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Preparing Hot Dip Galvanizing for Painting – It’s not Overly Complicated

熱浸鍍鋅層涂裝前表面處理-這并不太復雜

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文章來自防腐涂裝良心網站KTAuniversity,原文鏈接 請點這里

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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.

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在 ph 值為7和13的情況下, 僅憑鍍鋅就可以提供優異的腐蝕保護, 但鋅的腐蝕會隨著 ph 值變酸性 (小于 7) 或堿性 (大于 13) 而提高。 當鍍鋅層暴露在更低或更高的 pH 值范圍內時, 應當進行涂裝。 當外觀比較重要時, 無論 pH 值如何, 也應該涂裝。 不幸的是, 對鍍鋅表面涂裝時, 很可能發生涂層失效 (照片 1), 但通過適當的表面處理這很容易防止。

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Photo 1 – Typical failure of paint to galvanizing due to lack of surface preparation.

照片 1-由于缺乏表面處理造成的典型涂層失效。

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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?

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表面處理是保證涂層附著力和性能的關鍵。所需處理的程度取決于對以下四個問題的回答:

鍍鋅層鈍化處理了嗎?

鍍鋅層是新的嗎?

鍍鋅層是否部分風化?

鍍鋅層是否老化?

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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:

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鈍化處理 – 鈍化是鍍鋅時常用的一種方法,以減少鍍鋅件儲存時白銹印跡的形成。不幸的是,對鍍鋅件的鈍化處理(特別是鉻酸鹽處理)會影響涂層的附著力。如果鍍鋅件要涂裝,應提前通知鍍鋅人員,以便取消鈍化步驟。然而,實際情況通常是部件已經鍍鋅完成但用戶不知道是否進行了鈍化處理。如有疑問,應對表面進行測試。SSPC - SP16中提供了一個合適的程序,即“涂裝和未涂裝鍍鋅件、不銹鋼和有色金屬表面的掃砂處理標準。” 該標準第4.3.3節規定如下:

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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.

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是否使用過鉻酸鹽或其它鈍化處理可以通過硫酸銅溶液按照以下步驟檢測:

1. 確保表面沒有任何可見的氧化或氧化副產物

2. 通過將2g硫酸銅晶體溶解在100ml去離子水中制備溶液。

3. 在鍍鋅件表面標記3個相鄰的區域,每個面積約為6.45平方厘米(1平方英寸)

4. 保留第1個區域不做任何處理,用溶劑清洗第2和第3個區域,并使用砂紙徹底打磨第3個區域

5. 用滴液管或移液管將硫酸銅溶液浸透一只棉簽,涂于所有的3個區域,或直接將溶液滴在3個區域中

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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.

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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.

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如果3個區域均立刻變黑,則表面沒有鈍化處理。如果第1個區域沒有在10秒內變黑,而第2和第3區域立即變黑,則表面沒有鈍化處理,但可能有輕微的油漬存在。如果第1和第2區域沒有在10秒內變黑,而第3區域立即變黑,則表面有過鈍化處理。

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如果檢測有過鈍化處理,則通常需要機械工具來清理,盡管后面提到的一些蝕刻/清洗液也能去除某些鈍化層。如果使用蝕刻清洗液,建議先在小塊區域試驗,隨后進行上述測試,以確定是否去除了鈍化層。如果項目進度允許,涂裝前風化了12至18個月的構件也應去除可能存在的鈍化層。

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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.

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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 ).

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新的和部分風化的鍍鋅層 –  如果鍍鋅時間小于約48小時,可以直接涂裝,但如果鍍鋅層經過在幾天至約一年之間的風化(稱為部分風化),則需要進行表面處理。通常用于鍍鋅件的兩種表面處理方法是噴砂清理和用磷酸/洗滌劑混合物進行清理/蝕刻。

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蝕刻清洗液用于表面清理、表面暗化和粗糙化以獲得適當的涂層附著力。將溶液施加到表面上,通常用合成研磨片研磨,徹底沖洗,并烘干干燥。溶液種類可在“Master Painters Institute ( MPI )”認證產品列表#25“鍍鋅金屬的蝕刻清洗液”中找到。(http://www.specifypaint.com/APL/paintinfo_APL_new/MpiNumber.asp?ID=25000 )

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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.

