Five Effective Ways To Improve The Corrosion Resistance Of Magnesium Alloy Materials

Five effective ways to improve the corrosion resistance of magnesium alloy materials
01
The overall corrosion reaction of magnesium alloys usually reacts electrochemically with water to cause the dissolution of magnesium. At the same time, a hydroxide film with a hexagonal structure is formed, and hydrogen gas is generated. Magnesium ions and hydroxide ions are arranged alternately in the crystal structure, resulting in The base layer of the membrane is prone to cracking. The general corrosion products of magnesium alloys vary with the corrosion environment and the chemical composition of magnesium alloys.

02
The main forms of localized corrosion are filiform corrosion, crevice corrosion and pitting corrosion. Oxygen concentration difference battery driving is the main factor of filamentous corrosion, and the potential difference between the head and the tail is between 0.1 and 0.2 V. The research on AZ91 magnesium alloy shows that pitting corrosion and filiform corrosion are the main characteristics of its early corrosion, and the initial pitting corrosion will lead to filiform corrosion. The pitting corrosion of magnesium alloys mainly occurs on the active points on the surface, and once the electrical corrosion occurs, it will tend to develop into the interior of the alloy.

03
Magnesium has a low electrode potential, and it is very prone to galvanic corrosion when it is in contact with the cathode. Usually, the cathode is other metal materials in contact with the magnesium alloy, or the second phase and impurity elements inside the magnesium alloy, which are called External galvanic corrosion and internal galvanic corrosion, as shown in the figure below. Elements such as Fe, Ni, Cu, etc. have low hydrogen overpotential and usually act as highly efficient cathodes, which can lead to severe galvanic corrosion in magnesium alloys.

Schematic diagram of galvanic corrosion

04
In the corrosive environment containing chromate, sulfate, etc., when magnesium alloys are subjected to internal and external stresses at the same time, cast magnesium alloys, especially Mg and Al-based cast magnesium alloys, exhibit It is extremely sensitive to stress corrosion, and the existence of stress corrosion will greatly reduce the service performance of components.

Factors Affecting Corrosion Resistance of Magnesium Alloys
The main factors affecting the corrosion resistance of magnesium alloys are alloy composition, microstructure and corrosion medium. By testing the corrosion rate of magnesium alloys with different contents of impurity elements such as Fe, Ni, and Cu, it is shown that the corrosion rate of magnesium alloys decreases sharply as the content of impurity elements increases, as shown in the figure below. These impurity elements have low solid solubility in magnesium, often form intermetallic compounds, and form a primary battery with the magnesium alloy matrix, accelerating the corrosion of magnesium alloys.

Effect of Element Mass Fraction on Corrosion Rate of Magnesium Alloy
The microstructure has a great influence on the corrosion behavior of magnesium alloys. For example, for rapidly solidified magnesium alloys, due to the fast solidification speed, the distribution of alloy elements in the matrix is relatively uniform, which improves the corrosion resistance.
Magnesium alloys show different corrosion characteristics in different corrosive media. In a dry environment, a gray protective film is easily formed on the surface and is not easy to corrode; moderate corrosion occurs in rural and industrial atmospheres, but not in most organic media. corroded. The specific corrosion conditions in different media are as follows:

Based on the above analysis of the three major factors affecting the corrosion behavior of magnesium alloys, the improvement of the corrosion resistance of magnesium alloys can be studied from the following two aspects:

One is to improve the intrinsic corrosion resistance of magnesium alloys, that is, to improve the corrosion resistance of base materials by optimizing alloy composition and improving the microstructure of magnesium alloys;

The second is to use surface protection treatment technology to protect the substrate through the surface protection layer and isolate the contact between the corrosive medium and the substrate, thereby improving the corrosion resistance of magnesium alloys. This is currently the most commonly used technology for corrosion protection of magnesium alloys.

An Effective Way to Improve the Corrosion Resistance of Magnesium Alloys

chemical transformation
The treatment technology that the magnesium alloy reacts with the conversion solution to form a protective passivation film is called the chemical conversion method. Its main features are simple equipment and low cost, and it is suitable for the treatment of complex and large parts.

The chemical conversion coating layer is well combined with the substrate, has specific pores, and can form a good combination with the organic layer, and is suitable for components that do not require high environmental temperature and surface quality accuracy. Chromium-free conversion coating technology has received widespread attention, such as phosphate, permanganate, vanadium-based salt, rare earth metal salt, and stannate treatment. The non-chromate conversion methods and their advantages and disadvantages are as follows:

Anodizing
The process of forming a film on the metal surface by electrolysis is called anodic oxidation. The anodic oxidation film has a porous double-layer structure, the thicker porous layer is the outer layer, and the thinner dense layer is the inner layer. Oxide and deposited oxide together.

The anodized film has large pores, irregular and uneven distribution. If it is not sealed, the corrosion resistance will be very poor. Therefore, subsequent sealing treatment is required to make it both beautiful and corrosion-resistant. As shown in the table below, the treatment solution contains chromium compounds, causing serious pollution, so environmentally friendly anodizing processes such as phosphates have gradually been developed.

Micro-arc oxidation
Micro-arc oxidation is a technique for in-situ growth of ceramic layers on metal surfaces. It was first proposed by Gnterschulze and Betz in the early 1930s, and has been continuously improved by scientists from various countries. Compared with chemical conversion and anodic oxidation techniques, the thickness of the film prepared by micro-arc oxidation is controllable, and the corrosion resistance and wear resistance are also better. It has broad application prospects in the fields of aerospace, aviation, machinery and electronics.

Ion implantation and surface alloying
Ion implantation exposes the surface to a beam of ionized particles, and the ions are intercalated and neutralized at the interstitial positions of the substrate to form a solid solution, thereby changing the surface properties of the substrate. Ion implantation is also an effective technique to improve the corrosion resistance surface modification of magnesium alloys. A certain dose of ions can inhibit the corrosion of magnesium alloys. The implanted elements mainly include N, O, Ti, Al and Zn.

Surface coated with anti-corrosion coating
Another effective way to improve the corrosion resistance of magnesium alloys is to apply corrosion-resistant coatings on the surface to isolate magnesium alloys from corrosive media. At present, corrosion-resistant coatings mainly include organic coatings, corrosion-resistant metal coatings, and compound coatings.

1) Organic/polymer coating. The organic/polymer coating can be directly used for corrosion protection on the surface of the magnesium alloy, and can also be used for the outermost protective coating and sealing layer of the magnesium alloy to further improve the corrosion resistance.

2) Metal/compound coating. The metal coating is mainly to apply a layer of metal coating on the surface of the magnesium alloy substrate by means of electroless plating and electroplating. The research on AZ91 shows that after Ni-P and Ni-P-SiC electroless plating, the self-corrosion potential is significantly positive compared with the AZ91 substrate (see the table below), and the corrosion current is significantly reduced, indicating that the electroless plating can improve the corrosion resistance of AZ91 magnesium alloy. Corrosion resistance, while the addition of SiC particles has little effect on corrosion resistance.

Corrosion Electrochemical Parameters of AZ91 Magnesium Alloy Coatings in 3.5%NaCl

Compound coatings exhibit much higher chemical inertness than magnesium alloy substrates in neutral or acidic corrosive media. Preparing a layer of dense compound coating on the surface of magnesium and magnesium alloy substrate will greatly increase the corrosion potential of the substrate and improve the corrosion resistance.