Aluminum Materials Surface Treatment: Complete Introduction

 

Surface Treatment

It is a process that uses mechanical and chemical methods to create a protective layer on the surface of a product. This layer stabilizes in natural conditions, enhances corrosion resistance, improves aesthetics, and increases the value of the product. The choice of surface treatment depends on the environmental conditions, product lifespan, aesthetic considerations, and economic value.

 

The surface treatment process includes pre-treatment, film formation, post-film treatment, packaging, storage, and shipment. Pre-treatment involves mechanical and chemical processes.

 

Mechanical processes include shot blasting, sanding, polishing, waxing, and other procedures. The purpose of mechanical treatment is to eliminate surface irregularities and rectify other aesthetic defects. Chemical treatment removes oil and rust from the product's surface, forming a layer that enhances the bonding of the film-forming substance or reacts with active metal, ensuring the coating's stability and increasing the protective layer's binding force.

 

 

Section 1: Chromizing

 

Chromizing forms a chemically converted film on the product's surface, with a thickness ranging from 0.5-4um. This conversion film has good adsorption properties and serves as an excellent base for coatings. It comes in colors like gold, natural aluminum, and green. Chromizing provides good conductivity, making it a preferred option for electronic products like conductive strips in mobile phone batteries and magnetic devices. While suitable for all aluminum and aluminum alloy products, the soft nature of the conversion film makes it less suitable for external components.

 

Chromizing Process:

Degreasing -> Deoxidizing with aluminum acid -> Chromizing -> Packaging -> Storage

 

Chromizing is suitable for aluminum, aluminum alloy, magnesium, and magnesium alloy products.

 

Quality Requirements:

 

Uniform color, fine film, no scratches or damage; no roughness, dust when touched.

Film thickness: 0.3-4um.

 

Section 2: Anodizing

 

Anodizing forms a uniform, dense oxide layer (Al2O3·6H2O, commonly known as aluminum jade) on the product's surface, providing a hardness of 200-300HV. Hard anodizing can achieve a surface hardness of 400-1200HV, making it essential for surface treatment in hydraulic cylinders, transmissions, and other critical applications. Anodizing improves wear resistance, making it suitable for aerospace-related products.

 

Anodizing differs from hard anodizing in that it allows coloring, offering better decorative options. Common processes include brushed matte natural color, brushed bright natural color, brushed bright coloration, and matte brushed coloration (can be dyed in any color scheme). Anodizing process: degreasing -> alkali etching -> etching and polishing -> neutralization -> sealing -> anodizing -> coloring -> sealing -> hot water wash -> drying.

 

Common Quality Anomalies and Judgments:

 

A. Surface spotting: Typically caused by poor metal tempering or poor material quality. Remedies include re-tempering or material replacement.

B. Rainbow colors: Usually a result of anodizing errors, such as loose hanging during processing, leading to poor electrical conductivity. Remedies involve re-anodizing after fixing the electrical connection.

C. Surface damage and scratches: Often due to carelessness during transportation or processing. Remedies include re-anodizing and polishing.

D. White spots during coloring: Typically caused by oil or impurities in the water during anodizing.

 

Quality Standards:

 

Film thickness: 5-25um, hardness above 200HV, sealing experiment color change rate less than 5%.

Salt spray test for at least 36 hours, reaching level 9 or above according to CNS standards.

No scratches, scuffs, iridescence, or other defects on the surface. No hanging points or yellowing.

 

Section 3: Electroplating of Aluminum

 

Advantages of Aluminum and Aluminum Alloys:

Good conductivity, rapid heat transfer, lightweight, and easy to shape. However, they have drawbacks such as low hardness, poor wear resistance, susceptibility to intergranular corrosion, and difficulty in welding. Electroplating addresses these issues in modern industry.

