Overview
ISO 28721-2:2025 specifies how to designate and specify the resistance of vitreous and porcelain enamels used on glass-lined apparatus for process plants to chemical attack and thermal shock. The standard applies to enamels applied to low-alloy carbon steel substrates in chemical plant equipment and defines measurable quality criteria for ordering, testing and acceptance.
Key topics and requirements
- Designation for ordering: Enamel quality shall be specified in terms of measurable resistance parameters (corrosion rates, crack formation temperature), cover-coat structure and colour.
- Chemical corrosion limits:
- Resistance to condensing hydrochloric acid vapour: corrosion rate ≤ 0.08 mm/year (per ISO 28706-2).
- Resistance in 0.1 mol/L sodium hydroxide (NaOH) at 80 °C for 24 h: corrosion rate ≤ 0.40 mm/year with a specified volume/surface ratio (per ISO 28706-4).
- Thermal shock / crack formation temperature:
- Crack formation temperature (per ISO 13807) shall be ≥ 190 °C for primary equipment.
- For accessories (agitators, baffles, thermometer wells, immersion tubes, sensors, piping accessories, pumps) a minimum of 170 °C may be permissible.
- Cover-coat structure: Classification examples include all-vitreous, all-semicrystalline, vitreous over semicrystalline, or semicrystalline over vitreous.
- Normative references: ISO 13807, ISO 19496-1, ISO 28706-2, ISO 28706-4 are referenced for test methods and terminology.
Applications
- Specifying enamel performance for glass-lined reactors, piping, vessels, and accessories in the chemical and process industries.
- Creating procurement and inspection criteria for manufacturers, fabricators and suppliers of glass-lined apparatus.
- Defining acceptance criteria used by testing laboratories and quality assurance teams to verify resistance to chemical attack and thermal shock.
- Guiding material selection in corrosive or thermally cycling environments where long-term enamel integrity is critical.
Who uses this standard
- Chemical plant engineers and process designers
- Procurement and specification writers for industrial equipment
- Manufacturers of glass-lined equipment and enamel formulators
- QA/QC inspectors and independent testing labs
- Regulatory/compliance teams and asset integrity managers
Related standards
Keywords: ISO 28721-2, glass-lined apparatus, vitreous enamel, porcelain enamel, chemical attack resistance, thermal shock, crack formation temperature, corrosion rate, process plants.