Aluminium Radiators – Galvanic and Electrolytic Corrosion

I’d bought a super cheap aluminium kit car radiator from ebay. Was it big enough? What about Aluminium radiator corrosion?

I had done my sums to make sure it was big enough for my V8.

Start with 2 cubic inches of core for every cubic inch of engine. Increase or decrease that value by the following factors:


  • 0.1 for a vertical flow radiator core
  • 0.1 for an in-line engine
  • 0.1 for a small trailer towing
  • 0.1 for a 2 row radiator
  • 0.1 for double evaporators
  • 0.2 for outside temperatures of 105°F (40.5°C)
  • 0.2 for a medium trailer towing
  • 0.2 for a small engine fitted to a heavy car
  • 0.2 for a radiator fan with diameter less than 90% of smallest dimension
  • 0.3 for air conditioning
  • 0.3 for no fan shroud
  • 0.3 for an antique car with small engine compartment
  • 0.4 for large trailer towing
  • 0.6 for a diesel engine


  • 0.1 for remote transmission cooler (not within radiator)
  • 0.1 for standard in-line transmission
  • 0.1 for a single row radiator
  • 0.1 for a V6 / V8 engine
  • 0.2 for a spacious pickup truck engine compartment
  • 0.2 for outside temperatures less than 90°F (32.2°C)
  • 0.2 for a full fan shroud
  • 02 for a horizontal flow radiator core
  • 0.3 for a large engine in a small car

The equation I have is for brass / copper radiators only. I reckon you need to add 0.2 for Aluminium.

My radiator core dimensions:

35cm (13.8″) x 34cm(13.4″) x 5.5cm(2.16″) = 399.4ci (6545cc)

  • Electric Fan (14″) radiator (14″)
  • No trailer
  • Add 0.1 for vertical flow core
  • Temperature under 105°F (40.5°C)
  • add 0.1 3 Row Radiator
  • subtract 0.1 For V8 engine
  • subtract 0.3 For large engine in light car
  • subtract 0.1 For standard in-line transmission
  • add 0.2 for aluminium radiator

No fan shroud:

  • add 0.3 No Fan shroud

399ci ÷ 2.2 = 181ci (2974cc) Engine

With full fan shroud:

  • subtract 0.2 for full fan shroud

399ci ÷ 1.7 = 235ci (3850cc) Engine

This bargain radiator makes a VW Polo radiator look somewhat lame. However, although compact, it was a little too small for the engine and car.

Hopefully, by adding a Fan shroud, proper electric fan and oil cooler, I’ll be OK if I can get decent air flow through the engine bay.

A 4553cc (277ci) Range Rover V8 has a 494 x 568 x 40mm radiator, equal to 685ci. Air-flow to them isn’t perfect and they are designed to operate in extreme temperatures (add 0.2). They have air conditioning (add 0.3), are designed to tow heavy trailers (add 0.4) and have a mechanical fan (add 0.2).  They are horizontal flow rather than vertical (subtract 0.1) (685 ÷ 2.4 = 298ci Engine).

What are the advantages of an aluminium radiator over brass and copper?

Maybe this question should be reversed…..

Aluminium radiators are light, strong and relatively cheap. While aluminium isn’t as thermally efficient as copper or brass it performs within a few percent, so providing space isn’t super critical and the radiator is well made, then they are a good option.

However, if you neglect your radiator or you plan on racing down gravel tracks, then copper may be a better bet.

As far as maintenance goes, the anti-freeze solutions for all radiator types are the same and providing you follow a decent routine there shouldn’t be a problem. The problem comes with heavy damage. Not many aluminium radiators can be repaired. A brass/copper radiator can be re-cored and re-soldered, whereas an aluminium radiator; once damaged; is usually destined for the scrap heap.

Aluminium radiator corrosion

Damage isn’t the only problem:

  • Galvanic Corrosion
  • Electrolytic Corrosion

The trouble was, I knew the basics, but had forgotten the detail. So I went onto Google and searched for ‘car radiator corrosion’ and many other similar phrases.
The forums had plenty of people claiming all sorts of conflicting arguments, so I decided to take a different search route. Out with the chemistry books.

Galvanic Corrosion in Car Radiators

If you have 2 dissimilar metals and immerse them in an electrolytic solution you basically create a battery. The more noble metal will corrode the less noble metal by stripping electrons from it; this creates electrical charge.

Case 1

  • Iron engine block
  • Copper / Brass Radiator

Copper and Brass are more noble than iron, therefore the iron engine block will be slowly corroded away. This isn’t normally a problem as an engine block is pretty thick. Given long enough, any zinc in the radiators’ solder will corrode. This may cause the radiator to expand or leak at the seams.

