Weathering steel (atmospheric corrosion-resistant steel; Chinese grade standard GB/T 4171-2008) is a low-alloy structural steel that uses copper, chromium and nickel additions, with controlled phosphorus and silicon, to grow a dense, strongly adherent rust patina in service that progressively slows atmospheric corrosion. Whether it may stay unpainted is an environmental data question, not a material slogan.
Key takeaways
- Unpainted does not mean non-corroding. The patina lowers the corrosion rate; the decision criterion is the combination of airborne chloride deposition and the long-term corrosion rate, not the grade name.
- Published design guidance sets the ceiling for unpainted conventional weathering steel at a chloride deposition of 6-12 mg/(m2 d), with a long-term corrosion rate that should stay below 6 um/a.
- Marine air magnifies the gap between grades. At Male, Maldives (432.3 mg/(m2 d) Cl-) the predicted 20-year rate is 69.28 um/a for conventional weathering steel and 18.23 um/a for a 3% Ni grade - both far above the unpainted criterion.
- Wetting time controls patina quality. Where the surface stays wet less than about 20% of the time a protective patina appears within months; in stagnant water, crevices or slow-drying shading the rust flakes off and stops protecting.
- Painting remains necessary inside closed box sections and crevices, under de-icing salt, in high SO2 industrial air, in the splash zone, at galvanic contacts with stainless steel or aluminium, and wherever wall-thickness corrosion allowance cannot cover the measured loss.
- Make the decision auditable: classify the site with ISO 9223 / GB/T 19292.1-2018, expose coupons for at least one year, then extrapolate with GB/T 41756-2022 before choosing bare use, a reduced coating, a full ISO 12944 system or a metal spray coating.
Key data and acceptance criteria
| Parameter | Value and unit | Source and use |
|---|---|---|
| Atmospheric corrosivity, first-year loss of carbon steel | C1 <=1.3; C2 >1.3-25; C3 >25-50; C4 >50-80; C5 >80-200; CX >200 um | ISO 9223:2012 (CN: GB/T 19292.1-2018). The starting point of every bare-use argument. |
| Salinity ceiling for unpainted conventional grades | 6-12 mg/(m2 d) | Japanese construction guidance, 1993 edition, corresponding to a long-term rate of about 5-6 um/a. |
| Bare-use corrosion-rate criterion | <=6 um/a | Widely quoted limit for unpainted weathering steel; exceed it and keep a coating. |
| Loss of protection under high chloride | About 6x higher rate | US bridge patina study: with roughly 1.5 wt% chloride in the rust no protective patina forms. |
| Q355GNH chemistry and mechanics | C <=0.12%, Si 0.20-0.75%, Mn <=1.00%, P 0.07-0.15%, S <=0.020%, Cu 0.25-0.55%, Cr 0.30-1.25%, Ni <=0.65%; t <=16 mm ReL >=355 MPa, Rm 490-630 MPa, A >=22% | GB/T 4171-2008. The high-weathering (GNH) series trades higher P for patina stability, so thick highly restrained welds need a qualified procedure. |
| Accelerated patina stabilisation (patent example) | Dip 0.5 h; dry-wet cycle 12 h at 25+/-3 C and 60-70% RH, twice a day for 3 days; then about 80 um semi-permeable coat, cured 24 h at room temperature | Chinese patent CN202211055590.9, bridge weathering steel. |
| Coated route, nominal dry film thickness | C4 high durability 2 coats / 240 um; C5 very high 3 coats / 320 um | Minimum systems for new carbon steel in EN ISO 12944-5 Annex B (third-party summary; verify the official text before tendering). |
| Life-cycle cost gap vs painted bridges | 11%-21% lower (100/150/200 year assessment) | Wang Chunsheng et al., Journal of Traffic and Transportation Engineering 2024, 24(3): 69-81, DOI 10.19818/j.cnki.1671-1637.2024.03.004. |
Why the rust protects - and when it stops
A two-layer patina
Long-exposed weathering steel shows a dense inner layer, dominated by amorphous spinel-type iron oxide and goethite (alpha-FeOOH), beneath a looser outer layer rich in lepidocrocite (gamma-FeOOH). Copper, chromium, nickel and phosphorus accelerate densification of the inner layer, raise the alpha/gamma ratio and reduce the electrochemical activity of the rust, which is what makes the annual corrosion rate decay with exposure time.
