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Anti-rust treatment of coupling castings

作者: Published: 2020-01-18

Due to the particularity of the manufacturing process, the surface of the casting is easy to rust, and it is difficult to handle after rust. If it is not paid attention to, not only the anti-corrosion ability of the workpiece is poor, but also the decoration is not good. At the same time, according to the different working conditions of the castings, the castings (cast iron, cast steel) are required to be blackened at room temperature, phosphating treatment and other special anti-rust treatments.The difference between cast iron parts and other steel parts is that the structure is loose and the carbon content is high. If not handled properly, it will directly affect the bonding strength of the blackened film and the matrix. Therefore, the pretreatment is particularly important.

  The following focuses on the application of the HH902 normal temperature blackening series products on iron castings, and briefly introduces several anti-rust methods for castings and the situation of related products.General pre-treatment includes degreasing and rust removal. When degreasing, it is necessary to remove oil stains on the surface, and it is not possible to use strong alkali to avoid over-corrosion of the matrix.There are many degreasing methods, such as mechanical degreasing, organic solvent degreasing, chemical degreasing at various temperatures, and electrolytic degreasing. You can choose according to your own existing conditions.Derusting activation can be determined according to the condition of the substrate. If the rust and scale are heavy, first use sandblasting and then blacken. If the surface condition is better, use the hydrochloric acid activation method, but be careful not to over-corrosion.Otherwise, it will be difficult to clean the surface of the carbon film, and it will be easy to hang dust on the surface after blackening, which will also affect the binding force.

  Due to the loose structure and high porosity of iron castings, it is easy to leave the crab solution. It must be cleaned after blackening to avoid yellowing of the parts in the future.In order to improve the anti-corrosion ability of blackened iron castings, post-treatment must be done. Generally, the holes are sealed with a water-soluble sealant, and finally immersed in dehydrated anti-rust oil, so as to achieve anti-corrosion effect.

  1. The application of HH902 steel blackening products at room temperature in iron castings

At present, the demand for surface blackening treatment of iron castings is on the rise.Due to the traditional high-temperature alkaline oxidation blackening process, the appearance of the castings is brownish red or earth red, which cannot meet the requirements of manufacturers.The normal temperature blackening process has the advantages of beautiful surface, low cost, no influence on workpiece accuracy, and good corrosion resistance, and is welcomed by users.

Compared with general steel parts, the normal temperature blackening process of iron castings has both similarities and characteristics. The main difference lies in the pretreatment stage.

Process flow: including three stages of pre-treatment → room temperature blackening treatment → post-treatment

  1.1 Pre-treatment:

  Generally include degreasing and derusting activation, two processes, the quality of the pretreatment is the key to the success or failure of blackening at room temperature, that is, the degreasing must be thorough, and the surface activation must be moderate (that is, the fresh metal surface is exposed without excessive corrosion).

  Pay attention to the following three points for castings:

  ① Use mechanical methods to remove oxide scale.

  Such as: rolling, sandblasting, polishing, polishing, etc.Some castings can be directly blackened or treated with other anti-rust treatments after being treated by this method and then washed with water.

  ② First soak or wash with clean water.

  Before the workpiece is chemically degreasing and derusting, the workpiece should be cleaned (soaked). One is to clean the workpiece first, and the other is to fill the surface of the workpiece with water to facilitate cleaning after chemical treatment.

  ③ Degreasing and derusting activation.

  Due to the large number of pores on the surface of castings, it is difficult to remove pollution, especially oil stains. Poor treatment will affect the entire anti-rust treatment and fail to achieve the expected purpose, including iron, copper, aluminum, etc. and their alloys Castings.

  There are many degreasing methods.Such as: chemical degreasing, mechanical degreasing, baking degreasing, ultrasonic degreasing, electrolysis and organic solvent degreasing, etc.

  Derusting and activation generally adopts hydrochloric acid solution treatment or dilute sulfuric acid (15-20%) medium temperature treatment.

  Such products include: HH922A has excellent medium and normal temperature degreasing performance, and at the same time has a certain effect of activating the surface, easy to clean, for non-corrosive castings or zinc alloy die castings, processed in HH50A working fluid at 65~922℃5 ~15 minutes, after washing, it can directly enter the blackening tank without pickling to achieve normal temperature blackening without pickling, which is conducive to improving the environment and improving work efficiency. HH942 has multiple functions of degreasing, rust removal and surface activation, and can inhibit acid fog, improve working conditions, and is not prone to over-corrosion, which helps to overcome the tendency of iron castings to cause dust to hang due to over-corrosion.

  1.2 Black at room temperature

  Iron castings can be transferred to room temperature blackening treatment after being fully washed by pretreatment.

  When the cast iron turns black, it is necessary to dilute the normal temperature blackening agent with 5-6 times water to form a working fluid, the PH value is 2~2.5, and the treatment time is generally 1 to 3 minutes. Gray cast iron, ductile iron, etc. turn black faster. Some high-alloy cast irons turn black more slowly.During the blackening process, the workpiece can be stirred several times, as long as the black film on the surface of the workpiece is even.

  After the room temperature blackened piece is taken out of the bath, the chemical reaction of the surface layer has not been finally completed. Under the action of oxygen in the air, the chemical reaction is still going on, which is the phenomenon of "after-effect".The time can reach more than 24 hours (except for boiling oil at high temperature). This process can improve the strength, adhesion and uniformity of the conversion film.Therefore, the time of the workpiece in the bath should not be too long, and the black film on the surface should be uniform.This can reduce the consumption of blackening liquid, improve product quality, and reduce production costs.

  1.3 Electroless black nickel plating

  Iron castings can be directly plated with black nickel after pre-treatment and fully washed, or they can be plated with white nickel and then plated with black nickel. The color is uniform, the gun is black, and the black nickel has "after-effect" phenomenon. The longer the time, the iron castings The darker the surface.

  1.4 Post-processing

  The anti-corrosion ability and appearance of iron castings after blackening treatment depend on the quality of the post-treatment. Because of the loose structure of iron castings, they must be fully rinsed after blackening so that the acid residue in the pores can be released to avoid future damage. The erosion of the blackening film makes the workpiece yellow.The main purpose of post-treatment is to close the pores on the surface of the workpiece, thereby improving the ability of the workpiece to prevent corrosion and rust. Such products include: HH912 chromium-free passivation liquid, water-soluble sealant series, HH932 dehydrated rust preventive oil, etc.Practice has proved that they are all very good accessory products for normal temperature blackening treatment.Enterprises can choose appropriate methods for processing according to the requirements and conditions of the workpiece, so as to achieve the goal of not only meeting product requirements, but also reducing costs.


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