Atmospheric corrosion zinc hydroxide

Feitknecht has examined the corrosion products of zinc in sodium chloride solutions in detail. The compound on the inactive areas was found to be mainly zinc oxide. When the concentration of sodium chloride was greater than 0-1 M, basic zinc chlorides were found on the corroded parts. At lower concentrations a loose powdery form of a crystalline zinc hydroxide appeared. A close examination of the corroded areas revealed craters which appeared to contain alternate layers and concentric rings of basic chlorides and hydroxides. Two basic zinc chlorides were identified, namely 6Zn(OH)2 -ZnClj and 4Zn(OH)2 ZnCl. These basic salts, and the crystalline zinc hydroxides, were found to have layer structures similar in general to the layer structure attributed to the basic zinc carbonate which forms dense adherent films and appears to play such an important role in the corrosion resistance of zinc against the atmosphere. The presence of different reaction products in the actual corroded areas leads to the view that, in addition to action between the major anodic and cathodic areas as a whole, there is also a local interaction between smaller anodic and cathodic elements. 

The acidity of rain is very significant in zinc corrosion and, since the acidity occurs mainly from sulfur dioxide, further details are in Section II.B. Essentially, if rain is below about pH 5 (Fig. 2.7) the corrosion rate will be increased (see Fig. 1.11). Sulfur emissions are discussed by Likens et al. (1979) zinc is attacked only slightly by pure air, and zinc oxide forms, which is converted to hydroxide when moisture is present. Even if the moisture content is considerable, attack remains slight, but the hydroxide films formed (Schikorr, 1964 a,b) have a relatively minor protective effect. The zinc hydroxide reacts further with carbon dioxide in the atmosphere, forming a basic zinc carbonate. This film is very protective and is mainly responsible for the excellent resistance of zinc to ordinary atmospheres. 

The basic salts (hydroxychloride, hydroxysulfate and sodium zinc chlorohydroxysulfate) are formed discontinuously on the surface, looking like islands. These islands grow gradually to form a continuous layer that completely covers the metal and protects it from further attacks [24, 26]. However, if the surface moisture reaches a low pH, either permanently or occasionally (for example, due to higher pollution with SO2, acid rain or HCl-polluted atmospheres), zinc hydroxide and basic salts does not form [3, 15], Then, the formation of water soluble sulfates and chlorides is facilitated, which can be washed by rain and will not protect the metal. Thus, the corrosion rate could increase. 

The analysis by XPS can confirm that the ZnO was always present initially on the surface of zinc although the probe had not been exposed in the ehamber. A very thin film of this oxide forms instantaneously by chemical oxidation on the zine surfaee in contact with clean air at room temperature. This film does not affeet the later eleetroehemical corrosion process [5]. Once the humidity layer is established, zinc hydroxide is rapidly formed on the ZnO film via an electrochemical mechanism. Generally, it is considered that hydroxides are the initial compounds in zinc atmospheric corrosion studies [5, 15]. 

In natural atmospheres, once the moisture layer has been established, zinc hydroxide rapidly forms on this film, in this case due to an electrochemical mechanism. The formation of a moisture layer of sufficient thickness, together with the action of atmospheric CO2, leads to the formation of basic zinc carbonates from the initially formed hydroxide [3, 5]. Both the hydroxide and the carbonates are very stable and have a protective character, and they therefore tend to inhibit zinc corrosion in atmospheres without contamination. However, if the... 

Zinc is a relatively base metal. Atmospheric corrosion of zinc starts with the instantaneous formation of a thin film of zinc hydroxide, which may occur in different crystal structures, and the subsequent formation of basic zinc carbonate Zn,(CO02(OH)6. The pH of the aqueous layer controls the stability of initial corrosion products and results in the dissolution of Zn. ... 

Sulfur dioxide is a primary pollutant leading to the atmospheric corrosion of zinc. It controls the corrosion rate when the relative humidity is in the area of 70% or above. Sulfur oxides and other air pollutants are deposited on zinc surfaces either by dry or wet deposition. Regardless of the method of deposition, the sulfur dioxide deposited on the zinc surface forms sulfur-ous or other strong acids, which react with the protective zinc oxide, hydroxide, or basic carbonate film to form zinc sulfate. The film of protective corrosion products is destroyed by the acids, which reforms from the underlying metal, causing continuous corrosion by an amount equivalent to the film dissolved, hence to the amount of sulfur dioxide absorbed. Corrosion rates increase even further when the relative humidity exceeds 85%. 

