Preparing Hot Dip Galvanizing for Painting – It’s not Overly Complicated
热浸镀锌层涂装前表面处理-这并不太复杂
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Bare galvanizing provides excellent corrosion protection at a pH between 7 and 13, but corrosion of the zinc increases as the pH becomes more acidic (less than 7), or more alkaline (greater than 13). When galvanizing is exposed to the lower or higher pH ranges, it should be painted. Paint should also be applied when aesthetics is important, regardless of pH. Unfortunately, when galvanizing is painted, paint failures can occur (Photo 1), but they are easy to prevent through proper surface preparation.
在 ph 值为7和13的情况下, 仅凭镀锌就可以提供优异的腐蚀保护, 但锌的腐蚀会随着 ph 值变酸性 (小于 7) 或碱性 (大于 13) 而提高。 当镀锌层暴露在更低或更高的 pH 值范围内时, 应当进行涂装。 当外观比较重要时, 无论 pH 值如何, 也应该涂装。 不幸的是, 对镀锌表面涂装时, 很可能发生涂层失效 (照片 1), 但通过适当的表面处理这很容易防止。
Photo 1 – Typical failure of paint to galvanizing due to lack of surface preparation.
照片 1-由于缺乏表面处理造成的典型涂层失效。
Surface preparation is the key to assuring successful coating adhesion and performance. The extent of preparation required depends on the answers to these four questions:
Was the galvanizing passivated?
Is the galvanizing new?
Is the galvanizing partially weathered?
Is the galvanizing aged?
表面处理是保证涂层附着力和性能的关键。所需处理的程度取决于对以下四个问题的回答:
镀锌层钝化处理了吗？
镀锌层是新的吗？
镀锌层是否部分风化？
镀锌层是否老化？
Passivating Treatments – Passivation is a common practice used at the time of galvanizing to reduce the formation of white storage stain. Unfortunately, passivating treatments applied to galvanizing (especially chromate treatments) can interfere with the adhesion of coatings. If the galvanized steel is going to be painted, the galvanizer should be notified in advance, so that the passivation step can be eliminated. Frequently, however, parts have already been galvanized and the user does not know whether a treatment was applied. When in doubt, the surface should be tested. A suitable procedure is provided in SSPC-SP16, Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals.” Section 4.3.3 of the standard offers the following:
钝化处理 – 钝化是镀锌时常用的一种方法，以减少镀锌件储存时白锈印迹的形成。不幸的是，对镀锌件的钝化处理(特别是铬酸盐处理)会影响涂层的附着力。如果镀锌件要涂装，应提前通知镀锌人员，以便取消钝化步骤。然而，实际情况通常是部件已经镀锌完成但用户不知道是否进行了钝化处理。如有疑问，应对表面进行测试。SSPC - SP16中提供了一个合适的程序，即“涂装和未涂装镀锌件、不锈钢和有色金属表面的扫砂处理标准。” 该标准第4.3.3节规定如下：
The presence of chromates or other passivating treatments is detected by using a solution of copper sulfate, with the following procedure:
1. Ensure that surfaces are free of any visible oxidation or oxidation by-products
2. Prepare the solution by dissolving 2 grams of copper sulfate crystals in 100 ml of deionized water.
3. Mark off three adjacent areas on the galvanized part, approximately 6.45 cm2 (1 inch2)
4. Leave one area untouched, solvent wash the second and third areas, and also thoroughly sand the third area using emery paper.
5. Using an eyedropper or pipette, saturate a cotton swab with the copper sulfate solution and apply to all three areas, or apply the solution directly to the three areas.
是否使用过铬酸盐或其它钝化处理可以通过硫酸铜溶液按照以下步骤检测:
1. 确保表面没有任何可见的氧化或氧化副产物
2. 通过将2g硫酸铜晶体溶解在100ml去离子水中制备溶液。
3. 在镀锌件表面标记3个相邻的区域，每个面积约为6.45平方厘米（1平方英寸）
4. 保留第1个区域不做任何处理，用溶剂清洗第2和第3个区域，并使用砂纸彻底打磨第3个区域
5. 用滴液管或移液管将硫酸铜溶液浸透一只棉签，涂于所有的3个区域，或直接将溶液滴在3个区域中
If all three areas turn black immediately, there is no passivation on the surface. If the first area does not turn black within 10 seconds and the second and third areas turn black immediately, there is no passivation on the surface with the possible exception of light oil. If the first and second areas do not turn black within 10 seconds and the third area turns immediately, a passivator of some type is present.
If it is present, mechanical cleaning is typically required to remove it, although some of the etching/cleaning solutions discussed below suggest that they will also remove some treatments. Before using them, it is recommended that the etching cleaner be applied to discrete areas followed by the testing described above to determine if the treatment is removed. If the project schedule permits, weathering of 12 to 18 months prior to painting, should also eliminate any treatment that may have been present.
