Water continued nickel-iron alloys

The presence of water vapour adversely affects the selective oxidation of elements such as aluminium and chromium from iron-base and nickel-base ° alloys. This is illustrated in Figure 7.2 for a Ni-Cr-Al alloy. Exposure in dry air results in a continuous external alumina film while exposure in moist air results in profuse internal oxidation of the Al. It has also been found that water vapour affects the selective oxidation of aluminium from TiA1.2i... 

A similar method of test was used at the International Nickel Company s Corrosion Laboratory at North Carolina. The specimen discs are mounted on insulated vertical spindles and submerged in sea-water, which is supplied continuously to the tank in which the specimens are immersed. The maximum peripheral speed of the spinning disc is about 760cms , and the characteristic pattern of attack is shown in Fig. 19.3a. Studies of variation of depth of attack with velocity indicate that at low velocities (up to about 450 cm s ) alloys such as Admiralty brass, Cu-lONi and cupro-nickel alloys containing iron maintain their protective film with a consequent small and similar depth of attack for the diflferent alloys. At higher velocities the rate increases due to breakdown of the film. 

It is possible for the passivation (oxide) layer on the surface of a metal to be continuously removed or not allowed to develop, by erosion from particulate matter or gas bubbles. Not only is the surface eroded but the removal of the protective oxide layer allows corrosion to take place. The problem is accentuated by the presence of an obstruction or debris, on the metal surface that diverts and accelerates the flow near the surface along a defined path. Sato et al [1977] report experimental data on erosion-corrosion resistance of condenser tubes fabricated from various cupro nickel alloys. They suggest that high iron bearing cupro nickels are superior in respect of erosion corrosion by clean sea water. 

For more than a century, a number of different aluminum alloys have been commonly used in the aircraft industry These substrates mainly contain several alloying elements, such as copper, chromium, iron, nickel, cobalt, magnesium, manganese, silicon, titanium and zinc. It is known that these metals and alloys can be dissolved as oxides or other compounds in an aqueous medium due to the chemical or electrochemical reactions between their metal surfaces and the environment (solution). The rate of the dissolution from anode to cathode phases at the metal surfaces can be influenced by the electrical conductivity of electrolytic solutions. Thus, anodic and cathodic electron transfer reactions readily exist with bulk electrolytes in water and, hence, produce corrosive products and ions. It is known that pure water has poor electrical conductivity, which in turn lowers the corrosion rate of materials however, natural environmental solutions (e g. sea water, acid rains, emissions or pollutants, chemical products and industrial waste) are highly corrosive and the environment s temperature, humidity, UV light and pressure continuously vary depending on time and the type of process involved. ... 

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