Case 2

  • aluminium engine block
  • Copper / Brass Radiator

Copper and Brass are more noble than aluminium -see case 1

Case 3

  • Iron engine block
  • Aluminium Radiator

Iron is more noble than aluminium, therefore the aluminium radiator will be corroded away. This can be a problem for any radiator, as just a tiny amount of corrosion can create a hole.

Case 4

  • Aluminium block
  • Aluminium Radiator

Corrosion is minimal as although, their are differences between aluminium grades, there isn’t a particularly dominant metal. However, water within the core may contain electrolytes, which can cause electrolytic corrosion.

Electrolytic Corrosion in Car Radiators

Electrolytic corrosion is the evil cousin of Galvanic corrosion and can cause damage much more quickly. Electrical current actively accelerates the corrosion. Having used electrolysis and a battery charger to clean car parts, I have seen a stainless steel spoon completely dissolve in 2 hours.

To protect a radiator we must prevent the formation of electrolytes. This is where certain measures kick in:

  • A good engine ground
  • Anti-freeze
  • Disconnect the battery and drain the radiator when vehicle is in storage

When a component has a bad ground, electrolysis is accelerated – considerably. Therefore, engine ground straps should be given particular attention. If you have a bad ground, the current would try to find another path to ground using electrolytes in the water as a path for electrical current.

Many people think, because it isn’t winter, there is no need to run anti-freeze. The answers is simple, anti-freeze does more than one job. In fact, preventing freezing is really secondary to a lot of users.

Even if you were to run distilled water; which some professional teams do; it wouldn’t take too long for the water to pick up impurities and minerals from everything it touched. Pretty soon it would become an effective electrolyte. When dissimilar metals are present, you really need to do everything possible to keep the ‘ideal’ current path as short and direct as possible. Any stray current paths are going to be sources of electrolytic corrosion.

Anti-freeze doesn’t just change the boiling and freezing points of water it goes a long way to prevent the formation of these electrolytes with the addition of silicates and phosphates. There are different claims from various manufacturers, but after 1 year the effectiveness is significantly reduced and after 2 years, you really need to change it. If you buy a car, don’t just look at the water colour, change it asap and do it regularly. Lets face it, swapping the water isn’t hard and even Poundland sell anti-freeze. Personally, I’d get the best I could find, particularly if I could find some that claimed 5 years+. With an aluminium radiator, I’d still change it every year, just for peace of mind.

Electrolysis doesn’t just happen internally, rain water and spray can act externally on a radiator. If current is finding it’s way to ground through the radiator, corrosion will corrode the radiator from outside in.

The arguments on the forums were all about radiator grounding.

Checking for Electrolytic Corrosion

Using a digital multimeter, connect the negative lead directly to the negative battery terminal. Set the meter to read voltage. Take the positive lead, open the radiator filler cap and suspend it in the coolant. Make sure the positive lead doesn’t touch the filler neck or radiator core. If the meter reads over 0.3 volts, then you have high levels of electrolysis. The closer this reading is to 0 volts the better.

If you find electrolysis, turn every electrical component of or remove fuses. Check the voltage as each item is turned off and on. Don’t forget there can be more than one problem. When there is an increase in voltage, that components’ ground is suspect.

  • Fans can be an issue, so you must test the voltage with and without the fan running.
  • Check the voltage whilst the engine is turning over on the starter motor.
  • Check the voltage as revs are increased.

Old fashioned engines, with thick cast iron blocks, could tolerate high levels of electrolysis for longer but modern engines have been destroyed in a few hundred miles.

Should I Ground My Car Radiator?

If you had a bad engine ground, then the current would try to find a way to earth (chassis ground) via another path. These stray paths are where corrosion happens. If you were to ground the radiator, then current could flow through the electrolytes in the water, through the radiator ground to earth. In this situation, an aluminium radiator would corrode very quickly. For aluminium radiators, it is much more important to have a substantial and well maintained engine ground and insulated rubber mounts for the radiator.

If you have a copper-brass radiator and a decent engine ground, then rubber mounting the radiator is not so important but still a very good idea.

If you have a separate body and chassis, make sure that there is a decent ground between these 2 components.

If you have a metal fan shroud or housing, ideally it needs a decent ground to the chassis, but electrically insulated from the radiator. Personally, I’d use a plastic fan housing and insulate or make sure the motor doesn’t electrically touch the radiator at all. If it is necessary to have a metal shroud or housing mounted directly to the radiator, make sure there is a decent earth between the two. The fans’ motor casing, should be electrically isolated from the shroud, with a strong earth from it’s negative wire to chassis ground.

Very similar applies to the heater on the car, as this has a radiator in it too. Make sure you heater is grounded directly to the battery with a substantial earth or very well grounded to the chassis. In other words, don’t paint all the bolts!

