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ISO 28706-2:2020 – Vitreous and Porcelain Enamels — Determination of Resistance to Chemical Corrosion — Part 2: Determination of Resistance to Chemical Corrosion by Boiling Acids, Boiling Neutral Liquids, and Their Vapours
Test method for determining the chemical corrosion resistance of vitreous and porcelain enamels to boiling acids, neutral liquids, and their vapours
Scope and Industrial Significance
ISO 28706-2:2020 specifies a standardized test method for evaluating the chemical corrosion resistance of vitreous and porcelain enamel coatings when exposed to boiling acids (typically 20% hydrochloric acid or 10% citric acid), boiling neutral liquids (deionized water), and their vapours. This test is fundamental for quality assurance in industries where enamel-lined equipment serves in corrosive chemical environments — chemical reactors, storage tanks, pharmaceutical processing vessels, sanitary ware, heat exchangers, and kitchen appliances. Vitreous enamel offers unique advantages as a corrosion barrier: it is inorganic, non-porous when properly fired, resistant to virtually all organic chemicals, and maintains integrity at temperatures up to 400°C depending on formulation.
A properly formulated and fired vitreous enamel can reduce general corrosion rates to below 0.1 mm/year in boiling 20% HCl at 108°C — performance unmatched by most metallic alloys and organic coatings in this aggressive environment.
The test method enshrined in ISO 28706-2 has been refined over decades of industrial application. The 2020 revision introduced improved specimen preparation guidelines, updated the vapour exposure apparatus design to ensure uniform condensate distribution, and clarified the reporting requirements for different enamel classes. The standard recognizes that enamel corrosion resistance is not solely a material property but is profoundly influenced by manufacturing process parameters — frit composition, milling fineness, application technique, firing temperature and time, and coating thickness all play critical roles.
Test Apparatus and Procedure
The test employs a borosilicate glass reflux apparatus consisting of a boiling flask, a heating mantle with temperature control, a reflux condenser, and a specimen support system that suspends the enamel test piece in the boiling liquid or in the vapour phase. Test specimens — typically 50 mm × 50 mm flat panels or equivalent curved sections — are weighed to 0.1 mg precision before and after exposure. The test duration is 2.5 hours for acid tests and 6 hours for neutral liquid and vapour tests. After exposure, specimens are rinsed, dried at 110°C, and reweighed. Results are expressed as mass loss per unit area (g/m²) and classified according to the standard’s corrosion resistance classification scheme.
| Test Condition | Test Medium | Temperature | Duration (h) | Classification Criteria (mass loss g/m²) |
|---|---|---|---|---|
| Boiling acid — Class AA | 20% HCl or 10% citric acid | Boiling (~108°C for HCl) | 2.5 | < 0.3 g/m² (highest resistance) |
| Boiling acid — Class A | 20% HCl or 10% citric acid | Boiling | 2.5 | 0.3 to < 1.0 g/m² |
| Boiling acid — Class B | 20% HCl or 10% citric acid | Boiling | 2.5 | 1.0 to < 3.0 g/m² |
| Neutral liquid | Deionized water | Boiling (100°C) | 6.0 | < 0.2 g/m² typical for quality enamels |
| Vapour (acid) | Vapour over boiling acid | ~100°C vapour phase | 2.5 | Typically 50-70% of liquid phase attack |
Enamels achieving Class AA acid resistance are suitable for the most demanding chemical service — continuous contact with boiling mineral acids in chemical reactor vessels. Industrial reactors lined with Class AA enamel provide 10-15 years of maintenance-free service in hydrochloric acid service at concentrations up to 30% and temperatures to 150°C.
Factors Influencing Enamel Corrosion Resistance
Several interdependent factors determine the corrosion resistance of a vitreous enamel coating. The frit chemical composition is primary — silica (SiO₂) content of 45-65% provides the glass network structure, while additions of boron oxide (B₂O₃) lower the melting temperature and improve acid resistance. Zirconia (ZrO₂) at 5-15% dramatically enhances alkali resistance and overall chemical durability. Titanium dioxide (TiO₂) at 15-20% is used in opaque enamels and provides additional chemical resistance through its refractory nature. The firing process must achieve complete fusion and flow of the frit particles while avoiding overfiring that can cause bubble formation and surface degradation. Typical firing temperatures range from 780-900°C for ground coat enamels and 800-850°C for cover coat enamels applied to steel substrates.
