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ISO 28706-4:2016 — Alkaline Corrosion Resistance of Vitreous Enamel (Cylindrical Vessel Method)
Determination of resistance to chemical corrosion by alkaline liquids using a cylindrical vessel
1. Principle and Scope of the Cylindrical Vessel Method
ISO 28706-4:2016 specifies a test method for determining the resistance of vitreous and porcelain enamelled articles to attack by alkaline liquids at temperatures between 25 °C and 95 °C using a cylindrical vessel. Unlike the multi-specimen hexagonal vessel of ISO 28706-3, this method tests a single specimen per apparatus, with the solution unstirred during the test. This configuration is primarily designed for evaluating enamel coatings used in the chemical industry, where static alkaline exposure conditions better represent actual process environments such as storage tanks, reaction vessels, and pipeline sections where agitation is minimal or intermittent.
The test leverages the linear corrosion behavior of enamel in alkaline media. The silica network (SiO2) undergoes hydrolysis, and because silicates and metal hydroxides remain soluble in alkali, the attack proceeds linearly with time. This enables precise calculation of the rate of loss in mass per unit area (g/m²h) and the corrosion rate (mm/year) — both essential for predicting equipment service life in alkaline chemical processes.
The cylindrical vessel method is the preferred test for chemical industry enamels because its unstirred, static configuration more closely simulates actual process vessel conditions where agitation is minimal or intermittent. It is specifically referenced in ISO 28721-2 for enamel quality classification.
2. Apparatus Design and Material Specifications
The test apparatus comprises a cylindrical vessel made entirely of austenitic stainless steel, typically grade 1.4571 (X6CrNiMoTi17-12-2) with composition 16.5-18.5 % Cr, 10.5-13.5 % Ni, 2-2.5 % Mo, max 0.08 % C, and titanium addition for stabilization. The cylinder has a welded-on plate at one end and a circular flange with six threaded bolts at the other. The test specimen is enclosed in a protective envelope made of synthetic rubber (70 IRHD hardness per ISO 48, chloroprene or ethylene-propylene, resistant to alkaline solutions at 100 °C) and clamped between the cylinder and flange plate using six wing nuts.
| Component | Material | Specification |
|---|---|---|
| Cylinder | Austenitic stainless steel | 1.4571 (EN 10088-1) |
| Protective envelope | Synthetic rubber | 70 IRHD (ISO 48) |
| Flange plate | Austenitic stainless steel | 1.4571 (EN 10088-1) |
| Wing nuts (6) | Stainless steel | 1.4571 (EN 10088-1) |
| Stopper | Synthetic rubber | 70 IRHD (ISO 48) |
| Water bath | Demineralized water | ±0.1 °C up to 100 °C |
A filling nozzle closed with a rubber stopper allows introduction of 1 L of test solution. The apparatus sits in a thermostatically controlled water bath with stirrer, sealed against evaporation, maintaining temperatures up to 100 °C with ±0.1 °C stability. A calibrated thermometer graduated in 0.1 °C steps verifies the temperature during the test.
3. Test Procedures: Sodium Hydroxide Solutions
The standard defines two primary alkaline test conditions. The 0.1 mol/L NaOH test uses 4 g of NaOH dissolved in carbon dioxide-free water, made up to 1 L. The 1.0 mol/L NaOH test uses 40 g of NaOH prepared identically. Both tests run at 80 ± 1 °C for 24 hours. The test solution is preheated to the specified temperature before introduction into the apparatus, which has been preheated in the water bath for at least 10 minutes prior to filling.
The NaOH test solution must be prepared using carbon dioxide-free water. CO2 absorption by alkaline solutions can form carbonates, reducing the effective hydroxide concentration and leading to underestimated corrosion rates. Always use freshly prepared solutions stored in closed vessels.
After the 24-hour exposure period, the apparatus is removed from the bath, the solution discarded, and the interior rinsed with water. The specimen is wiped three times with cotton wool soaked in cold acetic acid solution (50 mL/L), rinsed with cold water, dried at 110 ± 5 °C for 2 hours, cooled in a desiccator for 2 hours, and weighed to 0.2 × 10−3 g precision. At least two specimens are tested per determination.
