Weathering Steel: A Guide to Corten and the A/B Equivalents, Origins & Standards

by AMC

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Posted on February 12, 2024 at 03:57 PM

Weathering steel (WS), often referred to as low‐alloy steel, represents a category where the carbon content is kept below 0.2 wt. %, with key alloying elements including Cu, Cr, Ni, P, Si, and Mn, collectively constituting no more than 3‐5 wt. %.

The unique chemical composition of these steel alloys is carefully tailored to encourage the early formation of a rust/iron oxide layer, serving as a natural weather-protective coat for the underlying steel.

The corrosion rate of Weathering steel is remarkably low, enabling bridges constructed from weathering steel to boast a design life of up to 120 years with minimal maintenance.

In practical terms, weathering steel construction offers an array of advantages, encompassing reduced overall lifecycle costs and enhanced safety due to the absence of a protective paint system, leading to reduced inspection cycles. The absence of a paint requirement not only eliminates the release of volatile organic compounds into the atmosphere but also accelerates the construction process.

Generally, the costs of weathering steel bridges are 5% lower than conventional painted steel alternatives, considering both initial material cost and maintenance perspectives. Introduced in 1933 with the primary aim of eliminating the need for painting and other maintenance on ore wagons, this alloy evolves to showcase a rust-like appearance or patina. This distinctive feature is now highly coveted by architects, gracing prominent structures such as The John Deere World Headquarters in Illinois and The Angel of the North monument in the UK. Beyond its aesthetic allure, the patina on Weathering steel not only provides superior corrosion resistance compared to mild steel but also contributes to its captivating appearance and inherent self‐healing capabilities. Weathering steel, with its rich history and multifaceted benefits, continues to shape both the functional and aesthetic aspects of construction projects worldwide.

Chemistry of Weathering Steel

The remarkable corrosion resistance of Weathering Steel (WS) finds its roots in a meticulously balanced alloy composition. While increased copper and nickel content play pivotal roles, other alloying elements, including Cr, Mn, and P, contribute their unique strengths to the mix. Copper, in particular, emerges as a key player, fostering the bond between the protective oxide layer and the metal, effectively decelerating the corrosion process.

The enchanting aspect of WS lies in its ability to rust deliberately. Unlike conventional steel, this alloy rusts at a leisurely pace, forming a protective coating that acts as a natural deterrent against future corrosion. Scientific studies reveal that a rhythmic cycle of wet and dry conditions is imperative for the optimal formation of a dense and adherent rust layer. Rainwater plays a crucial role in cleansing the steel surface, allowing accumulated moisture to drain efficiently, followed by swift drying.

Structures, therefore, need to be devoid of crevices where water could accumulate, as corrosion in such areas might occur without the formation of the protective patina, as evidenced by a recent study on motorway bridges in the Czech Republic.

The oxidation journey can span several years before achieving a steady-state ‘stabilisation’ of the surface, marked by a tightly bonded coating. The pace of this process is intricately tied to the prevailing atmospheric conditions. The contribution of phosphorus and sulphur to the formation of the patina layer is noteworthy, with low-solubility sulphates or phosphates developing between the base steel and existing corrosion, especially when the surface experiences cyclic wet and dry periods.

ISO 9223 suggests that atmospheres with SO2 pollution foster the creation of a more protective rust layer. While Leygraf and Graedel support this claim, they also caution against excessive non-metal oxides, which can lead to intense acidification, hindering patina formation. Phosphorus, in addition to forming a protective passive film over the steel surface, acts as a guardian against aggressive ions and moisture, supporting the formation of a coherent and dense patina layer. However, the inclusion of phosphorus may impact the alloy grain structure and mechanical strength of the steel, prompting the addition of low levels of boron or carbon to restore the necessary grain boundaries. In the alchemical dance of elements, Weathering Steel reveals its secrets, crafting both resilience and aesthetic allure through the intricate balance of its chemical composition.

Cor-Ten: Corrosion Resistance and Strength

In the world of weather-resistant steels, Cor-Ten stands as an illustrious maestro. The very name Cor-Ten encapsulates the two defining features that set it apart from conventional carbon steel (CS): corrosion resistance (Cor) bestowed by its copper component and exceptional tensile strength (Ten), showcasing superior mechanical properties. In fact, Cor-Ten proudly boasts a remarkable 30% enhancement in mechanical properties and a weather resistance capacity 4-8 times greater than traditional CS.

