What Is Anodizing?

Anodizing is a commonly used surface treatment in the process. Anodizing is actually an electrochemical process of a metal or alloy. Generally speaking, people often use metals or alloys as anodes. We take aluminum and its alloys as an example. During anodization, an oxide film is formed on aluminum products during the process of electrolysis.

The metals commonly used in are aluminum alloys, followed by magnesium alloys, and titanium alloys. Different electrolytes are used for different materials. According to the classification of electrolyte, anodizing is divided into Chromic acid anodizing (type I), Sulfuric acid anodizing (type II), and Oxalic acid anodizing.

Detailed anodizing types:

Sulphuric Acid Anodizing

First, let’s introduce sulfuric acid anodizing, the most commonly used oxidation form of sulfuric acid anodizing. The following is the direct current, the process parameters of sulfuric acid anodizing in general:

Craft Project	Indicator range	explanation
Oxidation voltage	12-18V	As the voltage decreases, the film thickness and hardness decrease.
current density	1-1.6A/dm³	When the current density is too high, the surface adhesion of the profile will decrease, and the corrosion resistance and wear resistance will decrease, resulting in poor sealing effect.
Bath Solution temperature	18-22℃	When the temperature of the Bath Solution increases, the film thickness and hardness decrease, and the surface adhesion increases.
Sulfuric acid concentration	15-200g/L	When the concentration of sulfuric acid decreases, the adhesion of the profile surface decreases, while the film thickness and hardness increase.
Aluminum ion concentration	＜20g/L	When the concentration of aluminum ions is high, the adsorption capacity of the oxide film decreases, and white spots appear on the surface of industrial aluminum profiles.
Oxidation time	20-30min	When the oxidation time decreases, the corrosion resistance and adsorption capacity decrease, and the hardness increases.

Sulfuric acid anodizing (5-20um) is easier to dye and has higher hardness. It is the most widely used surface treatment method, and is also the main decoration and protection method for aluminum and alloys. Its process is simpler and the operation is convenient.

Chromic Acid Anodizing

This is a more traditional process, formulated from a single chromic acid. Compared with sulfuric acid anodizing, the oxide film of chromic acid anodizing is thinner. Although the wear resistance is slightly lower than that of sulfuric acid anodizing, it can maintain the accuracy of the original metal parts. The following are the process parameters of chromic acid anodizing under direct current.

Polished parts with small dimensional deviations		General parts or weldments		explanation
Craft Project	Indicator range	Craft Project	Indicator range	Using the chromic acid anodizing method with a concentration of 3-10%, the obtained oxide film is only 2-5μm, which is thinner than sulfuric acid anodizing, and can maintain the tensile strength of the original material, but it has poor wear resistance and is not easy to dye. It is suitable for machining parts, sheet metal parts with small dimensional deviation, and difficult castings that are difficult to process with sulfuric acid. For industrial aluminum products with copper content greater than 4%, it is not suitable to use chromic acid anodizing treatment.
Chromic anhydride CrO₃ concentration	0-35g/L	Chromic anhydride CrO₃ concentration	95-100g/L
Oxidation voltage	0-40V	Oxidation voltage	0-40V
current density	0.2-0.6A/dm³	current density	0.3-0.5A/dm³
Bath Solution temperature	40±2℃	Bath Solution temperature	37±2℃
Oxidation time	60min	Oxidation time	35min
Cathode material	stereotype or graphite	Cathode material	stereotype or graphite
pH	0.65-0.8	pH	＜0.8
Cathode and anode area ratio	3:1

The oxide film of chromic acid anodizing (2-5um) is opaque and low in porosity, which can keep the original precision and surface roughness of the parts, and the film is thinner, which is more suitable for precision parts.

Oxalic Acid Anodizing

Oxalic acid anodizing can obtain a thicker film layer. However, due to the high cost and power consumption, it is only used under some special conditions, such as electrical insulating protective film, etc. The following are the process parameters of oxalic acid anodizing.

