Have you ever encountered a situation where the aluminium radiator in a customer’s vehicle failed repeatedly in fairly short order as a result of corrosion even though there was never any sign of rust, cloudiness, or discolouration of the coolant? You may have thought that aluminium radiator cores are simply no longer as durable as they used to be, but it is more likely that the repeated failures were the result of an electrochemical attack on the aluminium known as electrolytic corrosion. This article will discuss what electrolytic corrosion is, what its causes are, how to fix it, as well as how to prevent it from recurring, starting with this question-
Also known as, “stray current corrosion” electrolytic corrosion occurs when an electrical current passes through the coolant on its way back to battery negative, or ground. This can happen on any vehicle, and any electrical consumer that depends on an engine or chassis ground to work can be implicated. Typically, stray current corrosion occurs when the ground attachment point for an electrical consumer is absent or in a less than perfect condition, or if the ground cabling for accessories such as auxiliary driving lights, high-powered audio systems, or winches is not capable of coping with the current flow. As a practical matter, electricity will always follow the path of least resistance, and if an electrical consumer is not properly grounded through its wiring, the current will use the coolant in the cooling system as a convenient pathway to return to ground, since it offers less resistance than an imperfect ground connection.
NOTE: Electrolytic corrosion must not be confused with galvanic corrosion, which occurs when two dissimilar metals, such as the aluminium and cast iron in car engines, come into direct contact with each other. Galvanic corrosion depends on the difference in electrical potential of each metal, and while stray electrical currents can accelerate the process of galvanic corrosion, this form of corrosion does not require the presence of an electrical current to occur.
Look for pinhole-sized leaks anywhere on the radiator core
Electrolytic corrosion is a greatly accelerated version of “normal” corrosion and on radiator cores, it usually manifests as pinhole leaks that may or may not have black-ish edges or surrounds. Depending on the strength of the stray current and the condition of the antifreeze additive, there may be multiple leak sites, which can form within a few months, or sometimes just a few weeks even in brand new radiators.
However, since detecting pinhole-sized leaks can be difficult, especially if there is no discolouration of the coolant, it may be necessary to pressurise the cooling system to above its design pressure. Holding a piece of paper close to the core will usually pinpoint the sites of leaks as the escaping coolant wets the paper.
NOTE: Since cabin heater cores are equally vulnerable to electrochemical attack, it is imperative that the heater core also be inspected for signs of pinhole leaks- preferably with the heater core removed from the vehicle to make it easier to spot leak sites.
Look for coolant leaks around plastic intake manifolds
Some synthetic rubber compounds used in the manufacture of inlet manifold gaskets and/or seals are particularly vulnerable to the type of electrochemical attack that causes electrolytic corrosion in metals. As a result, pressurised coolant pushes past damaged rubber seals and gaskets, but note that simply replacing a damaged inlet manifold won’t necessarily resolve the problem.
If the inlet manifold seal is damaged or degraded, it is almost certain that the metal face it seals against will be pitted, cratered, or otherwise damaged as well, which in severe cases, may require removal of the cylinder head for the purposes of welding up and resurfacing the damaged surface.
Look for damaged radiator hoses
Since synthetic rubber radiator and heater hoses are particularly vulnerable to the effects of electrolyte corrosion, any inspection of the cooling system must include an inspection of the inside of the radiator hoses as well. Typical evidence of degradation caused by electrolytic corrosion includes pitting, cracking, and/or splitting of the inner walls. Damaged hoses must be replaced a matter of course, which brings us to-
It should be noted that while there is no single voltage value (as it pertains to electrolytic corrosion) that is accepted as being harmful by all vehicle manufacturers, all manufacturers do agree on three things, these being that-
Stray currents are largely unavoidable
Vehicle manufacturers agree that due to the nature of modern radiators, how modern radiators are mounted in vehicles, and the complexity of modern automotive electrical systems, it is largely inevitable that some stray currents will always be present in the cooling systems of most vehicles.
Stray currents of less than 300 mV in strength are harmless
Several studies performed by specialised, independent laboratories suggest that stray currents of less than 300 mV in strength do not cause damage in cooling systems if the engine block is made of cast iron.
Stray currents of more than 0.3 volts are harmful to all cooling systems
Although all vehicle manufactures seem to agree on the above point, not all manufacturers agree on the rate at which a 0.3-volt stray current will destroy a modern aluminium radiator, due to the large number of variables that have different effects on different applications.
Having said the above, below is a generic test procedure that should serve to detect the presence of stray currents in almost any application. Proceed as follows-
TIP: Results that are more accurate can be obtained if you use multimeter leads that have crocodile clips on their ends. Use the clip to clamp onto a short length of copper tubing, and use the copper tubing as a probe since copper is a better conductor than the steel of a normal multimeter probe.
