Corrosion Protection for Industrial Coating Contractors
Article 06: Surface Cleanliness and Contaminant Testing
What the Eye Cannot See Can Still Destroy the Coating
A steel surface can look clean and still carry oil, grease, soluble salts, dust, moisture, process chemicals, or other contaminants capable of causing poor adhesion, blistering, underfilm corrosion, and premature coating failure.
Big Idea: Visual cleanliness and chemical cleanliness are not the same. A surface must be clean enough for the specified coating system, service environment, and inspection requirements.
Why Surface Cleanliness Matters
A coating must establish and maintain contact with the prepared substrate. Anything trapped between the coating and the steel can interfere with that contact.
Some contaminants weaken adhesion. Others attract moisture or create conductive conditions beneath the coating. Dust can prevent complete wetting of the surface. Oil can cause craters, fisheyes, and loss of adhesion. Soluble salts can draw moisture through the coating and contribute to blistering and corrosion.
Abrasive blasting removes rust, mill scale, and existing coating, but it does not automatically remove every contaminant. Blasting an oily surface can spread contamination or drive it deeper into the surface profile.
Cleaning must occur in the correct order. Oil, grease, salts, and process contamination should be addressed using appropriate methods rather than relying on abrasive blasting to solve every surface problem.
Visual Cleanliness Is Only the Beginning
Visual inspection is essential, but it has limits. It can identify rust, mill scale, loose coating, dirt, grease deposits, weld spatter, dust, and visible moisture. It cannot reliably determine whether invisible soluble salts remain on the surface.
A freshly blasted surface may appear bright and uniform while chlorides remain inside pits or at the bottom of the surface profile. A surface wiped with solvent may look clean while a thin film of oil remains.
The inspection plan should combine visual examination with the tests required by the specification, coating manufacturer, owner, or service environment.
Field Rule: If the contaminant cannot be reliably seen, visual inspection alone cannot prove that it has been removed.
Oil and Grease
Oil and grease can come from manufacturing, maintenance, leaking equipment, hydraulic systems, compressors, tools, worker handling, cutting fluids, lubricants, and nearby operations.
These contaminants prevent the coating from properly wetting and bonding to the substrate. They may also spread when contacted by abrasive, compressed air, rags, or cleaning tools.
Possible Warning Signs
- Dark or glossy areas
- A greasy feel
- Water beading on the surface
- Fisheyes or craters in newly applied coating
- Poor adhesion in isolated areas
- Repeated coating failure near machinery or lubrication points
Cleaning Considerations
Oil and grease should be removed using the approved cleaning method before abrasive blasting or other mechanical preparation. The method may involve an approved detergent, cleaner, solvent, emulsifying product, pressure washing, or another specified process.
Rags and cleaning solution must be changed frequently. Reusing a contaminated rag can move oil from one area to another rather than remove it.
Cleaning residues must also be removed. Replacing an oily film with soap, detergent, or solvent residue does not produce a coating-ready surface.
Soluble Salts
Soluble salts dissolve in water and can remain on steel after the water evaporates. Common sources include seawater, road salt, process chemicals, wastewater, cooling water, firewater, fertilizers, industrial fallout, and contaminated abrasive.
Chlorides are a common concern, but other ionic contamination can also affect coating performance.
Salts can collect in pits, crevices, weld irregularities, and the valleys of the surface profile. Abrasive blasting may remove corrosion products while leaving some soluble contamination behind.
Why Salts Are Dangerous
Salts can attract moisture and increase electrical conductivity beneath a coating. They may contribute to osmotic blistering, loss of adhesion, underfilm corrosion, and early rusting.
A coating can trap the contamination against the steel. Once moisture reaches the contaminated area, corrosion can begin beneath a film that initially looked acceptable.
Testing for Soluble Contamination
Field testing generally involves extracting soluble material from a defined area of the surface using water or another approved extraction solution. The extracted liquid is then evaluated using conductivity measurements or a test specific to the contaminant of concern.
Patch cells, sleeves, swabs, or other approved extraction methods may be used depending on the specification and test procedure.
The contractor should follow the specified procedure exactly. Surface area, extraction-liquid volume, contact time, temperature, instrument condition, and calculation method can affect the result.
Conductivity Testing and Ion-Specific Testing
Conductivity testing indicates the general level of dissolved ionic material in the extracted solution. It does not identify every individual ion or explain where the contamination came from.
