Alloying elements, effect corrosion resistance

The anodic polarization of a given alloy base metal such as iron or nickel is sensitive to alloying element additions and to heat treatments if the latter influences the homogeneity of solid solutions or the kinds and distribution of phases in the alloy. The effect of chromium in iron or nickel is to decrease both EpP and icrit and hence to enhance the ease of placing the alloy in the passive state. The addition of chromium to iron is the basis for a large number of alloys broadly called stainless steels, and chromium additions to nickel lead to a series of alloys with important corrosion-resistant properties. 

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

In air, carburising atmospheres and sulphidising/oxidising atmospheres all alloys showed excellent corrosion resistance due to the formation of protective oxide scales. It was found that the concentration of aluminium or chromium does not significantly effect the corrosion behaviour in any of these environments. 10 wt% aluminium, probably even less, are sufficient to enable the formation of protective Al203-scales even at temperatures as low as 650°C. Overdoping with reactive elements (mischmetal), however, causes high oxidation rates in air and should be avoided. 

TABLE 23.1. Alloying Elements and Their Major Effects in Alloys for Aqueous Corrosion Resistance... 

Fitzer, E. and Schwab, J., Attack of Scaling-Resistant Materials by Vandium Pentoxide and Effect of Various Alloying Elements Thereon, Corrosion, Vol. 12,1956, p. 459. 

Effect of alloying. The additions of alloying elements to aluminum change the electrochemical potential of the alloy, which affects corrosion resistance even when the elements are in solid solution. Zinc and magnesium tend to shift the potential markedly in the anodic direction, whereas silicon has a minor anodic effect. Copper additions cause marked cathodic shifts. This results in local anodic and cathodic sites in the metal that affect the type and rate of corrosion. 

The corrosion resistance of ferritic steels has been extensively studied. The following expressions summarize the effects of different alloying elements on the resistance of ferritic steels exposed to boiling corrosive solutions during slow strain tests. The stress corrosion indices (SCls) in each environment integrate the beneficial (-) or deleterious (-I-) effect of the alloying elements (in %) when the steels are in contact with such a caustic environment. In boiling 4M NaNOa at pH 2 the stress corrosion index is... 

Solders. Modem dental solders are made from mostly corrosion-resistant, nontoxic metals. Minimal quantities of tin and other elements are often added, some of which could produce toxic effects in the unalloyed state. Each solder is used for specific appHcations (180—188) typical compositions and properties of solders used in dentistry are presented in Table 11. Most of the ingredients of solders are resistant to corrosion, and alloying them with other ingredients renders the alloy safe for use in appHances placed in the oral environment. Silver solders corrode, but are used only for temporary appHances. Available products do not contain cadmium, although cadmium was an ingredient of some silver solders up to ca 1980. 

The addition of small amounts of alloying materials greatly improves corrosion resistance to atmospheric environments but does not have much effect against liquid corrosives. The alloying elements produce a tight, dense adherent rust film, but in acid or alkaline solutions corrosion is about equivalent to that of carbon steel. However, the greater strength permits thinner walls in process equipment made from low-alloy steel. 

The improvement in rust resistance achieved through low-alloy additions obviously depends on the nature and amounts of the alloying elements — incidentally their effects are not additive — and to an even greater degree on the nature of the corrosive environment. To make a broad generalisation, weathering steels show to maximum advantage when they are freely exposed to the open air in industrial environments but, even then, their performance... 

Table 3.10 Effect of alloying elements on marine corrosion resistance...

Small variations in the composition of cast irons, or even the addition of small amounts of alloying elements, generally have little effect on the corrosion resistance. 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Amorphous Fe-3Cr-13P-7C alloys containing 2 at% molybdenum, tungsten or other metallic elements are passivated by anodic polarisation in 1 N HCl at ambient temperature". Chromium addition is also effective in improving the corrosion resistance of amorphous cobalt-metalloid and nickel-metalloid alloys (Fig. 3.67). The combined addition of chromium and molybdenum is further effective. Some amorphous Fe-Cr-Mo-metalloid alloys passivate spontaneously even in 12 N HCl at 60° C. Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acids of various concentrations and different temperatures are shown in Fig. 3.68 ... 

As can be seen in Fig. 3.67, the corrosion resistance of amorphous alloys changes with the addition of metalloids, and the beneficial effect of a metaU loid in enhancing corrosion resistance based on passivation decreases in the order phosphorus, carbon, silicon, boron (Fig. 3.72). This is attributed partly to the difference in the speed of accumulation of passivating elements due to active dissolution prior to passivation... 

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... 

Most commercial uses of aluminum require special properties that the pure metal cannot provide. The addition of alloying elements imparts strength, improves formability characteristics, and influences corrosion resistance properties. The general effect of several alloying elements on the corrosion behavior of aluminum has been reported by Godard et al. (2) as follows ... 

Ruthenium alloyed to platinum, palladium, titanium and molybdenum have many apphcations. It is an effective hardening element for platinum and palladium. Such alloys have high resistance to corrosion and oxidation and are used to make electrical contacts for resistance to severe wear. Ruthenium-palladium alloys are used in jewelry, decorations, and dental work. Addition of 0.1% ruthenium markedly improves corrosion resistance of titanium. Ruthenium alloys make tips for fountain pen nibs, instrument pivots, and electrical goods. Ruthenium catalysts are used in selective hydrogenation of carbonyl groups to convert aldehydes and ketones to alcohols. 

The addition of low concentrations of elements to steel such as manganese, titanium, or boron can greatly enhance the properties of steel. Improved hardness, strength, machinability, and resistance to corrosion can all be improved by alloying. The effect of various alloying elements is provided in TABLE 9-3. 

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