Electrolysis in the Cooling System
Electrolysis in an automotive cooling system can be hazardous to the lifespan of metal and in particular aluminum. It is often left unnoticed until it is too late and leaks have already started to form.
Electrolysis is extremely corrosive and eats away the inside lining of aluminum parts, especially thinner parts like aluminum heater cores. It occurs when two dissimilar metals are joined together in the presence of moisture. It’s a chemical or electrical process that corrodes the weaker of the two metals. It is said that a current of as low as 0.5V will corrode a cast iron block. It takes much less current to damage an aluminum engine block and many of its components. It's more like 0.15V to 0.20V. Use both A/C and D/C settings for this test. This sometimes happens after adding an aftermarket part or breaking a ground. To correct this type of electrolysis, the part or connection causing the excess current must be found and repaired.
The second and most common type of electrolysis is when the coolant breaks down or the ratio between the coolant and the water is predominantly water. As coolant breaks down, it acts like a catalyst for electrical current. A small amount of electrical current can flow through the coolant corroding the lesser metal; often an aluminum part like the water pump or engine coolant outlet.