1. Pickled & Oiled (P&O) Finish (POOR PERFORMANCE – Not Coastal-Suitable)
Surface characteristics: Smooth, scale-free surface (Ra 0.8–1.5 μm) with a thin protective oil film; minimal surface roughness, closed texture.
Effect on patina cycle: Oil residue repels moisture/oxidation initially (delays rust start by 2–4 weeks), but once the oil degrades (via salt spray/UV), uneven, rapid rusting begins. The smooth surface blocks alloy oxide adhesion, and salt ions trap in micro-gaps; the patina cycle is stuck in the loose rust stage permanently.
Patina outcome: No protective patina forms-only flaky, mottled red/black rust; severe pitting corrosion develops within 6–12 months as chlorides penetrate the unbonded rust layer.
2. Hot-Rolled Mill Scale Finish (WORST PERFORMANCE – Catastrophic for Coasts)
Surface characteristics: Unprocessed, rough surface covered in thick, brittle iron oxide mill scale (FeO/Fe₂O₃/Fe₃O₄); uneven texture, embedded dirt/oil, and loose scale flakes.
Effect on patina cycle: Mill scale delaminates rapidly in salt spray (1–2 weeks), exposing fresh, uneven steel substrate to random oxidation. Chloride ions trap between scale flakes and steel, causing under-scale pitting corrosion; the patina cycle never progresses past the initial rust stage-oxidation is uncontrolled and accelerated.
Patina outcome: Complete failure of patina formation; surface is covered in peeling scale, loose rust, and deep pitting within 3–6 months; structural integrity is at risk for load-bearing components.
3. Ground/Polished Finish (VERY POOR PERFORMANCE – No Protective Patina)
Surface characteristics: Extremely smooth, low-roughness finish (Ra 0.2–0.8 μm); machined surface with no micro-texture for oxide bonding.
Effect on patina cycle: Oxidation starts slowly (3–4 weeks) due to low surface area, but the smooth finish prevents the formation of dense iron-alloy oxides. Salt/moisture forms a thin, stagnant film on the surface, accelerating uniform surface corrosion (vs. pitting) but no protective patina development.
Patina outcome: A thin, non-adherent orange rust layer forms and is continuously washed away by salt spray; the steel surface dulls and corrodes uniformly, with no reddish-brown protective patina ever forming.
4. Wire-Brushed Finish (MODERATE PERFORMANCE – Mild Coastal Only, ≥3km from Sea)
Surface characteristics: Lightly textured finish (Ra 1.5–3.0 μm); removes loose mill scale/oil but leaves a thin, tight oxide layer; minimal micro-roughness.
Effect on patina cycle: Enables slow, partial patina formation only in mild coastal areas (≥3km from sea, low salt spray). The light texture allows limited alloy oxide adhesion; the patina stabilisation stage takes 6–12 months (vs. 4–8 weeks for sandblasted) and is easily disrupted by heavy salt events.
Patina outcome: A patchy, semi-dense reddish-brown patina forms in mild coastal conditions; pitting corrosion still occurs (but slower than P&O/ground finishes); requires a transparent sealer to maintain protection.
