The following information is condensed from "2^nd Edition Handbook of Corrosion Data" Edited by B.D. Craig, D.S. Anderson, published by ASM International, 1995

Stainless Steels

Stainless steels are iron-base alloys containing at least 12% chromium. Maximum corrosion protection occurs, generally, with the highest chromium content, which may range up to about 30%. Corrosion resistance of stainless steels is a function not only of composition, but also of heat treatment, surface condition, and fabrication procedures, all of which may change the thermodynamic activity of the surface and thus dramatically affect the corrosion resistance. It is not necessary to chemically treat stainless steels to achieve passivity. The passive film forms spontaneously in the presence of oxygen. Most frequently, when steels are treated to improve passivity (passivation treatment), surface contaminants are removed by pickling to allow the passive film to reform in air, which it does almost immediately.

The principal alloying elements that affect the corrosion resistance of stainless are discussed below.

Chromium. The one element essential in forming the passive film or high-temperature, corrosion-resistant chromium oxide is chromium. Other elements can influence the effectiveness of chromium in forming or maintaining the film, but no other element can, by itself, create the stainless characteristics of stainless steel. The passive film is observed at about 10.5% chromium, but it affords only limited atmospheric protection at this point. As chromium content is increased, the corrosion protection increases. When the chromium level reaches the 25 to 30% level, the passivity of the protective film is very high, and the high-temperature oxidation resistance is maximized.

Nickel. In sufficient quantities, nickel is used to stabilize the austenitic form of iron and so produce austenitic stainless steels. A corrosion benefit is obtained as well, because nickel is effective in promoting repassivation, especially in reducing environments. Nickel is particularly useful in promoting increased resistance to mineral acids. When nickel is increased to about 8 to 10% (a level required to ensure austenitic structures in a stainless which has about 18% chromium) resistance to stress-corrosion cracking is decreased. However, when nickel is increased beyond that level, resistance to stress-corrosion cracking increases with increasing nickel content.

Manganese. An alternative austenite stabilizer is sometimes present in the form of manganese, which in combination with lower amounts of nickel than otherwise required will perform many of the same functions of nickel in solution. The effects of manganese on corrosion are not well documented; it is known that manganese combines with sulfur to form sulfides. The morphology and composition of these sulfides can have substantial effects on the corrosion resistance of stainless steels, especially their resistance to pitting corrosion.

Other Elements. Molybdenum in moderate amounts in combination with chromium is very effective in terms of stabilizing the passive film in the presence of chlorides. Molybdenum is especially effective in enhancing the resistance to pitting and crevice corrosion. Carbon does not seem to play an intrinsic role in the corrosion characteristics of stainless, but it has an important role by virtue of the tendency of carbide formation to cause matrix or grain boundary composition changes that may lead to reduced corrosion resistance. Nitrogen is beneficial to austenitic stainless in that it enhances pitting resistance, retards formation of sigma phase, and may help to reduce the segregation of chromium and molybdenum in duplex stainless steels.

Aluminum and Aluminum Alloys

Aluminum and its alloys resist attack by a wide range of environments and many chemical compounds. Consequently, aluminum (along with stainless steel) is the metal most thought of by the public and engineering community when lower temperature corrosion resistance is considered. The widespread acceptance of aluminum in cookware, the beverage industry, and as home siding testifies to the utility and applicability of alloys of aluminum as a corrosion-resistant material. Table 5 gives comparative corrosion characteristics of aluminum alloys.

The acidity or alkalinity of the immediate environment (chemical, soil, atmospheric, or aqueous) significantly affects the corrosion of aluminum. The Pourbaix diagram for aluminum with a hydrated aluminum oxide film shows immunity or passive behavior in the pH range of about 3 to 8.5. When aluminum is exposed to higher pH (alkaline) conditions, corrosion may occur, and when the oxide film is perforated locally, accelerated attack occurs, because aluminum is attacked more rapidly than its oxide under alkaline conditions. The result is pitting corrosion. In acidic conditions, the oxide is more rapidly attacked than aluminum, and more general attack should result.

