1. Core Criterion 1: Uniform, Consistent Physical Appearance
Color: The entire surface should be a consistent dark brown to black (not patchy). Unstable rust shows bright orange, red, or yellow streaks-signs of ongoing corrosion.
Texture: The patina should feel smooth and compact to the touch (when lightly brushed with a gloved hand), with no loose, flaky, or powdery rust. Loose rust (that falls off when touched) means the layer is not bonded to the steel and is still forming.
Coverage: No bare steel or shiny spots should remain. Even hard-to-reach areas (e.g., weld joints, crevices between panels) must have the same patina color and texture as the main surface-these "exposed" spots indicate incomplete stabilization.
2. Core Criterion 2: No Further Rust Progression Over Time
Step 1: Mark 2–3 test areas (50mm × 50mm each) on the steel surface (include high-risk spots like welds or edges).
Step 2: Take clear photos of these areas (note the date and lighting conditions for consistency).
Step 3: Re-inspect and re-photograph the same areas after 1 month (for coastal/humid environments) or 2 months (for inland/dry environments).
Judgment: If there is no visible change (no new orange rust, no expansion of existing rust spots, no flaking), the patina is stabilized. Any new corrosion activity means the layer is still maturing.
3. Core Criterion 3: Water Repellency and Leachate Test
Water spray test: Use a low-pressure (≤30 bar) fresh water spray to wet the surface. After 10 minutes, wipe the area with a white cloth.
Stabilized patina: The cloth remains clean (no orange/red stains from leached rust). Water beads up or runs off smoothly (no absorption into the rust layer).
Unstable patina: The cloth picks up orange stains, and water soaks into the rust (causing dark, wet patches that take hours to dry).
Salt spray test (for critical structures): For coastal or industrial applications, conduct a simplified salt spray test (per ASTM B117 principles):
Place a small S355J2W coupon (with the same rust layer as the installed structure) in a sealed container.
Mist the coupon with a 5% NaCl (salt) solution once daily for 7 days.
After 7 days, check for new rust: Stabilized patina shows no change; unstable patina develops new orange spots.
4. Core Criterion 4: Adhesion of the Rust Layer
Use a standard pressure-sensitive tape (e.g., 3M Scotch 600 tape, 25mm width).
Press the tape firmly onto the rust layer (apply uniform pressure with a roller) and wait 1 minute.
Pull the tape off quickly at a 90° angle.
Result: If the tape has no rust particles or only tiny, powdery specks (less than 5% of the tape area), the patina is well-adhered and stabilized. If large flakes or clumps of rust stick to the tape, the layer is loose and unstable.
Summary of Stabilization Verification Steps
Check for uniform dark brown/black color and smooth texture (no bare spots or flaking).
Compare photos of test areas over 1–2 months (no new corrosion).
Perform a water spray test (no stained cloth, water repellency).
Do a tape pull test (minimal rust on tape).
