Humidity with zinc carbonate

Salt spray tests, humidity tests, and other accelerated tests, some using sulfur dioxide and carbon dioxide, have shown favorable results for tin—zinc in comparison with zinc, cadmium, and tin deposits. Chromating improves the performance. 

This chapter covers the effect of light from a GE sunlamp (high in UV light) on papers containing zinc oxide under moist (60% relative humidity) and dry (8% relative humidity) conditions in comparison with the effects on untreated controls, the effects of iodide as a negative catalyst, and the effect of converting the zinc oxide to zinc carbonate before exposure to avoid the catalytic photochemical reaction. 

Since there was no significant difference in the loss in folding endurance between the controls with no zinc carbonate and the treated papers containing about 2% zinc carbonate when exposed to light and humidity, it is obvious that no accelerated degradation results from the zinc carbonate. 

With this demonstration that zinc carbonate does not catalyze the photodegradation of paper in the presence of UV light and high humidity, we feel that the diethyl zinc process, as modified by the final carbon dioxide exposure, is now commercially feasible. 

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. 

Seasonal fluctuations of corrosion are similar in industrial and urban environments, particularly the more severe corrosion that occurs in winter (Schikorr and Schikorr, 1943). The difference in the seasonal corrosion rate can be explained as follows even for the same sulfur dioxide content of the air, it can be expected that in summer the absorption of SO2 by the zinc surface is less than in winter because of the lower relative humidity in the warmer months. The same values for absorption in the Liesegang bell were found whether the covering was impregnated with potassium carbonate or with sodium carbonate (i.e., two salts with very different hygroscopic properties). This suggests that the bell absorbs sulfur dioxide irrespective of the actual relative humidity (Schikorr, 1961a). 

Zinc carbonate is a white material with composition ZnCOj which occurs in nature as the mineral smithsonite (q.v). It is Hsted in the Colour Index (1971) as Cl 77950, where it is described as synthesised by precipitating a zinc salt with sodium bicarbonate and as used in the USA as a white pigment (Merck Index, 1996). As mentioned by Kiihn (1986), zinc carbonate was used with copper as a starting material in the manufacture of brass in ancient times from which the by-product of white zinc oxide (q.v.) was first produced. Zinc carbonate may also form as an alteration product of zinc oxide in a humid environment. 

Insoluble solids, regardless of particle size, that have a relatively low interfacial tension and are readily wetted by water are called hydrophilic solids. These solids include clays (bentonite, kaolin, talc, magnesium aluminum silicate) bismuth salts, barium sulfate, carbonates, hydroxides, or oxides of calcium, magnesium, zinc, and aluminum and titanium dioxide. The hydro-philicity of a powder surface can be investigated with the help of moisture absorption studies in which the solid particles are exposed to varying relative humidities. Insoluble powders that absorb moisture below relative humidities of 70-80% at room temperature are said to be hydrophilic solids. 

Concrete is an alkaline environment where iron passivates and the amphoteric materials, aluminum and zinc, react with fresh concrete by evolving hydrogen. In the case of zinc, the reaction can be quenched by addition of chromates to the canent or prepassivating the zinc used as a protective layer for reinforcing iron bars in concrete. Lead in concrete, in conditions of high humidity, corrodes, and the concrete prevents the formation of the protective layers of basic lead carbonate, which would be formed in its absence. 

The advantage of zinc-air cells also presents a big challenge for battery designers. Remaining open to the atmosphere renders zinc-air cells exposed to detrimental environmental conditions, especially humidity. Water in humid air can be absorbed by the basic electrolyte solution diluting it and subsequently flooding the cathode. Arid air may evaporate water from the electrolyte and dry the cathode. Both conditions lead to reduced cell performance and battery life. Carbon dioxide in the air can enter the cell, react with the basic electrolyte solution and precipitate carbonates, also decreasing performance. 

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%. 

Carbonation of Electrolyte. Carbon dioxide, which is present in the atmosphere at a concentration of approximately 0.04%, reacts with an alkaline solution (electrolyte) to form an alkali metal carbonate and bicarbonate. Zinc/air batteries can be satisfactorily discharged using a carbonated electrolyte, but there are two disadvantages of extreme carbonation (1) vapor pressure of the electrolyte is increased, aggravating water vapor loss in low-humidity conditions, and (2) crystals of carbonate formed in the cathode stracture may impede air access, eventually causing cathode damage with subsequent deterioration of cathode performance. As indicated in Fig. 13.16 carbonation must be extreme to be detrimental to cell performance in most applications. ... 

The decreasing levels of SO2 and increasing frequency of car traffic has resulted in a new multi-pollutant situation in many urban and industrial areas. In order to possibly quantify the corrosion effects caused by this new multi-pollutant situation an extended exposure program was performed that took place between 1997 and 2001 and involved some 30 test sites in 18 countries in Europe and North America [91]. Specimens of carbon steel, zinc, copper, bronze, limestone, paint-coated steel, and glass representative of medieval stained glass windows were exposed for up to four years. At each site, the environmental data measured included climatic parameters (temperature, relative humidity, and sunshine radiation), gaseous pollutants (SO2, NO2, HNO3, and O3), particles (presented as PMio, i.e., concentration of particles with diameter < 10 pm), and precipitation (total amount, conductivity, and concentration of, i.e., H+, S04 , N03, Cl", NH4+). 

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