You will find the figures mentioned in this article in the German issue of ATZ 09/2005 beginning on page 784.

Verdampfer mit Behroxal – Chromfrei in die Zukunft

Evaporators with Behroxal Chrome-Free into the Future

Authors: Snjezana Boger, Jochen Eitel, Oliver Mamber and Ingo Trautwein

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ATZ 09/2005 Volume 107

The new Behroxal process by Behr creates an environment-friendly corrosion protection on evaporator surfaces, replacing Chrome VI according to the new EU Life Vehicles Directive. Moreover, this coating improves drainage of condensate, in that water runs off as a thin film instead of drops, improving dehumidification and enabling smaller evaporators with less odour formation.

Behr

1 Introduction Under the End of Life Vehicles Directive (2000/53/EC), as from 1 July 2007 it will not be permissible for any vehicle component to contain chromium with a valency of six (Cr VI). With our new development, the Behroxal process, an environment-friendly corrosion protection coating is generated on evaporator surfaces as a substitute of equivalent quality for yellow chromium conversion coating. Moreover, the Behroxal coating is hydrophilic, so promoting drainage of condensate from the evaporator. This complements the new geometry of the evaporator core that is designed to improve water drainage behaviour, as exhibited by Behr’s new flat tube evaporators. Both coating and geometry together permit the construction of more compact evaporators that economize on packaging space, and moreover are not susceptible to odour formation. Like the coating it generates, the Behroxal process itself is environmentally friendly, in contrast to most other coating technologies for which frequently highly acidic and aggressive chemicals are needed.

2 Danger of Corrosion through Condensation Condensation water always collects during operation of A/C evaporators. Depending on the type of surface and geometry of the evaporator core, a conventional plate-andfin evaporator may contain, due to capillary attraction, up to 200 g of water. In unfavourable weather conditions and locations, several hours may pass until, after the A/C unit is shut off and the vehicle is stationary, it dries out completely. The presence of this water greatly accelerates corrosion. For this reason, evaporators must not only have a surface that is resistant to corrosion but also their geometry must reduce water-retainability by optimizing water drainage.

3 The Behroxal Coating Process In the Behroxal process, the evaporators are first heated and then sprayed with an aqueous non-toxic solution, Figure 1. This vaporizes immediately upon contacting the hot surface, with the vapour spreading out and penetrating the entire evaporator core even into the tiniest voids. It then condenses in the form of fine droplets uniformly over the surfaces to form a closed homogeneous oxide layer that is passive to corrosion processes.

The micrographs of various evaporator surfaces taken with a scanning electron microscope –untreated, yellow-chromated and Behroxal surfaces – show the differences in corrosion protection, Figure 2. The bare surface with flux residues adhering to it from the manufacturing process is vulnerable to corrosion attack. Within the micro-fissures that typically form with the yellow chromating process, corrosion promoting contaminants, for example de-icing salt, may collect. In contrast, the Behroxal coating is fully closed; the flux residues are incorporated into the surface layer and there are no fissures, and consequently no starting points for fouling and corrosion. The improved corrosion protection action of the Behroxal coating over a yellow chromated surface is also revealed by the standard SWAAT corrosion test (Sea Water Acetic Acid Test), Figure 3.

4 Odour Generation in Evaporators 4.1 Intrinsic Odour In an odour test, a brand-new plate-and-fin evaporator with a Behroxal coat is assessed as being odour neutral. The same evaporator, but with a chrome-plated surface, was attested as having an odour between perceptible and clearly perceptible, Figure 4. This analysis and evaluation of odours was done on an odour test stand developed inhouse by Behr to allow standardized testing based on a recommendation by VDA, the German Association of the Automotive Industry. During testing, the evaporator odour can be tested by three persons at the same time, so largely compensating for individual variations in odour sensitivity, Figure 5.

4.2 Odours Arising from Operation If pollutants that are always present in fresh air are captured on a wet evaporator surface, a growth medium for micro organisms can build up. However, this will only turn into a habitat if water is present, as it is only then that micro organisms can multiply and trigger the biological degradation processes that are the cause of unpleasant odours.

5 Reducing Odour

SUPPLEMENT

Figure 6: Two-tank flat tube evaporator with high performance density, partially cut open

allel to the Behroxal process, a new geometry was developed for Behr’s flat tube evaporators through which, by widening the apex angle between the corrugated fins, the capillary forces are reduced, Figure 7. The Behroxal coating is not only resistant to corrosion but is also hydrophilic. Chrome-plated surfaces, in contrast, tend to be hydrophobic. Water vapour condenses on these in spherical droplets that do not run off easily. On the Behroxal coating, though, first flat droplets are deposited and then, when they join up, a thin film forms that readily runs off from the surface, Figure 8. Compared to a plate-and-fin evaporator, the amount of water retained by a flat tube evaporator with the new geometry of equivalent performance is some 30 to 50% lower, so it will dry correspondingly quicker after the A/C unit is switched off. This makes it more difficult for a habitat for micro organisms to form. Additionally, the water film that continually runs off from the evaporator surface has a self-cleaning action that at least in part washes away pollution and odorous substances. This also applies to substances originating from within the vehicle, like cigarette smoke, which can reach the evaporator, too, when the A/C unit is switched to air recirculation mode. The low intrinsic odour of Behr’s new evaporator and the fact that it hinders generation of external odours are genuine gains for driving comfort. ■

Of decisive importance for odour generation in an evaporator is its ability to retain water, and this depends on its water drainage behaviour during operation. This in turn is a function of the geometry of the evaporator core, Figure 6, and the characteristics of the core surface. Therefore, in parATZ 09/2005 Volume 107

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