Resistance to Aqueous Corrosion

Water hardness is an important variable in zinc corrosion. Hard waters are usually less corrosive toward zinc because they deposit protective scales 

Raising the temperature increases die dissolution of zinc and the ZA alloys (Belisle and DuFresne, 1986) in water. A marked increase occurs up to about 60°C, followed by a decrease at higher temperatures. Intergranular corrosion of the zinc-aluminum casting alloys is a risk above about 70 C in wet or humid conditions (e.g., in steam), when no protective layer can form and selective dissolution of the structure occurs. A steam test at 95°C is indeed a standard method of checking whether alloys have been correctly produced. 

At temperatures usually between 60 and 90°C (but varying with the water composition), a reversal of potential may occur in which zinc coatings become cathodic to the ferrous substrate. 

The well-known Pourbaix diagrams (showing specific zones in which no corrosion and passivation occur), in which pH is plotted against electrode potential, are valid only in pure water at 25°C. A comparison of the behavior of zinc and other metals offers a deeper insight into corrosion by water, however. 

For research purposes, to identify the nature of protective films or to study the mechanism of corrosion, many investigators have used distilled water, which although highly corrosive is relatively pure. 

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Special Metals Corporation. In High-Performance Alloys for Resistance to Aqueous Corrosion Special Metals Corporation New Hartford, NY, 2000. 

Because of the oxide layer (Ti02) that forms instantaneously on its surface when exposed to O2 or H2O. titanium is generally considered resistant to aqueous corrosion up to 589 K. assuming that the oxide layer is present on the surface of titanium, absorption or release of impurities or interstitials, such as hydrogen, is unlikely. 

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