The principal alloying elements (copper, magnesium, silicon, and zinc) added to aluminum reduce the atmospheric corrosion resistance somewhat. More significantly, they affect other localized corrosion resistance (stress-corrosion cracking and exfoliation corrosion, for example).

Magnesium. Magnesium in nonhardening type alloys (5000 series) makes them especially immune to aqueous corrosion. The magnesium-silicide alloys (6000 series) offer excellent aqueous corrosion resistance in a stronger material. The copper (2000 series) and zinc (7000 series) alloys are generally much less resistant to aqueous corrosion. Cladding techniques, as discussed below, afford protection for many applications.

Magnesium somewhat increases the resistance of aluminum to corrosion in alkaline solutions, but when present in the grain boundaries as an anodic magnesium aluminum compound, it may promote stress-corrosion cracking and intergranular corrosion.

Silicon. Silicon in small amounts (0.1%) has little effect on pitting corrosion of aluminum, yet greater amounts reduce resistance to pitting. Silicon has a detrimental influence on the resistance of aluminum to seawater.

Chromium. Chromium when added to aluminum-magnesium or aluminum-magnesium-zinc alloys, is used in very small amounts (0.1 to 0.3%) and has a beneficial effect on corrosion resistance. Chromium improves resistance to stress-corrosion cracking in high-strength alloys, but in super-purity aluminum it increases the pitting potential in water. Iron, although not an intentional alloy addition, is the main cause of pitting in aluminum alloys. Its effects may be mitigated by other alloying additions.

Copper. Copper reduces the corrosion resistance of aluminum more than any of the common alloying elements. It can lead to a higher rate of uniform corrosion, greater occurrence of pitting, intergranular corrosion, and stress-corrosion cracking. At low levels (about 0.15%) of copper, the pitting resistance of commercial aluminum is decreased, especially in seawater. In higher copper content alloys (2000 series), the effect of the element is related directly to the fabrication process and heat treatment. At higher copper contents, intergranular corrosion or stress-corrosion cracking are areas of concern.

Zinc. Zinc has only a small influence on the corrosion resistance of commercial aluminum. It may reduce the resistance to acidic media, but improve the resistance to alkaline solutions. When zinc is present in higher levels and in combination with magnesium and copper, the influence of zinc is a function of the fabrication and heat treatment of the alloy. The high-zinc alloys may be susceptible to intergranular corrosion, stress-corrosion cracking, and exfoliation corrosion

Lithium. Lithium additions are now made to some aluminum alloys. Although lithium is a highly reactive element, the addition of up to 3% lithium makes aluminum only slightly more anodic. Little has been published on the corrosion resistance of lithium-containing alloys, however. In an electrochemical comparison of an aluminum-lithium-magnesium alloy to AA-7075 (aluminum-zinc-magnesium), the lithium alloy exhibited more active corrosion and pitting potentials, along with a higher current density for passivation.

Copper and Copper Alloys

Copper and copper alloys have been widely used for centuries in many applications because of their excellent corrosion resistance and moderate cost. Despite the formation of the common green patina in natural environments, copper and its alloys corrode at negligible rates in unpolluted water or air and in deaerated nonoxidizing acids. For marine applications, it is known that copper shows resistance to biofouling. Copper alloys resist many saline solutions, neutral or slightly alkaline solutions, and organic chemicals. In strongly reducing conditions at temperatures from about 290 to 400 ºC (550 to750 º F), copper alloys are often superior to stainless steels and other stainless alloys.

Though classed as corrosion resistant, neither copper nor its alloys form the truly passive corrosion-resistant film that characterizes most true corrosion-resistant alloys. In aqueous environments at ambient temperatures, cuprous oxide forms the protective scale. Alloy additions of aluminum, tin, zinc, and nickel are used to dope the corrosion product films to enhance the natural corrosion resistance of copper and to produce the range of corrosion-resistant capability for which copper alloys are noted.