 

Advantages of Aluminum Electroplating:

1) Improved decoration

2) Increased surface hardness and wear resistance

3) Reduced friction coefficient, improved lubrication

4) Enhanced surface conductivity

5) Improved corrosion resistance (including with other metal combinations)

6) Easy to weld

7) Improved adhesion during hot pressing with rubber

8) Increased reflectivity

9) Repaired dimensional tolerances

 

Electroplating Process for Aluminum:

Degreasing -> Alkali etching -> Activation -> Zinc displacement -> Activation -> Electroplating (e.g., nickel, zinc, copper) -> Chromium plating or passivation -> Drying

 

Common Electroplating Coatings:

Nickel plating (pearl nickel, sand nickel, black nickel), silver plating (surface silver, thick silver), gold plating, zinc plating (color zinc, black zinc, blue zinc), copper plating (cyanide copper, acid copper), chromium plating (bright chrome, hard chrome, black chrome), etc.

 

Common Uses of Electroplating Coatings:

Black coatings, such as black zinc and black nickel, are used in optical electronics and medical devices.

Gold and silver are excellent conductors used in electronic products. They also enhance high-end decorative properties, but they are relatively expensive.

Copper, nickel, and chromium are popular mixed coatings with widespread use in terms of decoration and corrosion resistance. They are cost-effective and find applications in sports equipment, lighting, and the majority of the electronics industry.

Among copper-based coatings, white tin-copper is an environmentally friendly coating developed in the 1970s and is preferred in the jewelry industry. Bronze (lead-tin-copper can be used as a gold substitute, making it a good decorative coating), but copper has relatively poor anti-tarnishing properties, leading to slow development.

Zinc electroplating: The zinc layer appears bluish-white and is soluble in acid and alkali. Due to zinc's lower standard electrode potential than iron, it provides reliable electrochemical protection for steel and iron products used in industrial atmospheres and marine atmospheres.

Hard chrome: Chromium-plated layers deposited under certain conditions have high hardness and wear resistance, reaching HV900-1200kg/mm. Chromium is the hardest among common coatings and can enhance the wear resistance of parts, extending their service life. It is indispensable for cylinders, hydraulic systems, and transmissions.

 

Common Anomalies and Improvement Measures:

Peeling: a. Poor zinc displacement, too long or too short time, improvement measures involve adjusting displacement time, bath temperature, and concentration. b. Insufficient activation, change activation method. c. Incomplete pre-treatment, surface with oil, strengthen pre-treatment.

Surface roughness: a. Improper amount of electroplating brightener, softener, and leveler, readjust the amounts. b. The surface of the body itself is rough, re-polish before electroplating.

Surface yellowing is due to incorrect current distribution trends, change the hanging method, add an appropriate amount of leveling agent.

Surface tooth loss: Electroplating solution is too dirty, strengthen filtration, perform appropriate bath treatment.

 

Quality Requirements:

No discoloration, pinholes, burrs, foaming, scratches, or other defects on the surface.

Film thickness above 15um, salt spray test for 48 hours, meeting military standards level 9 or above, potential difference in the range of 130-150mv.

Adhesion can pass a 60-degree bending test.

For products in special environments, appropriate modifications can be made.

 

Operational Precautions for Electroplating Aluminum and Aluminum Alloys:

Aluminum parts for electroplating should use aluminum alloy as hanging fixtures.

Processes after aluminum and aluminum alloy have been etched must be performed rapidly, with minimal intervals between procedures to prevent re-oxidation.

The second zinc displacement time should not be too long to avoid over-corrosion.

Ensure thorough rinsing during water washing.

Guard against power interruptions during electroplating.

 

 

Section 4: Painting of Aluminum Materials

 

Painting includes dipping, spraying, pouring, rolling, etc., with dipping and spraying being the main methods. Dipping includes electrophoretic painting, which uses electrochemical methods to deposit organic resin particles on the surface of the workpiece, forming transparent or various colored organic coatings. Among them, cathodic electrophoresis, developed in the 1970s, is a new process and a major technique in the painting industry. It features excellent corrosion resistance, strong colorfastness, and good adhesion. Spraying includes powder coating and liquid coating, with electrostatic coating being a popular process in the industry.

