New Technology of Thermal Spray Coating Sealing

       Technical Guide for Sealing Thermal Spray Coatings

   Sealing is a necessary post-processing process for many thermal spray coatings. Sealing is a must in coating design when the coating is exposed to corrosion and oxidation (sometimes at high temperatures). This paper deals with the purpose and requirements of the sealing operation, and introduces the types, properties, 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 coating porosity range is quite large. The gas-oxygen flame sprayed layer is sometimes as high as 15% or more, while the high-velocity oxygen-fuel sprayed (HVOF) layer is below 1%. In practical applications, porous structures are 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, porosity of the coating is undesirable. When the coating is exposed to the atmosphere, steam, industrial atmosphere, chemically active substances, corrosive gases and high-temperature environments, the pores introduce corrosive elements, causing the coating to chemically or electrochemically erode the substrate and cause the coating to fail. In this case , the coating must be sealed.

   1. Performance requirements of 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. enough permeability

   Because the pores are curved and small, they are not sufficiently permeable to be filled to the necessary extent.

   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 erosion of these media, the sealing will be ineffective.

   3. Can 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, the sealing agent cannot undergo physical changes such as melting, evaporation or decomposition at the working temperature.

   5. No chemical reaction with coating or substrate

   Chemically stable to coatings and substrates under operating conditions.

   6. Does not degrade the properties of the coating or base metal.

   7. Non-toxic to food when the coating is used in the food industry.

   8. During construction, ensure safety.

   Second, the selection of sealing agent and construction points

   In general, all sealing should be done after spraying and before grinding. Except for the microcrystalline paraffin 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 rapidly or the sealing agent curing prematurely. Air-drying glyceride resins will cure prematurely and non-uniformly due to heat generated during machining. Therefore, the sealant should cure after grinding. All sealants other than agent, should be cured before grinding. Sealants used in anti-corrosion coatings must be fully cured before being put into use. General sealants are not dangerous, but some are volatile and flammable. During storage and use, special care should be taken to avoid open fire or heat. Use adequate ventilation, so as not to affect normal breathing.

   Sealing agents are generally divided into two types: organic and inorganic. The main basis for the selection of the sealing agent is the corrosion of the medium and the working temperature. Organic sealing agents are mainly vinyl resin, phenolic, modified epoxy resin and polyurethane resin, etc. They are mostly used for sealing of zinc or aluminum coatings on steel substrates. The working temperature is not high, and they are exposed to atmospheric oxidation, industrial atmosphere, Seawater and other environments; for high-temperature oxidizing media, oxide ceramic powder, asphalt-based aluminum paste, alumino-silicone resin, and some inorganic substances such as silicate and chromate are used. Different sealing agents have different requirements, generally follow the following What time is it:

   1. Many sealants require diluent dilution to ensure good permeability.

   2. Before the coating is sealed, it is best to pre-treat with an etch coating (usually phosphoric acid) to form a complex phosphate compound film that helps the sealing agent bond.

   3. Many sealing agents are mixed with flake aluminum powder to improve the temperature resistance and corrosion resistance of the sealing agent.

   4. The sealant can be sprayed with a spray gun, applied with a brush or impregnated by submersion, some of which can be used with all three of the above, and some with only one or two of them. Sometimes just one coat, sometimes two or more coats, as needed.

   5. To increase permeability, some use vacuum infiltration operations.

   6. Sealing agent can be divided into one-component and two-component, air-drying and baking-curing. In use, it is necessary to correctly grasp each step of the procedure.

   3. Main types and properties of sealing agents

   my country has not yet formed a series of large-scale sealing agent products. Since coating sealing is an important part of thermal spraying technology, this work needs to be developed and promoted urgently. Referring to related foreign products, the important sealing agents are introduced as follows:

   1. Air-drying glycerol ester resin

   Features: The resin content is as high as 95%, the penetration and hole filling ability is very high, it can be directly processed after the hole is sealed, and the machinability is obviously improved.

   Operating temperature: 145°C for continuous operation.

   Dosage of sealing agent: 11.6m2/liter/0.6mm

   Construction method: dip, brush or spray. Normal temperature operation, the coating surface should be dry and clean. If necessary, after the first pass is implemented, the second pass can be implemented after 15 minutes.

   Curing procedure: air drying at room temperature, it will take a week to fully cure; if it is baked at 145°C, it can be cured in 1 hour.

   Typical application: The working environment is weak corrosive media such as dilute acid, solvent, oil and salt water, and the temperature does not exceed 145°C.

   2. Air drying phenolic resin

   Features: It is a modified air-drying phenolic paint. Suitable for metal and ceramic coatings, good wettability, strong adhesion, good chemical stability, strong corrosion resistance and impact resistance. Stable in many organic substances.

   Working temperature: 145～205°C for continuous work, 205～260°C for intermittent work

   Dosage of sealing agent: 7.4～8.0m2/L.

   Construction method: brush or spray. When spraying, use ketone, aromatic hydrocarbons and absolute ethanol as diluents for dilution, and the dosage is 1/2 to 1/3.

   Curing procedure: Air dry for 30 to 60 minutes, but it will take 1 to 10 hours to fully cure, depending on the thickness of the sealant. If it is baked at a temperature of about 135 degrees Celsius, it only takes 15 to 30 minutes.

   Typical application: suitable for working environments such as hot water, salt spray, acid, oil, gasoline and grease, and has strong adaptability to high-pressure working conditions.

   3. Aluminum Vinyl

   Features: It is a corrosion-resistant, air-drying aluminum-containing ethylene copolymer, commonly used in zinc and aluminum topcoats, while the opposite primer is passivated zinc chromate paste.