A discussion of the role in corrosion resistance of the principal alloy additions that produce these systems follows.

Zinc. Zinc content in copper can range from a few percent to about 40%. The resistance of brasses to corrosion does not change markedly as long as the zinc content is 15%, or less. When the zinc content exceeds 15%, the alloy may be susceptible to dezincification. This is a process involving the leaching of the zinc from the alloy. It results in a porous, reduced ductility, reddish copper matrix. What remains may support a given load until an increase of pressure or weight exceeds the local ductility and causes fracture. Soft, stagnant, or slow moving waters or saline solutions can lead to dezincification of unmodified brasses. The brasses may be more prone to dezincification in stressed regions (for example, in the bent region of a float arm on a water closet fixture) or where a bend exists (as in an elbow in a fresh water supply line).

Tin. Tin bronzes are essentially solid solutions of tin in copper. Phosphorus is commonly used as a deoxidizer, and the residual phosphorus content gives rise to the term "phosphor bronze." The addition of tin to copper promotes good resistance to fresh and seawaters. Under some conditions, when more than 5% tin is present, the corrosion resistance in marine applications is enhanced. Where the water velocity is high, tin content for marine applications of copper alloys should exceed 5%. Alloys containing between 8 and 10% tin have high resistance to impingement attack.

Tin bronzes tend to have intermediate pitting resistance. When tin is added to some brasses, the corrosion resistance is substantially increased; in fact, 1% tin inhibits dealloying (dezincification) in 70 copper-30 zinc alloys.

Nickel. Nickel and copper are mutually soluble, and in alloys where copper is the dominant element, commercial alloys range from about 54 to over 90% copper. Nickel provides the best general resistance to aqueous corrosion of all the commercially important alloy elements. It promotes resistance to impingement corrosion and to stress-corrosion cracking.

The addition of nickel to copper-zinc alloys produces nickel-silver alloys. Most commonly these have about 18% nickel and 55 to 65% copper. Such alloy additions promote good resistance to corrosion in both fresh and salt waters. The nickel inhibits dezincification. Nickel-silvers are much more corrosion resistant in saline solutions than brasses of similar copper content.

Other elements. Other elements are added to copper alloys in varying amounts to introduce favorable corrosion characteristics. For example, 2% aluminum is added to 76 copper-22 zinc solutions to produce aluminum brass, and a small amount of arsenic (less than 0.10%) is added to the alloy to inhibit dezincification. Significant amounts of aluminum (5 to 12%) are added to the copper-nickel-iron-silicon-tin system to produce the aluminum bronzes. Aluminum bronzes provide a substantial range of corrosion resistance. Environments that should be avoided include nitric acid, some metallic salts (ferric chloride), moist chlorinated hydrocarbons, and moist ammonia. Exposure to the latter medium could trigger stress corrosion.

Nickel and Nickel Alloys

Nickel-base alloys generally are extremely effective corrosion-resistant materials in service environments that range from subzero to elevated temperatures. Nickel-base alloys are known for their ability to resist severe operating conditions involving liquid or gaseous environments, high stresses, and combinations of these factors. Nickel itself has good resistance to corrosion in reducing environments and can be used in oxidizing environments that act to promote the formation of a passive, corrosion-resistant oxide film.

The principal elements used in nickel-base alloys are copper (as in the Monels) and chromium plus aluminum (as in superalloys such as the Hastelloy or Inconel/Incoloy materials). Chromium and aluminum provide elevated temperature oxidation resistance, and chromium and titanium provide resistance to hot corrosion. Other elements also are used to enhance oxidation/hot corrosion resistance. Chromium is a prime promoter of corrosion resistance in some liquid media at lower temperatures, but other alloy elements also are quite significant in the enhancement or promotion of corrosion resistance. Notable among the latter are copper, molybdenum, and tungsten. Other alloy elements are used to a considerable degree in nickel-base alloys, although not primarily for corrosion (or oxidation) resistance. Superalloys, in particular, may contain, intentionally, up to a dozen elements at controlled levels. The role of the major alloy element additions to nickel are summarized below.

