Any engineer knows that galvanic corrosion is a real problem. Yet, corrosion does occur at times when dissimilar metals are in contact with each other. This often occurs when aluminium is used. Cooling systems in particular are often affected by it. What is the underlying problem and how can it be solved?
Galvanic Corrosion in Cooling Systems
An engineer’s preference for aluminium is understandable, because it has a number of unique properties, while it is not extraordinarily expensive:
- Excellent thermal conductivity
- Very light in comparison with stainless steel or brass
- Easy to machine
- Easy to anodise
So what is the problem? An electrolyte must be present for galvanic corrosion to occur. Electrolytes are present in large quantities in cooling systems in the form of the coolant. Furthermore, cooling systems generally contain different metals that are in contact with each other (another condition for galvanic corrosion). For example: a cooling system designed using aluminium because of its thermal properties. A plastic hose is used to connect the liquid to the cooling system. This is a practical choice to facilitate routing or to accommodate the transition from moving to static parts. In this case, you require a coupling. While aluminium couplings are available, they contain between 2%-10% copper. This is where the problems start.
More than 0.2% copper is already too much
The culprit in this form of corrosion is that two dissimilar materials touching each other form a galvanic couple, whereby the least noble metal creates a potential difference that accelerates its corrosion. Copper is a far more noble metal than aluminium.
Aluminium alloys containing more than 0.2% copper can cause galvanic corrosion. This is not only the case for aluminium couplings commonly available on the market (2% - 10% copper), but also for aluminium welds. The applied welding material also contains more than 0.2% copper. The copper causes free copper ions to be released into the cooling medium. Furthermore, because it is a closed system, the volume of such ions increases over time. This causes corrosion that sometimes only surfaces after many years.
How do you solve this problem? PVDF or aluminium?
The requirements to be met by the couplings in a cooling system are abundantly clear. There is a need for a light-weight coupling with excellent thermal and mechanical properties. PVDF meets these requirements. At least in some cases. In any case, pay careful attention to the product’s specifications relating to:
- Mechanical properties
- Chemical properties
- Clean packaging
- Performance in a vacuum
PVDF may be an excellent solution, one that we often recommend. However, when mechanical reliability is a requirement, aluminium is superior. Tests have been conducted whereby 100 PVDF couplings were tested simultaneously. In such tests, one coupling stripped its threads during assembly (in accordance with instructions of course). What should be working in theory proved to be different in actual practice.
If you are interested in the maximum achievable, then an aluminium coupling is the preferred option, but then made from that difficult to machine alloy. It was not until 2018, before a manufacturer took on this challenge. Serto has been supplying couplings for 30 years and has developed a great radial assembly system as a result of which the couplings take up less space and can be removed without the need for disassembling the piping. They use a metal compression ferrule, which makes it possible to assemble the coupling multiple times and to use it for pipe as well as hose fittings (through means of a support sleeve inside the hose).
The aluminium alloy Serto uses for this purpose contains exceptionally little copper (< 0.2%) (AlSi1MgMn / DIN 3.2315). Alloying elements, such as magnesium, manganese and silicon give this aluminium alloy its special properties. The aluminium ferrule coupling is anodised making it corrosion-resistant.
Leak-tightness of aluminium couplings
In addition, the SERTO connections are extremely leak-tight, regardless of the temperature and the medium used. This is because they do not use any soft sealing materials that can harden or create deposits.
This is a particular benefit over the longer term and when used in extreme temperature environments. The usual standard for an industrial seal is defined as a leakage rate of 10-6 mbar l/s. All of SERTO’s products are rated at a much improved standard of 10-8 mbar l/s. Furthermore, when all separate components are perfectly harmonised and the pipes are processed in accordance with the applicable directives, leakage rates of 10-9 mbar l/s can be achieved. This exceeds the industrial standard by a factor of 1,000.
Vacuum and aluminium: additional challenges
Anodising aluminium works extremely well. This makes the material less prone to corrosion and makes it smooth. Anodising works well for objects with internal cavities and difficult shapes can also easily be treated this way. However, the oxide layer formed during the anodising process is porous. Even after the pores are sealed, the surface remains pocketed with holes that retain gas molecules due to adhesion. The gradual release of these gas molecules makes application in a vacuum difficult. This makes it impossible to reach a very deep and stable vacuum. The Serto couplings are anodised blue as standard. But if the application is a deep vacuum, Serto can also supply treated couplings without a blue finish. If needed, custom work is also possible. For example, a non-standard version with a straight union and a specific support sleeve is in production for one of our customers.
Typical customer applications
Liquid cooling of optical elements
There was a problem with corrosion in a machine with some 800 couplings. The corrosion primarily affected the manifold assemblies with stainless steel couplings, as well as the welds in the aluminium pipe sections. These will now be cut and replaced with aluminium couplings. It goes without saying that the new coupling was extensively tested for this application.
Uncoupling mechanical motion
The wafer in a chip manufacturing machine was floating on an electromagnetic field and thus constituted a moving entity. This unit had to be cooled as well. Because of the transition corrosion occurring in this case, there was a need for galvanic separation.
Traditional heat exchanger
In a third application, the coupling is used in a traditional cooler situation. The radiator through which liquid flows very precisely regulates the air flow’s temperature.