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Application of Thermal Sprayed Aluminum to Subsea Structures

1.0 Scope

This specification defines the minimum requirements for the surface preparation, application and inspection of thermal sprayed aluminum (TSA) coatings to subsea structures (e.g., risers, catenary risers, tethers and tendons, air buoyancy cans, conductor guides, wellhead systems, pipelines).  In addition, requirements for application of a sealer to TSA coatings are also detailed.  For a specific structure, this specification should be used in conjunction with the Data Sheets and Technical Requirements for that system.  Supplier should submit any exception to this Specification in writing for resolution by the Buyer.
Advantages of TSA over other corrosion control measures include:
  • TSA is flush with the surface and does not interfere with reeling of pipelines or pulling of risers through J-tubes
  • TSA  can provide corrosion protection under thermal insulation where use of conventional anodes with coatings is not possible due to electrical insulation provided by materials such as syntactic foam
  • TSA provides long-term, maintenance-free corrosion protection in locations where coating repair and/or anode replacement is not feasible due to lack of physical access, water depth or hazardous conditions
  • TSA can be used on structures where welding is not permitted for attachment of conventional anodes, such as TLP tendons or risers
  • TSA surfaces have a low current demand when connected to platforms or floating bulls and their life can be supplemented by attached CP systems.
Disadvantages of TSA include:
  • Initial cost may be higher than conventional systems using coatings and galvanic anodes
  • Subsea TSA coated structures must be electrically isolated from attached steel structures not protected with CP to achieve expected life
  • TSA is not suitable for service environments with a pH below 3.5 or higher than 9.0.

2.0 References

The following documents are considered part of this specification. Use the edition of each referenced document in effect on the date of the publication of this specification.
ANSI/AWS C2.18, "Guide for the Protection of Steel with Thermal Sprayed Coatings of Aluminum, Zinc, and their Alloys and Composites"
ASTM C633, "Test Method for Adhesion or Cohesive Strength of Flame-sprayed Coatings"
ASTM D4541, "Test Method for Pull-Off Strength of Coating Using Portable Adhesion Testers"
AWS C2.2, "Recommended Practices for Metalizing with Aluminum and Zinc for Protection of Iron"
SSPC-SP 1, "Solvent Cleaning"
SSPC-SP5/NACE No.1, "White Metal Blast Cleaning" (Joint Standard)
SSPC-VIS 1, "Visual Standard for Abrasive Blast Cleaned Steel"
SSPC-PA-2, "Measurement of Dry Coating Thickness with Magnetic Gages"
ISO 8501-1, "Pictorial Surface Preparation Standard for Painting Steel Surfaces
MIL-W-6712C, "Wire Metalizing"
ISO 209-1, "Wrought aluminum and aluminum alloys - chemical composition and forms of products - part 1: Chemical composition"
DOD-STD-2138A (SH), Military Standard, "Metal Sprayed Coatings for Corrosion Protection aboard Naval Surface Ships"
In the event of a conflict among the above listed standards or between the standards and other instructions listed in the specifications and drawing, it should be the Suppliers responsibility to notify the Buyer in writing for a resolution of the conflict prior to starting work.

3.0 Surface Preparation

3.1 General

  • Weld spatter and other surface defects, such as pits or crevices, should be removed by disc grinding or other suitable mechanical methods prior to blast cleaning.  Repairs requiring significant removal of metal (e.g. by disc grinding) should be subject to Buyer approval.
  • Localized contamination of surfaces by oil and grease should be solvent cleaned in accordance with SSPC-SP1 before blast cleaning.
  • Hot water/ steam cleaning should be used to remove dirt and oil or grease from large areas of unprepared surface.  Clean potable water should be used and the minimum operating temperature of the water should be 180oF.
  • Weld preparation or field welding areas should be masked for a minimum distance of one inch, so that the TSA coating comes no closer than 1-inch (25 mm) to the weld.
  • Ambient air relative humidity should be measured during blast cleaning and thermal-spray operations and recorded in permanent documentation made available to the Buyer at the conclusion of TSA operations.

