1、 1 Item No. 24243 NACE International Publication 6G186 This Technical Committee Report has been prepared by NACE International Task Group 142,*“Surface Preparation of Contaminated Steel Surfaces.” Surface Preparation of Soluble Salt Contaminated Steel Substrates Prior to Coating March 2010, NACE Int
2、ernational This NACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products, processes,
3、 or procedures not included in this report. Nothing contained in this NACE International 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 pr
4、otecting anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumsta
5、nces may negate the usefulness of this report in specific instances. NACE International assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE International report are responsible for reviewing appropriate health, safety, and regulatory documents
6、and for determining their applicability in relation to this report prior to its use. This NACE International report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to
7、 within this report. Users of this NACE International report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory r
8、equirements prior to the use of this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE International reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if
9、more than 10 years old. Purchasers of NACE International reports may receive current information on all NACE International publications by contacting the NACE International FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1-281-228-6200). Foreword The subject of
10、 soluble salts on steel substrates and their effect on coating performance is an important and widely debated topic. The concentration at which soluble salts begin to have a detrimental effect on coating performance varies widely, depending on factors such as the type of service, coating thickness,
11、generic coating type, and the presence of moisture. This technical committee report is intended to provide information concerning the use of coatings in service environments where soluble salt contamination of the substrate is suspected. Ideally, it is desirable to have no soluble salts present on t
12、he surface to be coated; however, there is a cost associated with their detection, _ * Chair Ken Tator, KTA-Tator Inc., Pittsburgh, PA. NACE International 2 removal, and testing. These associated surface preparation costs provide incentive for risk assessment to balance the cost-to-benefit ratio of
13、reducing or removing salts that may be present. The purpose of this technical committee report is to increase the industry awareness of the following: The effects of various nonvisible soluble salt contaminants on a coatings performance; An approach to risk assessment regarding the costs of soluble
14、salt removal versus the risk of future coating failure; Identification of the indicators of salt contamination; and Various methods of salt contamination removal. Coating manufacturers can provide recommendations regarding tolerable soluble salt levels based on coating type, service, and desired ser
15、vice life. With this information, the user typically makes a more informed decision regarding the most effective surface preparation methods. The level of salt contamination that a specific coating system can tolerate in a given service environment depends on the type and severity of the environment
16、, extent of surface preparation, coating material formulation, and a number of other parameters. Because of the variability of these parameters, numerical levels are not discussed in this report. Numerical levels of tolerable salt contamination are being addressed by ISO(1)/TC 35/SC 12/WG-5.1The rem
17、oval of visible contaminants such as iron oxides, previously applied coatings, dirt, oil, grease, water, or microbiological contamination is not covered in this report. Removal of these contaminants is covered in joint NACE/SSPC(2)surface preparation standards.27Those standards rely on a visual asse
18、ssment of surface cleanliness. However, premature coating failures have been attributed to salt contamination even where surfaces have been prepared to appropriate visual standards. This report is intended for use by engineers, specification writers, contractors, and anyone interested in reducing pr
19、emature coating failures caused by nonvisible soluble salts. It addresses only chlorides, nitrates, and sulfates, because of insufficient research and data regarding other soluble salts. This report consists of the following sections: Introduction Effect of Salt Contamination on Coating Performance
20、Risk Assessment Salt Contamination Sources Water-Soluble Salt Contaminants (Chlorides, Nitrates, and Sulfates) Recognition and Identification of Salt Contaminants Field Tests to Detect the Presence of Salts Salt Removal Methods Project-Specific Sampling Protocols and Acceptance Criteria References A
21、ppendixes: A. Area-Based Sampling Protocol and Testing B. Preparing a Specification C. Example Lining Specification (1) International Organization for Standardization (ISO), 1 ch. de la Voie-Creuse, Case postale 56, CH-1211 Geneva 20, Switzerland. (2)The Society for Protective Coatings (SSPC), 40 24
22、thStreet, 6thFloor, Pittsburgh, PA 15222-46546. NACE International 3 This technical committee report was originally issued in 1986 by NACE Task Group (TG) T-6G-22, “Surface Preparation of Contaminated Steel Surfaces,” a component of Unit Committee T-6G, “Surface Preparation for Protective Coatings.”
