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
Most epoxy and polyurethane coatings resist mineral oil, diesel, and gasoline well, though silicone performance varies by formulation. Synthetic oils and specialty fluids are less predictable, and compatibility needs to be verified rather than assumed. Epoxy formulated specifically for fuel resistance, or polyurethane, is the safer default for this category.
Water and Steam
Fresh water is tolerated by most coatings indefinitely, but saltwater or brackish water is a different story — standard coatings absorb it, while marine-grade coatings resist it. Steam condensate, mildly acidic from dissolved CO₂, is only mildly corrosive and most epoxy and polyurethane systems handle it fine. Marine-grade epoxy with inhibitors, or polyurethane, holds up best across this whole category.
Acids
Dilute acids above pH 3 are tolerated briefly by most industrial coatings, but strong acids below pH 2 overwhelm nearly all standard formulations, requiring specialized chemically resistant coatings instead. Phenolic or furan-based specialty coatings perform best here; standard epoxy and polyurethane are only marginal against strong acid exposure.
Bases and Alkalis
Weak bases below pH 12 are tolerated by most coatings, while strong bases above pH 12 are more selective — epoxy is often acceptable, but polyurethane can degrade under sustained exposure. Epoxy, especially formulated with chemical-resistant additives, is the more reliable choice against alkaline exposure.
Solvents
Aliphatic solvents such as mineral spirits or diesel are tolerated by most coatings, but aromatic solvents like benzene and toluene can dissolve or soften both epoxy and polyurethane, and halogenated solvents such as methylene chloride or chloroform dissolve most coatings outright. Polyurethane holds up somewhat better than epoxy against solvent exposure generally, and specialized coatings exist for extreme solvent environments where neither is adequate.
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. Immersion testing per ASTM D543, the standard practices for evaluating the resistance of plastics to chemical reagents, is the basis most manufacturers use to generate these compatibility ratings, so a data sheet that cites the standard is more trustworthy than one with an unreferenced in-house scale.
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
Chemical exposure rarely occurs in isolation from oxidative attack; see how to prevent oxidation with high-temperature coatings for how the same barrier properties discussed here also govern oxidation resistance, and how to choose the right high-temperature coating for steel, aluminum, and cast iron for how substrate chemistry changes which coating is chemically compatible.
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. Stack and chimney applications combine chemical exposure with sustained high temperature; see best high-temperature coating for industrial chimneys and stacks for how that combination changes the selection calculus.
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.
Contact Our Team to review your chemical exposure profile and confirm a coating system tested for your specific reagents.
Visit www.incurelab.com for more information.