Types of Corrosion in Magnesium Alloys
1. The general corrosion reaction of magnesium alloys often occurs with water, leading to the dissolution of magnesium. This reaction forms a hexagonal structured hydroxide film, releasing hydrogen gas.
Magnesium ions and hydroxyl ions alternate in the crystal structure, making the base layer of the film susceptible to cracking. The products of general corrosion of magnesium alloys vary based on the corrosive environment and the chemical composition of the alloy.
2. Localized corrosion mainly takes the form of filamentary corrosion, crevice corrosion, and pitting. Oxygen concentration cell drive is a significant factor in filamentary corrosion, with a potential difference of 0.1-0.2V between the head and tail.
Studies on AZ91 magnesium alloy have shown that pitting and filamentary corrosion are its primary early corrosion characteristics, and initial pitting can lead to filamentary corrosion. Pitting corrosion of magnesium alloys mainly occurs on active points on the surface, and once electrolytic corrosion occurs, it tends to develop inward into the alloy.
3. Magnesium has a lower electrode potential, and it is prone to galvanic corrosion when in contact with a cathode.
Typically, the cathode is other metal materials in contact with the magnesium alloy, or the second phase and impurity elements within the alloy, known as external and internal galvanic corrosion, respectively, as shown below. Elements such as Fe, Ni, and Cu have low hydrogen overpotential and often act as efficient cathodes, leading to severe galvanic corrosion in magnesium alloys.
4. In corrosive environments containing chromates and sulfates, when magnesium alloys are subjected to both internal and external stress, cast magnesium alloys, especially Mg-Al series cast magnesium alloys, exhibit a strong sensitivity to stress corrosion under stress lower than the yield strength. Furthermore, the presence of stress corrosion significantly reduces the service performance of the components.
Factors Affecting the Corrosion Resistance of Magnesium Alloys
The main factors affecting the corrosion resistance of magnesium alloys include alloy composition, microstructure, and corrosive medium. Corrosion rate testing of magnesium alloys with different concentrations of impurity elements such as Fe, Ni, and Cu shows that as the content of impurity elements increases, the corrosion rate of magnesium alloys drastically decreases, as shown in the diagram below.
These impurity elements have low solubility in magnesium, often forming intermetallic compounds, and creating galvanic cells with the magnesium alloy substrate, accelerating the corrosion of the magnesium alloy.
The microstructure greatly impacts the corrosion behavior of magnesium alloys. For rapidly solidified magnesium alloys, due to the fast solidification speed, the distribution of alloy elements in the matrix is relatively uniform, enhancing their corrosion resistance.
Magnesium alloys exhibit different corrosion characteristics in various corrosive mediums. In dry environments, they easily form a gray protective film on their surface, making them resistant to corrosion. However, they experience moderate erosion in rural and industrial atmospheres, while in most organic media, they are not affected by corrosion. The specific corrosion conditions in different media are as follows:
|Freshwater, seawater, humid atmosphere||Corrosion damage|
|Organic acids and their salts||Severe corrosion damage|
|Inorganic acids and their salts||Intense corrosion destruction|
|Ammonia solution, ammonium hydroxide||Severe corrosion damage|
|Formaldehyde, acetaldehyde, trichloroacetaldehyde||Corrosion damage|
|Methyl ether, ethyl ether, acetone||No corrosion|
|Petroleum, gasoline, kerosene||No corrosion|
|Sodium hydroxide solution||No corrosion|
|Dry air||No corrosion|
|Aromatic compounds||No corrosion|
Based on the analysis of the three major factors affecting the corrosion behavior of magnesium alloys, research on improving the corrosion resistance of magnesium alloys can be conducted from the following two aspects:
Firstly, improving the inherent corrosion resistance of magnesium alloys, that is, enhancing the corrosion resistance of the base material through optimizing alloy composition, improving the microstructure of magnesium alloys, and so on.
Secondly, adopting surface protection treatment technology, which boosts the corrosion resistance of magnesium alloys by protecting the base material with a surface protection layer, thereby isolating the contact between the corrosion medium and the base material. This is the most commonly used technology for corrosion protection of magnesium alloys at present.
Effective Ways to Improve Corrosion Resistance of Magnesium Alloys
1. Chemical Conversion
The process of treatment technology that involves a chemical reaction between magnesium alloy and conversion fluid to form a layer of protective passivation film is known as chemical conversion. It is characterized by simple equipment and low cost, suitable for handling components with complex structures and large parts.
The chemical conversion film layer is well bonded with the base material, possessing specific pores that can form a good bond with the organic layer. It is suitable for components where the environmental temperature and surface quality accuracy requirements are not high.