   Dosage of sealing agent: 5.5m2/L.

   Operating temperature: no more than 80°C.

   Construction method: pressure spraying or brushing.

   Curing procedure: dry for 30 minutes, diluent is xylene or ketone.

   4. Passivated zinc chromate

   Features: It is a primer for aluminum vinyl topcoat. Air-drying type, because it can form a porous film, which provides strong adhesion to the surface sealant. The main ingredient is zinc chromate.

   Dosage of sealing agent: 6.0～7.4m2/L.

   Typical application: Used as a primer in combination with aluminum vinyl topcoat to provide sealing protection for aluminum-zinc coatings on steel structures or equipment in industrial atmospheres, salt spray atmospheres, and non-potable water.

   5. baked phenolic resin

   Features: It is used for sealing of metal and ceramic coatings. It is a baking type synthetic phenolic slurry. In terms of protection performance, the baking type is higher than the air drying type. It maintains good chemical stability (but not alkali resistance) in many organic media, and has good heat resistance. The main components are phenolic and aromatic hydrocarbons.

   Working temperature: continuous work can reach 145 ~ 205 ° C, intermittent work is 205 ~ 260 ° C.

   Dosage of sealing agent: 4.6m2/L.

   Construction method: brush and spray can be. Dilute with 25% isopropanol when spraying. For single coats, bake at 230°C for 10 minutes or 205°C for 20 minutes; for multiple coats, bake the bottom layer at 145°C for 10 minutes and the top layer at 190°C for 60 minutes.

   Typical applications: Same as air-drying phenolic resin, suitable for media such as boiling water, salt spray, acid, oil, gasoline and grease.

   6. Asphalt-based aluminum paste

   Features: It is a high melting point, asphalt containing paint, the main components are flake aluminum powder, asphalt coal tar and corresponding solvent.

   Dosage of sealing agent: 4.9～7.4m2/L.

   Working temperature: 870～980°C.

   Construction method: spray or brush. Dilute with mineral alcohol when spraying, preferably two coats.

   Typical application: When the substrate is steel, nickel or nickel-chromium alloy, the coating is aluminum, and it is in the environment of high temperature oxidation and high temperature sulfur-containing gas corrosion, this sealing agent is the first choice.

   7. Silicon aluminum resin

   Features: It is an air-drying silicone resin containing flake aluminum powder, and is a relatively high-temperature-resistant sealing agent in an anti-oxidative atmosphere.

   Dosage of sealing agent: 3.6m2/L.

   Operating temperature: 480°C.

   Construction method: brush or spray, each thickness is 0.018 - 0.02mm. No dilution is required for brushing, but ethanol, aromatic hydrocarbons and methyl ketone are used for flushing when spraying, and the amount of diluent is 1/3.

   8. Air-drying silicone

   Features: It is a two-component silicone resin, recommended for medium corrosion or in environments with temperatures below 205°C and long-term exposure to air, resistant to dilute acids, alkalis, oils, liquids, water and salt water and other media corrosion. The solids content was 25%.

   The amount of sealing agent: 5.0 - 11.6m2/L (related to the porosity of the coating).

   Operating temperature: The long-term operating temperature is 205°C.

   Construction method: Two-component, spraying method or soaking method can be used, air drying at room temperature for about 1 hour, but it takes 3 days for full curing, and curing can be accelerated by drying at 145°C for 2 hours.

   9. Baked polyimide resin

   Features: It belongs to baking polyimide resin, which is used in weak or moderate corrosive environment. Solids content up to 30%, low viscosity, good permeability and strong film formation. It is very stable to media such as water, solvents, organics, inorganic oils, fuels, liquid streams and dilute acids.

   Dosage of sealing agent: 5.3 - 9.5m2/L/0.6mm.

   Working temperature: Long-term working temperature is 260°C.

   Construction method: can be soaked, sprayed or brushed. Work at room temperature until the coated surface does not absorb, after 15 minutes, seal again. Cure time is 8 hours at room temperature and 2 hours at 180°C.

   Typical application: It is suitable for conveying pumps in petrochemical systems and paper industry, with corrosion resistance and wear resistance. These pumps operate in alkaline feeds such as soda mortar, calcium hydroxide, magnesium hydroxide and sodium hydroxide.

   10. Microcrystalline wax

   Characteristics: It is an inert, petroleum-based microcrystalline wax that provides sealing and lubrication to coatings. It is stable in fresh water, sea water and almost all acids and alkalis, but it is not resistant to corrosion by solvents and hydrocarbons, and is especially suitable for high-pressure lubrication environments. It provides effective "dry lubrication" where lubricant cannot be applied.

   Dosage of sealing agent: 10m2/kg.

   Operating temperature: no more than 80°C.

   Construction method: After preheating the workpiece to 95°C, apply directly on the surface.

   Typical application: It is usually used to seal shafts or other workpieces to ensure clean turning or grinding, prevent sand particles from entering the pores, and at the same time have good corrosion resistance.

   11. Oxide ceramic inorganic sealing agent

   Features: The main components are zirconia fine powder, alumina fine powder and ternary dispersant, etc. It is mainly used for sealing holes after metal spraying in high temperature furnaces such as boilers. It has no chemical reaction with the spray coating, and has the characteristics of strong permeability, high temperature resistance, and good film-forming effect at high temperature.

   Usage and Dosage: Mix A and B groups evenly on site according to the ratio of 1:1, and evenly apply them to the surface of the metal spray layer with a brush or spray gun. The amount per square meter is about 1 kg.

   Operating temperature: 300°C -1250°C

   Typical application: anti-wear and anti-corrosion coatings made by thermal spraying technology for coal-fired boilers