Wetting time is the hidden switch
Published exposure work gives an empirical boundary: if the surface is wet for less than about 20% of the time, a protective patina starts to develop within months and continues to densify over a decade. Near water, in fog, or where drainage is poor, maghemite increases, nano-scale goethite falls, the rust exfoliates and protection is lost. Two faces of the same member can therefore behave very differently.
How chloride defeats the mechanism
Chloride penetrates the rust and concentrates at the steel interface, lowering local pH and forming electrochemically active akaganeite (beta-FeOOH). Conventional grades usually contain less than 0.4% Ni and resist this poorly; 3% Ni grades change the ion selectivity of the patina and push chloride outwards - a nine-year marine exposure at 79 mg/(m2 d) reported roughly twenty times the resistance of conventional weathering steel. That relative gain does not make heavy marine air suitable for bare use.
The bare-use boundary, category by category
| Cl- deposition mg/(m2 d) | Conventional: year 1 um / 20-year um per a | 3% Ni: year 1 um / 20-year um per a | Recommended route |
|---|---|---|---|
| <=6 (rural, dry urban) | typically below 6 um/a | n/a | Bare use possible; still design for drainage and avoid crevices |
| about 18 (mild marine) | 13.78 / 7.57 | 19.66 / 4.44 | Conventional already exceeds 6 um/a - reduce the coating or re-verify a Ni grade |
| about 30 (moderate marine) | 35.56 / 7.49 | 28.53 / 3.83 | Coat conventional steel; Ni grade arguable only with strict detailing |
| about 75 (severe marine) | 54.78 / 11.54 | 42.58 / 5.72 | Outside bare-use scope: ISO 12944 C5 system or metal spray coating |
| 432.3 (tropical marine, Male) | 74 / 69.28 | 49 / 18.23 | Coating mandatory; bare use is not acceptable risk |
Grade equivalence
| GB/T 4171-2008 | P content | EN 10025-5 | JIS | ASTM | Role |
|---|---|---|---|---|---|
| Q355GNH | 0.07-0.15% | S355J0WP / S355J2WP | SPA-H (G3125) | A242 Type 1 | High-weathering series; strong patina stability, cautious welding of thick plate |
| Q355NH | <=0.030% | S355J0W / J2W / K2W | SMA490AW/BW/CW (G3114) | A588M (e.g. Gr. K) | Weldable weathering series for primary bridge members |
| Q295GNH | 0.07-0.12% | - | - | - | Light structures and fittings; ReL >=295 MPa, Rm 430-560 MPa (t <=16 mm) |
These are grade correspondences from the GB/T 4171-2008 appendix, not equivalence of quality subclasses; import projects must still be inspected against the EN, JIS or ASTM text.
Detailing failures are more common than material failures
- Crevice and water traps: internal diaphragms, stiffener roots and bearing plates stay damp and lose metal far faster than the open air suggests.
- Orientations differ: the windward face collects salt, the shaded face dries slowly, so inspection must be recorded face by face.
- Runstreak staining: early rust wash-off marks concrete and paving; guide the run-off or stabilise the patina first.
- Dissimilar metal contact with stainless steel or aluminium accelerates local attack unless insulated or coated.
- Weld seams, flame-cut edges and heat-straightened zones have hard, tenacious mill scale and rust unevenly; published work recommends grinding or blasting these areas first.
When a coating is still required
- Marine air above roughly 30 mg/(m2 d) chloride, splash zones and members exposed to de-icing salt spray.
- High SO2 industrial atmospheres, chemical plants and fertiliser or livestock buildings where ammonia plus humidity is aggressive.
- Internal voids, laminations and areas behind insulation that cannot dry.
- Architecture where colour uniformity is specified or rust staining is unacceptable.
