Copper and its alloy castings are widely used in fields such as mechanical manufacturing, electronic and electrical engineering, and building decoration due to their excellent conductivity, thermal conductivity, and mechanical properties. However, copper is prone to oxidation and discoloration in natural environments, which affects the appearance quality and functional characteristics of products.

1、 Mechanism analysis of oxidation discoloration of copper castings

1. Chemical properties of copper

Copper is a chemically active metallic element, located in Group 11 of the periodic table with an atomic number of 29. The standard electrode potential of copper is+0.34V, indicating that it has a certain degree of reducibility and is prone to losing electrons and undergoing oxidation reactions. Copper is relatively stable in dry air; But in humid or corrosive environments, copper quickly reacts with oxygen, water vapor, carbon dioxide, sulfides, etc., producing various oxides, sulfides, and carbonates.

2. Oxidation discoloration process

The oxidation discoloration of copper is a complex electrochemical process that typically goes through the following stages:

Initial oxidation: Copper surface reacts with oxygen to form a red cuprous oxide (Cu ₂ O) thin film

Further oxidation: Copper oxide continues to oxidize to form black copper oxide (CuO)

Environmental corrosion: Formation of black copper sulfide (CuS) or brownish black cuprous sulfide (Cu ₂ S) in sulfur-containing environments

Composite corrosion: Green basic copper carbonate [CuCO ∝· Cu (OH) ₂], also known as copper green, is generated in a humid environment containing carbon dioxide

3. Factors affecting oxidation rate

Environmental humidity: Relative humidity exceeding 60% will accelerate corrosion

Temperature: High temperature promotes oxidation reaction

Pollutants: SO ₂, H ₂ S, Cl ⁻ and other pollutants greatly accelerate corrosion

Alloy composition: Alloy elements such as zinc and nickel affect oxidation behavior

Surface condition: Rough surfaces are more prone to corrosion than smooth surfaces

2、 Practical methods for preventing oxidation and discoloration of copper castings

1. Alloying treatment

By adjusting the alloy composition, the oxidation resistance of copper can be improved:

Adding aluminum (Al): forming a dense Al ₂ O ∝ protective film

Adding nickel (Ni): Improving the corrosion resistance of alloys, commonly using white copper (BZn15-20)

Adding tin (Sn): Bronze (Sn6-9%) has good corrosion resistance

Adding Zinc (Zn): Brass (Zn20-45%) has low cost and good oxidation resistance

2. Surface treatment technology

(1) Chemical passivation

Benzotriazole (BTA) treatment: Formation of Cu (I) - BTA polymer protective film

Chromate passivation: forms a composite film containing Cr ₂ O ∝, but has poor environmental friendliness

Organosilane treatment: forming a protective layer of Si-O-Si network structure

(2) Electrochemical protection

Electroplated nickel plating: 5-10 μ m nickel layer provides a good barrier

Electroplated tin plating: low cost and good weldability

Silver electroplating: high-end application, high cost

(3) Physical protection

Coating protective paint: organic coatings such as acrylic and polyurethane

Vapor deposition: PVD coating technology for preparing metal films such as Al and Cr

Hot dip tin plating: forming an alloy diffusion layer

3. Environmental control measures

Storage environment: Control RH<40%, temperature 15-25 ℃

Packaging method: Use VCI (vapor phase inhibitor) rust proof paper

Deoxygenation treatment: nitrogen filling or vacuum packaging

Desiccant: Place moisture absorbing materials such as silicone gel

4. Production process optimization

Rapid cooling after casting reduces high-temperature oxidation

Adopting oxygen free copper process (TU1, TU2)

Precision casting reduces surface defects

Timely remove residual release agent on the surface

3、 Selection of protection schemes for different application scenarios

1. Electronic and electrical components

Preferred method: Nickel plating+organic protective film

Process parameters: Nickel layer thickness of 8-12 μ m, BTA concentration of 0.5-1.0%

Attention: Ensure conductivity and solderability

2. Architectural decorative components

Preferred method: chemical coloring+varnish protection

Process parameters: Copper green artificial coloring, polyurethane topcoat

Attention: High weather resistance requirements

3. Mechanical components

Preferred method: hard chrome plating or hot-dip tin plating

Process parameters: chromium layer thickness 20-50 μ m

Attention: Consider wear resistance

4. Art Casting

Preferred method: Microcrystalline wax protection

Process parameters: High temperature wax immersion process

Attention: Maintain the original color and texture

4、 Quality inspection and maintenance

1. Testing method

Salt spray test: evaluation of corrosion resistance performance

Electrochemical testing: measuring polarization resistance

Film thickness measurement: X-ray or eddy current method

Adhesion test: grid method or tensile method

2. Maintenance suggestions

Regular cleaning: use neutral cleaning agents

Avoid mechanical damage: prevent damage to the protective layer

Environmental monitoring: Control warehouse temperature and humidity

Regular inspection: quarterly sampling testing

5、 The development trend of new protective technologies

Nanocomposite coating: Graphene reinforced protective film

Self repairing coating: microencapsulated corrosion inhibitor technology

Environmental passivator: rare earth element conversion film

Intelligent Packaging: Humidity Indicator VCI Material

Surface Nanotechnology: Surface Mechanical Grinding Treatment (SMAT)

The anti oxidation discoloration of copper castings is a systematic engineering that requires comprehensive consideration from multiple aspects such as material selection, process design, surface treatment, and environmental control. With the increasing demand for environmental protection and the development of new technologies, copper casting protection technology is moving towards high efficiency, environmental protection, and intelligence. Production enterprises should choose suitable protective solutions based on product usage, cost budget, and environmental requirements to ensure that copper castings maintain stable appearance and performance over the long term.