If the procedure outlined above does not yield positive results, proceed as follows-
Test the starter motor
Still keeping the probe in contact with the coolant, have the assistant crank the engine. Short circuits in the starter and/or poor engine grounds will cause the current to spike, thus revealing the starter motor and/or its associated wiring as the source of the stray current.
Test the charging system
If the starter motor checks out, start the engine and increase its speed to allow the alternator to deliver a full charge, but be sure to change the multimeter over to AC voltage for this test. If the alternator over charges, or if a one or more diodes in the rectifier is even marginally defective, the fault will produce an AC current in the coolant.
Test the winch
If the vehicle is fitted with a winch and none of the previous tests yielded positive results, inspect the winch’s power and ground cables. Pay particular attention to the diameter of the ground cable, as well as its attachment point. If the winch is grounded at any point other than directly to the battery, replace the ground cable and attach it directly to the battery negative pole with a suitable terminal.
This test is best performed with a heavy load on the winch, so if needs be, attach the winch cable to a suitable object, and activate the winch to pull the vehicle forward while testing the coolant for stray current with the multimeter in DC mode. If the stray current voltage remains the same, the problem is obviously not with the winch’s grounding. If however, the stray voltage is no longer present or dips below 300 mV, the winch was the cause of the issue.
Check for static electricity
Static electricity can be described as an accumulation of electrical charges on the surface of a material, most commonly on insulators or other non-conductive materials, and has been shown to contribute to stray electrical currents in automotive cooling systems.
While static electricity is a form of electricity that does not flow in any direction, it can accumulate to the point where it jumps from the non-conductive material to a conductive material. If this happens, the current may flow through the rear axle of a RWD vehicle, up the drive shaft, and through the transmission into the engine if the gearbox is mounted on rubber mounts.
Thus, on vehicles that have air suspension systems, it is important to check all grounding points between the chassis and the body, or to fit additional earth cables between suitable points on the suspension, and suitable points on the chassis.
While the tests described above should resolve any stray current issues on most vehicles, there are no guarantees that one or more grounding points won’t fail again at some point in the future, or that the antifreeze additive in the coolant will be replaced when it becomes degraded. Below are some tips and tricks on-
Replace the coolant
It is understood that antifreeze of the correct formulation must be used on the affected vehicle, and a reputable supplier such as Penrite will be able to supply the correct formulation for any vehicle.
However, the main thing to remember is that when distilled water and antifreeze are mixed in the correct proportions, the mixture must have a pH of 7.0, which is considered to be neutral. A pH of less than 7.0 is acidic, while a pH of more than 7.0 is alkaline but note that excessive alkalinity can be just as harmful as excessive acidity.
When antifreeze additives break down, the active ingredients become progressively more acidic as the process of degradation progresses, and under these conditions, stray currents that would not be considered harmful can become harmful in combination with an acidic coolant mixture. If the pH balance of the coolant is in doubt, obtain a dedicated Ph test kit to ensure that the composition of the coolant is not contributing to the rate of corrosion.
Do NOT ground the radiator
If a stray current is present in the engine and chassis, these stray currents are offered an excellent entry point into the cooling system if the radiator and/or heater core is grounded to the chassis.
Bear in mind that the rubber mounts of a radiator are not only intended to reduce the effects of vibration on the radiator; they are also intended to insulate the radiator from any stray currents that might develop, which means that grounding the radiator and/or heater core effectively defeats this insulating mechanism.
Use a sacrificial anode
In scientific terms, electrolytic corrosion occurs when a current flows from an anode to a cathode through an electrolyte, which in this case, is the coolant that contains free ions. In this context, the soft aluminium of the radiator is the anode, while the harder cast iron of the engine block is the cathode.
In cases where electrolytic corrosion is stubborn, or very difficult to eliminate, it helps to use a sacrificial anode that is softer than the aluminium the radiator is made of. In practice, this anode is just a piece of soft metal that dangles from the radiator cap, with the soft metal serving as a target for the corrosion, and since the sacrificial anode “wears” away in place of the radiator, it can simply be replaced when it has corroded away completely. Most parts dealers carry these sacrificial anodes, and while they do not resolve the problem of electrolytic corrosion, they can prevent repeated radiator failures.
At first glance, it might appear that electrolytic corrosion (and its causes) is a complex, perplexing and mystifying problem, but it is possible to resolve the issue definitively simply taking a logical approach to the problem.
Stray currents are just that- currents that cannot return to battery negative via their dedicated paths, and resolving it usually involves nothing more complex than restoring a defective circuit’s natural path to ground. It’s really that simple.