Results are expressed as the arithmetic mean of individual determinations. The total loss in mass per unit area A = (ms − mf)/A. For the NaOH test where corrosion proceeds linearly, the rate of loss v = A/t (g/m²h) and corrosion rate w = 3.504v (mm/year, assuming homogeneous enamel density of 2.5 g/cm³). Individual values must not deviate from the mean by more than 20 %.
4. Engineering Significance
The cylindrical vessel method is the definitive test for chemical industry enamel qualification. The corrosion rate in mm/year directly translates to expected service life — a rate of 0.40 mm/year (the maximum permitted by ISO 28721-2 for standard chemical enamel) corresponds to approximately 2.5 mm enamel loss over 6.25 years. This must be considered against the typical enamel coating thickness of 1.0-2.2 mm for vessels and 0.8-2.0 mm for pipes and fittings. For aggressive processes involving hot alkaline media, specifying enamel with an A24 value below 1.0 g/m² (equivalent to approximately 0.18 mm/year) provides a substantial safety margin for long-term equipment reliability.
For processes involving sodium hydroxide concentrations exceeding 1.0 mol/L or temperatures above 80 °C, consult ISO 28706-5 for autoclave testing in closed systems under controlled pressure conditions. The cylindrical vessel method is also referenced in ISO 28721-2 for enamel quality designation, making it an integral part of the glass-lined equipment specification chain.
5. Frequently Asked Questions
Q1: Why is the solution unstirred in the cylindrical vessel method?
A: The unstirred condition intentionally simulates chemical process environments where bulk agitation is minimal. This method was specifically designed for evaluating chemical industry enamels, whereas the hexagonal vessel method (ISO 28706-3) simulates washing machine conditions with continuous stirring to represent detergent solution exposure.
A: The unstirred condition intentionally simulates chemical process environments where bulk agitation is minimal. This method was specifically designed for evaluating chemical industry enamels, whereas the hexagonal vessel method (ISO 28706-3) simulates washing machine conditions with continuous stirring to represent detergent solution exposure.
Q2: What is the significance of the 0.1 mol/L vs 1.0 mol/L NaOH test distinction?
A: These two concentrations represent mild and severe alkaline exposure conditions respectively. Most chemical enamel specifications require testing at 0.1 mol/L for qualification purposes. The 1.0 mol/L test is reserved for processes involving highly alkaline media and provides a more stringent assessment of enamel durability under aggressive conditions.
A: These two concentrations represent mild and severe alkaline exposure conditions respectively. Most chemical enamel specifications require testing at 0.1 mol/L for qualification purposes. The 1.0 mol/L test is reserved for processes involving highly alkaline media and provides a more stringent assessment of enamel durability under aggressive conditions.
Q3: How does the protective envelope prevent edge corrosion from affecting results?
A: The synthetic rubber protective envelope completely seals the specimen edges and any uncoated areas from contact with the alkaline solution. This ensures that only the intended enamel-coated face is exposed to the corrosive medium, preventing artificially high mass loss values that would otherwise result from steel substrate corrosion at unprotected edges.
A: The synthetic rubber protective envelope completely seals the specimen edges and any uncoated areas from contact with the alkaline solution. This ensures that only the intended enamel-coated face is exposed to the corrosive medium, preventing artificially high mass loss values that would otherwise result from steel substrate corrosion at unprotected edges.
Q4: Can other alkaline liquids be tested with this apparatus?
A: Yes, Clause 11 of ISO 28706-4 describes procedures for testing with other alkaline solutions, provided they do not damage the stainless steel apparatus. Any agreed test solution, temperature, and duration are documented in the test report. This flexibility allows the method to be adapted for specific process fluids beyond the standard NaOH tests.
A: Yes, Clause 11 of ISO 28706-4 describes procedures for testing with other alkaline solutions, provided they do not damage the stainless steel apparatus. Any agreed test solution, temperature, and duration are documented in the test report. This flexibility allows the method to be adapted for specific process fluids beyond the standard NaOH tests.
