A588 Weathering Steel Properties — Yield Strength, Patina & Applications
ASTM A588 is a high-strength low-alloy (HSLA) weathering steel with a yield strength of 50 ksi (345 MPa) and tensile strength of 70 ksi (485 MPa). When exposed to atmospheric conditions, A588 forms a stable, protective oxide patina that eliminates the need for painting in suitable environments. This page covers A588 mechanical properties, patina development, environmental limitations, and AISC 360 design guidance.
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A588 Weathering Steel Mechanical Properties
Yield and Tensile Strength
| Property | Imperial | Metric | AISC 360 Symbol |
|---|---|---|---|
| Yield strength (Fy) | 50 ksi | 345 MPa | Fy |
| Tensile strength (Fu) | 70 ksi | 485 MPa | Fu |
| Modulus of elasticity (E) | 29,000 ksi | 200,000 MPa | E |
| Shear modulus (G) | 11,200 ksi | 77,200 MPa | G |
| Poisson's ratio | 0.30 | 0.30 | — |
| Density | 490 lb/ft³ | 7,850 kg/m³ | — |
Elongation Requirements
| Specimen | Minimum Elongation |
|---|---|
| 8-inch gauge length | 18% |
| 2-inch gauge length | 21% |
Key Property Difference from A572-50
A588 has 8% higher tensile strength (70 ksi vs 65 ksi) compared to A572 Grade 50, while maintaining the same 50 ksi yield. This provides additional reserve strength and slightly better fatigue performance.
A588 Chemical Composition
Heat Composition Requirements
| Element | Plate Max (%) | Shapes Max (%) | Typical (%) |
|---|---|---|---|
| Carbon (C) | 0.19 | 0.20 | 0.15 |
| Manganese (Mn) | 0.80-1.25 | 0.75-1.35 | 1.00 |
| Phosphorus (P) | 0.035 | 0.04 | 0.02 |
| Sulfur (S) | 0.04 | 0.05 | 0.025 |
| Silicon (Si) | 0.30-0.65 | 0.15-0.40 | 0.35 |
| Nickel (Ni) | 0.40 | 0.40 | 0.25 |
| Chromium (Cr) | 0.40-0.65 | 0.40-0.70 | 0.55 |
| Copper (Cu) | 0.25-0.40 | 0.20-0.40 | 0.30 |
| Vanadium (V) | 0.02-0.10 | 0.01-0.10 | 0.04 |
Weathering Alloy Elements
The corrosion resistance of A588 comes from its alloying elements:
- Chromium (0.40-0.70%) — Forms a dense, adherent oxide layer
- Copper (0.20-0.40%) — Accelerates patina formation and improves resistance
- Nickel (up to 0.40%) — Enhances patina stability
- Phosphorus (up to 0.04%) — Aids in patina formation (controlled addition)
How Weathering Steel Works
Patina Formation Process
- Initial corrosion (0-6 months) — Steel corrodes like ordinary carbon steel. Rust appears as a reddish-brown layer.
- Patina development (6 months - 3 years) — The oxide layer becomes denser and changes color from reddish-brown to dark brown. Chromium and copper ions incorporated into the oxide create a protective barrier.
- Stable patina (3+ years) — The patina reaches equilibrium. Corrosion rate drops to 0.3-0.5 mils per year (compared to 5-10 mils per year for unpainted carbon steel).
Patina Appearance
| Stage | Time | Color | Corrosion Rate |
|---|---|---|---|
| Initial | 0-6 months | Orange-red | 5-10 mpy |
| Developing | 6-36 months | Brown | 2-5 mpy |
| Stable | 3+ years | Dark brown/black | 0.3-0.5 mpy |
When to Use Weathering Steel
Suitable Environments
A588 performs best in environments with alternating wet and dry cycles:
- Rural environments — Low pollution, good wet/dry cycling
- Suburban environments — Moderate conditions, generally suitable
- Urban environments — Acceptable if not adjacent to industrial sources
- Highway bridges — Excellent application (road salt requires detail design)
Unsuitable Environments
Do NOT use A588 without painting in:
- Marine environments — Persistent salt spray prevents stable patina
- Industrial environments — Sulfur dioxide and chloride concentrations destroy patina
- Continuously humid — No dry cycle to stabilize patina
- In contact with soil — Persistent moisture and chlorides
- Standing water — No atmospheric wet/dry cycling
- Salt storage areas — Direct chloride exposure
Decision Matrix
| Environment | Use A588 Unpainted? | Alternative |
|---|---|---|
| Rural bridge | Yes | — |
| Suburban building | Yes | — |
| Highway overpass | Yes (with drainage details) | — |
| Coastal bridge | No | Paint A588 or use A572 + paint |
| Industrial plant | No | A572 + paint/coating |
| Parking garage | No (de-icing salts) | Galvanized or painted |
A588 for Bridge Design
AASHTO/AREMA Requirements
Weathering steel is widely used in bridge construction. Key specifications:
- AASHTO M270 — Bridge steel specification (A709-50W is the bridge equivalent of A588)
- AREMA — Railway bridge design
- AISC 360 — Building design
Bridge Design Considerations
- Drainage details — Prevent water from pooling on steel surfaces. Provide minimum 2% cross-slope.
