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

Chemistry

Weathering steel, also referred to as low-alloy steel, is characterized by its carbon content of less than 0.2 wt. %, with additional alloying elements such as Cu, Cr, Ni, P, Si, and Mn added in controlled amounts totaling no more than 3‐5 wt. %. This specific chemical composition enables the rapid development of a rust or iron oxide layer upon exposure to the elements. This layer acts as a protective coat, offering weather resistance to the underlying steel.

The corrosion resistance of weathering steel is exceptionally high, allowing bridges constructed from this material to easily achieve a design life of up to 120 years with minimal maintenance. Weathering steel offers various advantages, including lower overall lifecycle costs and improved safety due to the absence of a protective paint system and reduced inspection cycles. The absence of a paint requirement also eliminates the release of volatile organic compounds into the atmosphere and facilitates faster construction.

In terms of cost, weathering steel bridges typically have a 5% lower cost compared to conventional painted steel alternatives, considering both initial material cost and maintenance expenses. Originally introduced in 1933 to eliminate the need for painting and maintenance on ore wagons, weathering steel develops a rust-like appearance or patina over time. This patina is now highly valued by architects and is featured in many renowned buildings and monuments, such as The John Deere World Headquarters in Illinois and The Angel of the North monument in the UK. The patina not only enhances corrosion resistance compared to mild steel but also contributes to the material’s appealing aesthetic and self-healing properties.

The superior corrosion resistance of weathering steel (WS) is primarily attributed to its increased copper and nickel content, along with contributions from other alloying elements like chromium (Cr), manganese (Mn), and phosphorus (P). Copper plays a crucial role in bonding the protective oxide layer to the metal surface, thereby slowing down the corrosion process.

WS is designed to rust gradually, but its alloy composition enables it to rust at a slower rate compared to conventional steel. This rust formation creates a protective coating that hinders future corrosion. Research indicates that wet/dry cycling is vital for the formation of an optimal, dense, and adherent rust layer. Rainwater helps wash the steel surface, allowing accumulated moisture to drain easily and promoting fast drying.

Structures made from WS should be free of crevices where water could collect, as corrosion could occur in these areas without the formation of a protective patina. The oxidation process can take several years before the surface stabilizes with a tightly bonded coating, influenced by atmospheric conditions.

Phosphorus and sulfur also play crucial roles in patina formation. Low-solubility sulfates or phosphates can develop between the base steel and existing corrosion layers, provided there are cyclic wet and dry periods. Environments with sulfur dioxide (SO2) pollution are conducive to forming a more protective rust layer according to ISO 9223 standards. However, excessive non-metal oxides can lead to intense acidification of the aqueous layer, hindering patina formation.

Phosphorus can form a protective passive film over the steel surface, preventing aggressive ion ingress and supporting the formation of a dense patina layer. To counteract potential negative effects on alloy grain structure and mechanical strength, low levels of boron or carbon are added to restore required grain boundaries.

Cor-Ten Steel

Cor-Ten Steel is a well-known brand of weather-resistant steel, distinguished by its corrosion resistance (Cor) due to its copper component and superior tensile strength (Ten). Compared to conventional carbon steel (CS), Cor-Ten is said to offer a 30% improvement in mechanical properties and 4-8 times more weather resistance.

There are two main variations of Cor-Ten available: Cor-Ten A and Cor-Ten B.

ASTM standard designation A 242 refers to Cor-Ten A for thicknesses up to 12.7mm, while the newer ASTM grades, like A 588, represent Cor-Ten B for thicknesses over 12.7mm.

Welding of Cor-Ten can be done using various methods such as gas shield, spot, or submerged arc welding, depending on the steel’s thickness. It’s important to choose a welding method that allows the rusting process to occur in a manner consistent with the rest of the structure.

Although Cor-Ten A and B are similar, Cor-Ten A alloys typically contain a higher phosphorus content for additional corrosion resistance.

Careful consideration during welding and maintenance is crucial to ensure the continued integrity and weather resistance of Cor-Ten structures.

Corten A Chemical Composition

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

Corten A Mechanical Properties

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

Corten B Chemical Composition

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

Corten B Mechanical Properties

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 - Type of Weathering Steel

PATINAX® is a type of weathering steel manufactured in accordance with the European standard EN 10025-5, and it is also produced as a special mill grade. The rust coating formed during its weathering process is considered nearly impermeable to oxygen. PATINAX® is typically used without painting or coating due to its natural weathered rust color, making it suitable for various applications such as bridges, landscaped structures, power line derricks, containers, mine cars, facades, tanks, and steel sculptures. It is available in different grades, including PATINAX® 355 (S355J2W) and the more corrosion-resistant PATINAX® 355P (S355J2WP) with higher phosphorus content, as well as the thinner cold-rolled sheet version PATINAX® 275PK, which also contains a higher phosphorus alloy (S355J2WP).

Grade	Max. plate thickness [mm]	Grade acc. to EN 10025-5	Alloying elements	R_{e, min}[MPa]	R_{m [MPa]}	A_min[%]	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

PATINAX 355P Mechanical Properties

Steel Grade	*Minimum yield point R_eHMPa )**	Tensile strength R_mMPa	Minimum elongation A (L_o=5.65√S_o) %
PANTINAX 355P	355	470 -630	20

PATINAX 275PK Mechanical Properties

Steel Grade	*Minimum yield point R_eHMPa )**	Tensile strength R_mMPa	Minimum elongation A %
PATINAX 275PK	275	410	25

PATINAX 355 Chemical Composition

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

European Standard Weathering Steels

European EN 10025 weathering steels, such as Cor-Ten and PATINAX, are comparable in terms of corrosion resistance and mechanical properties. Cor-Ten A and PATINAX 355P are equivalent to EN 10025 S355 J0WP, with similar phosphorus content (0.20% for Cor-Ten A and 0.15% for PATINAX) for enhanced corrosion resistance. On the other hand, Corten B and PATINAX 355 are equivalents of European specifications EN 10025 S355 J2W and EN 10025 S355 J2W+N, respectively. These equivalents maintain similar tolerances of vanadium, copper, manganese, aluminum, and chromium.

For EN 10025 S355 J2W+N and PATINAX 355, achieving a finer grain structure may require alloying the steel with different combinations of nitrogen-binding elements, such as aluminum (≥0.02%), niobium (0.015-0.060%), vanadium (0.02-0.12%), and titanium (0.02-0.10%). These adjustments help enhance the properties of the steel for specific applications.

Weathering Steels Thickness

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

Corten A Thickness Range

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

Corten B Thickness Range

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

S355JOWP Thickness Range

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

S355JOW Thickness

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

S355J2W+N Thickness

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

S355J2WP+N Thickness Range

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

Conclusion

Weathering steel, with its proven track record of structural integrity and low maintenance requirements spanning over 50 years, remains a reliable choice for various applications. Equivalent products like Cor-Ten, PATINAX, and their European counterparts offer excellent corrosion resistance and mechanical strength, making them suitable for diverse uses ranging from buildings to bridges.