Surface Preparation of a Rusty Tank Interior
A contractor is hired to coat the interior of a 10,000-liter carbon steel tank that has been in service for 20 years. The tank previously stored water and has extensive rust and pitting on the interior surfaces. The specified coating is a two-coat epoxy system with a total DFT of 300-350 μm. The specification requires surface preparation to Sa 2.5 (near-white metal blast cleaning) according to ISO 8501-1. The contractor plans to use a centrifugal blast machine to clean the interior. However, due to the tank's size and configuration, the contractor finds it difficult to achieve uniform blast cleaning, especially in the corners and weld seams. The contractor proposes to use power tool cleaning to St 3 (very thorough hand and power tool cleaning) instead of Sa 2.5, arguing that the epoxy coating can tolerate a less clean surface. The project engineer is concerned about the coating's performance.
1. What are the risks of using St 3 instead of Sa 2.5 for this tank interior?
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Using St 3 instead of Sa 2.5 carries significant risks:
- Residual contaminants: St 3 does not remove all rust, mill scale, or previous coatings; it only removes loose material. Sa 2.5 removes nearly all rust and mill scale, providing a clean surface. Residual rust can cause early coating failure.
- Anchor profile: Blast cleaning creates a profile that enhances adhesion. Power tool cleaning produces a much lower profile, which may not be sufficient for the epoxy coating to achieve adequate adhesion, especially in pitted areas.
- Pitting: The tank has pitting; St 3 may not clean out pits completely, leaving rust in the pits that can continue to corrode under the coating.
- Specification non-compliance: The specification explicitly requires Sa 2.5. Deviating from it without approval is a non-conformance and may void the coating warranty.
- Coating performance: Epoxy coatings are sensitive to surface cleanliness; a poorly prepared surface can lead to blistering, delamination, and reduced service life.
Therefore, the contractor should make every effort to achieve Sa 2.5, possibly by using different equipment or methods, or seek approval for an alternative with appropriate justification and testing.
2. If the contractor cannot achieve Sa 2.5 in the corners and weld seams, what alternative actions could be taken?
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If Sa 2.5 is unattainable in specific areas, the contractor could:
- Use a smaller blast nozzle or a different abrasive to reach confined areas.
- Supplement with hand-held blast equipment for corners and weld seams.
- Apply a stripe coat of the epoxy coating to these areas after achieving the best possible cleaning, to ensure adequate coverage and thickness.
- Consult the coating manufacturer for guidance on surface preparation requirements for their product; some coatings may tolerate a slightly lower standard if a suitable primer is used.
- Seek approval from the project engineer for a deviation, supported by test patches and adhesion testing.
However, the preferred approach is to achieve the specified cleanliness, as the long-term performance of the coating depends on it. If a deviation is approved, it should be documented and may require additional measures such as a higher DFT or a more tolerant coating.