Coastal offshore PV suffers long-term erosion of high-concentration chloride ions and alternating dry-wet tidal environment. Static neutral salt spray cannot truly simulate marine working conditions, and composite test of long-term artificial seawater immersion plus alternating salt spray is mandatory warehouse inspection item for DNV offshore projects. Forming temperature and passivation sealing craft of duplex steel directly determine pitting corrosion resistance. This article unifies full-set corrosion test control parameters of 2507 duplex steel bolts for offshore PV and clarifies qualified judgment red line for offshore projects.
| Project Scenario | Artificial Seawater Immersion Duration | Total Alternating Salt Spray Cycle Hours | Max Allowable Pitting Depth(μm) |
| Inland Mountain PV | 1000h | 1000h | ≤30 |
| Coastal Offshore PV | 3000h | 5000h | ≤10 |
1. Pre-treatment of samples: Bolt degreasing, pure water rinsing and chrome-free passivation sealing to restore complete factory surface protective film; 2. Constant temperature artificial seawater immersion: 35℃ constant temperature simulating surface seawater of coastal areas, replace standard seawater formula every month; 3. Dry-wet alternating salt spray cycle: 2h spray + 2h hot air drying cycle to reproduce day-night dry-wet change of tides; 4. Metallographic section observation: Cut thread root and shank for metallography after test completion to measure pitting depth; 5. Retensile test: Re-test tensile strength after corrosion, strength attenuation shall not exceed 5%.
1. Dot pits at thread root: Stress concentration during cold heading forming and tiny pores on passivation film; Rectify: Add secondary low-temperature stress relief procedure and increase thickness of passivation sealing film. 2. Uniform rust on shank: Excessively high duplex steel forming temperature leads to precipitation of brittle ferrite phase; Rectify: Control cold heading mold temperature ≤220℃ and adopt segmented buffer forming. 3. Crevice corrosion at thread fitting position: Excessive assembly clearance leads to seawater retention; Rectify: Unify reserved thread coating machining allowance and perform complete set test with duplex steel nuts of same material.
5 pieces are extracted from each batch of offshore projects to complete 5000-hour composite corrosion test. Goods can only be supplied to offshore power stations if no visible pitting corrosion exists on thread and shank and tensile strength attenuation is less than 5%.