- Load-bearing parts whose corrosion allowance cannot absorb the measured rate over the design life.
| Case | Surface preparation | System (coats x nominal DFT) | Note |
|---|---|---|---|
| Inland C3, long maintenance-free period | Sa 2 1/2, blast profile medium to coarse (ISO 8503) | Epoxy primer + polyurethane topcoat, 2 coats, 160-200 um | Weathering steel does not relax the preparation requirement |
| Coastal C4, 1-3 km from shore | Sa 2 1/2 plus edge precoat | Epoxy zinc-rich + epoxy MIO + polyurethane, 3 coats, 240 um | Welds and free edges to ISO 8501-3 |
| Long-life bridge scheme from the life-cost study | Sa 2 1/2 | Zinc-rich epoxy 80 um + MIO epoxy 40 um + topcoat 60-70 um | Alkyd, polyurethane or fluorocarbon topcoat |
| Marine C5 / splash zone | Sa 2 1/2 or Sa 3 | Thermal spray Zn-Al + sealer + topcoat (duplex) | Cathodic protection from the metal layer, barrier from the sealer |
| Repaint after years of bare exposure | Remove loose rust to St 3 / Sa 2 1/2, check soluble salts | Converter or barrier epoxy + finishing system | Chloride left under the new film causes early blistering |
Faying surfaces and high-strength bolting
Published Chinese work on stabilised patina treatment reports that weld seams, heat-straightened and cut surfaces rust slowly and unevenly, and that high-strength bolted joints may fail the slip-resistance requirement before treatment, improving afterwards. Slip coefficients are a structural-safety matter: test them to the design code, never by assumption.
Three problems when converting a bare structure to paint
- The dense patina is neither a primer nor easy to abrade; blasting to the specified grade costs more than new construction.
- Residual chloride in the rust causes blistering within the first years; test soluble salt (Bresle) before deciding on a sealer.
- Complex geometry - angles, lattices, box interiors - makes stripe coating and pre-treatment the deciding factor in service life.
Stabilising the patina on purpose
The first one to three years of a bare structure are the complaint period: uneven colour, rust dust, yellow run-off. Two industrial routes compress it. The common one is water-spray or cyclic immersion, which accelerates alpha-FeOOH formation; the patent example above gives auditable parameters (dip 0.5 h, 12 h cycles at 25+/-3 C and 60-70% RH twice daily for three days, then an 80 um semi-permeable coating). For landscape and facade work, waterborne acrylic films with sodium molybdate type stabilisers and iron oxyhydroxide additives are reported in Chinese literature; clear or semi-transparent topcoats are also used, provided the re-coating interval is written into the maintenance plan, because a failed sealer shows as patchiness.
Testing and acceptance: what to write into the specification
- Site classification: ISO 9225 / GB/T 19292.3 for SO2 and chloride deposition, ISO 9226 / GB/T 19292.4 for the first-year corrosion rate, categories per ISO 9223:2012 / GB/T 19292.1-2018.
- Material verification: chemistry and mechanics to GB/T 4171-2008; relative resistance by the Legault-Leckie index of GB/T 41756-2022 Method 2 (Larabee-Coburn basis, valid only for the Cu-Ni-Cr-Si-P systems); long-term loss by Method 1 log-power extrapolation with at least three data points, the earliest one after a stable patina has formed - not less than one year of exposure in industrial and marine air.
- Patina assessment: cross-section SEM plus XRD for the inner/outer structure and the alpha/gamma ratio; a clear rise of beta-FeOOH signals chloride ingress.
- Coated work: film thickness statistics per ISO 19840 / ISO 2808, adhesion per ISO 4624, ageing rating per the ISO 4628 series.
- Follow-up: define 3-, 5- and 10-year depth-of-rust sampling points, and write the trigger "measured rate above the criterion converts the element to a painted maintenance scheme" into the technical agreement.
