High-temperature coating for automotive exhaust system sheet metal: Withstands 800°C, extending component life by three times.

　　The automotive exhaust system is a core component subject to the greatest heat stress in the vehicle. From the engine exhaust manifold's transient temperatures of 800-1200°C, to the muffler's sustained temperatures of 400-600°C, to the tailpipe's 300-500°C heat radiation, its sheet metal components (such as stainless steel exhaust manifolds, sheet metal muffler housings, and aluminum alloy tailpipes) are constantly exposed to the triple stresses of high-temperature oxidation, acid and alkali corrosion, and vibration and shock. Without professional high-temperature coating protection, sheet metal components are prone to rust, perforation, surface peeling, and structural deformation. This not only shortens the service life (normal uncoated sheet metal only lasts 1-2 years), but can also lead to reduced engine power and excessive exhaust emissions due to exhaust leakage. This article focuses on high-temperature-resistant surface spray coating technology for automotive exhaust system sheet metal components. From coating selection, core processes, application key points, to performance verification, it explains how spray coating can help sheet metal components withstand high temperatures and resist corrosion, providing practical, professional solutions for component manufacturing and repair.

　　1. Why is high-temperature-resistant spray coating necessary for automotive exhaust system sheet metal components?

　　Understanding the harsh operating conditions of the exhaust system is crucial to understanding the irreplaceable role of high-temperature spray coatings:

　　High-temperature oxidation corrosion: At engine startup, the exhaust manifold's temperature soars from room temperature to over 800°C. The metal surfaces (such as stainless steel and iron) react rapidly with oxygen in the air, forming loose oxide scale (such as Fe₂O₃ and Cr₂O₃). The shedding of this scale exposes the metal substrate, accelerating corrosion.

　　Acid-base corrosion: Exhaust gas from fuel combustion contains acidic gases such as SO₂ and NOx. When it encounters condensed water, it forms sulfuric acid and nitric acid, which adhere to the surface of sheet metal and "dissolve" the protective metal layer. Simultaneously, road deicing agents (calcium chloride, sodium chloride) and rainwater seep through the tailpipe, causing alkaline corrosion to the sheet metal inside the muffler.

　　Thermal fatigue and vibration shock: During vehicle operation, exhaust system sheet metal components repeatedly experience high-temperature-low-temperature cycles (e.g., when the temperature difference between start-stop and start-stop is as high as 100°C). 600°C), which is prone to thermal stress cracking. Combined with engine vibration, uncoated sheet metal welds are susceptible to fatigue fracture.

　　The core value of high-temperature spray coating is to form a dense, heat-resistant, and corrosion-resistant protective coating on the sheet metal surface, like a "fireproof and corrosion-resistant armor" for the sheet metal. Actual test data shows that exhaust system sheet metal parts that have undergone professional high-temperature spray coating can extend their lifespan from 1-2 years to 3-5 years, and can reduce rust and perforation problems by over 90%.

　　II. Core Technology: Selecting Three Types of High-Temperature-Resistant Coatings to Match Different Exhaust System Components

　　The temperatures in different parts of the automotive exhaust system vary significantly (300-1200°C), requiring targeted coating selection. Blindly selecting a high-temperature-resistant coating will increase costs, while selecting a lower-temperature coating may not meet the requirements:

　　1. Silicone Aluminum Powder Coating (400-600°C temperature resistance, suitable for mufflers and tailpipes)

　　Applicable Areas:

　　Muffler housings and exhaust tailpipes (relatively low temperatures, and aesthetic considerations are required).

　　Key Advantages:

　　High Cost-Effectiveness: Raw material costs are lower than ceramic coatings, making it suitable for mass-produced civilian vehicles.

　　Strong Adhesion: Aluminum-containing particles form a "mechanical and chemical bond" with the sheet metal substrate, achieving an adhesion of over 5MPa after drying at room temperature (national standards require ≥3MPa).

　　Appearance Options: Available in matte or glossy finishes such as silver and black to meet the design requirements of automotive manufacturers, and resists discoloration at high temperatures (no yellowing after long-term use at 400°C). Note: Not suitable for exhaust manifolds (temperatures exceeding 600°C will cause the silicone resin to decompose and the coating to peel).

