Resistance aqueous corrosion

In dry air at room temperature this reaction is self-limiting, producing a highly impervious film of oxide ca 5 nm in thickness. The film provides both stabihty at ambient temperature and resistance to corrosion by seawater and other aqueous and chemical solutions. Thicker oxide films are formed at elevated temperatures and other conditions of exposure. Molten aluminum is also protected by an oxide film and oxidation of the Hquid proceeds very slowly in the absence of agitation. 

Corrosion. Aqueous solutions of citric acid are mildly corrosive toward carbon steels. At elevated temperatures, 304 stainless steel is corroded by citric acid, but 316 stainless steel is resistant to corrosion. Many aluminum, copper, and nickel alloys are mildly corroded by citric acid. In general, glass and plastics such as fiber glass reinforced polyester, polyethylene, polypropylene, poly(vinyl chloride), and cross-linked poly(vinyl chloride) are not corroded by citric acid. 

The oil enmeshes in the tail, as shown in Figure 4-480, and provides a mechanical barrier to attack of the aqueous corrodents on the base metal. The oily film also increases the resistance to corrosion current flow and, thus, stifles the rate of corrosion. An advantage of using organic film-forming inhibitors... 

The majority of metals and alloys available depend for their resistance to corrosion on the properties of an oxide film or corrosion product which is formed initially by the corrosion process. In many cases the protectiveness of the oxide film is determined by its stability in aqueous solutions in a specific pH range, either chemically dissolving to form aquocations at lower pH values or complex anions (aluminate, ferroate, plumbate, zincate, etc.) at higher pH values (Fig. 2.22). An important property of the chemical is therefore the pH value that it develops when dissolved in water. For many materials and many chemicals this is the overriding factor and in many cases... 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

Aqueous corrosion resistance, 13 513 Aqueous dispersion polymerization, 18 291 of acrylonitrile, 11 197-200 Aqueous dispersions, 13 292. See also Aqueous polytetrafluoroethylene dispersions... 

Platinum and carbon are frequently used as counter electrode materials for both anode and cathode. Platinum is resistant to corrosion while carbon is cheap and can be discarded after use. Nickel is a suitable counter cathode material in aqueous solution because of the low overpotential for hydrogen evolution. Titanium coated with platinum and then over coated with mthenium dioxide is a stable counter anode material with a low overpotential for oxygen evolution. 

In general, polymers are much more resistant to corrosives than metals. Most water-insoluble polymers are not attacked by aqueous salt solutions, and polymers without hydrolyzable or reactive groups are resistant to most nonoxidizing acids and alkalis. 

It is possible to summarize the general features that can be attributed to the alloying elements in the alloy with respect to aqueous corrosion resistance as given... 

Table 4.55 Nominal composition of aqueous corrosion and wear-resistant cobalt alloys...

The low cross-section for absorption of neutrons and high-temperature (330-350°C) aqueous corrosion resistance as well as its good mechanical properties promote the use of zirconium alloys in the nuclear reactors. In the development of zirconium alloys care must be taken that the added minor elements do not posses high cross-sections for the absorption of neutrons and contribute to greater corrosion resistance and improved mechanical properties. The good corrosion resistance of the alloys in acids and bases favors the use of zirconium alloys in chemical plants. 

Type 317 has the highest aqueous corrosion resistance of all AISI stainless steels. 

The first ionisation potentials are all fairly low and those for ionisation up to arc known. As usual, the small 4+ ion which is invoked in compounds does not exist as such, but takes a considerable share of the electron density of the ligands. Nor does the ion exist in aqueous solution the most stable cation is the complex ion MO + and the redox potentials above refer to the MO2+/M couples. The metals are evidently strongly reducing but they are so easily rendered passive that they are not very reactive at room temperature and have a remarkable resistance to corrosion. 

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

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