 

Painting Process Range:

Due to its good corrosion resistance, affordable cost, and a wide range of colors (can be in various colors), painting is suitable for all aluminum material types. However, for materials with a loose structure, such as sand-cast products, dusting work is needed, and the relative quality may be lower.

 

Electrostatic Coating Working Principle:

Under the action of a pump, the coating enters the electric field, and the coating particles carry a negative charge. With the pressure of atomizing air, the coating becomes finer and more uniform. Then, under the action of working air pressure, the coating is evenly sprayed onto the surface of the workpiece through the action of a rotating cup or nozzle. Since the distance between the spray gun and the workpiece is equal on the same vertical plane, and the electric field intensity at the same interface is the same, positive and negative charges attract each other, resulting in a uniform layer of organic resin coating on the surface of the workpiece. Subsequently, under the conditions of high-temperature baking, the resin reaches a state of leveling and curing. It firmly embeds in the surface of the workpiece, achieving the purpose of protecting the workpiece.

 

Aluminum Painting Process:

Mechanical grinding -> Degreasing -> Deoxidizing -> Chromizing -> Powder or liquid spraying -> Baking -> Hanging -> Inspection -> Packaging -> Storag

 

1）Common abnormalities in powder and liquid operations and their solutions:

 

 

 

2）Quality standard reference

 

 

 

Section Five: Chemical Plating of Aluminum

 

1) Principles of Chemical Plating:

 

Chemical plating is a method of depositing metal on the surface of a part through a redox reaction in the same solution, using a reducing agent, without the need for external current. Therefore, chemical plating can be described as a controllable, self-catalytic chemical reduction process for depositing metal. The general reaction formula is:

 

2) Characteristics of Chemical Plating:

 

With industrial development, chemical plating has become a promising technological process. Compared with other coating methods, chemical plating possesses the following characteristics:

 

It can plate metal on parts made of various materials, including metals, semiconductors, and non-conductive materials.

Regardless of the complexity of the part's geometry, uniform coating thickness can be achieved wherever the solution contacts.

For self-catalytic chemical plating, significant coating thickness can be obtained, and even electroforming is possible.

No external current is required.

The coating is dense with few pores.

The coating often exhibits special chemical and mechanical properties.

As chemical plating offers numerous advantages, it has gained rapid development in the industrial and electronic sectors. The following focuses on chemical nickel plating as an example:

 

3) Chemical Nickel Plating:

 

Chemical nickel plating uses sodium hypophosphite as a reducing agent, resulting in a nickel-phosphorus alloy coating. Depending on the phosphorus content, it can be classified into low phosphorus (1%-4%), medium phosphorus (4%-10%), and high phosphorus (10%-12%) alloys.

Different phosphorus content coatings can be obtained from plating solutions with varying pH values. Medium and high phosphorus alloys can be obtained at pH=4-5, while low and medium phosphorus alloys can be obtained at pH=8-10 in a weak alkaline solution.

Alloys with phosphorus content above 8% are non-crystalline coatings, exhibiting excellent corrosion resistance due to the absence of grain boundaries. Through heat treatment at 300-400 degrees, they become a mixture of non-crystalline and crystalline phases, achieving a hardness of HV=1150. Chemical nickel plating, after heat treatment, approaches the hardness of hard chrome and is an ideal alternative coating, applicable to all aluminum and aluminum alloy products, including forging, die-casting, and aluminum profiles.

Drawback: The coating has high brittleness, but heat-treated products show significantly improved ductility (generally after heat treatment above 400 degrees for four hours, the ductility improves significantly).

 

4) Chemical Nickel Plating Process for Aluminum:

 

Ultrasonic degreasing—> Deoxidization—> First zinc displacement—> Activation—> Second zinc displacement—> Chemical nickel plating—> Passivation—> Ultrasonic cleaning—> Baking—> Full inspection—> Packaging

Removal of Defective Coatings:

 

Chemical nickel coatings on steel, aluminum, copper, plastic, etc., can be removed using concentrated nitric acid. However, parts must be introduced into the tank after drying, as introducing moisture will lead to corrosion of the base metal. Operating temperature should be below 35 degrees.