Ion-specific tests are intended to measure a particular contaminant, such as chloride. The project documents should state whether total soluble contamination or a particular ion is being evaluated.
Results from different test methods are not automatically interchangeable. Each method has its own procedure, units, detection limits, and calculation requirements.
Acceptance limits should come from the project specification, owner, coating manufacturer, or responsible technical authority. The contractor should not invent an allowable limit after testing is complete.
Selecting Test Locations
Testing only the cleanest and easiest-to-reach area does not provide a useful picture of the structure.
Representative and high-risk locations may include:
- Lower portions of tanks and equipment
- Areas exposed to splash or runoff
- Pits and heavily corroded areas
- Welds, seams, and crevices
- Areas beneath deposits or failed coatings
- Waterlines
- Marine or coastal-facing surfaces
- Areas near chemical leaks or process vents
- Previously repaired locations
- Locations identified by the inspector or specification
The number and location of tests should follow the approved inspection plan. If contamination varies across the structure, additional testing may be required.
Dust and Loose Particles
Dust can come from abrasive blasting, nearby construction, degraded concrete, insulation, process operations, wind, and poorly cleaned containment.
Dust left on a prepared surface can prevent the coating from contacting the steel. It may create weak adhesion, roughness, pinholes, or points where moisture can collect.
Checking for Dust
Visible dust should be removed. A clean, contrasting cloth or approved surface-checking method can help identify residue that is difficult to see.
Adhesive-tape methods may be specified to evaluate the quantity and particle size of dust remaining on a prepared surface. The tape is applied using the required procedure and compared with the applicable reference or acceptance criteria.
Compressed air used for blowdown must be clean and dry. Contaminated compressed air can deposit oil or moisture onto a surface that was previously acceptable.
Moisture and Condensation
Moisture can prevent adhesion, interfere with cure, cause flash rusting, and become trapped beneath the coating.
Sources include:
- Rain and fog
- Condensation
- Waterjetting and wet abrasive blasting
- Cleaning and rinsing
- Leaking equipment
- Moist compressed air
- Moisture trapped in pits, seams, and crevices
- Cold contents inside tanks or piping
A surface may appear dry while moisture remains inside deep pits, lap joints, or crevices. The required drying period and inspection method should reflect the geometry and service.
Environmental conditions must be checked before and during coating application. Article 09 will cover air temperature, surface temperature, relative humidity, and dew point in detail.
Compressed-Air Cleanliness
Compressed air may be used for abrasive blasting, blowdown, pneumatic tools, breathing-air systems, and spray equipment. Air used in one operation may have different quality requirements from air used in another.
Air used for blasting or surface blowdown should not deposit oil or water on the prepared surface. Compressor condition, aftercooling, moisture separation, filtration, piping, and maintenance all influence air quality.
A blotter-type test may be required to check for visible oil or water in the compressed-air stream. The test should be performed using the approved procedure and at the appropriate point in the air system.
Do not assume that compressed air is clean because the compressor is operating normally. Air quality must be verified when required.
Surface pH and Chemical Residue
Chemical cleaning, alkaline detergents, acid treatment, process exposure, concrete contact, and industrial residue can alter surface chemistry.
An acidic or alkaline residue may interfere with adhesion, cure, or coating resistance. Some projects require surface pH testing after cleaning or chemical treatment.
A surface-pH test generally uses an approved wetting and measurement procedure. The result should be evaluated against the project or coating manufacturer's acceptance criteria.
The surface must be adequately rinsed and dried after chemical cleaning. Cleaning chemicals left behind become contaminants.
Process Residue and Unknown Contamination
Industrial structures may be exposed to chemicals, fuels, lubricants, cooling water, wastewater, acids, alkalis, fertilizers, food products, biological material, and other process substances.
Unknown contamination should not be treated as ordinary dirt. The material may present health, fire, reactivity, disposal, or coating-compatibility concerns.
Before cleaning begins, determine:
- What substance may be present
- Whether the system has been emptied and isolated
- Whether specialized testing is required
- Which cleaning method is approved
- What personal protective equipment is required
- How rinse water and waste will be contained
- How cleanliness will be verified
When contamination cannot be identified, stop and request direction from the responsible facility representative.
Contaminated Abrasive and Recycled Media
Abrasive can introduce contamination if it is improperly stored, exposed to moisture, mixed with process debris, or reused without adequate control.
Recycled abrasive may accumulate paint debris, rust, dust, fines, salts, oil, and other material removed from the structure. The recycling system must properly clean and classify the abrasive before reuse.
