The corrosion resistance of magnesium or magnesium parts depends
on similar factors that are critical to other metals. However,
because of the electrochemical activity of magnesium, the relative
importance of some factors is greatly amplified.
This article will discuss the effects of heavy-metal impurities,
the type of environment (rural atmosphere, marine atmosphere,
elevated temperature etc), the surface condition of the part
(such as as cast, treated, and painted), and the assembly practice.
In some environments magnesium part can be severely damaged unless
galvanic couples are avoided by proper design or surface protection.
Unalloyed magnesium is not extensively used for structural purposes.
Consequently, the corrosion resistance of magnesium alloys is of
primary concern. Two mayor magnesium alloy systems are available
to the designer.
The first includes alloys containing 2 to 10% Al, combined with minor
additions of zinc and manganese. These alloys are widely available at
moderate costs and their mechanical properties are good up
to 95 to 120oC. Beyond this, the properties deteriorate rapidly
as temperature increases.
The second group consists of magnesium alloyed with various elements
(rare earth, zinc, thorium, silver etc) except aluminum, all
containing a small but effective zirconium content that imparts
a fine grain structure and thus improved mechanical properties.
These alloys generally possess much better properties at elevated
temperature, but their more costly alloying additions, combined
with the specialized manufacturing technology required, result
in significantly higher costs.
Six elements (aluminum, manganese, sodium, silicon, tin and lead)
plus thorium, zirconium, beryllium, cerium, praseodymium and yttrium
are known to have little if any effect on the basic saltwater corrosion
performance of pure magnesium when present at levels exceeding their
solid solubility or up to a maximum of 5%.
Four elements (cadmium, zinc, calcium and silver) have to
mild-to-moderate accelerating effects on corrosion rates,
whereas four others (iron, nickel, copper and cobalt) have
extremely deleterious effects because of their low solid
solubility limits and their ability to serve as active cathodic
sites for the reduction of water at the sacrifice of elemental magnesium.
The effects of increasing iron, nickel and copper contamination on
the standard ASTM salt spray performance of die cast AZ91 test
specimens as compared to the range of performance observed for
cold-rolled steel and die-cast aluminum alloy 380 samples.
Such results have led to the definition of the critical
contaminant limits for two magnesium-aluminum alloys in both
low- and high-pressure cast form and the introduction of improved
high-purity versions of the alloys.
The iron tolerance for the magnesium-aluminum alloys depends on the
manganese present, a fact suggested many years ago but only recently
proved. For AZ91 with a manganese content of 0,15%, this means that
the iron tolerance would be 0,0048% (0,032 x 0,15%).
It should also be noted that the nickel tolerance depends strongly on
the cast form, which influences grain size, with the low-pressure cast
alloys showing just 10-ppm tolerance for nickel in the as cast (F)
temper. The zirconium as an alloying element is effective in this
case because it serves as a strong grain refiner for magnesium
alloys and it precipitates the iron contaminant from the alloys
before casting. However, if alloys containing more than 0,5 to 0,7% Ag
or more than 2,7 to 3% Zn are used, a sacrifice in corrosion resistance
should be expected. Nevertheless, when properly finished, these alloys
provide excellent service in harsh environments.
Heat treating, Grain Size and Cold-Work Effects
Using controlled purity AZ91 alloy cast in both high-and-low pressure
forms, the contaminant tolerance limits have been defined as cast
(F), the solution-treated (T4, held 16h at 410oC and quenched),
and the solution treated and aged
(T6, held 16h at 410oC,
quenched and aged 4h at 215oC).
In the case of the high-iron-containing AZ91C, none of the variations
tested significantly affected the poor corrosion performance resulting
from an iron level 2 to 3 times the alloy tolerance.
In the case of the high-purity alloy, however, the T5 and T6 tempers
consistently gave salt spray corrosion rates under 0,25 mm/yr, whereas
the as-cast and solution -treated samples exhibited an inverse response
to grain size and/or the grain-refining agents.
Cold working of magnesium alloys, such as stretching or bending,
has no appreciable effect on corrosion rate. Shot- or grit-blasted
surfaces often exhibit poor corrosion performance, not from
induced cold work but from embedded contaminants.
Atmospheres. A clean, unprotected magnesium alloy surface exposed
to indoor or outdoor atmospheres free from salt spray will develop
a gray film that protects the metal from corrosion while causing
only negligible losses in mechanical properties.
Chlorides, sulfates and foreign materials that hold moisture on
the surface can promote corrosion and pitting of some alloys unless
the metal is protected by properly applied coatings. The surface film
that ordinarily forms on magnesium alloys exposed to the atmosphere
gives limited protection from further attack. Unprotected magnesium
and magnesium alloy parts are resistant to rural atmospheres and
moderately resistant to industrial and mild marine atmospheres provided
they do not contain joints or recesses that entrap water in association
with an active galvanic couple.
Corrosion of magnesium alloys increases with relative humidity.
At 9,5% humidity, neither pure magnesium nor any of its alloys
exhibit evidence of surface corrosion after 18 months. At 30% humidity,
only minor corrosion may occur. At 80% humidity the surface may exhibit
considerable corrosion. In marine atmospheres heavily loaded with
salt spray, magnesium alloys require protection for prolonged survival.
Fresh Water. In stagnant distilled water at room temperature,
magnesium alloys rapidly form a protective film that prevents
further corrosion. Small amounts of dissolved salts in water,
particularly chlorides or heavy metal salts, will break down
the protective film locally, which usually results in pitting.
Dissolved oxygen plays no mayor role in the corrosion of magnesium
in either freshwater or saline solutions. The corrosion of magnesium
alloys by pure water increases substantially with temperature.
Salt Solutions. Severe corrosion may occur in neutral solutions
of salts of heavy metals, such as copper, iron and nickel. Such corrosion
occurs when the heavy metal, the heavy metal basic salts or both plate
out to form active cathodes on the anodic magnesium surface. Chloride
solutions are corrosive because chlorides, even in small amounts,
usually break down the protective film on magnesium. Fluorides form
insoluble magnesium fluoride and consequently are not appreciable
corrosive. Oxidizing salts, especially those containing chlorine or
sulfur atoms, are more corrosive than nonoxidizing salts, but
chromates, vanadates, phosphates and many others are film forming
and thus retard corrosion, except at elevated temperatures.
Acids and Alkalis. Magnesium is rapidly attacked by all mineral
acids except hydrofluoric acid (HF) and H2CrO4.
Hydrofluoric acid does not attack magnesium to an appreciable extent,
because it forms an insoluble, protective magnesium fluoride film on the magnesium,
however pitting develops at low acid concentrations. Pure H2CrO4
attacks magnesium and its alloys at a very low rate.
Organic compounds. Aliphatic and aromatic hydrocarbons, ketones,
ethers, glycols and higher alcohols are not corrosive to magnesium
and its alloys. Ethanol causes slight attack, but anhydrous methanol
causes severe attack. The rate of attack in the latter is reduced by
the presence of water. Pure halogenated organic compounds do not attack
magnesium at ambient temperatures. At elevated temperatures or if water
is present, such compounds may cause severe corrosion, particularly
those compounds having acidic final products.
Gases. Dry chlorine, iodine, bromine and fluorine cause
little or no corrosion of magnesium at room or slightly elevated
temperature. Even when it contains 0,02% H2O, dry bromine
causes no more attack at its boiling temperature (58oC) than at
room temperature. The presence of a small amount of water causes
pronounced attack by chlorine, some attack by iodine and bromine,
and negligible attack by fluorine.