The aviation accidents at the end of 2024 raised global concerns about flight safety. As aerospace professionals, ensuring quality control is our primary responsibility. Aviation safety relies on operational management and component manufacturing processes. This article delves into the critical surface treatment technologies used in aerospace component manufacturing, covering two core areas: oxidation treatment and anti-corrosion coatings. It discusses their importance in protecting aluminum alloys from various forms of corrosion, ensuring the safety and effectiveness of aerospace components.

I. Introduction to Aerospace Surface Treatment

1. Importance of Surface Treatment in Aerospace Engineering

Surface treatment is a cornerstone of aerospace engineering, which is pivotal in ensuring aerospace components’ durability, safety, and performance. Aluminum alloys, known for their lightweight and strong properties, are widely used in the aerospace industry but face significant corrosion-related challenges, including pitting and stress corrosion cracking. The aerospace industry relies heavily on these technologies to shield aircraft components from the harsh environmental conditions encountered during flight. These conditions include extreme temperatures, high humidity, and significant mechanical stresses, all of which can lead to corrosion, wear, and fatigue. By applying a protective layer to the component’s surface, surface treatment technologies enhance corrosion resistance and maintain the structural integrity of aerospace components, ensuring they can withstand the rigors of flight.

2. Brief Overview of Corrosion Resistance and Its Significance in Aerospace

Corrosion resistance is a vital attribute of aerospace components, directly impacting their safety and operational performance. Aluminum alloys, known for their lightweight and strong properties, are particularly susceptible to various forms of corrosion, making surface treatment technologies crucial for their protection. Corrosion can compromise the structural integrity of components, potentially leading to catastrophic failures. To combat this, aerospace engineers employ various surface treatment technologies designed to bolster the corrosion resistance of aircraft components. Techniques such as anodizing, electroplating, and thermal barrier coatings are commonly used to apply a protective layer to the surface, preventing corrosion and extending the lifespan of the components. These measures are essential for maintaining the reliability and safety of aircraft in service.

3. Historical Context of Surface Treatment Technologies

The evolution of surface treatment technologies in aerospace has a rich history, tracing back to the early days of aviation. Initially, these treatments were rudimentary and aimed primarily at providing essential corrosion protection. However, as the aerospace industry advanced, so did the sophistication of surface treatment technologies. Today, aerospace engineers utilize cutting-edge techniques that offer superior corrosion resistance, wear resistance, and thermal protection. These advancements have been instrumental in enhancing the safety and performance of aircraft components, reflecting the industry’s commitment to continuous improvement and innovation.

II. Surface Oxidation Treatment Technologies

III. Aerospace Metal Finishing and Surface Oxidation Treatment Technologies

Aerospace metal finishing and surface oxidation treatment technologies are pivotal in enhancing the performance and longevity of aerospace components. These technologies involve the application of specialized coatings and surface treatments designed to protect against corrosion, wear, and environmental degradation. By employing these advanced techniques, aerospace engineers can ensure that components maintain their structural integrity and functionality even under the most demanding conditions.

Surface oxidation treatments, such as anodizing and chromic acid, create a robust protective layer on metal surfaces. Anodizing, for instance, is widely used for aluminum components, forming a durable alumina coating that significantly enhances corrosion resistance and wear properties. Similarly, chromic acid treatment is essential for magnesium alloys, providing a protective chromate layer that shields against environmental stressors.

These metal finishing processes are meticulously controlled to meet stringent industry standards, ensuring that each component receives the optimal level of protection. By integrating these technologies into the manufacturing process, the aerospace industry can produce reliable components capable of withstanding the harsh environmental conditions encountered during flight.

I. Aerospace Metal Finishing and Surface Oxidation Treatment Technologies

1. Anodizing Process

Principle: An electrolytic process that generates an oxide layer on metal surfaces, primarily used for aluminum alloys to form an alumina coating.

Process Flow:

• Pre-treatment: Degreasing, cleaning, and acid pickling to ensure a clean substrate.
• Anodizing: In an electrolytic bath, the component acts as the anode, with lead/graphite as the cathode. Key parameters include electrolyte composition (sulfuric/chromic acid), voltage (10-20V), current density (1-3 A/dm²), and time (20-60 minutes).
• Post-treatment:
• Cleaning: Removal of residual electrolytes.
• Sealing: Filling micro-pores using boiling water, nickel salts, or silicate solutions to enhance corrosion resistance (salt spray test ≥500 hours), wear resistance (hardness HV300-500), and insulation (breakdown voltage >200V).

Standards:

• MIL-A-8625 (general aluminum)
• AMS2470 (chromic acid anodizing)
• AMS2472 (sulfuric acid anodizing with dyeing)

2. Chromic Acid Treatment

Principle: As part of a comprehensive metal finishing process, the principle is to form a protective chromate layer through chemical reactions, specifically for magnesium alloys (manganese content < 1.5%).

Features:

• Suitable for cast/forged parts, with a coating thickness of 0.5-3μm.
• Salt spray resistance: 72 hours (unsealed) to 300 hours (sealed).

Limitations: The treated surface is brittle and should avoid further mechanical processing.

3. Electrolytic Treatment

Principle: Surface passivation through electric current in electrolytes is one of the key metal finishing processes, and it is compliant with AMS2476.

Applications: Magnesium alloy components, improving:

• Corrosion resistance (salt spray test ≥500 hours)
• Paint adhesion (cross-cut test ≥4B)
• Wear resistance (Taber abrasion < 50mg/1000 cycles)

Note: The electrolytic coating has a high roughness (Ra >1.6μm) and requires resin sealing or painting.

