1、SSPC-TU 1July 1, 1995Editorial Revisions November 1, 200410-1SSPC: The Society for Protective CoatingsTECHNOLOGY UPDATE NO. 1Surface Tolerant Coatings for SteelPainting under these conditions is not recommended, but in some cases may be unavoidable.SSPC members were surveyed as to the importance of
2、surface-tolerant coatings for various industries. Most industries indicated signifi cant use, with the highest importance noted for bridge, petrochemical, pulp and paper, and utility facilities. Respondents indicated that rust, existing coatings, and soluble salts were the three most frequent applic
3、ations of surface-toler-ant coatings.3.2 COMPROMISED STEEL SUBSTRATES: Surface-tolerant coatings are used on many types of compromised substrates including surfaces with soluble salt, petroleum product, and moisture deposits. Corrosion products and aged coatings are the most common forms of compromi
4、sed surfaces. Oftentimes multiple contaminants are present. Bridges may have salt deposits, as well as corrosion products, and an existing coating. Petroleum storage facilities may have rusty painted surfaces with oily deposits. Corroded surfaces in industrial at-mospheres may be contaminated with f
5、errous sulfate. Sulfate and chloride deposits are often the limiting factor when painting corroded steel. High concentrations of these contaminants will cause most coatings to fail (See Section 3.4).3.3 SURFACE-TOLERANT COATING TECHNOLOGIES: Surface-tolerant coatings may be subdivided into two broad
6、 categories: corrosion inhibitive and barrier coatings. Inhibitive coatings are primarily oil-based, alkyd, and water borne acrylic coatings. A wide variety of rust-inhibitive pigments are commer-cially available. Various mechanisms of corrosion inhibition are postulated for these pigments. Low visc
7、osity in these formula-tions improves the penetration and encapsulation of corrosion products. Corrosion-inhibitive surface-tolerant coatings are primarily used over existing coatings, corrosion products, and soluble salt contaminants. Some oil and alkyd coatings may be oil tolerant. The performance
8、 of inhibitive surface-tolerant coatings is enhanced by the addition of a barrier topcoat.Barrier coatings work by impeding the transmission of water vapor, oxygen, and ions to the substrate/coating inter-face.2,3Both the binder and the pigmentation are important in achieving good barrier properties
9、. Two-component epoxies and urethanes have excellent barrier properties when properly formulated. Lamellar pigments such as aluminum fl ake, mica, glass fl ake, and micaceous iron oxide can further improve the performance of surface tolerant coatings by enhancing their barrier properties. Two common
10、 surface-tolerant barrier-type coatings that use lamellar pigments are epoxy mastics and moisture-cure urethanes. 1. ScopeThis document defi nes surface-tolerant coatings and discusses their uses and limitations. It is widely recognized that the application of protective coatings over improperly pre
11、pared surfaces is poor painting practice. It is also recognized that proper surface preparation is not always possible or feasible and that the use of surface-tolerant coatings may offer an economical compromise. The types of coatings for specifi c end uses are dependent on the condition of the subs
12、trate. This technology update is for general information purposes only.2. Description and UseSurface-tolerant coatings may be defi ned as coatings for substrates that have been prepared for painting to a lesser degree of cleanliness than provided by SSPC-SP 6/NACE No. 3, “Commercial Blast Cleaning.”
