Materials of construction for caustic storage Which is the cheapest suitable material of construction for storage tanks handling sodium hydroxide (caustic soda) solutions up to about 75% concentration under typical ambient plant conditions?

Introduction / Context:
Sodium hydroxide (caustic soda) is a common bulk chemical used for neutralisation, pH control, pulping, and cleaning. Selecting a material of construction for storage tanks depends on chemical compatibility, mechanical integrity, and life-cycle cost. This question asks which common engineering metal provides the most economical service for caustic solutions up to about 75% concentration at ordinary storage temperatures.

Given Data / Assumptions:

• Sodium hydroxide concentration ≤ 75% by mass.
• Storage at ambient temperatures (no unusual high-temperature service).
• Conventional atmospheric tanks in typical chemical plant duty.
• Goal: choose the cheapest suitable material, not the most corrosion-resistant premium alloy.

Concept / Approach:
Caustic soda is non-oxidising and generally non-aggressive to ferritic steels at ordinary temperatures; iron forms adherent magnetite in strong caustic, granting good service if chloride contamination and stress corrosion conditions are controlled. Plain carbon steel is therefore widely used for caustic storage up to high concentrations. Copper and nickel are susceptible to caustic corrosion or are unnecessarily expensive for this duty. Stainless steels can be used, but for ambient storage they add cost without proportional benefit compared with plain carbon steel.

Step-by-Step Solution:

Verification / Alternative check:
Industry storage guidelines and vendor bulletins specify carbon steel tanks, often lined or coated only for cleanliness, for 50–75% NaOH at ambient temperatures; heating coils and stress cracking precautions apply mainly at elevated temperatures.

Why Other Options Are Wrong:

• Stainless steel: Acceptable but significantly more expensive; typically not required for ambient caustic storage.
• Nickel: Premium alloy with niche use; cost is unjustified for this service.
• Copper: Not recommended; caustic attack occurs.

Common Pitfalls:
Over-specifying stainless steel by default; ignoring the effect of temperature and impurities (chlorides) on caustic stress corrosion cracking; forgetting to segregate aluminum components, which caustic attacks aggressively.

Final Answer:
Plain carbon steel