What matters should be paid attention to in the process of manufacturing anodized aluminum plates?

The manufacturing of anodic aluminum oxide plate is a complex process that combines electrochemistry, machining and surface treatment. It is necessary to strictly control the parameters of each link to ensure the product performance (such as corrosion resistance, wear resistance and appearance consistency) and production safety. The following five key links: pretreatment, anodic oxidation core process, post-treatment, machining, safety and environmental protection, explain in detail the matters needing attention:

1. Pretreatment: Basic cleaning and surface regulation (determines the adhesion of oxide layer)

The core of pretreatment is to remove the oil stain, oxide film and impurities on the surface of the plate, ensure that the base metal (aluminum and aluminum alloy) is exposed and pure, and lay the foundation for the uniform growth of the subsequent oxide layer. Pay attention to the following three points:

Degreasing thoroughness

Appropriate degreasers (alkaline degreasers and solvent degreasers) should be selected according to the types of oil stains (such as stamping oil and cutting oil) to avoid "bubbling" and "pinholes" of oxide layer caused by residual oil stains.

After degreasing, it must be thoroughly cleaned with deionized water (at least 2-3 times) to prevent the degreasing agent residue from reacting with the subsequent acid etching process and causing surface spots.

Parameter control of pickling/alkali etching

Pickling (usually mixed solution of nitric acid and hydrofluoric acid) mainly removes natural oxide film, and the concentration (10%-20% of nitric acid and 1%-3% of hydrofluoric acid) and temperature (20-30℃) should be controlled to avoid over-corrosion of aluminum matrix and rough surface caused by too high acidity or too long time.

Alkaline etching (commonly used sodium hydroxide solution) is used to eliminate machining traces and refine the surface. The alkali concentration should be controlled at 30-50g/L and the temperature should be ≤60℃, otherwise it will easily lead to "over-alkali etching" of the plate, and the surface will be dark or the size will be out of tolerance.

Light emission and activation

After alkali etching, nitric acid should be used to "shine" to remove the residual aluminum hydroxide film on the surface and make the surface appear uniform silvery white; The activation process (such as soaking in dilute sulfuric acid) needs to ensure the time (1-3 minutes) to avoid the reduction of the bonding force of the oxide layer caused by insufficient activation.

Second, the core process of anodic oxidation: the key control of oxide layer growth (determining the core performance of the product)

Anodizing is the process of forming Al₂O₃ oxide film on aluminum surface by electrolysis, and it is necessary to control electrolyte parameters, electrolysis conditions and oxide film thickness to avoid oxide layer defects.

Electrolyte formula and state

The commonly used electrolyte is sulfuric acid solution (15%-20% concentration), and the impurity content should be strictly controlled:

Avoid heavy metal ions such as Fe³⁺ and Cu²⁺ exceeding the standard (≤50mg/L), otherwise the oxide film will be unevenly dyed and black spots will appear;

Filter the electrolyte regularly to remove aluminum ion (Al) accumulation (controlled at ≤20g/L) to prevent the growth rate of oxide film from decreasing and the surface from being rough.

Precise control of electrolytic parameters

Matters needing attention in parameter control range

Temperature 18-22℃ (sulfuric acid system) is too high → the dissolution speed of oxide film is accelerated, the film becomes thinner and the hardness decreases; The temperature is too low → the film is brittle and easy to crack. A constant temperature cooling system is required.

The current density is 1-2a/dm, the current is too high → the film grows too fast, the internal stress is large, and it is easy to appear "ablation" (the surface is black and bulging); The current is too low → the film is dense, but the growth is slow and the efficiency is low.

The oxidation time of 30-60 minutes (according to the film thickness requirements) needs to be adjusted according to the target film thickness (conventional decorative film thickness of 5-15μm, functional film thickness of 20-50μm), and the time is insufficient → the film thickness is insufficient, resulting in poor corrosion resistance.