3.2 Blast Cleaning Equipment

  • Automatic blast cleaning should be performed in an enclosed recycling Wheelabrator that is fitted with controls to allow control of anchor profile and abrasive cleaning equipment.
  • Manual blast cleaning should be performed using conventional compressed air driven abrasive blasting equipment.
  • Compressed air for blast cleaning should be free of oil and condensed moisture.  Filters, desiccant dryers, separators and traps should have a continuous bleed to remove liquids.  An after cooler of suitable size should be attached to each compressor, and have separate oil and water traps on its discharge side.
  • Air pressure used during blast cleaning operations should be adequate to provide a 3 to 5 mil (67 to 125 micron) surface profile and should be adjusted to ensure this profile is achieved.  Air pressure should be measured by means of an in-line pressure gauge in the main air supply adjacent to the blast pot.
  • Abrasive should be a mixture of #16, #20 and #25 steel grit or aluminum oxide (or as specified by Buyer), selected as required to maintain a working mixture.
  • Blasting abrasives should be clean, dry, sharp and angular and should be free of clay, salt, oil or other foreign matter.
  • Abrasives may be recycled; using properly maintained equipment to remove dust, corrosion products and other contaminants prior to reuse.
  • The cleanliness of the abrasive mix should be checked at the beginning of each work shift using the following procedure (or Buyer approved replacement).  If abrasive mix is contaminated, it should be removed from the machine and replaced with clean abrasive.
    • Fill a small, clean 100-200 ml (4-6 oz.) bottle half full of abrasive
    • Fill the remainder of the bottle with distilled water
    • Cap and shake the bottle
    • Inspect water for surface oil sheen.  If any sheen is observed, clean the blasting equipment and replace with new media and retest.
    • Inspect solution for suspended dust particles.  If excessive, recycling screens should be adjusted and the abrasive recycled prior to use.

3.3 Blast Cleaning

  • Blast cleaning should not be performed when the surfaces to be blasted are wet, or are less than 5oF above the dewpoint temperature, or where the relative humidity of the blast cleaning area is greater than 90%.
  • Surfaces to be coated with TSA should be blast cleaned to SSPC-SP5/ NACE No. 1 (white metal).
  • Minor laminations, or other steel surface defects, exposed by the blast cleaning process should be removed by grinding and blast cleaning again prior to application of TSA coating.  Major laminations (or other surface defects) exposed by blast cleaning should be reported to Buyer.
  • Abrasive blasting operations should be separated from the TSA, sealer or overcoat application operations.
  • After blast cleaning, dust and grit should be removed by stationary bristle brushes or with clean, dry compressed air.

3.4 Inspection and Verification of Blast Cleaned Surfaces

  • Blast cleaned surfaces should be visually inspected and verified to match surface preparation meeting SSPC-VIS 1 or ISO 8501-1 Sa 3 Standards.
  • The anchor profile should be determined on representative samples or on the work piece itself using replica tape.
  • Anchor profile should be 3 mils (67 microns) minimum or no greater than 5 mils (125 microns) maximum.
  • An anchor profile determination consisting of three measurements should be made at the start of each shift during which blast cleaning is performed.  Thereafter, frequency of measurement of anchor profile should be one measurement for every two hours of continuous blasting and at least once per component, or after any change of nozzle, nozzle pressure or abrasive size.  Profile measurements should be evenly distributed on the surface measured.
  • Pipe transportation and storage information and details.

4.0 Thermal Spray Coating

4.1 General

Blast cleaned steel should be thermal sprayed within four hours after blast cleaning.  Surfaces that show rust bloom, surface oxidation or other surface coloration should be re-blasted prior to TSA application.

