A coated steel component shows rust spots after 6 months of outdoor storage. The coating was applied correctly and survived temperature cycling without visible damage. Yet rust grew underneath the coating—defeating the purpose of coating.

Coating peeling and corrosion under the coating are distinct failure modes. Rust under coating indicates one of five root causes, all preventable through proper specification and pre-treatment.

Root Cause 1: No Pre-Treatment or Primer

Without conversion coating or primer, the coating adheres to bare steel but doesn’t prevent corrosion initiation at uncoated edges, holidays (small gaps in coating), or scratches.

Water and oxygen penetrate through microscopic coating defects, reaching bare steel. Corrosion initiates immediately and grows under the coating since the coating seals in moisture.

Fix: Apply chromate or phosphate conversion coating before high-temperature topcoat. This neutralizes surface corrosion and provides a true corrosion barrier.

Root Cause 2: Coating Holidays and Edge Corrosion

Thin coating application leaves tiny gaps (holidays) where the coating doesn’t completely cover. These microscopic defects are invisible but become rust initiation points.

Fix: Apply in thin multiple coats ensuring complete coverage. Verify coverage under UV light with fluorescent dyes if available.

Root Cause 3: Moisture Under the Coating

Coating traps moisture against the substrate. Even dry steel contains absorbed moisture; once sealed under coating, this moisture initiates corrosion.

Fix: Dry substrate thoroughly before coating. Bake at 80°C for 2 hours if necessary to remove absorbed moisture.

Root Cause 4: Coating Incompatibility with Substrate

Some coatings react with certain steel types (stainless, weathering steel) or galvanized surfaces, initiating corrosion through electrochemical mechanisms.

Fix: Verify coating-to-substrate compatibility. Use primer compatible with specific substrate material.

Root Cause 5: Inadequate Coating Thickness

Thin coating (1–2 mils) provides minimal protection. Water penetrates through the coating in weeks. Thick coating (4–6 mils) resists moisture penetration for years.

Fix: Apply minimum 4–6 mils dry film thickness in multiple coats.

Prevention Strategy

Complete surface preparation: Grit-blast to bare metal; remove all mill scale, rust, and contamination.

Conversion coating: Apply chromate (best corrosion prevention) or phosphate before topcoat.

Adequate thickness: Apply 4–6 mils minimum; monitor with wet-film thickness gauge.

Dry substrate: Moisture-test before coating; surface must be <3% moisture content.

Quality materials: Use industrial-grade high-temperature coatings from reputable suppliers; avoid consumer-grade products.

Incure specializes in complete coating systems with conversion primer and topcoat designed to prevent under-coating corrosion.

Email Us to specify a complete coating system preventing corrosion and rust growth.

Visit www.incurelab.com for more information.