System corosion

JM_2

I have seen threads on this board discussing galvanic corrosion caused by dissimilar metals contacting each other in a hydronic heating system. I wonder why you coudnt use a sacraficial zinc anode like boaters use for corrosion protection. Any thoughts?

Matt Undy

Its a sealed system so the solution reaches equilibrium at some point (except 1 pipe steam to some extent). Anodes are used in open systems such as potable water heating. Zinc does not work in hot water systems because in hot water the system reverses and favors depostion of zinc and dissolution of iron. I believe that anodes are only effective where the system is made of similar metals. It will protect an iron system but will not protect a copper-iron connection.

Matt

Aidan (UK)

sacrificial anodes

It may work, but a corrosion inhibitor solution would stop galvanic corrosion. The inhibitor should remain in the system, unless there's a leak. Annual testing and topping-up is all that's then required.

I've seen sacrificial anodes on steel water storage tanks and steel water heaters, but they were made of magnesium alloy. I was told recently, by a water heater manufacturer, that sacrificial anodes are now prohibited by the UK Building Regulations, because of the products of corrosion of the alloy that are left in the water system.