For many thermal spray coatings, sealing is a necessary post-treatment process. When the coating is exposed to corrosion and oxidation (sometimes at high temperatures), sealing is a must for the coating design. This article involves the purpose and requirements of the sealing operation; introduces the types, performance, construction points, selection basis and application scope of the sealing agent.
Any kind of thermal spraying method, especially the coating deposited by gas-oxygen flame spraying (ie flame spraying), is a porous structure. Overall, the porosity range of the coating is quite large. The gas-oxygen flame spray coating is sometimes as high as 15%, while the high velocity oxygen-gas spray (HVOF) layer is below 1%. In practical applications, a porous structure is sometimes advantageous. Coating pores help to store oil, promote lubrication, and reduce wear; in some special applications, thermal spraying is used to prepare porous materials for heat exchange devices or other components. However, in more cases, the porosity of the coating is undesirable. When the coating is exposed to the atmosphere, steam, industrial atmosphere, chemically active material, corrosive gas and high temperature environment, the pores introduce corrosive elements, causing chemical or electrochemical corrosion of the coating and the substrate, causing the coating to fail. In this case , The coating must be sealed.
Performance requirements of the sealing agent There are many types of sealing agents. In order to achieve a good sealing effect, the sealing agent must meet the following requirements:
1. Sufficient permeability: Because the pores are curved and small, they do not have high enough permeability to fill the necessary level.
2. The role of chemical solvent resistance: because the coating itself is in a certain chemical and solvent corrosive medium, if the sealing agent cannot withstand the corrosion of these media, the sealing will be invalid. To
3. It can withstand certain mechanical action: that is, the sealing agent will not fall off or break due to mechanical action.
4. Stable performance at working temperature: that is, the sealing agent cannot undergo physical changes such as melting, evaporation or decomposition at working temperature.
5. Can not chemically react with the coating or the substrate: maintain chemical stability with the coating and the substrate in the working environment.
6. Does not reduce the performance of the coating or base metal.
7. When the coating is used in the food industry, it is non-toxic to food.
8. Ensure safety during construction.
Two. Selection of sealing agent and key points of construction Generally speaking, all sealing operations should be carried out after spraying and before grinding. Except for the microcrystalline paraffin wax sealing agent, all other sealing agents should be carried out when the temperature of the workpiece is lower than 80°C to prevent the solvent from evaporating quickly or the sealing agent solidifies prematurely. For air-drying glyceride resin, the heat generated during its machining will cause it to cure prematurely and non-uniformly. Therefore, the sealing agent should be cured after grinding. All other sealing The agent should be cured before grinding. The sealing agent used for anticorrosive coating must be fully cured before being put into use. Common sealing agents are not dangerous, but some are volatile and flammable. During storage and use, you should be extra cautious and avoid open flames or heat. Enough ventilation is required during use to avoid affecting normal breathing. There are two types of sealing agents: organic and inorganic. The main basis for choosing the sealing agent is the corrosion condition of the medium and the working temperature.
The article is reproduced from the WeChat public platform: Aluminum Friends Community