Cor-Ten unfolds in two variations: Cor-Ten A and Cor-Ten B. The ASTM standard designates A 242 for Cor-Ten A (up to 12.7mm thick), while the newer ASTM grades, A 588, encapsulate Cor-Ten B (over 12.7mm thickness). Welding Cor-Ten demands precision, with options ranging from gas shield to spot or submerged arc welding, tailored to the steel's thickness. A meticulous approach is crucial to ensure that the welding method allows the rusting process to mirror the natural evolution of the entire structure. While Cor-Ten A and B share similarities, the former tends to have a higher phosphorus content for an added layer of corrosion resistance, enhancing its durability against the elements. In the grand symphony of steel, Cor-Ten harmonizes corrosion resilience with formidable strength, creating a masterpiece that weathers the tests of time.

Grade	C [%]	Si [%]	Mn [%]	P [%]	S [%]	AI [%]	Cu [%]	Cr [%]	Ni [%]
Corten A	0.12	0.25/0.75	0.20/0.50	0.07/0.15	0.030	0.015/0.06	0.25/0.55	0.50/1.25	0.65

Grade	Thickness (mm) Strip Products	Plate Products	Yield Strength RelN/mm² Minimum	Tensile Strength RmN/mm² Minimum	Elongation Aso% Minimum
Corten A	2-12	6-12	345	485	20

Grade	C [%]	Si [%]	Mn [%]	P [%]	S [%]	AI [%]	Cu [%]	V [%]	Cr [%]	Ni [%]
Corten B	0.19	0.30/0.65	0.80/1.25	0.035	0.030	0.02/0.06	0.25/0.40	0.02/0.1	0.40/0.65	0.40

Grade	Thickness (mm) Strip Products	Plate Products	Yield Strength Rel N/mm² Minimum	Tensile StrengthRm N/mm² Minimum	Elongation A50 % Minimum
Corten B	2 – 13	6 – 40	345	485	19

PATINAX

In the world of weathering steels, European EN 10025 specifications unfold as counterparts to the familiar tunes of Cor-Ten and PATINAX, showcasing similarity in both corrosion resistance and mechanical properties [7]. Cor-Ten A and PATINAX 355P align seamlessly with EN 10025 S355 J0WP, sharing a comparable phosphorus content (Cor-Ten A at 0.20% vs. S355 J0WP and PATINAX at 0.15%), fostering heightened corrosion resistance. On the other side of the spectrum, Corten B and PATINAX 355 take the stage as counterparts to European specifications EN 10025 S355 J2W and EN 10025 S355 J2W+N, each echoing similar tolerances of vanadium, copper, manganese, aluminum, and chromium.

Yet, the performance nuances emerge in the dance of alloying elements. For EN 10025 S355 J2W+N and PATINAX 355, a quest for a finer grain structure prompts the inclusion of a diverse set of nitrogen-binding elements in the steel composition (e.g., ≥0.02% Aluminum, Nb 0.015-0.060%, V 0.02-0.12%, Ti 0.02-0.10%). In this symphony of equivalence, European standard weathering steels resonate with the corrosion-resistant melodies and mechanical harmonies of their global counterparts.

Grade	Max. plate thickness [mm]	Grade acc. to EN 10025-5	Alloying elements	Re, min [MPa]	Rm [MPa]	Amin [%]	T27 [°C]
PATINAX 355P	12.5	S355J2WP+N	Cr Cu P	355	470-630	20	-20
PATINAX 355	50	S355J2W+N	Cr Cu	355 (t≤ 16mm)	470-630	20	-20

Composition of PATINAX 355P

C	Si	Mn	P	S	Cr	Cu	Ni
≤0.12	0.25 - 0.75	0.20 - 0.50	0.07 - 0.15	≤ 0.030	0.50 - 1.25	0.25 - 0.55	≤ 0.65

Steel Grade	Minimum yield point ReH MPa *)	Tensile strength Rm MPa	Minimum elongation A (Lo =5.65√So) %
PANTINAX 355P	355	470 -630	20

Steel Grade	Minimum yield point ReH MPa *)	Tensile strength Rm MPa	Minimum elongation A %
PATINAX 275PK	275	410	25

Weathering fine grain structural steel   Heavy plates	Steel grade		Material No.	Material Specification 532 July 2014
	TKSE-Short name	EN-Short name
	PATINAX 355	S355J2W+N	.8965

PATINAX 355

C	Si	Mn	P	S	Cr	Cu	V	Ni
≤ 0.16	0.30 - 0.50	0.80 - 1.25	≤ 0.030	≤ 0.030	0.40 - 0.65	0.25 - 0.40	0.02 - 0.10	≤ 0.40

