What is Thermal Pre-Cleaning?
A NACE and SSPC Thermal Pre-Cleaning Informational Report and Technology Update
NACE International (NACE) and SSPC: The Society for Protective Coatings (SSPC) issue this technical committee report in conformance with the best current technology regarding the speciﬁc subject. This technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. It is intended to aid the manufacturer, the consumer, and the general public. Its acceptance does not in any respect preclude anyone, whether he has adopted the report or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not addressed in this report. Nothing contained in this NACE/SSPC technical committee report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This technical committee report represents current technology and should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of this technical committee report in speciﬁc instances. NACE and SSPC assume no responsibility for the interpretation or use of this technical committee report by other parties and accept responsibility for only those ofﬁcial interpretations issued by NACE or SSPC in accordance with their governing procedures and policies which preclude the issuance of interpretations by individual volunteers.
Users of this technical committee report are responsible for reviewing appropriate health, safety, and regulatory documents and for determining their applicability in relation to this report prior to its use. This NACE/SSPC technical committee report may not necessarily address all safety problems and hazards associated with the use of materials, operations, and/or equipment detailed or referred to within this document.
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Approved October 1994 Copyright (c)1994, NACE International and SSPC
NOTICE TO THE READER: The NACE and SSPC releases of this publication contain identical wording in the same sequence. Publication format may differ.
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Although thermal pre-cleaning has long been a standard procedure in the oil and gas industry as a method of surface preparation for the application of high-bake coatings to the interior surfaces of oilﬁeld tubular goods,(1) it is now commonly used in the process industry as well. The surfaces of tanks, rail tank cars, tubular goods, and process equipment that have been exposed to a corrosive environment are usually pitted and scaled and contain chemical contaminants both on the surface and within the grain boundaries of the substrate. Failure to remove deleterious amounts of these contaminants ultimately results in blistering and premature failure of the coating.(2) Years of industry experience have shown that abrasive blasting alone will not adequately remove all contaminants, especially in the bottom of pits.
Thermal pre-cleaning is not used exclusively; rather, it is a surface preparation method that, when used in conjunction with other cleaning methods, can achieve the degree of cleanliness required for a successful coating application.
(1) Thermal pre-cleaning procedures for the oilﬁeld tubular goods are a special case requiring higher temperature ranges for adequate degradation. For speciﬁc information on these procedures, refer to NACE Standard RP0191(latest revision), “The Application of Internal Plastic Coatings for Oilﬁeld Tubular Goods and Accessories.”1
(2) Trimber cites the most commonly used contemporary methods for detecting contaminants and then lists the most recent and generally industry-accepted levels of residual contaminants that will not adversely affect coating performance.2 The list includes the following information:
- Weldon, et. al., whose laboratory work indicates that chloride levels need to be less than 5 µg/cm2 and sulfate levels less than 10 µg/cm2.
- Swedish Corrosion Institute, studies indicate levels less than 2 µg/cm2 and 10 µg/cm2 respectively.
- British Maritime studies indicate levels less than 7 µg/cm2 and 16 µg/cm2 respectively.
- When coating thickness exceeds 250 µm (10 mils), the tolerance level appears to be good at concentrations up to 50 µg/cm2 for both types of contaminants.
- The conclusion is that the data indicates levels of chloride contamination on the order of 2 to 10 µg/cm2 and sulfate contamination on the order of 10 to 20 µg/cm2 can adversely affect the performance of most coatings.
The bibliography lists other articles that address this subject.
Thermal pre-cleaning is typically used in conjunction with abrasive blasting, high-pressure water cleaning, steaming, chemical treatment (e.g., phosphoric acid), or several repetitive applications of thermal precleaning and abrasive blasting in order to facilitate the removal of deleterious levels of salts and carbonaceous materials produced as a result of thermal pre-cleaning.
Within industry there is sufﬁcient experience with thermal pre-cleaning, particularly by coating application shops, to warrant the issuance of this state-of-the-art report by means of which industry can refer to a consensus document for thermal pre-cleaning in coating speciﬁcations.
This state-of-the-art report was prepared by NACE/SSPC Task Group B on Surface Preparation by Thermal Cleaning,(3) which is a component of NACE Unit Committee T-6G on Surface Preparation for Protective Coatings and the SSPC Surface Preparation Committee. This report is issued by NACE International under the auspices of Group Committee T-6 on Protective Coatings and Linings and by SSPC: The Society for Protective Coatings.
