1. Low-Temperature Environments (≤10℃): Accelerate Rust Layer Maturation
Pre-form the rust layer: Artificially accelerate rusting before installation (20-25℃, 60-80% humidity): Sandblast to remove mill scale → Activate with 5% NaCl salt spray for 12-24h → Cure via cyclic wet-dry cycles for 7-10 days to build a dense rust layer in advance.
Alloy optimization: Slightly increase Cu (0.30-0.50%) and Cr (0.80-1.20%) content (within GB/T 4171-2008) to promote α-FeOOH formation at low reaction rates.
Stabilizer coating: Apply breathable rust stabilizers (e.g., water-based PVA) after pre-rusting to lock in the dense layer and prevent brittleness.
2. High-Temperature Environments (≥35℃): Slow Corrosion & Prevent Cracking
Al-Zn thermal spraying: Deposit a 100-150μm Al-Zn alloy coating (75% Al, 25% Zn) to act as a sacrificial anode and form a heat-resistant oxide film (withstands ≤300℃).
Thermal management: Design shading/ventilation for structures to keep surface temp ≤40℃, avoiding heat accumulation and rust cracking.
Regular maintenance: Clean loose rust annually and reapply high-temp rust inhibitors (e.g., molybdate-based) to reinforce the layer.
3. Fluctuating Temperature Environments: Resist Thermal Stress
Composite coating: Use "inorganic zinc-rich primer + fluorocarbon topcoat" (with Cr₂O₃ nanoparticles) to match steel's thermal expansion and block chloride ions.
Cathodic protection: For moisture-exposed parts (e.g., offshore piles), use Zn/Al sacrificial anodes to protect the rust layer from cyclic corrosion.
Routine monitoring: Check for rust detachment quarterly with eddy current testing; repair promptly via sanding, activation, and stabilizer recoating.
4. Universal Measures (All Temperatures)
Surface preparation: Sandblast to Sa 2.5 grade or pickle to remove contaminants, ensuring uniform rust nucleation.
Trace alloying: Add Ni (0.20-0.30%) and Mo (0.10-0.15%) to enhance rust layer toughness and inhibit pitting.
Avoid dissimilar metal contact: Insulate Q355NH from carbon steel/stainless steel with gaskets to prevent galvanic corrosion.
