Research on the Corrosion Resistance of Aluminum Magnesium Alloy Powder Metallurgy Materials: Failure Mechanism and Protective Coating Design in Cl ⁻ Environment

Aluminum magnesium alloy powder metallurgy materials are widely used in aerospace, marine engineering and other fields due to their lightweight and high specific strength characteristics. However, they are prone to corrosion failure in Cl ⁻ - containing environments such as salt spray and seawater. Studying the corrosion mechanism induced by Cl ⁻ and developing efficient protective coatings are key to expanding their engineering applications.

1、 Corrosion Failure Mechanism in Cl ⁻ Environment

The corrosion damage of Cl ⁻ on aluminum magnesium alloys is mainly achieved through electrochemical corrosion and pitting corrosion extension:

Electrochemical corrosion drive

There are multiphase structures in aluminum magnesium alloys (such as Mg ₁₇ Al ₁ ₂ strengthening phase, Al Mn solid solution), and Cl ⁻ destroys the surface oxide film (Al ₂ O3/MgO), forming a "microbattery" effect:

Mg preferentially dissolves as the anode (electrode potential -1.66 V), while the Al phase (-0.76 V) becomes the cathode, accelerating the corrosion of the magnesium substrate.

The precipitation of Mg ₁₇ Al ₁₂ phase at the grain boundary leads to a grain boundary potential difference (about 0.3 V), causing intergranular corrosion and forming intergranular cracks.

Formation and expansion of pitting pits

The strong penetrability of Cl ⁻ causes it to adsorb on the weak points of the oxide film (such as the powder sintering interface and the edge of the second phase particles), forming a local acidic microenvironment (pH<3), resulting in the dissolution of the oxide film and the formation of pitting corrosion pits.

The concentration of Cl ⁻ in the pitting pit can reach more than 10 times that of the external solution, forming a "closed cell" effect. The metal at the bottom of the pit self catalyzes dissolution, and the depth can reach 100-300 μ m/year.

Environmental synergy

In humid environments, Cl ⁻ forms complexes (such as MgCl ₂ · 6H ₂ O) with hydrated ions (such as Mg ² ⁺, Al ³ ⁺), hindering the dense accumulation of corrosion products (Mg (OH) ₂, Al (OH) ∝) and allowing corrosion to continue.

2、 Design Strategy and Performance Optimization of Protective Coatings

For the Cl ⁻ corrosion mechanism, protective coatings need to have both physical barrier and chemical inhibition functions. Common technical paths include:

Ceramic Conversion Membrane Technology

Micro arc oxidation (MAO): In situ formation of porous ceramic layer (mainly composed of MgAl ₂ O ₄, α - Al ₂ O ∝) on the surface of aluminum magnesium alloy, with a thickness of 5-20 μ m and a porosity of 8% -15%. By subsequent sealing treatment (such as sodium silicate solution), the corrosion current density can be reduced from 10 ⁻⁴ A/cm ² to below 10 ⁻𔑂 A/cm ².

Anodizing: Forming an Al ₂ O ∝ film in sulfuric acid or chromic acid solution, combined with rare earth salts (such as Ce (NO ∝) ∝) to seal the pores, can reduce the corrosion rate by more than 90% in NaCl solution with a Cl ⁻ concentration of 3.5%.

Metal composite coating

Chemical nickel phosphorus (Ni-P) plating: An amorphous Ni-P coating (thickness 10-20 μ m) is formed by electroless deposition. The best corrosion resistance is achieved when the P content is 10% -12%, and there is no red rust after 1000 hours in the salt spray test. The P element in the coating can inhibit Cl ⁻ adsorption and delay the penetration of corrosive media through the "maze effect".

Zinc aluminum composite coating: A Zn Al alloy layer (such as Zn-11Al) is deposited using hot-dip or electroplating processes, and the sacrificial anode effect of Zn is used to protect the substrate. The Al element forms a dense Al ₂ O3 barrier, and the salt spray resistance life can reach more than 2000 hours.

Organic-inorganic composite coating

Graphene modified epoxy resin: Add 1% -3% graphene nanosheets to the coating to form a "layered barrier network", which extends the diffusion path of Cl ⁻ by 3-5 times. By using silane coupling agents (such as KH-560) to pretreat the interface, the adhesion is improved to over 5 MPa, and the salt spray resistance is three times higher than that of pure epoxy resin.

Polydopamine (PDA) biomimetic coating: Utilizing the self aggregation property of dopamine under alkaline conditions, a 2-5 μ m thick polyphenol layer is formed on the alloy surface, which contains catechol groups that can chelate metal ions and adsorb corrosion inhibitors (such as benzotriazole), achieving a "double locking" of corrosion inhibition.