Why Replace BS 3059 Tubes with Corten Steel Tubes in Air Pre Heaters

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If you have ever pulled a block of air preheater tubes out of a boiler, you already know where the trouble is. The hot end usually looks fine. The cold end, where the flue gas leaves and fresh air comes in, is thin, pitted and full of holes. Most of those tubes are plain carbon steel ERW tubes made to BS 3059, and they fail there for one reason: sulphuric acid condenses on them and eats the steel. 

Corten steel pipes, which contain small quantities of copper, chromium, and nickel, resist acid damage and increase the longevity of the cooler end. While a transition to Corten steel pipes may be beneficial in some cases, it is not in all boilers. Below we explain why APH tubes fail, what Corten changes, what the test data says, where Corten will not help, and how to plan the change. Amardeep Steel supplies both BS 3059 and Corten APH tubes, so this is the conversation we have with plant engineers every week.

What Air Pre Heater Tubes Do

In a tubular air pre heater, hot flue gas leaving the boiler flows through a bank of thin wall steel tubes while cold combustion air flows across the outside. Heat passes through the tube wall, so the air reaches the furnace warmer and the boiler burns less fuel for the same steam. The tubes are expanded or welded into tube sheets and grouped in blocks. Every APH tube does the same job, but they do not all live the same life: the cold end block, where gas is coolest and the incoming air is coldest, decides how often the unit needs retubing. Sizes and specifications are on our air pre heater tubes page.

Why APH Tubes Fail at the Cold End

Sulphur in coal, oil or bagasse burns to sulphur dioxide, and a small part of it turns into sulphur trioxide. That combines with water vapour in the flue gas to form sulphuric acid vapour. As long as the gas stays hot, the acid stays a vapour. The moment it touches a surface colder than its acid dew point, it condenses into a liquid film on the metal.

In an air pre-heater, the coldest metal is at the cold end, because the incoming air cools the tubes well below the gas temperature. The acid film corrodes the tube wall, wet deposits trap ash and block the tubes, and once a tube is holed, air leaks straight into the gas side. The fan works harder, the APH loses efficiency, and the boiler loses output. This is APH tube corrosion in almost every plant we see.

What Is Acid Dew Point Corrosion?

Acid dew point corrosion is the attack that happens when sulphuric acid condenses out of flue gas onto a surface below the acid dew point. The important word is surface. In one published field test on an oil-fired boiler, the flue gas was 124 to 130°C and the acid dew point was 130°C, yet the tube metal at the cold end was only 70 to 80°C. The gas temperature looked safe. The metal temperature was not. The acid dew point itself changes with the sulphur and moisture in the fuel, so it is different from boiler to boiler.

What Makes Corten Steel Different

Corten is a weathering steel: a low carbon steel with small, deliberate additions of copper, chromium, nickel and phosphorus. Outdoors, those elements build a tight, adherent rust layer that slows further corrosion, which is why weathering steel tubes and plates are used unpainted on bridges and structures.

Inside an air pre-heater, the benefit comes from the same chemistry. Copper in particular slows attack in dilute sulphuric acid condensate, which is why the special acid dew point steels used in boilers are also built on copper-bearing chemistry. Corten APH tubes are usually ERW tubes made from Corten A strip, or tubes supplied to ASTM A423 Grade 1, a copper-bearing low-alloy tube specification with a similar chemistry. For European projects, the nearest equivalent is S355J2WP Corten tubes. Available sizes are listed on our Corten steel APH tubes page.

BS 3059 vs Corten APH Tube: Side by Side

The quickest way to see the difference is the chemistry. A BS 3059 tube has no corrosion resisting elements at all; a Corten A tube or an ASTM A423 Grade 1 tube carries copper, chromium and nickel.

