Water and Wastewater Protective Coating Systems
Article 08 of 24
Evaluating Steel Tanks, Pipe, Fittings, Welds, and Existing Coatings
Steel condition assessment must identify corrosion, section loss, surface contamination, fabrication details, existing coating failures, and conditions requiring repair before surface preparation and recoating begin.
Do Not Treat Every Rusted Surface the Same
Visible rust confirms that corrosion has occurred, but it does not explain the complete condition of the steel. The project team must determine where corrosion is located, how severe it is, what caused it, whether section loss has occurred, and whether the steel remains suitable for preparation and coating.
Uniform atmospheric rust, deep isolated pitting, corrosion beneath a failed lining, crevice corrosion around bolts, and section loss at a water-retaining seam are not equivalent conditions. Each can require a different repair, preparation, coating, and inspection response.
The coating contractor should document the condition and identify work outside the coating scope. Decisions concerning structural adequacy, plate replacement, weld repair, reinforcement, or remaining service capacity belong to qualified personnel authorized by the owner.
Separate Coating Condition from Structural Condition
A coating can fail while the underlying steel remains serviceable. Steel can also experience serious section loss beneath a coating that appears mostly intact.
Refer conditions for engineering or qualified owner evaluation when they include:
- Deep or widespread pitting
- Measurable loss of plate, pipe, structural member, or fastener thickness
- Perforation or active leakage
- Cracked, incomplete, or severely corroded welds
- Distorted, buckled, displaced, or damaged steel
- Severe corrosion at supports, anchorages, columns, rafters, roofs, seams, or connections
- Corroded ladders, platforms, railings, or access systems
- Unknown alterations or previous welded repairs
- Any condition beyond the contractor’s authority or technical qualification
Begin with Service and Maintenance Records
Before inspecting the steel, review available information about the asset and its previous service.
- Original drawings and fabrication details
- Tank, pipe, or equipment age
- Water or wastewater service
- Normal and maximum operating levels
- Water chemistry and treatment chemicals
- Previous coating systems and application dates
- Previous inspection reports
- Leak and repair history
- Cathodic-protection system information
- Known modifications, welding, attachments, or antenna installations
- Operational problems, overflows, condensation, or chemical upsets
Records help direct the inspection, but field conditions must still be verified.
Map Corrosion by Location and Form
Corrosion should be mapped to specific elevations, components, exposure zones, and details. Record both the amount of corrosion and its form.
Uniform Corrosion
Relatively even metal loss over a broad area. The surface may appear generally rusted, but thickness measurement may be required to quantify section loss.
Pitting Corrosion
Localized cavities that may be much deeper than the surrounding corrosion. Pits can retain salts, moisture, abrasive, and air.
Crevice Corrosion
Localized attack at lap joints, bolted connections, gaskets, seams, supports, and other tight spaces that retain moisture and contamination.
Galvanic Corrosion
Accelerated corrosion associated with electrically connected dissimilar metals in the presence of an electrolyte.
Underfilm Corrosion
Corrosion spreading beneath a coating from holidays, damaged areas, edges, pinholes, cracks, or loss of adhesion.
Microbiologically Influenced Corrosion
Localized corrosion affected by microorganisms, deposits, and changes in the surface environment. A qualified investigation may be required.
Inspect by Exposure Zone
Corrosion patterns often correspond to the service environment.
| Exposure Zone |
Conditions to Examine |
Typical Details |
| Immersion |
Underfilm corrosion, pitting, holidays, abrasion, deposits |
Floors, lower walls, columns, pipe, fittings |
| Waterline |
Wet-and-dry cycling, deposits, differential aeration |
Maximum and normal operating levels |
| Interior headspace |
Condensation, vapor exposure, corrosion at roofs and rafters |
Roof plates, columns, rafters, hatches, supports |
| Exterior atmospheric |
Weathering, ultraviolet exposure, condensation, trapped water |
Roof, shell, ladders, platforms, stiffeners, foundations |
Edges, Welds, Bolts, and Crevices Are Critical
Coatings tend to draw away from sharp edges and irregular details during application and cure. These locations may receive less dry-film thickness than adjacent flat surfaces.
Closely inspect:
- Sharp plate edges
- Weld seams and weld spatter
- Undercut, porosity, rough welds, and incomplete welds
- Bolts, nuts, rivets, and fasteners
- Lap joints and seams
- Stiffeners, rafters, brackets, and supports
- Pipe connections and flanges
- Ladder attachments and platform connections
- Horizontal ledges that retain water or debris
- Inaccessible or difficult-to-spray areas
The repair and coating plan may require edge rounding, weld grinding, removal of spatter, filling of pits, sealing of crevices, stripe coating, or other preparation before full-coat application.
Measure Section Loss Where Required
Visual inspection cannot reliably determine remaining steel thickness. Ultrasonic thickness measurement or another approved nondestructive method may be needed where pitting, corrosion, or section loss is suspected.
Measurement locations should be selected systematically and should include areas of visible deterioration, waterlines, low points, roof and floor plates, seams, supports, pipe, fittings, and difficult details.
The coating contractor may assist with access, cleaning, and documentation, but acceptance of the remaining steel and design of repairs should be performed by qualified personnel acting for the owner.
Evaluate the Existing Coating
Record the existing coating type when known, its approximate age, the service environment, and the observed failure modes.
Look for:
- Rusting through the coating
- Blistering
- Cracking and checking
- Peeling, flaking, and delamination
- Undercutting at damaged areas
- Pinholes and holidays
- Erosion and abrasion
- Chalking, fading, and ultraviolet degradation
- Chemical staining or softening
- Poor adhesion between coats
- Failure concentrated at edges, welds, fasteners, and transitions
Use Standardized Condition Ratings
Standardized rating methods allow the owner, engineer, contractor, and inspector to describe coating deterioration consistently.
