Selecting a Coating for a Chemical Storage Tank
A new carbon steel tank will store 50% sulfuric acid at ambient temperature. The tank is 10 meters in diameter and 8 meters high. The owner wants a coating system with a service life of at least 15 years. The coating applicator proposes a two-coat epoxy system (total DFT 250 microns) and argues that it is resistant to sulfuric acid. The inspector must evaluate the proposal and consider alternatives.
1. Evaluate the proposed epoxy system for 50% sulfuric acid service.
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The proposed two-coat epoxy system at 250 microns DFT is likely inadequate for 50% sulfuric acid service. While some epoxies have limited resistance to dilute sulfuric acid, 50% concentration is aggressive and can cause blistering, softening, and permeation. The service life of 15 years is unlikely to be achieved. The coating manufacturer's chemical resistance chart should be consulted; typically, epoxies are not recommended for sulfuric acid concentrations above 20-30%. A thicker film (e.g., 300-400 microns) or a different coating type, such as a vinyl ester or a phenolic epoxy, may be required. The inspector should reject the proposal and request a coating system with proven resistance to 50% sulfuric acid, supported by test data or case histories.
2. What alternative coating systems would you recommend and why?
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Recommended alternatives: 1) Vinyl ester coating: Excellent resistance to acids, including sulfuric acid up to 50-70% concentration. Typically applied at 300-500 microns DFT. 2) Phenolic epoxy: Good acid resistance, but may be less flexible; suitable for immersion. 3) Glass-flake filled epoxy: Enhanced barrier properties and chemical resistance. 4) Rubber lining (e.g., chlorobutyl): For severe acid service, but more expensive. The selection should consider the tank's operating conditions, surface preparation (near-white metal blast), and application constraints. The inspector should require the applicator to provide a coating system with a documented service history for 50% sulfuric acid at ambient temperature, and ensure the DFT meets the manufacturer's recommendations. Additionally, consider lining with acid-resistant bricks or thermoplastics if the coating is not feasible.