Corrosion Allowance: How Much to Add
Corrosion allowance (CA) is the extra wall thickness added to the pressure design thickness to compensate for material loss due to corrosion, erosion, or wear over the design life of a piping system. It is expressed in millimeters (or inches) and is specified by the process or materials engineer based on service conditions, fluid corrosivity, and the expected service life (typically 20-30 years).
When Corrosion Allowance Is Applied
CA is added during the minimum wall thickness calculation per ASME B31.3, Paragraph 304.1.1. It appears as “c” in the formula:
t_min = t + c
Where t is the pressure design thickness and c is the total mechanical, corrosion, and erosion allowance.
CA is applied to the internal surface of the pipe. For external corrosion (e.g., buried or submerged piping), external coatings and cathodic protection are used instead of additional wall thickness.
How to Determine Corrosion Allowance
- Identify the corrosion rate: The corrosion rate (mm/year or mils/year) is determined from laboratory testing, field experience, or published data for the specific fluid-material combination.
- Define the design life: Typical design life is 20-25 years for process piping.
- Calculate CA: CA = corrosion rate x design life. For example, a corrosion rate of 0.1 mm/year over 25 years gives CA = 2.5 mm.
- Apply project minimum: Most projects specify a minimum CA (e.g., 1.5 mm or 3.0 mm) regardless of the calculated value.
Typical Corrosion Allowance Values
| Service / Fluid | Material | Typical CA |
|---|---|---|
| Clean hydrocarbons (dry crude, gas) | Carbon steel | 1.5 mm (1/16 in.) |
| Wet/sour hydrocarbons | Carbon steel | 3.0 mm (1/8 in.) |
| Produced water (oilfield) | Carbon steel | 3.0-6.0 mm (1/8-1/4 in.) |
| Steam (power plant) | Carbon steel | 1.5 mm (1/16 in.) |
| Cooling water (treated) | Carbon steel | 1.5-3.0 mm |
| Seawater | Carbon steel | 3.0-6.0 mm (with coating) |
| Amine (MEA/DEA) | Carbon steel | 3.0 mm (1/8 in.) |
| Caustic soda (NaOH) | Carbon steel | 3.0-6.0 mm |
| Sulfuric acid (>93%) | Carbon steel | 3.0-6.0 mm |
| Stainless steel services | 304/316 SS | 0 mm (nil) |
| Duplex / super duplex | DSS/SDSS | 0 mm (nil) |
| Nickel alloys (Inconel, Hastelloy) | Ni alloys | 0 mm (nil) |
| CRA-clad or lined pipe | CRA layer | 0 mm (nil) |
| Instrument air / nitrogen | Carbon steel | 1.5 mm (1/16 in.) |
| Firewater | Carbon steel (galvanized) | 1.5 mm (1/16 in.) |
Key Points
- Corrosion allowance is not a substitute for material selection. If the corrosion rate exceeds 0.25 mm/year (10 mils/year), upgrading the material (e.g., from carbon steel to stainless or CRA) is generally more economical than increasing wall thickness.
- Erosion allowance is added separately for services with high-velocity flow, particulates, or two-phase flow. Typical erosion allowance is 1.0-3.0 mm.
- The owner/operator may specify different CA values based on operating experience with similar facilities.
- Corrosion rates and CA values are documented in the piping material specification (PMS) and the corrosion study report.
CA is verified during fabrication and pre-commissioning as part of piping inspections and wall thickness measurements.
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