Magnesium features an attractive hexagonal close-packed crystal structure and is lighter than other metals, yet highly robust enough to withstand extreme temperatures without damage from corrosion. However, corrosion can have its effects as well.

Corrosion can initiate crack formation from air inclusions or defects or corrosion pits in solutions like Simulated Body Fluid (SBF). Therefore, it's essential to understand how this process occurs.

Temperature

Magnesium's corrosion behavior depends on the temperature of its surroundings. At low temperatures, magnesium plating has outstanding corrosion resistance in water and aqueous salt solutions; at higher temperatures its corrosion performance deteriorates rapidly.

An natural surface film on magnesium controls its corrosion by restricting outward flow of cations while resisting inward flow of anions and oxidants that damage it. The thickness and composition of this protective layer depends on environmental, metallurgical and alloy microstructural considerations.

Uneven microstructure of magnesium-aluminum alloys reduce their corrosion resistance. Impurities like iron, nickel, copper and cobalt have a negative impact on its corrosion properties by decreasing tolerance limit for hydrogen evolution [15]. Through manipulation techniques or creating new alloys with different element distributions, deleterious elements may be decreased below their threshold value or their concentration reduced below it; grain size and intermetallic particles also influence corrosion behavior while rapid solidification can create more refined microstructure.

Environment

Corrosion occurs when metal reacts with elements in its environment and forms oxides, hydroxides, or sulphides that then decompose into oxides, hydroxides, or sulphides. Mg alloys' corrosion rate largely depends on how aggressively their environment corrodes them while their corrosion resistance may also be affected by type of conversion coating applied over them; resistant coatings help extend Mg alloy fatigue limits when exposed to acids like gear oil or sodium sulfate solutions.

Potentiodynamic polarization can be used to assess the corrosion properties of magnesium alloys. A lower corrosion current density indicates a more compact protective passive film and improved corrosion resistance of your sample, while weight loss methods may result in errors due to hydrogen dissolving into corrosive medium, leading to steady state conditions which aren't accurate - therefore hydrogen evolution methods are usually preferred when testing Mg alloy corrosion properties.

Chemical Composition

Alloys with low concentrations of iron, nickel and copper tend to be less vulnerable to corrosion; however, impurities such as Fe, Mn, Cu and Chr act as active cathodic sites during galvanic corrosion, increasing magnesium oxidation rates and overall corrosion rates.

Magnesium, like most metals and alloys, relies on a natural surface film to protect it against corrosion. This barrier controls outward flow of cations while blocking inward-flow of potentially damaging anions or oxidants from damaging itself locally, quickly repairing itself if localized breaks arise, with its structure and composition highly dependent upon environmental and metallurgical considerations.

Example: in a 3 percent NaCl solution at room temperature, MgO (a phase) is more corrosive than pure magnesium while Mg17Al12 (b phase) provides far better corrosion protection. Utilizing techniques such as grain refinement can decrease a-phase fractions while simultaneously increasing those for the latter phase (Mg17Al12).

Microstructure

Researchers have shown that alloy microstructure has an enormous effect on its corrosion resistance. Rapid solidification results in more refined microstructure and stronger corrosion resistance than sand casting due to precipitates of intermetallic compound Mg17Al12 pining down grain boundaries, making microgalvanic corrosion cells harder to form.

Corrosion studies reveal that impurities present in magnesium and its alloys negatively impact its corrosion behavior, with iron, nickel teflon plating and copper having particularly detrimental effects. When their concentration exceeds tolerance limits in the metal itself, their harmful effects worsen; similarly for other elements. Nonetheless, research has proven that magnesium alloy corrosion behavior can be greatly improved by either bringing their impurity concentrations back within tolerance levels or developing new alloys with reduced levels of deleterious elements.

Formation of a protective oxide film is an integral component in Mg and its alloy corrosion behavior. Oxidizing anions such as chromates, fluorides and phosphates as well as silicates improve magnesium's corrosion resistance in aqueous salt solutions, while humidity also plays a crucial role; although no crystalline oxide haze forms on pure uncoated magnesium at 10% humidity in air, less ordered, amorphous film forms at 30% humidity which offers excellent surface protection.