Corrosion Resistance of Zinc-Rich Metallic Coatings on Aluminum

Abstract

Thin walled aluminum heat exchanger tubes, with solid-solution Zn-rich surface are widely used by the automotive industry and considered by the HVAC&R (Heat, Ventilation, Air Conditioning and Refrigeration) market for improved corrosion resistance against pitting. It is well known that alloying Al with Zn causes decrease in the corrosion potential in chloride solution.1 Thus, Zn-rich coating can provide cathodic protection to the aluminum substrate, in particular prevent pitting corrosion. However, corrosion rate of Zn itself is quite high in chloride solution.2 This work investigates the extent to which the self-corrosion of the Zn-rich layer can be a limiting factor in determining the service life time of the heat exchanger tubes. Zinc was applied on AlMn alloy extruded tubes by thermal arc spraying immediately after extrusion. Typical Zn load on the surface was 0.8 ± 0.2 mg/cm2. The tubes were subsequently subjected to heat treatment at various temperatures (350 - 430°C) and durations to obtain solid-state AlZn alloy diffusion layer with varying thickness and Zn-concentration profiles at the surface of the AlMn substrate. The Zn depth profiles were characterized by glow discharge optical emission spectroscopy (GD-OES). Corrosion rate was determined by weight loss resulting from immersion for predetermined times in acidified artificial sea water solution of pH 3 at 25°C for slight acceleration of the corrosion rate, during which the open circuit potential change was recorded with respect to time. The morphology and composition of the Zn-rich layer before and after corrosion were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).The Zn concentration distribution in the layer could be modeled by the solution to Fick's 2nd law with appropriate boundary conditions, giving an effective diffusion coefficient for Zn in the Al substrate of order 10-10 cm2/s in the temperature range of interest, varying according to the Arrhenius law with respect to temperature. Zn-rich layer thickness obtained on the samples heat treated at selected conditions varied approximately in the range 35 to 80 µm with increasing heat treatment temperature and time, corresponding to Zn surface concentrations of 28 to 7 wt%, respectively. Reduction in thickness due to corrosion, calculated from the weight loss measurements by taking into consideration the density depth-profile of Zn, which was obtained from the GD-OES data, increased with increasing thickness of the Zn-rich layer. Unexpectedly, decreasing Zn concentration level in the layer did not have a significant effect on the corrosion rate, unless it was lower than a threshold of a few wt%, which appeared to be sufficient to give the necessary protection. The results confirmed also that Zn gave satisfactory protection against pitting corrosion. Corrosion potential of samples as coated with Zn (no heat treatment) reached the pitting potential of the AlMn substrate in about 4 days. After 11 days of immersion, corrosion potential of the samples heat treated at 350°C for 2 hours increased up to about 30 mV below the pitting potential of the substrate, while the corrosion potential of the samples heat treated at 430°C for 4 hours remained about 120 mV lower than the pitting potential of the substrate. The reduction in thickness after 11 days of immersion for those heat treatment conditions was 23 µm and 54 µm, respectively. These results indicate that optimal Zn concentration and Zn-rich layer thickness, in terms of service life and loss of thickness, lie at levels much lower than those used in this study. The optimal concentration needs to maintain the corrosion potential at a level lower than the critical pitting potential. At the same time, the Zn concentration level in the layer has to be low enough to restrict the rate of self-corrosion, such that the layer thickness can also be reduced to give sufficiently long service life time. The well-known relationship between the corrosion potential of AlZn alloys and Zn concentration1 was extended over a wide concentration range and related further to the corrosion rate in acidified synthetic seawater. The database thus developed is useful for optimizing the Zn-rich layer thickness and Zn concentration profile in the layer to minimize uniform corrosion, and therefore maximize service lifetime, without reducing the protection against pitting.