- ASTM D610: Evaluates the degree of visible rusting on painted steel surfaces.
- ASTM D714: Evaluates coating blistering using photographic reference standards.
- AMPP SSPC-VIS 2: Provides a standard method and visual references for evaluating rusting on painted steel surfaces.
- Other specified ASTM or AMPP methods: May be used to rate cracking, flaking, chalking, adhesion, and other coating conditions.
State the method and edition used. A statement such as “coating is in poor condition” is less useful than a documented rating tied to defined locations and photographs.
Determine Whether the Existing Coating Can Remain
An existing coating should not remain merely because removal is difficult or expensive. The project requirements should define whether complete removal, spot repair, or overcoating is permitted.
If overcoating is considered, evaluate:
- Coating identity and chemistry
- Existing adhesion and cohesive strength
- Number and thickness of existing coats
- Rusting and underfilm corrosion
- Blistering, cracking, and embrittlement
- Contamination and soluble salts
- Compatibility with the proposed system
- Ability to clean and prepare without destabilizing the coating
- Effect of additional coating stress
- Manufacturer and specification approval
A test patch can provide useful compatibility and adhesion information, but it represents only the tested location. It does not prove that every portion of an aging coating is suitable for overcoating.
Identify Hazardous Existing Coatings Before Disturbance
Older coatings may contain lead, chromium, cadmium, or other hazardous constituents. Abrasive blasting, grinding, scraping, welding, and coating removal can create hazardous dust, fumes, debris, and waste.
Testing and hazard assessment should occur before the coating is disturbed. The work plan may require containment, exposure monitoring, respiratory protection, regulated-area controls, hygiene facilities, worker training, waste characterization, environmental controls, and specialized contractor qualifications.
Do not assume that the color or apparent age of a coating establishes whether hazardous constituents are present.
Soluble Salts May Be Invisible
Chlorides, sulfates, and other soluble contaminants can remain in pits, beneath failed coatings, around seams, or on apparently clean steel. If they remain beneath a new coating, they can contribute to osmotic blistering, underfilm corrosion, and premature failure.
The specification should define:
- Whether soluble-salt testing is required
- The extraction and analytical method
- Testing locations and frequency
- Acceptance limits
- Required cleaning or decontamination
- Retesting requirements after cleaning
Do not report a result without identifying the test method, extraction area, location, unit of measurement, and acceptance criterion.
Inspect Cathodic-Protection Interfaces
Steel water-storage tanks may use impressed-current or sacrificial-anode cathodic protection to supplement the interior coating system. The coating and cathodic-protection system should be treated as interacting parts of corrosion control.
Document anodes, reference electrodes, cables, penetrations, attachments, insulation, damaged areas, and evidence of coating disbondment. Coordinate coating work with the owner’s cathodic-protection specialist.
Do not alter, coat over, remove, reconnect, energize, or de-energize cathodic-protection components without authorized instructions.
Create a Steel Condition Map
The condition map should identify the location and extent of:
- Uniform rusting
- Pitting and suspected section loss
- Perforations and leaks
- Weld and edge deficiencies
- Corroded bolts, fasteners, seams, and attachments
- Coating rust grade and blistering
- Loose, peeling, or delaminated coating
- Soluble-salt test locations and results
- Previous steel repairs and coating patches
- Cathodic-protection components
- Hazardous-coating test areas
- Locations requiring engineering review or additional testing
Define Repair Responsibility Before Preparation
Steel repairs should be identified, designed, authorized, and scheduled before coating application whenever possible.
Clarify responsibility for:
- Plate, pipe, fitting, and structural-member replacement
- Welding and weld inspection
- Pit filling and fairing compounds
- Grinding sharp edges and weld irregularities
- Replacement of bolts and fasteners
- Repair of ladders, platforms, railings, and access systems
- Cathodic-protection modifications
- Repreparation after welding or repair work
- Final acceptance of repaired steel before coating
Welding after coating application can burn, contaminate, or damage the new system. Coordinate repair sequencing before final surface preparation.
Article 08 Knowledge Check
- Why should coating condition and structural steel condition be evaluated separately?
- What steel conditions should be referred for engineering or qualified owner evaluation?
- How does pitting differ from uniform corrosion?
- Why are edges, welds, bolts, and crevices common coating-failure locations?
- Why may ultrasonic thickness measurement be necessary?
- What should be evaluated before an existing coating is approved for overcoating?
- Why must hazardous existing coatings be identified before surface preparation?
- How can residual soluble salts contribute to premature coating failure?
Technical References and Further Study
- American Water Works Association: AWWA D102-24, Coating Steel Water-Storage Tanks.
- ASTM International: ASTM D610, Standard Practice for Evaluating Degree of Rusting on Painted Steel Surfaces.
- ASTM International: ASTM D714, Standard Test Method for Evaluating Degree of Blistering of Paints.
- AMPP: SSPC-VIS 2, Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces.
- AMPP: Current visual guides and standards for steel surface preparation and cleanliness.
- AMPP: SSPC Guide 15, Field Methods for Extraction and Analysis of Soluble Salts on Steel and Other Nonporous Substrates.
- AMPP: SP0716, Soluble Salt Testing Frequency and Locations on Previously Coated Surfaces.
- Project documents: Use the specified inspection methods, condition ratings, thickness measurements, repair criteria, surface-preparation standards, and acceptance requirements.
Standards are revised periodically. Always verify the edition required by the project and obtain current documents from the issuing organization.
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