Material thickness mm	Minimum yield point ReH MPa*)	Tensile strength Rm MPa	Minimum elongation A (Lo =5.65√So) %
≤ 16	355	470 - 630	20
> 16 ≤ 50	345

European Standard Weathering Steels

In the world of weathering steels, European EN 10025 specifications unfold as counterparts to the familiar tunes of Cor-Ten and PATINAX, showcasing similarity in both corrosion resistance and mechanical properties [7]. Cor-Ten A and PATINAX 355P align seamlessly with EN 10025 S355 J0WP, sharing a comparable phosphorus content (Cor-Ten A at 0.20% vs. S355 J0WP and PATINAX at 0.15%), fostering heightened corrosion resistance. On the other side of the spectrum, Corten B and PATINAX 355 take the stage as counterparts to European specifications EN 10025 S355 J2W and EN 10025 S355 J2W+N, each echoing similar tolerances of vanadium, copper, manganese, aluminum, and chromium.

Yet, the performance nuances emerge in the dance of alloying elements. For EN 10025 S355 J2W+N and PATINAX 355, a quest for a finer grain structure prompts the inclusion of a diverse set of nitrogen-binding elements in the steel composition (e.g., ≥0.02% Aluminum, Nb 0.015-0.060%, V 0.02-0.12%, Ti 0.02-0.10%). In this symphony of equivalence, European standard weathering steels resonate with the corrosion-resistant melodies and mechanical harmonies of their global counterparts.

Thickness Range
	0.6mm	0.8mm	1mm	1.2mm	1.5mm	1.7mm	2mm	2.5mm	3mm
Corten A	x	x	x	x	x	x	x	x	x
Corten B									x
S355JOWP							x	x	x
S355JOW								x
S355J2W+N								x	x
S355J2WP+N
PATINAX 355P									x

Thickness Range
	4mm	5mm	6mm	7mm	8mm	9mm	10mm	12mm	14mm
Corten A	x	x	x		x	x	x	x
Corten B	x	x	x		x		x	x	x
S355JOWP	x	x	x	x	x		x	x
S355JOW		x		x
S355J2W+N	x	x	x	x	x		x	x
S355J2WP+N								x	x
PATINAX 355P	x	x	x		x		x

Thickness Range
	15mm	16mm	18mm	20mm	25mm	30mm	35mm	40mm	50mm	60mm
Corten A
Corten B	x	x	x	x	x	x	x	x	x	x
S355JOWP	x
S355JOW
S355J2W+N	x	x	x	x	x	x	x	x	x	x
S355J2WP+N	x	x	x	x	x	x
PATINAX 355P	x	x	x	x	x	x	x	x	x	x

Corten A	Thickness Range	Available Widths
	0.6mm to 12mm	1000mm, 1250mm, 1500mm, 1800mm, 2000mm, 2500mm

Corten B	Thickness Range	Available Widths
	3mm to 60mm	1000mm, 1250mm, 1500mm, 2000mm, 2500mm, 3000mm

S355JOWP	Thickness Range	Available Widths
	2mm to 15mm	1000mm, 1140mm, 1250mm, 1310mm, 1400mm, 1500mm, 1800mm, 2000mm, 2500mm

S355JOW	Thickness Range	Available Widths
	2.5mm to 7mm	1400mm, 1500mm, 2000mm

S355J2W+N	Thickness Range	Available Widths
	2.5mm to 60mm	1000mm, 1100mm, 1250mm, 1500mm, 2000mm, 2500mm, 3000mm

S355J2WP+N	Thickness Range	Available Widths
	12mm to 30mm	1000mm, 1500mm, 2000mm

PATINAX 355P	Thickness Range	Available Widths
	3mm to 60mm	1000mm, 1250mm, 1500mm, 2000mm, 2500mm, 3000mm

Conclusion

For over five decades, weathering steel has stood as a stalwart choice for structural applications, boasting a commendable legacy of integrity and low maintenance. In this robust lineage, notable counterparts such as Cor-Ten, PATINAX, and their European equivalents have graced the market, offering equivalent excellence in corrosion resistance and mechanical/tensile strength. From the architectural grandeur of buildings to the sturdy spans of bridges, these products have consistently delivered the requisite attributes for a diverse array of applications. As the journey of weathering steel continues, its endurance and adaptability mark it as a timeless solution in the realm of structural engineering.

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