Enamel defects such as pinholes, fish-scaling, and copper-heading are not merely cosmetic issues — they create direct pathways for corrosive attack to reach the substrate metal. Even a single pinhole per square centimeter can reduce the effective service life of enamel-lined chemical equipment by 50% or more. High-voltage spark testing (15-20 kV per ISO 2746) is essential for detecting such defects before equipment is placed in service.
Coating thickness is another critical variable. A thickness of 0.8-1.5 mm is typical for chemical service enamels applied to steel. Thicker coatings provide greater corrosion barrier but increase susceptibility to thermal shock failure and chipping due to the differential thermal expansion between enamel (α ≈ 8-12×10⁻⁶ K⁻¹) and steel (α ≈ 12×10⁻⁶ K⁻¹). Modern two-coat enamel systems — a ground coat for adhesion followed by a cover coat for chemical resistance — achieve optimal performance by combining the adhesive properties of cobalt-rich ground coats (cobalt oxide at 0.5-2.0%) with the chemical durability of zirconia-toughened cover coats.
Never use enamel-lined equipment in hydrofluoric acid (HF) service — even trace concentrations (above 50 ppm) will rapidly attack the silicate network of the enamel, causing catastrophic coating failure within hours. This is the single most important chemical compatibility limitation of vitreous enamel.
FAQs
Q: How does ISO 28706-2 relate to other parts of the ISO 28706 series?
A: The series includes Part 1 (determination of resistance to chemical corrosion by acids at room temperature), Part 2 (boiling acids, neutral liquids, and vapours — the most commonly referenced part), Part 3 (determination of resistance to chemical corrosion by alkaline liquids using a hexagonal vessel), Part 4 (determination of resistance to chemical corrosion by alkaline liquids using a cylindrical vessel), Part 5 (determination of resistance to chemical corrosion by hot acids), and Part 6 (determination of resistance to chemical corrosion by hot chemical liquids for equipment). Together they form a comprehensive chemical resistance evaluation framework.
A: The series includes Part 1 (determination of resistance to chemical corrosion by acids at room temperature), Part 2 (boiling acids, neutral liquids, and vapours — the most commonly referenced part), Part 3 (determination of resistance to chemical corrosion by alkaline liquids using a hexagonal vessel), Part 4 (determination of resistance to chemical corrosion by alkaline liquids using a cylindrical vessel), Part 5 (determination of resistance to chemical corrosion by hot acids), and Part 6 (determination of resistance to chemical corrosion by hot chemical liquids for equipment). Together they form a comprehensive chemical resistance evaluation framework.
Q: What is the difference between Class AA and Class A acid resistance?
A: Class AA requires mass loss below 0.3 g/m² in the boiling acid test and represents the highest corrosion resistance category. Class A allows 0.3-1.0 g/m². In practical terms, Class AA enamels are specified for continuous chemical reactor service with concentrated mineral acids, while Class A may be acceptable for batch processing or less aggressive conditions.
A: Class AA requires mass loss below 0.3 g/m² in the boiling acid test and represents the highest corrosion resistance category. Class A allows 0.3-1.0 g/m². In practical terms, Class AA enamels are specified for continuous chemical reactor service with concentrated mineral acids, while Class A may be acceptable for batch processing or less aggressive conditions.
Q: Can enamels be repaired after corrosion damage?
A: Yes, but with limitations. Small-area damage (< 25 cm²) can be repaired using noble metal (tantalum or platinum) patch kits or specialized enamel repair putties. However, the repaired area will never match the original corrosion resistance. Large-area damage typically requires complete strip and refire, which is only economically viable for high-value equipment.
A: Yes, but with limitations. Small-area damage (< 25 cm²) can be repaired using noble metal (tantalum or platinum) patch kits or specialized enamel repair putties. However, the repaired area will never match the original corrosion resistance. Large-area damage typically requires complete strip and refire, which is only economically viable for high-value equipment.
Q: What causes fish-scaling defects in enamel coatings?
A: Fish-scaling is caused by hydrogen gas evolution at the enamel-steel interface during firing. The hydrogen originates from moisture on the steel surface or from the acid pickling step. Defect prevention strategies include using low-hydrogen steel, adequate degassing firing cycles, and ground coat formulations with controlled gas retention capacity.
A: Fish-scaling is caused by hydrogen gas evolution at the enamel-steel interface during firing. The hydrogen originates from moisture on the steel surface or from the acid pickling step. Defect prevention strategies include using low-hydrogen steel, adequate degassing firing cycles, and ground coat formulations with controlled gas retention capacity.