4.2 Thermal Spray Equipment

  • Thermal spray aluminum may be applied using a flame spray or arc spray process.  Manual or automated spraying techniques may be used.  Thermal spray equipment should be set up, adjusted and operated in accordance with the equipment manufacturer's instructions and manuals.
  • Thermal spray systems should be capable of uniform spray application during continuous operations as well as during start and stop operations cycles.  Wire fusing, sputtering, or globular spitting is not acceptable.
  • Compressed air should be free of oil and condensed moisture.  Filters, dryers, separators and traps should be used as necessary to remove moisture.

4.3 TSA Coating Material

  • Thermal spray aluminum wire should conform to the requirements of MIL-W-6712C or ISO 209-1, Type A1 99.5.  The manufacturer or supplier should provide a certificate of conformance for each batch of wire supplied.
  • Wire should be supplied in protective wrapping that clearly identifies the type of material, the heat (batch) number and the supplier.
  • A sample of each batch of wire used should be clearly labeled and retained for a period of 12 months after the application process by Supplier.  The retained samples should be stored in a location designated for retention of TSA samples.

4.4 TSA Coating Quality Plan

Supplier should provide Buyer a written Quality Plan (QP) applicable to the structures to be coated with TSA.  The QP should list all essential steps of the process, including the following quality control requirements:
  • Visual inspection and acceptance criteria
  • Adhesion test requirements and acceptance criteria
  • Non-conformance processing
  • Requirements for qualified operators
  • Requirements for bend test coupons and production test panels
  • Reports required and appropriate data handling and disposition

5.0 Thermal Spray Application

  • Thermal spray equipment should be set up, calibrated, and operated in accordance with the equipment manufacturer's instructions and technical manuals.
  • Prior to flame spray application of TSA coatings, the surface to be coated should be preheated to 100oF, using the oxy-acetylene flame of the spray gun.  Surface temperatures should not exceed 150oF.  No surface preheating is required for arc spray application of TSA.
  • Spray guns used to apply thermal arc sprayed aluminum should be held approximately perpendicular to the surface to be coated and at a distance required to deposit a uniform TSA layer.  Gun distance from the work piece should not exceed 8 inches.  Passes should overlap approximately 50%.
  • The TSA coating thickness should be within the range agreed to by Buyer and Supplier.  Typically this range is no less than 7 mils (175 microns) and no more than 14 mils (350 microns).
  • Flame spray TSA coatings should be applied in a series of passes of the spray nozzle (with a minimum of two passes) moving, whenever practicable, in contrasting directions.  Coatings should be applied in a smooth, continuous movement, and should be overlapped sufficiently to ensure a consistent film thickness.  The surface of the finished coating should be free of lumps, coarse or rough areas, loosely adherent particles, blisters, cracks, chips or pits.
  • TSA should not be applied closer than 1 inch to any area known to be reserved for future (e.g. field) welding, or on to any designated "cut-back" area.
  • Application of materials should be made in accordance with the principles of good workmanship described in AWS C2.2 and DOD-STD-2138A (SH).

6.0 TSA Coating System Check

6.1 Manually Applied TSA Check

  • A production test panel, made from similar material and having a surface area of at least 36 square inches, should be blast cleaned and thermal sprayed, but not sealed or overcoated, at the same time as, and in an identical manner to the subsea structure or component being coated.
  • Production test panels should be labeled in such a way as to permanently identify the test sample and the companion structure or component.
  • One production panel per 10 components and per operator is acceptable if the surface area for each component does not exceed 10 square feet.
  • Three adhesion tests should be performed on each production test panel using ASTM D4541 test procedures with a HATE Mark VII or Elcometer 106/3 portable adhesion tester.  All three, adhesion tests should meet or exceed 1000 psi.
  • If any of the adhesion values is less than 1000 psi, three adhesion tests will be made at random on the structure or components.  If one production test panel represents multiple components, then three adhesion tests should be made on each component coated by Supplier Operator since the Operator's last successful production panel tests.  All three of these adhesion tests should meet or exceed 1000 psi.  If any of the adhesion test results are lower than 1000 psi, the structure or component should be completely blast cleaned and re-coated in accordance with the requirements of this document.
  • Production test panels should be retained by Supplier for a period of one year after delivery of the structure or components and should be available for inspection by Buyer upon request.

