White rust

White pine White rust White s agar White Shoulders White smoke Whitewares Whitewash White water Whitfield s Ointment... 

However, in this section emphasis is placed upon damp and wet atmospheric corrosion which are characterised by the presence of a thin, invisible film of electrolyte solution on the metal surface (damp type) or by visible deposits of dew, rain, sea-spray, etc. (wet type). In these categories may be placed the rusting of iron and steel (both types involved), white rusting of zinc (wet type) and the formation of patinae on copper and its alloys (both types). 

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. 

This type of corrosion can take place on any new surface of zinc and is best prevented by storing the metal in a dry, airy place until a protective layer has been formed. Zinc which has been properly aged in this way is safe against white-rust formation. Various methods are employed to prevent white rust. A chromate treatment is widely used for zinc-plated articles and for galvanised sheet, and occasionally for zinc die castings. Fatty substances, such as oils or lanolin, are sometimes used to protect larger items. 

Chromating Chromating is considered in Section IS.3. The chromate film on zinc is adherent and can be drab, yellow-green or colourless in appearance the colour varies considerably with the method of application. It retards white rust , the white deposit which sometimes forms on fresh zinc surfaces which are kept under humid conditions (see Section 4.7). A chromate film is damaged by heat and if used as a basis for paint adhesion, should preferably not be heated above 10°C, nor for longer than 1 h. 

Zinc High alkalinity causes white rust... 

Although this form of corrosion has been known for many years, the reported incidence (and therefore awareness) of white rust corrosion on new galvanized steel cooling towers has grown significantly since the late 1980s and early 1990s. 

The high incidence of white rust today has been due to several factors, including ... 

Requirements for white rust control in a new cooling system are ... 

Chemical programs employing molybdate or orthophosphate chemistry may be especially useful in helping to promote a stable, basic zinc carbonate film of sufficient thickness that permits eventual high alkalinity, high cycle operating conditions to be permanently used, without fear of regression to a white rust situation. 

White rust and similar visible surface corrosion problems, resulting from a failure to provide a proper chemical passivation program. 

Is the cooling tower for HVAC purposes and constructed of galvanized steel Did the initial treatment program encompass a passivation program to prevent the risk of white rust Can the tower cope with peak summer cooling demands ... 

If it is a galvanized steel tower, are there signs of zinc corrosion (white rust) or steel corrosion (especially under screw-down, tower top, distribution channel covers, and in the floor pan) Unscrew the covered distribution channels to inspect them ... 

Chromate conversion coatings are used widely on aluminum alloys as a pretreatment for painting, though in some applications, where noncondensing atmospheric exposure is expected, they may be used as the primary means of corrosion protection. Chromate conversion coatings are used on magnesium, cadmium, and zinc, and on galvanized steel to suppress the formation of white rust. 

Rust of iron (the most abundant corrosion product), and white rust of zinc are examples of nonprotective oxides. Aluminum and magnesium oxides are more protective than iron and zinc oxides. Patina on copper is protective in certain atmospheres. Stainless steels are passivated and protected, especially in chloride-free aqueous environments due to a very thin passive film of Cr2C>3 on the surface of the steel. Most films having low porosities can control the corrosion rate by diffusion of reactants through the him. In certain cases of uniform general corrosion of metals in acids (e.g., aluminum in hydrochloric acid or iron in reducible acids or alkalis), a thin him of oxide is present on the metal surface. These reactions cannot be considered hlm-free although the him is not a rate-determining one.1... 

Extension of such new culture technology to other orange and white rust pathogens would enable the development of countless other rust species to control their specific weed hosts. 

Leaves with pale yellow patches on upper surfaces. Causes Downy mildew white rust. If spots develop a grayish mold on the under-... 

If spots develop white blisters on the undersides of the leaves, they have white rust, another fungal disease. Destroy infected leaves or plants. Prevent problems by thinning plants to increase air circulation and by planting tolerant cultivars such as Fall Green . 

The slopes and the constituents associated with those slopes are different for zinc and galvanized steel. This was unexpected since, to the atmosphere, they should appear the same. However, there was one significant difference between the two materials. The surface of the galvanized steel received a chromate treatment to improve its resistance to "white rusting" during storage (31, 32). 

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