Design of polar plate and hanger

The hanger should be in close contact with the aluminum plate (avoid virtual connection) to prevent "local no film" or "edge burning" caused by concentrated current at the contact;

Anode (aluminum plate) and cathode (lead plate or stainless steel plate) should be arranged in parallel, with a uniform distance (10-15cm) to avoid the difference in film thickness caused by uneven convection of electrolyte.

Third, post-treatment: hole sealing and dyeing (improving appearance and durability)

The core of post-treatment is to seal the micropores of the oxide film to prevent the invasion of impurities, and to achieve the appearance requirements through dyeing. The following two points should be noted:

Sealing quality control

Commonly used hot water sealing (deionized water at 95-100℃ for 20-30 minutes) or "nickel salt sealing" (normal temperature, suitable for thick film):

Hot water sealing needs to ensure that the water temperature is ≥95℃, otherwise the micropore sealing is not complete, which is easy to absorb dirt and uneven dyeing;

After sealing the hole, it needs to be washed with cold water to remove the residual hole sealing agent on the surface and prevent the surface from "white spots".

Dyeing uniformity and fastness

Before dyeing, it is necessary to ensure that the pores of the oxide film are clean (no residual electrolyte), otherwise "color spots" will easily appear;

The pH value (4-6) and temperature (20-30℃) of acid dyes (commonly used) should be controlled, and the dyeing time should be adjusted according to the requirements of color depth (5-20 minutes) to avoid dye decomposition caused by too high temperature or dark color caused by too long time;

Immediately after dyeing, the hole should be sealed to prevent dye loss and ensure the color fastness (sun resistance and friction resistance).

4. Mechanical processing: avoid oxide damage (for the scene of "oxidation first and then processing")

The mechanical processing of anodized aluminum plate can be divided into "processing first and then oxidizing" (conventional process, suitable for complex shapes) and "oxidizing first and then processing" (suitable for high-precision surfaces). If you choose the latter, you should pay attention to:

Tool and cutting parameters

Choose cemented carbide tools (such as WC-Co alloy) to avoid the rapid wear of high-speed steel tools, resulting in rough machining surface;

The cutting speed is controlled at 100-200m/min and the feed rate is 0.1-0.2mm/r, so as to avoid the oxide film from "edge collapse" or falling off due to excessive cutting force.

Surface protection after machining

After machining, compressed air should be used to blow clean the surface chips to avoid the chips from scratching the oxide film;

If subsequent assembly is required, it is necessary to avoid striking the surface of the oxide layer with sharp tools to prevent local film damage.

V. Safety and Environmental Protection: Compliance and Personal Protection

The anodic oxidation process involves strong acid (sulfuric acid, nitric acid), high temperature and high voltage, and the safety and environmental protection requirements should be strictly observed:

Personnel safety protection

Wear acid-resistant gloves, protective glasses and anti-corrosion apron during operation to avoid electrolyte contact with skin (sulfuric acid can cause chemical burns);

Insulating pads should be set around the electrolyzer to prevent electric shock; It is necessary to prevent burns when sealing holes at high temperature.

Environmental compliance treatment

Wastewater from degreasing, pickling and oxidation processes should be treated by classification (such as neutralizing acidic wastewater to pH 6-9 and removing heavy metal ions), and then discharged after reaching the standard;

Waste gas (such as hydrofluoric acid fog during pickling) needs to be treated by gas collecting hood+spray tower to avoid unorganized discharge;

Waste aluminum slag and waste cathode plates need to be classified and recycled to avoid solid waste pollution.

summary

The processing and manufacturing of anodic alumina plate should focus on the logic of "surface cleaning → uniform film growth → micropore sealing → mechanical protection", strictly control the parameters of each link (concentration, temperature, current and time), and give consideration to safety and environmental protection. The negligence of any link (such as incomplete degreasing, temperature fluctuation and insufficient hole sealing) will lead to product defects, which will affect its corrosion resistance, appearance consistency and service life.