If corrosion resistance is desired, the ribbon may be galvanized - i.e., coated with a protective layer of zinc. Upon exposure to the atmosphere, the Zn coating sacrifi-cially oxidizes to form a protective layer of zinc oxide. Further interaction with moisture results in the formation of zinc hydroxide (Eq. 12), which may also react with carbon dioxide in the atmosphere to form a thin, impermeable, and water insoluble coating of zinc carbonate (Eq. 13). 

In normal atmospheric conditions, zinc reacts with oxygen to form a thin oxide layer. This oxide layer in turn reacts with water in the air to form zinc hydroxide (Zn[OH]2), which in turn reacts with carbon dioxide in air to form a layer of basic zinc carbonate [15-17]. Zinc carbonate serves as a passive layer, effectively protecting the zinc underneath from further reaction with water and reducing the amount of corrosion. 

The atmospheric corrosion of nickel is similar to that of zinc. Nickel exists solely as Ni + and instantaneously forms nickel hydroxide and, subsequently, NiS04 6H2O, has been observed [61b]. After prolonged exposure, an amorphous basic nickel sulfate is formed, frequently mixed with small amounts of carbonate, with less protective abUity. This phase can crystallize and form another basic nickel sulfate, with higher stabUity and protective abUity [61a]. No evidence of other anions in the corrosion products has been found so far. The corrosion rates of nickel are comparable to those of copper [70]. 

The zinc hydroxide and the basic zinc salts form patina which protects the zinc surface from further attacks. Sulfur dioxide is the most harmful pollutant in the atmosphere. The presence of 0.1% SO2 in a polluted atmosphere causes a marked increase in the rate of corrosion as the basic salts, such as ZnOH(C03)o.5 and Zn(OH)i.5 ( 04)0.25. formed earlier may be dissolved. 

The rate of corrosion of zinc is primarily influenced by the time of wetness, and the presence of pollutants, such as SO, Cl" and CO2 in the air. Corrosion products, such as Zn(OH)2, Zn5(C03)2(0H)6 and ZnS04, are predominantly observed. Zinc hydroxide and basic zinc salts react to form a patina which protects the surface fi-om corrosion attack. Atmospheric pollutants, such as SO2, lead to an increase in acidification of the electrolyte and dissolution of the protective films. 

Both zinc and zinc alloys have excellent resistance to corrosion in the atmosphere and in most natural waters. The property which gives zinc this valuable corrosion resistance is its ability to form a protective layer consisting of zinc oxide and hydroxide, or of various basic salts, depending on the nature of the environment. When the protective layers have formed and completely cover the surface of the metal, the corrosion proceeds at a greatly reduced rate. 

Vernon claims that in outdoor atmospheres the corrosion product consists largely of zinc oxide, hydroxide and combined water, but also contains zinc sulphide, zinc sulphate and carbonate. The following table gives the composition of typical films formed in an industrial atmosphere. 

Zinc and zinc-coated products corrode rapidly in moisture present in the atmosphere. The corrosion process and its mechanism were studied in different media, nitrate [283], perchlorate [259], chloride ions [284], and in simulated acid rain [285]. This process was also investigated in alkaline solutions with various iron oxides or iron hydroxides [286] and in sulfuric acid with oxygen and Fe(III) ions [287]. In the solution with benzothia-zole (BTAH) [287], the protective layer of BTAH that formed on the electrode surface inhibited the Zn corrosion. 

CUPRIETHYLENE DIAMINE HYDROXIDE SOLUTION (13426-91-0) Dissolves wood, cotton, and other cellulosic material. Reacts violently with water. Forms unstable peroxides under normal conditions of temperature and storage. A powerful reducing agent. Reacts violently with oxidizers, organic materials, and many other substances. Corrosive to copper, aluminum, zinc, and tin. Store under inert atmosphere such as nitrogen. 

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