Chromium. Chromium additions promote improved resistance of nickel to oxidizing acids such as nitric and chromic. Chromium also improves resistance to high-temperature oxidation in binary alloys, provided the chromium level exceeds about 5%. In practice, when chromium is the element providing oxidation resistance, a level of 20% or greater was found to be desirable for maximum corrosion protection.

A principal role of chromium in modern superalloys has been to promote resistance to hot corrosion. Hot corrosion-resistant nickel-base alloys contain no less than about 14% chromium, and the most resistant alloys contain as much as 22%.

Copper. Copper has long been a prime alloying element with nickel, because the two elements are mutually soluble in one another. Each has good ductility and good corrosion resistance, along with the ability to be hardened. Additions of copper provide improvement in the resistance of nickel to nonoxidizing acids. In particular, alloys containing 30 to 40% copper offer useful resistance to nonaerated sulfuric acid and offer excellent resistance to all concentrations of nonaerated hydrofluoric acid. The Monel series of alloys are based on the 70% nickel/30% copper composition and are widely used in the marine, chemical, petroleum, and process industries.

In addition to its role as a major alloying element, copper has been found to confer improved resistance to hydrochloric acid and phosphoric acid when 2 to 3% copper is added to nickel-chromium-molybdenum-iron alloys.

Molybdenum. Molybdenum in nickel substantially improves resistance to nonoxidizing acids. Commercial alloys for ambient temperature applications have employed up to 28% molybdenum for service in severe nonoxidizing solutions of hydrochloric, phosphoric, and hydrofluoric acids as well as in sulfuric acid. On the other hand, molybdenum, when used for elevated temperature strength, drastically degrades the hot corrosion resistance of nickel-base superalloys. Molybdenum in nickel superalloys rarely exceeds 6%. However, 9% molybdenum has been combined with 21.5% chromium, 3.5% niobium, and minor other elements in nickel to produce Incone1 625, a non-age-hardenable superalloy with outstanding resistance to reducing or oxidizing conditions.

Tungsten. Tungsten behaves in a similar way to molybdenum, in that it degrades the hot corrosion resistance of superalloys, but provides improved resistance to nonoxidizing acids and to localized corrosion.

Iron. Iron is not added to nickel to improve corrosion resistance, but rather to reduce costs. However, iron does provide nickel with improved resistance to sulfuric acid at concentrations above 50% sulfuric.

Silicon. Silicon typically is present in minor amounts as a residual element. In general, silicon is restricted to low levels to minimize processing problems and the potential for embrittling reactions in certain alloys. Under some circumstances, silicon has been intentionally added to promote the elevated temperature oxidation resistance of superalloys where it probably promotes scale retention of the protective oxides formed by chromium or aluminum.

Aluminum. Aluminum is added to nickel-base alloys principally to produce high-temperature strength through the precipitation of the gamma prime phase in the nickel-chromium matrix. An unexpected benefit of the addition of aluminum was the formation of oxidation-resistant aluminum oxide scales on alloys containing greater than about 4% aluminum. Aluminum may actually be detrimental in promoting hot corrosion resistance in superalloys, depending on the level of chromium and aluminum in the alloy, as well as the temperature of exposure to hot corrosion-producing environments.

Titanium. Titanium is not added in any significant amounts to nickel alloys for lower temperature applications; if present, it can tie up carbon, nitrogen, or oxygen, which may be beneficial under some conditions of stress-enhanced corrosion. Titanium is found as a constituent of superalloys, where it acts similarly and in concert with aluminum to produce strength through gamma-prime hardening.

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