Resistance to Forms of Corrosion

Magnesium and magnesium alloys can be highly susceptible to a number of corrosion forms including general corrosion. Issues unique to Mg alloys will be covered in this section. Corrosion prevention and protection methods are almost always necessary when using magnesium materials.

Uniform Corrosion

Magnesium alloys have the highest uniform corrosion rates of any metal as shown previously in Figure 22 in Section 1.1-Uniform Corrosion. An oxide protective layer will form on magnesium once exposed, however this film is easily degraded by a number of environmental conditions and chemical compounds. Corrosion protection methods are almost always used with magnesium alloys. Magnesium alloys are not normally used in moving components where the coatings are easily damaged.

Galvanic Corrosion

All metals are cathodic to (more noble than) magnesium. Aluminum alloys are closest to magnesium in the galvanic series, although some aluminum alloys may still pose galvanic corrosion problems when in contact with magnesium alloys. Copper, nickel, and iron cause severe galvanic corrosion to magnesium, and thus aluminum alloys absent of these elements (5000 and 6000 Al series) are preferred for use when in contact with magnesium alloys. Aluminum alloys 5052, 5056, and 6061 have been found to have the least galvanic effect on Mg alloys in a marine atmospheric environment.

Stress Corrosion Cracking

The SCC susceptibility of magnesium is generally more severe in alloys containing Al and/or Zn. The addition of aluminum, above 0.15 to 2.5%, creates the highest susceptibility in Mg alloys. The susceptibility increases with increasing Al content to a peak at about 6%, see Figure 38. The addition of zinc increases SCC susceptibility, but not to the extent as does Al containing alloys. Mg alloys absent of both Al and Zn are the most resistant to SCC.