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ISO 28706-3:2017 — Alkaline Corrosion Resistance of Vitreous Enamel (Hexagonal Vessel Method)
Determination of resistance to chemical corrosion by alkaline liquids using a hexagonal vessel or tetragonal glass bottle
1. Understanding Alkaline Attack on Vitreous Enamel
Vitreous and porcelain enamels are glass-ceramic coatings fused onto metal substrates at high temperatures (typically above 800 °C). The primary structural component is silicon dioxide (SiO2), which forms a three-dimensional silica network. When exposed to alkaline liquids, this network undergoes hydrolysis — the Si-O-Si bonds break, forming silicic acid and silicates that dissolve into the attacking medium. Other metal oxide components hydrolyze as well, producing hydrated metal ions or hydroxides. The entire process manifests as a measurable loss in mass per unit area.
For aqueous alkaline solutions like 0.1 mol/L NaOH at 80 °C, the silica network undergoes significant attack. Silicates and most hydrolyzed components remain soluble in alkali, causing corrosion to proceed linearly with time. This linear relationship allows engineers to calculate precise corrosion rates in g/m²h and mm/year.
The corrosion behavior varies fundamentally with pH and temperature. In weak acids at room temperature (e.g., citric acid per ISO 28706-1), only minor surface leaching occurs — highly resistant enamels show no visible change. In boiling acids (ISO 28706-2), the attack is more aggressive but self-limiting due to silica saturation of the solution. In alkaline media, however, the corrosion products remain soluble, enabling continuous linear attack — making alkaline resistance a critical design parameter for chemical process equipment.
2. Test Apparatus: Hexagonal Vessel vs. Tetragonal Glass Bottle
ISO 28706-3 specifies two distinct apparatus configurations. The hexagonal vessel method uses a six-sided stainless steel vessel (austenitic stainless steel, e.g., 1.4571) where six enamelled specimens are simultaneously tested. Each specimen (80 mm diameter exposed area) is pressed against circular openings using gripping plates with wing nuts, sealed with synthetic rubber rings (70 IRHD hardness, chloroprene or EPDM, resistant to alkaline solutions at 100 °C). The vessel holds 4.5 L of test solution, stirred by a paddle agitator operating at 1350 ± 50 min&supmin;¹, with two 600 W immersion heaters and a contact thermometer accurate to ±1 °C.
The alternative tetragonal glass bottle apparatus (borosilicate glass 3.3 per ISO 3585) holds four specimens with 50 mm exposed diameter, using 600 mL of test solution. It features a magnetic stirrer with heating, a contact thermometer, and a Liebig-West reflux condenser. The condensate volume change is monitored via a graduated collector (0.1 mL graduation).
3. Standard Detergent Solution Test Procedure
The standard detergent test simulates washing machine conditions. The test solution (4.5 L) comprises: 27.0 g sodium tripolyphosphate (Na5P3O10), 9.0 g anhydrous sodium carbonate (Na2CO3), 2.7 g hydrated sodium perborate (NaBO2·H2O2·3H2O), 1.8 g sodium silicate (~81 % Na2SiO3), and 4.5 g alkylsulfonate.
| Parameter | Hexagonal Vessel | Tetragonal Glass Bottle |
|---|---|---|
| Test solution volume | 4.5 L | 600 mL |
| Number of specimens | 6 | 4 |
| Exposed area diameter | 80 mm | 50 mm |
| Test temperature | 95 ± 2 °C | 95 ± 2 °C |
| Standard duration | 24 h (extend to 168 h) | 2.5 h |
| Stirring method | Paddle, 1350 min&supmin;¹ | Magnetic stirrer |
Specimens are degreased, dried at 120 ± 5 °C for 2 h, cooled in a desiccator for 2 h, and weighed to 0.2 mg before and after testing. The loss in mass per unit area A = (ms − mf)/A is calculated. If the average loss is less than 8 mg after 24 h, the test extends to 168 h with solution replacement every 24 h. Results below 1.6 g/m² are reported as “<1.6 g/m²”.
The test solution must be replaced every 24 h for extended tests exceeding 24 h. Always use a freshly prepared solution for each test to ensure consistent alkalinity and avoid silica saturation effects that could inhibit corrosion.
4. Engineering Design Insights
For chemical process engineers specifying glass-lined equipment, understanding the alkaline corrosion mechanism is crucial. The linear corrosion rate in alkali means that equipment life can be estimated with reasonable accuracy — unlike acid attack where inhibition effects make long-term prediction difficult. The crack formation temperature (typically ≥190 °C for standard enamels per ISO 28721-2) and thermal shock limits must be simultaneously considered. When designing detergent manufacturing or handling equipment, specify enamel grades tested per ISO 28706-3 with documented A24 values below 1.6 g/m² for optimal service life.
The hexagonal vessel method’s ability to test six specimens simultaneously with continuous stirring closely replicates real washing machine conditions. For quality control in enamel production, this multi-specimen approach reduces testing time while improving statistical confidence through parallel replicates.
5. Frequently Asked Questions
Q1: Why does alkaline corrosion proceed linearly while acid corrosion is often logarithmic?
A: In alkaline media, the corrosion products (silicates and metal hydroxides) remain soluble, maintaining continuous attack on fresh enamel surface. In acids, dissolved silica quickly saturates the solution, forming an inhibitory layer that slows further corrosion.
A: In alkaline media, the corrosion products (silicates and metal hydroxides) remain soluble, maintaining continuous attack on fresh enamel surface. In acids, dissolved silica quickly saturates the solution, forming an inhibitory layer that slows further corrosion.
Q2: Can the hexagonal vessel be used for non-detergent alkaline solutions?
A: Yes. Clause 11 of ISO 28706-3:2017 describes procedures for other alkaline test solutions at agreed temperatures between 40 °C and 95 °C. However, never use solutions that could damage the austenitic stainless steel apparatus.
A: Yes. Clause 11 of ISO 28706-3:2017 describes procedures for other alkaline test solutions at agreed temperatures between 40 °C and 95 °C. However, never use solutions that could damage the austenitic stainless steel apparatus.
Q3: What is the significance of the 8 mg threshold in standard detergent testing?
A: The 8 mg threshold (equivalent to ~1.6 g/m²) indicates very high alkaline resistance. Below this level, the mass loss is too small for reliable corrosion rate calculation, so the test extends to 168 h for better differentiation.
A: The 8 mg threshold (equivalent to ~1.6 g/m²) indicates very high alkaline resistance. Below this level, the mass loss is too small for reliable corrosion rate calculation, so the test extends to 168 h for better differentiation.
Q4: How do specimen preparation defects affect test results?
A: Specimens with pinholes down to the metal, chipped edges, or edge corrosion must be discarded and replaced. Such defects provide direct pathways for alkaline attack on the steel substrate, producing artificially high mass loss values unrelated to enamel quality.
A: Specimens with pinholes down to the metal, chipped edges, or edge corrosion must be discarded and replaced. Such defects provide direct pathways for alkaline attack on the steel substrate, producing artificially high mass loss values unrelated to enamel quality.
