1、SSPC -TR 1/NACE 6G194October 1, 1994Editorial Revisions November 1, 20043-1SSPC-TR 1/NACE 6G194INFORMATIONAL REPORT AND TECHNOLOGY UPDATEThermal Pre-CleaningNACE International (NACE) and SSPC: The Society for Protective Coatings (SSPC) issue this technical committee report in conformance with the be
2、st current technology regarding the specifi c 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
3、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 i
4、mplication 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
5、 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 specifi c instances. NACE and SSPC assum
6、e no responsibility for the interpretation or use of this technical committee report by other parties and accept responsibility for only those offi cial interpretations issued by NACE or SSPC in accordance with their governing procedures and policies which preclude the issuance of interpretations by
7、 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 addres
8、s all safety problems and hazards associated with the use of materials, operations, and/or equip-ment 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document.CAUTIONARY NOTICE: NACE/SSPC technical committee reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice
9、. The user is cautioned to obtain the latest edition. NACE and SSPC require that action be taken to reaffi rm, revise, or withdraw this technical committee report no later than fi ve years from the date of initial publication. Approved October 1994Copyright (c)1994, NACE International and SSPCNOTICE
10、 TO THE READER: The NACE and SSPC releases of this publication containidentical wording in the same sequence.Publication format may differ.SSPC: The Society for Protective Coatings40 24th Street, Sixth FloorPittsburgh, PA 15222+1 412-281-2331NACE International1440 South CreekHouston, TX 77218-8340+1
11、 281-228-6200Printed by SSPCForewordAlthough 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 oilfi eld tubular goods,(1)it is now commonly used in the process i
12、ndustry 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 delet
13、erious 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 precleaning is not used exclusively
14、; rather, it is a surface preparation method that, when used in conjunc-tion with other cleaning methods, can achieve the degree of cleanliness required for a successful coating application. (1) Thermal precleaning procedures for the oilfi eld tubular goods are a special case requring higher tempera
15、ture ranges for adequate degra-dation. For specifi c information on these procedures, refer to NACE Standard RP0191(latest revision), “The Application of Internal Plastic Coatings for Oilfi eld Tubular Goods and Accessories.“1(2) Trimber cites the most commonly used contemporary methods for detectin
16、g contaminants and then lists the most recent and generally industry-accepted 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
17、5 g/cm2 and sulfate levels less than 10 g/cm2.2. Swedish Corrosion Institute, studies indicate levels less than 2 g/cm2and 10 g/cm2 respectively.3. British Maritime studies indicate levels less than 7 g/cm2and 16 g/cm2 respectively.4. When coating thickness exceeds 250 m (10 mils), the tolerance lev
18、el appears to be good at concentrations up to 50 g/cm2for both types of contaminants.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 coating
19、s. The bibliography lists other articles that address this subject.SSPC-TR 1/NACE 6G194October 1, 1994Editorial Revisions November 1, 20043-2acids such as hydrochloric acid, alkalies, and other chemical contaminants such as sulfates and chlorides that either reside on or have permeated the grain bou
20、ndaries of ferrous and non-ferrous surfaces. Preparation for Thermal PrecleaningAll heavy deposits of wax, grease, oil, etc., some of which may autoignite when heated, are typically removed in accor-dance with SSPC-SP 1, “Solvent Cleaning.”3Heavy rust scale, nodules, tubercles, and other encrusted c
21、ontaminants 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 h
22、and or power tool cleaning (in accordance with SSPC-SP 2,6or SSPC-SP 37).Application of Thermal PrecleaningThermal precleaning is time and temperature related. Previous experience is generally the governing factor in the length of time required to effectively remove deleterious amounts of contaminan
23、ts from the substrate. The specifi c temperature and duration of the heat application vary with the heat method, type of contaminant, substrate material, and complexity of substrate confi guration.CAUTION: Some exterior paints or other components (such as alloys, wooden bolsters, elastomeric materia
24、ls 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 typic
25、ally performed in these cases. The item to be thermally precleaned is typically inspected for stress corrosion cracking before precleaning, 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 th
26、e removal of hydrogen in steel.Dry HeatThermal precleaning 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
27、 certain conditions lower tempera-tures are sometimes used; general practice is that the thermal precleaning 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
28、 contaminated structure, the structure is thermally precleaned at a temperature in excess of the fi nal bake temperature of the coating being applied. This procedure Thermal precleaning is typically used in conjunction with abrasive blasting, high-pressure water cleaning, steaming, chemical treatmen
29、t (e.g., phosphoric acid), or several repeti-tive 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 suffi cient experience with t
30、hermal 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 specifi cations. This state-of-the-art report was prepared by NACE/SSPC Task Group B
31、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 L
32、inings and by SSPC: The Society for Protective Coatings.GeneralThis 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 ap-plication of high-performance or high-bake coating and lin
33、ing systems.Defi nitionsThermal Precleaning: Thermal precleaning is the ap-plication 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
34、blast cleaning and coat-ing 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, tar, drawing
35、 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 come out d
36、uring 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, mineral
37、(3)Chaired by the late Carroll Steely, formerly with Vickers Industrial Coatings, Lyons, Texas.SSPC -TR 1/NACE 6G194October 1, 1994Editorial Revisions November 1, 20043-3ensures that contaminants whose volatile temperatures are at or near the fi nal bake temperature of the coating will have been rel
38、eased and will not interfere with the fi nal cure and integrity of the coating being applied. General practice has been that the dwell time of the thermal precleaning heat exceed the fi nal cure time of the previously applied coating by a minimum of 30 minutes.Since ovens are not always practical wh
39、en 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 vessel from portable, non-contaminating natural gas or LPG heaters, thereby raising the temperature of the metal substrate to the desired thermal prec
40、leaning temperature.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 reach
41、ed and continues for as long as necessary to achieve partial or complete decom-position of organic contaminants.Salt deposits and carbonaceous residues are typically removed using high-temperature steam or a hot-water rinse prior to abrasive blasting.Wet HeatThermal precleaning using high-temperatur
42、e 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 pressur
43、es 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
44、 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)o
45、r 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
46、 is typically performed after thermal pre-cleaning operations are completed.Verifi cation of Surface Cleanliness After Thermal PrecleaningThe most common method used by many coatings applica-tors to determine surface cleanliness is to observe the prepared surface and carefully note any rapid discolo
47、ration. Rapid discol-oration 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 a
48、re commercially available for use in the fi eld; several published references and test methods are cited in Table 1 and the bibliography. Specifi c ApplicationsThermal precleaning is a valuable aid in the removal of pre-existing coatings that might be time- and cost-prohibitive to remove by conventi
49、onal abrasive blast cleaning. When charred by thermal precleaning, most coatings lose their adhesion so that abrasive blasting more readily removes them.Oilfi eld 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 oilfi eld tubular goods at 371 to 426 C (700 to 800 F) for four to six hours after the metal reaches
