Zinc Diffusion Coatings on Aluminium for Improved Pitting Resistance

Abstract

The automotive industry has replaced copper tube based heat exchangers with all-aluminium solutions during the past years in order to reduce weight, material costs and environmental impact. Conversely, the use of aluminium tubing in the HVAC&R (Heat, Ventilation, Air Conditioning and Refrigeration) heat exchanger market is limited due to the much more demanding lifetime requirements. One way of obtaining the required level of safety against failure of aluminium tubing in such applications is corrosion protection by surface modification. This work investigates the effect of surface alloying with zinc on the pitting resistance of heat exchanger tubes. Pure zinc was applied on flat, thin-walled (≈ 400 µm) multi-port extruded Al-Mn alloy tubes by thermal arc spraying immediately after extrusion, resulting in a sub-micron thick metallic Zn layer on the surface. The tubes were subsequently heat treated to obtain various Zn-rich diffusion layers ranging up to about 70 µm into the alloy substrate. Zn depth profiles were determined by glow discharge optical emission spectroscopy (GD-OES). Corrosion testing was performed by exposure to a cyclic acidified (pH 3) synthetic seawater fog test (ASTM G85 annex A3), normally referred to as the SWAAT test. Open circuit potentials were measured on separate samples as a function of time during immersion in the SWAAT solution at 25 °C. The extent of corrosion as a function of time was determined by weight loss on samples retrieved from the salt spray cabinet at predetermined time intervals for a period of up to eight weeks. The maximum pit depth on each sample was measured by use of a light microscope with a micrometre focusing dial. Ten replicate samples (each 25 mm x 100 mm in size) for each substrate/coating combination were tested, allowing analysis of the maximum pit depth data from extreme value probability plots. The open circuit potential of the Zn diffusion coated samples in artificial seawater solution was about -0.95 V (SCE), shifting only 10 mV in positive direction during 24 h of immersion. The corrosion potential of the Al-Mn alloy substrate remained constant at -0.74 V (SCE) under similar conditions. The cathodic protection provided by the Zn-rich diffusion layers effectively protected the Al-Mn alloy tubes against formation of deep pits during SWAAT testing. No pits deeper than about 50 µm had formed on the Zn diffusion coated samples after eight weeks of exposure, whereas pit depths exceeding 200 µm was observed on uncoated reference samples already after two weeks. However, weight loss measurements showed that the (uniform) corrosion rate of the anodic Zn-rich diffusion layer was nearly two orders of magnitude higher than that of the Al-Mn alloy substrate under the aggressive low pH conditions prevailing during SWAAT exposure. The results suggest that a lower Zn load than that realizable by thermal arc spraying is required to obtain optimal Zn diffusion profiles in terms of service life of the heat exchanger tubing.