6.2 Mechanized TSA Application Check (Typically Tubular Products)

  • Supplier should perform a TSA systems application check at the start of each production run, not to exceed 10 sequential components, or after any change to the machine set-up parameters.  The system check consists of TSA spraying one flat coupon approximately 2 x 6 x 0.050-inches (50 x 150 x 1.25 mm) and one 1-inch diameter test cylinder.  The flat coupon and test cylinder should be of a material similar to the structure or components to be TSA coated.  The coupon and test cylinder should be fixed so that they are approximately the same distance from the application guns as the structure or components to be coated.  The test coupons should then be coated using the same methods and parameters as those that will be used for the structure or components.
  • The coupon and cylinder should be tested as follows:
    • The flat coupon should be bent to 180o around a 0.5-inch (13-mm) diameter mandrel in two places.  After bending, the coupon should be free from significant cracking showing exposed substrate and should conform to the requirements of bend test coupons described in ANSI/AWS C2.18-93.  Small hairline cracks or "alligatoring" of the coating in the vicinity of the bend are permissible.
    • An adhesion test should be made on the 1-inch diameter test cylinder as detailed in ASTM C633.  The adhesion test cylinder should be glued to another test cylinder of bare substrate that has been profiled by abrasive blasting.  It should then be removed using pull-test equipment similar to that used for ASTM C633 testing.  The adhesion value obtained should be equal to or greater than 2000 psi after calculation.
  • If the TSA coating on the test coupon or cylinder does not meet the requirements of Section 4.2.2, the process should be corrected, and testing repeated until Supplier can demonstrate that the problems have been solved, and the tests can be successfully completed.
  • Any structures or components that receive TSA coating between the time the test coupons were coated and unacceptable coupon test results become available should be quarantined to allow detailed inspection and testing.  Buyer has the right to reject or accept coated equipment based on test and inspection results.

7.0 Thermal Spray Coating Inspection

7.1 General

Thermal spray coatings should be inspected after all thermal spraying and sealing have been completed but before top coating (if applicable).  Defective or non-conforming coatings should be repaired in accordance with Section 10 of this specification.

7.2 Visual Inspection

All coated surfaces should be visually inspected.  The surfaces should be free from lumps, coarse areas, loosely adherent particles, blisters, cracks, chips or pits.  The coating should have a uniform appearance with no discoloration.

7.3 Coating Thickness

  • Coating thickness should be measured and recorded for each coated pipe, component or structure and for the coating system check coupons.  Thickness of the coating should be within the range agreed to by Buyer and Supplier.
  • Thickness gages should be calibrated using certified coating thickness calibration standards at the start of each shift in which they are used.
  • For manual application of TSA, the dry film thickness (DFT) should be measured and recorded for each coat at the frequency stated in SSPC-PA-2.  At least one determination (i.e., 15 individual measurements) per coat should be made on each structure, component or unit of subsea equipment.  Each coat should have a thickness between 3 and 7 mils (76 and 175 microns).
  • For automated application of TSA, DFT measurements should be made on each coated pipe at three locations, and in five equally spaced positions at each location.  The three test locations should normally be at each end of the pipe and at the mid-position.  Additional measurements should be made on any area that is manually sprayed.

7.4 Coated Component Adhesion Tests

  • Additional adhesion tests should be performed using a HATE Mark VII or Elcometer 106/3 tester on TSA coating applied to components when there is a failure with the system check coupon.  Adhesion testing should include a minimum of one measurement on the previously coated component or structure.  Adhesion tests will follow ASTM D4541 procedures.
  • The minimum acceptable adhesion value for any single measurement is 1000 psi.
  • If any adhesion measurement value is below 1000 psi, three additional measurements should be made on the component.  One of the three additional measurements must be in the "vicinity" of the failed test and the other two tests should be performed at random locations on the component.  If any of the three additional tests are below 1000 psi, then components before and after the failed component will be tested in accordance with Section 7.4 to determine the extend of the defective coating.  If three additional measurements meet or exceed 1000 psi on any given component, then the component is acceptable.  Defective coatings should be removed by blast cleaning and re-coated in accordance with this Specification.
  • Adhesion test on components may be discontinued prior to failure, provided that the value achieved is equal to or greater than 1000 psi.