- Splash zones — Avoid or protect areas where road spray contacts steel continuously.
- Salt exposure — In northern climates, road salt accelerates corrosion. Design for drainage away from steel.
- Expansion joints — Detail to prevent salt-laden water from dripping onto steel below.
- Bearing areas — Paint or coat bearing surfaces to prevent crevice corrosion.
A709-50W vs A588
A709-50W is the bridge specification for weathering steel:
| Attribute | A588 | A709-50W |
|---|---|---|
| Fy | 50 ksi | 50 ksi |
| Fu | 70 ksi | 70 ksi |
| Application | Buildings | Bridges |
| Charpy testing | Supplementary | Required (Zone-based) |
| Certification | Mill cert | MTR with CVN |
Weldability
A588 has good weldability, similar to A572 Grade 50.
Preheat Requirements
| Thickness | Preheat (AWS D1.1) |
|---|---|
| Up to 3/4 in. (19 mm) | Not required (above 32°F) |
| 3/4 to 1-1/2 in. (19-38 mm) | 50°F (10°C) minimum |
| 1-1/2 to 2-1/2 in. (38-64 mm) | 150°F (65°C) minimum |
| Over 2-1/2 in. (64 mm) | 200°F (93°C) minimum |
Electrode Selection for Weathering Steel
Use weathering steel electrodes to match the corrosion resistance:
| Process | Electrode | Notes |
|---|---|---|
| SMAW | E8018-W2 | Weathering steel electrode |
| FCAW | E81T1-W2C | Weathering flux-cored wire |
| GMAW | ER80S-G (weathering) | Match copper-chrome-nickel |
| SAW | F8A6-ECNi2 | Weathering submerged arc |
Important: If A588 is left unpainted, weld metal must also resist corrosion. Standard E7018 electrodes will corrode preferentially at welds, creating an unsightly appearance. Use weathering-matching electrodes.
AISC 360 Design with A588
Available Strength Values
A588 has higher Fu (70 ksi) than A572-50 (65 ksi), which affects:
| Limit State | A588 | A572-50 | Difference |
|---|---|---|---|
| Flexure (yielding) | 50Zx | 50Zx | Same |
| Tension (yielding) | 50Ag | 50Ag | Same |
| Tension (fracture) | 70An | 65An | +8% |
| Bolt bearing | 70 | 65 | +8% |
| Weld rupture | 70 | 65 | +8% |
Worked Example — A588 Plate in Tension
Problem: A 3/4" x 12" A588 plate with two lines of 7/8" diameter bolts. Find the available tensile strength.
Given:
- Plate: 3/4" x 12" (Ag = 9.00 in²)
- Bolts: 2 lines of 7/8" dia, 4 bolts per line
- Hole diameter: 7/8" + 1/16" = 15/16" per AISC
Solution:
- Net area: An = 9.00 - 2(15/16)(3/4) = 9.00 - 1.406 = 7.594 in²
- Yielding: phiPn = 0.90 × 50 × 9.00 = 405 kips
- Fracture: phiPn = 0.75 × 70 × 7.594 = 399 kips
- Governs: 399 kips (fracture)
Note: With A572-50, fracture would be 0.75 × 65 × 7.594 = 370 kips. A588 provides 8% more fracture strength.
Calculator
Design steel members with A588 weathering steel using our free calculators:
- Beam Capacity Calculator → — Check flexural strength for A588 beams
- Column Buckling Calculator → — AISC compression capacity
- Bolted Connection Calculator → — Connection design with higher Fu
- Section Properties → — Full AISC section database
FAQ
Q: What is the yield strength of A588 weathering steel? A: ASTM A588 has a minimum yield strength of 50 ksi (345 MPa) and a minimum tensile strength of 70 ksi (485 MPa). The yield is the same as A572-50, but the tensile strength is 8% higher.
Q: When should I use A588 weathering steel? A: Use A588 when the structure is exposed to weather in rural, suburban, or moderate urban environments with alternating wet/dry cycles. The steel forms a protective rust patina that eliminates painting. Do not use in marine, industrial, or continuously humid environments.
Q: How long does it take for the patina to form? A: The stable patina typically develops in 2-5 years, depending on the environment. During the initial 6 months, the steel will rust and stain like ordinary carbon steel. The patina stabilizes as chromium and copper ions become incorporated into the oxide layer.
Q: Can I paint A588 weathering steel? A: Yes, A588 can be painted if desired or if the environment does not support stable patina formation. Painting A588 follows the same procedures as painting A572 or A36. The weathering alloy elements do not interfere with paint adhesion.
Q: Is A588 more expensive than A572 Grade 50? A: Yes, A588 typically costs 15-25% more than A572 Grade 50 due to the chromium, copper, and nickel alloying. However, the life-cycle cost is often lower because painting is eliminated. A typical bridge paint system costs $8-15/ft² initially plus repainting every 15-25 years.
Q: What is the difference between A588 and A709-50W? A: A588 is the building specification for weathering steel. A709-50W is the bridge specification. Both have Fy=50 ksi and Fu=70 ksi. A709-50W requires Charpy V-notch testing based on the bridge's temperature zone. A588 does not require Charpy testing unless specified.
Related: US Steel Grades → | A572 Grade 50 → | US Steel Comparison → | Beam Design Guide →