　　2. Ceramic-based composite coating (heat resistant 800-1200°C, suitable for exhaust manifolds and three-way catalytic converter housings)

　　Applicable areas:

　　Exhaust manifold (maximum temperature up to 1200°C), three-way catalytic converter housing (temperature 800-1000°C) - These areas are the "high-temperature core" of the exhaust system and require a high-temperature-resistant coating.

　　Core Advantages:

　　Extreme Heat Resistance: Made primarily of aluminum oxide (Al₂O₃) and zirconium oxide (ZrO₂), it boasts a melting point exceeding 2000°C and withstands long-term use at 800°C without cracking or flaking.

　　Excellent Thermal Insulation: The coating's thermal conductivity is only 0.15W/(m・K) (compared to 45W/(m・K) for ordinary steel), reducing heat radiation from the exhaust system and lowering engine compartment temperature (measured reductions of 15-20°C).

　　Strong Corrosion Resistance: The dense ceramic structure isolates acidic gases and deicing agents, preventing rust on sheet metal parts. It is particularly suitable for vehicles in snowy northern regions.

　　Technical Details:

　　The coating solids content must be ≥85% (lower solids content will result in a porous coating and reduced corrosion resistance). The recommended coating thickness is 80-120μm (too thick to easily crack due to differences in thermal expansion coefficients, while too thin to provide adequate protection). 3. Metal-Ceramic Composite Coating (Heat Resistance 600-900°C, Suitable for Intercooler Piping and Exhaust Transition Sections)

　　Applicable Locations:

　　Exhaust system transition sections (connecting the manifold and muffler, temperature 600-900°C), intercooler sheet metal piping (needing both heat resistance and vibration resistance).

　　Core Advantages:

　　Balanced Heat Resistance and Toughness: The addition of metal particles such as nickel and chromium to the ceramic matrix preserves the ceramic's heat resistance while improving the coating's toughness (elongation at break ≥ 2%), enabling it to withstand exhaust system vibration and shock, preventing cracking.

　　Excellent Welding Compatibility: Spray coating does not affect subsequent welding processes (such as laser welding and argon arc welding) on sheet metal parts, making it suitable for the production of parts requiring secondary processing.

　　III. Professional Application Process: From Pretreatment to Curing, Six Steps to Ensure Spray Coating Results

　　Automotive exhaust system sheet metal components often have irregular shapes (such as complex manifolds and muffler welds). The spray coating process must balance "full coverage" with "uniform coating." The following is a standard application process:

　　1. Sheet Metal Pretreatment: This is crucial for coating adhesion (accounting for 40% of spray coating effectiveness).

　　Step 1: Degreasing

　　Soak the sheet metal in a neutral degreaser (pH 7-8) for 15-20 minutes, or use a high-pressure spray (0.3-0.5 MPa) to remove stamping oil and cutting fluid from the surface. Oil contamination can cause craters and peeling of the coating and must be thoroughly removed.

　　Step 2: Rust and Scale Removal

　　For rusted sheet metal components (such as those undergoing repair or refurbishment), sandblast (use quartz sand, 80-120 mesh, and a pressure of 0.5-1.5 MPa). 0.5-0.7MPa), so that the surface roughness of the sheet metal reaches Ra 3.2-6.3μm (a rough surface enhances the mechanical bond between the coating and the substrate). New parts should have their surface scale removed (acid pickling can be used for passivation to avoid acid residue).

　　Step 3: Drying

　　The pretreated sheet metal should be dried in an oven at 80-100°C for 30 minutes to remove surface moisture – moisture can cause bubbles in the sprayed coating.

　　2. Paint Mixing: Mix according to the proper ratio to ensure stable performance.

　　Organic silicon aluminum powder coating: Mix base: curing agent at a weight ratio of 10:1. Stir thoroughly and let stand for 10 minutes to eliminate bubbles. Use within 4 hours (any longer than this will cause the coating to fail).

　　Ceramic-based coating: Add a dedicated thinner (5%-8%) and adjust the coating viscosity to 25-30 seconds (tested with a 4-cup viscometer). Too high a viscosity will cause the coating to sag, while too low a viscosity will result in uneven coverage.

