Preparing Steel for Protective-Lining Application

The Lining Begins at the Steel

A lining can only adhere to what it touches. If it is applied over oil, salts, rust, mill scale, dust, moisture, or weak existing material, it is bonded to the contamination instead of the steel.

Immersion and chemical service can expose weaknesses that might remain hidden beneath an atmospheric coating. Water and chemicals can migrate through small defects, collect at the steel-lining interface, and cause blistering, underfilm corrosion, delamination, or rapid loss of protection.

Follow the Project Specification

The required preparation must be established by the project specification, approved lining manufacturer, owner, or qualified corrosion professional. The contractor should confirm:

• The required degree of blast cleanliness
• The required surface-profile range
• The permitted abrasive type and size
• The maximum permitted soluble-salt level
• The dust-cleanliness requirement
• Required treatment of edges, welds, pits, and defects
• Environmental limits for preparation and priming
• The maximum time permitted between blasting and coating
• Inspection methods, frequency, and hold points

Requirements should be resolved before equipment and personnel are mobilized. A contractor should not guess at cleanliness, profile, or contamination limits after blasting has begun.

Cleaning Comes Before Blasting

Abrasive blasting removes rust, mill scale, and coatings while producing a surface profile. It is not the preferred method for removing oil and grease. Blasting an oily surface can spread contamination or drive it into the newly created profile.

Remove oil, grease, cutting fluids, fuel, soap, and similar contamination using the cleaning method required by the specification. This may involve detergents, emulsifying cleaners, approved solvents, steam, or pressure washing.

Cleaning residues must also be removed. A strong cleaner that leaves a film behind can create another adhesion problem.

Repair Fabrication Defects Before Blasting

Steel intended for immersion lining should be examined for fabrication conditions that may prevent continuous coverage.

Edges and corners

Liquid coatings tend to pull away from sharp edges while curing. Cut edges, plate corners, stiffeners, bolt heads, and projections may require rounding or grinding to the condition specified for the lining system.

Welds

Remove weld spatter, slag, sharp ridges, and rough starts and stops. Undercut, porosity, pinholes, incomplete welds, and other weld defects must be evaluated and repaired as required.

Pits and irregular surfaces

Deep pits can trap abrasive, air, contamination, and lining material. They may also create thin areas and holidays. The approved repair may involve welding, grinding, filling, fairing, or plate replacement.

Crevices and inaccessible areas

Lap joints, skip welds, back-to-back members, and tight spaces may be impossible to prepare and line properly. These conditions should be addressed before blasting rather than buried beneath an incomplete lining.

Selecting the Abrasive

Abrasive selection affects cleaning rate, surface profile, dust generation, contamination, visibility, equipment wear, and waste volume. The abrasive must be suitable for the substrate, lining, blast equipment, and project requirements.

Important abrasive characteristics include:

• Particle size and size distribution
• Particle shape and angularity
• Hardness and breakdown rate
• Cleanliness and soluble-salt content
• Dust generation
• Ability to produce the required profile
• Suitability for recycling, when applicable
• Potential worker and environmental hazards

A coarse, angular abrasive may produce a deeper profile than a fine or rounded abrasive. The contractor should verify the actual profile produced under field conditions instead of selecting abrasive by habit.

Cleanliness and Profile Are Different

Blast cleanliness describes how thoroughly visible rust, mill scale, coating, and foreign matter have been removed. Surface profile describes the peak-to-valley texture created by the abrasive.

Steel can meet the visual cleanliness requirement and still have the wrong profile. It can also have an acceptable profile while failing the required cleanliness standard.

Blast-Cleaning Standards

Immersion lining specifications commonly require a very high degree of steel cleanliness. Two commonly specified conditions are white-metal blast cleaning and near-white-metal blast cleaning.

White-metal blast cleaning

White-metal blast cleaning removes visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter from the surface. It represents the highest level of abrasive-blast cleanliness.

Near-white-metal blast cleaning

Near-white-metal blast cleaning permits only limited staining within the area defined by the standard. It is cleaner than commercial blast cleaning but is not identical to white-metal preparation.

The contractor must follow the exact standard named by the specification. Descriptions such as “blast it clean” or “blast to bright metal” are not adequate acceptance criteria.

Producing the Correct Surface Profile

The lining manufacturer normally specifies an acceptable profile range. The profile must be deep enough to support mechanical adhesion but not so deep that the lining cannot cover the peaks.

If the profile is too shallow, adhesion may be reduced. If it is too deep, coating peaks may remain insufficiently covered, increasing the possibility of pinpoint rusting and premature failure.

Profile is affected by:

• Abrasive type, size, hardness, and shape
• Nozzle size and condition
• Blast pressure and air volume
• Nozzle distance and angle
• Operator technique
• Steel hardness and original condition
• Existing profile from previous blasting

Measure profile throughout the work. One acceptable test at the beginning of the shift does not prove that the entire structure meets the requirement.

