6.3 Tank Bottom Interior Linings, Surface Prep & Inspection (API RP 652)

6.3 Tank Bottom Interior Linings, Surface Prep & Inspection (API RP 652)

API RP 652 Core Principle: Internal tank bottom linings serve as the primary defensive barrier against aggressive internal corrosion mechanisms, including water-bottom drop-out, microbial attack, and chemical contamination. Achieving maximum lining service life depends directly on rigorous surface preparation cleanliness, correct anchor profile geometry, and comprehensive quality assurance testing.

While cathodic protection shields the external soil-side of an aboveground storage tank bottom, internal surfaces face severe degradation driven by stored hydrocarbons and operational contaminants. Crude oil and refined fuels frequently contain entrained water that separates by density and settles directly onto the tank floor. This aqueous layer becomes saturated with dissolved salts, organic acids, and hydrogen sulfide. API Recommended Practice 652 (Linings of Aboveground Petroleum Storage Tank Bottoms) provides engineers, inspectors, and coating applicators with comprehensive standards for selecting, installing, and inspecting internal protective lining systems.

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1. Internal Degradation Mechanisms & Purpose of Linings

Internal corrosion of storage tank floors is rarely uniform; it predominantly manifests as aggressive, localized pitting beneath sludge and water layers.

                INTERNAL CORROSION CELL UNDER WATER BOTTOM

     [ Stored Hydrocarbon / Crude Oil Layer ]
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     [ Water Bottom & Heavy Sludge Layer (Chlorides, H2S, Acidic Salts) ]
     -------------------------------------------------------------------------
            (SRB Bacterial Colony)           (Acidic Pitting Cell)
                 [ MIC Pits ]                     [ Pitting ]
                      |                                |  Anodic Metal Loss:
                      v                                v  Fe -> Fe2+ + 2e-
     +=======================================================================+
     |                     STEEL TANK BOTTOM PLATES                          |
     +=======================================================================+

Primary Internal Corrosion Drivers

• Water Bottom Drop-Out: Heavy water layers accumulate in floor depressions, sumps, and perimeter sketch plates. Free water contains dissolved oxygen, sodium chloride, carbon dioxide ($CO_2$), and hydrogen sulfide ($H_2S$), forming a highly conductive, corrosive electrolyte.
• Microbiologically Influenced Corrosion (MIC): Anaerobic bacteria—predominantly Sulfate-Reducing Bacteria (SRB) such as Desulfovibrio—thrive underneath stagnant sludge and sediment at the oil-water interface. SRB utilize organic nutrients from hydrocarbons to reduce sulfates into highly corrosive biogenic hydrogen sulfide ($H_2S$), producing deep, sharp-edged pits with corrosion rates that can exceed 100 mils/year (2.5 mm/year).
• Galvanic Pitting Under Sludge: Heavy rust scale and iron sulfide deposits acting as cathodes relative to the exposed steel bottom plates create localized galvanic couples that accelerate pit penetration.

Primary Purpose of API RP 652 Linings

An internal lining acts as an impermeable dielectric barrier isolating the carbon steel substrate from corrosive water bottoms and chemical attack. Per API RP 652 Section 4, the internal lining is typically applied across the entire tank bottom and carried up the lower shell course 18 to 24 inches (450 to 600 mm) above the bottom-to-shell corner weld, fully encompassing the maximum anticipated water-bottom elevation.

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2. Classification of Lining Systems: Thin-Film vs. Thick-Film

API RP 652 classifies internal tank bottom linings into two fundamental categories based on applied nominal Dry Film Thickness (DFT) and physical construction:

