Rust formation

Stresses may develop resulting from the increased volume of coiTosion products, e.g. rust formation involves a seven-fold increase in volume. 

Laminar Scale—rust formation in heavy layers. 

Many systems are idle for long periods after operating at high temperatures. This permits moisture to condense in the system, resulting in rust formation. Certain corrosion-and rust-preventive additives are added to hydraulic liquids. Some of these additives are effective only for a limited period. Therefore, the best procedure is to use the liquid specified for the system for the time specified by the system manufacturer and to protect the liquid and the system as much as possible from contamination by foreign matter, from abnormal temperatures, and from misuse. 

This type of corrosion can take place on any new surface of zinc and is best prevented by storing the metal in a dry, airy place until a protective layer has been formed. Zinc which has been properly aged in this way is safe against white-rust formation. Various methods are employed to prevent white rust. A chromate treatment is widely used for zinc-plated articles and for galvanised sheet, and occasionally for zinc die castings. Fatty substances, such as oils or lanolin, are sometimes used to protect larger items. 

Stanners have designed a test rig (Fig. 19.12) which provides results that can be correlated with actual atmospheric exposure data. The rig has been designed to investigate a wide range of alloying elements in a development programme on slow-weathering steels for which it was essential to have a rapid, reliable and reproducible test that incorporated the specific atmospheric factors responsible for rust formation. 

Acids, for example those in fruit juices, catalyze rust formation because they furnish H+(aq) to accept electrons from the iron, causing it to dissolve faster. Oxygen gas is necessary to oxidize Fe+S to Fe2Oa. The presence of water facilitates the migration of Fe+2 from the reaction site. The resulting reduction in Fe+2 concentration allows more to be formed. Support for these ideas comes from the frequent observation that when... 

It resists pinpoint rust formation before enameling and filiform corrosion after enameling. 

FIGURE 12.18 The mechanism of rust formation in a drop of water, (a)... 

To prevent contamination, the tanks will be lined with epoxy, except for the bottom and the lower 2 ft of the sides. The lower portions will be lined with an inorganic zinc silicate such as Dimetcote to prevent rust formation and to allow static charges that may develop in the liquid to drain off through the tank.23... 

Misawa, T. Kyuno, T. Suetaka, W. Shimodaira, S. (1971) Mechanism of atmospheric rusting and the effect of copper and phosphorus on the rust formation of low alloy steels. Corrosion Sci. 11 35-48... 

Figure 14.11 illustrates the electrochemical rust formation process. 

Rust is a product of electrochemical reactions occurring on a piece of iron or steel. Iron metals are never uniform. There are always minor irregularities in both the composition and physical structure of the metal. These minute differences give rise to the anodic and cathodic regions associated with rust formation. During the rusting process, iron is oxidized at the anode according to the reaction ... 

The area of rust formation is different from the point where oxidation of iron takes place. As noted, water serves as an electrolyte through which iron ions migrate. This explains why vehicles rust much more rapidly in regions where road salts are used to melt winter ice. The salts improve the... 

The x in this equation indicates a positive whole number corresponding to the variable amount of water molecules that takes part in the reaction. The rusting process is illustrated in Figure 52.1. The area of rust formation differs from where the oxidation of iron takes place. 

FIGURE 12.15 The mechanism of rust formation, (a) Oxidation of the iron occurs at a point out of contact with the oxygen of the air, and the surface of the metal acts as an anode in a tiny galvanic cell, (b) Further oxidation of Fe2+ to Fe3+ results in the deposition of rust on the surface. 

Another example of corrosion is electrochemical corrosion. A well-known example of this is the rust formation on the body of a car. Such a process requires two electrochemical elements, transport of electrons and a closed circuit . Suppose two steel tubes are connected by means of a copper fitting and water flows through them. In that case electrons from the iron atoms in the steel will move in the direction of the fitting and the thus formed Fe2+ ions will be given off to the flowing water. The tube decays . 

Ona-Nguema, G., Abdelmoula, M., Jorand, F. et al. (2002) Iron(n,m) hydroxycarbonate green rust formation and stabilization from lepidocrocite bioreduction. Environmental Science and Technology, 36(1), 16-20. 

Figure 1.5 The electrochemical process involved in rust formation...

Cosmetic corrosion Surface attack at spots where paint is damaged related to (i) red rust, (ii) paint creep, (iii) chipping factors involved are (i) coating composition and thickness, (ii) surface treatment, (iii) type of paint damage, (iv) type of paint generally evaluated by (i) measuring length of underpaint creep, (ii) extent of rust formation or paint loss... 

Corrosion of reinforcement is probably the most widespread cause of deterioration in concrete. The expansion produced by rust formation causes the surrounding concrete to crack and spall. In a sound concrete, rusting is prevented by the high pH of the pore solution, which stabilizes an oxide film on the steel that inhibits further attack. This film is unstable at lower pH values, which can result from carbonation or leaching, or in the presence of Cl . Sources of the latter include sea water or salt spray, de-icing salts used on roads, certain aggregates, especially those available in desert climates, and CaClj used as an accelerator. Though now widely prohibited, this can... 

The adhesive is applied to the fat-free thread before the screw is screwed in immediately. Sufficient initial strength sets in after approximately 30 minutes, the final functional strength after approximately 3 hours. Fixing by means of adhesives has the additional advantage of absolute tightness of the screw connection, and it prevents the possible rust formation within the boundary layer. 

By the precipitation of iron hydroxide the equilibrium shifts in favor of further rust formation. But rust is not pure iron hydroxide - it is a mixture of oxides and hydroxides of iron. 

Figure 19.14 shows the mechanism of rust formation. The electrical circuit is completed by the migration of electrons and ions this is why rusting occurs so rapidly in salt water. In cold climates, salts (NaCl or CaCl2) spread on roadways to melt ice and snow are a major cause of rust formation on automobiles. 

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