Stress corrosion cracking (SCC) in carbon steel storage tanks occurs when three conditions converge: tensile stress, a corrosive environment, and susceptible material. Here's a structured approach to prevent SCC:

• 1. Material Selection and Design

  (1) Use SCC-Resistant Steels

  • Opt for low-carbon steels (e.g., ASTM A516 Grade 70) with reduced impurity levels (e.g., sulfur, phosphorus).
  • For high-stress applications, specify quenched-and-tempered steels (e.g., ASTM A352) or add alloying elements (e.g., nickel) to enhance resistance.

  (2) Minimize Residual Stress

  • Post-Weld Heat Treatment (PWHT): Apply PWHT (e.g., 600–650°C for carbon steel) to relieve welding-induced stresses.
  • Cold Work Control: Avoid excessive cold forming, which introduces residual stresses. If unavoidable, stress-relieve the component.

  (3) Design Optimization

  • Eliminate sharp corners and geometric discontinuities that cause stress concentrations.
  • Ensure smooth transitions between tank sections (e.g., shell-to-bottom connections).

• 2. Environmental Control

  (1) Corrosion Inhibitors

  • Add chemicals (e.g., nitrites, phosphates) to neutralize corrosive species (e.g., chloride, hydroxide).
  • For water-based systems, maintain pH between 8.5–10.5 to passivate the steel surface.

  (2) Moisture and Oxygen Management

  • Keep tanks dry through desiccant breathers or inert gas blanketing (e.g., nitrogen).
  • For buried tanks, use cathodic protection (CP) in combination with coatings to mitigate soil-side corrosion.

  (3) Contaminant Removal

  • Filter or treat stored liquids to reduce chloride, sulfide, or cyanide levels.
  • For atmospheric tanks, install air scrubbers to remove pollutants (e.g., SO₂, H₂S).

• 3. Coatings and Linings

  Type	Application	Benefit
  Epoxy or Polyurethane Coatings	Standard atmospheric tanks	Isolate steel from environment
  Glass-Fused-to-Steel (GFS) Linings	Harsh chemical storage	Durable, corrosion-resistant barrier

  Note: Regularly inspect coatings for damage (e.g., holidays, blistering) and repair promptly.

• 4. Cathodic Protection (CP)

  Sacrificial Anodes

  Use zinc or aluminum anodes for small tanks or low-current applications.

  Impressed Current CP

  Employ rectifiers for large tanks or high-corrosion environments (e.g., seawater).

  Monitoring

  Ensure CP systems maintain -0.85 V (vs. Cu/CuSO₄ reference electrode) to prevent SCC.

• 5. Operational and Maintenance Practices

  (1) Stress Reduction

  • Avoid over-pressurization or rapid temperature changes that induce thermal stresses.
  • Use expansion joints in piping systems to absorb movement.

  (2) Inspection and Monitoring

  • Non-Destructive Testing (NDT): Regularly inspect critical areas (e.g., welds, nozzles) via ultrasonic testing (UT) or magnetic particle testing (MT).
  • Stress Analysis: Perform finite element analysis (FEA) during design to identify high-stress zones.

  (3) Preventive Maintenance

  • Replace gaskets and seals to prevent leaks that could create corrosive environments.
  • Drain accumulated water or condensate from tank bottoms.

• 6. Compliance with Standards

  • API 653: Follow guidelines for tank inspection, repair, and alteration.
  • NACE SP0169: Adhere to standards for controlling external corrosion on buried tanks.
  • ASME BPVC Section VIII: Ensure design and fabrication meet pressure vessel codes.