+-------------------------------------------------------------------------+
|                    API RP 652 LINING CLASSIFICATION                     |
+------------------------------------+------------------------------------+
| THIN-FILM SYSTEMS (<= 20 MILS)     | THICK-FILM SYSTEMS (> 20 MILS)     |
+------------------------------------+------------------------------------+
| • Nominal DFT: 8 to 20 mils        | • Nominal DFT: 25 to 100+ mils     |
|   (200 to 500 microns)             |   (650 to 2500+ microns)           |
| • Multi-coat unreinforced epoxies  | • High-solids / 100% solids resins |
| • Smooth, economical, cleanable    | • Often reinforced with chopped    |
| • CONFORMS to steel contour;       |   fiberglass, woven mat, or flakes |
|   CANNOT bridge holes or gaps      | • BRIDGES mechanical gaps, pits,   |
| • Vulnerable to mechanical abuse   |   and small through-floor holes    |
+------------------------------------+------------------------------------+

1. Thin-Film Lining Systems ($\le 20$ mils / 500 $\mu$m)

• Formulation: Multi-coat amine-cured epoxies, epoxy-polyamides, epoxy-phenolics (novolac epoxies), and polyurethanes. Typically applied in 2 to 3 coats to achieve a total DFT of 10 to 20 mils.
• Mechanical Performance: Forms a smooth, chemically resistant film that conforms intimately to the underlying steel profile. It provides excellent chemical barrier resistance at moderate cost.
• Limitations: Thin-film linings possess zero structural bridging capability. If applied over deeply pitted steel, lap-weld edges, or plates subject to structural flexing, thin films can crack or suffer pinhole holidays. They are also vulnerable to mechanical gouging from cleaning squeegees, maintenance footwear, and floating roof landing legs.

2. Thick-Film Reinforced Lining Systems (> 20 mils / 500 $\mu$m)

• Formulation: 100% solids epoxies, vinyl esters, polyesters, or polyurea resin systems applied at thicknesses ranging from 30 to 60 mils (unreinforced or glass-flake filled) up to 80 to 120+ mils (2 to 3+ mm) for laminate-reinforced systems.
• Fiberglass Reinforcement Types:
  • Glass Flake: Microscopic overlapping glass platelets suspended in the resin matrix. The platelets align parallel to the substrate, creating an exceptionally tortuous path that reduces vapor and moisture permeability by over 90%.
  • Chopped Strand Mat / Roving: Continuous glass strands sprayed simultaneously with catalyzed resin using a chopper gun, or rolls of chopped strand fiberglass mat pressed into a wet resin base coat.
  • Woven Roving Fabric: Heavy bidirectional fiberglass cloth rolled into the resin matrix to provide immense tensile strength and impact resistance.
• Structural Bridging Capability: Thick-film reinforced linings form a continuous, fiber-reinforced composite plastic membrane across the tank floor. They can bridge localized pitting, mechanical crevices, weld seams, and small through-floor perforations, effectively acting as a secondary containment membrane even if the underlying carbon steel suffers localized exterior corrosion.

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3. Surface Preparation Standards & Anchor Profile

Industry statistics confirm that over 75% of all premature coating failures stem directly from inadequate surface preparation or surface contamination. Clean steel is chemically active and provides mechanical tooth for coating adhesion.

Blast Cleaning Cleanliness Standards

In accordance with API RP 652 Section 6 and joint NACE/SSPC (AMPP) standards, tank steel must be abrasive blast cleaned to one of the following levels:

1. SSPC-SP 5 / NACE No. 1 (White Metal Blast Cleaning):
   • Complete removal of all visible rust, mill scale, previous paint coatings, oxides, and foreign matter.
   • Steel surface must exhibit a uniform gray-white metallic color.
   • Application: Mandatory for severe immersion service, critical chemical storage, high operating temperatures (> 140°F / 60°C), and thick-film reinforced lining installations.
2. SSPC-SP 10 / NACE No. 2 (Near-White Metal Blast Cleaning):
   • At least 95% of each unit area (approx. 9 sq in.) must be completely free of all visible residues; slight shadows, streaks, or minor discolorations are permitted on the remaining 5%.
   • Application: The recognized industry standard benchmark for most petroleum storage tank bottom linings.

