Introduction and process requirements of hard anodizing.

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

  • Process requirements
  • In order to obtain a good quality hard anodized film and to ensure the required size of the parts, it must be processed according to the following requirements.
  • Sharp angle chamfer
  • The processed parts are not allowed to have sharp corners, burrs and other sharp corners Because hard anodize generally takes a long time for anodizing, and the oxidation process (Al+O2→A12O3+ Q) itself is an exothermic reaction. In addition, because the corners of general parts are often the places where the current is relatively concentrated, these parts are most likely to cause local overheating of the parts then burn the parts. Therefore, all edges and corners of aluminum and aluminum alloys should be chamfered, and the radius of the chamfer should not be less than 0.5 mm.
  • Surface finish
  • After hard anodizing, the surface finish of the parts will be changed. For rough surfaces,  it can be smoother after hard anodizing. For parts with higher original finish, the surface finish is often reduced after hard anodizing , and the reduction range is about Grade 1 to Grade 2.
  • Size margin
  • Due to the high thickness of the hard anodized film,  the aluminum parts that need to be further processed or the parts that need to be assembled in the future, a certain amount of processing allowance should be reserved in advance, and the clamping position should be designated.
  • Because the size of the parts needs to be changed during hard anodizing, the possible thickness and dimensional tolerance of the anodized film should be predicted in advance during machining, then the actual size of the parts before anodize should be determined,so that it can meet the specified tolerance range after anodized.
  • Generally , the increased size of the part is roughly half of the thickness of the anodized film.
  • Fixture
  • Because the hard anodized parts have to withstand high voltage and high current during the anodizing process, the fixture and parts must be kept in very good contact, otherwise the part will breakdown or burn due to poor contact. Therefore, it is required to design and manufacture special fixtures for parts with different shapes and specific requirements after the parts are anodized.
  • Partial protection
  • If there are both common anodized and hard anodized parts on the same part, the specific process should be arranged according to the finish and precision of the part. Ordinary common anodize first, then hard anodize. Insulate surfaces that do not need to be hard anodized. The method of insulation can be :Use a spray gun or a brush to apply the prepared nitrocellulose adhesiveto the surface that does not require anodize,the insulating layer should be thin and even. Each coat should be dried at low temperature for 30-60 minutes, and a total of 2~4 coats are enough.

The so-called “Alodine” process is the use of chromium phosphating treatment technology for aluminum and aluminum alloys. The phosphating solution is mainly composed of phosphate and hexavalent chromium, and fluoride ions are generally added. The formed phosphating film is emerald green, which has good compatibility with paint and corrosion resistance.

Among the aviation structural materials, the potential of aluminum alloy materials is relatively low, and it is easy to electrochemically corrode with other metal materials in a solution immersion or humid environment.

The surface treatment of aviation aluminum alloy materials refers to the application of Alodine solution on the surface of the material to cause the solution to chemically react with the surface material to form a chemical conversion layer on the surface of the aluminum alloy material.

The role of the chemical conversion layer mainly includes: isolating the aluminum alloy material from the corrosive medium such as the liquid in the surrounding environment to play an anti-corrosion effect; improving the bonding force between the aviation primer coating and the aluminum alloy structure surface, and improving the adhesion of the primer Effect: Maintain the electrical conductivity of the aviation aluminum alloy structure and meet the electrical conductivity requirements between aircraft parts. 1 Commonly used Alodine solution and its precautions for use According to different aluminum alloy material types, primers, and color requirements, commonly used Alodine solutions include: Alodine 600, the surface of the aluminum alloy is yellow after treatment; Alodine 1200, 1200S, 1201, the surface of the aluminum alloy is light golden to brown-yellow after treatment; Alodine 1000, 1000L, 1001, 1500, keep the original color of the aluminum alloy after treatment. The commonly used Alodine solution formula is shown in Table 1. When using Alodine solution, it should be noted that Alodine is flammable. High-concentration Alodine solution or powder is easy to catch fire when it comes into contact with organic solvents. Avoid close to high temperature and fire sources; textiles, Paper etc. should be soaked in water before being discarded. In addition, the Alodine solution is toxic. Avoid contact with the body during operation and avoid direct inhalation.

Surface treatment process: electrostatic spraying, baking varnish, galvanizing, chrome plating, nickel plating, titanium plating, gold plating, silver plating, aluminum anode, impregnation, oil injection, sandblasting, DLC treatment, Teflon treatment, black dyeing, cold plating

Electrostatic spraying: Electrostatic spraying is a spraying method that uses high-voltage electrostatic electric field to make negatively charged paint particles move in the opposite direction of the electric field and adsorb the paint particles on the surface of the workpiece. The electrostatic spraying equipment is composed of spray gun, spray cup and electrostatic spraying high voltage power supply.

Baking paint: Put primer and top coat on the substrate, and send it to a dust-free and temperature-controlled baking room for baking every time the paint is applied.

Galvanizing: refers to the surface treatment technology that coats the surface of metal, alloy or other materials with a layer of zinc for aesthetics and rust prevention. There are many colors, generally blue-white, silver-white, etc.

