Auto Service World
Feature   November 1, 2004   by Jim Anderton

Under Attack

Underhood hoses are under attack by electrochemical degradation. It's a rubber killer that's invisible to the naked eye

In the old days, it was simple. The telltale bulge, weeping around the clamp and the obvious green staining told technician and consumer alike that a hose was about to blow. And with road side boil overs a relatively common sight, it didn’t take much to sell a replacement, either. Today, the situation is different, although the consequences of gambling with worn hoses are even more severe than they were twenty or thirty years ago.

Hoses, then and now, are moulded from synthetic rubber and like all rubber products, degrade from several factors. Heat, solvent attack from oil or gasoline, abrasive wear and plain old age all act to reduce the life expectancy of under hood hoses. In the days of slow-revving V-8’s, however, under hood temperatures were modest, coolant changes a seasonal ritual, and running temperatures were low and consistent. Today’s engine compartment is a relatively hellish environment with every hose-killing mechanism working to shorten product life. One issue, however, is still largely misunderstood, especially by vehicle owners: electrochemical attack.

Electricity attacking rubber? Isn’t rubber an insulator? The answer is yes to both questions, although it’s far from a “no-brainer” that electric currents can degrade rubber hoses. The electrochemistry is complex, but put simply, put any two different metals into a current-carrying solution and you have a battery. The familiar lead acid batteries use two forms of lead as the dissimilar metals and the acid solution as the current-carrying medium. Saltwater boaters know that a magnesium block attached to a stern drive or outboard motor will protect it against corrosion, by corroding itself as a “sacrificial anode”. It’s the same principle as the lead acid battery. Now consider the under hood environment. There are different metals everywhere, from differing aluminium alloys to steel and cast iron. Hoses run between these different metals carrying, you guessed it, an electrically conductive fluid, in this case, anti-freeze solution. The wet inside surface of the hose is an especially good conductive path, but unfortunately, current takes the path of least resistance, which in time, is a series of longitudinal grooves inside the hose’s inner walls. In time, hot coolant will penetrate to the hoses reinforcement layer, and ultimately the hose won’t hold pressure. Failure comes soon after that. How fast? Gates testing has demonstrated failures in as little as 40,000 kilometres. It’s especially prevalent in hot, air-entrained coolant loops such as the upper radiator hose and heater bypass hoses.

Solutions? There are few ways to design around this problem. We can’t design vehicles entirely out of one metal, or sacrifice the benefits of glycol-based coolant formulations, but major hose manufacturers have developed hose materials that minimize the damage. Some are modified versions of the standard EDPM ethylene-based rubber that’s the industry standard, but silicone hoses are very resistant to ECD attack. Silicones aren’t the answer in every application, though. Major drawbacks of some silicone formulations are poor adhesion to nipples and fittings and surprisingly, permeability. While permeability is strange enough in a fluid hose, in this case, water migrates faster than glycol coolant, actually changing the mixture ratio of the coolant solution. The colours may be pretty, but avoid unknown or off-brand silicone hoses. Another factor to consider with silicones is the attachment to the fittings. Silicones conform well to irregularities, but aren’t known for high levels of adhesion to fittings. Clamps are critical to good sealing here, both because of the need for consistent clamp force at all temperatures, and because of the softer, easily cut silicone material itself. The key to silicones is to stick with a reputable brand, and always use the clamps recommended by the manufacturer.

What can you do about electrochemical degradation? The simple solution is to change the hoses before the bulge lets you now that the system is ready to blow. Can’t sell the service? Even discounting the cost of a tow and the inconvenience, describe the cost of replacing a warped cylinder head and your customer should see the light. Unfortunately for the aftermarket repair industry, most owner’s manuals list “inspect hoses” as the maintenance procedure, as opposed to “change hoses”. And ECD can’t be seen with a simple inspection, or felt as softness until it’s almost too late. Consider saving a particularly bad example of a degraded hose with a good exterior, and show it to your customer. Four years is a good change interval, and as a moderately priced procedure, shouldn’t cause excessive “sticker-shock”, especially if it’s packaged with winterization service or as part of other cooling system service. Regular hose changeover is a true win-win for you and your customer: good for your bottom line and the best way to save your customer from the ECD-related roadside breakdown.


Where do you most commonly see ECD damage? According to Gates, the following areas are the most common regions of concern:

Within 5 cm of hose ends

In high temperature hoses

In small diameter hoses like heater or bypass hoses

In hoses that contain significant air as well as coolant

In engines subjected to extended idle or stop-and-go driving

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