Chemical Composition (%)

Element	BS 3059 Part 1 ERW 320	Corten A	ASTM A423 Grade 1
Carbon	0.16 max	0.12 max	0.15 max
Manganese	0.30 to 0.70	0.20 to 0.50	0.55 max
Phosphorus	0.040 max	0.07 to 0.15	0.06 to 0.16
Sulphur	0.040 max	0.030 max	0.060 max
Silicon	As per standard	0.25 to 0.75	0.10 min
Copper	Not specified	0.25 to 0.55	0.20 to 0.60
Chromium	Not specified	0.50 to 1.25	0.24 to 1.31
Nickel	Not specified	0.65 max	0.20 to 0.70

Mechanical Properties

Property	BS 3059 Part 1 ERW 320	Corten A	ASTM A423 Grade 1
Tensile strength (MPa)	320 to 480	485 min	415 min
Yield strength, min (MPa)	195	345	255
Elongation, min (%)	25	20	25
Corrosion resisting elements	None	Copper, chromium, nickel, phosphorus	Copper, chromium, nickel, phosphorus

Corten A values are for hot rolled strip and plate (SSAB COR TEN A data). BS 3059 values from BS 3059 Part 1:1987. Always check the mill test certificate on the tubes you receive.

Five Reasons to Replace BS 3059 Tubes with Corten

• Slower cold end corrosion: copper bearing steels corrode much more slowly than plain carbon steel in sulphuric acid condensate. In a Nippon Steel field test, a copper bearing acid resistant tube lost about one fifth of the metal that a carbon steel tube lost in the same air pre heater, over the same period.
• Fewer holed tubes and air leaks: more wall left at the end of each campaign means fewer failures, less air bypassing into the gas side and a steadier fan load.
• Longer gaps between retubing: the cold end block is the part most often replaced. Slowing its corrosion stretches the interval between shutdowns.
• A direct replacement: Corten tubes are made in the same outside diameters and wall thicknesses as BS 3059 ERW tubes, so they fit the existing tube sheet holes and are expanded the same way.
• More strength from the same wall: Corten A has a minimum yield strength of 345 MPa against 195 MPa for ERW 320, which helps tubes resist sagging and handling damage.

A fair note on that test figure: the steel in the Nippon Steel trial was S TEN 1, a purpose made acid dew point steel, not Corten. It shows how much copper bearing chemistry can help. How much Corten helps in your boiler depends on your fuel, ash and metal temperature, which is why the next section matters.

How to Plan an APH Tube Replacement

1. Map the damage. Record which rows and blocks fail first; it is almost always the cold end.
2. Measure the cold end metal temperature and check the fuel sulphur, so you know how harsh the conditions really are.
3. Decide the scope. Many plants change only the cold end block to Corten and keep carbon steel tubes at the hot end.
4. Keep the same tube size. Order the same outside diameter and wall thickness so the tube sheet holes and expansion settings stay unchanged.
5. Specify the material clearly: Corten A tube or ASTM A423 Grade 1, with mill test certificates that show copper, chromium and nickel.
6. Inspect the tubes on arrival: certificate chemistry, dimensions and, if you specified them, flattening and flaring tests.
7. Measure wall thickness at the next shutdown and compare it with your old carbon steel tubes. That record is your proof for the next budget.

Other Ways to Reduce Air Preheater Cold End Corrosion

Tube material is one tool. These work alongside it:

• Raise the metal temperature: a steam coil air heater or hot air recirculation warms the incoming air so the cold end tubes stay closer to, or above, the dew point.
• Bypass air at low load: part load and start up are when the cold end runs coldest.
• Lower sulphur fuel: less sulphur means less acid to condense.
• Regular soot blowing: clean tubes do not hold acid soaked deposits against the metal.

For a wider look at how material choice affects corrosion, read our guide on the difference between stainless steel and carbon steel.

Planning an APH retubing? Send us the tube size, quantity and your current material, and we will quote BS 3059 and Corten options side by side. Browse our APH tube range, Corten steel pipes and tubes, boiler tubes and ERW welded pipes, or contact Amardeep Steel.