All-aluminium heat exchangers and corrosion resistance
More than 30 years of experience within the automotive industry shows that the average service life of a copper/copper radiator used for heavy trucks is 6-to-10 years, the average service life of an aluminium/aluminium radiator is 8-to- 12 years and the difference in service life is due to the superior corrosion characteristics offered by the all-aluminium concept and well-designed products.
For HVACR heat exchangers, a hybrid copper tube/aluminium fin solution is often chosen. These heat exchangers are prone to excessive fin corrosion and subsequent loss of heat transfer already after one-to-two years in a coastal high humidity climate due to poor design against corrosion.
Today, all-aluminium heat exchangers are virtually the only concept in the automotive industry. With the development of cheaper and easier brazing and soldering methods, the technical benefits of all-aluminium heat exchangers now have become increasingly attractive for industries with smaller production volumes, like the HVACR industry.
Design against corrosion
Design against corrosion covers the process of designing a system of components, in which the corrosive components have been combined and sometimes protected in a manner that optimizes the overall lifetime with respect to corrosion. With design against corrosion, the goal is the elimination of at least one of the three conditions listed in the table at right. The presence of an electrolyte, most often in terms of water or humidity, is virtually impossible to avoid in most designs. Thus, design against corrosion will usually focus on the two other premises by eliminating electrical potential differences or the conductor between such potential differences. This will be carefully considered during the early design phase, regarding:
- Material and/or alloy choice
- Joining methods
With respect to the use of aluminium, there is a wide variety of alloys that meet different needs for strength, corrosive characteristics and ductility. Combined with general design specifications, design against corrosion will have to be an iterative process.
For corrosion to take place, all three of the following conditions must be met:
- Presence of materials or regions with different electrical potentials that form an anode and cathode (for instance copper and aluminium or local potential differences on a material’s surface due to metallurgical or other electro potential variations)
- Presence of an electrolyte (commonly solutions of acids, bases or salts)
- Presence of a conductor between the anode and cathode (for instance a brazing filler material, rivet or direct contact between anode and cathode)