Corrosion alloying

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

The main febricated parts of the units are carbon steel, with suitable corrosion allowance for the conditions of the chilled and condensing water. When brackish or sea water is used in a barometric condenser, steel construction with a V4 -in. to -in. corrosion allowance is suggested, and minimum wall plates of V2 -in. to -in. may be justified. Internal splash plates should be V2 -in. to -in. minimum, because the atmosphere of water vapor-air is very corrosive. Alloy construction is not justified except in exceptional cases. 

The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

Tin and lead are often used together in a variety of useful alloys, partly because the presence of a few percent of tin mixed with lead causes the alloy to be considerably harder than lead is alone. For example, common solder consists of about 50% Sn and 50% Pb. Type metal consists of about 82% Pb, 15% Sb, and 3% Sn. Pewter, an alloy used to make ornamental objects and vessels for food and beverages, consists of about 90% Sn with the remainder consisting of copper and antimony. Large quantities of tin are used as a coating for other metals, particularly steel, to retard corrosion. Alloys used as bearings often contain tin, antimony, and copper or aluminum. One such alloy is babbit, which consists of 90% Sn, 7% Sb, and 3% Cu. Several other commonly encountered alloys have various specialty uses. 

Fig. 7.48 Response of five austenitic stainless steels to pitting and crevice corrosion. Alloys exposed 1 month at room temperature in indicated concentrations of FeCI3 solutions. (Numbers represent weight percent.)...

Linearly extrapolated oxidation-corrosion data for the 1150 hr first exposure in the HYGAS gasifier offgas are plotted in Fig. 7. In this location at 580°F (average) carbon steel, AISI 410, AISI 304, IN-800, and titanium exhibited very limited corrosion. Alloy IN-600 and Monel 400 had corrosion rates of 42 and 124 mpy, respectively. 

As described in Chapter 4.2 on localized corrosion, alloys that are protected by a thin passive film tend to be susceptible to locaKzed corrosion resulting from a breakdown of that film and aggressive dissolution at the breakdown sites. The tendency for breakdown increases as the potential increases. The electrochemical techniques described in Sect. 7.3 can be used to address localized corrosion. Other techniques specific to localized corrosion... 

This chapter is devoted to the corrosion performance of non-ferrous alloys, i.e. those not based in the element iron (Fe). Most of the structural and corrosion alloys that are used worldwide are just plain steel (>95% iron) and stainless steels which are iron-based alloys (>50% Fe) containing different amounts of chromium (Cr), nickel (Ni), molybdenum (Mo), and other elements. 

This nonhardenable chromium steel exhibits good resistance to reducing sulfurous gases and fuel-ash corrosion. Alloy S44600 has good general corrosion resistance in mild atmospheric environments, fresh water, mild chemicals, and mild oxidizing conditions. 

The presence of these acids in crude oils and petroleum cuts causes problems for the refiner because they form stable emulsions with caustic solutions during desalting or in lubricating oil production very corrosive at high temperatures (350-400°C), they attack ordinary carbon steel, which necessitates the use of alloy piping materials. 

CO2 corrosion often occurs at points where there is turbulent flow, such as In production tubing, piping and separators. The problem can be reduced it there is little or no water present. The initial rates of corrosion are generally independent of the type of carbon steel, and chrome alloy steels or duplex stainless steels (chrome and nickel alloy) are required to reduce the rate of corrosion. 

Another method by which metals can be protected from corrosion is called alloying. An alloy is a multicomponent solid solution whose physical and chemical properties can be tailored by varying the alloy composition. 

For example,copper has relatively good corrosion resistance under non-oxidizing conditions. It can be alloyed with zinc to yield a stronger material (brass), but with lowered corrosion resistance. Flowever, by alloying copper with a passivating metal such as nickel, both mechanical and corrosion properties are improved. Another important alloy is steel, which is an alloy between iron (>50%) and other alloying elements such as carbon. 

Marcus P, Teissier A and Oudar J 1984 The influence of sulfur on the dissolution and the passivation of a nickel-iron alloy. 1. Electrochemical and radiotracer measurements Corrosion Sc . 24 259... 

Cihal V 1984 Intergranular Corrosion of Steels and Alloys (Amsterdam Elsevier)... 

Intermetallic compounds with gallium are used as semiconductors. Indium is used to coat other metals to protect against corrosion, especially in engine bearings it is also a constituent of low-metal alloys used in safety sprinklers. The toxicity of thallium compounds has limited the use of the metal, but it does find use as a constituent of high-endurance alloys for bearings. 

Reactor-grade zirconium is essentially free of hafnium. Zircaloy(R) is an important alloy developed specifically for nuclear applications. Zirconium is exceptionally resistant to corrosion by many common acids and alkalis, by sea water, and by other agents. Alloyed with zinc, zirconium becomes magnetic at temperatures below 35oK. 

---