What Weathering Steel Plate Actually Is
Weathering steel plate, usually shortened to WS and described in older texts as atmospheric-corrosion-resistant low-alloy steel, is a structural plate product in which small, deliberately controlled additions of copper, chromium and nickel change the behaviour of the rust layer that forms on an exposed surface. Instead of the loose, flaking oxide found on ordinary carbon steel, the plate develops a dense and tightly adherent oxide film that slows the inward transport of oxygen and moisture. The mechanism depends on alternating wet and dry cycles, which is why the material is normally supplied bare and unpainted for exposed bridge, architectural and marine-atmosphere service.
Composition and the Protective Oxide Film
Leaving aside the alloying package, the base chemistry of a weathering plate is close to that of a plain carbon structural steel: carbon, silicon, manganese, phosphorus and sulfur are all present, with copper, chromium and nickel added on top. Copper promotes the formation of the dense oxide layer, chromium and nickel improve passivation so that the corrosion rate stays low even in aggressive atmospheres, and phosphorus supports film stability while being restricted to avoid embrittlement. Sulfur is treated as a detrimental element and is held low, because sulfide inclusions become initiation sites for pitting.
| Element | Function in the plate | Typical range in WS plate |
|---|---|---|
| C | Strength and hardenability, kept low for weldability | 0.16 % max |
| Si | Deoxidation and solid-solution strengthening | 0.50 % max |
| Mn | Strength and toughness balance | 0.50 - 1.50 % |
| P | Supports atmospheric resistance, restricted against embrittlement | 0.035 % max |
| S | Inclusion former, tightly controlled | 0.035 % max |
| Cu | Primary driver of the protective rust layer | 0.25 - 0.55 % |
| Cr | Passivation and film stability | 0.30 - 1.25 % |
| Ni | Toughness and corrosion resistance | 0.12 - 0.65 % |
Those bands describe normalized weathering structural plate of the Q355NH family in GB/T 4171 and of the S355J2W grade in EN 10025-5. A carbon steel of the same strength class carries none of the deliberate copper, chromium and nickel additions, and that chemistry difference, not the strength level, is what separates the two products.
Grades and Standards in Common Supply
GB/T 4171 covers high atmospheric corrosion resistant structural steels, including Q235NH, Q355NH and Q415NH, with copper and phosphorus controlled as part of the corrosion resistance requirement.
EN 10025-5 covers structural steels with improved atmospheric corrosion resistance, for example S235J0W, S355J2W and S355K2W, delivered in normalized or thermomechanical condition depending on grade.
ASTM A588 / A588M covers high-strength low-alloy structural steel for weight-saving exposed service, while ASTM A242 and ASTM A606 Type 4 cover similar unpainted applications in thinner sections.
Minimum yield strength across the weathering family runs from 235 MPa to 355 MPa, with tensile strength roughly 470 to 630 MPa for the 355 MPa class.
Weathering performance is not assumed from the grade stamp alone. Under the guidance of ASTM G101 a weathering grade is expected to reach an atmospheric corrosion resistance index of at least 6.0, which is approximately four times that of plain carbon steel.
Selecting a Welding Electrode for Weathering Steel
Electrode selection follows three matching rules: the deposit chemistry should resemble the parent plate so that the joint corrodes at a similar rate, the deposit strength should match the plate rather than exceed it, and the electrode must run cleanly in the position and thickness involved.
Composition matching: choose copper-bearing covered electrodes of the low-alloy basic type classified under AWS A5.5, for example E8018-G, or the equivalent classification under EN ISO 2560-A, so that the joint develops its own protective film.
Strength matching: match, do not overmatch. An over-matched deposit on a restrained thick joint raises residual stress and hydrogen cracking risk without adding service value.
Process matching: for panel lines and long runs, gas-shielded metal arc welding with a copper-bearing solid wire to AWS A5.28 ER80S-G is common; for flat and horizontal fillets, a seamless flux-cored wire to AWS A5.29 gives higher deposition efficiency. Submerged arc welding suits long straight seams where a matching agglomerated flux is available.
Thin plate and light components: a rutile or basic low-alloy electrode sized to the section is normally sufficient and gives good arc stability and slag detachability.
Thick plate, restrained joints and low-temperature service: use hydrogen-controlled basic electrodes, dry them to the electrode maker's schedule, and hold a controlled interpass temperature.
Welding Practice and Joint Protection
Clean and dry the plate and the electrodes before striking an arc; free the joint of scale, oil and moisture to avoid porosity and slag inclusions.
Keep welding current, voltage and travel speed within the qualified range, and cap heat input so that the toughness of the fine-grained heat affected zone is preserved.
Preheat is not usually needed for ordinary thickness, but for heavy sections a controlled preheat in the region of 50 to 100 degrees C reduces the risk of cold cracking.
Complete the root before the joint cools excessively, remove slag between passes, and inspect the finished weld by visual examination, ultrasonic or radiographic testing and mechanical property checks as the specification requires.
The critical point on weathering steel is often overlooked: the weld metal and the heat affected zone do not form the protective film as readily as the parent plate. Paint the weld and a band either side of it with a suitable coating, then keep the coating in repair so that the joint is not the first place to corrode.
Frequently Asked Questions
Q: What kind of steel plate is weathering steel?
It is a low-alloy structural plate with deliberate copper, chromium and nickel additions, typically covered by GB/T 4171, EN 10025-5 or ASTM A588, in which the surface oxide film protects the base metal instead of flaking away.
Q: Which welding rod should be used on weathering steel?
Use a copper-bearing low-alloy electrode such as an AWS A5.5 E8018-G basic covered electrode, or a matching copper-bearing wire for gas-shielded or flux-cored welding, so that the deposit chemistry and the protective film behaviour stay close to the plate.
Q: Can ordinary E7018 electrodes be used instead?
They will produce a sound joint on thin, lightly loaded components, but the deposit lacks the copper and nickel that build the protective film, so the weld becomes the preferred corrosion path. Matching electrodes are the safer choice for exposed service.
Q: Does weathering steel need to be painted?
For exposed architectural and bridge service it is normally left bare so that the stable oxide layer can form. Joints, welds and sheltered or salt-laden locations should still be coated, and unpainted plate needs free drainage and dry cycles to behave as intended.
Q: What happens if the protective film is damaged?
The damaged area re-oxidises and the film re-forms, provided the surface stays exposed to alternating wet and dry cycles. Standing water, trapped debris and chloride salt deposits from de-icing or marine spray prevent film formation and must be avoided or washed off.
Q: How is weathering performance verified before supply?
Chemistry and mechanical testing on the plate, together with the corrosion resistance index assessment described in ASTM G101, give an indication of behaviour, and mill test certificates should be reviewed against the ordered grade before the plate is released to fabrication.
