Selecting a Coating System for an Immersed Sump
An owner is specifying a coating for the interior of a concrete sump that will be continuously immersed in mildly acidic process water (pH 4–5) at 40 °C. The sump also sees occasional steam cleaning at 90 °C. The owner's draft specification lists a generic 'epoxy coating, 300 µm DFT' with no product family, no surface preparation standard for concrete, and no immersion or temperature qualification. A contractor has proposed a standard amine-cured epoxy at 250 µm. The inspector is asked to review the specification before tender.
1. Identify the gaps in the draft specification that would make it unenforceable, and explain why each matters.
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Gaps:
- No product family or qualification: 'Generic epoxy' does not define chemical resistance. Immersion at pH 4–5 and 40 °C requires a product qualified for that service; not all epoxies are suitable for acid immersion.
- No concrete surface preparation standard: Concrete preparation (e.g., abrasive blast, scarification, moisture content limits) is essential for adhesion; without it, the contractor cannot be held to a defined surface.
- No DFT basis or measurement method: 300 µm with no method or acceptance criteria is not verifiable.
- No temperature qualification: The 90 °C steam cleaning exceeds many standard epoxy service limits; the product must be rated for intermittent high temperature or the cleaning procedure must be restricted.
- No moisture/vapor transmission consideration: Concrete substrates require assessment of moisture and may need a primer or moisture-tolerant system.
Each gap converts a requirement into an opinion, which is unenforceable at inspection.
2. Evaluate the contractor's proposed 250 µm standard amine epoxy against the service conditions.
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Chemical resistance: Standard amine-cured epoxies have limited resistance to continuous acid immersion, particularly at pH 4–5; resistance depends on the specific product's qualification data. The proposal does not demonstrate qualification for this service.
Temperature: Continuous 40 °C is within many epoxy limits, but intermittent 90 °C steam cleaning may exceed the product's rated service temperature and could cause softening, blistering, or loss of adhesion. The data sheet must be checked for intermittent temperature limits.
Thickness: 250 µm is below the draft's 300 µm and may be insufficient for immersion service; immersion systems often require higher DFT and sometimes a lining-grade product.
Conclusion: The proposal is not demonstrated to be suitable. The inspector should require product-specific immersion and temperature qualification data, and a system designed for the service rather than a generic epoxy.
3. What should the revised specification include to be enforceable at inspection?
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Revised specification should include:
- Product: Named or qualified product family with documented resistance to pH 4–5 at 40 °C and intermittent 90 °C, supported by manufacturer's chemical resistance data.
- Surface preparation: Concrete preparation standard (e.g., abrasive blast or equivalent), surface profile, moisture content limit, and repair of defects.
- Primer/system: Full system (primer, build coat, optional seal coat) with each coat's DFT.
- DFT and measurement: Minimum and maximum DFT, measurement method, number of readings, and acceptance criteria.
- Application conditions: Ambient and substrate temperature, RH, dew point margin, and recoat windows.
- Cure and verification: Cure schedule, inspection hold points, and acceptance tests (adhesion, holiday detection if applicable).
- Cleaning restriction: Maximum temperature and duration for steam cleaning, consistent with the product rating.
This converts the specification into measurable requirements the inspector can verify.