This state-of-the-art report addresses the use of thermal pre-cleaning for tanks, vessels, rail tank cars and hopper cars, and process equipment when preparing surfaces for the application of high-performance or high-bake coating and lining systems.
Thermal Pre-cleaning: Thermal pre-cleaning is the application of high temperatures to aid in the partial or complete degradation, embrittlement, and/or dilution and subsequent removal of contaminants and failed or old coatings from the surface of a substrate prior to abrasive blast cleaning and coating application. Dry heat and wet heat are two common types of thermal pre-cleaning.
Dry Heat: The structure to be thermally pre-cleaned is subjected to elevated temperatures by appropriate means, such as an oven, in order to: (1) thermally degrade wax, grease, oil, tar, drawing compounds (if the proper temperatures are achieved), and some hydrocarbon-based volatiles; and (2) embrittle existing coatings to facilitate their removal from ferrous and nonferrous substrates. Thermal pre-cleaning removes all volatile contaminants from the substrate that might otherwise come out during the curing process and result in blistering of the coating.
Wet Heat: The structure to be thermally pre-cleaned is heated to elevated temperatures by steam (pressurized or unpressurized) for the purpose of diluting and removing salts of oxidizing acids such as nitric and sulfuric acid, mineral acids such as hydrochloric acid, alkalies, and other chemical contaminants such as sulfates and chlorides that either reside on or have permeated the grain boundaries of ferrous and nonferrous surfaces.
Preparation for Thermal Pre-cleaning
All heavy deposits of wax, grease, oil, etc., some of which may autoignite when heated, are typically removed in accordance with SSPC-SP 1, “Solvent Cleaning.”3 Heavy rust scale, nodules, tubercles, and other encrusted contaminants can be removed prior to thermal precleaning in order to facilitate removal of embedded contaminants. The methods of removal include abrasive blasting (such as NACE No. 4/SSPC-SP 74), water blasting or water jetting (in accordance with NACE Standard RP01725), or mechanical means such as hand or power tool cleaning (in accordance with SSPC-SP 2,6 or SSPC-SP 37).
Application of Thermal Pre-cleaning
Thermal pre-cleaning is time and temperature related. Previous experience is generally the governing factor in the length of time required to effectively remove deleterious amounts of contaminants from the substrate. The speciﬁc temperature and duration of the heat application vary with the heat method, type of contaminant, substrate material, and complexity of substrate conﬁguration.
CAUTION: Some exterior paints or other components (such as alloys, wooden bolsters, elastomeric materials in valves, gasket materials, etc.) of the item being heated may be altered or adversely affected by the applied temperature. Some compounds/chemicals that are in contact with the substrate may cause stress corrosion cracking in welds and base metals, and more elaborate testing/inspection is typically performed in these cases. The item to be thermally pre-cleaned is typically inspected for stress corrosion cracking before pre-cleaning, if possible, or before lining application if base metal is obscured by existing linings or corrosion deposits. Thermal pre-cleaning is not intended for use in the removal of hydrogen in steel.
Thermal pre-cleaning using dry heat may degrade or char existing coatings and/or remove some contaminants from the surfaces of tanks, vessels, piping, and other hydrocarbon-contaminated surfaces. Oven temperatures are typically 232 to 426 °C (450 to 800 °F). Under certain conditions lower temperatures are sometimes used; general practice is that the thermal pre-cleaning temperature be a minimum of 28 °C (50 °F) above the curing temperature of the coating to be applied or the operating temperature of the equipment. When high-bake coatings are to be applied to a contaminated structure, the structure is thermally pre-cleaned at a temperature in excess of the ﬁnal bake temperature of the coating being applied. This procedure (3) Chaired by the late Carroll Steely, formerly with Vickers Industrial Coatings, Lyons, Texas.
Decomposition of organic materials is related to time and temperature. Thermal precleaning proceeds slowly with a gradual rise in temperature until the metal substrate is evenly heated to the desired temperature. The decomposition time period begins when this temperature is reached and continues for as long as necessary to achieve partial or complete decomposition of organic contaminants.
Salt deposits and carbonaceous residues are typically removed using high-temperature steam or a hot-water rinse prior to abrasive blasting.
