Corrosion Allowance in Shell Thickness: How It Works and When It Applies

What Is Corrosion Allowance, and Why Does It Matter?

Corrosion allowance (CA) is extra steel thickness added to your tank shell to account for metal loss over time. As your tank stores its product — whether crude oil, sour gas, water, or chemicals — the walls corrode. CA is the margin that keeps the tank structurally safe until your next scheduled inspection or until the tank reaches end-of-life.

Here's why this matters to your bottom line: if your tank is 30 meters in diameter with five shell courses, and your corrosion allowance is 3 mm instead of 1.5 mm, you're ordering significantly more steel. That translates directly into higher fabrication costs, heavier foundation loads, and taller project timelines. Getting CA right — not too conservative, not too thin — is a real cost lever on every project.

Where Does CA Fit in the Thickness Formula?

This is the part that trips up most engineers. API 650 says you calculate the minimum required thickness using the one-foot method or variable design point method, then you add CA to that result, and finally you round up to the nearest gauge. Many engineers reverse steps two and three — they round first and add CA second, which inflates the ordered thickness by 1–2 mm per course.

The correct sequence is:

1. Calculate the structurally required minimum thickness using the API 650 formula (based on tank diameter, liquid height, specific gravity, and allowable stress)
2. Add your corrosion allowance to that minimum
3. Round up to the next available gauge (available thicknesses are fixed: 6 mm, 7 mm, 8 mm, etc.)

If you reverse steps 2 and 3, you end up ordering a plate that is thicker than necessary, and that extra thickness serves no structural purpose. On a large tank, this mistake can mean ordering an extra tonne or two of steel unnecessarily.

What Are Typical Corrosion Allowance Values?

API 650 does not specify a single CA value. The owner or operator chooses it based on the product being stored, the environment, and the desired inspection interval. Here are the most common values you'll see in practice:

• 1.5 mm (1/16 inch): Mild service. Use this for clean products with low corrosivity — clean hydrocarbons, clean water, products that are not aggressive chemically. This assumes a regular inspection interval of 5–7 years.
• 3 mm (1/8 inch): Crude oil, sour service, or products containing hydrogen sulfide or chlorides. This is the most common value in the industry. It assumes a 10-year inspection interval and accounts for both internal and external corrosion.
• 6 mm (1/4 inch) or more: Aggressive chemical service, high-temperature products, or tanks with very long inspection intervals (15+ years). Use this when the product is known to be highly corrosive or when inspection access is difficult and you want extra margin.

Do not simply copy a CA value from a previous project and assume it applies to yours. Each project should have a documented corrosion study or a specific reason for the CA value chosen. That reason should appear in your design basis document and be confirmed with the owner before detailed design starts.

Where Does CA Apply — and Where Does It Not?

CA applies only to surfaces that are exposed to the product or to potentially corrosive environments:

• Shell plates (all courses): Always apply CA — all shell surfaces are in contact with the stored product.
• Bottom plate: Always apply CA — the bottom is in contact with sludge, water bottoms, and corrosive residue.
• Annular plate ring: Always apply CA — this ring bridges between the shell and bottom and is exposed to the same corrosive environment.
• Nozzle neck walls: Always apply CA — nozzle necks are in contact with the product or vapors inside the tank.

CA does NOT apply to:

• Roof plates: In normal operation, roof plates are not submerged in the product and are not subject to corrosion from the stored liquid. The code does not add CA to roof plate minimum thicknesses by default.
• Structural members (rafters, wind girder, top angle): These are sized using different criteria and do not get a blanket CA addition.
• Internal attachments (ladders, platforms, clips): These are not covered by API 650's CA rules.

One caveat: if your product is known to generate a corrosive vapor phase — for example, crude with significant hydrogen sulfide vapor — the project specification may require a roof CA. If this is the case, it should be stated explicitly in the project datasheet, not assumed.

The Three Mistakes That Cost the Most

Mistake 1: Rounding the thickness before adding CA. This is the biggest offender. You calculate the minimum thickness (say, 12.3 mm), round it up to 13 mm, then add 3 mm CA for a total order of 16 mm. The correct approach: 12.3 + 3 = 15.3 mm, round up to 16 mm. On a large tank with seven courses, this mistake adds 5–8 tonnes of steel you don't need.

Mistake 2: Applying shell CA to roof plates. Roof plates are not submerged. Applying 3 mm CA on top of a 5 mm minimum roof plate gives you an 8 mm roof — 60% thicker than required — with no benefit to your inspection interval, because the roof is not the component subject to liquid-phase corrosion.

Mistake 3: Forgetting the hydrotest thickness check. API 650 requires each course to satisfy two independent checks: the design condition (with CA) and the hydrotest condition (without CA, using water SG = 1.0 and a higher allowable stress). On upper courses where the hydrostatic head is low, the hydrotest check can govern, and you must use whichever thickness is greater.

Practical Tips for Getting CA Right

• Lock CA into your design basis before you start calculations. Changing it mid-project forces you to recalculate every course, the nozzle schedule, the weight summary, and the foundation loads. Thirty minutes of discussion with the owner at the kickoff meeting saves two days of rework.
• Document where your CA value comes from. Your design package should reference the project corrosion specification, a corrosion study, or the owner's standard. This makes it easy for future engineers or inspectors to understand the design intent.
• Apply CA consistently across all sheets. Shell thickness, nozzle schedule, annular plate, weight summary — they must all use the same CA. A common error is updating CA in the shell sheet and forgetting to update the nozzle schedule.
• On large tanks, the material savings of reducing CA by 1 mm are significant. For a 35-meter diameter tank with five lower courses, reducing CA by 1 mm can save 5–8 tonnes of steel. If you have good corrosion data to support a lower CA, it's worth putting that conversation in front of the owner with a cost comparison.

Related reading: How Does Corrosion Affect My Design Over 10, 20, 30 Years?, Rerating a Tank: Do I Need to Re-Design Everything if I Thicken the Shell?, and Stainless Steel in API 650: Can I Use 304/316L, and What Changes?.