                  SURFACE PROFILE (ANCHOR PATTERN) GEOMETRY

          Peak-to-Valley Height (Anchor Profile: 1.5 to 3.0 mils)
                         |<--------->|
             /\          /\          /\          /\
            /  \  /\    /  \        /  \  /\    /  \       <-- Angular Grit Blast
    _______/    \/  \__/    \______/    \/  \__/    \____     (Mechanical Tooth!)
    =========================================================
                     Steel Substrate Base Plate

    * ROUND STEEL SHOT PROHIBITED: Produces smooth, rounded peened craters
      lacking the sharp mechanical undercuts necessary for lining adhesion.

Surface Anchor Profile Depth and Shape

• Abrasive Media Selection: Only sharp, angular abrasive media (such as aluminum oxide, copper/coal slag, garnet, or crushed steel grit) may be used. Round steel shot is strictly prohibited because it produces peened, hemispherical indentations without the sharp mechanical undercuts required to anchor high-build resin coatings.
• Profile Depth Specifications:
  • For Thin-Film Linings: 1.5 to 2.5 mils (38 to 65 $\mu$m).
  • For Thick-Film Linings: 2.5 to 4.0 mils (65 to 100 $\mu$m).
• Field Profile Verification (ASTM D4417 Method C): Measured using Replica Tape (Testex Press-O-Film). An emulsion-coated plastic tape is burnished into the blasted steel surface; the microscopic peaks compress the tape. The thickness is read with a calibrated anvil spring micrometer and the 2.0-mil backing thickness is subtracted, yielding the exact peak-to-valley profile depth.

Soluble Salt Contamination and Osmotic Blistering

Even a visually pristine White Metal surface can harbor microscopic, invisible soluble salts (chlorides and sulfates) trapped within steel micro-crevices:

• Osmotic Blistering Mechanism: Soluble salts left on the steel draw moisture through the semi-permeable lining film via osmosis, creating high localized hydraulic pressures that burst the lining and form water-filled blisters.
• Acceptance Thresholds: API RP 652 specifies that soluble chloride concentrations must be tested prior to lining application (via Bresle patch or extraction sleeve methods per ISO 8502-6/9). Chlorides must not exceed 3 to 5 $\mu$g/cm² for critical immersion linings.

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4. Environmental Application Controls

Applying high-performance protective linings requires strict atmospheric regulation inside the confined space of a storage tank.

+-------------------------------------------------------------------------+
|                   MANDATORY ENVIRONMENTAL THRESHOLDS                    |
+-----------------------+-----------------------+-------------------------+
| Environmental Factor  | Mandatory Limit       | Failure Mode Prevented  |
+-----------------------+-----------------------+-------------------------+
| Surface Temperature   | Minimum 5°F (3°C)     | Microscopic flash rust; |
| vs. Dew Point         | ABOVE Dew Point       | moisture entrapment     |
+-----------------------+-----------------------+-------------------------+
| Relative Humidity     | Strictly < 85%        | Amine blush / blooming; |
| (RH)                  | (Often < 80%)         | curing inhibition       |
+-----------------------+-----------------------+-------------------------+
| Steel Substrate Temp  | 50°F to 90°F          | Under-cure (<50°F) or   |
| Range                 | (10°C to 32°C)        | solvent entrapment (>90)|
+-----------------------+-----------------------+-------------------------+

The Dew Point Rule

At all times during abrasive blasting, vacuuming, coating application, and initial curing, the steel substrate temperature must be maintained at least 5°F (3°C) above the calculated dew point of the internal air. If the steel temperature drops within 5°F of the dew point, invisible microscopic moisture condenses onto the freshly blasted steel, triggering instant flash rusting and destroying coating adhesion.

Relative Humidity & Amine Blush

• Humidity Limit: Relative humidity must be maintained below 85% (preferably below 50% using industrial desiccant dehumidification units).
• Amine Blush (Sweating): When amine-cured epoxies are applied under conditions of high humidity (> 85%) or in the presence of carbon dioxide ($CO_2$) from unvented fuel-burning heaters, the amine curing agent reacts with atmospheric moisture and $CO_2$ to form a carbamate compound (an oily, waxy surface film known as amine blush). Amine blush ruins the chemical bond of subsequent coats, causing catastrophic intercoat delamination.

