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

Introduction

Weathering steel (WS), which is also called low-alloy steel, is steel with a carbon content of less than 0.2 wt.% to which alloying elements, mostly Cu, Cr, Ni, P, Si, and Mn, add up to no more than 3–5 wt.% [1]. The chemicals in these steel alloys are put together in a way that makes it easy for a rust/iron oxide layer to form quickly and protect the steel underneath from the weather.

The rate of corrosion in WS is so low that bridges made from it can easily last for 120 years with very little maintenance. In reality, weathering steel construction has a lot of other benefits, such as lower overall lifecycle costs and better safety because you don’t need a paint system to protect it and you don’t have to check it as often. Since there is no need for paint, no volatile organic compounds are put into the air, and the building process goes faster. From a material cost and maintenance point of view, the costs of weathering steel bridges are, on average, 5% less than the costs of painted steel bridges. The alloy was first sold in 1933 to keep ore waggons from needing to be painted or repaired in other ways. Over time, it gets a rusty look, called a patina, which is now very popular among architects and can be seen on many famous buildings and monuments, such as the John Deere World Headquarters in Illinois and the Angel of the North monument in the UK. The patina on WS makes it more resistant to corrosion than mild steel and also gives it an attractive look and the ability to heal itself.

Chemistry

WS has better corrosion resistance than other alloys because it has more copper and nickel than other alloys. Other alloying elements, such as Cr, Mn, and P, also help. Copper is important because it helps the protective oxide layer stick to the metal and slows down corrosion.

The steel is left to rust. Because it is made of an alloy, it rusts more slowly than regular steel, and the rust forms a protective coating that slows the rate of corrosion in the future. Research has shown that wet/dry cycling needs to happen over and over again for the best dense and stuck-on rust layer to form. Rainwater should wash the steel surface well, moisture should drain easily, and the surface should dry quickly. Structures need to be free of cracks where water can collect because corrosion would happen there if a protective patina didn’t form. This was shown by a recent study of S355J2W steel motorway bridges in the Czech Republic.

The oxidation process can go on for years before the surface “stabilises” in a steady state with a coating that sticks tightly together. This depends a lot on the weather conditions at the time. Phosphorus and sulphur also play an important role in the formation of the patina layer. Low-soluble sulphates or phosphates form between the base steel and the existing corrosion if the surface is exposed to alternating wet and dry periods. ISO 9223 says that “SO2 pollution in the air makes it possible for a thicker layer of rust to form.” Leygraf and Graedel agree with this statement, but they also think that large amounts of non-metal oxides make the water layer very acidic, which stops the patina from forming. Phosphorus species have also been shown to form a passive protective film over the surface of the steel. This film keeps aggressive ions and water from getting in, which helps a dense patina layer form. Phosphorus, on the other hand, can hurt the grain structure of the alloy and, as a result, the mechanical strength of the steel. To fix this, low levels of boron or carbon are added to bring back the required grain boundaries.

Cor-Ten

Cor-Ten is one of the best-known types of steel that doesn’t rust. Cor-brand Ten’s name comes from the two things that make it different from carbon steel (CS): its resistance to corrosion (Cor), which comes from the copper in it, and its tensile strength (Ten), which comes from its better mechanical properties. In fact, it is said that Cor-Ten has 30% better mechanical properties and 4–8 times better weather resistance than regular CS.

There are two kinds of Cor-Ten: Cor-Ten A and Cor-Ten B. ASTM standard designation A 242 is for Cor-Ten A (up to 12.7mm thick), and the newer ASTM grades are A 588 for Cor-Ten B. (over 12.7mm thickness). Depending on how thick the steel is, you can weld Cor-Ten with gas shield, spot, or submerged arc welding. When welding, it’s important to use a method that lets the rusting happen the same way as the rest of the structure. In general, Cor-Ten A and B are the same, but Cor-Ten A alloys have more phosphorus to make them more resistant to corrosion.

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/mm²Minimum	Tensile Strength RmN/mm²Minimum	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/mm²Minimum	Tensile StrengthRm N/mm²Minimum	ElongationA50 %Minimum
Corten B	2 – 13	6 – 40	345	485	19

PATINAX

PATINAX is a weathering steel made to the European standard EN 10025-5 and also as a special mill grade. During the weather resistance process, an oxide rust coating is made that is thought to be “virtually impermeable to oxygen”. PATINAX® is mostly used unpainted or uncoated. Its natural weathered rust colour makes it a good choice for bridges, landscaped structures, power line derricks, containers, mine cars, facades, tanks, and steel sculptures. PATINAX comes in several grades, such as PATINAX® 355 (S355J2W) and PATINAX® 355P (S355J2WP), which has a higher phosphorus content and are more resistant to corrosion. There is also PATINAX® 275PK, which is a thinner cold-rolled sheet with a higher phosphorus alloy (S355J2WP).

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 Specification532July 2014
TKSE-Short name	EN-Short name
PATINAX 355	S355J2W+N	1.8965

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 ReHMPa*)	Tensile strength RmMPa	Minimum elongation A(Lo =5.65√So)%
≤ 16	355	470 – 630	20

European Standard Weathering Steels

Specifications for the European EN 10025 Weathering steels are very similar to Cor-Ten and PATINAX, and they are the same in terms of how well they resist corrosion and how strong they are. Cor-Ten A and PATINAX 355P are both the same as EN 10025 S355 J0WP because they both have the same amount of phosphorus (0.20% for Cor-Ten A and 0.3% for PATINAX 355P).

S355 J0WP and PATINAX at 0.15%) to make them less likely to rust. On the other hand, Corten B and PATINAX 355 are the same as European specification EN 10025 S355 J2W and EN 10025 S355 J2W+N, with the same tolerances for vanadium, copper, manganese, aluminium, and chromium. But in the case of EN 10025 S355 J2W+N and PATINAX 355, different combinations of nitrogen-binding elements can be added to the steel to make it have a finer grain structure. For example, 0.02% aluminium, Nb 0.015-0.060%, V 0.02-0.12%, and Ti 0.02-0.10%.

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

The information above is based on what is readily available from stock. Where there are enough pieces for mill production, the thicknesses and widths that are available vary.

Conclusion

Weathering steel has been used for building structures for more than 50 years, and it has a good track record for being strong and requiring little upkeep. On the market, there are a number of similar products, such as Cor-Ten, PATINAX, and European counterparts. All of these products are equally good at resisting corrosion and having the mechanical/tensile strength that is needed for a wide range of uses, from buildings to bridges. Lakshya International is a Worldwide Supplier of Corten Steel (Weathering Steel) and its products.