Coolant System Materials Choice for Aluminium Engines:
Quite a few conversions get done using a 'what is readily available / cheap' approach, and this can result in some very unsuitable materials being used for coolant system components. These include copper and stainless steel. Another reason these materials get used is a lack of understanding of galvanic corrosion by whoever chooses the materials. To help resolve this, here is an explanation of how to choose materials for coolant system components on aluminium engines:
The key to which metals are suitable is it's galvanic voltage. Aluminium has a galvanic voltage of 0.7 to 0.9V, depending on grade. In a harsh environment, the galvanic voltage of any metal in contact with (or connected to via an electrolyte - such as coolant) aluminium should be plus or minus 0.15V maximum. Ideally less, as this reduces the corrosion rate.
Some common materials, their galvanic voltages, and their suitability for use as coolant system components with alloy engines are listed below:
- Plastic is an ideal material, as being inert, it will not affect the corrosion of any metals used within the coolant circuit.
- Aluminium obviously won’t corrode when uses with an alloy engine - it has the same galvanic voltage.
- Steel is a near perfect material for use in a coolant system on an aluminium engine - it's galvanic voltage is 0.85V. Almost identical to that of aluminium. This is why it is used on production cars. Yes, steel corrodes, but from the outside only on an alloy engine.
- Galvanised (as in hot dip) steel is not so good - zinc's galvanic voltage is 1.2V. Being hot dipped, the inside of the pipes is likely to be zinc plated, as well as the outside.
- Zinc plated (electroplated, NOT galvanised) steel is a good choice. Because the closed nature of a pipe forms a Faraday cage, the electric field used to apply the zinc plating can't get inside the tube, so only the outside gets plated. Therefore you get the zinc coating where needed - on the outside, but more suitable plain steel inside.
- Stainless steel is a very bad choice - it's galvanic voltage is 0.5V. DO NOT use stainless coolant pipes!
- Copper is an even worse choice (despite being popular due to cheap and readily available plumbing fittings) - it's galvanic voltage is 0.35V. DO NOT use copper either.
Galvanic voltage increases with the reactivity of the metal (gold = 0V, beryllium = 1.85V). The two metal types form a battery. The one with the highest galvanic voltage will corrode at an accelerated rate, whilst reducing the corrosion rate of the other metal. Compare magnesium (1.75V) with steel (0.85V), and you can understand why magnesium VW engine and gearbox cases corrode very badly when in contact with plain steel fasteners, but the fasteners are often not corroded at all, despite being 30 years old. Or why aluminium Land Rover bodies can corrode really badly, but the steel to which they are attached is fine. Or why galvanised steel has a ‘self healing’ property. Scratch right through the zinc, and the steel exposed still won’t corrode.
You can prevent galvanic corrosion by making sure that there is no electrical contact between the two different metals, but to rely on this is not good engineering practice. Better to chose a suitable material in the first place. Also consider temperature senders, etc usually require that the pipe into which they are fitted is earthed (i.e. connected to the engine) to work. This is why production cars stopped using copper radiators when aluminium engines came along.
What about small brass copper fittings? Small quantities of the metal with the lower galvanic voltage are acceptable when the difference in galvanic voltage would otherwise rule out that material combination. There is so much alloy in an engine, that a few small brass (0.45V) fittings will have no noticeable effect on the corrosion rate of the engine, but you wouldn't want lots of it, such as long 1.5" pipes in a rear engined VW.
DO NOT use small quantities of a material which has a considerably higher galvanic voltage - it will corrode away very fast.
If you want to know more about galvanic compatibility, check out the Corrosion Doctors site. This page from the Dutch Secretariat for Aluminium and Environment uses copper and aluminium as an example of a bed material combination choice and explains why. It also contains lots of useful info on minimising galvanic corrosion.