Product Description and Features

Chemical oxidation is a widely used technique in metal surface treatment, primarily employed to enhance corrosion resistance, provide a coating substrate, or achieve specific decorative effects.

Chemical oxidation, as the name suggests, refers to the process where a metal, immersed in a solution containing an oxidizing agent, forms a dense oxide film on its surface through chemical rather than electrochemical means.

1. Core Principle: Controlled Corrosion and Film Formation

Its essence is a controlled corrosion and conversion process of a metal surface in a liquid medium. This process does not rely on an external power source but is driven by chemical reactions between oxidizing agents in the solution and the metal substrate.
Dissolution Process: The metal (represented as M, such as aluminum, iron, magnesium, etc.) loses electrons in the solution medium, oxidizing into metal ions that enter the solution or form oxides.
Film Formation Process: Oxidizing agents in the solution (e.g., sodium nitrite, chromate salts) gain electrons and undergo reduction. Simultaneously, dissolved metal ions react with anions in the solution (e.g., O²⁻, OH⁻, CrO₄²⁻) to form a dense, water-insoluble layer of metal oxide or metal salt conversion coating that adheres firmly to the substrate.
Key point: "Controllable": Unlike uncontrolled corrosion like rusting, chemical oxidation precisely controls the solution's composition, pH, and temperature. This achieves a dynamic equilibrium between the dissolution rate of the metal surface and the formation rate of the oxide film. Once this film reaches a certain thickness, it prevents further contact between the solution and the underlying metal, gradually halting the reaction and forming a uniform, continuous protective layer.

2. Characteristics and Functions of the Oxide Film

Porosity: Unlike anodic oxide films on aluminum, many chemical oxide films are thin and porous, exhibiting excellent adsorption capacity. They serve as superior substrates for paints and dyes, significantly enhancing coating adhesion.

Corrosion Resistance: The oxide film isolates the metal from the external environment, markedly improving the substrate's corrosion resistance.

Decorative Properties: Through formulation adjustments or post-treatment, various colors can be achieved (e.g., blue or black patinas on steel, colored oxides on aluminum).

Friction Reduction: Certain oxide films (such as phosphating films) also provide lubrication and friction-reducing effects.

3. Primary Types (by Base Material)

Chemical Oxidation of Steel (Bluing/Blackening): Performed in high-temperature concentrated alkali solutions, producing a magnetic oxide film primarily composed of Fe₃O₄, appearing blue or black.

Chemical Oxidation of Aluminum and Aluminum Alloys: Primarily conducted in solutions containing chromate or phosphate, forming oxide films containing chromium or phosphorus. This low-temperature, easy-to-operate process is commonly used for complex parts or as a primer for painting.

Chemical Oxidation of Magnesium Alloys: Provides basic corrosion protection and serves as a coating primer.

Chromate Passivation of Zinc: Forms a chromate conversion coating on galvanized surfaces, delivering excellent corrosion resistance and producing iridescent, blue, or black finishes.

Process Requirements for the Chemical Oxidation Line

Although chemical oxidation production lines are relatively simpler than electroplating lines, they still impose strict requirements on process control to ensure the quality and consistency of the oxide film.

Chemical Oxidation Process Requirements (Core Steps)

The process parameters for different types of chemical oxidation vary significantly. The following examples illustrate the most widely applied processes: steel bluing and chromate oxidation of aluminum.

1. Chemical Oxidation of Steel (Bluing)

Solution Composition:

Oxidizing Agent: Sodium nitrite (NaNO₂) or sodium nitrate (NaNO₃).

Main Component: Sodium hydroxide (NaOH), providing a strongly alkaline environment and reaction temperature.

Additives: Certain formulations incorporate trisodium phosphate or similar agents to enhance coating properties.

Process Parameter Control:

Temperature: Extremely critical. Typically requires control within the high-temperature range of 135-145°C. Too low a temperature prevents oxide film formation, while excessively high temperatures result in a loose film with poor adhesion.

Time: Varies between 15-90 minutes depending on steel grade and solution concentration. Film color deepens progressively over time.

Concentration: Regularly analyze and replenish sodium hydroxide and oxidizing agent to maintain total alkalinity and oxidizing agent concentration.

1. Chemical Oxidation of Aluminum and Aluminum Alloys

Solution Composition:

Chromate System: Utilizes chromic acid (CrO₃) or dichromate salts as the primary film-forming agent and oxidizing agent, generating a highly corrosion-resistant chromate conversion coating (yellow or iridescent).

Phosphate-Chromate System: Contains phosphoric acid and chromic acid, forming a green phosphochromate conversion coating. This serves as an excellent primer for subsequent coatings and is the most widely used system.

Process Parameter Control:

Temperature: Typically controlled between 20-60°C (39-140°F), with specific requirements varying by formulation.

Time: Relatively short, generally ranging from 30 seconds to 10 minutes.

pH Value: Critical. Must be strictly controlled within specified ranges using precision pH meters (e.g., pH 1.5–2.5 for phosphate-chromate systems). Minor fluctuations severely impact coating quality and color.

Chemical oxidation lines offer advantages including simple equipment, low cost, high efficiency, and suitability for complex workpieces. They serve as a vital method for enhancing the corrosion resistance, decorative properties, and functionality of metal products.

Product Specifications

Scope of Application

Applications of Aluminum Alloy Chemical Oxidation (Conversion Coatings)

This is the most widely applied field, primarily divided into chromium-containing chromate conversion and environmentally friendly chromium-free conversion.

Applications of Chemical Oxidation of Steel (Bluing/Blackening)

This process generates a thin film of magnetic iron oxide (Fe₃O₄) on the steel surface, appearing blue or black.