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Wet Storage Stain (White Rust)

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Wet Storage Stain (White Rust) Wet Storage Stain WET STORAGE STAIN galvanized coatings. (WHITE RUST) When this surface has access to freely moving air in normal atmospheric exposure, it reacts with rainfall Wet storage stain, also known as white rust, is the or dew to form a porous, gelatinous zinc hydroxide cor- voluminous white or gray deposit formed by accelerat- rosion product. During drying, this product reacts with ed corrosion of the zinc coating when closely-packed, carbon dioxide in the atmosphere and converts into a newly-galvanized articles are stored or shipped under thin, compact and tightly adherent layer of corrosion damp and poorly ventilated conditions. It is found most products consisting mainly of basic zinc carbonate gen- often on stacked and bundled items, such as galvanized erally written as 2ZnCO3·3Zn(OH)2. sheets, plates, angles, bars, and pipe. Weathered zinc The long life normally associated with galvanized surfaces which have already formed their normal pro- coatings in atmospheric service is entirely dependent tective layer of corrosion products are seldom attacked. upon the protection of the basic zinc carbonate. Being Due to their configuration, many products galvanized relatively insoluble, the basic zinc carbonate layer is after fabrication are less susceptible to wet storage stain- weather-resistant and, once formed, minimizes further ing than plain galvanized wire and sheet. corrosion. After a period of time, this whitish-gray Wet storage stain results from the exposure condi- powdery film tends to mask the underlying zinc crystals tions and is not indicative of inferior or poor quality gal- on the surface of the galvanized coating. vanizing. The degree of protection obtained in a given case The bulky white or gray corrosion product associ- varies with the nature of the environment. The presence ated with wet storage stain should not be confused with of chlorides and sulfur gases in the air, for example, the protective layer of zinc corrosion products which modify the composition of the carbonate layer and tend form under normal atmospheric exposure of galvanized to increase its solubility allowing rain and moisture to coatings. Even though the corrosion products on fully dissolve the carbonate layer more rapidly. Thus the life exposed galvanized surfaces may be white or light gray, of a galvanized coating is generally shorter in marine they are not the product of wet storage stain. Their color and industrial atmospheres than it is in the cleaner air of is solely a function of the environment and the zinc-iron a rural environment. On a galvanized part, the basic alloy content of the galvanized coating. chemical conversion on the surface of zinc metal to zinc When wet storage staining is found on galvanized carbonate is the mechanism that is interrupted by the materials, it is not usually in sufficient quantity to be formation of wet storage stain. detrimental to coating protection. Normally it disap- pears with weathering. However, with ill advised trans- WET STORAGE STAIN CHEMISTRY portation, handling and storage methods, it can become When galvanized articles are closely packed, serious. deprived of freely moving air and exposed to moisture, While the galvanizer has a responsibility to pro- an entirely different set of conditions arises in the chain duce a galvanized coating of good quality in confor- of zinc chemical changes. Without the free flow of car- mance with governing specifications, the galvanizer is bon dioxide over the galvanized surface, the surface no longer in control after the work leaves the plant. The retains water and forms an “oxygen concentration cell.” purchaser must ensure the proper handling and storage The effect may be best explained in terms of the of material at the construction site to ensure a stain-free reactions which occur under a drop of water flattened product at the time of final installation. between two surfaces. The resultant water film obvi- ously has only a minimal surface exposed to air; mean- NORMAL GALVANIZED COATING CHEMISTRY ing that the zinc surface near the middle of the water Before considering how wet storage stain occurs drop or film gets a different supply of oxygen than does on a galvanized part, it is important to understand how the zinc surface at the periphery of the water drop. This freshly coated zinc surfaces interact with the atmos- difference in turn sets up a difference in the electrolytic phere. Once the basics of the zinc-atmosphere interac- potential of the zinc. The central area becomes anodic tions are understood, the formation and prevention of and the edge area becomes cathodic, thus creating the wet storage stain can be explained. oxygen concentration cell (Figure 1). Since zinc is very reactive, any zinc surface in The attack on the zinc occurs at the anodic area. contact with the surrounding air will quickly form a The corrosion products