Chrome plating: A dense chromium oxide film is plated on the surface of metal products, which can make the metal products more durable. There are two types of chrome plating, one is decorative chrome and the other is hard chrome. Hard chrome plating generally uses more machines that are often used under high temperature conditions, such as molds, etc., as the name implies, the main purpose of decorative chrome plating is to brighten the surface, beautiful appearance, rust prevention, etc.

Nickel plating: A method of depositing a layer of nickel on the surface of a metal part by redox without using external current. Used to improve corrosion resistance and wear resistance, increase gloss and beauty. It is suitable for bright nickel plating of tubular or small parts with complex shapes without polishing. Steel parts can be directly plated with nickel. Tin, copper and copper alloy parts should first be contacted on the surface with an aluminum sheet for 1-3 minutes to accelerate the electroless nickel plating.

Titanium plating: is to coat the surface of the object with a layer of titanium metal film by electrochemical method. There are many common types such as: TIN, TIAIN, TIALN, etc.

Gold plating: two types of gold plating are commonly used, one is gold plating of homogeneous materials, and the other is gold plating of heterogeneous materials. Homogeneous gilding refers to gilding the surface of gold jewelry. Its meaning is to improve the brightness and color of jewelry. The gilding of heterogeneous materials refers to the gilding of non-gold materials, such as silver and copper. Its meaning is to replace the color of the plated material with the luster of gold, thereby enhancing the ornamental effect of jewelry.

There are also industrial gold-plated circuit boards and terminals. The main functions of gold-plating are to reduce electrical resistance, improve electrical conductivity, and prevent corrosion.

Nickel must be plated before gold plating to prevent the gold layer from penetrating into the copper layer and cause gold infiltration, so that the nickel layer is used for isolation.

Silver plating: Although it is a traditional process, there is less labor in silver plating compared with other plating species.

The difference in art, such as special treatment before and after silver plating, different substrates and different processes

The silver plating process of the parts has different requirements. For this reason, silver plating is difficult to master in the electroplating process

One of the plating species.

Aluminum anode: The so-called aluminum anodization is an electrolytic oxidation process, in which the surface of aluminum and aluminum alloy is usually transformed into an oxide film, which has protective, decorative and other functional properties . Anodizing of aluminum starting from this definition includes only the part of the process of forming anodized film.

The color of aluminum anode: generally white and black. Other colors can also be made, but the oxidation plant will charge you for the pigment.

There is also a problem related to the thickness of the oxide film. If the thickness is different, there will be chromatic aberration. So you should pay attention to these two points when processing.

Infiltration: It is a process of infiltration and sealing of micropores (fine slits). The sealing medium (usually a low-viscosity liquid) is infiltrated into the micropores (fine slits) through natural penetration (ie, self-priming of micropores), vacuuming and pressure, etc., filling the gaps, and then passing natural (room temperature) and cooling Or heating and other methods to solidify the sealing medium in the gap to achieve the role of sealing the gap.

Oil spraying: spray the paint on the surface of the product and let it dry naturally.

Sandblasting: It uses compressed air as the power to form a high-speed jet beam to spray the spray material (copper ore, quartz sand, emery, iron sand, sea sand) on the surface of the workpiece to be treated at a high speed, so that the outer surface of the workpiece surface Or the shape changes, due to the impact and cutting action of the abrasive on the surface of the workpiece, the surface of the workpiece obtains a certain degree of cleanliness and different roughness, so that the mechanical properties of the workpiece surface are improved, thereby improving the fatigue resistance of the workpiece and increasing It improves the adhesion between it and the coating, prolongs the durability of the coating film, and is also conducive to the leveling and decoration of the coating.

Teflon treatment:

After Teflon coating, it has the following characteristics:

  1. Non-sticky: Almost all materials are not bonded with Teflon film. Very thin films also show good non-stick properties.
  2. Heat resistance: Teflon coating film has excellent high temperature and low temperature resistance. It can withstand high temperatures up to 300°C in a short time, and generally can be used continuously between 240°C and 260°C. It has significant thermal stability. It can work at freezing temperatures without embrittlement and does not melt at high temperatures.
  3. Sliding property: Teflon film has a low friction coefficient. The friction coefficient changes when the load is sliding, but the value is only between 0.05-0.15.
  4. Moisture resistance: The surface of the Teflon coating film does not stick to water and oil, and it is not easy to stick to the solution during production operations. If there is a small amount of dirt, simply wipe it off. Short downtime, saving working hours and improving work efficiency.
  5. Wear resistance: It has excellent wear resistance under high load. Under a certain load, it has the dual advantages of wear resistance and non-sticking.
  6. Corrosion resistance: Teflon is hardly corroded by chemicals and can protect parts from any kind of chemical corrosion.

Black bluing

Blackening is also called bluing. When the workpiece is heated to an appropriate temperature at room temperature or in a solution of air, water vapor or chemical drugs, a blue or black oxide film is formed on the surface of the workpiece to improve its corrosion resistance and appearance. Treatment process. Generally, only 45# steel (carbon steel) and A3 steel can be made very black, like SKD11 and CR12MOV are not very black.

In machinery manufacturing, NaOH solution is often used to heat the workpiece to blacken the workpiece. Compared with the galvanizing and chrome plating, the cost is lower and the effect is good.

The black oxide film formed by blackening has a thickness of 0.5-1.5μm, and its corrosion resistance is lower than other chemical films.

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