　　3. Spraying Method: Select the appropriate method based on part shape.

　　Electrostatic spraying: Suitable for mass-produced, regular parts (such as tailpipes and muffler housings). Utilizing the principle of electrostatic adsorption, it achieves a paint utilization rate of over 90%, produces a uniform coating thickness (with an error of ±5μm), and covers the sides and grooves of sheet metal parts.

　　Air spraying: Suitable for irregular-shaped parts (such as exhaust manifolds). Using 0.3-0.5MPa compressed air, hold the spray gun 20-30cm from the sheet metal part, and employ a "horizontal reciprocating, 1/3 overlapping" spraying technique to ensure that complex manifolds and welds are sprayed thoroughly.

　　High-pressure airless spraying: Suitable for large-sized sheet metal parts (such as heavy truck mufflers). Airless atomization eliminates paint rebound, resulting in a denser coating and enhanced corrosion resistance. 4. Leveling and Pre-baking: Avoiding Coating Defects

　　After spraying, allow the sheet metal to rest for 15-20 minutes (leveling time) to allow the coating to level naturally and eliminate spray marks.

　　Pre-bake in a 60-80°C pre-oven for 30 minutes (pre-baking) to allow the solvent in the coating to evaporate slowly, preventing cracking during subsequent high-temperature curing due to rapid solvent evaporation.

　　5. High-Temperature Curing: Activating the Coating's Heat Resistance

　　Organic Silicone Aluminum Powder Coating: Cure at 180-200°C for 60 minutes to crosslink the silicone resin and form a film.

　　Ceramic-Based Coating: Preheat at 300°C for 30 minutes, then increase the temperature to 550-600°C for 90 minutes (use a stepwise temperature increase to prevent cracking due to thermal shock).

　　Metal Ceramic Coating: Cure at 400°C for 120 minutes to ensure a thorough bonding of the metal particles to the ceramic substrate.

　　6. Post-Processing: Inspection and Repair

　　Appearance Inspection: The coating surface should be free of sags, pinholes, or spray leaks. Observe with the naked eye or a magnifying glass (10x magnification). The defect area must be ≤0.5%.

　　Thickness Inspection: Use a coating thickness gauge (magnetic method) to measure at five points at different locations on the sheet metal. The thickness must meet the design requirements (e.g., 80-120μm for ceramic coatings).

　　Adhesion Test: Use the cross-hatch method (cross-hatch spacing 1mm, depth to the substrate) to test. The coating peeling level must reach level 0 (the highest level in the national standard, indicating no peeling).

　　IV. Selection and Compliance: 3 Key Criteria to Avoid Pitfalls

　　1. Select coatings based on the "actual maximum temperature" and avoid blindly pursuing high-temperature resistance.

　　Exhaust manifold (up to 1200°C) → Ceramic-based coating;

　　Transition section (900°C) → Metal-ceramic coating;

　　Muffler (600°C) → Silicone-aluminum coating;

　　Mistake: Choosing a 1200°C ceramic coating for a muffler, while heat-resistant, increases costs by over 30%, and the coating lacks toughness and is prone to cracking due to vibration. 2. Prioritize environmentally friendly coatings that meet automotive industry standards.

　　The VOC content of the coating must be ≤420g/L (in compliance with GB 24409-2020, "Limits of Hazardous Substances in Vehicle Coatings").

　　Heavy metal content (lead, cadmium, mercury, hexavalent chromium) must comply with RoHS 2.0 (total heavy metal content per kilogram of coating ≤1000mg).

　　For export vehicles, compliance with the EU ELCD Directive (environmental requirements for exhaust emission-related coatings) is required.

　　3. Select additional features based on the "use scenario."

　　For snowy northern regions: Choose salt spray-resistant coatings (salt spray test ≥1000 hours, neutral salt spray standard) to avoid corrosion from de-icing agents.

　　For modified vehicles: Choose high-temperature resistant and heat-insulating coatings (such as ceramic-based coatings) to reduce the impact of exhaust system heat radiation on other cabin components.

　　For commercial vehicles (trucks and buses): Choose vibration-resistant coatings (such as metal ceramic coatings) to withstand the constant vibrations of long-distance driving.