如果3个区域均立刻变黑，则表面没有钝化处理。如果第1个区域没有在10秒内变黑，而第2和第3区域立即变黑，则表面没有钝化处理，但可能有轻微的油渍存在。如果第1和第2区域没有在10秒内变黑，而第3区域立即变黑，则表面有过钝化处理。
如果检测有过钝化处理，则通常需要机械工具来清理，尽管后面提到的一些蚀刻/清洗液也能去除某些钝化层。如果使用蚀刻清洗液，建议先在小块区域试验，随后进行上述测试，以确定是否去除了钝化层。如果项目进度允许，涂装前风化了12至18个月的构件也应去除可能存在的钝化层。
New and Partially Weathered Galvanizing – If the age of the galvanizing is less than approximately 48 hours, it may be possible to coat it as is, but if it is weathered between a couple days and approximately one year (termed partially weathered), surface preparation will be required. Two methods that are commonly used to prepare the galvanizing are cleaning/etching with a phosphoric acid/detergent blend and abrasive blast cleaning.
Etching solutions are used to clean, dull, and roughen the surface for proper coating adhesion. The solution is applied to the surface, typically scrubbed with a synthetic abrasive pad, thoroughly rinsed, and force dried. Examples of solutions can be found in the Master Painters Institute (MPI) qualified products list #25, Cleaner, Etching, for Galvanized Metal (http://www.specifypaint.com/APL/paintinfo_APL_new/MpiNumber.asp?ID=25000).
新的和部分风化的镀锌层 – 如果镀锌时间小于约48小时，可以直接涂装，但如果镀锌层经过在几天至约一年之间的风化(称为部分风化)，则需要进行表面处理。通常用于镀锌件的两种表面处理方法是喷砂清理和用磷酸/洗涤剂混合物进行清理/蚀刻。
蚀刻清洗液用于表面清理、表面暗化和粗糙化以获得适当的涂层附着力。将溶液施加到表面上，通常用合成研磨片研磨，彻底冲洗，并烘干干燥。溶液种类可在“Master Painters Institute ( MPI )”认证产品列表#25“镀锌金属的蚀刻清洗液”中找到。(http://www.specifypaint.com/APL/paintinfo_APL_new/MpiNumber.asp?ID=25000)
Abrasive blast cleaning is performed according to SSPC-SP16. Excerpts from a few paragraphs of the standard describing the cleaning of bare, uncoated metal (galvanizing) are repeated below. The sentences that were redacted address the criteria for any paint remaining on the surface, which is not applicable to this article.
2.1 A brush-off blast cleaned non-ferrous metal surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, metal oxides (corrosion products), and other foreign matter… Bare metal substrates shall have a minimum profile of 19 micrometers (0.75 mil).
2.1.1 The entire surface shall be subjected to the abrasive blast to achieve the specified degree of cleaning and to produce a dense and uniform surface profile on the bare metal substrate. The peaks and valleys on the surface shall form a continuous pattern, leaving no smooth, unprofiled areas…
2.1.2 …If the surface profile is not specified in the procurement documents, the abrasive selected shall roughen the cleaned surface to the degree required by the product data sheet for the coating to be applied.
喷砂清理根据SSPC - SP16进行。以下是标准中描述清理裸露未涂装金属(已镀锌)的几段摘录。节略的部分是关于表面残留油漆的标准，不适用于本文因此省略。
2.1 在不放大的情况下观察时，扫砂清理的有色金属表面应无任何可见油、油脂、污垢、灰尘、金属氧化物(腐蚀产物)和其他异物……裸露金属底材的表面粗糙度应大于19微米( 0.75密耳)。
2.1.1 应对整个表面进行喷砂处理，以达到规定的表面清洁度，并在裸露金属底材上形成致密且均匀的表面粗糙度。表面的波峰和波谷应形成连续的纹路，不留下平滑、无粗糙度的区域…
2.1.2 …如果采购文件中未规定表面粗糙度，则所选磨料应使清理后的表面粗糙度符合所使用涂料产品说明书的要求。
Paragraph 4.3.1 of the standard requires the removal of wet storage stain by means other than blast cleaning since the “dwell time” required to remove it by blast cleaning can damage the galvanizing. Other methods are addressed in a non-mandatory Appendix. Appendix Section A3 indicates that salts from wet storage stain can be removed with a nylon brush and water, or a dilute solution of acetic or citric acid, lime juice, or white vinegar, followed by rinsing with a large amount of water.
Appendix Section A8.1 indicates that to reduce the risk of damage, blast cleaning is done with relatively low nozzle pressures (A8.2) and a softer abrasive with a Mohs hardness of 5 or less (A8.1.1.).
Appendix Section A9 recommends that the thickness of the galvanizing be measured before blast cleaning and again after blast cleaning to confirm that it meets the specified thickness. The thickness requirements may be found in the procurement documents, or as required in ASTM A123, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, or ASTM A153/153M, Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware.
The Appendix (A9.2) cautions that in the presence of moisture, zinc oxides can form quickly on the surface after preparation, so it should remain dry, and painted as soon as possible. The same recommendations for a minimum 5°F (3°C) spread between surface temperature and dew point temperature when preparing steel (steel temperature being the higher of the two), also apply to galvanizing.