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噴砂清理根據SSPC - SP16進行。以下是標準中描述清理裸露未涂裝金屬(已鍍鋅)的幾段摘錄。節略的部分是關于表面殘留油漆的標準,不適用于本文因此省略。

2.1 在不放大的情況下觀察時,掃砂清理的有色金屬表面應無任何可見油、油脂、污垢、灰塵、金屬氧化物(腐蝕產物)和其他異物……裸露金屬底材的表面粗糙度應大于19微米( 0.75密耳)。

2.1.1 應對整個表面進行噴砂處理,以達到規定的表面清潔度,并在裸露金屬底材上形成致密且均勻的表面粗糙度。表面的波峰和波谷應形成連續的紋路,不留下平滑、無粗糙度的區域…

2.1.2…如果采購文件中未規定表面粗糙度,則所選磨料應使清理后的表面粗糙度符合所使用涂料產品說明書的要求。

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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.

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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.).

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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.

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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.

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該標準第4.3.1段要求通過噴砂清理以外的方式清除儲存時產生的白銹印跡,因為噴砂清理白銹印跡所需的“停留時間”可能會損壞鍍鋅層。其他方法在非強制性附錄中討論。附錄部分A3指出,儲存時產生的白銹印跡可以用尼龍刷和水,或乙酸或檸檬酸、檸檬汁或白醋的稀溶液除去,然后用大量水沖洗。

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附錄部分A8.1指出,為了降低鋅層破壞風險,噴砂清理采用相對較低的噴嘴壓力( A8.2 )和莫氏硬度為5或更低的較軟磨料( A8.1.1 )。

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附錄A9建議在噴砂清理前和噴砂清理后測量鍍鋅層厚度,以確認其符合規定膜厚。厚度要求見采購文件,或ASTM A123《鋼鐵制品鍍鋅(熱浸鍍鋅)標準規范》或ASTM A153/153M《鋼鐵五金件鍍鋅(熱浸鍍鋅)規范》。

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附錄( A9.2 )警告稱,在潮濕條件下,噴砂清理后會在表面會迅速形成鋅氧化物,因此應保持干燥,并盡快涂裝。與鋼結構表面處理相同,鍍鋅層表面處理時同樣建議表面溫度超出露點溫度至少5°F ( 3°C )。

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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).

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鍍鋅層老化 – 當鍍鋅層已經風化12至18個月時,表面將被氧化,并且通常只需要高壓清洗處理就可以準備涂裝。關鍵是清除表面污物和鋅渣以及鋅鹽,并確認表面暗化(圖2 )。

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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.

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圖2– 鍍鋅層的外表面已經風化,可以在高壓清洗除去鹽分后進行涂裝。但請注意保護區(紅色箭頭)內氧化物不多,有可見亮色的金屬鋅,應在該區域小范圍涂漆測試,以確定未粗糙暗化的表面涂料附著力是否足夠。

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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.

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根據鋼材(表面高壓沖水)的經驗,旋轉噴嘴的壓力在3000psi(約20MPa)應該足夠,但最好在鋅層表面高壓沖水后小范圍涂漆測試,并檢查附著力以確保合格。如果對涂層的附著力有任何疑問,則應使用上述部分風化后鍍鋅層的任一表面處理方法(蝕刻清洗液或SP-16掃砂)。

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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

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結論 – 鍍鋅過程中的大多數涂層失效是由于涂層與鍍鋅層的附著力差造成的。附著力差通常是由于光滑鍍鋅層的粗糙度不足、涂層下面鋅鹽的大量存在或抑制涂層粘附的鈍化處理而引起的。經過適當的涂裝前表面處理,涂層將很好地粘附到鍍鋅層上,并改善鍍鋅層的美觀性和防腐性。本文介紹了幾種常用的涂裝前鍍鋅層表面處理方法。其他方法記錄于如下標準:

ASTM D6386, 鍍鋅(熱浸鍍鋅)鋼鐵制品和五金件涂裝前表面處理的標準操作規程

ASTM D7396, 新的連續鍍鋅鋼材涂裝前表面處理的標準指南

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原文作者介紹

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公司的總裁,SSPC表面處理委員會和SSPC目視標準委員會主席,SSPC標準審查委員會和ASTM DOl成員,負責制定、審查和批準類似文中所述的標準。作者郵箱地址 [email protected]

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