Soluble Salts

Chlorides, sulfates, nitrates, and other soluble salts may remain on steel or inside corrosion pits after visible rust has been removed. These salts can draw moisture through the lining and contribute to osmotic blistering and underfilm corrosion.

Salt contamination may be found in marine structures, wastewater equipment, chemical tanks, fertilizer facilities, food plants, road-salt equipment, and structures exposed to process water.

The specification should define:

• The extraction and test method
• The maximum acceptable contamination level
• The number and location of tests
• When testing must be performed
• The required cleaning and retesting procedure

Contaminated surfaces may require pressure washing, approved chemical treatment, repeated blasting and washing, or another specified process. Retest after cleaning to verify that the required level has been reached.

Compressed-Air Cleanliness

Compressed air used for blasting and surface cleaning can carry oil or water from the compressor and air system. Either contaminant can be deposited on the prepared steel.

Test the air supply at the required frequency and whenever equipment conditions change. Position the test where it represents the air reaching the work. Separators, aftercoolers, filters, and drains must be properly maintained.

If oil or water is detected, correct the source and determine whether previously prepared steel has been contaminated.

Dust and Abrasive Removal

After blasting, remove dust, spent abrasive, and debris from the surface, ledges, pits, joints, scaffolding, tank bottoms, and overhead members. Material left above the work can fall into wet lining.

Use the cleaning method permitted by the specification, such as clean dry compressed air or industrial vacuuming. Avoid ordinary sweeping that simply redistributes dust.

Inspect difficult locations with adequate lighting. Corners, welds, underside surfaces, and deep pits often retain abrasive after large flat surfaces appear clean.

Environmental Control

Prepared steel can rust rapidly when exposed to moisture or high humidity. Record air temperature, steel temperature, relative humidity, and dew point before and during preparation and coating work.

The required difference between surface temperature and dew point must follow the specification and lining manufacturer's instructions. Conditions must remain acceptable until the coating has been applied and cured sufficiently.

Dehumidification, heating, and ventilation may be needed to maintain the prepared steel. Environmental-control equipment should not introduce combustion products, oil, dust, or moisture into the work area.

Preventing Flash Rust and Recontamination

Once steel is accepted, protect it from condensation, flash rust, dirt, worker traffic, fingerprints, exhaust, leaking equipment, and residues from nearby work.

Personnel entering the prepared area should use clean footwear, gloves, and protective clothing as required. Hoses, tools, lighting, scaffolding, and inspection equipment must not contaminate the surface.

Apply the primer or lining within the specified period. If rusting or contamination develops, restore and reinspect the affected surface before coating.

Stripe Coating and Difficult Areas

Welds, edges, corners, pits, bolts, attachments, and other irregular areas may receive less coating thickness during spray application. A stripe coat may be specified to provide additional material and improve coverage.

Stripe-coat material, application method, sequence, thickness, and recoat time must follow the approved system. Excessive stripe-coat thickness can create runs, trapped solvent, or recoat problems.

Stripe coating does not replace proper grinding, preparation, cleaning, or repair of fabrication defects.

Inspection Hold Point Before Coating

The prepared surface should be formally inspected and accepted before primer or lining application. The hold-point inspection should confirm:

• The required visual cleanliness has been achieved
• The surface profile is within the specified range
• Soluble salts meet the acceptance limit
• Dust and abrasive have been removed
• Welds, edges, pits, and defects have been properly treated
• Compressed air is free of detectable oil and water
• Environmental conditions are acceptable
• No flash rust or recontamination is present
• Inspection records are complete

Do not cover a rejected or uninspected surface simply to protect the production schedule.

Abrasive-Blasting Safety

Abrasive blasting can generate high concentrations of dust, toxic metals, noise, rebounding abrasive, reduced visibility, and static electricity. Existing coatings and the abrasive itself may contain hazardous substances.

• Evaluate the abrasive, existing coating, substrate, and process residues.
• Use containment and ventilation appropriate for the work.
• Keep unauthorized personnel outside the blasting area.
• Use NIOSH-approved respiratory protection required for abrasive blasting.
• Maintain a compliant respiratory-protection program.
• Provide hearing, eye, face, head, body, hand, and foot protection.
• Ground and bond equipment where required.
• Inspect blast hoses, couplings, deadman controls, and safety restraints.
• Use suitable fall protection and confined-space procedures.
• Remove accumulated dust using approved vacuum or wet-cleaning methods.

Common Preparation Failures

• Blasting before removing oil and grease
• Meeting visual cleanliness but ignoring soluble salts
• Producing a profile outside the lining manufacturer's range
• Leaving abrasive inside pits, crevices, and attachments
• Failing to test compressed air for oil and water
• Allowing condensation or flash rust after blasting
• Coating over dust or worker contamination
• Leaving sharp edges, weld spatter, and rough welds untreated
• Using one test result to represent an entire structure
• Applying coating before the inspection hold point is released

Contractor Field Checklist

Knowledge Check

Key Takeaway