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New sealing technology of thermal spray coating
source： author： date：2020-01-08 hits：3689
Technical guide for sealing holes of thermal spray coating
For many thermal spraying coatings, sealing is a necessary post-treatment process. When the coating is faced with corrosion and oxidation (sometimes in high temperature) environment, sealing is a necessary part of coating design. This paper deals with the purpose and requirements of hole sealing operation, introduces the type, performance, key points of construction, selection basis and application scope of hole sealing agent.
Any kind of thermal spraying method, especially the coating deposited by gas oxygen flame spraying, is a porous structure. In general, the porosity range of the coating is quite large. Sometimes the gas oxygen flame spray coating is more than 15%, while the HVOF coating is less than 1%. In practical application, porous structure is sometimes advantageous. In some special applications, porous materials are made by thermal spraying, which are used in heat exchange devices or other components. However, in more cases, the pores of the coating are not desirable. When the coating is exposed to atmosphere, steam, industrial atmosphere, chemical active substances, corrosive gas and high temperature environment, corrosion elements are introduced into the pores, which causes chemical or electrochemical corrosion between the coating and the substrate, leading to coating failure. In this case, the coating must be sealed.
1、 Performance requirements of sealing agent
There are many kinds of sealing agent. In order to achieve good sealing effect, the sealing agent must meet the following requirements:
1. Sufficient permeability
It is difficult to fill the pores to the necessary degree because of their small bending and insufficient permeability.
2. Resistance to chemical solvents
Because the coating itself is in a certain chemical and solvent corrosion medium, if the sealing agent can not withstand the corrosion of these media, the sealing will be invalid.
3. Able to withstand certain mechanical action
That is, the sealing agent does not fall off or break due to mechanical action.
4. Stable performance at operating temperature
That is to say, the pore sealing agent can not undergo physical changes such as dissolution, evaporation or decomposition under the working temperature.
5. No chemical reaction with coating or substrate
Keep chemical stability with coating and substrate in working environment.
6. Do not reduce the performance of 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.
2、 Selection of sealing agent and key points of construction
Generally, all sealing operations shall be carried out after spraying and before grinding. Except for microcrystalline wax sealing agent, all other sealing agents shall be carried out when the workpiece temperature is lower than 80 ℃ to prevent rapid evaporation of solvent or premature curing of sealing agent. The heat produced in the machining of air drying glyceride resin will make it cure unevenly early. Therefore, the sealing agent should be cured after grinding, and all other sealing agents should be cured before grinding. The sealing agent for anticorrosive coating must be fully solidified before being put into use. General sealing agents are not dangerous, but some are volatile and flammable. In the process of storage and use, special care should be taken to avoid near open fire or heating. During use, ventilate sufficiently to avoid affecting normal breathing.
There are two kinds of sealing agents, organic and inorganic. The selection of sealing agent is mainly based on the corrosion of the medium and the working temperature. The organic sealing agents are mainly vinyl resin, phenolic resin, modified epoxy resin and polyurethane resin. They are mostly used for sealing holes of zinc or aluminum coating on steel substrate, with low working temperature, in atmospheric oxidation, industrial atmosphere, sea water and other environments. For high-temperature oxidation medium, oxide ceramic powder, asphalt based aluminum slurry, aluminum silicone resin and some silicates, chromates and other inorganic materials are used There are different requirements for different sealing agents, which generally follow the following points:
1. Many sealing agents need diluent to dilute to ensure good permeability.
2. Before sealing the coating, it is better to use the etch coating (usually phosphoric acid) as pretreatment to form a composite phosphate composite film, which is conducive to the bonding of the sealing agent.
3. Many sealing agents are mixed with flake aluminum powder to improve the temperature resistance and corrosion resistance of the sealing agent.
4. The pore sealing agent can be sprayed by spray gun, wiped by brush or soaked by submergence, some of which can be used in the above three ways, some can only use one or two of them. As needed, sometimes just one coat, sometimes two or more.
5. In order to increase the permeability, some adopt vacuum infiltration operation.
6. The pore sealing agent can be divided into single component and two component, air drying and baking curing, and each procedure must be correctly mastered in use.
3、 Main types and properties of pore sealer
There is no series and large-scale sealing agent product in China. As the coating sealing is an important part of thermal spraying technology, this work needs to be developed and promoted urgently. Referring to the relevant foreign products, several important sealing agents are introduced as follows:
1. Air drying glyceride resin
Features: the resin content is up to 95%, the permeability and filling capacity is very high, and the machining performance can be improved obviously after sealing.
Working temperature: continuous working 145 ° C.
Amount of sealing agent: 11.6m2/l/0.6mm
Construction method: soak, brush or spray. The coating surface shall be dry and clean during normal temperature operation. According to the needs, after the implementation of the first channel, the second channel can be implemented after 15 minutes.
