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Key takeaways
- Corrosion allowance (CA) is extra wall thickness added to the pressure design thickness to allow for expected metal loss over the design life.
- Typical carbon steel values are 1.5, 3 or 6 mm. Stainless and corrosion-resistant alloys usually have zero CA.
- CA handles slow, uniform corrosion. It does little against pitting, erosion, external corrosion under insulation or cracking.
Where CA sits in the wall
The B31.3 wall thickness calculation works in layers:
- Pressure design thickness (t) from the pressure design equation.
- Plus allowances (c): corrosion allowance, plus thread or groove depth where relevant. This gives the minimum required thickness, tm.
- Plus mill tolerance: seamless pipe can be up to 12.5% thinner than nominal, so the nominal wall must allow for it.
- Choose the next standard schedule above the result.
You can try this with the wall thickness calculator on this site.
How CA is chosen
CA is a materials and corrosion decision, made from the expected corrosion rate multiplied by the design life:
CA = corrosion rate (mm/year) × design life (years)
A line expected to lose 0.1 mm per year over 25 years needs at least 2.5 mm, which rounds up to 3 mm in most class systems. In practice, CA is set for each piping class:
| Material and service | Typical CA |
|---|---|
| Carbon steel, dry gas or clean hydrocarbons | 1.5 mm |
| Carbon steel, general process | 3 mm |
| Carbon steel, wet CO2 or produced water | 6 mm or change material |
| Stainless, duplex, CuNi, GRE | 0 mm |
When the calculated CA goes beyond about 6 mm, it is usually cheaper and more reliable to change material, add inhibition or use a lined pipe than to keep adding steel.
What CA does not protect against
- Pitting and localised attack: a pit can go through the wall while the average loss is small.
- Erosion and erosion-corrosion: at high velocity, sand or droplets can remove metal much faster than predicted.
- Corrosion under insulation (CUI): external corrosion of carbon steel under wet insulation, most active for carbon steel between roughly −12 and 175 °C. Coating and insulation design, not CA, are the defence.
- Microbiologically influenced corrosion (MIC): in stagnant water, deadlegs and hydrotest water left in place.
- Cracking: sulfide stress cracking, chloride stress corrosion cracking and fatigue don’t care about extra wall.
Onshore
Onshore lines can be inspected and replaced relatively easily, so carbon steel with CA is often the economic choice.
Offshore
Offshore, replacement is expensive and weight matters, so operators often prefer corrosion-resistant alloys with zero CA for wet services, even at higher initial cost.
What to take back to your desk
Treat CA as one tool among several. If a line’s failure mode is pitting, erosion or CUI, adding millimetres won’t help; changing material, velocity, coating or inspection will.
Educational content. Always design to your project specification.
