Surface treatment 3: hard oxidation

Time：2024-05-14 09:17:03 / Popularity： / Source：

For previous read, please refer to Surface treatment 2: aluminum oxidation .

Introduction

Hard anodic oxide films generally require a thickness of 25-150um. Most hard anodized films have a thickness of 50-80um. Hard anodized films with a film thickness less than 25um are used for parts such as tooth keys and spirals. Thickness of anodic oxide film used for wear resistance or insulation is about 50um. Under certain special process conditions, it is required to produce a hard anodized film with a thickness of more than 125um. However, it must be noted that the thicker anodized film, the lower microhardness of its outer layer can be, and roughness of film surface increases. Bath liquid for hard anodizing is usually a sulfuric acid solution and sulfuric acid added with organic acids, such as oxalic acid, sulfamic acid, etc. In addition, hard anodizing can be achieved by lowering anodizing temperature or reducing sulfuric acid concentration. For deformed aluminum alloys with copper content greater than 5% or silicon content greater than 8%, or high silicon die-cast aluminum alloys, some special measures for anodizing may also be considered. For example: for 2XXX series aluminum alloy, in order to prevent aluminum alloy from being burned during anodization process, 385g/L sulfuric acid plus 15g/L oxalic acid can be used as electrolytic bath liquid, and current density should also be increased to above 2.5A/dm.

Process method

There are many electrolysis methods for hard anodizing, such as: sulfuric acid, oxalic acid, propylene glycol, sulfosalicylic acid and other inorganic salts and organic acids. Power supply used can be divided into DC, AC, AC-DC superposition, pulse and superposition pulse power supply, etc. Currently, following types of hard anodization are widely used.
(1) Sulfuric acid hard anodizing method;
(2) Oxalic acid hard anodizing method.
(3) Mixed acid hard anodizing
Among them, sulfuric acid method is a hard oxidation method that is currently widely used.

Principle

1 Principle of hard anodizing
Principle of pure sulfuric acid type aluminum alloy hard anodizing is not essentially different from ordinary anodizing. If it is mixed acid type hard anodizing, there will be some side reactions. reaction nature:
1 Cathodic reaction:
4H+ +4e=2H2↑
2 Anode reaction:
4OH－－4e=2H2O+O2↑
3 Aluminum oxidation: Oxygen precipitated on anode is in atomic state, which is more active than molecular oxygen and is more likely to react with aluminum:
2A1+3O→A12O3
4 Dynamic balance of oxidation and dissolution of anode film: As energization time increases, current increases and oxide film thickens. At the same time, due to dual nature of chemical properties of (Al2O3), that is, it appears as an alkaline oxide in an acidic solution and an acidic oxide in an alkaline solution. There is no doubt that oxide film liquid dissolves in sulfuric acid solution. Only when formation rate of oxide film is greater than its dissolution rate, can oxide film be thickened. When dissolution rate is equal to formation rate, oxide film will no longer thicken. When oxidation rate is too much greater than dissolution rate, powdery oxide films are likely to form on the surfaces of aluminum and aluminum alloy parts.

Process requirements

In order to obtain a good quality hard anodized film and ensure required size of parts, processing must be carried out according to following requirements.

Acute angle rounding

Parts to be processed are not allowed to have sharp corners, burrs and other sharp edges. Because of hard oxidation, anodizing time is generally very long, and oxidation process (A1+O2→A12O3+ Q) itself is an exothermic reaction. And because edges and corners of general parts are often where current is concentrated, these parts are most likely to cause local overheating of parts and cause parts to be burned. Therefore, all edges and corners of aluminum and aluminum alloys should be chamfered, and radius of chamfering circle should not be less than 0.5 mm.
Surface finish

Surface finish

After hard anodizing, surface finish of parts has changed. For rougher surfaces, they can appear smoother than before after this treatment. For parts with higher original smoothness, they often go through this process. After treatment, brightness of displayed surface is reduced, and degree of reduction is about 1 to 2 levels.

Allowance for part size

Due to high thickness of hard oxide film, if aluminum parts require further processing or parts that need to be assembled later, a certain processing allowance should be left in advance and a designated clamping location should be reserved.
Since size of part needs to be changed during hard anodizing, it is necessary to predict possible thickness and dimensional tolerance of oxide film in advance during machining, then determine actual size of part before anodizing so that it meets specified tolerance range after processing.
Generally speaking, increased size of part is roughly half thickness of resulting oxide film.

Special fixture

Because hard anodized parts have to withstand high voltages and high currents during oxidation process, it is important to maintain excellent contact between fixture and parts, otherwise poor contact will cause breakdown or burns to contact parts of parts. Therefore, it is required to design and manufacture special fixtures for parts with different shapes and specific requirements after oxidation of parts.
Local protection
For example, if there are both ordinary anodized and hard anodized parts on same part, specific process will be arranged according to smoothness and precision of part. Usually, ordinary anodizing is performed first, then hard anodizing is performed. Surface that does not require hard anodizing is insulated. Insulation method is to use a spray gun or brush. Apply prepared nitro glue or hydrogenated vinyl glue to the surface that does not need to be treated. Insulation layer should be applied thinly and evenly. Each layer should be dried at low temperature for 30 to 60 minutes, and a total of 2 to 4 layers can be applied.

Process characteristics

Electrolyte for hard anodization is electrolyzed at a temperature of about -10℃ to +5℃. Since oxide film layer generated by hard anodization has high resistance, it will directly affect oxidation effect of current intensity. In order to obtain a thicker oxide film, it is necessary to increase external voltage. Purpose is to eliminate influence of high resistance and keep current density constant. However, when current is large, intense heating will occur. In addition, a large amount of heat will be released when oxide film is formed, causing temperature of electrolyte around parts to rise sharply. Temperature increase will accelerate dissolution of oxide film and prevent oxide film from becoming thicker. In addition, heating phenomenon is most serious at contact point between film layer and metal. If not solved in time, local surface of processed parts will be burned due to temperature rise.
Solution is to use a combination of cooling equipment and stirring.Cooling equipment forcibly cools down electrolyte, and stirring is to make temperature of electrolyte in the entire tank uniform, so as to obtain a higher-quality hard oxide film.

Process influencing factors

Influence of various factors on hardness and growth rate of oxide film
Whether a high-quality hard oxide film layer can be formed on the surface of aluminum and aluminum alloys mainly depends on component concentration, temperature, current density of electrolyte, and composition of its raw materials.
Electrolyte concentration
When using sulfuric acid electrolyte for hard anodization, it is generally in concentration range of 10% to 30%. When concentration is low, hardness of oxide film is high, especially for pure aluminum, except for aluminum alloys with high copper content (CY12). Because aluminum alloys with higher copper content are prone to generate CuAl2 compounds, this compound dissolves quickly during oxidation and can easily burn aluminum parts. Therefore, it is generally not suitable to use low-concentration sulfuric acid electrolyte. It must be oxidized in high concentration (H2SO4 at 300~400g/L) or treated by AC and DC superposition method.
Effect of temperature on film layer
Temperature of electrolyte has a great influence on wear resistance of oxide film. Generally speaking, if temperature decreases, wear resistance of anodized film of aluminum and aluminum alloys increases. This is due to decrease in dissolution rate of electrolyte to the film, in order to obtain a higher hardness oxide film. We must control temperature within range of ±2℃ for hard anodizing treatment.

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