Use of zinc in an engine coolant system

The use of zinc is wide-spread in the world of corrosion control. Many methods exist, but the best known is certainly the application of a protective zinc coating to steel or iron. The material obtained is called galvanised steel. The electrochemical protection provided to steel by zinc coatings is a vital element in the effectiveness of galvanised coatings in protecting steel from corrosion. The way that zinc is used in corrosion control is by forming a galvanic cell where the construction metal is protected by sacrificing the zinc layer. By combining two metals, a galvanic cell is formed whereby the least noble metal will corrode to protect the more noble metal in the system. As long as the two metals are electrically connected, the sacrificial anode principle can theoretically be applied to any combination of metals.

A precise science
In the earlier coolant systems, the use of zinc sacrificial anodes was wide-spread and the system was built in such a way that an anodic material was used to protect critical aluminium or copper parts. The use of this simple component allowed the electrolysis process to "eat" the zinc rather than the aluminium parts of the cooling system. The proper placement of sacrificial anodes is a precise science.

Modern cooling systems
However, the world of cooling systems has become more complex, and engine designs tend towards more efficient and environmental systems. Today the "ancient" way of using zinc to protect the materials comprising the cooling system would not be adequate to deal with the more demanding requirements present in the cooling system. The glycol added to the cooling system to provide freezing protection is extremely aggressive towards zinc, especially at the higher temperatures used in modern cooling systems. The "intended" release of zinc by the created galvanic cell has a tendency to precipitate as fluffy, adhesive precipitate, clogging narrows pathways.

Besides forming insoluble materials, the inhibitors used in a typical engine coolant will attempt to protect the corroding zinc. Consequently, the corrosion inhibitors present to give long-lasting protection to a diversity of metals will be consumed for a material intentionally added in order to corrode. The reaction products of the zinc and the corrosion inhibitors will, in turn, result in additional sludge formation. By reacting with zinc, the inhibitors will be depleted. Once consumed below the critical threshold, the additive package of the coolant will no longer have the correct balance of additives and will no longer be capable of protecting the metal surfaces of the cooling system.

As a result, engine coolant manufacturers and Original Equipment Manufacturers (OEMs) issue warnings not to use galvanised steel for pipes or any other part of the storage/mixing installation or in the cooling system itself.