8.0 Sealers and Overcoats

8.1 Materials

  • Sealers and/or overcoats used in multi-coat systems should be provided by one manufacturer only, and should be of contrasting colors.  A low viscosity seal or prime coat is applied as a filler for porosity in the TSA coating, while preserving the electrical conductivity critical to maintaining the cathodic protection qualities.  The purpose of the seal coat is only to fill the porosity in the TSA coating and extend its cathodic protection life.  Additional overcoats may be applied but these will electrically isolate the TSA from the subsea environment and negate the overall cathodic protection qualities of the TSA coating.
  • Coating materials should be stored in accordance with the manufacturer's recommendations.
  • Coating materials have a limited shelf life and must not be used after their expiration date.  If any portion of the coating material in a container shows signs of degradation after storage, the entire contents should be discarded.
  • The contents of all coating containers should be thoroughly stirred before use.  Multi-component systems should be mixed in the sequence specified by the manufacturer, but no more should be mixed than can be used before the pot life is exceeded.  Pots should be agitated using mechanical mixers, if necessary, to keep the coating materials well mixed.
  • Thinning is permissible only when specified by the coating manufacturer for a given type of application.  The types and amounts of thinner materials used should be as recommended by the coating manufacturer.

8.2 Sealer or Overcoat Application

  • Sealer or overcoat application equipment should be thoroughly cleaned before use.
  • Before the seal or overcoat is applied, Supplier should verify the following conditions exist:
    • TSA surfaces are not wet or damp
    • TSA surface temperature is at least 5oF above the Dew Point temperature
    • The relative humidity of the ambient air is below 90%
    • The TSA surface and ambient air temperature are both above 45oF
    • No more than 48 hours have elapsed since the TSA was applied.
  • Acceptable sealer/overcoat primer materials for subsea applications are given in Table 1.  The sealer materials and the solvents used should be compatible produces provided by the same manufacturer.
Table 1 - TSA Seal Coat Systems for Subsea Structures Designed to Provide Cathodic Protection to Substrate
Coat
No.
Properties Coating Manufacturer / Material
Carboline Ameron Carboline Hempel International
1 Carboline 893
Epoxy Primer
Amercoat 878
Silicone Aluminum
Carboline 4700
Silicone
Aluminum
Hempel 5691
Silicone
Aluminum
Interbond
HTA 154
Silicone
Aluminum
DFT (max) 0.5 mils 0.5 mils 0.5 mils 0.5 mils 0.5 mils
Color
Thinning 2 parts product to 1 part solvent 4 parts product to 1 part solvent
2 Carboline 893
Epoxy Primer
Amercoat 878
Silicone Aluminum
Carboline 4700
Silicone
Aluminum
Hempel 5691
Silicone
Aluminum
Interbond
HTA 154
Silicone
Aluminum
DFT (max) 1.0 mils 1.0 mils 1.0 mils 1.0 mils 1.0 mils
Color Break from coat 1
Thinning 1 part product to 1 part solvent No more than 10% solvent
  • Application of sealer coating should be carried out in accordance with the standards of workmanship set forth in ASW C2.2 and DOD STD 2138A.  The application equipment and coating materials will be used in conformance with manufacturer recommendations or specifications listed below or elsewhere in this Specification.
    • Solvents should not be added to a sealer for any other purpose than viscosity reduction to improve application and penetration of material into the porosity in the TSA coating.  The addition of solvents should not exceed 10% of base material by volume, except as stated in Table 1.  The thinned sealer should be contained in a manner appropriate to minimize or eliminate evaporation until application is complete.
    • Application of the first seal layer by conventional spray techniques should take place only after the temperature of the TSA coating has fallen below 100oF but no later than 48 hours after application of the TSA coating.  During the intervening period, the part must be kept in a dry, clean, protected area.
    • A brush may be used for the pre-spray application of sealer in difficult to spray (corners, edges, brackets, etc.) or inaccessible (recesses, narrow channels, etc.) areas.  No rollers, pads, or other devices may be used.
    • Forced drying of a coated surface should not take place unless approved by Buyer.
  • Supplier will perform comparative coating thickness tests to assure that the application of the sealer does not produce a measurable increase in thickness of the TSA coating.