Non-chromium conversion film technology, such as phosphate, permanganate, vanadium-based salt, rare earth metal salt, and stannic salt treatments, has attracted widespread attention. The advantages and disadvantages of non-chromate conversion methods are as follows:
|Phosphate Conversion Coating||Excellent bonding strength.||The solution gets consumed rapidly and has poor corrosion resistance, making it suitable only for pretreatment.|
|Stannate Conversion Coating||Low cost, minimal pollution.||It has poor corrosion resistance and is used for the pretreatment of organic coatings.|
|Rare Earth Metal Salt Conversion Coating||Free of toxic substances and heavy metal ions.||The processing technique requires high temperatures and long durations, leading to a loose film structure and poor corrosion resistance.|
|Phytate Salt Chemical Conversion Coating||The conversion coating is more uniform and compact than a chromic acid film, offering high corrosion resistance, eco-friendliness, and non-toxicity.||Waste liquid treatment is challenging.|
|Tungstate Conversion Coating||Affordable, non-toxic, and pollution-free.||Its corrosion resistance is inferior to that of chromate conversion coatings.|
|Fluorozirconate Conversion Coating||The coating has a three-dimensional structure, indicating significant potential for development.||The solution formulation is difficult, and fluoride ions are harmful to people and the environment, as well as having poor corrosion resistance.|
|Permanganate (Manganate) Conversion||Good corrosion resistance.||Manganese ions pollute the environment, and the solution is unstable.|
|Key Acid Salt Conversion Coating||Environmentally friendly with superior corrosion resistance.||The cost is relatively high.|
2. Anodic Oxidation
Anodic oxidation is a process that uses electrolysis to form a film on a metal surface. The anodic oxide film has a porous double-layer structure. The thicker porous layer is the outer layer, while the thinner dense layer is the inner layer. The composition of the film layer is made up of the oxides of alloy elements and the deposited oxides.
The anodic oxide film has large, irregular, and unevenly distributed pores. If not sealed, its corrosion resistance is extremely poor. Therefore, subsequent pore-sealing treatment must be carried out to make it both aesthetically pleasing and corrosion-resistant.
As shown in the table below, the treatment liquid contains chromium compounds, which are heavily polluting. Hence, environmentally friendly anodic oxidation processes using phosphates and the like are gradually being developed.
3. Microarc Oxidation
Microarc oxidation, a technique for in-situ growth of ceramic layers on metal surfaces, was first proposed by Gnterschulze and Betz in the early 1930s, and has been continuously improved by scientists worldwide.
Compared to chemical conversion and anodizing techniques, the thickness of the film prepared by microarc oxidation is controllable, and its corrosion resistance and wear resistance are superior. It has broad application prospects in aerospace, aviation, mechanical, and electronic fields.
4. Ion Implantation and Surface Alloying
Ion implantation is a process where the surface is exposed to a beam of ionized particles. The ions are embedded and neutralized in the interstitial positions of the matrix, forming a solid solution and thereby changing the surface properties of the matrix.
Ion implantation is also an effective technology for improving the corrosion-resistant surface modification of magnesium alloys. A certain dosage of ions can inhibit the corrosion of magnesium alloys, with the main elements being N, O, Ti, Al, and Zn.
5. Surface Coating with Corrosion-Resistant Layers
Coating the surface with a corrosion-resistant layer to isolate the magnesium alloy from the corrosive medium is another effective approach to improving the corrosion resistance of magnesium alloys. The main types of corrosion-resistant coatings currently are organic coatings, corrosion-resistant metal coatings, and compound coatings.
1) Organic/Polymer Coatings.
Organic/polymer coatings can be directly applied for corrosion protection on the surface of magnesium alloys, and can also be used as the outermost protective layer and sealant layer of magnesium alloys, further enhancing corrosion resistance.
2) Metal/Compound Coatings.
Metal coatings primarily involve applying a layer of metal coating to the surface of the magnesium alloy matrix using electroplating and chemical plating methods. Studies on AZ91 show that after Ni-P and Ni-P-SiC chemical plating, the self-corrosion potential significantly shifts positive compared to the AZ91 matrix (see below), and the corrosion current significantly decreases.
This indicates that the chemically plated layer can improve the corrosion resistance of the AZ91 magnesium alloy, while the addition of SiC particles has little effect on corrosion resistance.
Corrosion electrochemical parameters of AZ91 magnesium alloy coating in 3.5% NaCl.
|AZ91D Magnesium Alloy||-1.593||209.46||7.06||39.01|
Compound coatings demonstrate a much higher chemical inertia than magnesium alloy substrates in neutral or acidic corrosive media.
Preparing a dense layer of compound coating on the surface of magnesium and magnesium alloy substrates will significantly enhance the substrate’s corrosion potential, thereby improving corrosion resistance.