What Is Alloy 20? Acid Resistant Steel
Alloy 20 (UNS N08020), also known as Carpenter 20 or Alloy 20Cb-3, is a nickel-iron-chromium alloy with copper and molybdenum additions specifically developed for resistance to sulfuric acid. It was originally engineered to handle the full range of sulfuric acid concentrations at temperatures up to boiling, making it the workhorse material for sulfuric acid piping in chemical plants and refineries.
| Property | Value |
|---|---|
| UNS designation | N08020 |
| Common names | Alloy 20, 20Cb-3, Carpenter 20 |
| Nickel | 32.0-38.0% |
| Chromium | 19.0-21.0% |
| Iron | Balance (~37%) |
| Molybdenum | 2.0-3.0% |
| Copper | 3.0-4.0% |
| Niobium (Columbium) | 8x C min, 1.0% max |
| Carbon | 0.07% max |
| Manganese | 2.00% max |
| Tensile strength (min) | 551 MPa (80 ksi) |
| Yield strength (min) | 241 MPa (35 ksi) |
| Elongation (min) | 30% |
| Density | 8.08 g/cm3 |
| Heat treatment | Solution anneal 925-1010°C, water quench |
Why Alloy 20 for Sulfuric Acid?
The 3-4% copper content is the critical differentiator. Copper dramatically reduces corrosion rates in sulfuric acid at concentrations between 20% and 80% and temperatures up to boiling. Standard austenitic stainless steels (316/CF8M) corrode rapidly in hot sulfuric acid above 30% concentration.
| H2SO4 Concentration | 316 SS | Alloy 20 | Hastelloy C-276 |
|---|---|---|---|
| 10%, 80°C | <0.5 mm/yr | <0.1 mm/yr | <0.1 mm/yr |
| 40%, 80°C | >5 mm/yr | <0.5 mm/yr | <0.5 mm/yr |
| 70%, 80°C | >10 mm/yr | <0.5 mm/yr | 1-5 mm/yr |
| 96%, boiling | <0.5 mm/yr | <0.5 mm/yr | >5 mm/yr |
Alloy 20 maintains low corrosion rates across the full concentration range. At 96%+ concentration, concentrated sulfuric acid passivates the surface. Below 20%, dilute acid conditions are less aggressive. The critical zone is 20-80% concentration at elevated temperature, where Alloy 20 excels.
ASTM Specifications
| Product Form | ASTM Specification |
|---|---|
| Forgings | B564 |
| Seamless pipe and tube | B729 |
| Welded pipe | B464 |
| Plate, sheet, strip | B463 |
| Castings | A494 CN7M |
Applications
- Sulfuric acid plants: piping, heat exchangers, absorber towers, and acid coolers
- Phosphoric acid plants: evaporator piping and reactor circuits
- Chemical processing: mixed acid environments, dye manufacturing
- Pharmaceutical: process piping with acid cleaning cycles
- Food processing: concentrated acid handling and pickling circuits
Niobium Stabilization
The niobium (columbium) addition, at a minimum of 8 times the carbon content, prevents sensitization by forming stable niobium carbides instead of chromium carbides at grain boundaries. This makes Alloy 20 resistant to intergranular corrosion in the as-welded condition without requiring post-weld solution annealing.
Testing
Mill test certificates per EN 10204 3.1 must report full chemistry (including Cu and Nb), mechanical properties, and heat treatment records. Intergranular corrosion testing per ASTM A262 Practice B or E is commonly required. NDT and hydrostatic testing follow the applicable piping or equipment standard.
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