Early White Rust on Hot-Dip Galvanized Steel Structure for Outdoor Substation – Process Investigation and Material Adjus

H2: Operating Conditions & Environmental Parameters
Structural Material: Q355B steel welded columns, hot-dip galvanized

Environment: Coastal outdoor substation, annual humidity 75%~90%, corrosivity category C5-M (ISO 12944-2)

Service Duration at Failure: 8 months after installation

Specified Coating Thickness: Average ≥85μm (GB/T 13912-2002), no passivation or sealing required

H2: Problem Description
Field inspection revealed:

Large areas of white powder and gray-white spots on galvanized surface

Local coating peeling with underlying red rust

No mechanical damage or galvanic contact observed

H2: Technical Investigation
Measured Coating Thickness:

Spec ≥85μm, actual range: 72–91μm (local under-spec)

Thin areas concentrated at welds and corners

Surface Composition (handheld XRF):

White rust areas: high levels of zinc oxides and carbonates

No abnormal chlorine or sulfur – ruled out direct industrial acid rain attack

Process Traceback:

Post-galvanizing: water quench + chromate passivation

Missing bath log for passivation – measured pH >2.5 (spec 1.5–2.0)

No sealing coat applied

H2: Corrective Actions – Material & Process Changes
Parameter    Original State    Revised State    Standard / Reference
Surface prep (abrasive)    Sa2    Sa2.5, Rz 40–70μm    ISO 8501-1
Galvanized coating thickness    ≥85μm (locally below)    Min. 100μm, corners ≥120μm    GB/T 13912-2002 Annex B
Passivation    Chromate, pH >2.5    Trivalent chromium, pH 1.8–2.0, coating weight ≥0.3g/m²    ASTM B633
Sealing coat    None    Epoxy/polyurethane clear coat, dry film 20–30μm    NORSOK M-501
Post-passivation rinse    Tap water    Deionized water + hot air drying (80°C, 5min)    Control residual moisture
H2: Validation Results (12-month re-inspection)
Coating thickness: 98–115μm – meets revised spec

No new white rust observed

Neutral salt spray test (ASTM B117):

Original process: white rust at 168 hours

Revised process: no white rust at 672 hours

H2: Technical Conclusion
Primary causes of white rust:

Locally insufficient galvanized coating + ineffective passivation

No sealing layer under high humidity and salt exposure

Residual rinse water accelerating oxidation

Applicable to: Galvanized steel structures in coastal or high-humidity environments, especially those with complex weld geometry and outdoor service.