Stone and Weather Damage

Damage from stones can be a real problem, especially for aluminium radiators. Any grill or mess that can protect the radiator is going to be helpful. On modern cars, the air travels a rather bendy route before hitting the radiator, whereas on an old Model T Ford, the radiator is the first thing a stone could hit. Therefore, for an old fashioned car, maybe a copper/brass radiator is a better idea.

Salt water and road spray can damage a radiator. Hence modern cars having water traps in the air passage to the radiator.

A fan shroud not only helps radiator performance, it can help protect the radiator, however a front mounted fan, in some circumstances, can increase the speed that stones and water hit the radiator.

Fan Shrouds

When there is a fan shroud, more air is directed through the radiator and through the blades of the fan. On modern cars there is often a large, often low, grill opening. The air is caught by the grill, directed around a few bends on a  route that gets smaller the closer it gets to the fan shroud. This compressing the air, cools it slightly and amplifies air speed. The bendy route prevents a stone from directly hitting the radiator. This route effectively has dirt and water traps. Fitting a fan shroud can reduce the operating temperature of an engine by 50% at certain speeds. If your car doesn’t have one and you are considering a bigger radiator, try a fan shroud first.

Rubber Mounting Your Radiator

I used the following mounts – M6 x 20mm diameter.

Rubber Radiator Mounts - Aluminium radiator corrosion

I made sure my mounting brackets were short and on the hefty side (3mm plate steel). After all, vibration is a big killer of aluminium radiators. I’ve seen dozens that use floppy sheet steel brackets on aluminium radiators. This isn’t ideal. If the brackets mount directly to the radiator, keep the metals the same and the metal thick, if they can be soldered or welded together – do it. But, remember to rubber mount the fan.
Copper radiator = copper bracketry,
Aluminium radiator = ally bracketry

I have seen people cable tie their fans through the rad. The ties will eventually melt or wear through, but even before that happens, the mass will amplify vibration and destroy the rad quickly.

The problem I am going to have, is the radiator core is quite exposed. I will use a very open mess to stop the large stones (hidden behind the main grill).

When you fit a  radiator make sure there is tons of clearance, cramming in the biggest radiator possible, is just asking for something to rub. If the radiator doesn’t have air flow, a smaller radiator might out perform it anyway.

Can I fit too big a radiator?

The engine’s thermostat is responsible for keeping the car at normal working temperature (roughly 180-200°F), so providing the engine has been running for a while, the temperature is controlled by the thermostat. If the engine gets too cold the thermostat opens and as it warms back up it will close again.

In the winter, the cabin heater will draw around 20% of the engine’s heat in order to heat you and demist the screen. All the time an engine is running outside of it’s normal operating range, it will have increased wear and will run less efficiently.

As a vehicles speed increases the heat from the engine will increase. Modern cars run lower gearing than older cars and produce less heat at speed. In days gone past, people used to partially cover their radiators in cold weather to get their engines to heat up more quickly. I used to drive a Ford 100E 3 speed manual. With a cold engine, top speed was around 30mph, but as it warmed, nearly 50mph was possible. I’d fitted a heater from an old Land Rover, but even on a 10 mile journey it would not kick out any heat in cold weather; partially covering the radiator helped in run much more smoothly and I could see out!

When a huge radiator is fitted to a small engine, as soon as the thermostat opens, the cold water from the radiator is released into the system producing a large drop in temperature. With the radiators’ increased surface area and the greater amount of water inside, the temperature of water in the rad can drop quickly. In this situation a thermostat needs to open and close at a much higher frequency to maintain correct temperature.

Maintaining a constant temperature

In order to keep the car at temperature the thermostat instantly opens and closes. As a thermostat ages, wears or become clogged, the rate at which it opens and the frequency it happens becomes much slower. As the thermostats’ performance decreases with age, it might not open or close as fast required, making it impossible to keep the car at normal working temperature.

If a radiator was too small, the engine temperature would increase too quickly. In cold temperatures this could cause the block to crack or the radiator to split. At speed, the engine could overheat and blow a gasket.

Providing your radiator is not too small, the thermostat is in good condition, the coolant has been freshly changed and you are happy to tolerate cold air from the cars heater, then going too big on a radiator isn’t too much of an issue.

However, section 34 (Defrost and Demist) of the IVA exam states:

  1. The vehicle must be fitted with a system capable of defrosting /demisting the windscreen.
  2. A system using warm air to clear the screen must employ fan assistance and ducting to direct the air onto the screen, to ensure effective operation under cold weather conditions.
  3. An electrically heated screen must provide adequate heat and distribution to ensure effective operation.

