Zinc, corrosion rate

The corrosion product on zinc exposed 3 years at the New Jersey site, where short-term zinc corrosion rates are low, is uniform, somewhat nodular, and fine grained on both the skyward and ground-ward sites. Figure 7. The same was true for samples from the District of Columbia site. On the other hand, the corrosion film on... 

The near coastal effect of wetness and sea salt is evident in the data, particularly for 1 and 2 year exposures. Zinc corrosion rates at Cape Kennedy, FL, dropped by a factor of 4 as distance from the coast increased from 60 to 880 m. At Kure Beach,NC, Zn rates dropped by a factor of 3 as distance increased from 25m to 250m from sea corresponding A1 corrosion rates fell by a factor of 2. Clearly sites within a few hundred meters of the sea must be segregated from other sites in any further analysis. 

By adding 1 % of platinum to pure zinc, the zinc corrosion rate drastically increases from the corrosion rate for pure zinc (2 x 10 K/cm). This is mainly due to the large hydrogen evolution exchange current densities on Pt (10 A/cm ). 

Fig. 2.1 User guidelines for zinc corrosion rates linear corrosion rates in different types of atmosphere (BS 5493, 1977). Note The atmospheric corrosion rates shown here are typically 50% higher than those now prevailing.

The results are too scattered for close correlation to be made, but general guidance from the results is that (unless chloride is greater than 120 mg/m Cl ), zinc corrosion rate is a basic 0.5-2p.m/year plus 1 p,m for every 40 p.g/m SO2 in the atmosphere. Where chloride concentrations exceed 120 mg/m Cl , specific mass loss data must be provided for each site. 

Table 2.15 Effect of Time of Day on Zinc Corrosion Rates at Palmerton, Pennsylvania ...

Haynie and Upham (1970) compiled data for the atmospheric sulfur dioxide level, relative humidity, and zinc corrosion rate for eight U.S. cities. Using regression and curve-fitting techniques, they developed the equation ... 

Goodwin (1991) has however observed that a simple chemical reaction model overestimates the zinc corrosion rates due to wet deposition. The actual corrosion rates are only 15-90% of the theoretical values. The difference is believed to be caused by (a) incomplete dissociation of available hydrogen, (b) the presence of previously reacted zinc on the surface, which limits the amount of available fresh zinc, and (c) simple washing away of reactants before they can react with the zinc surface. Rainfalls with pH 3.5 and higher are believed to decrease zinc corrosion rates because they wash the panel, removing many of the potential reactants before they have a chance to corrode the zinc surface. 

To determine the contribution of dry versus wet deposition, Goodwin initiated laboratory tests using synthetic rainfall of different pH values. Dry deposition was attained by setting panels in a louvered box outdoors. Rain intensities between 0 and 12 mm/h were used. This technique made it possible to vary sulfate concentration independently of pH. Rolled pure zinc panels were used for all tests. Atmospheric corrosion rates of zinc is Scandinavia, when only dry deposition was allowed, were measured to be 12 g/m / year based on an exposure period of 2.5 months. Exposure of these dry panels to artificial rainfalls with pH values of 4.5 and 3.5 decreased their corrosion rates. Only when rainfall pH was lowered to 2.5 did zinc corrosion rate increase over that seen with solely dry deposition (Fig. 2.7). 

Zinc corrosion rates at five sites for four exposure periods (Cramer et al.,... 

Fig. 2.23 Zinc corrosion rate as a function of distance from British coasts (Bawden and Ferguson, 1989).

Which comes first, the chicken or the egg The same question must be asked about attempts to define atmospheres (in terms of time of wetness and deposition rates of SO2 and chloride) so that corrosion rates of zinc can be assessed, whereas others have used zinc corrosion rates to assess the corrosivity of the atmosphere. In this connection, the 10-year tests at seven sites (Fig. 2.28) by Holler, Knotkova, et al. (1986-1987) mostly show straight-line relationships with time this analysis of corrosion rate versus sulfur dioxide plus chloride contents of the atmosphere showed a linear rate for freely exposed zinc, but in semienclosed conditions with high sulfur dioxide plus chloride, the corrosion was slightly less than if a straight-line relationship applied. 

Because corrosion occurs differently in different parts of the world, qualitative descriptions of atmospheres as urban, industrial, marine, or rural are of significance only within a specific geographic region, and multiplication factors need to be determined for any other region. Nevertheless, it can be helpful to show the range of zinc corrosion rates in this form (Tables 2.3-2.6). SO2 content and time of wetness should be obtained wherever possible (see Tables 2.7A and B). 

Trials in a North American municipal wastewater treatment plant have shown variations in zinc corrosion rates in the sedimentation tanks ranging from 28 to more than 140 p.m/year, whereas 85 xm zinc coatings in nomi-... 

From this study, it can be seen that raising the chloride load has a mnch stronger effect on painted zinc-coated substrates than on painted carbon-steel snbstrates. It is known that for bare metals, the zinc corrosion rate is more directly dependent than the carbon steel corrosion rate on the amonnt of pollutant (NaCl in this case). This relationship may be the cause of the results in the table above. In addition, higher salt levels leave a heavier hygroscopic residue on the samples (see Section 7.2.3) this may have caused a thicker moisture film at RH levels above 76%. 

Zinc alloys containing copper, iron, antimony, arsenic, or tin are known to increase the zinc corrosion rate while zinc alloyed with cadmium, aluminum, or lead will reduce corrosion rates. - If enough gas pressure is generated, the sealed button cell could leak or rupture. In commercial use, the high surface area zinc powder is amalgamated with small amounts of mercury (3 to 6%) to bring the corrosion rate within a tolerahle limit. 

As with copper, zinc runoff rates in atmospheric environments have turned out to remain relatively constant over exposure periods of a few years. Measured zinc runoff rates ranged from around 3 to 5 g/m per year and could be correlated with the SO2 concentration at sites with comparable annual amounts of precipitation [114]. For most sites the quotient between zinc mnoff rate and zinc corrosion rate varied between 50 and 60% during the first 2 years of exposure. 

In any situation where zinc is corroded sacrificially to protect exposed steel, the mass of available zinc will determine the corrosion protection performance. Corrosion rates of zinc coatings required to cathodically protect uncoated steel in aggressive environments (salt-water/marine) may be 25 times higher than the normal zinc corrosion rate. 

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