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5. Quality Assurance Inspection & Testing Protocols

Quality verification during and following lining application requires systematic non-destructive and destructive testing in compliance with recognized standards:

1. Dry Film Thickness (DFT) Verification (SSPC-PA 2)

• Measurement Tool: Calibrated Type 2 electronic magnetic or eddy-current thickness gauges (e.g., Positector, Elcometer).
• Measurement Protocol: For storage tank bottoms, SSPC-PA 2 mandates taking 5 spot measurements per 100 sq ft (10 m²) of coated surface. Each "spot" consists of the mathematical average of at least 3 localized gauge readings obtained within a 1.5-inch (4 cm) diameter circle.
• Acceptance Criteria: Individual spot readings must satisfy the specified thickness range (e.g., 80% minimum to 120% maximum of nominal DFT), and no individual point reading may fall below 80% of minimum specified DFT.

2. Holiday (Pinhole) Detection (NACE SP0188)

Holidays are microscopic pinholes, air voids, voids over weld seams, or inclusions that penetrate the lining to bare metal. Even a microscopic pinhole concentrates internal corrosion into an intense localized cell.

+-------------------------------------------------------------------------+
|                      NACE SP0188 HOLIDAY TESTING                        |
+-----------------------------------+-------------------------------------+
| LOW-VOLTAGE WET SPONGE TESTER     | HIGH-VOLTAGE SPARK TESTER           |
+-----------------------------------+-------------------------------------+
| • Thickness: <= 20 mils (500 um)  | • Thickness: > 20 mils (500 um)     |
| • Operating Voltage: 67.5 V DC    | • Operating Voltage: 1,000 to 15,000|
| • Wetting agent in potable water  |   Volts (V = 525 * sqrt(Thickness)) |
| • Current flows through wet sponge| • Voltage arcs across air void to   |
|   to grounded steel, sounds horn  |   grounded steel, generating spark  |
| • SAFE: Will not puncture thin    | • DANGER: Using high voltage on     |
|   coatings                        |   thin films punctures sound coating|
+-----------------------------------+-------------------------------------+

• Low-Voltage Wet Sponge ($\le 20$ mils / 500 $\mu$m): Operated at 67.5 V DC with an open-cell sponge saturated with potable water and a non-sudsing surfactant. When dragged across a pinhole, water penetrates to the bare steel, completing the electrical circuit and sounding an audible alarm.
• High-Voltage Spark Tester (> 20 mils / 500 $\mu$m): For thick-film coatings, wet sponges cannot penetrate deep tortuous micro-cavities. A high-voltage DC spark tester utilizing a brass wire brush or conductive spring is used. Voltage is calibrated per NACE SP0188: $V = 525 \times \sqrt{T_{\text{mils}}} \quad \text{or approximately } 100\text{ to }125\text{ V per mil of DFT}$ When the probe passes over a void, electrical breakdown of the air gap occurs, creating a bright visible spark and sounding an alarm.

  Caution: Applying high-voltage spark testing to thin-film coatings (< 20 mils) will exceed the dielectric breakdown strength of the resin, burning holes directly through sound lining!

3. Cure Verification Testing

Before returned to petroleum service, the lining must achieve complete molecular cross-linking:

• Solvent Rub Test (ASTM D4752 / ASTM D5402): A cotton rag saturated with Methyl Ethyl Ketone (MEK) is rubbed firmly across the cured lining surface for 50 double rubs. If the lining softens, scratches with a fingernail, or transfers pigment to the rag, it is under-cured. Fully cured epoxy resists 50 double rubs with zero softening (Rating 4 or 5).
• Barcol Hardness Test (ASTM D2583): For thick-film fiberglass-reinforced polyester or vinyl ester systems, a spring-loaded indenter (Barcol Impressor) is pressed against the lining surface. The depth of penetration yields an instantaneous hardness value (typically required to exceed 35 to 50), directly verifying full polymer polymerization.