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ISO 28721-2:2025 — Designation of Chemical Attack and Thermal Shock Resistance
Vitreous and porcelain enamels — Part 2: Designation and specification of resistance to chemical attack and thermal shock
1. Enamel Quality Designation System
ISO 28721-2:2025 specifies requirements for the resistance of chemical enamels to chemical attack and thermal shock, establishing a standardized designation system for use in procurement, quality assurance, and material specification. The enamel quality is defined by five essential parameters: the hydrochloric acid corrosion rate measured per ISO 28706-2 (boiling HCl test), the sodium hydroxide corrosion rate measured per ISO 28706-4 (cylindrical vessel method at 80 °C), the crack formation temperature determined per ISO 13807 (differential thermal analysis), the cover coat structure (all-vitreous, all-semicrystalline, or layered), and the colour of the enamel surface. This designation system provides a complete performance profile that enables engineers to select the most appropriate enamel grade for specific process conditions.
Including “Enamel quality requirements in accordance with ISO 28721-2” in procurement specifications ensures that the delivered glass-lined equipment meets internationally recognized minimum performance standards. The designation system eliminates ambiguity by providing quantified performance targets rather than qualitative descriptions.
The designation follows a structured format that communicates all five parameters concisely. For example, an enamel designated as “ISO 28721-2 – E0.08/A0.40/190/V/BL” indicates: HCl corrosion rate E ≤ 0.08 mm/year, NaOH corrosion rate A ≤ 0.40 mm/year, crack formation temperature of 190 °C, all-vitreous (V) cover coat structure, and blue (BL) colour. This standardized notation allows manufacturers and end-users to communicate enamel quality requirements precisely across international supply chains without language barriers.
2. Quantitative Performance Requirements and Their Significance
The standard establishes specific numerical limits for each performance parameter. The HCl corrosion rate must not exceed 0.08 mm/year when tested by the boiling hydrochloric acid method of ISO 28706-2. This limit is derived from decades of experience with glass-lined equipment in acidic chemical processes, where hydrochloric acid is one of the most common and aggressive process fluids. At a corrosion rate of 0.08 mm/year, a standard 1.6 mm enamel coating has a theoretical service life of approximately 20 years before complete penetration, assuming uniform corrosion and no other degradation mechanisms.
| Property | Requirement | Test Method |
|---|---|---|
| HCl corrosion rate (boiling 20 % HCl, 6 h) | ≤ 0.08 mm/year | ISO 28706-2 |
| NaOH corrosion rate (0.1 mol/L, 80 °C, 24 h) | ≤ 0.40 mm/year | ISO 28706-4 |
| Crack formation temperature (standard enamels) | ≥ 190 °C | ISO 13807 |
| Crack formation temperature (accessories) | ≥ 170 °C | ISO 13807 |
It is essential to understand that the crack formation temperature determined by ISO 13807 is a material property measured on standardized test specimens. It does not directly represent the safe operating limit of a complete apparatus. The actual thermal shock limits for installed equipment depend on the apparatus design, steel grade, enamel thickness, and service conditions, and must be determined in accordance with ISO 28721-3 thermal shock diagrams.
The NaOH corrosion rate limit of 0.40 mm/year is notably higher (less stringent) than the HCl limit. This reflects the fundamental material science reality that vitreous enamel is inherently more susceptible to alkaline attack because the silica network (SiO2) dissolves readily in alkaline media, forming soluble silicates. In acidic media, the dissolution products remain largely insoluble and form a protective layer that inhibits further attack, whereas in alkaline media, the continuous dissolution prevents any such protective mechanism from developing. The 0.40 mm/year limit represents the maximum acceptable corrosion rate for standard chemical enamel grades, with higher-performance enamels achieving rates below 0.20 mm/year.
3. Testing Framework and Certification Requirements
The performance parameters specified in ISO 28721-2 are verified through standardized test methods that form an integrated testing framework. ISO 28706-2 evaluates acid resistance by measuring the mass loss of enamel specimens exposed to boiling 20 % hydrochloric acid for 6 hours, with results expressed as a corrosion rate in mm/year assuming uniform corrosion and an enamel density of 2.5 g/cm³. ISO 28706-4 evaluates alkaline resistance using the cylindrical vessel method with 0.1 mol/L NaOH at 80 °C for 24 hours, also expressed as a corrosion rate in mm/year. ISO 13807 determines the crack formation temperature by measuring the differential thermal analysis (DTA) signal during controlled heating of an enamel-coated specimen, identifying the temperature at which the enamel begins to crack due to thermal expansion mismatch with the steel substrate.
When requesting ISO 28721-2 compliance from a manufacturer, always ask for a certified test report showing all three measured parameters (HCl corrosion rate, NaOH corrosion rate, and crack formation temperature) with the corresponding test methods and dates. This provides traceable, auditable evidence of enamel quality that can be referenced throughout the equipment lifecycle, from factory acceptance to periodic in-service inspection.
4. Engineering Applications and Material Selection
The practical value of the ISO 28721-2 designation system lies in its direct applicability to equipment specification. For a glass-lined reactor handling chlorinated organic compounds with periodic alkaline cleaning cycles, the specification should address both acid and alkaline resistance. The HCl corrosion rate of ≤ 0.08 mm/year ensures adequate resistance to hydrochloric acid generated by hydrolysis of chlorinated organics, while the NaOH corrosion rate of ≤ 0.40 mm/year ensures the enamel can withstand the alkaline cleaning solutions used between batches. The crack formation temperature of ≥ 190 °C provides a safety margin for the thermal cycling inherent in batch operations, where the reactor may be heated to reaction temperature and cooled for product discharge and cleaning.
