Surface Preparation for Pipeline Coatings
Cleanliness and profile
Surface preparation is the largest single influence on coating performance. Abrasive blasting removes mill scale, rust, and old coating and creates an anchor profile. Standards include SSPC-SP 5/NACE No. 1 (white metal), SSPC-SP 10/NACE No. 2 (near-white), and SSPC-SP 6/NACE No. 3 (commercial); ISO 8501-1 uses Sa 3, Sa 2½, and Sa 2. For pipeline coatings, near-white (Sa 2½) is common; FBE often requires a profile of roughly 50–100 µm.
Contamination and climate
Soluble salts such as chlorides and sulfates cause osmotic blistering and disbondment. Measure with Bresle or conductivity methods; many pipeline specifications limit chlorides to a few µg/cm². Blasting does not remove salts - washing may be required. Climate control: surface temperature at least 3 °C above dew point and relative humidity within specification, often ≤85%.
Traps
“Near-white equals white metal,” assuming blasting removes salt, and ignoring dew point because the air “looks dry.” Excessive profile can cause peaks to protrude through thin films; too little profile reduces adhesion.
Rule of thumb
Keep the surface 3 °C above dew point, verified at the pipe, and match profile to coating thickness. If in doubt, re-clean.
Worked example
A Bresle test reads 8 µg/cm² chloride against a 3 µg/cm² limit. Re-blast and wash, then retest before coating; do not coat and hope.