Overview
EN ISO 28721-2:2025 specifies how to designate and specify the resistance of vitreous and porcelain enamels used in glass-lined apparatus for process plants to chemical attack and thermal shock. Applicable to enamels applied on low-alloy carbon steel substrates, this standard sets minimum, test-based quality requirements used for ordering, acceptance and classification of enamel coatings for chemical process equipment.
Key topics and technical requirements
- Scope and purpose
- Defines designation elements and minimum performance requirements for enamel resistance to chemical corrosion and thermal shock.
- Intended for glass-lined apparatus, piping and associated components in chemical plants.
- Designation elements
- Rate of corrosion in hydrochloric acid (condensing HCl vapour).
- Rate of corrosion in sodium hydroxide solution (0.1 mol/L, 80 °C, 24 h).
- Crack formation temperature (thermal shock characteristic).
- Structure of cover coat (e.g., all‑vitreous, semicrystalline combinations).
- Enamel colour.
- Standard designation label: “Enamel quality requirements in accordance with ISO 28721-2.”
- Quantified quality requirements
- Resistance to condensing hydrochloric acid vapour: ≤ 0.08 mm/year.
- Corrosion rate in 0.1 mol/L NaOH at 80 °C (24 h): ≤ 0.40 mm/year (volume:surface ratio 20 cm3:1 cm2 per ISO 28706-4).
- Crack formation temperature: ≥ 190 °C (minimum 170 °C permitted for accessories such as agitators, thermometer wells, pumps and piping accessories).
- Note: crack formation temperature is a specimen characteristic determined per ISO 13807 and does not directly represent the assembled apparatus.
- Referenced test methods
- ISO 28706-2 (corrosion by boiling acids/neutral liquids)
- ISO 28706-4 (alkaline corrosion using cylindrical vessel)
- ISO 13807 (thermal shock / crack formation temperature)
Applications and who uses this standard
- Manufacturers of glass-lined reactors, vessels, piping and pumps to specify enamel performance.
- Procurement and specification engineers in the chemical, pharmaceutical and process industries to ensure material compatibility and durability.
- Quality assurance, inspection bodies and testing laboratories performing acceptance tests and verification.
- Designers and maintenance teams selecting enamel types for corrosion resistance and thermal-shock resilience.
Related standards
EN ISO 28721-2:2025 is a practical reference for specifying and verifying enamel coating performance where corrosion resistance and thermal stability are critical to safe, long-lasting process equipment.