 

5)Quality Requirements:

 

a. Coating thickness in the range of 5-30um.

b. Salt spray test lasting more than 24 hours, meeting CNS standard 9 or above.

c. Passes the 60-degree bending test without peeling.

d. No surface defects such as dents, scratches, water marks, etc.

e. Uniform coating without fogging, white spots, or exposed plating.

 

 

Common exception handling

 

 

 

Section Six: Classification and Selection of Coating Usage Conditions

 

Considering the requirements for corrosion resistance, the surface treatment design should take the following into account:

 

Precious metals (gold, platinum), stainless steel with chromium content above 18%, magnetic alloy materials, and nickel-copper alloys generally do not require additional protective layers.

 

Components made of carbon steel, low-alloy steel, and cast iron are prone to corrosion in the atmosphere and should be coated with a protective layer.

 

Parts made of copper and copper alloys can be protected using bright acid pickling, passivation, electroplating, or paint, depending on different usage conditions. Precision parts made of phosphor bronze or beryllium bronze may not require surface treatment.

 

Parts made of aluminum and aluminum alloys can undergo anodization and sealing treatment. For small parts unsuitable for anodization, chemical oxidation treatment can be applied. Cast aluminum alloys can be protected with paint.

 

Parts made of zinc alloys can be protected using phosphating, passivation, electroplating, or paint.

 

Section Seven: Electrophoretic Treatment of Aluminum

 

Colorful electrophoretic coating for aluminum is an innovative surface treatment process. It employs an electrochemical method to deposit colloidal particles of organic resin on components, forming a transparent or variously colored organic coating. Depending on the charged state of resin particle ionization in electrophoretic paint, it can be classified into anodic electrophoresis (resin particles ionize into negative ions) and cathodic electrophoresis (resin particles ionize into positive ions).

 

The electrophoretic coating exhibits excellent corrosion resistance (able to withstand over 400 hours of neutral salt spray test), strong colorfastness, good adhesion to the base metal for various machining processes, and vivid colors customizable as per user requirements (common colors include gold, coffee, gunmetal, black, etc.). Compared to paint, it has better construction performance, causing minimal pollution and harm to the environment. Therefore, it finds applications in automotive shells and accessories, bicycle handlebars and accessories, various daily hardware accessories, furniture, crafts, etc.

 

As cathodic electrophoresis is more advanced, we will focus on introducing it:

 

Cathodic electrophoretic coating is a complex electrochemical and colloid chemical process. Electrophoretic paint itself is a multi-component system of colloid and suspension, consisting of dispersed phase (resin, pigment particles) and continuous phase (water). There are four processes in cathodic electrophoresis:

 

Electrophoresis: Positively charged water-soluble resin particles and their adsorbed pigment move toward the cathode.

 

Electrodeposition: Positively charged resin particles reach the surface of the part (cathode) and discharge, forming an insoluble deposit layer, which turns into a film after baking.

 

Electro-osmosis: Moisture seeps out from the deposit layer. Baking occurs when the water content drops to 5%-15%.

 

Electrolysis of Water: Direct current electrolysis of water releases hydrogen and oxygen. Since electrolysis reduces penetration, affects the appearance of the paint film, decreases adhesion of the paint film, and increases power consumption, weakening water electrolysis is necessary.

 

Electrophoretic Process Flow:

 

Degreasing -> Oxide Removal -> Chromating -> Electrophoresis -> Drying -> Packaging

 

 

Quality Standards:

 

Visual Inspection: No color difference, orange peel, pinholes, rainbow effects, impact damage, scratches, or other abnormalities.

Film Thickness: Film thickness should be within the range of 40-60 micrometers.

Salt Spray Test: The coating must withstand a salt spray test for at least 400 hours, meeting or exceeding CNS standards at level 9 or above.

Bend Test: The coating should pass a 60-degree bend test without exhibiting peeling.

Additional Tests: Both the cross-cut adhesion test (hundred grid test) and the falling ball impact test should meet the relevant national standards.