For specialized applications, such as processes involving highly concentrated alkalis at elevated temperatures, the standard allows for enhanced performance grades with lower corrosion rates (e.g., A24 ≤ 1.0 g/m² equivalent to approximately 0.18 mm/year NaOH corrosion) to be specified by agreement between manufacturer and purchaser. These enhanced grades may require testing per ISO 28706-5 (closed-system autoclave method) in addition to or instead of the standard cylindrical vessel test, particularly when the process temperature exceeds 95 °C.
For processes that involve both acidic and alkaline media, the more demanding requirement typically governs material selection. If a process exposes the enamel to both boiling HCl and hot NaOH, the enamel must simultaneously satisfy both the ≤ 0.08 mm/year HCl limit and the ≤ 0.40 mm/year NaOH limit. Always verify both parameters rather than assuming that good acid resistance implies good alkaline resistance, as the two properties are governed by different chemical mechanisms and are not necessarily correlated.
5. Frequently Asked Questions
Q1: What is the practical significance of the 0.08 mm/year HCl corrosion rate limit?
A: At a corrosion rate of 0.08 mm/year, a standard enamel coating thickness of 1.6 mm would theoretically last 20 years before complete penetration under continuous exposure to boiling 20 % HCl. In practice, actual service life is longer because industrial processes do not operate continuously at boiling conditions, and the self-limiting nature of acid corrosion slows the attack as the surface layer becomes enriched in insoluble corrosion products. The 0.08 mm/year limit thus provides a conservative design basis with substantial safety margins for most applications.
A: At a corrosion rate of 0.08 mm/year, a standard enamel coating thickness of 1.6 mm would theoretically last 20 years before complete penetration under continuous exposure to boiling 20 % HCl. In practice, actual service life is longer because industrial processes do not operate continuously at boiling conditions, and the self-limiting nature of acid corrosion slows the attack as the surface layer becomes enriched in insoluble corrosion products. The 0.08 mm/year limit thus provides a conservative design basis with substantial safety margins for most applications.
Q2: Why is the NaOH corrosion rate limit (0.40 mm/year) five times higher than the HCl limit (0.08 mm/year)?
A: The five-fold difference reflects the fundamentally different corrosion mechanisms. In HCl, the corrosion is self-limiting due to silica saturation of the solution at the enamel surface, which forms a protective gel layer. In NaOH, the corrosion products (silicates) remain fully soluble, allowing continuous attack. The 0.40 mm/year limit represents the best practical balance between enamel performance and manufacturing feasibility for standard chemical enamels.
A: The five-fold difference reflects the fundamentally different corrosion mechanisms. In HCl, the corrosion is self-limiting due to silica saturation of the solution at the enamel surface, which forms a protective gel layer. In NaOH, the corrosion products (silicates) remain fully soluble, allowing continuous attack. The 0.40 mm/year limit represents the best practical balance between enamel performance and manufacturing feasibility for standard chemical enamels.
Q3: How does the crack formation temperature relate to actual thermal shock limits in service?
A: The crack formation temperature (ISO 13807) is a material property measured on small, standardized test specimens under controlled laboratory conditions. It represents the temperature at which the enamel-steel system first develops cracks due to differential thermal expansion. The actual safe operating limits for a complete apparatus are typically lower than the crack formation temperature and depend on the apparatus design, steel thickness, enamel thickness distribution, and heating/cooling rates. These operational limits are determined using the thermal shock diagrams in ISO 28721-3, which account for full-scale apparatus behavior.
A: The crack formation temperature (ISO 13807) is a material property measured on small, standardized test specimens under controlled laboratory conditions. It represents the temperature at which the enamel-steel system first develops cracks due to differential thermal expansion. The actual safe operating limits for a complete apparatus are typically lower than the crack formation temperature and depend on the apparatus design, steel thickness, enamel thickness distribution, and heating/cooling rates. These operational limits are determined using the thermal shock diagrams in ISO 28721-3, which account for full-scale apparatus behavior.
Q4: What does the cover coat structure designation mean in practice?
A: The cover coat structure describes the microstructure of the enamel layer. All-vitreous (V) enamels are completely glassy with no crystalline phases, offering maximum chemical resistance but lower mechanical strength. All-semicrystalline (SC) enamels contain controlled crystalline phases throughout the coating, providing enhanced mechanical strength and abrasion resistance at the expense of slightly lower chemical resistance. Layered (L) enamels combine a vitreous base coat for chemical resistance with a semicrystalline top coat for mechanical durability, offering a balance of properties for demanding applications where both chemical and mechanical resistance are required.
A: The cover coat structure describes the microstructure of the enamel layer. All-vitreous (V) enamels are completely glassy with no crystalline phases, offering maximum chemical resistance but lower mechanical strength. All-semicrystalline (SC) enamels contain controlled crystalline phases throughout the coating, providing enhanced mechanical strength and abrasion resistance at the expense of slightly lower chemical resistance. Layered (L) enamels combine a vitreous base coat for chemical resistance with a semicrystalline top coat for mechanical durability, offering a balance of properties for demanding applications where both chemical and mechanical resistance are required.
