1、SSPC-TU 3May 1, 1997Editorial Revisions November 1, 200410-511. Scope1.1 This technology update discusses the risks associated with the maintenance painting practice known as overcoating. Factors affecting overcoating application, service and costs are discussed. 1.2 This document is intended to ser
2、ve as a resource for facility owners and others charged with developing and implementing maintenance painting programs.1.3 Overcoating is one of several maintenance painting options. This document is not intended to provide a detailed description or comparison of the relative merit and cost consider
3、-ations of overcoating versus other maintenance painting options. For a more complete and detailed discussion of maintenance painting practices, the reader should refer to SSPC-PA Guide 5, Guide to Maintenance Painting Programs.2. Description and Defi nitions2.1 DESCRIPTION2.1.1 This document contai
4、ns discussions of the risks associated with overcoating, methods of assessing risk, and means by which risks may be managed and reduced. 2.1.2 Overcoating is generally defi ned as the practice of painting over an existing coating as a means of extending its useful service life. Overcoating may be a
5、cost-effective alter-native to complete coating removal and repainting. When the old coating contains lead, cadmium, or chromium, overcoating may be a particularly attractive option due to economic consid-erations. Overcoating presents certain risks as well (see risk defi nition below).2.2 DEFINITIO
6、NSFor the purposes of this document, the following defi ni-tions will be used:Coating stress: The tension that a coating has, which is capable of being imparted to the steel substrate or other coating.Embrittled coating: Coating that has degraded to a fri-able condition but still has enough elastici
7、ty to adhere to the substrate or existing coating.Flaking: The detachment of small pieces of the coating fi lm, usually preceded by cracking, checking or blistering.Loose coating: Coating that has delaminated and disbond-ed from the substrate or other coats, but has not fallen off.Marginally adheren
8、t coating: A coating that exhibits tape adhesion of 2A or less (per ASTM D 3359), such that the overcoating risk is moderate or high.Overcoating: Application of coating materials over an existing coating in order to extend its service life, including use of the appropriate cleaning methods. The proc
9、edure includes preparation of rusted or degraded areas, feathering edges of existing paint, low-pressure water washing of the entire structure to remove contaminants, application of a full intermediate coat over repaired areas, and optional application of a full topcoat over the entire structure. Ov
10、ercoating may be a cost effective alternative to complete coating removal and repainting. When the old coating contains lead, cadmium, or chromium, over-coating may be a particularly attractive option due to economic considerations. Overcoating presents certain risks as well.Repaint: Complete remova
11、l of the existing coating system followed by application of a new coating system (including appropriate cleaning methods.)Risk: As used herein, “risk” refers to the chance that the overcoated system (old paint plus newly applied overcoat) will either fail catastrophically (e.g., delamination of the
12、system) or will not provide the desired period of protection (e.g., early rust back).Spot repair: A procedure entailing surface cleaning of isolated corrosion or paint breakdown areas using appropriate cleaning methods, and subsequent coating of these areas.Zone painting: A procedure entailing surfa
13、ce preparation using appropriate cleaning methods and painting of a defi ned area of a structure. Zone painting may involve (a) many spot repairs within a defi ned area or (b) removal of all coating in a defi ned area, followed by application of a new coating system to that area.3. Discussion3.1 RIS
14、KS ASSOCIATED WITH OVERCOATING3.1.1 Delamination: A primary risk associated with over-coating is that the overcoating system could cause delamination. If a delamination failure occurs, the overcoating investment is lost. Delamination is diffi cult to predict; however, an understand-ing of the underl
15、ying principles will help the coatings engineer SSPC: The Society for Protective CoatingsTECHNOLOGY UPDATE NO. 3OvercoatingSSPC-TU 3May 1, 1997Editorial Revisions November 1, 200410-52reduce the chance of a delamination failure.Delamination is primarily the result of internal stresses in the overcoa
16、t material being transferred to underlying or exist-ing coating layers. Internal stress occurs as the applied paint shrinks. Several factors affect the degree of internal stress in the overcoat material, including the type of coating, the formulation, the fi lm-forming conditions, the temperature an
17、d the coatings age and thickness. A good example of an increased internal stress is the oxidative curing of alkyds. Temperature fl uctuations may also affect the level of internal stress. Brittle coatings are more apt to crack during temperature changes. The application of an overcoat may also affec
18、t the internal stress of the existing coating because the stress present in the overcoat is transmitted to the existing coating.The internal stress of the overcoat is counteracted by its adhesion to the existing coating. A loss of adhesion of the existing paint system at either the steel/coating int
19、erface or within the layers of the existing coating may result in cracking of the overcoat.Good overcoating systems should be designed so that there is higher tensile strength and rigidity in the existing or original coating than in the overcoat.3.1.2 Early Rust Back or Poor Coating Performance: Ano
20、ther primary risk involved in overcoating is that the system will not provide an adequate period of service. The overcoat may not experience a catastrophic failure, such as delamination, but nonetheless may fail prematurely because of the severity of the service environment. This type of degradation
21、 may be manifested by pinpoint rust, undercutting at small breaks in the coating system, or blistering. The amount and type of surface preparation used prior to applying the overcoat can also affect the degree of protection afforded by the overcoat material.3.2 FACTORS AFFECTING RISK IN OVERCOATING3
22、.2.1 Infl uential Factors: The risk of delamination or other coating failure described in 3.1 is infl uenced by the condition of the existing coating, substrate factors, compatibility of new and old system, the type of structure and the exposure environment, etc.3.2.2 Condition of Existing Coating3.
