Surface Treatment of DNV Grade B Marine Steel Plates in Marine Corrosive Environments

I. Steel Properties and Corrosion Environment

DNV Grade B is a standard-strength carbon-manganese steel that lacks the corrosion resistance of alloy steels. In marine environments (atmospheric, splash, immersion, and sub-sediment zones), it is susceptible to salt spray electrochemical corrosion, pitting corrosion, and weld corrosion; therefore, a combined protective approach of “surface preparation + coating + cathodic protection” must be employed.

II. Surface Preparation Standards (Based on DNV-CG-0288, ISO 8501-1, PSPC)

1. Surface Cleanliness (Mandatory Requirement)

Sandblasting/Shot Blasting: Sa2.5 Grade (Near-White)

Remove more than 95% of scale, rust, and oil contamination; only uniform, slight traces are permitted.

Local Repairs: Sa2 Grade (Industrial)

Remove 80% of contaminants.

Welds/Edges: Sa2.5 grade, with a roughness of Ra ≤ 12.5 μm

Prohibitions:

Manual rust removal (St2 / St3) is prohibited in splash zones and immersion zones.

Salt content ≤ 50 mg/m² (calculated as NaCl); exceeding this limit may cause coating blistering or peeling.

2. Surface Roughness (Critical Parameter)

Recommended range:

40 - 75 μm (uniform peak-to-valley profile to ensure coating adhesion)

Abrasive requirements:

Use a mixture of steel grit (G25/G40) and steel shot (angular + round) to avoid excessively deep grooves; the use of river sand (which contains salt) is strictly prohibited.

Process Parameters:

Pressure: 0.6 - 0.8 MPa; Spray angle: 60° - 75°; Distance: 150 - 200 mm.

3. Pretreatment Process (7-Step Method)

3.1 Degreasing: Remove oil and grease using solvents or alkaline cleaners; rinse with clean water and dry thoroughly to ensure no salt residue remains.

3.2 Water-soluble salt detection: ≤ 50 mg/m²; If exceeded, rinse with fresh water until compliant.

3.3 Sandblasting for rust removal: Achieve Sa2.5 grade, surface roughness 40 - 75 μm; apply coating within 4 hours after dust removal (to prevent rust recurrence).

3.4 Edge treatment: Round all sharp edges to R ≥ 2 mm (or sand three times) to prevent coating cracking at sharp corners.

3.5 Weld Preparation: Remove slag, spatter, and oxidation, then grind smooth to Ra ≤ 12.5 μm.

3.6 Dust Removal and Cleaning: Clean the surface using a vacuum or oil-free compressed air to ensure it is free of dust and oil.

3.7 Environmental Control: Relative humidity ≤ 85%; steel plate temperature ≥ dew point temperature + 3°C; do not apply under conditions where condensation may occur.

III. Marine Zone Coating Systems (DNV Approved)

1. Atmospheric Zone (Superstructure, Deck)

Primer: Epoxy zinc-rich, 60 - 80 μm, zinc content ≥ 80%

Intermediate Coat: High-build epoxy, 120 - 150 μm

Topcoat: Aliphatic polyurethane, 60 - 80 μm, salt spray resistance ≥ 5000 h

Total Film Thickness: 240 - 300 μm

2. Splash Zone (Most Severe, Subject to Tidal Wet-Dry Cycling)

Primer: Epoxy glass flake, 150 - 200 μm, impact-resistant, resistant to wet-dry cycling

Intermediate Coat: High-build epoxy, 120 - 150 μm

Topcoat: Modified epoxy or polyurethane, 80 - 100 μm, salt spray resistance ≥ 8,000 h

Total Film Thickness: 350 - 450 μm, compatible with sacrificial anodes

3. Immersed Zone (Hull, Ballast Tanks)

Primer: Epoxy, 80 - 100 μm, compatible with cathodic protection

Intermediate Coat: High-build epoxy, 150 - 200 μm

Topcoat: Epoxy, 80 - 100 μm, seawater resistant

Total Film Thickness: 300 - 400 μm, forced cathodic protection (ICCP or sacrificial anodes)

4. Submerged-in-mud zone (buried in seabed mud)

System: Same as submerged zone, total film thickness ≥ 400 μm, enhanced cathodic protection

IV. Key Quality Control Points

Shop Primer: After sandblasting, apply a zinc silicate shop primer (20 - 30 μm) that is compatible with the topcoat and does not interfere with welding.

Rust Recurrence Control: Coating must be applied within 4 hours of sandblasting; if this time is exceeded, the surface must be re-sandblasted to Sa2.5 grade.

Inspection Standards:

Cleanliness: Visual inspection using ISO 8501-1 comparison plates

Surface Roughness: Surface roughness tester (Ra) or anchor pattern depth gauge

Salt Content: Conductivity method (≤ 50 mg/m²)

Film Thickness: Dry film thickness gauge; measure 5 points per 10 m² and calculate the average

V. Common Misconceptions and Solutions

VI. References

1. DNV-CG-0288:2021 “Corrosion Protection of Ships”

2. ISO 8501-1:2007 Surface Cleanliness Grades

3. ISO 8503-2:2012 Surface Roughness Grading

4. PSPC (IMO MSC.215(82)) Ballast Tank Coating Standards