Thermal precleaning using high-temperature steam dilutes acid salts, fatty acids, alkalies, waxes, and other water-soluble contaminants so that they are more readily removed by high-pressure water cleaning. Wet heat (steam) is also used to remove grease and oil in accordance with SSPC-SP 1, “Solvent Cleaning.” Steam tables list the pressures used to achieve the desired thermal precleaning temperature. Only pressure-rated structures are subjected to pressurized thermal precleaning steam temperatures. Nonpressure-rated vessels are typically isolated, vented, and insulated before and during the injection of steam. This procedure ensures that the injected steam will be able to sustain a substrate temperature at or near 100 °C (212 °F), usually 93 °C (200 °F).
Because wet heat is usually applied at a lower temperature than dry heat, longer dwell times are generally used, particularly when contaminants are embedded in grain boundaries(4) or the bottom of pits or craters. With severe cases of grain-boundary or pit contamination, repeated applications of wet heat and abrasive blasting are often used to remove deleterious levels of contaminants such that the surface will not immediately discolor after abrasive blasting.
Abrasive blasting is typically performed after thermal pre-cleaning operations are completed.
Veriﬁcation of Surface Cleanliness After Thermal Pre-cleaning
The most common method used by many coatings applicators to determine surface cleanliness is to observe the prepared surface and carefully note any rapid discoloration. Rapid discoloration indicates that contaminants remain on the surface. This method is not effective in dry environments because moisture is not present to react with contaminants that may remain on the surface. Test kits for detecting the presence of visible and/or nonvisible contaminants are commercially available for use in the ﬁeld; several published references and test methods are cited in Table 1 and the bibliography.
Thermal pre-cleaning is a valuable aid in the removal of pre-existing coatings that might be time- and cost-prohibitive to remove by conventional abrasive blast cleaning. When charred by thermal precleaning, most coatings lose their adhesion so that abrasive blasting more readily removes them.
Oilﬁeld tubular lining shops employ thermal precleaning as a means of volatilizing grease, oils, waxes, and other contaminants that might otherwise be released during the baking and curing cycle of high-bake thermoset coatings and cause blistering of the applied coating.
A commonly accepted practice is to thermally preclean oilﬁeld tubular goods at 371 to 426 °C (700 to 800 °F) for four to six hours after the metal reaches the desired temperature in order to thermally degrade oil, grease, wax, old coatings, and other organic contaminants encountered in the oil and gas industry. Some companies successfully treat chloride-contaminated oilﬁ eld tubular goods by performing the following: (1) abrasive blast, (2) preheat to 66°C (150 °F), (3) phosphoric acid wash, (4) rinse with water, and (5) reblast.
Process industry lining shops use thermal precleaning in their work with: (1) process tanks, especially chemical process and storage tanks, that have old or failed linings and require reapplication; (2) unlined tanks and equipment that have suffered metal loss and pitting and now require a lining to provide extended service; and (3) equipment that has become chemically contaminated during storage, shipment, or fabrication. In the power industry, wet heat followed by a hydrogen peroxide rinse has been used to disinfect water boxes that have pitted and corroded due to microbiologically induced corrosion (MIC).11
Mill varnish on pipe has been removed successfully by thermal precleaning at 288 °C (550 °F) held for one hour, although thermal precleaning at 260 °C (500 °F) for the same period of time has not achieved the desired results. (4) “In a severe case of grain-boundary corrosion, entire grains are dislodged due to complete deterioration of their boundaries. In such a case, the surface will appear rough to the naked eye and will feel sugary because of the loose grains.” Source: M. Henthorne, “Fundamentals of Corrosion,” Chemical Engineering 78, 11 (1971): p. 131.