Inspect:
- Abrasive storage conditions
- Moisture exposure
- Oil or chemical contamination
- Excessive fines and dust
- Debris-removal performance
- Required abrasive tests
- Compliance with project requirements
Clean steel cannot be produced consistently with contaminated abrasive.
Cleaning and Retesting
When a surface fails a contamination test, the response should follow an approved corrective procedure.
A typical process may include:
- Identify the likely source of contamination
- Prevent additional contamination
- Clean the affected area using the approved method
- Rinse or remove cleaning residue when required
- Allow the surface to dry
- Repeat mechanical preparation if required
- Retest the affected and representative surrounding areas
- Document the corrective action and results
Do not continue coating because an average test result appears acceptable while known contaminated areas remain untreated.
Contamination is often uneven. One acceptable location does not automatically represent the entire structure.
Avoid Recontaminating the Surface
A surface that passed inspection can become contaminated again before coating.
Common sources of recontamination include:
- Workers touching steel with bare or dirty hands
- Oil from tools and equipment
- Contaminated compressed air
- Dust from adjacent work
- Rain, fog, and condensation
- Dirty scaffolding or containment
- Leaks from operating equipment
- Contaminated footwear, hoses, or access platforms
- Overspray and debris from nearby operations
After final preparation and acceptance, protect the surface and apply the primer within the required time and environmental limits.
Documenting Contaminant Testing
A complete test record should identify:
- Project and structure identification
- Date and time
- Exact test location
- Surface condition
- Test method and equipment
- Instrument identification
- Verification or calibration status when applicable
- Extraction-solution information
- Test result and units
- Acceptance criterion
- Pass or fail determination
- Corrective cleaning performed
- Retest results
- Name of the person performing the test
Record the units exactly. A number without units or a location cannot be reliably interpreted later.
Pre-Coating Cleanliness Checklist
- Have oil and grease been removed before mechanical preparation?
- Has the contamination source been stopped?
- Have required soluble-salt tests been completed?
- Were representative and high-risk locations tested?
- Are test results below the approved acceptance limits?
- Has dust been removed and inspected?
- Is compressed air clean and dry?
- Is the surface visibly dry and free of condensation?
- Have chemical-cleaning residues been removed?
- Is the abrasive clean and properly stored?
- Have failed areas been cleaned and retested?
- Has the accepted surface been protected from recontamination?
- Have all inspection results been documented?
Bottom Line: If contamination remains beneath the coating, the failure has already been placed into the system. Testing and cleaning must happen before the surface is covered.
Key Takeaways
- A surface can look clean while still carrying harmful contamination.
- Visual cleanliness and chemical cleanliness are different conditions.
- Oil and grease should be removed before abrasive blasting.
- Soluble salts can attract moisture and contribute to blistering and underfilm corrosion.
- Conductivity tests and ion-specific tests do not provide identical information.
- Testing should include representative and high-risk locations.
- Dust can prevent proper coating contact with the steel.
- Compressed air can contaminate a prepared surface with oil or water.
- Cleaning residues must be removed before coating.
- Failed areas must be cleaned, retested, and documented.
- An accepted surface must be protected from recontamination.
Knowledge Check
- Why can abrasive blasting an oily surface make the problem worse?
- Why can soluble salts cause coating failure even when the surface looked clean?
- What is the difference between general conductivity testing and ion-specific testing?
- Why should contamination testing include high-risk locations?
- How can compressed air contaminate a prepared surface?
- What should happen after a surface fails a contamination test?
View Knowledge Check Answers
1. Blasting can spread oil across the surface or drive it deeper into the surface profile instead of removing it.
2. Salts can remain invisible in pits and profile valleys, attract moisture, increase conductivity, and promote blistering or corrosion beneath the coating.
3. Conductivity testing indicates the general amount of dissolved ionic material. Ion-specific testing measures a particular contaminant, such as chloride.
4. Contamination is often uneven and may be concentrated in pits, lower areas, waterlines, crevices, or locations exposed to leaks and splash.
5. The air stream can deposit oil, water, or other contamination from the compressor, piping, or air-treatment system.
6. Identify the source, prevent additional contamination, clean the surface using the approved method, retest, and document the corrective action and results.
Coming Next
Article 07: Abrasive Blasting and Surface-Preparation Standards
The next article examines abrasive selection, blasting equipment, surface-cleanliness grades, visual standards, flash rust, inspection, and the contractor's responsibility for producing a coating-ready surface.