4. Black Oxide Coating

Principle: Generates a Fe₃O₄ oxide layer (0.5-1.5μm thick) on carbon/alloy steel surfaces.

Properties:

• High oil absorption (oil retention >90%)
• Mild corrosion resistance (24-hour neutral salt spray without red rust)
• Temperature resistance ≤150℃ (compatible with lubricants)

Typical Applications: Gears, bearings, and other oil-immersed parts, compliant with AMS2485, ensuring superior metal finishing.

IV. Anti-Corrosion Coating Technologies

Anti-corrosion coating technologies are integral to aerospace engineering. They provide a critical line of defense against corrosion and environmental degradation. These coatings form a barrier protecting aerospace components from the corrosive effects of moisture, chemicals, and other environmental stressors, extending their service life and maintaining performance.

Zinc chromate primer is one of the most commonly used anti-corrosion coatings in the aerospace industry. It is particularly effective in preventing galvanic corrosion, which occurs when different metals come into contact. The primer offers excellent adhesion and fast drying properties and provides a lubrication effect, reducing the components’ friction and wear.

Phosphate coatings are another essential anti-corrosion technology used in aerospace applications. These coatings create a porous layer on the surface of steel parts, enhancing paint adhesion and reducing mechanical stress. By improving the surface characteristics of the components, phosphate coatings contribute to their overall durability and performance.

By applying these advanced anti-corrosion coatings, aerospace engineers can ensure that components remain protected and functional, even in the most challenging environments.

II. Anti-Corrosion Coating Technologies

1. Zinc Chromate Primer

Function: Zinc chromate primer is crucial in aerospace metal finishing, preventing galvanic corrosion (metal contact with potential difference >0.25V).

Advantages:

• Single-component alkyd base, fast drying (surface drying < 30 minutes)
• Excellent adhesion (cross-cut test 5B)
• Temperature resistance up to 200℃, with lubricating properties (40% reduction in friction coefficient)

Standard: TT-P-1757 series, suitable for pre-coating magnesium alloy bolts.

2. Phosphate Coating Base

Process: Immersion in zinc phosphate solution (85℃, pH 2.5-3.5) for 10-20 minutes.

Effects:

• Forms a porous zinc phosphate layer (1-5μm thick)
• Reduces surface friction coefficient to 0.1-0.15
• Reduces mechanical stress and enhances paint adhesion (pull-off strength >5MPa)

Standard: AMS2480, suitable for steel parts below 300℃.

3. Paint Topcoat Systems

Classification and Properties:

Process Requirements:

• Substrate surface roughness Ra ≤0.8μm (steel parts require sandblasting to Sa2.5 grade)
• Multi-layer coating (primer 20-30μm + topcoat 50-80μm)

V. Emerging Surface Treatment Technologies

Emerging surface treatment technologies are at the forefront of innovation in aerospace engineering. They offer enhanced corrosion protection and durability for aerospace components. These cutting-edge technologies leverage advanced materials and techniques to provide superior performance and reduced maintenance costs, addressing the industry’s evolving needs.

1. Laser Cladding: Process and Applications

Laser cladding is a revolutionary surface treatment technology that uses a high-powered laser beam to fuse a protective layer onto the surface of components. This process creates a metallurgically bonded coating with exceptional corrosion resistance and wear properties. In aerospace engineering, laser cladding enhances the durability of critical components, such as turbine blades and landing gear, which are subjected to extreme mechanical stresses and environmental conditions.

The precision and control offered by laser cladding make it ideal for applications where traditional coating methods may fail. By providing a robust protective layer, laser cladding helps extend the service life of aerospace components, ensuring they can withstand the rigors of flight.

3. Nanotechnology: Role in Corrosion Protection and Surface Treatment

Nanotechnology is transforming the landscape of surface treatment technologies in aerospace engineering. Engineers can develop advanced coatings with unparalleled corrosion protection and durability using nanomaterials. These nanocoatings are designed to provide a uniform and defect-free protective layer, significantly enhancing the performance of aerospace components.

The application of nanotechnology in surface treatments allows for the creation of coatings with unique properties, such as self-healing capabilities and enhanced resistance to environmental stressors. These innovations improve the longevity of components and reduce maintenance costs, making them a valuable addition to the aerospace industry’s arsenal of surface treatment technologies.

In conclusion, the continuous development of emerging surface treatment technologies, such as laser cladding and nanotechnology, drives the aerospace industry toward excellent reliability and efficiency. By embracing these advancements, aerospace professionals can ensure that components remain protected and perform optimally, even in the most demanding conditions.

III. Core Design Guidelines

1. Process Sequence: Core design guidelines for aerospace materials and components include process sequence, coating thickness control, and compatibility testing. Welding/machining must precede surface treatment to avoid damaging protective layers.
2. Coating Thickness Control: Must be specified on drawings (e.g., anodizing layer 10-25μm).
3. Compatibility Testing: Coating systems must pass 2000-hour combined environmental tests (humidity + salt spray + UV).

Conclusion

Surface treatment technologies are critical to aviation safety and various aerospace applications. From anodizing to composite coatings, each process must adhere to strict standards. In the future, with increasing environmental regulations (e.g., replacing hexavalent chromium processes), green surface treatment technologies will become a focus of development. Aerospace professionals must continue to innovate to enhance component reliability and ensure the safety of every flight.