The IVA exam has strict requirements for emissions. If your car struggles to maintain a normal working temperature, then emissions will be up. It emissions are an issue use an infra-red thermometer to monitor engine temperature and check for cold / hot spots on the radiator, sticky thermostats and the average working temperature of the engine.

I have read about several cases where the cars’ heater wasn’t producing sufficient warm air to defrost the screen after 10 minutes and the subsequently causing the vehicle to remain un-roadworthy. Therefore, although a slowly heating engine isn’t a massive issue for modern engines, it is for demisting and the IVA inspector.

Choosing and Fitting a fan

Always choose a fan that is roughly the width of the radiators’ smallest dimension. My radiator is 14″ wide, therefore I bought a 14″ fan. Never fit a fan smaller than 90% of the smallest dimension.

A lot of electric fans have blades that do not twist or flex with speed, they are held fairly rigid by an outer strengthening ring. Even so, there should be at least 3cm’s clearance between fan blades and the radiator. My electric fan is a typical 14″ cheap Chinese import. Although, it looks good quality, I’m expecting the bearings to loosen over time. As the bearings loosen, because of the diameter, it wouldn’t take too much ‘wobble’ for it to come in contact with the radiator. Always read manufacturers recommendations on clearance (unfortunately mine were Chinese). I’m fitting a fan shroud, which means my fan is almost 20cm in front of the radiator – plenty!

When using a traditional fan mounted to the water pump, the blades twist as the engine revs build up. The fan maybe almost flat whilst stationary but 10 times as thick at speed. The engine can move forward and backwards on it’s mounts and can foul fan housing causing them to collapse towards the radiator.

Basically, fit the biggest fan you can, fit a fan shroud and leave as much room as possible!

Checking a Thermostat

If a radiator is too small, it can soon damage a good thermostat.

There are several ways to check a thermostat.

Start the car from cold and feel the top hose. The hose should not feel too hot to hold. When the thermostat opens, the top hose, will soon heat up. If the hose, remains cold, the thermostat is not opening.

If the expected opening temperature is below around 91ºC (195ºF), remove the thermostat and drop it into a pan of water and bring it to the boil. The exact opening temperature can be measured with a thermometer. Anti-freeze and pressurization, within a cars coolant system, will increase the boiling point of water, therefore for thermostats over 195ºF simply boiling plain water will never reach the intended opening temperature. Adding anti-freeze may increase the boiling point by a few degrees (roughly 107°C).

Thermostat Temperature

If you need to replace your thermostat, check the owners manual. As a rough guide, on modern engines, it’s going to be roughly 192 – 195°F. Fitting a cooler thermostat to a modern engine (e.g. 160°F) can increase fuel consumption, emissions and wear. Modern engines have tighter tolerances and clearances. Operating outside the ideal range accelerates wear much more on these engines compared to vintage ones. Relatively constant operating temperature is absolutely essential for proper emissions control, good fuel economy, performance and wear.

There are exceptions to this, for instance to achieve modern emission standards, Range Rover increased the thermostat temperature of the 1996, 4.6ltr version of it’s ageing V8 engine from 82°C to 96°C (180°F). This was at the expense of reliability; liners slipped and blocks cracked. Coolant temperatures would ultimately settle at ~100°C(212°F). After-market fitment of a lower thermostat, to keep peak water temperature below 80°C (176°F), can dramatically increase reliability, however passing an MOT emission test won’t be so easy.  For these engines, a bigger radiator, fan shroud and powerful fan should also be considered.

How much coolant should be added?

With a 50/50 mixture of water to antifreeze, with the cap removed, will boil at 225°F. Once the cap is in place (my cap is 15psi) this should increase to 265°F. If you run 70/30 (the maximum recommended) the boiling point rises to 276°F

If the coolant starts to boil, old hoses, weak radiators, water pumps and engines will quickly fail. Water will be vented, meaning future cooling will be affected.


  • Fresh Anti-freeze and clean water is important
  • Replace your thermostat regularly
  • Anti-freeze needs changing regularly – especially with aluminium radiators
  • Engine, Chassis and Bodywork grounds protect your radiator
  • Rubber mount and electrically insulate your radiator
  • Aluminium Radiators are more susceptible to damage
  • When batteries are are several feet away from the engine; e.g. in the boot; then the size of cables needs to be upgraded and particular attention paid to chassis and bodywork grounds.
  • Don’t mount your fan directly to the radiator (especially with cable ties)
  • Rubber mounts protect damage from vibration (a big problem for aluminium radiators)
  • Plastic fan housings are good for absorbing vibration and electrical insulation
  • Disconnect the battery and drain the radiator when vehicle is in storage
  • Where possible, fit a fan shroud
  • All of the above applies to the cars heater / demister


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