The Influence of SO₂on the Corrosion of Mg and Mg-Al Alloys

Abstract

The SO 2 -induced atmospheric corrosion of some magnesium-aluminum (Mg-Al) alloys, including Mg alloy AZ91D, and commercially pure Mg (CP Mg) was investigated using well-controlled laboratory exposures and included real-time measurements of SO 2 deposition. The influence of SO 2 concentration, alloy composition, humidity, and ppb level additions of O 3 or NO 2 on the rate of SO 2 deposition was investigated. SO 2 accelerates the corrosion of Mg and Mg alloys causing localized corrosion, MgSO 3 6H 2 O being the dominant corrosion product. At 60% RH, traces of O 3 or NO 2 strongly increased both the SO 2 deposition and the corrosion rate. The rate of SO 2 deposition was strongly dependent on humidity; at 70% RH and higher the SO 2 deposition rate was very rapid and constant in time while it was transient below 50% RH. At 60% RH, a change from transient to rapid, steady-state, SO 2 deposition occurred. The sudden activation is explained by the onset of electrochemical corrosion. The activation behavior was shown to depend on SO 2 concentration, the thickness of the surface film and by the presence of ambient O 2 . © The Author Magnesium-aluminum (Mg-Al) alloys are widely used as structural materials in many engineering sectors as a result of their good castability and good mechanical properties at room temperature. Due to their low weight/strength ratio, these types of Mg alloys have found widespread applications; from portable microelectronics to automobiles and aircraft. However, Mg alloys are sensitive towards corrosion and exhibit the lowest corrosion potentials among the engineering metals.1-4 Also, Mg and Mg-Al alloys exhibit high corrosion rates when immersed in NaCl (aq) solution, corrosion being more severe compared to other engineering alloys, including carbon steel. Nevertheless, Mg alloys exhibit somewhat better corrosion properties than steels under atmospheric conditions. [5][6][7][8] The causes behind the relatively slow corrosion of Mg-Al alloys in the atmosphere are still not fully clear.Because Mg-Al alloys are often used in atmospheric conditions, a better understanding of the atmospheric corrosion properties is of importance. A large number of parameters affect the atmospheric corrosion process of Mg-Al alloys. Therefore, systematic studies are required to clarify their role on the corrosion. There have been extensive researches on the atmospheric corrosion behavior of Mg alloys using field studies that correlate corrosion rate to selected environmental parameters. [9][10][11][12][13][14][15][16][17][18][19] The variability and complexity of the outdoor atmosphere constitute a major problem when trying to correlate the corrosion rate of a metal to the concentration of, e.g., a particular trace gas in the environment. Laboratory studies under controlled conditions provide the opportunity to study the effect of a specific parameter and/or the synergic effects of atmospheric variable on the corrosion of metallic materials. For example, laboratory investigations have shown that carbon dioxide (CO 2 ...

A detailed microstructural and corrosion analysis of an additively manufactured magnesium alloy produced by selective laser melting

“…Although there exists a plethora of studies on the processing, microstructure and physical properties of Mg-based alloys (see e.g., [9][10][11][12][13][14][15][16][17]), to date, there remains a limited reporting, characterisation and rationalisation of processing-structure-properties relationships for AM produced Mg alloys [18][19][20]. In part this is because the production of Mg-based alloys by AM has been considered complicated, owing to a range of factors that could include: safety concerns associated with the reactive nature of Mg powder; the superheating and boiling / evaporation of Mg via interaction with laser based methods; a tendency for ignition in the presence of common environments (Mg can burn under air or N 2 ), and even supply chain issues for Mg powder of suitable size and composition (relevant to powder based AM).…”

Localised atmospheric corrosion of magnesium-aluminium alloys produced by semi solid casting - A 2D and 3D investigation

“…In addition to such electrochemical processes, the pH of the electrolyte on the alloy surface is strongly influenced by the gases present in the atmosphere. For instance, in highly polluted areas and under humid conditions, the Mg surface becomes an efficient getter for SO2 owing to the high pH of the thin electrolyte layer [6]. The outcome is the formation of two highly acidic species, namely hydrogen sulfite (HSO3 -) and sulfite (SO3 2-) that results in a rapid dissolution of the MgO/Mg(OH)2 surface film [6].…”

“…For instance, in highly polluted areas and under humid conditions, the Mg surface becomes an efficient getter for SO2 owing to the high pH of the thin electrolyte layer [6]. The outcome is the formation of two highly acidic species, namely hydrogen sulfite (HSO3 -) and sulfite (SO3 2-) that results in a rapid dissolution of the MgO/Mg(OH)2 surface film [6]. Another significant example is ambient concentrations of CO2 gas, which is always present in air at a concentration of around 400 ppm [5].…”

Corrosion Characteristics of Hydrotalcite and Mg(OH)2 Surfaces of AZ31 Alloy Accomplished via Hydrothermal Surface Modification

“…However, in practical applications, magnesium alloys are not standing as strong competent to others due to the poor corrosion resistance, which is a main disadvantage. The high chemical reactivity, magnesium alloys exhibits higher corrosion rates even in atmospheric and other mediums of corrosion tests [5][6][7][8][9][10][11][12]. Two types of approaches were employed on Mg alloys to improve corrosion resistance; one is alloying [13] and the second is application of external coatings.…”