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How to read a zinc-plating salt spray test report: NSS and coating thickness essentials
2026-09-10
Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.
Why zinc-plated parts need a salt spray report
When zinc-plated fasteners show white rust or red rust after outdoor or humid service, it is rarely a defect of the zinc itself — it usually means the coating thickness and salt-spray rating did not match the actual environment. A zinc salt spray report links lab data to real service life: the same white zinc finish can fail in 24 hours of neutral salt spray, while hot-dip galvanising at 80–120 μm easily exceeds 1000 hours. Reading the report correctly is the only way to pick a zinc finish that truly fits the job, instead of being misled by a 'passes' label.
How NSS is run and how to read the result
The international standard test for zinc plating is the Neutral Salt Spray test (NSS), run per ASTM B117 or ISO 9227: 5% NaCl solution, pH 6.5–7.2, chamber at 35±2°C, continuous spray. Three observations are recorded: (1) white-rust onset time (zinc oxidation products, loose white deposits first); (2) red-rust onset time (steel substrate oxidation, meaning the coating has been penetrated); and (3) corroded area percentage. Rating is normally by GB/T 6461 or ISO 10289 in 10 grades (grade 10 best). A typical '96 h no red rust, 48 h white rust' line in a report refers to exactly these metrics.
Coating thickness vs salt spray life
Salt spray life roughly scales with zinc coating thickness. Electroplated zinc (white, yellow, blue-white) is typically 5–15 μm and gives 24–96 hours to first red rust; hot-dip galvanising per GB/T 13912 ranges 45–85 μm and above, delivering 500–1500 hours salt spray; Dacromet at 8–12 μm achieves 500–1000 hours with much better heat resistance than pure zinc. When reading a report, watch the units — coating thickness in μm or g/m² (hot-dip often quoted as 100 g/m² ≈ 14 μm), and red-rust time as a single value or an average. Need help interpreting a salt spray report or selecting the right zinc finish for your environment? Contact JIALI Industrial (164.hk) — we provide ASTM B117 / ISO 9227 data with every batch.
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