If I join two metals together, how does this influence their corrosion?

Bob the Builder

If we immerse a metal in an aqueous environment e.g. seawater, anodic and cathodic reactions (as described in this answer) occur. Because these are electrochemical in nature, they take place at a certain voltage or potential, as measured against a standard reference electrode. Active metals such as zinc and aluminium have a high negative potential, whilst noble metals such as copper and stainless steel have less negative or even positive potential. The order of the metals and alloys in a particular environment forms what is termed a galvanic series, as shown below.

If two different metals are joined together in the particular environment, then corrosion of the more active metal is increased while corrosion of the less active or more noble metal is decreased. It is therefore not a good idea to couple aluminium alloys or mild steel to copper, since the corrosion rate of the steel and especially the aluminium will be increased. On the other hand, metals can be protected by coupling with more active metals, which corrode sacrificially. An example of this is ships hulls, which are protected by strapping zinc or magnesium anodes below the waterline.

In reality, if the potential difference between two metals or alloys is less than ~200mV, then the galvanic effect will be slight. Note also that the galvanic series cannot be used to predict corrosion rates.

 

Galvanic Series in Stagnant Sea Water:

Noble (Cathodic End)

Gold

Silver

Titanium

Stainless steel (316 passive)

Stainless Steel (304 passive)

Silicon bronze

Stainless Steel (316 active)

Monel 400

Phosphor bronze

Admiralty brass

Cupronickel

Molybdenum

Red brass

Brass plating

Yellow brass

Naval brass 464

Niobium 1% Zr

Tungsten

Stainless Steel (304 active)

Tantalum

Chromium plating

Nickel (passive)

Copper

Nickel (active)

Cast iron

Steel

Lead

Tin

Aluminum

Cadmium

Zinc plating

Magnesium

Active (anodic end)

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