Corrosion allowance policy: above what wall thickness is a separate corrosion allowance generally not required in many mechanical design practices?

Introduction / Context:
Pressure parts often include a corrosion allowance (CA) to account for expected metal loss during service. However, when components are very thick, some practices omit a separate CA because expected loss is small relative to thickness and inspection/retirement criteria govern.

Given Data / Assumptions:

• General industrial practice for carbon/low-alloy steels in non-severe corrosion.
• CA is a policy choice subject to applicable codes and service environment.
• Question refers to a typical threshold used in many objective handbooks.

Concept / Approach:
Heavier sections (beyond a threshold) can meet design life without a formal CA, relying on periodic inspection and minimum-thickness criteria. Many design question banks cite 30 mm as the rule-of-thumb limit beyond which explicit CA may be omitted.

Step-by-Step Solution:
Recognize typical CA values (e.g., 1.5–3 mm for many services).Compare to very thick components where CA is proportionally small.Adopt the commonly cited threshold: above 30 mm, separate CA often not specified.

Verification / Alternative check:
Project specifications may still mandate a CA regardless of thickness for severe corrosion or regulatory reasons; the 30 mm threshold is a general practice reference, not a universal law.

Why Other Options Are Wrong:
5–20 mm are common component thicknesses where CA is routinely applied.

Common Pitfalls:
Treating rules of thumb as universal; ignoring localized corrosion mechanisms where even very thick sections can perforate.

Final Answer:
30 mm