该标准第4.3.1段要求通过喷砂清理以外的方式清除储存时产生的白锈印迹，因为喷砂清理白锈印迹所需的“停留时间”可能会损坏镀锌层。其他方法在非强制性附录中讨论。附录部分A3指出，储存时产生的白锈印迹可以用尼龙刷和水，或乙酸或柠檬酸、柠檬汁或白醋的稀溶液除去，然后用大量水冲洗。
附录部分A8.1指出，为了降低锌层破坏风险，喷砂清理采用相对较低的喷嘴压力( A8.2 )和莫氏硬度为5或更低的较软磨料( A8.1.1 )。
附录A9建议在喷砂清理前和喷砂清理后测量镀锌层厚度，以确认其符合规定膜厚。厚度要求见采购文件，或ASTM A123《钢铁制品镀锌(热浸镀锌)标准规范》或ASTM A153/153M《钢铁五金件镀锌(热浸镀锌)规范》。
附录( A9.2 )警告称，在潮湿条件下，喷砂清理后会在表面会迅速形成锌氧化物，因此应保持干燥，并尽快涂装。与钢结构表面处理相同，镀锌层表面处理时同样建议表面温度超出露点温度至少5°F ( 3°C )。
Aged Galvanizing – When galvanizing has weathered for 12 to 18 months, the surface will have oxidized and can often be prepared for painting by pressure washing alone. The key is to remove surface dirt and debris as well as zinc salts and confirm that the surface is dull (photo 2).
镀锌层老化 – 当镀锌层已经风化12至18个月时，表面将被氧化，并且通常只需要高压清洗处理就可以准备涂装。关键是清除表面污物和锌渣以及锌盐，并确认表面暗化(图2 )。
Photo 2 – Exterior surfaces of the galvanizing have weathered and can likely be painted after pressure washing to remove the salts, but note the lack of oxidation and the visible spangle in the in the protected area (red arrow). A test patch should be applied to determine if the adhesion is adequate without roughening and dulling the surface.
图2– 镀锌层的外表面已经风化，可以在高压清洗除去盐分后进行涂装。但请注意保护区(红色箭头)内氧化物不多，有可见亮色的金属锌，应在该区域小范围涂漆测试，以确定未粗糙暗化的表面涂料附着力是否足够。
Depending on the condition of the steel, 3,000 psi with a rotating tip may be adequate, but it’s best to apply a test patch of the coating after cleaning and check the adhesion to make certain. If there is any doubt regarding the adhesion of the coating, either of the methods described above for partially weathered galvanizing (etching cleaner or SP-16 Brush-Off Blast) should be used.
根据钢材（表面高压冲水）的经验，旋转喷嘴的压力在3000psi（约20MPa）应该足够，但最好在锌层表面高压冲水后小范围涂漆测试，并检查附着力以确保合格。如果对涂层的附着力有任何疑问，则应使用上述部分风化后镀锌层的任一表面处理方法(蚀刻清洗液或SP-16扫砂)。
Conclusion – Most coating failures on galvanizing are the result of poor adhesion of the coating to the galvanizing. Poor adhesion is typically caused by a lack of roughening of the smooth galvanizing, an abundance of zinc salts beneath the coating film, or the presence of a passivating treatment that inhibits adhesion of the coating. When properly prepared, coatings will adhere well to galvanizing and improve the aesthetics and corrosion protection afforded by the galvanizing. This article described a few common methods or preparing galvanizing for painting. Other methods are described in:
ASTM D6386, Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting
ASTM D7396, Standard Guide for Preparation of New, Continuous Zinc-Coated (Galvanized) Steel Surfaces for Painting
结论 – 镀锌过程中的大多数涂层失效是由于涂层与镀锌层的附着力差造成的。附着力差通常是由于光滑镀锌层的粗糙度不足、涂层下面锌盐的大量存在或抑制涂层粘附的钝化处理而引起的。经过适当的涂装前表面处理，涂层将很好地粘附到镀锌层上，并改善镀锌层的美观性和防腐性。本文介绍了几种常用的涂装前镀锌层表面处理方法。其他方法记录于如下标准:
ASTM D6386, 镀锌(热浸镀锌)钢铁制品和五金件涂装前表面处理的标准操作规程
ASTM D7396, 新的连续镀锌钢材涂装前表面处理的标准指南
原文作者介绍：
Ken Trimber is the President of KTA-Tator, Inc. He is Chairman of the SSPC Surface Preparation Committee and the SSPC Visual Standards Committee, as well as a member of the SSPC Standards Review Committee and ASTM DOl responsible for developing, reviewing and approving standards like the ones described in this article. He can be reached at [email protected]
ken Trimber，KTA-Tator公司的总裁，SSPC表面处理委员会和SSPC目视标准委员会主席，SSPC标准审查委员会和ASTM DOl成员，负责制定、审查和批准类似文中所述的标准。作者邮箱地址 [email protected]