1、 SSPC-SP-TR 1 NACE 6G194 Informational Report and Technology Update Thermal Precleaning NACE International (NACE) and the Steel Structures Painting Council (SSPC) issue this technical committee report in conformance with the best current technology regarding the specific subject. This technical comm
2、ittee 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,
3、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 a
4、ny 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
5、 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 committ
6、ee 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 respons
7、ible 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, ope
8、rations, and/or equipment detailed or referred to within this document. CAUTIONARY NOTICE: NACE/SSPC technical committee reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. The user is cautioned to obtain the latest edition. NACE and SSPC require
9、 that action be taken to reaffirm, revise, or withdraw this technical committee report no later than ten years from the date of initial publication. Approved October 1994 Copyright 1994, NACE International and SSPC NOTICE TO THE READER: The NACE and SSPC releases of this publication contain identica
10、l wording in the same sequence. Publication format may differ. SSPC: The Society for Protective Coatings 40 24thStreet, Sixth Floor Pittsburgh, PA 15222 telephone +1 412/281-2331 NACE International P.O. Box 218340 Houston, TX 77218-8340 telephone +1 281/228-6200 Printed by NACE International NACE 6G
11、194/SSPC-SP-TR 1 2 FOREWORD Although thermal precleaning 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 a
12、s 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 am
13、ounts 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 precleaning is not used exclusively; rather
14、, 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. Thermal precleaning is typically used in conjunction with abrasive blasting, high-pressure water cleaning, steaming, c
15、hemical 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 precleaning. Within industry there is sufficient exp
16、erience with thermal precleaning, 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 precleaning in coating specifications. This state-of-the-art report was prepared by NACE/SSPC
17、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 C
18、oatings and Linings and by the Steel Structures Painting Council. GENERAL This state-of-the-art report addresses the use of thermal precleaning for tanks, vessels, rail tank cars and hopper cars, and process equipment when preparing surfaces for the application of high-performance or high-bake coati
19、ng and lining systems. DEFINITIONS Thermal Precleaning: Thermal precleaning 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
20、 abrasive blast cleaning and coating application. Dry heat and wet heat are two common types of thermal precleaning. Dry Heat: The structure to be thermally precleaned is subjected to elevated temperatures by appropriate means, such as an oven, in order to: (1) thermally degrade wax, grease, oil, ta
21、r, 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 precleaning removes all volatile contaminants from the substrate that might otherwise
22、come out during the curing process and result in blistering of the coating. Wet Heat: The structure to be thermally precleaned 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
23、, mineral acids such as hydrochloric acid, alkalies, and other chemical contaminants such as sulfates and chlorides that (1)Thermal precleaning procedures for oilfield tubular goods are a special case requiring higher temperature ranges for adequate degradation. For specific information on these pro
24、cedures, 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
25、levels of residual contaminants that will not adversely affect coating performance.2The list includes the following information: 1. Weldon et al., whose laboratory work indicates that chloride levels need to be less than 5 g/cm2and sulfate levels less than 10 g/cm2. 2. Swedish Corrosion Institute st
26、udies indicate levels less than 2 g/cm2and 10 g/cm2respectively. 3. British Maritime studies indicate levels less than 7 g/cm2and 16 g/cm2respectively. 4. When coating thickness exceeds 250 m (10 mils), the tolerance level appears to be good at concentrations up to 50 g/cm2for both types of contamin
27、ants. 5. The conclusion is that the data indicates levels of chloride contamination on the order of 2 to 10 g/cm2and sulfate contamination on the order of 10 to 20 g/cm2can adversely affect the performance of most coatings. The bibliography lists other articles that address this subject. (3)Chaired
28、by the late Carroll Steely, formerly with Vickers Industrial Coatings, Lyons, Texas. NACE 6G194/SSPC-SP-TR 1 3 either reside on or have permeated the grain boundaries of ferrous and nonferrous surfaces. PREPARATION FOR THERMAL PRECLEANING All heavy deposits of wax, grease, oil, etc., some of which m
29、ay 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 inclu
30、de 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 2,6or SSPC-SP 37). APPLICATION OF THERMAL PRECLEANING Thermal precleaning is time and
31、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, substr
32、ate 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 compoun
33、ds/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 precleaned is typically inspected for stress corrosion cracking before precleaning,
34、if possible, or before lining application if base metal is obscured by existing linings or corrosion deposits. Thermal precleaning is not intended for use in the removal of hydrogen in steel. Dry Heat Thermal precleaning using dry heat may degrade or char existing coatings and/or remove some contami
35、nants from the surfaces of tanks, vessels, piping, and other hydrocarbon-contaminated surfaces. Oven temperatures are typically 232 to 426C (450 to 800F). Under certain conditions lower temperatures are sometimes used; general practice is that the thermal precleaning temperature be a minimum of 28C
36、(50F) 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 precleaned at a temperature in excess of the final bake temperature of the coating being ap
37、plied. This procedure ensures that contaminants whose volatile temperatures are at or near the final bake temperature of the coating will have been released and will not interfere with the final cure and integrity of the coating being applied. General practice has been that the dwell time of the the
38、rmal precleaning heat exceed the final cure time of the previously applied coating by a minimum of 30 minutes. Since ovens are not always practical when working with large equipment, an alternative method of thermal precleaning is to insulate the tank or vessel and use ducts to convey heat into the
39、vessel from portable, non-contaminating natural gas or LPG heaters, thereby raising the temperature of the metal substrate to the desired thermal precleaning temperature. Decomposition of organic materials is related to time and temperature. Thermal precleaning proceeds slowly with a gradual rise in
40、 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 residu
41、es are typically removed using high-temperature steam or a hot-water rinse prior to abrasive blasting. Wet Heat 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-pressu
42、re 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 ste
43、am 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 (4)”In a severe case of grain-boundary corrosion, entire grains are dislodged due to complete de
44、terioration 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. NACE 6G194/SSPC-SP-TR 1 4 substrate temperature at or near
45、 100C (212F), usually 93C (200F). 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 contaminat
46、ion, 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 precleaning operations are completed. VERIFICAT
47、ION OF SURFACE CLEANLINESS AFTER THERMAL PRECLEANING 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 me
48、thod 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 m
49、ethods are cited in Table 1 and the bibliography. SPECIFIC APPLICATIONS Thermal precleaning 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 t