23、2.2.1 Existing Conditions: Visual and physical inspections, patch testing, and previous experience with similar systems for the expected exposure and conditions are proven tools in assessing the risk.3.2.2.2 Existing Coating System Type (Oil, Alkyd, Vi-nyl, Epoxy, Urethane): It is important to be ab
24、le to determine if multiple coating system types exist on the structure and to identify them in order to determine basic chemical composition, so that the proper overcoat system can be selected and special hazardous conditions can be identifi ed.3.2.2.3 Thickness: Thicker, aged coatings tend to be m
25、ore highly stressed. Strong peeling forces can be generated during curing and aging of the overcoat. When overcoated, thicker, more highly stressed coatings are more likely to delaminate than thinner coatings with lower internal stress. Delamination may also be caused by thermal cycling that may dis
26、rupt the integ-rity of thick, aged coatings that have been overcoated. Rapid thermal cycling may accelerate system deterioration. Thicker, more highly stressed coatings are also more likely to sustain damage from blast media or other mechanical processes. This often results in a subsequent loss of a
27、dhesion that may affect the performance of the overcoat system.3.2.2.4 Number of Coating Layers: Many layers of paint increase the chance of poor intercoat adhesion and may lead to delamination.3.2.2.5 Coating Age: Depending on the curing mechanism, certain coatings tend to embrittle more with age t
28、han others. Alkyds are particularly susceptible to embrittlement with age. 3.2.2.6 Chalking and Erosion: Epoxy and alkyd coat-ings may chalk and erode with prolonged exposure. Generally this does not present a problem for overcoating as long as the loose chalk is removed prior to painting. Even seve
29、rely eroded coatings with exposed primer may be good candidates for overcoating, provided the remaining coating has good adhesion and rusting is nominal.3.2.2.7 Delaminated Paint Films: Paint fi lms that exhibit delamination or other undesirable characteristics, such as crack-ing, are not good candi
30、dates for overcoating.3.2.2.8 Coating Brittleness: Embrittled coatings tend to crack, providing sites for stress-induced peeling.3.2.2.9 Coating Adhesion: The adhesion of the existing coating to itself and to the substrate is a critical factor. However, it is diffi cult to precisely defi ne a satisf
31、actory adhesion value. At present, adhesion is generally evaluated by either ASTM D 3359 or ASTM D 4541. Systems exhibiting low adhesion values in these tests are more likely to delaminate when overcoated than are aged coatings with higher adhesion values. Generally, the aged coating system will fai
32、l at its weakest point. Coating type, age, thickness, and surface preparation all affect the adhesion of the aged coating system.3.2.2 Substrate Factors and Corrosion Pattern: The condition and type of the substrate under the existing coating system must be determined. Mill scale, because it is smoo
33、th and slick, generally presents the weakest point of adhesion of the coating, even if the mill scale itself is tightly adherent to the steel. Filiform rust or undercutting could continue beneath the fi lm unless the source is removed. The condition of the substrate may affect the performance of the
34、 overcoat system. Generally, the more corrosion present, the higher the degree SSPC-TU 3May 1, 1997Editorial Revisions November 1, 200410-53of surface preparation required. This may cause localized problems on structures that were not cleaned uniformly prior to receiving the original coating. Locali
35、zed rusted areas may dictate a different strategy than would spot rust over the entire surface area. There is a point at which it may no longer be cost-effective to overcoat. 3.2.3.1 Surface Preparation: The performance of the system is infl uenced by surface preparation prior to initial coat-ing ap
36、plication. A surface that was previously blast cleaned is more likely to have satisfactory adhesion values, and is gener-ally a better overcoat candidate, than a surface with existing mill scale.3.2.3.2 Surface Contaminants: Surface contaminants, such as chloride and sulfates, can lead to decreased
