Corrosion allowance policy: in the design of pressure vessels and process equipment, when is an explicit corrosion allowance generally not provided?

Introduction / Context:
Corrosion allowance (CA) is an extra thickness added to account for expected uniform metal loss during service. It is distinct from mechanical allowances such as mill tolerance or fabrication margins. The need for CA depends on corrosion resistance of the chosen material and the expected environment, not simply on initial wall thickness.

Given Data / Assumptions:

• Standard pressure vessel codes followed; material selection aligned with process corrosion data.
• Question focuses on when CA is not typically required.
• High alloy steels (e.g., austenitic stainless, nickel alloys) are used in suitably compatible services.

Concept / Approach:
When highly corrosion-resistant alloys are correctly matched to the process environment—such that expected metal loss is negligible—designs commonly omit a specific CA and rely on the intrinsic resistance of the alloy. In contrast, merely increasing wall thickness (e.g., > 30 mm) does not remove the need for CA; corrosion would still proceed and could compromise life if not accounted for.

Step-by-Step Solution:

Verification / Alternative check:
Owner standards often specify zero CA for stainless/nickel alloys in non-corrosive or passivating media, while mandating CA for carbon steels even at large thicknesses.

Why Other Options Are Wrong:

• Plain carbon steel/cast iron: usually require a CA in corrosive environments.
• Wall thickness > 30 mm: thickness does not eliminate corrosion; life would still diminish.
• Both (b) and (c): includes an unjustified condition (b), hence incorrect.

Common Pitfalls:
Confusing CA with mill tolerance; assuming localized corrosion modes are covered by CA; neglecting cladding/lining strategies.

Final Answer:
Material of construction is high alloy steel.