5.1 What was tested
| Item |
Detail |
| Sample ID |
SG-TP-2026-0187 |
| Batch number |
GE/EN/26-B142 |
| Glass type |
Acid/alkali-resistant enamel |
| Test piece |
105 mm dia. × 2 mm thick disc; 2 pieces (one per test) |
| Coating thickness |
1.55 mm average (1.42 – 1.70 mm), 2 ground coats + 4 cover coats |
| Condition before test |
No pinholes; passed 20 kV spark test |
| Date of testing |
27 – 28 May 2026 |
5.2 Standards followed
| Standard |
How it was applied |
| ISO 28706-2 |
Resistance of vitreous and porcelain enamels to boiling acids and their vapours — used for
the hydrochloric acid vapour-phase test |
| ISO 28706-4 |
Resistance to alkaline liquids — used for the sodium hydroxide liquid-phase test |
| DIN EN ISO 28721-2 |
Glass lined apparatus for process plants — source of the maximum permissible corrosion
rates used as acceptance limits |
5.3 How the test was done
Each weighed disc was exposed for 24 hours — one to hydrochloric acid vapour under reflux, and one
to boiling sodium hydroxide solution in full immersion. The mass lost per unit of exposed area was
measured on a 0.01 mg analytical balance and converted to an equivalent uniform corrosion rate. All
instruments used are calibrated and traceable to national standards.
| Parameter |
Test 1 — acid (ISO 28706-2) |
Test 2 — alkali (ISO 28706-4) |
| Reagent |
Hydrochloric acid, 20% w/w, AR grade |
Sodium hydroxide, 10% w/w, AR grade (pH 13.9) |
| Phase of exposure |
Vapour, above boiling liquid |
Liquid, full immersion |
| Temperature |
Boiling, 108 ± 2 °C |
Boiling, 103 ± 2 °C |
| Duration |
24 h continuous reflux |
24 h continuous reflux |
| Exposed area |
80.0 cm² |
80.0 cm² |
| Weight loss |
2.1 mg |
6.0 mg |
5.4 Results
Permissible limits per DIN EN ISO 28721-2 for glass lined apparatus in continuous service
| Test |
Measured rate |
Permissible limit |
Allowance used |
Status |
| Acid resistance (HCl vapour) |
0.067 mm/year |
≤ 0.080 mm/year |
84% |
Pass |
| Alkali resistance (NaOH liquid) |
0.174 mm/year |
≤ 0.400 mm/year |
44% |
Pass |
| Characteristic |
Requirement |
Observed |
Status |
| Surface integrity after exposure |
No pinholes, blisters, cracks or spalling |
None detected at 10× |
Pass |
| High-voltage continuity |
No holiday at 20 kV DC |
No breakdown |
Pass |
| Gloss retention |
≥ 85% of initial 60° gloss |
93% acid, 89% alkali |
Pass |
5.5 What the results tell us
Both coupons kept a continuous, uniformly glazed surface. The acid-exposed coupon showed no visible
change; the alkali-exposed coupon showed only slight matting at the vapour line, in line with its slightly
higher mass loss. No colour shift of the cobalt blue enamel was seen against the reference coupon.
Gloss retention stayed well above the 85% threshold. Examination at 10× magnification found no
pinholes, blisters, craters, crazing or edge spalling, and a 20 kV DC test after exposure showed no
breakdown — the coating remained electrically continuous.
The behaviour observed is typical of mild leaching of the alkali-borosilicate network rather
than bulk dissolution. Surface recession of about 0.11 µm (acid) and 0.30 µm (alkali) over 24
hours is negligible against the 1.55 mm mean coating thickness.
Overall assessment
Pass
2 of 2 chemical resistance tests within permissible
limits. Suitable for continuous duty in mixed acid/alkali service within the operating envelope of the
equipment data sheet.