Craft Project	Indicator range	explanation
Oxidation voltage	50-65V	Oxalic acid anodizing can use direct current, alternating current or superposition of direct current and alternating current, but direct current is prone to film pitting corrosion. The thickness and color of the films obtained by oxidation vary.
current density	DC1-1.5A/dm³
Bath Solution temperature	30℃
Oxalic acid concentration	5-10%
Oxidation time	10-30min
film thickness	15μm

Oxalic acid anodizing (8-20um, up to 60um thick) is easy to produce thicker film layers, with higher hardness, low porosity, and high corrosion resistance. It can be used as a good insulating part, but the cost is high, so it is generally used Surface treatment for special requirements.

What is the general principle of anodizing?

Metal products such as aluminum alloys are placed in the electrolyte solution as anodes
The process of forming an oxide film on the surface through electrolysis
The cathode in this device is a material with high chemical stability in the electrolytic solution
The principle is actually the principle of water electrolysis
When current is passed, hydrogen gas is released from the cathode
The oxygen evolved on the anode includes atomic oxygen and ionic oxygen
Aluminum as anode is oxidized by oxygen to form anhydrous aluminum oxide film

How does anodizing work in practice?

（If the original color of the oxide film is used, the step of adding color is not required）

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Surface Pretreatment

Before anodizing, some treatments are actually done on the surface, such as Unoil, alkali washing, pickling, polishing, etc. Unoil , also known as degreasing, is to remove the oil stains on the surface of the workpiece. During this process, be careful not to collide with the barrel tank. alkali washing is to remove the oxide layer on the surface and expose the metal body, and a degreasing agent is generally used.

Pickling is to remove the dirt on the surface, using nitric acid, put it in the pickling tank at room temperature. Polishing is to add shine, phosphoric acid is used for this step, and temperatures are as high as 92°C.

Anodizing

After the above steps are completed, anodization can be carried out to convert the surface into an oxide film, which has certain properties.

Add Color

Coloring is to meet customer’s color requirements. The process of anodizing and coloring is completed in the same solution, and a colored oxide film can be formed directly on the metal. This process is called electrolytic coloration. caused by reflection.Material composition, solution type, oxide film thickness, and operating conditions have important effects on electrolytic color development.

Electrolytic coloring is to place the metal to be colored in the electrolyte as an electrode. When the current passes through, the metal particles, metal oxides or the mixture of metal particles and oxides are electrolyzed, and then deposited on the surface of the metal to form a color.

Hole Sealing Treatment

Sealing is also an important step in anodic oxidation, which can reduce the porosity and adsorption capacity of the oxide film, seal the fuel in the micropores, and improve the corrosion resistance and wear resistance of the oxide film. Sealing holes include heat sealing holes, cold sealing holes, medium temperature sealing holes and organic sealing holes.

The heat sealing hole has a high sealing effect. It can be boiling water sealing or high temperature steam sealing. The advantage of high temperature steam sealing is that the speed is relatively fast and the risk of fading is small.

Cold sealing is the most common and most basic sealing technology. It can be operated at room temperature, so it can also be called normal temperature sealing, and the time is shorter than that of heat sealing.

The medium temperature sealing is between the hot sealing and the cold sealing. The temperature requirement of the medium temperature sealing is not high, the energy consumption is also low, the efficiency is also good, and the sealing effect is also good, and the main sealing agent is inorganic salt, which is stable Good performance, easier to control, and more environmentally friendly.

Organic sealing is to coat an organic polymer coating on the oxide film, and electrophoresis is the best example of organic sealing.

Drying Treatment

During the anodizing process, the metal needs to be immersed in various solutions and washed several times, so it needs to be dried before it can be packaged and put into storage. Drying includes natural drying, backing dry, and blow-drying.

Natural drying is natural drying, which is more suitable for summer. Backing dry is a commonly used drying method, suitable for winter or humid weather. Backing dry is divided into low temperature drying, medium temperature drying and high temperature drying. The oxidized aluminum is more suitable for low temperature drying, and high temperature is more suitable for electrophoresis, spraying etc. Blow-drying is blowing against one end of the metal, which is suitable for various surface treatments.

What should we pay attention to when anodizing?

Check the electrical conductivity before anodizing, and clean the fixtures that have been used before to avoid the residual oxide film on the furniture surface.

During anodizing, various instruments such as voltage, current and temperature should be inspected frequently to eliminate abnormalities in time.