37、coating life and vastly accelerated corrosion, while grease and oil can result in poor wetting and adhesion of the overcoating system. Coatings differ widely in their ability to protect under these condi-tions. The problems associated with surface contaminants are not necessarily specifi c to overco
38、ating; however, contaminants are less likely to be removed during overcoating because typically much less surface preparation is done. Less surface preparation holds down costs and reduces environmental and worker exposures to hazardous dusts. Extensive surface preparation is also more likely to cau
39、se mechanical damage to an old, marginally adherent, embrittled coating that may later delaminate.3.2.3.3. Pitting: Pits often contain active corrosion cells (due to chlorides or sulfates) which can cause early failure of overcoat materials.3.2.4 Coating Compatibility: Patch testing is a good method
40、 of determining whether the new coating is compatible with the existing one. The test should be performed so that the worst-case exposure to the patch is achieved. A method for patch testing is described in SSPC-PA Guide 5 and ASTM D 5064. (See Appendix A.1.2 for an example utilizing a test patch.)3
41、.2.5 Type of Structure3.2.5.1 Confi guration of Surfaces: Wide planar areas may delaminate fi rst, particularly if the coating is applied over mill scale.3.2.5.2 Flexing: The rigidity of the coated surface affects how the internal stresses in the coating are translated to interfaces. Flexible beams
42、and wide planar areas tend to contribute more stress to a coating system than more angular, smaller planar areas.3.2.6 Exposure Environment: The coating selected must be able to withstand the environmental conditions to which it will be exposed, as well as the surface conditions over which it is app
43、lied. Rapid thermal cycles tend to stress aged coatings, causing delamination at the weaker interfaces.3.3 APPLICATION, SERVICE AND COST CONSIDER-ATIONS: The items below should be considered when determin-ing whether overcoating is the most appropriate maintenance strategy for a particular situation
44、. 3.3.1 Application Considerations3.3.1.1 Limitations on Surface Preparation Methods: In some locations, because of noise or emission considerations, some methods of surface preparation cannot be used. As a result, complete removal and replacement of existing coating may not be an option. The prefer
45、red treatment would then be a limited surface preparation and overcoating. 3.3.1.2 Limitations on Application Methods: Some environmental or local restrictions prohibit certain application methods or coating products. It must be determined if the per-missible application methods (e.g., brushing and
46、rolling) are suitable for the overcoating product selected or considered. 3.3.1.3 Overcoatability of the Coating: Certain existing coatings can only be overcoated after major surface preparation such as scarifying the surface by sweep blasting or power tool cleaning. This situation may reduce or eli
47、minate the economic and environmental advantages of the overcoat strategy.3.3.2 Service Considerations3.3.2.1 Expected Remaining Service Life of the Structure: The specifi er should determine the remaining service life (in years) of the structure. The cost of overcoating (including surface preparati
48、on, containment, materials, application, etc.), may not be justifi ed for a structure to be replaced or decom-missioned in a short time.3.3.2.2 Expected Life of the Overcoat System (in years): The expected service life of the overcoat system is also a criti-cal factor. Unfortunately there is a high
49、degree of uncertainty in these projections. This uncertainty can be conveyed by assigning a range of years for the overcoating system lifetime (e.g., 4-12 years).3.3.2.3 Risk Threshold of Failure: The specifi er must recognize that there is some chance of a catastrophic or premature failure of an overcoat system. Overcoating may not be a viable option if the risk of a coating failure cannot be tolerated. While the decision to use an overcoating strategy is independent of the lead paint issue (lead may not be present), the presence of lead makes all the options more costly. If the overcoat cau
