Corrosionist all you need to know on Corrosion & Rust

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Magnesium is highly active in the electromotive force (EMF) series, and it has little tendency to polarize in most environments.

Therefore, magnesium and magnesium alloys are also highly active in the galvanic series of metals and this mean that there is a high likelihood of magnesium galvanic corrosion when coupled to other metals.

The dissimilar metals most frequently involved in structures with magnesium alloys are

1. Aluminum alloys
2. Iron, copper, and nickel alloys.

Very pure aluminum (alloy 1099) is almost completely compatible with magnesium alloys in chloride solutions.

 

Aluminum containing magnesium, manganese, or silicon alloying elements has equivalent compatibility.
Zinc, however, has a small adverse effect, iron, copper, and nickel can have large adverse effects as alloying elements in aluminum.

Nevertheless, the adverse effect of these elements on galvanic compatibility with magnesium alloy may be mitigated by magnesium in the aluminum alloy or magnesium ion in the electrolyte solution.
Rivets of Type 5056 aluminum which contain about 5% magnesium, have been used extensively in magnesium alloy structures without causing significant galvanic corrosion on magnesium.

Series 5XXX and 6XXX aluminum alloys are the most compatible in magnesium-aluminum galvanic couples.
Corroding couples of magnesium alloys and aluminum alloys often generate an alkaline environment that may result in corrosion of the aluminum alloy and in magnesium galvanic corrosion that is more reactive.

Iron alloys, copper alloys, and nickel alloys are incompatible with magnesium alloys. There is little significant difference among these metal alloys in the galvanic corrosion on magnesium alloys.

Titanium base alloys are only slightly less cathodic to magnesium alloys. When these metals are constituents of the magnesium alloy, they may be considered to react galvanically with magnesium resulting in high rates of uniform corrosion in electrolyte solutions.

Magnesium alloys are widely used as galvanic sacrificial anodes in the cathodic protection of steel.
Steel is frequently electroplated with cadmium to reduce its severe galvanic attack on magnesium.

Electroplates of chromium, lead, silver, and manganese on steel are roughly comparable to cadmium plate in compatibility with magnesium. Tin is rapidly gaining favor as being superior to cadmium. Zinc is approximately equal to tin in compatibility, but in severe marine environments zinc coatings rapidly bare the steel substrate.

Copper, nickel, gold, and platinum electroplates are incompatible with magnesium.

Because high-purity aluminum is much more compatible with magnesium than tin, aluminum coatings on steel are potentially useful in preventing galvanic attack on magnesium alloys. The coating, however, must be extremely sound.

 

If underlying steel is exposed through a pore, the alkaline environment generated at the surface of the steel will rapidly strip the adjacent aluminum coating.

ref: Material Deterioration Prevention and Control Guide MIL-HDBK-735

See also

Galvanic Corrosion

Galvanic Corrosion Chart