between the open scratch and the not injured areas of the coating (b) with the change in immersion time.
With this technique, we monitor and predict adhesion (or delamination) of the coating under corrosive attack. The underfilm corrosion propagation velocity (more precise: the corrosion potential propagation velocity, due to the first delamination of the coating before even first corrosion occurs) can be quantitatively measured.
In contrast to the sample design by Stratmann [38], we are measuring on samples like those used in our scratch test (4.2.). This leads to the phenomenon, that in the best of our systems, no potential negative enough for even to start any corrosion develops, so that no delamination or even rust formation or underrusting can occur.
Fig. 6 is showing a development of the Volta potentials in the open scratch and in the neighbourhood under the coating with time (CORRPASSIV™ primer plus top coat). We are interested to follow the potential change especially in the first 24 hours of immersion in salt water (after usual conditioning in humid atmosphere). Typically, in the best system, a „w“ form of the potential distribution forms, with no potential approaching strong enough negative values (Fig. 7), to allow corrosion. Only, when the bottom of the „W“ form reaches values of -200mV or lower, a delamination (the prerequisite for corrosion) can be observed and corrosion may start. The delamination velocity is estimated with widening of the negative corrosion potential „W“ valley, if it occurs at all (Fig. 8).
In contrast, scratches in coatings without Organic Metal primer have a big potential difference, a broad and deep valley, resulting in a high underrusting propagation velocity (Table 1). Very quickly, in the middle of the scratch the potential is strongly increasing now as the sign of rust formation.
We differentiate various CORRPASSIV™ systems under development by comparison with the actually best performing benchmark (4900, a commercial product), where no corrosion potential is reached in the scratch, hence no delamination occurs.
Three systems, each built up on the same primer, but top coated with different paints (Table 1), have all passed the scratch test but were different in their OCP. The question to be evaluated was: Do we see a difference in the delamination velocity? Table 1 contains the answer „Yes“: after 24 hours, significant differences can be noticed. Note, that the performance order is in parallel with all other results. Primer formulations (same basis) without polyaniline under even the best top coats have 5-10 times quicker delamination. The best top coat on CORRPASSIV™ primer alone (on epoxy primer or even on Zn-rich epoxy primer) does not perform comparably well at all. It is the most important conclusion, that underrusting or persistent passivation, resp., is also strongly influenced by the top coat, a conclusion which is not at all self-explanatory, but can be understood in view of [17, 7] and Fig. 1.
We set our internal specification as an underfilm corrosion propagation velocity of between 3 and 5 µm/h or less. Epoxy coating systems (primer plus top coat) are generally showing a velocity of around 20 to 60 µm/h, a factor of 10 or more faster than CORRPASSIVTM systems.
Zn-rich epoxy primers (with epoxy top coat) do even show much quicker underfilm propagation, at least in the first 1-2 days. Such systems are not scratch-tolerant, they perform very well only with intact coatings. In comparison to that, our systems based on Organic Metals are performing extremely well both with and without scratches.
Only the best cataphoretic coatings on Zn-Phospate electrochemically pretreated steel in car manufacturing are showing figures (3-5 µm/h) comparable with CORRPASSIVTM (0 - 5 µm/h). We are convinced to be able to achieve these numbers also with future product developments (as we actually see with our developmental product CORRPASSIV™ 4500, intended to be used in coil coating or with the new water based CORRPASSIV™ system under development).