9.0 Sealer and Overcoat Inspection

9.1 Visual Inspection

Each coat should be visually inspected and should be free from blisters, cracks, holidays (with the exception of the first seal coat), sags or other defects.  Minor runs, drips or other flaws that are clearly cosmetic as determined by Buyer should not be considered defects.  Each coat should have a uniform appearance.

9.2 Coating Thickness

  • Dry film thickness (DFT) of seal and overcoats should be measured and recorded for each component in accordance with Section 7.3.
  • Seal coats applied after TSA are intended to fill the porosity of the aluminum and not to build total film thickness.  DFT specified for these seal coats are intended to be maximum values only.  Seal coat thickness may be inspected at 30-power magnification to ensure that there are no voids or missed areas in coverage of the TSA.
  • The overall coating system (TSA plus seal and/or overcoat) DFT should be within the range agreed to by Buyer and Supplier.
  • Thickness gages should be calibrated using certified coating thickness calibration standards at the start of each shift in which they are used.

10.0 Coating System Repair

  • Defective work should be identified and should receive no subsequent coating until the non-conformance is rectified.
  • Where only the seal or overcoat coatings have been damaged, the surface should be prepared by abrasion with No. 60 abrasive paper until a clean bright surface is evident.  All edges of existing coatings will be feathered.  The appropriate seal/overcoat coating(s) should then be re-applied to the damaged areas.
  • When the underlying TSA coating has been damaged exposing the substrate, the entire coating system should be completely removed in the damaged area by blast cleaning and the full coating system replaced in that area.  Edges of the coating system around the damaged area should be feathered and cleaned to accept an overlap of thermal sprayed aluminum.  Seal and overcoat materials should be overlapped onto existing coatings around the repaired area.  Care should be taken to ensure that no cracked or poorly bonded overlaps exist.
  • Coatings which are to be over coated and which have become contaminated with oil, grease or other foreign matter, should be cleaned with water-soluble cleaners and then water rinsed.  No traces of oil or grease should remain in the pores of the aluminum prior to overcoating.
  • Coating repairs should be re-inspected to ensure compliance with the requirements of this Specification.

11.0 Documentation

  • Detailed records of the TSA coating process should be maintained including records of surface preparation procedures, anchor profile results, materials batch numbers, film thickness records, production test panel adhesion results, and any results for adhesion tests run on subsea structures or components.
  • Nonconformance reports should be prepared to document all significant variances and corrective action taken.  A nonconformance report is typically written to document deleterious events that require Buyer notification and feedback.
  • In the event that nonconforming product is deemed to be acceptable, though still nonconforming, a concession (or waiver) request should be produced and submitted to Buyer for approval.
  • Traceability records showing the batch numbers of all materials used should be maintained together with manufacturer's certification for materials.  Traceability should be maintained through the Supplier's quality system.  Records of TSA work performed should be maintained in computer data form for an indefinite period.
  • Supplier should provide a Certificate of Conformance or a Buyer approved Concession (Waiver) Request to Buyer in a Documentation Package to accompany all subsea structures or components that have received a TSA coating from Supplier.
  • Copies of all pertinent test results for TSA coatings applied to subsea structures or components by Supplier should also be supplied to Buyer in the Documentation Package accompanying the equipment.
Tags: Thermal Sprayed Aluminum Subsea Structure