Curing procedure: air drying at room temperature, one week before complete curing; if baking at 145 ° C, one hour can be cured.
Typical application: the working environment is weak corrosion such as dilute acid, solvent, oil and salt water
Any kind of thermal spraying method, especially the coating deposited by gas oxygen flame spraying, is a porous structure. In general, the porosity range of the coating is quite large. Sometimes the gas oxygen flame spray coating is more than 15%, while the HVOF coating is less than 1%. In practical application, porous structure is sometimes advantageous. In some special applications, porous materials are made by thermal spraying, which are used in heat exchange devices or other components. However, in more cases, the pores of the coating are not desirable. When the coating is exposed to atmosphere, steam, industrial atmosphere, chemical active substances, corrosive gas and high temperature environment, corrosion elements are introduced into the pores, which causes chemical or electrochemical corrosion between the coating and the substrate, leading to coating failure. In this case, the coating must be sealed.
1、 Performance requirements of sealing agent
There are many kinds of sealing agent. In order to achieve good sealing effect, the sealing agent must meet the following requirements:
1. Sufficient permeability
It is difficult to fill the pores to the necessary degree because of their small bending and insufficient permeability.
2. Resistance to chemical solvents
Because the coating itself is in a certain chemical and solvent corrosion medium, if the sealing agent can not withstand the corrosion of these media, the sealing will be invalid.
3. Able to withstand certain mechanical action
That is, the sealing agent does not fall off or break due to mechanical action.
4. Stable performance at operating temperature
That is to say, the pore sealing agent can not undergo physical changes such as dissolution, evaporation or decomposition under the working temperature.
5. No chemical reaction with coating or substrate
Keep chemical stability with coating and substrate in working environment.
6. Do not reduce the performance of 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.
2、 Selection of sealing agent and key points of construction
Generally, all sealing operations shall be carried out after spraying and before grinding. Except for microcrystalline wax sealing agent, all other sealing agents shall be carried out when the workpiece temperature is lower than 80 ℃ to prevent rapid evaporation of solvent or premature curing of sealing agent. The heat produced in the machining of air drying glyceride resin will make it cure unevenly early. Therefore, the sealing agent should be cured after grinding, and all other sealing agents should be cured before grinding. The sealing agent for anticorrosive coating must be fully solidified before being put into use. General sealing agents are not dangerous, but some are volatile and flammable. In the process of storage and use, special care should be taken to avoid near open fire or heating. During use, ventilate sufficiently to avoid affecting normal breathing.
There are two kinds of sealing agents, organic and inorganic. The selection of sealing agent is mainly based on the corrosion of the medium and the working temperature. The organic sealing agents are mainly vinyl resin, phenolic resin, modified epoxy resin and polyurethane resin. They are mostly used for sealing holes of zinc or aluminum coating on steel substrate, with low working temperature, in atmospheric oxidation, industrial atmosphere, sea water and other environments. For high-temperature oxidation medium, oxide ceramic powder, asphalt based aluminum slurry, aluminum silicone resin and some silicates, chromates and other inorganic materials are used There are different requirements for different sealing agents, which generally follow the following points:
1. Many sealing agents need diluent to dilute to ensure good permeability.
2. Before sealing the coating, it is better to use the etch coating (usually phosphoric acid) as pretreatment to form a composite phosphate composite film, which is conducive to the bonding of the sealing agent.
3. Many sealing agents are mixed with flake aluminum powder to improve the temperature resistance and corrosion resistance of the sealing agent.
4. The pore sealing agent can be sprayed by spray gun, wiped by brush or soaked by submergence, some of which can be used in the above three ways, some can only use one or two of them. As needed, sometimes just one coat, sometimes two or more.
5. In order to increase the permeability, some adopt vacuum infiltration operation.
6. The pore sealing agent can be divided into single component and two component, air drying and baking curing, and each procedure must be correctly mastered in use.
3、 Main types and properties of pore sealer
There is no series and large-scale sealing agent product in China. As the coating sealing is an important part of thermal spraying technology, this work needs to be developed and promoted urgently. Referring to the relevant foreign products, several important sealing agents are introduced as follows:
1. Air drying glyceride resin
Features: the resin content is up to 95%, the permeability and filling capacity is very high, and the machining performance can be improved obviously after sealing.
Working temperature: continuous working 145 ° C.
Amount of sealing agent: 11.6m2/l/0.6mm
Construction method: soak, brush or spray. The coating surface shall be dry and clean during normal temperature operation. According to the needs, after the implementation of the first channel, the second channel can be implemented after 15 minutes.
Curing procedure: air drying at room temperature, one week before complete curing; if baking at 145 ° C, one hour can be cured.
Typical application: the working environment is weak corrosion such as dilute acid, solvent, oil and salt water
