Weathering steel and ASTM A572 Grade 50 are both high strength low alloy structural steels, and on a strength sheet they look similar. What separates them is chemistry, and specifically the deliberate additions that let a weathering steel form its own protective surface layer instead of being painted. Understanding where the two compositions diverge is the fastest way to decide which one a given structure actually needs.
Base Chemistry of A572 Grade 50
A572 Grade 50 is a columbium or vanadium micro-alloyed structural steel. Strength comes from a modest alloy addition combined with controlled rolling, not from a large alloy content, which is why it remains cheap and readily weldable. The composition requirements are broad by design.
| Element, mass % | A572 Grade 50, max |
|---|---|
| C | 0.23 |
| Mn | 1.35 |
| P | 0.04 |
| S | 0.05 |
| Si | 0.40 |
Thickness up to about 20 mm is commonly supplied, with minimum yield strength of 345 MPa and minimum tensile strength of 450 MPa. Elongation in 50 mm is specified at a minimum of 18 percent. The grade is used for structural plate, shapes and bar where ordinary atmospheric exposure is accepted and a coating system is applied.
Chemistry of Weathering Steels
Weathering grades keep the low carbon and low alloy character of a structural steel but add copper, chromium, nickel and in some specifications phosphorus and vanadium. The aim is not higher strength; it is a different corrosion product. Copper and chromium change the chemistry and porosity of the rust, phosphorus accelerates the early formation of the protective layer, and nickel improves toughness and helps the layer stay intact through wet and dry cycles.
| Element, mass % | Typical weathering structural plate | Role |
|---|---|---|
| C | max 0.19 | Strength with weldability |
| Mn | 0.80-1.25 | Strength, hardenability |
| P | max 0.04 | Accelerates protective layer formation |
| S | max 0.05 | Residual, kept low for toughness |
| Si | 0.30-0.65 | Deoxidation, strength |
| Cu | 0.25-0.40 | Key element for corrosion resistance |
| Cr | 0.40-0.65 | Refines and stabilises the rust layer |
| Ni | max 0.40 | Toughness, layer adherence |
Minimum mechanical properties for the weathering plate grades are broadly comparable to the plain structural grade: minimum yield strength of 345 MPa and minimum tensile strength of 485 MPa for the widely used structural weathering grades, with elongation of at least 18 percent. The extra alloy content costs money, and it is only justified when the steel will be left exposed.
Standard Coverage by Product Form
The weathering family is spread across several specifications rather than a single one, and the correct choice depends on the product form and thickness being bought.
| Specification | Coverage | Practical note |
|---|---|---|
| High strength structural weathering grades, copper bearing | Structural shapes, plate and bar | Most common for bridges and building frames |
| High strength low alloy structural grades | Plate, generally up to about 12.5 mm | Minimum copper content of 0.20 percent applies |
| Sheet and strip specifications, weathering type | Coil and sheet | Used for cladding and architectural panels |
| Structural plate and bar | Heavy plate and bar | Higher minimum copper and strength |
| Bridge specifications with weathering suffix | Bridge plate and shapes | Bridges designates the weathering option separately |
How the Protective Layer Works
In a plain structural steel the rust layer is porous and flaky. Water and oxygen keep reaching the metal, so attack continues at a roughly constant rate and thickness loss accumulates. In a weathering steel the alloy additions change the rust into a dense, tightly adherent oxide that limits the supply of oxygen to the surface. The rate of further corrosion falls sharply, in the same way that the parabolic stage of high temperature oxidation slows an oxide film. The layer is not permanent. It regenerates after mechanical damage, but only while the surface keeps alternating between wet and dry; if water stands on the steel, or if chloride deposits are heavy, the protective layer breaks down and the weathering steel corrodes like an unalloyed one.
Fabrication and Detailing Consequences
The chemistry that makes the material weather well also makes it slightly harder to fabricate. Phosphorus and copper increase susceptibility to hot cracking if the material is welded without adequate filler control, and copper can cause surface hot shortness during reheat if the furnace atmosphere is not managed. Welding consumables for weathering steel normally match the alloy content so the joint weathers at the same rate as the parent metal. Detailing is where projects succeed or fail: provide drainage paths, avoid pockets and crevices that trap water, keep the steel out of continuous immersion and splash zones, and specify the material only for those parts of the structure that can stay clean and dry.
Choosing Between the Two
If the structure will be painted, coated or fully protected for its service life, a plain high strength low alloy plate such as A572 Grade 50 is usually the economical choice. If the structure will be exposed, difficult to repaint and detailed so that it drains freely, a weathering grade can remove the coating maintenance burden and often the whole lifecycle cost of the finish. The two steels should not be mixed in a single exposed assembly without thought, because the uncoated weathering member will sacrifice itself to protect the plain steel at any direct connection.
Frequently Asked Questions
Q: Is A572 Grade 50 a weathering steel?
A: No. It is a plain high strength low alloy structural steel without the copper, chromium and nickel additions that produce a protective rust layer, and it normally needs a coating system in exposed service.
Q: What are the main alloy additions in weathering steel?
A: Copper, chromium and nickel, with phosphorus and vanadium used in some grades. Copper and chromium control the structure of the rust, phosphorus speeds up its formation and nickel supports toughness and adhesion.
Q: Are the mechanical properties much higher?
A: Not much. Minimum yield strength around 345 MPa is common to both, and the weathering grades typically guarantee a higher minimum tensile strength, but the real difference is corrosion behaviour rather than strength.
Q: Can weathering steel be used in coastal locations?
A: Only with care. Chlorides in air and sea spray keep the protective layer from stabilising, so coastal and splash zone exposure normally calls for a coating even on weathering grades.
Q: Does the protective layer need any maintenance?
A: Very little, provided the steel drains freely and does not collect debris. Where water stands, or where run-off from uncoated plain steel or concrete reaches the surface, localised attack will develop.
Q: Can the two steels be joined in the same structure?
A: They can be welded, but an uncoated weathering member in electrical contact with plain carbon steel will corrode preferentially. Connections in mixed assemblies are normally detailed with insulation or with matching material.
