Factors that affect the corrosion properties of magnesium are alloy composition, heavy-metal impurities, casting variables, metallographic structure, environment, surface condition, and contact with other materials. Thermodynamically, magnesium should react completely with oxygen and as well as water. The fact that it reacts with neither of those is caused by passive behavior in many environments. In a corrosive environment, pitting or other forms of local corrosion occur as a result of film breakdown.
Magnesium alloys of suitable composition and purity are corrosion resistant. They are being used successfully in a variety of applications. Automotive parts made of commercial high-purity die-cast AZ91D alloy located in the worst splash zone under a car, illustrate the excellent corrosion resistance of magnesium.
Effect of Composition and Structure
Composition
The corrosion of magnesium alloys is commonly measured in a sodium chloride solution, by using either immersion or salt spray tests. These tests relate to important practical uses of magnesium alloys in automotive, aircraft, and military applications.
Most of the elements present in, or added to, magnesium alloys have limited solid solubility in the alloy, and therefore occur as precipitated phases. In virtually all cases, these phases are more noble (i.e., have a higher redox potential) than the matrix. Their influence on saltwater corrosion depends heavily on their potential relative to the matrix, as well as their efficiency as cathodic sites, i.e., the ease with which they liberate hydrogen gas (overvoltage).
Elements that are generally present in commercial magnesium alloys, which influence saltwater corrosion can be classified as follows:
1) generally benign or beneficial: aluminum, beryllium, manganese, rare earths, silicon, zinc, and zirconium;
2) moderately deleterious: silver;
3) severely deleterious: nickel (and cobalt), iron, and copper.
The commercially important Mg-Al-Zn alloys used for die-casting and sand casting have received intensive study, resulting in the development of alloys with outstanding saltwater corrosion resistance. These alloys have a very low critical impurity content (Ni, Fe, Cu), and a controlled manganese content.
Structure
The size and distribution of the cathodic phases play an important role in corrosion and are influenced by process parameters and heat treatment. Homogenized and artificially aged specimens of AZ91E (T6) show considerably lower corrosion than cast (F) and homogenized (T4) specimens. Heat treatment influences mainly the distribution of the intermetallic Beta-phase (Mg17Al12) in the alloy. Aging to T6 temper causes precipitation of this phase as an almost continuous network of secondary particles along the grain boundaries. In the T4 condition, the Beta-phase is fully dissolved. By air cooling from T4, only traces of Beta-phases can have the same effect as a full T6 treatment. Tolerance limits in cast AZ91 for the most important impurity elements (iron, copper, and nickel) are influenced by the cooling plate. In the early stages of corrosion, filiform attack develops from an initiating pit adjacent to intermetallic particles, and the role of Mg17Al12 concentrated in grain boundaries can be clearly illustrated. Cold working of magnesium alloys (e.g., by stretching or bending) has no appreciable effect on corrosion rate.
Surface Contamination
Producers of magnesium have demonstrated the importance of high-purity alloys for structural applications. However, surface contamination from handling and mechanical treatment can greatly degrade the corrosion resistance of high-purity alloys. This helps explain why ceramic blasting media containing iron oxide can be just as harmful to the corrosion properties of magnesium as steel grit.
Atmosphere
A magnesium alloy surface exposed to a salt-free atmosphere develops a gray film consisting mainly of magnesium hydroxide that protects the metal from corrosion. Chlorides, sulfates, or other hydrophilic substances promote corrosion by destroying this film. Structural magnesium alloys are resistant to rural atmospheres and moderately resistant to industrial or mild marine atmospheres. The corrosion rate in marine atmospheres is significantly lower for the high-purity Mg-Al-Zn alloys.
The surface film that usually 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.
Water
When magnesium is immersed in distilled water without the possibility of carbon dioxide absorption, the initial corrosion rate decreases rapidly to a very low value. A protective film of magnesium hydroxide forms on the surface. The solubility product of magnesium hydroxide in the solution is quickly reached, dissolution of the hydroxide is inhibited, and corrosion essentially stops. If the water is replenished, corrosion continues and increases on absorption of carbon dioxide due to dissolution of the protective film. Raising the temperature of distilled or natural water also increases the corrosion rate of magnesium alloys. Aluminum is beneficial as an alloying ingredient because it promotes the formation of protective hydrotalcite [Mg6Al2 (OH)16CO3 • 4H2O] films.
Acids
Magnesium is attacked by all acids except hydrofluoric or chromic acid. Passive films are formed in most concentrations of these acids, accounting for their use in many conversion-coating processes.
