Chemical attack and corrosion are distinct threats to high-temperature coatings. Chemical resistance means the coating does not dissolve or degrade when exposed to specific chemical products. Corrosion resistance means the coating prevents electrochemical corrosion of the substrate. Both are critical in industrial service.
High-temperature coatings vary widely in chemical and corrosion resistance. Selecting the wrong coating for a chemically aggressive environment guarantees rapid failure.
Chemical Attack Mechanisms
Dissolution: The coating dissolves directly when exposed to a solvent that matches the coating chemistry (e.g., epoxy dissolves in strong solvents like methylene chloride).
Swelling: The coating absorbs the chemical, swells, and loses adhesion. The substrate corrodes beneath the swollen coating.
Crazing or cratering: The coating develops fine cracks when exposed to incompatible chemicals.
Embrittlement: Exposure to certain chemicals makes the coating brittle and prone to cracking.
Common Industrial Chemicals and Coating Compatibility
Oils and Fuels
Mineral oil, diesel, gasoline: Most epoxy and polyurethane coatings resist these well. Silicone varies by formulation.
Synthetic oils, specialty fluids: Some coatings fail; compatibility must be verified.
Best coatings: Epoxy formulated for fuel resistance, or polyurethane.
Water and Steam
Fresh water: Most coatings resist indefinitely.
Saltwater or brackish water: Standard coatings absorb water; marine-grade coatings resist.
Steam condensate (slightly acidic from CO₂): Mildly corrosive; most epoxy and polyurethane resist.
Best coatings: Marine-grade epoxy with inhibitors; polyurethane.
Acids
Dilute acids (pH > 3): Most industrial coatings tolerate briefly.
Strong acids (pH < 2): Few standard coatings resist. Specialized chemically resistant coatings required.
Best coatings: Phenolic or furan-based coatings (specialty); standard epoxy and polyurethane are marginal.
Bases and Alkalis
Weak bases (pH < 12): Most coatings tolerate.
Strong bases (pH > 12): Epoxy is often acceptable; polyurethane can degrade.
Best coatings: Epoxy, especially with chemical-resistant additives.
Solvents
Aliphatic solvents (mineral spirits, diesel): Most coatings tolerate.
Aromatic solvents (benzene, toluene): Can dissolve or soften epoxy and polyurethane.
Halogenated solvents (methylene chloride, chloroform): Dissolve most coatings.
Best coatings: Polyurethane is somewhat more solvent-resistant than epoxy. Specialized coatings available for extreme solvent exposure.
Corrosion Prevention
Beyond chemical attack, coatings prevent corrosion by:
Moisture barrier: Blocking water access to the metal surface, eliminating electrochemical corrosion.
Galvanic isolation: Separating dissimilar metals (aluminum-to-steel) which would otherwise corrode galvanically.
Oxygen barrier: Preventing oxidation of the metal surface.
Coatings Ranked by Chemical Resistance (Best to Weakest)
- Phenolic: Excellent chemical resistance; extreme temperature limits (different class)
- Furan: Excellent chemical resistance; specialty products
- Polyester: Good chemical resistance; lower temperature rating than epoxy
- Epoxy: Good chemical resistance; excellent for most industrial chemicals
- Polyurethane: Moderate chemical resistance; better than epoxy for some applications
- Silicone: Adequate for low-stress chemical exposure; weaker than epoxy
- Enamel: Minimal chemical resistance; cosmetic applications only
Selection Strategy for Chemically Aggressive Environments
Step 1: Identify the Chemicals
Know what chemicals the coating will contact:
– What substance touches the coating?
– What is the concentration?
– What is the temperature of exposure?
– Is exposure continuous or intermittent?
Step 2: Consult Chemical Resistance Data
Manufacturers provide chemical resistance charts showing compatibility with hundreds of substances.
- A (Excellent): No degradation
- B (Good): Minor degradation acceptable
- C (Fair): Noticeable degradation
- D (Not Recommended): Unacceptable degradation
Select coatings rated A or B for your specific chemicals.
Step 3: Test if Uncertain
If the data sheet doesn’t list your specific chemical, request a test sample:
– Apply coating to a test panel
– Expose to the chemical under expected conditions
– Inspect for swelling, dissolution, embrittlement, or adhesion loss
This empirical test is worth the cost if the application is critical.
Step 4: Combine Multiple Protection Methods
For severe chemical exposure:
- Coating as primary barrier
- Topcoat or sealant for secondary protection
- Material selection: Stainless or corrosion-resistant alloy for extra margin
- Mechanical isolation: Prevent direct chemical contact where possible
Common Chemical Exposure Failures
Failure 1: Automotive underbody
– Road salt is applied during winter
– Cheap enamel paint dissolves in salt brine
– Corrosion begins within months
– Solution: Use epoxy-based or polyurethane coating with salt-water resistance
Failure 2: Industrial pipes in chemical plants
– Equipment handles dilute acid or solvent
– Standard epoxy selected without checking chemical compatibility
– Coating swells and delaminates
– Solution: Select coating tested specifically for the chemical being handled
Failure 3: Marine equipment
– Saltwater and salt spray are present
– Standard industrial coating lacks corrosion inhibitors
– Failure within 1–2 years
– Solution: Use marine-grade coating with zinc or aluminum inhibitors
Long-Term Durability in Chemical Service
With proper coating selection:
- Chemically compatible coating: 10–20+ years durability
- Marginal chemical compatibility: 2–5 years before degradation
- Incompatible coating: 6–12 months before failure
Selection discipline is the primary factor in durability.
Email Us if you need help selecting a chemical-resistant coating for your specific application, or if you’re troubleshooting coating failure in a chemically aggressive environment.
The Bottom Line
High-temperature coatings can resist both chemical attack and corrosion—but only if properly selected for the specific application. Epoxy and polyurethane are the workhorses of industrial coating; phenolic and furan are available for extreme chemical service. Always consult the manufacturer’s chemical resistance chart for your specific chemical exposure. Test if uncertain. For severe chemical exposure, combine coating with additional protection (dual barriers, material selection, mechanical isolation). A coating selected without regard to chemical exposure will fail prematurely; the same coating selected carefully for chemical compatibility can last 10–20+ years.
Visit www.incurelab.com for more information.