TABLE 1 CANDIDATE PHYSICAL OR CHEMICAL TESTS FOR CONTAMINANTS REMAINING ON “CLEANED STEEL”
Class of Contaminant Contaminant Suggested Test Existing Standard
Soluble in Water Moisture Cobalt chloride test paper None
Soluble iron salts Potassium ferricyanide test BSI 5493, Appendix G(B) SABS Method 770 (C) DIN 55928 Part 4(D)
Soluble chlorides Silver nitrate test paper Aquaquant test kit Ion selective electrodes SABS Method 770 None None
Soluble sulfates Barium chloride/potassium permanganate test paper SABS Method 770
Soluble in Organic Solvents Grease, oil, and waxy residues Fettrot test Fluorescence Water break test Solvent spotting test DIN 55928 Part 4 DIN 55928 Part 4 None None
Insoluble or of Low Solubility Mill scale Copper sulfate test Etch with 15% HNO3 Differential resistance probe SABS Method 771(E) None None
Mill scale rust and dust The “surclean” test BS5493 Appendix F
Loose rust and dust Reﬂ ectometer Tape Method SABS Method 768(F) SABS Method 771
Weld ﬂuxes pH test paper None
(A) Source: A.N. McKelvie, “Can Coatings Successfully Protect Steel, What Are the Ingredients of Success?” Materials Performance 19, 5 (1980): p. 13.
(B) BS5493 (latest revision), “Code of Practice for Protective Coating of Iron and Steel Structures Against Corrosion,” (Milton Keynes, United Kingdom: British Standards Institution).(6)
(C) SABS Method 770 (latest revision, now SANS 5770), (Pretoria, South Africa: South African Bureau of Standards)(7)
(D) DIN 55928, Parts 1-8 (latest revision), “Protection of Steel Constructions from Corrosion by Organic and Metal Coatings” (Berlin, Germany: Deutsches Institut fur Normung).(8)
(E) SABS Method 771 (latest revision, now SANS 5771), (Pretoria, South Africa: South African Bureau of Standards).
(F) SABS Method 768 (latest revision, now SANS 5768), (Pretoria, South Africa: South African Bureau of Standards).
(6) British Standards Institution (BSI), Linford Wood, Milton Keynes, Milton Keynes MK146LE, United Kingdom.
(7) South African Bureau of Standards (SABS), 1 Dr. Lategan Rd., Private Bag X191 Groenkloof, Pretoria, South Africa 00001, Republic of South Africa.
(8) Deutsches Institut fur Normung (DIN), Burggrafenstrasse 4-10, 1000 Berlin 30, Germany.
Some owners of rail tank cars (e.g., cars used to transport sulfuric acid) have the cars ﬁrst neutralized with a 10% caustic solution then subjected to high heat (232°C [450°F]) to thermally degrade organic contaminants. Chloride and sulfate contaminants are removed by high-pressure water jetting followed by abrasive blasting.
Steam cleaning helps remove chlorides and sulfates. Frondistou-Yannas concludes that, “In terms of effectiveness in removing chlorides and sulfates from a rusted steel surface, the methods of surface preparation rank as follows: (1) Water blasting; (2) steam, detergent, and wire-brushing; (3) solvent cleaning and wire-brushing; and (4) single wire-brushing.”13
Testing for Surface Contaminants
Table 1 lists candidate physical or chemical tests for the presence of contaminants that still may be present on cleaned steel. The International Organization for Standardization (ISO)(5) has drafted documents that, if approved, may replace the cited standards. Although no U.S. standards exist for physical and chemical testing, NACE Publication 6G186, “Surface Preparation of Contaminated Steel Surfaces,”14 gives additional information on currently available ﬁeld tests for detecting contaminants.
1. NACE Standard RP0191 (latest revision), “The Application of Internal Plastic Coatings for Oilﬁeld Tubular Goods and Accessories” (Houston, TX: NACE International).
2. K.A. Trimber, “Detection and Removal of Chemical Contaminants in Pulp and Paper Mills,” Journal of Protective Coatings and Linings 5, 11 (1988): pp. 30
3. SSPC-SP 1 (latest revision), “Solvent Cleaning” (Pittsburgh, PA: SSPC).
4. NACE No. 4/SSPC-SP 7 (latest revision), “Brush-Off Blast Cleaning” (Houston, TX: NACE International and Pittsburgh, PA: SSPC).
5. NACE Standard RP0172 (latest revision), “Surface Preparation of Steel and Other Hard Material by Water Blasting Prior to Coating or Recoating” (Houston, TX: NACE International).
6. SSPC-SP 2 (latest revision), “Hand Tool Cleaning” (Pittsburgh, PA: SSPC).
7. SSPC-SP 3 (latest revision), “Power Tool Cleaning” (Pittsburgh, PA: SSPC).
8. NACE Standard RP0181 (latest revision), “Liquid Applied Internal Protective Linings and Coatings for Oilﬁeld Production Equipment’’ (Houston, TX: NACE International, 1981).
