Brass crevice corrosion

Wastage was caused by crevice corrosion, accelerated by the difference in tube and tube sheet metallurgies. The brass tube, being more noble, was cathodically protected by corrosion of the surrounding mild steel tube sheet. However, the galvanic effect was secondary to the primary cause of failure, namely, crevice corrosion. 

Structural materials that are direcdy exposed, such as in cabinets, housings, and heat sinks, are fabricated fix>m materials such as steels, stainless steels, brass, zinc, aluminum, and other metals and alloys with appropriate prof>erties. The types of corrosion encountered in these structures depend on the environment and material as would be exp>ected. Both uniform and localized corrosion can be important when cosmetic corrosion is of concern. In structural applications, crevice corrosion, corrosion fatigue, stress corrosion cracking (SCC), galvanic corrosion, and intergranular corrosion causing reduction in mechanical properties are important. 

Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen as well as minerals, several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper-nickel alloys, and nickel-chromium-molybdenum alloys are highly corrosion resistant in seawater. 

For reasons of ease of manufacture, the majority of solid electrodes have a circular or rectangular form. External links are through a conducting epoxy resin either to a wire or to a solid rod of a metal such as brass, and the whole assembly is introduced by mechanical pressure into an insulating plastic sheath (Kel-F, Teflon, Delrin, perspex, etc.) or covered with epoxy resin. It is very important to ensure that there are no crevices between electrode and sheath where solution can enter and cause corrosion. Examples of electrodes constructed by this process will be shown in Chapter 8. 

P + Pi brasses of similar composition (Cu—38 wt.% Zn and Cu—41 wt.% Zn, respectively), it has been concluded that this is not simply due to an increased Zn concentration in the p phase, but is likely to reflect the structural differences of hoth phases [67]. Other factors that stimulate the dezincification of both O - and a p brasses are the presence of chlorides, high concentrations of CuCl2 ions, stagnant environments, differential aeration cells and elevated temperatures. In chloride solutions, such conditions are typically encountered at a later stage of corrosion when mass transport restrictions by a deposit, corrosion product, or crevice have heen established and create a local environment, in which the copper in the brass is nearly in equilibrium with CU2O, CuCl, and accumulated CuCl2 anions [68,69]. Experimentally, the necessity... 

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