Wet storage stain

In dry air the stability of zinc is remarkable. Once the protective layer of zinc oxide formed initially is complete, the attack ceases. Even under under normal urban conditions, such as those in London, zinc sheet 0 -8 mm thick has been found to have an effective life of 40 years or more when used as a roof covering and no repair has been needed except for mechanical damage. The presence of water does, of course, increase the rate of corrosion when water is present the initial corrosion product is zinc hydroxide, which is then converted by the action of carbon dioxide to a basic zinc carbonate, probably of composition similar to ZnCOj 3Zn(OH)2 . In very damp conditions unprotected zinc sometimes forms a loose and more conspicuous form of corrosion product known as wet storage stain or white rust (see p. 4.171). 

White rust If a fresh zinc surface is allowed to stand with large drops of dew on it, as may easily happen if it is stored in a closed place in which the temperature varies periodically, it is attacked by the oxygen dissolved in the water, owing to differential aeration between the edges and the centres of the drops. A porous form of zinc oxide builds up away from the surface and quickly takes up carbon dioxide from the air to form the basic carbonate known as white rust or wet storage stain. 

Extra treatments Conversion coatings—chromates prevent wet storage stain phosphates good on new sheet as a base for paints. Weathered coatings often painted (after 10-30 years) for longer service. Conversion coatings (e.g., chromates used to prevent wet storage stain. Frequently used as a base for paints. 

Zinc-plated coatings are usually passivated to prevent wet storage staining, an unsightly form of mild corrosion that occurs where zinc surfaces encounter heavy condensation. Historically, a simple chromate dip has been widely used, but other treatments are being developed. Phosphates are often favored when the dipped surface is to be painted. 

The one form of general corrosion that is not protective is white rust, more properly called wet storage stain because it occurs in storage, where there is access for water but limited supply of oxygen and carbon dioxide. The presence of chlorides and sulfates accelerates wet storage stain formation. 

The special form of corrosion known as wet storage stain (white rust) that sometimes appears on hot dip galvanized objects after a short period of storage shall not be regarded as sufficient cause for not granting approval, unless a special agreement has been reached that wet storage stain shall not be allowed. 

The effect of atmospheric humidity on the corrosion of zinc is related to the conditions that may cause condensation of moisture on the metal surface and to the frequency and duration of the moisture contact. If the air temperature drops below the dew point, moisture will be deposited. The thickness of zinc, its surface roughness, and its cleanliness also influence the amount of dew deposited. Lowering the temperature of a metal surface below the air temperature in a humid atmosphere will cause moisture to condense on the metal. If the water evaporates quickly, corrosion usually is not severe and a protective film is formed on the surface. If water from rain or snow remains in contact with zinc when access to air is restricted and the humidity is high, the resulting corrosion can appear to be severe (wet storage stain, popularly known as white rust ), since the formation of a protective basic zinc carbonate is prevented. 

To retard weathering discoloration and improve atmospheric corrosion resistance, in addition to preventing wet storage staining, chromate passivation procedures have been developed. 

A chromate conversion coating over the zinc coating prevents wet storage stain while phosphate conversion coatings provide a good base (on a new sheet) for paints. Weathered coatings are often painted after 10-30 years for longer... 

Chromate conversion coatings are used to prevent wet storage stain while phosphate conversion coatings are used as a base for paint. 

In fact, the compound Zr COz (OH)e or ZnCOz 3Zn OH)2 is zinc carbonate or white mst or wet-storage stain (porous). Atmospheric corrosion of aluminum is due to a passive oxide film formation instead of a porous layer. The gray/black-color film may form as follows... 

Short-term protection against wet-storage staining can be provided by chromating or phosphating. Painting after galvanizing will also provide protection. 

White Rust n (ca. 1848) White corrosion products (zinc hydroxide and zinc oxide) on zinc-coated articles. They form when the parts are stored so close together that condensed moisture is entrapped between them and the air circulation is inadequate to assist drying. Known also as Wet Storage Stain. (Corrosion Engineer s Handbook, 3 Ed, Baboian, R., NACE International -The Corrosion Society, Houston, TX, 2002)... 

Zinc is one of the most important tools used by engineers to control the corrosion of steel in the atmosphere. Zinc coated steels are used in general building, constructions in automobile bodies, in hardware, containers, tubes, pipe fittings, wires, ropes and hundreds of other applications. Zinc is little susceptible to corrosion even at high humidity. At 99% relative humidity the atmospheric corrosion of zinc amounts to 0.007 g/m /day. But a white hygroscopic corrosion product called wet-storage stain can form on new zinc wet surfaces unless these have been pre-stored for several weeks in low humidity air. 

High humidity and high salt contents increases the rate of corrosion. The average rate of corrosion of zinc is 1-7 pm/year in coastal areas. Corrosion is also intensified by erosion from sand particles. In tropical climates, zinc corrodes only a little unless high humidity causes wet storage stain on a new zinc surface (mentioned earlier). The average corrosion rate of galvanized steel is 2 xm in humid tropical atmosphere to about 6 iim in a tropical sea atmosphere. 

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