13、1Surface-tolerant coat-ings exhibit a greater propensity toward satisfactory service performance than conventional coatings not intended for such applications.Section 3 is subdivided into narratives for surface-tolerant coating usage, compromised steel substrates, and surface-tolerant coating techno
14、logies and their limitations.3. Discussion3.1 SURFACE-TOLERANT COATING USAGE: Surface-tolerant coatings may be selected for many reasons. They are chosen for jobs where less-than-ideal surface cleanliness can be achieved, such as areas where abrasive blasting cannot be performed. Surface-tolerant co
15、atings may be specifi ed for overcoating existing aged coatings. In some instances, this has been done to cover and extend the service life of lead-based paint systems. Due to its geographical location and function, steel may become contaminated with deposits that are diffi cult to remove. For examp
16、le, deicing and marine salt deposits may remain on steel even after abrasive blast cleaning. Sometimes surface-tolerant coatings are specifi ed to reduce overall painting costs by lowering the surface prepara-tion requirements. Although the use of surface tolerant coat-ings over minimally prepared s
17、urfaces will generally reduce performance compared to thoroughly prepared and coated surfaces, proper selection and use of these systems may result in only modest performance reductions. Certain environmental conditions may suggest the need for a surface-tolerant coating. Moisture may be readily dep
18、osited on surfaces due to fog, cold wall condensation, and seepage. SSPC-TU 1July 1, 1995Editorial Revisions November 1, 200410-2Barrier coatings are most commonly used over rust, existing coatings, and soluble salt contaminants. Specialized barrier coatings are sometimes used on moist and oily surf
19、aces. Some epoxies are formulated such that small amounts of oil may be absorbed into the coating without seriously impairing performance.4Some urethane coatings may be especially tolerant of surface moisture.Other types of surface-tolerant coatings include penetrating primers, tape coatings, and ru
20、st convertors. Penetrating primers are typically unpigmented, low-viscosity, two-component epox-ies or urethanes that are used in conjunction with barrier-type topcoats. Tape coatings provide a barrier to moisture and oxygen migration and may also contain corrosion inhibitors. They are typically han
21、d applied with the most common application being corroded piping systems. Rust convertors typically consist of a conversion chemical such as tannic acid combined with an acid-tolerant binder such as a vinyl-type emulsion.5The acid component stabilizes the corrosion products and the binder forms a fi
22、 lm. Rust convertors are usually topcoated with a barrier coating.3.4 LIMITATIONS OF SURFACE-TOLERANT COATINGS: The performance limitations of surface-tolerant coatings are not always well recognized within the industry. Premature failures attributed to surface-tolerant coatings are more often the r
23、esult of unrealistic expectations. High levels of surface contaminants, incompatible or poorly adherent aged coatings, and severe exposure environments all contribute to early coating failure. State-of-the-art characteristics of surface-tolerant coatings indicate that most are compatible with specif
24、i c contaminants and are not necessarily tolerant to a combination of contaminants.Excessively high concentrations of soluble salts can result in premature coating failure. Research has shown that concen-trations greater than 500 mg/m2of chloride or 1000 mg/m2of sulfate are associated with blisterin
25、g and premature coating failure.5Maximum recommended permissible concentrations vary considerably for chloride (10-500 mg/m2) and sulfate (100-1000 mg/m2) contamination.2Coating performance will be improved by the use of water to remove salt contaminants. Waterjetting has been shown to remove iron s
26、ulfate more rapidly and completely than grit blasting.6Wet blasting techniques both with and without abrasive are more effi cient at removing soluble contaminants.Excessive amounts of water and oil are not easily displaced, absorbed, or transmitted through the fi lm by surface-tolerant coatings and
27、may cause premature coating failure. Some water-tolerant coatings displace water from the substrate and allow the water to pass through the fi lm and evaporate into the air. This is accomplished with hydrophilic solvents and polymers that can displace water effectively. Moisture on the surface or in
28、 the pores of the rust will react with the polyisocyanate in the coating. Excess moisture, however, can cause excessive carbon dioxide to be released, causing pinholing, bubbling, and foaming of the Higher fi lm thicknesses of the urethane fi lm may further aggravate this problem. Oil-tolerant coati
