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Water and Wastewater Protective Coating Systems | Article 12 of 24

Steel Surface Preparation, Soluble Salts, Dust, and Flash Rust

Successful steel lining work requires more than removing visible rust. The completed surface must satisfy requirements for cleanliness, profile, soluble contamination, dust, weld preparation, and environmental condition.

A Bright Steel Surface Is Not Necessarily a Clean Surface

Abrasive blasting can produce steel that appears visually clean while nonvisible soluble salts, fine dust, oil, moisture, or embedded contamination remain. These materials can interfere with wetting and adhesion or contribute to blistering, underfilm corrosion, and premature failure.

Steel preparation should therefore be accepted as a combination of visible cleanliness, nonvisible cleanliness, surface profile, physical condition, and environmental control.

Define the Required Surface Before Work Begins

The project specification and coating manufacturer should identify the required preparation standard, surface-profile range, soluble-salt limits, dust acceptance, flash-rust tolerance, and maximum time permitted between preparation and coating.

The contractor should confirm:

• The substrate type and existing coating condition
• The specified AMPP surface-cleanliness standard
• Whether dry abrasive blasting, wet abrasive blasting, waterjetting, or power-tool cleaning is permitted
• The required profile range and permitted measurement method
• The soluble-salt test method, frequency, locations, and acceptance limit
• The permissible dust level
• The permitted degree of flash rust after wet preparation
• Required treatment of welds, edges, pits, bolts, and irregular geometry
• Inspection hold points and documentation requirements

The Specification Must Name the Standard

Phrases such as “sandblast the steel,” “blast to clean metal,” or “remove all rust” do not define a complete acceptance condition. The contract should identify the applicable standard and edition.

The specified cleanliness level should be selected for the coating system and service exposure. A lining intended for continuous immersion or severe wastewater service may require a more demanding surface than an atmospheric maintenance coating.

Common Abrasive-Blast Cleaning Standards

White Metal Blast Cleaning

AMPP SSPC-SP 5/NACE No. 1 defines white-metal blast cleaning. The visible surface is cleaned of oil, grease, dust, dirt, mill scale, rust, coating, corrosion products, and other visible foreign matter. This degree of preparation is frequently associated with severe service, but it should be specified rather than assumed.

Near-White Metal Blast Cleaning

AMPP SSPC-SP 10/NACE No. 2 defines near-white-metal blast cleaning. Limited staining may remain within the restrictions of the standard, but the surface must otherwise meet its visible-cleanliness requirements.

Other Degrees of Cleaning

Commercial, industrial, brush-off, hand-tool, and power-tool standards serve different purposes. They should not be substituted for white-metal or near-white-metal preparation unless the coating system, service exposure, and written project requirements permit the change.

Visible-Cleanliness Standards Do Not Establish Soluble-Salt Cleanliness

The AMPP white-metal and near-white-metal standards primarily establish requirements for visible surface contaminants. A surface can satisfy the visual standard and still contain nonvisible soluble contamination.

When soluble salts are a concern, the specification must separately identify testing requirements and acceptance criteria. Visual inspection alone cannot confirm that salts have been reduced to the required level.

Remove Oil and Grease Before Blasting

Oil, grease, and similar contaminants should be removed before abrasive blasting. Blasting an oily surface can spread contamination, drive it into pits and profile valleys, contaminate abrasive, and transfer it to adjacent steel.

Use cleaning methods compatible with the substrate, existing contamination, facility restrictions, and subsequent coating system. Remove the cleaning agent and loosened contamination before mechanical preparation begins.

Prepare Welds, Edges, and Irregular Steel

Welds, sharp edges, pits, crevices, bolts, nuts, rivets, lap joints, and attachments are common sites of coating breakdown. Coating tends to pull away from sharp edges, bridge irregularities, and leave thin film or holidays.

When required by the project documents:

• Remove weld spatter, slag, sharp projections, and temporary attachments.
• Smooth rough welds and unacceptable weld undercut.
• Round or ease sharp edges to the specified condition.
• Open or repair crevices as directed.
• Remove corrosion products from pits and irregular surfaces.
• Inspect fabrication defects before they are concealed by coating.

