ASTM A588 Grade A: Standard and Designation
ASTM A588/A588M covers high strength low alloy structural steel shapes, plates and bars with a corrosion resistance substantially better than that of plain carbon structural steel, intended for welded, riveted or bolted construction. Grade A of this standard, usually written A588GrA, is the plate product of the family, supplied with a specified minimum yield point of 345 MPa (50 ksi) and a specified minimum tensile strength of 485 MPa (70 ksi) for thicknesses up to 100 mm (4 in).
The grade is selected where a structure is to be left unpainted and is expected to give a long maintenance free life in an atmospheric environment. It is not a stainless steel and it is not intended for continuous immersion or for heavy chloride exposure; the corrosion benefit depends entirely on the protective oxide layer being able to form and stay intact.
Chemical Composition of A588 Grade A
| Element | Content (%) |
|---|---|
| C | 0.19 max |
| Mn | 0.80 to 1.25 |
| Si | 0.30 to 0.65 |
| P | 0.04 max |
| S | 0.05 max |
| Cu | 0.25 to 0.40 |
| Cr | 0.40 to 0.65 |
| Ni | 0.40 max |
| V | 0.02 to 0.10 |
The copper, chromium and nickel addition is the core of the corrosion behaviour: these elements are what let the surface oxide become dense and adherent instead of flaking off as loose rust. Vanadium contributes strength through precipitation, silicon supports the stability of the oxide film, and the carbon and manganese levels are kept moderate to preserve weldability and toughness.
Weathering Behaviour and the Protective Patina
Left bare and exposed to alternating wet and dry cycles, A588 Grade A develops a dark, tightly adhering oxide layer within the first one to three years of service, and the corrosion rate then falls to a small fraction of the rate for ordinary carbon steel in the same environment. This is the reason the grade is specified for bridges, transmission structures and building facades where access for repainting would be costly.
The patina does not form properly under continuous wetting, in buried service, in splash and tidal zones, or where chlorides and industrial pollutants concentrate on the surface. Runoff discolouration on adjacent concrete or paving is also normal in the early years and is usually dealt with by detailing rather than by changing the material. Good detailing keeps water off horizontal surfaces, avoids pockets and crevices, and directs runoff away from architectural finishes.
Mechanical Properties and Product Forms
| Property | Requirement |
|---|---|
| Minimum yield point | 345 MPa (50 ksi) for thickness up to 100 mm |
| Minimum tensile strength | 485 MPa (70 ksi) |
| Product forms | plates, shapes and bars under A588/A588M |
| Available sheet thickness | commonly 3 mm to 150 mm, wider range on agreement |
Elongation requirements are stated in ASTM A588/A588M for each product form and plate thickness, and the inspection document reports the measured values together with the heat analysis. For welded work the grade is welded with procedures qualified under the applicable structural welding code, using consumables that match the strength and the weathering characteristics of the base metal so that the joint does not become the weak point of the corrosion system.
Fabrication and Detailing Notes
Fabrication follows normal structural steel practice with a few extra rules. Cut edges are usually left as cut or lightly ground; when the steel is to remain unpainted, grinding marks and weld spatter should be removed so that the oxide layer develops evenly. Where weathering steel is joined to plain carbon steel or to a coated component, the dissimilar surfaces should be insulated or detailed so that water does not run from the more noble material onto the less noble one. In bolted work, fasteners with a compatible weathering finish are used where the appearance of the connection matters.
Frequently Asked Questions
Q: What is A588 Grade A steel?
A: It is a high strength low alloy structural steel plate covered by ASTM A588/A588M, with a minimum yield point of 345 MPa and a minimum tensile strength of 485 MPa, developed for atmospheric corrosion resistance.
Q: Does A588 Grade A have to be painted?
A: No. It is designed to be used bare and allowed to form a protective patina, which is the usual specification for bridges and exposed structures. Painted use is also possible when a particular finish is required.
Q: How long does the patina take to form?
A: The oxide layer normally develops and darkens over the first one to three years of exposure, depending on the climate, the orientation of the surface and the presence of pollutants or chlorides.
Q: Can A588 Grade A be used in marine environments?
A: It is not recommended for continuous salt water exposure, splash zones or heavy airborne chloride conditions, because the protective layer cannot stabilise and pitting may occur.
Q: Which grades are comparable to A588 Grade A?
A: Comparable weathering grades in other systems include S355J0W and S355J2W of EN 10025-5 and Q355NH of the Chinese GB/T 4171 series, subject to a property by property comparison.
Q: Which documents accompany the plate?
A: The mill issues an inspection document with the heat analysis and the mechanical test results for the plate, and ultrasonic or other additional testing can be specified where the application requires it.