29、ngs are comprised of polymers, which are compatible with oil yet can produce a hard-wearing, resilient coating. Some coatings absorb the oil into the fi lm, forming a gel, while others absorb and pass the oil through the coating. Pooled and visible moisture should be wiped dry with clean rags to the
30、 greatest extent possible. Visible oil deposits should be absorbed or solvent cleaned to the extent practicable prior to painting.Grease deposits are perhaps the most diffi cult contaminant to paint. Generally, grease cannot be absorbed or displaced by the coating. Painting over grease will impair a
31、dhesion and fi lm integrity and should be avoided. Grease deposits should be removed by solvent cleaning prior to painting.Loose rust and pack rust (rust that has accumulated in joints or crevices) are incompatible with good performance. Rusted substrates should be hand tool cleaned to a tight rust
32、at a minimum. Surface-tolerant coatings should not be applied over loose rust or pack rust.Much has been written about the overcoating of existing coating systems. This is a particularly popular approach for delaying the removal of hazardous lead-based paints. Overcoat-ing is also employed as a mean
33、s of upgrading OEM coatings. The primary concern in using surface-tolerant coatings for these applications is coating compatibility. The existing coat-ing must not be lifted or excessively softened by the solvents in the surface-tolerant coating. Strong solvents found in some surface-tolerant coatin
34、gs such as epoxies and urethanes may cause the existing coating system to fail. In the case of aged coatings, this may be especially pronounced. Good intercoat adhesion between the surface-tolerant coating and the aged or OEM coating is necessary. Refer to ASTM D 5064, “Stan-dard Practice for Conduc
35、ting a Patch Test to Assess Coating Compatibility.” Surface preparation may also affect the perfor-mance of the overcoated system. Methods that may disrupt the adhesion of the existing coating to the substrate should be avoided. Brush-off blasting of existing aged coatings has been implicated in the
36、 premature failure of surface-tolerant overcoat systems.7Excessive shrinkage of the overcoat may also result in adhesive failure of the old coating. Proper coating selection and the application of test patches prior to overcoating will help prevent premature failures.Severe exposure environments may
37、 also contribute to the early failure of surface tolerant coatings.1Situations to avoid include high temperature, frequent or continuous immersion, and exposure to corrosive chemicals.The name “surface-tolerant coating” implies a certain level of comfort or forgiveness when these coatings are used.
38、While tolerant of relaxations from ordinarily rigorous surface cleanliness requirements, surface-tolerant coatings should be specifi ed with due diligence, as their use is dictated by surface conditions that are known to cause coating failures. While they are tolerant of surface conditions of which
39、ordinary coatings are not, surface-tolerant coatings also have their practical limits in use.urethane fi lm. SSPC-TU 1July 1, 1995Editorial Revisions November 1, 200410-34. Disclaimer4.1 This technology update is for information purposes only. It is neither a standard nor a recommended practice. Whi
40、le every precaution is taken to ensure that all information furnished in SSPC technology updates is as accurate, complete, and useful as possible, SSPC cannot assume responsibility nor incur any obligation resulting from the use of any materials, coatings, or methods specifi ed herein, or of the tec
41、hnology update itself.4.2 This technology update does not attempt to address problems concerning safety associated with its use. The user of this specifi cation, as well as the user of all products or prac-tices described herein, is responsible for instituting appropriate health and safety practices
42、 and for ensuring compliance with all governmental regulations.5. References1 Kapsanis, K. A., Appleman, B. A., “Myths and Realities of Surface-Tolerant Coatings for Bridges,” Journal of Protective Coatings and Linings, January, 1992.2Thomas, N. L., “The Protective Action of Coatings on Rusty Steel,
43、” Journal of Protective Coatings and Linings, De-cember, 1989.3Hare, C. H., “Barrier Coatings,” Journal of Protective Coatings and Linings, February,1989.4Masciale, M., “Coatings Compatible with Moist, Oily, Rusty Surfaces,” Journal of Protective Coatings and Linings, May, 1988.5Bagchi, D., “Coating
44、s for Rusted Steel,” Journal of Protective Coatings and Linings, September, 1991.6Morcillo, M., et al, “Some Observations on Painting Contaminated Rusty Steel,” Journal of Protective Coatings and Linings, September, 1987.7Kline, E., and Corbett, W. D., “Benefi cial Procrastina-tion: Delaying Lead Paint Removal Projects by Upgrading the Coating System,” Journal of Protective Coatings and Linings, March, 1992.
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