SSPC -TR 1/NACE 6G194 October 1, 1994 Editorial Revisions November 1, 2004
9. “ ‘News from the Field,’ Tenneco Gas Deals with Chloride-Contaminated Pipe,” Journal of Protective Coatings and Linings 6, 2 (1989): p. 34.
10. M.A. Kazemi, B.T. Nosé, “Fusion-Bonded Epoxy Pipe Coating: Preparation and Application Make a Big Difference,” Journal of Protective Coatings and Linings 9, 5 (1992): pp. 52-56.
11. G.V. Spires, “Epoxy Linings for Stainless Steel and a Circulating Water System,” Journal of Protective Coatings and Linings 5, 12 (1988): p. 38-45.
12. K.W. Ferry, “Cleaning Lined Tank Cars and Unlined Tank Cars for Lining Application,” Materials Performance 30, 5 (1991): p. 37.
13. S. Frondistou-Yannas, “Effectiveness of Nonabrasive Cleaning Methods for Steel Surfaces,” Materials Performance 25, 7 (1986): p. 53.
14. NACE Publication 6G186 (latest revision), “Surface Preparation of Contaminated Steel Surfaces” (Houston, TX: NACE International).
Appleman, B.R. “Painting over Soluble Salts: A Perspective.” Journal of Protective Coatings and Linings 4, 6 (1987): pp. 68-82.
Calabrese, C., J.R. Allen. “Surface Characterization of Atmospherically Corroded and Blast Cleaned Steel.” Corrosion 34, 10 (1978): pp. 331-338.
Frenzel, L.M., J. Nixon. “Surface Preparation Using High-Pressure Water Blasting.” CORROSION/89, paper no. 397. Houston, TX: NACE International, 1987.
Johnson, W.C. “New Concepts for Coating Protection of Steel Structures.” ASTM Special Publication 841. Philadelphia, PA: American Society for Testing and Materials, 1984.
McKelvie, A.N. “Can Coatings Successfully Protect Steel, What Are the Ingredients of Success?” Materials Performance 19, 5 (1980): p. 13.
McKelvie, A.N. “Steel Cleaning Standards-A Case for their Reappraisal.” Journal of the Oil and Colour Chemists’ Association 60 (1977): pp. 227-237.
Munger, C.G. “Sulﬁdes-Their Affect on Coatings and Substrates.” CORROSION/77, paper no. 1 (Houston, TX: NACE International, 1977).
Munger, C.G. “The Coating of Contaminated Ferrous Surfaces.” NACE International Coatings Symposia Regional Meeting, Niagara, New York (Houston, TX: NACE International, 1979).
NACE Publication 6G186 (latest revision). “Surface Preparation of Contaminated Steel Surfaces.” Houston, TX: NACE International.
(5) International Organization for Standardization (ISO) 1 rue de Varembe, Case Postale 56, CH 1121 Geneva 20, Switzerland.
SSPC-TR 1/NACE 6G194 October 1, 1994 Editorial Revisions November 1, 2004
NACE Standard TM0170 (latest revision). “Visual Standard for Surfaces of New Steel Airblast Cleaned with Sand Abrasive.” Houston, TX: NACE International.
Rex, J. “A Review of Recent Developments in Surface Preparation Methods.” Journal of Protective Coatings and Linings 7, 10 (1990): pp. 50-58.
Systems and Speciﬁcations: Steel Structures Painting Manual, Sixth Edition. Pittsburgh, PA: SSPC, 1991.
Trimber, K.A. “An Investigation into the Removal of Soluble Salts Using Power Tools and Steam Cleaning.” SSPC: The Society for Protective Coatings Seventh Annual Symposium, paper SY-3.1. Pittsburgh, PA: SSPC, 1988.
Trimber, K.A. “Detection and Removal of Chemical Contaminants in Pulp and Paper Mills.” Journal of Protective Coatings and Linings 5, 11 (1988): pp. 30-37.
Weldon, D.G., A. Bochan, M. Schleiden. “The Effect of Oil, Grease, and Salts on Coating Performance.” Journal of Protective Coatings and Linings 4, 6 (1987): pp. 46-58.
Thermal Pre-Cleaning – A NACE and SSPC Informational Report and Technology Update