CoatingsPro Magazine

JAN 2013

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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lefT Steel pieces are shown above the hot caustic tank. Dipping the steel into the tank with hot caustic removes oil, grease and dirt. 10% Fe in the delta layer, 94%Zn and 6% Fe in the zeta layer, and 100% zinc in the outermost eta layer). The 100% Zn eta layer is highly reactive. Therefore, immediately after the steel is removed from the kettle, the eta layer begins to react with oxygen and moisture in the surrounding air — and with any chemicals present in the environment. As a result of these reactions, zinc oxide (ZnO) and zinc hydroxide (Zn(OH) 2) particulates begin forming on the surface of the eta layer, and the pure zinc is transformed. These particulates are highly water-soluble and are not tightly adhered to the surface. Therefore, applying a duplex coating over the HDG when it is in this state results in many premature coating failures. In contrast, after uncoated HDG is exposed to the environment — and numerous wet-dry cycles combined with air movement across the surface for approximately six to twenty-four months — it eventually develops a complex, zinc carbonate film, ZnCO.3Zn(OH)2, a stable, inert, and highly corrosion resistant state. The length of time required to attain this state is dependent upon the particular environment to which it is exposed. Many premature coating failures occur when a duplex coating is applied to HDG during the time it is in the unstable zinc hydroxide state. This occurs when moisture (as a vapor) permeates through the coating film after the coating cures. This moisture continues to react with the zinc underneath the coating. Since the HDG surface under the coating receives moisture without sufficient carbon 82 Coatingspro g January 2013 dioxide, it cannot attain a stable zinc carbonate state and eventually the zinc is entirely consumed. The visible appearance of this phenomenon is evident when the coating is lifted off the surface as voluminous amounts of a white powdery material (zinc salts) form on the underlying HDH surface. Once the zinc is totally consumed, rust begins to form. Best tiMe AnD plACe to Apply DUplex systeM The best time to apply a duplex coating system is as soon as possible after the article is removed from the kettle and cooled, when the outermost layer is in the pure zinc state (Zn), or after several months or years when it reaches the complex zinc carbonate state, ZnCO.3Zn(OH) 2. However, applying a duplex coating when the HDG is in either of these states presents several challenges. Again, this article focuses on challenges associated with applying the duplex coating in a HDG shop as opposed to continuous sheet galvanizing. Applying a coating when the HDG is in the pure zinc state generally requires the galvanizing shop to apply it. To accomplish this successfully, without performing additional surface preparation, requires a properly climate-controlled area to place the HDG immediately after it is removed from the kettle. Such areas are mandated in most parts of the world. The shop must maintain strict environmental climatecontrols in this area (such as keeping the relative humidity less than 40% and sustaining required air and surface temperatures) until the coating system is cured. Failure to maintain a climatecontrolled environment results in zinc oxide and zinc hydroxide formation on the surface. If the specification requires duplex coating application in the shop and the HDG facility is not climate controlled, the shop may perform some method of surface preparation in an attempt to remove zinc oxides and zinc hydroxides that form on the surface. However, in this author's experience, this approach often produces duplex coating failures for a number of reasons including, but not limited to, the three discussed below. First, even if the shop applies the coating in a "controlled" area, the HDG is often prepared in an "uncontrolled" area of the plant, and often remains exposed in unsuitable conditions for an extended time before the coating is applied. During one failure investigation involving a duplex coating system, the author reviewed records indicating the relative humidity readings exceeded 90% at all times when the shop performed surface preparation. This resulted in rapid formation of new zinc oxides and zinc hydroxides on the highly reactive HDG surface—to which the shop applied the duplex coating system. Eventually, an expensive coating failure developed in the field. Second, during the surface preparation process (e.g. abrasive blasting or power tool cleaning) occurs in the below Steel exiting caustic tank shows importance of designing drain holes when tubular pieces receive HDG.

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