What 09CrCuSb Steel Is
09CrCuSb is a low-alloy structural steel developed specifically to resist low-temperature sulphuric acid dew point corrosion. It is widely known in the trade as ND steel, and it was created to solve a problem that affects boiler flue gas equipment: when flue gas cools below the acid dew point, sulphuric acid condenses on metal surfaces and attacks ordinary carbon steel rapidly. The grade is used for air preheater tubes, economiser tubes, flue gas ducting and similar components in boilers and industrial furnaces where the gas stream contains sulphur oxides.
The design intention is narrow and deliberate. Instead of resisting a wide range of chemical media, the steel concentrates a small quantity of copper, chromium and antimony in a surface layer that passivates in the medium concentration sulphuric acid that forms at the dew point, so the tube survives the corrosive conditions of the cold end of the gas path.
Chemical Composition to GB 150.2
| Element, percent | Requirement |
|---|---|
| Carbon, max | 0.12 |
| Silicon | 0.20 to 0.40 |
| Manganese | 0.35 to 0.65 |
| Chromium | 0.70 to 1.10 |
| Copper | 0.25 to 0.45 |
| Antimony | 0.04 to 0.10 |
| Phosphorus, max | 0.030 |
| Sulphur, max | 0.020 |
The low carbon and manganese contents keep the steel weldable and ductile, while chromium, copper and antimony provide the corrosion resistance. Antimony is the distinguishing element: it enriches the passive film that forms on the tube surface in the acid condensate and is the main reason the grade outperforms ordinary corrosion resistant steels in this specific service.
Mechanical Properties
| Property | Wall thickness up to 8 mm |
|---|---|
| Tensile strength | 390 to 550 MPa |
| Minimum lower yield strength | 245 MPa |
| Minimum elongation | 25 percent |
These values place the material in the low strength range, which is appropriate for tubing that operates at moderate temperature and must be bent, expanded into tube sheets and welded into headers. Higher strength is not the design driver; corrosion resistance and fabricability are.
The Passivation Mechanism and the Standard Corrosion Test
When 09CrCuSb is exposed to medium concentration sulphuric acid at the temperatures found at the cold end of a flue gas path, the initial dissolution of iron is accompanied by enrichment of the surface in copper, chromium and antimony. These elements build a thin protective layer that suppresses further dissolution, so the corrosion rate falls steeply after the first period of exposure. That is why the material performs far better than plain carbon steel, and better than chromium-only low alloy steels, in the same medium.
GB 150.2 requires the corrosion resistance to be verified by test rather than assumed. Specimens are taken from two tubes of each batch, each specimen being a short length of tube about 10 mm long, and are immersed in sulphuric acid solution with a mass fraction of 50 percent at 70 degrees Celsius, with a tolerance of plus or minus 2 degrees Celsius, for 24 hours. The average corrosion rate of the two specimens must remain within the limit set for the grade, with the specific acceptance figure stated in the order contract. Testing the finished tube rather than the steel alone accounts for the effect of tube manufacture on surface condition.
Tube Supply, Fabrication and Welding
Tubes are supplied as seamless product with the dimensional tolerances, straightness and surface finish required for air preheater and economiser assemblies, and each batch is accompanied by a certificate recording the chemical analysis, the tensile and elongation results, and the corrosion test data. The material bends readily to the tight radii used in air preheater elements. Welding is carried out with low-hydrogen consumables matched to the parent chemistry, using conventional arc processes at controlled heat input, and preheat is not normally required for the thin walls typical of this application. The completed joints are cleaned and inspected, since any residual scale or spatter becomes a preferential corrosion site in the acid condensate.
Frequently Asked Questions
Q: What does ND steel mean?
A: ND is the trade name for 09CrCuSb, a low-alloy steel developed for resistance to low temperature sulphuric acid dew point corrosion in boiler flue gas equipment.
Q: Why is antimony added to 09CrCuSb?
A: Antimony enriches the passive layer that forms in the acid condensate and is the element chiefly responsible for the grade's resistance in this specific medium.
Q: How is the corrosion resistance tested?
A: Per GB 150.2, samples from two tubes per batch are immersed in 50 percent sulphuric acid at 70 degrees Celsius for 24 hours, and the average corrosion rate must stay within the order contract limit.
Q: Where is the steel used?
A: Air preheater tubes, economisers, flue gas ducts and other components in the cold end of boiler and furnace gas paths where acid condensation occurs.
Q: Can 09CrCuSb tubes be welded normally?
A: Yes, with low-hydrogen consumables matched to the parent metal using standard arc processes. Thin walls normally need no preheat, and welds are cleaned thoroughly after welding.
Q: What records accompany a tube shipment?
A: Chemical composition, tensile and elongation results, dimensional inspection and the corrosion test report, issued as an inspection certificate traceable to each heat and batch.