Hydrofluoric acid does not attack magnesium to an appreciable extent, because it forms an insoluble, protective magnesium fluoride film on the surface; however, pitting develops at low acid concentrations. With increasing temperature, the rate of attack increases at the liquid line, but to a negligible extent elsewhere.
Pure H2CrO4 attacks magnesium and its alloys at a very low rate. However, traces of chloride ion in the acid will markedly increase this rate. A bolting solution of 20% H2CrO4 in water is widely used to remove corrosion products from magnesium alloys without attacking the base metal. Magnesium resists dilute alkalis, and 10% caustic solution is commonly used for cleaning at temperatures up to the boiling point.
Salt Solutions
Neutral solutions of salts of heavy metals such as nickel, iron, and copper are corrosive to magnesium alloys. Such corrosion occurs when the heavy metal plates 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 tend to passivate. Oxidizing salts, especially those containing chlorine or sulfur atoms, are more corrosive than non-oxidizing salts, but chromates, vanadates, phosphates, and others are film forming, and thus retard corrosion, except at elevated temperatures.
Gases
Iodine, bromine, fluorine, and dry chlorine 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 (58 oC, or 136 oF) 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. Wet chlorine, iodine, or bromine below the dew point of any aqueous phase causes severe attack on magnesium. Dry, gaseous sulfur dioxide causes no attack at ordinary temperatures. If water vapor is present, some corrosion may occur.
Organic compounds
Aliphatic and aromatic hydrocarbons, ketones, and ethers are not corrosive to magnesium and its alloys. Ethanol and higher alcohols are not corrosive at ordinary temperatures, but they may react destructively at high temperature (150 oC, or 300 oF). Anhydrous methanol attacks magnesium alloys catastrophically at room temperature; however, the rate of attack is reduced by the presence of water. Gasoline-methanol fuel blends, in which the water content equals or exceeds about 0.25 wt% of the methanol content, do not attack magnesium.
Pure halogenated organic compounds do not attack magnesium at ambient temperatures. At elevated temperatures, or if water is present, such compounds can cause serious corrosion, particularly those compounds having acidic hydrolysis products.
Dry fluorinated hydrocarbons, such as the freon refrigerants, do not attack magnesium alloys at room temperature, but when water is present they may stimulate significant attack. At elevated temperatures, fluorinated hydrocarbons may react violently with magnesium alloys.
Acidic foodstuffs, such as fruit juices and carbonated beverages, attack magnesium seriously. Milk causes attack, particularly when souring.
At room temperatures ethylene-glycol solutions cause minor corrosion of magnesium that is used alone or galvanically connected to steel; at elevated temperatures such as 115 oC (240 oF), the rate increases and the corrosion is serious enough to preclude the use of solutions of ethylene glycol and water in liquid-cooled magnesium engines. Anhydrous propylene glycol coolant is reported to be successfully used in prototype magnesium-alloy engines having modified cooling systems.
Galvanic Corrosion
Two conditions must be satisfied for galvanic corrosion to occur: (1) dissimilar metal-to-metal contact and (2) bridging of the bimetal junction by a conductive solution (electrolyte). The electrochemical process, which normally occurs at the galvanic couple containing magnesium is:
Anode: Mg (metal) → Mg2+ + 2e-
Cathode: 2H2O + 2e- → H2 + 2(OH)-
In salt environments, the high solubility and acidic nature of the magnesium chloride formed at the anode can result in rapid penetration of magnesium alloys.
Proper protection against galvanic corrosion begins with good design. This includes good drainage to prevent entrapment of electrolyte, selection of the most compatible metals, sealing of faying surfaces, small ratios of cathode to anode area, and use of alkali-resistant barrier coatings.
The closest approach to compatibility with magnesium is provided by aluminum alloys of the 5000-6000 series. Tin, cadmium, and zinc platings on steel fasteners reduce galvanic action on magnesium in salt spray by 60%-70%, which is sufficient for many practical applications. Supplementary polymer coatings on the plating can reduce galvanic corrosion further.
Additional resistance in the metallic or electrolytic portions of the galvanic cell circuit can reduce or eliminate galvanic current flow. Such resistance can be supplied by insulating materials such as nonmetallic bolts, insulating washers or tapes, or organic coatings.
The combination of a small magnesium anode and a large cathode area can lead to intense corrosion penetration of the magnesium. In painting a galvanic couple, the cathode or the entire couple must be coated. In no case should the magnesium alone be coated. Small areas of magnesium exposed to paint defects or scratches could be subjected to intense corrosion penetration. |
