Impurity effect corrosion resistance

Refining and Isomerization. Whatever chlorination process is used, the cmde product is separated by distillation. In successive steps, residual butadiene is stripped for recycle, impurities boiling between butadiene (—5° C) and 3,4-dichloto-l-butene [760-23-6] (123°C) are separated and discarded, the 3,4 isomer is produced, and 1,4 isomers (140—150°C) are separated from higher boiling by-products. Distillation is typically carried out continuously at reduced pressure in corrosion-resistant columns. Ferrous materials are avoided because of catalytic effects of dissolved metal as well as unacceptable corrosion rates. Nickel is satisfactory as long as the process streams are kept extremely dry. 

Few general statements can be made regarding the effect on corrosion resistance of alloying elements or impurities. A useful summary of the information has been prepared by Whitaker. Copper is usually harmful causing increased susceptibility to intercrystalline or general attack, so that alloys... 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

The impurities likely to be present in nominally pure tin are unlikely to affect its corrosion resistance, except for minor effects on the rate of oxidation in air. Small aluminium contents, however, may result in a severely embrittling intercrystalline attack by water. The addition of antimony counteracts this effect. Although 0-1% magnesium appears to be tolerable, larger amounts produce effects similar to those of aluminium. 

Impure metals and alloys exhibit all the structural features and crystal defects of the pure meteils already discussed. In addition, however, impure metals and alloys exhibit many structures which are not observed in pure metals, and which, in many instances, have an extremely important effect on the properties, particularly the corrosion resistance. However, before dealing with the structure of impure metals and alloys, it is necessary to consider the concept of metallurgical components, phases, constituents and equilibrium phase diagrams. 

Tinplate and Solder. Metallurgical studies were performed to determine the effect of irradiation at low temperature on the corrosion resistance of tinplate and on the mechanical properties and microstructure of tinplate and side-seam solder of the tinplate container. The area of major interest was the effect of low-temperature irradiation on the possible conversion of the tin from the beta form to the alpha form. In the case of pure tin, the transition occurs at 18 °C. It was feared that low-temperature irradiation would create dislocations in the crystal lattice of tin and enhance the conversion of tin from the silvery form to a powdery form rendering the tin coating ineffective in protecting the base steel. Tin used for industrial consumption contains trace amounts of soluble impurities of lead and antimony to retard this conversion for several years. 

Very pure single crystals have defects that can effect corrosion, but impurities and alloying elements, grain boundaries, second phases, and inclusions often have serious effects. Welded structures invariably corrode first at the welds because of metallurgical heterogeneities that exist in and near welds. The most susceptible site or defect in a metal will be the first to be attacked on exposure to a corrosive environment. Sometimes such attack simply results in innocuous removal of the susceptible material, leaving a surface with improved corrosion resistance. (Frankel)5... 

Although several impurities are known to segregate in these boundaries, correlations have been made with essentially continuous films of iron carbide or segregated carbon. Maximum susceptibility occurs in the range of 0.005% C it is proposed that lower carbon contents do not Table 7.9 Effect of alloying elements on stress-corrosion resistance ... 

Corrosion of Magnesium in Neutral and Alkaline Solutions Magnesium is highly susceptible to galvanic corrosion. Small amounts of impurities in the alloy can have a tremendous influence on the corrosion susceptibility. In Fig. 30, the influence of various elements is demonstrated. Small additions of copper, iron, nickel, and cobalt have an extremely negative effect on the corrosion resistance. The tolerance Kmit for iron is 0.015%, for nickel 0.0005%, and for copper 0.1% [35]. Because of the low solid solubility of these elements, they precipitate as inclusions. These act as active cathodic sites for the... 

Alloying elements or impurities in the zinc coating have been found to have little effect other than that observed on addition of copper or aluminum. The action of copper, which increases the corrosion resistance of the coating, is the complete reverse of this elanent s effect on aluminum coatings. Aluminum decreases the corrosion resistance. 

The usual alloying additions to aluminum in order to improve physical properties include Cu, Si, Mg, Zn, and Mn. Of these, manganese may actually improve the corrosion resistance of wrought and cast alloys. One reason is that the compound MnAle forms and takes iron into solid solution. The compound (MnFe)Alg settles to the bottom of the melt, in this way reducing the harmful influence on corrosion of small quantities of alloyed iron present as an impurity [27]. No such incorporation occurs in the case of cobalt, copper, and nickel, so that manganese additions would not be expected to counteract the harmful effects of these elements on corrosion behavior. 

The environment in which an article is used may influence bond durability (see also Durability fundamentals). Atmospheric ozone can cause time-dependent crack growth in vulcanized elastomers in addition, ozone can induce failure at a bond with certain bonding agents. Although water is only slightly soluble, it can permeate elastomers by an osmotic mechanism induced by salt-Uke impurities. As a result, the uptake in salt water is generally less than that in pure water. Rubber to metal bond failure has been found to occur in a time-dependent manner under salt water in the presence of electrochemical activity but much more slowly, if at all, in its absence (see also Cathodic disbondment). In the absence of imposed electrochemical activity, effects are likely to depend particularly on the metal used and its corrosion resistance. Provision of a bonded rubber cover layer over all metal surfaces subject to immersion is likely to enhance bond durability. 

As a general rule, assuming impurity and dissimilar metal effects are controlled and not overriding, the lower the nickel content of an alloy, the better is its corrosion resistance in liquid metals. The materials shown in Table 1 have proven to be corrosion-resistant to the specified liquid metals up to the temperature limit indicated. For additional information on materials compatibility, the reader is referred to the Liquid Metals Handbook [92,93]. 

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