ISO 28721-2:2025

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

• ISO 28721-1 / ISO 28721-3 (other parts of the series - see ISO 28721-3 for thermal shock resistance specifics)
• ISO 13807, ISO 28706-2, ISO 28706-4, ISO 19496-1 (test methods and terminology)

Keywords: ISO 28721-2, glass-lined apparatus, vitreous enamel, porcelain enamel, chemical attack resistance, thermal shock, crack formation temperature, corrosion rate, process plants.