are modifications of the non- layer of zinc oxide. The formation of this thin, hard, protective relatively soluble zinc hydroxide. Since this layer is the first step in the development of the protec- hydroxide remains rather stable under these conditions, tive corrosion product film normally associated with zinc ions constantly leave the coating to be bound by FIGURE 1 PREVENTION OF WET Wet Storage Stain Chemical Reactions STORAGE STAIN Zinc Coating Water (Moisture) STORAGE AND TRANSPORTATION Whenever galvanized articles are packed closely together for appreciable periods of time, adequate pre- cautions should be taken against wet storage stain. Wet storage stain can be minimized by maintaining a low humidity environment around the material and by pro- viding adequate ventilation between the stacked pieces. Low O2 Concentrations High O Concentrations Points to be observed are: 2 1. The galvanized steel, as produced, should be - - Cathodic Reaction: O2 + 4e + 2H2O 4OH clean and free from flux residues. Anodic Reaction: Zn Zn++ + 2e- 2. The material should be stored under cover in water, thus accelerating corrosion. The limitation of dry, well ventilated conditions, with heating facilities. It access to oxygen is also a limitation of access to carbon is very important to store the materials away from open dioxide and this hinders conversion of zinc hydroxide to doorways. the protective zinc carbonate. The corrosion products in this case are not protective and corrosion proceeds as 3. If outdoor stacking is unavoidable, the articles long as the original conditions prevail. should be raised from the ground and separated with The moisture necessary for the formation of wet strip spacers to provide free access of air to all parts of storage stain may originate in various ways. It may be the surface. They also should be inclined in a manner present on the galvanized parts at the time of stacking or which will give maximum drainage. Do not store gal- packing, as a result of incomplete drying after quench- ing. It may also be a result of direct exposure to rain or FIGURE 2 sea water, or from condensation caused by atmospheric temperature changes. Close packing can result in mois- ture being retained by capillary action between the sur- faces in contact because drying is delayed by the lack of circulating air. The extent of the damage by wet storage stain depends on the duration of exposure to retained mois- ture and the environment. The attack is accelerated when the retained moisture contains chlorides from sea water, sulfur compounds from industrial environments, or flux residues from the galvanizing operations. Each of these contaminants heightens the oxygen concentra- tion cell’s effect by increasing the water’s electrical con- ductivity. The important thing to realize, however, is that severe damage can be done in a relatively short time by water alone, without any other contributing factors. Because the corrosion products of wet storage stain are voluminous, any attack may appear more seri- ous than it actually is. The volume change from zinc- metal to zinc-oxide or zinc-hydroxide is 3 to 5 times greater. Medium or even heavy layers of wet storage stain are very unsightly, but they represent the loss of very little zinc from the base coating. The thicker zinc To help prevent wet storage stain from occurring on newly coatings provided by after fabrication hot dip galvaniz- galvanized surfaces, structures should be stored at an ing usually result in wet storage stain having little or no incline and oriented so water does not collect. Wood spac- significant effect on the durability and intended service er boards are placed between the layers so that crevices are life of the coating. not created between zinc surfaces. FIGURE 3 FIGURE 4 Figure 3 shows improperly stored galvanized guardrail. Storing galvanized steel in vegetation with no spacers between the products creates a highly conducive environment for wet storage stain to take place. By contrast, the items on this truck, Figure 4, are ideally stacked for transportation. FIGURE 5 Figure 5 shows an example of heavy wet storage staining. Some parts with heavy wet storage stain build-up can be cleaned, but most must be stripped and regalvanized before being put into service. FIGURE 6 Most parts with a medium layer of wet storage stain, Figures 6 and 7, can be cleaned and then put into service. FIGURE 8 FIGURE 7 In general, galvanized steel with a light build-up of wet storage stain, Figure 8, does not need to be cleaned. The light layer of wet storage stain will be converted to zinc carbonate during normal service. vanized steel on wet soil or decaying vegetation. be painted within six months, a post treatment may The use of spacers is also recommended during any interfere with paint adhesion. Communicating with shipping if there is the likelihood of condensation. the galvanizer will establish how to prepare the sur- For example, where material is chilled in traveling face for painting.
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