Economics, stated honestly
The Chinese life-cycle study compared three unpainted weathering steel composite-girder bridges with four painted schemes over 100, 150 and 200 year design lives and found the unpainted option 11%-21% cheaper over the life cycle, with the difference sitting in steel and welding consumables, patina inspection, coating maintenance and the environmental and user cost of traffic disruption. Two sensitivities matter: raising the washing frequency from once every six years to once a year consumes 5%-11% of that saving, and once about 70% of the surface has to be painted anyway, the unpainted bridge becomes the more expensive option.
For export projects we therefore issue two routes side by side in the quotation and technical agreement: a bare-use route with site classification, coupon plan and inspection thresholds, and a reduced- or full-coating route built on EN ISO 12944-5 Annex B minima (240 um for C4 high durability, 320 um for C5 very high). Supply terms are EXW or FOB, with TDS, MSDS and application data sheets.
FAQ
Q1 Can weathering steel be painted like ordinary carbon steel?
Yes, and in most industrial duty it should be. Mill scale and rust must still be removed to Sa 2 1/2 with a controlled profile; "weathering" changes nothing about surface preparation.
Q2 Our site is 2 km from the sea. Can we go bare?
Distance is not the criterion - wind, terrain and shading dominate. Measure chloride deposition: above roughly 12 mg/(m2 d) the bare-use argument weakens for conventional grades, and above 30 mg/(m2 d) it is only arguable for Ni-enhanced grades with careful detailing.
Q3 How do landscape pieces keep the rust look without staining walls?
Stabilise the patina in the shop, then apply a clear or semi-transparent sealer, and detail so rust water cannot run over light substrates. A sealer is a coating: it ages and needs a re-application interval.
Q4 What about high-strength bolt faying surfaces on a bare structure?
Test them. Published work reports slip-resistance shortfalls before patina treatment and improvement after it; values and re-inspection follow the design code.
Q5 Is weathering steel cheaper than galvanising or Zn-Al-Mg coating?
Only inside one common frame. The three routes fail differently, so compare them over the same assessment period and the same corrosivity category using measured rates, and remember Zn-Al-Mg has its own risk in ammonia-rich air.
Q6 Can a structure that rusted for ten years be repainted?
Yes, with heavier preparation: remove loose and exfoliated rust to the required grade, check soluble salts, then convert or seal before the finishing system. Painting straight over active rust typically re-blister within one or two wet-dry seasons.
Q7 How do we avoid ordering the wrong grade?
Put standard number, grade, phosphorus range, impact temperature subclass, delivery condition and UT requirements in the technical annex. Q355GNH (P 0.07-0.15%) and Q355NH (P <=0.030%) differ in weldability as well as weathering performance.
References
- GB/T 4171-2008 Weathering structural steel; GB/T 30790 series (IDT ISO 12944).
- GB/T 19292.1-2018 (issued 2018-05-14, effective 2019-02-01, replacing GB/T 19292.1-2003) and GB/T 19292.2 / .3 / .4-2018; ISO 9223:2012.
- GB/T 41756-2022 Corrosion of metals and alloys - Method for estimating the atmospheric corrosion resistance of low-alloy steels (issued 2022-10-12, effective 2023-02-01).
- EN ISO 12944-5:2020 / ISO 12944-5:2018, Protective paint systems.
- Development and optimization of Ni-advanced weathering steel: a review, 2021 (salinity ceiling, 6 um/a criterion, Male and Spain exposure data).
- Wang Chunsheng et al., Life-cycle economic evaluation model of long lasting weathering steel bridges, Journal of Traffic and Transportation Engineering 2024, 24(3): 69-81, DOI 10.19818/j.cnki.1671-1637.2024.03.004.
- Chinese patent application CN202211055590.9, patina stabilisation with a semi-permeable coating for bridge weathering steel.
- Modern Paints and Coatings 2025, 28(12): 69-72, research on stabilised patina treatment (weld seams, flame-cut edges, bolted joints).
Figures are taken from published standards, journal papers and patent documents; items marked as third-party summaries must be checked against the official text before contractual use.
Last updated: 2026-10-23 | Prepared by the technical department of Kexin New Materials (Guangdong) Co., Ltd., covering industrial heavy-duty anticorrosion coatings; supply terms EXW / FOB with TDS, MSDS and application data sheets.