Welding and structural corrections must be performed by qualified personnel under the approved project requirements.

Abrasive Selection and Control

Abrasive type, hardness, particle shape, size distribution, cleanliness, and operating pressure affect cleaning rate and profile. The same equipment can produce different results when abrasive or steel condition changes.

• Confirm that the abrasive is permitted by the specification.
• Store abrasive where it will remain dry and uncontaminated.
• Check recycled abrasive for contamination, breakdown, and improper particle distribution.
• Prevent cross-contamination from previous blasting operations.
• Adjust nozzle size, pressure, distance, and angle to achieve consistent cleaning.
• Verify the resulting profile instead of assuming it from the abrasive selected.

Surface Profile

Abrasive blasting creates peaks and valleys that support mechanical attachment of the coating. Profile must fall within the specified range for the lining system.

A profile that is too shallow may reduce coating attachment. A profile that is too deep can leave peaks insufficiently covered, increase coating consumption, and contribute to pinpoint rusting or holidays.

Profile can be evaluated using approved visual comparators, depth micrometers, replica tape, or stylus instruments. Use the method and sampling frequency required by the specification. Record individual readings and the resulting area determination rather than reporting only an unsupported average.

Soluble Salts

Chlorides, sulfates, nitrates, and other soluble ionic contamination may remain in corrosion pits and surface irregularities. In immersion or continuously wet service, excessive contamination can attract moisture and contribute to osmotic blistering and underfilm corrosion.

Soluble-salt testing generally involves extracting contaminants from a defined surface area and analyzing the extract by conductivity or an ion-specific method. Results from different extraction and analysis methods may not be directly interchangeable.

The specification should identify the test method, units, test locations, frequency, acceptance limit, retesting procedure, and required corrective action. The contractor should not invent a numerical acceptance limit in the field.

Where Should Salt Tests Be Performed?

Random testing alone may miss the areas most likely to be contaminated. The inspection plan should include representative areas and locations where salts are most likely to concentrate.

• The normal liquid line and splash zone
• Low points, sumps, and poorly drained areas
• Corrosion pits and heavily rusted locations
• Weld seams, lap joints, crevices, and attachments
• Areas exposed to chemical treatment or industrial discharge
• Surfaces beneath failed or blistered coatings
• Areas that previously failed acceptance and were recleaned

Reducing Soluble Contamination

Dry abrasive blasting removes corrosion products and visible contamination but may not remove salts held in pits. Washing with clean water, pressure washing, approved chemical treatments, wet abrasive blasting, or waterjetting may be required.

If a chemical salt remover is used, follow its written instructions and confirm compatibility with the coating system. The treatment and displaced contamination must be thoroughly removed.

Retest after corrective cleaning. Do not assume the surface passes merely because it has been washed or blasted again.

Dust and Fine Debris

Abrasive dust, fractured corrosion products, and fine debris can remain in pits, welds, corners, and profile valleys. The primer may bond to the dust instead of the steel.

Final cleaning may include:

• Industrial vacuuming
• Brushing followed by vacuuming
• Clean, dry, oil-free compressed air
• Recleaning pits, welds, crevices, and horizontal ledges
• Inspection after airborne dust has settled

When required, dust can be evaluated using a specified tape method or other documented procedure. The acceptance level and test frequency should be established before production work begins.

Verify Compressed-Air Cleanliness

Compressed air used for abrasive blasting or final cleaning can introduce oil or water onto the steel. The system should be equipped and maintained to deliver air of suitable cleanliness and dryness.

Perform the required compressed-air contamination test before work and at the frequency specified. Correct any contamination source and reclean affected steel before coating.

Wet Preparation and Flash Rust

Wet abrasive blasting and waterjetting reduce airborne dust and can help remove soluble contamination. They also leave water on freshly prepared steel, allowing rapid oxidation known as flash rust.

Flash rust is not accepted simply because wet preparation was used. The project must define the permitted degree, usually by reference to the applicable wet abrasive blasting or waterjetting standard and the coating manufacturer’s limitations.

If flash rust exceeds the permitted condition, the surface must be recleaned. Rust that is powdery, loose, heavy, or capable of being wiped from the surface should not be concealed with coating unless the approved system expressly permits that condition.