23、 It was revised in 2010 by NACE TG 142, “Surface Preparation of Contaminated Steel Surfaces.” TG 142 is administered by NACE Specific Technology Group (STG) 04, “Coatings and Linings, Protective: Surface Preparation.” It is also sponsored by STG 02, “Coatings and Linings, Protective: Atmospheric;” S
24、TG 03, “Coatings and Linings, Protective: Immersion and Buried Service;” and STG 43, “Transportation, Land.” This report is issued by NACE under the auspices of STG 04. NACE technical committee reports are intended to convey technical information or state-of-the-art knowledge regarding corrosion. In
25、 many cases, they discuss specific applications of corrosion mitigation technology, whether considered successful or not. Statements used to convey this information are factual and are provided to the reader as input and guidance for consideration when applying this technology in the future. However
26、, these statements are not intended to be recommendations for general application of this technology, and must not be construed as such. Introduction Long-term coating performance is directly related to proper coating material selection, correct coating formulation, appropriate and adequate surface
27、preparation, and satisfactory coating application. In addition, soluble salt contamination in the form of chlorides, nitrates, and sulfates sometimes has a deleterious effect on long-term performance of some coating materials in some exposure conditions. Coating specification writers, facility owner
28、s, and coating applicators have avoided problems by understanding when soluble salts could be an issue and the steps that could be taken to mitigate the consequences of salt contamination. Soluble salts on a surface potentially affect the substrate or coating in two principal ways: By accelerating c
29、orrosion of an underlying steel surface. Dissolved salt solutions may accelerate oxidation of steel, resulting in undercutting of a coating system applied over that substrate. Corrosion can occur without the presence of any salts. Crevice corrosion, oxygen concentration corrosion, pitting, and gener
30、al corrosion all occur without salt contamination in the presence of moisture on susceptible metal surfaces, such as steel. If salts are present, the rate of such corrosion accelerates. As a cause of osmotic blistering. Blistering of a coating sometimes occurs when moisture permeates through the coa
31、ting film and dissolves entrapped water-soluble salts beneath the coating. The coating acts as a semipermeable membrane between a dilute salt solution (the liquid outside the coating), and a concentrated salt solution (liquid with dissolved salts within the blister). Coating systems in immersion or
32、wet environments are sometimes susceptible to osmotic blistering, and blistering may occur at the steel surface and between coats. Osmotic pressure beneath the coating, in excess of the bond strength of the coating, may cause blisters. Ultimately, corrosion sometimes develops within the blisters, de
33、pending on oxygen availability. Blistering of a coating system also occurs in the absence of soluble salts. Entrapped polar solvents and overdriven cathodic protection may draw moisture through a coating film to cause blistering. The best life-cycle performance is usually achieved when the coating s
34、ystem is applied over an uncontaminated or less-contaminated surface. However, a level of nonvisible soluble salt surface contamination that does not significantly compromise a coating systems life-cycle performance may also exist. At this level, the additional cost of removal of the salt contaminat
35、ion sometimes is not warranted. The owner or specifier sometimes conducts a risk assessment to evaluate the costs of soluble salt removal versus the risk of reduced coating performance. The owner and specifier often determine whether the increased performance anticipated by achieving a nonvisible so
36、luble salt decontaminated surface is justified from a cost standpoint. NACE International 4 Effect of Salt Contamination on Coating Performance Little definitive information is available regarding how the amount of salt contamination relates to coating performance. Difficulty in objectively evaluati
37、ng the detrimental effects of soluble salts is caused in part by the diverse and variable resins, pigments, and coating materials available that may be formed into single-coat or multicoat systems. Other variables include the thickness of a given coat or coating system and the nature and range of se
38、verity of exposure environments. These variables combine to make it difficult to prepare a simple, convenient table or chart that establishes acceptable tolerance levels for a soluble salt beneath a coating. Some coatings are more tolerant of the presence of water-soluble salts than others. For exam
39、ple, inorganic zinc-rich coatings and metallized coatings are generally considered to be more tolerant than organic coatings such as fusion-bonded epoxies and epoxy-phenolics. The total thickness of a coating system usually has an impact on the ability of a coating system to tolerate the effects of
40、salts on a surface. For a given coating system, thicker systems are typically more impervious to water, and therefore have a greater salt tolerance, than thinner systems. Some soluble salt contaminants are more corrosive to steel than others. For example, nitrates appear to be slightly less corrosiv
41、e than chlorides and sulfates at low concentrations, but not necessarily at higher concentrations.8The effect of the cation (positive ion) typically is also considered. For example, for chloride salts, the order of corrosiveness (from most corrosive to least corrosive) to steel is: lithium chloride,
42、 sodium chloride, potassium chloride, and calcium chloride.8 The corrosiveness of a salt is, up to a point, directly proportional to the conductivity of the electrolyte formed when it dissolves (for sodium chloride, the maximum corrosiveness occurs when the salt solution is approximately 3.5 wt%). T
43、he conductivity of a given salt solution is a function of its ionic species. The relative corrosiveness of salts to steel sometimes does not correlate with the effect of these same salts in compromising coating performance. Osmotic blistering of coatings is independent of the species of salt, and is
44、 entirely dependent on the number of ions in solution.9For example, the solubilities in 100 mL of ambient water at 20 oC (68 oF) are: 87.6 g for sodium nitrate, 35.9 g for sodium chloride, and 19.5 g for sodium sulfate.10Despite the dissociation of sodium sulfate into three ions, and sodium nitrate
45、and sodium chloride into only two ions each, the lesser solubility of sodium sulfate results in less ions in solution. Therefore, of these salts, nitrates are expected to be most destructive with regard to blistering of a coating, even though they typically are the least corrosive salt to a steel su
46、rface at low concentrations. To determine allowable levels of salts, users typically refer to the coating manufacturers product data sheet, or contact the coating manufacturer. Frequently, the allowable level of any salt is relative to the expected service life for the specified coating system in a
47、particular environment. Contamination levels are most commonly measured in micrograms per square centimeter (g/cm2) or milligrams per square meter (mg/m2). Risk Assessment The approach to risk assessment varies depending on a number of factors, not the least of which is the owner/specifiers attitude
48、s and concerns regarding the potential consequence of coating over a salt-contaminated surface. In some service environments, salts may not be a problem. There may be no evidence of salt-induced corrosion or salt contamination. In general, coating systems in atmospheric exposures are more tolerant t
49、o soluble salt contamination that those in immersion service. In environments where salt-induced corrosion is seen, or where salt contamination is suspected, salts may be present. In these environments, testing is usually performed to ensure that unacceptable levels of soluble salt contamination are not present before any coating operations are performed. To reduce risk, some owners and specifiers desire to minimize any salt contamination prior to coating. Other owners and specifiers attempt to assess the risk associated with the presence of so