Control the Environment

Freshly prepared steel is highly reactive. Condensation or elevated humidity can cause rapid rusting before coating is applied. Measure and document air temperature, steel-surface temperature, relative humidity, and dew point at the required intervals.

Maintain the temperature separation and environmental limits required by the coating manufacturer and project specification. If conditions move outside those limits, stop coating, protect the work, and reinspect the steel before operations resume.

Protect Prepared Steel from Recontamination

• Restrict entry into prepared areas.
• Use clean gloves and footwear where required.
• Prevent bare-hand contact with prepared steel.
• Keep blast hoses, ventilation ducts, lighting, and equipment clean.
• Prevent diesel exhaust, compressor discharge, water, and process contamination from entering the work area.
• Remove spent abrasive and dust from scaffolds, ledges, and overhead surfaces.
• Reinspect steel immediately before primer or lining application.

Final Steel Acceptance

Before coating begins, verify and document:

• Compliance with the specified visible-cleanliness standard
• Surface profile within the specified range
• Soluble-salt results within the specified limit
• Acceptable dust and abrasive removal
• Acceptable welds, edges, pits, bolts, and irregular surfaces
• Flash rust within the permitted condition
• Absence of oil, grease, moisture, and recontamination
• Acceptable air temperature, surface temperature, humidity, and dew-point separation
• Completion of required repairs and stripe-coat preparation
• Written release of the area for coating application

Contractor’s Field Checklist

• Is the exact preparation standard and edition identified?
• Were oil and grease removed before blasting?
• Are welds, edges, pits, bolts, and crevices properly prepared?
• Is the abrasive clean, dry, and capable of producing the required profile?
• Is the compressed air free from detrimental oil and water?
• Does the surface satisfy the visual-cleanliness requirement?
• Is the profile within the specified range?
• Have soluble salts been tested at representative and high-risk locations?
• Have dust and spent abrasive been removed?
• Is flash rust within the permitted condition?
• Has the steel been accepted immediately before coating?

Knowledge Check

1. Does compliance with a visual blast-cleaning standard prove that soluble salts are acceptable?

No. Soluble salts are nonvisible contaminants and require the specified extraction, analysis, sampling, and acceptance procedure.

2. Why can an excessively deep blast profile cause problems?

It can increase coating consumption and leave profile peaks with inadequate film thickness, contributing to pinpoint rusting, holidays, or early breakdown.

3. When is flash rust acceptable?

Only when its degree and condition comply with the project specification, applicable preparation standard, and coating manufacturer’s written requirements.

Technical References and Further Study

• AMPP SSPC-SP 5/NACE No. 1-2026, White Metal Blast Cleaning. This standard defines the visible surface condition produced by white-metal abrasive blast cleaning.
• AMPP SSPC-SP 10/NACE No. 2-2024, Near-White Metal Blast Cleaning. This standard defines the visible surface condition produced by near-white-metal abrasive blast cleaning.
• AMPP SSPC-Guide 15-2026, Field Methods for Extraction and Analysis of Soluble Salts on Steel and Other Nonporous Substrates. This guide addresses extraction and analysis methods, including conductivity and ion-specific testing.
• AMPP SSPC-PA 17-2020, Procedure for Determining Conformance to Steel Profile/Surface Roughness/Peak Count Requirements.
• ASTM D4417, Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel.
• ASTM D4285, Standard Test Method for Indicating Oil or Water in Compressed Air.
• NACE SP0287-2024, Measurement of Surface Profile of Metal Surfaces Using a Replica Tape.
• Applicable AMPP wet abrasive blasting or waterjet-cleaning standard when wet preparation is specified.
• The coating manufacturer’s current technical data sheets, safety data sheets, surface-preparation requirements, profile limits, salt limits, flash-rust tolerances, and written project recommendations.

Standards and manufacturer instructions may be revised. Confirm the current edition and the project-specific acceptance criteria before surface preparation begins.

Professional responsibility: This article provides foundational education and does not replace the project specification, approved coating system, site-specific safety plan, inspection plan, or manufacturer instructions. When the specified standard, salt limit, profile requirement, or flash-rust criterion is unclear, obtain written clarification before preparing or coating the steel.

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