Aluminum corrosion characteristics

The problem is somewhat different with an oxidizer such as N204. While N204 is compatible with aluminum and many stainless steels, the presence of a small amount of water as an impurity can increase its corrosive characteristics considerably. In addition, if a leak, even of micron size, is present in the propellant tank, a corrosive condition occurs, caused by reaction of the metal with nitric acid, which formed when the N204 contacts water from the surrounding atmosphere. 

Should not exhibit corrosivity characteristics when placed in intimate contact with steel and aluminum alloy motors. 

The previous paragraph assumes that the ethanol will be dry (containing no water) and contain only very small amounts of contaminants such as chloride and sulfate ions that would greatly increase the corrosivity of ethanol. Ethanol produced for fuel purposes in the past has contained up to 5 volume percent water and ion concentrations that made it much more corrosive than pure ethanol [3.7]. For an ethanol fuel with these corrosion characteristics, it was found that aluminum and steel could be coated with cadmium, hard chromium, nickel, or anodized aluminum to make them compatible. Coatings such as zinc, lead, and phosphate were found to be inadequate to prevent corrosion [3.7]. 

In an undamaged corrosion protection system, the corrosion characteristics of the surface of the substrate (i.e., pure aluminum oxide, mixed oxides, chromium... 

These alloys containing 5 to 12 % aluminum have exceUent resistance to impingement corrosion and high-temp)erature oxidation. The corrosion characteristics of aluminum bronzes are related primarily to aluminum content. Alloys with up to 8 % aluminum have face-centered cubic alpha structures and good resistance to corrosion attack. 

A corrosion mechanism similar to brass dezincification, known as dealuminification, can occur to the beta phase or eutectoid structure depending on environmental conditions. Proper quench and temper treatments produce a tempered beta structure with reprecipitated acicular alpha crystals, a combination often superior in corrosion resistance to the normal annealed structure. The nickel component in the more complex nickel aluminum bronzes alters the corrosion characteristics of the beta phase because of the nickel additive and gives greater resistance to deaUoying and cavitation-erosion in most liquids. 

Standard Test Method for Cavitation Corrosion and Erosion-Corrosion Characteristics of Aluminum Pumps with Engine Coolants... 

The localized corrosion of aluminum is characteristically autocatalytic in nature, as has already been mentioned in Section 3.3.4. To know whether or not localized corrosion will occur, it is important to know the relative value of Econ and the threshold potential E. In other words, to prevent aluminum from pitting, it is necessary to consider alloy design and environmental conditions and to shift corr a less noble directions than pj, (ASM International,... 

Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen as well as minerals, several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper-nickel alloys, and nickel-chromium-molybdenum alloys are highly corrosion resistant in seawater. 

Another test described in ASTM D 2809, Test Method for Cavitation Erosion-Corrosion Characteristics of Aluminum Puir ts with Engine Coolants. This test can be used to evaluate automotive water pump materials (aluminum castings) or the abOity of a coolant formulation to prevent metal loss. Chance (Ref... 

The corrosion characteristics of aluminum in propionic acid are shown in Fig. 22. The rates of attadc are voy amilar to those in acetic add. Again, the rates can be significantly affected by contamination. 

This chapter discusses the ambient-temperature corrosion characteristics of aluminum MMCs. Enqrhasis is placed on marine environments. Coatings and design criteria for optimum protection of MMCs arc also discussed. 

Aluminum and aluminum alloys are employed in many appHcations because of the abiHty to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. This film is 5 —10 nm thick when formed in air if dismpted it begins to form immediately in most environments. The weathering characteristics of several common aluminum alloy sheet products used for architectural appHcations are shown in Eigure 30. The loss in strength as a result of atmospheric weathering and corrosion is small, and the rate decreases with time. The amount of... 

The electrolysis protection process using impressed current aluminum anodes allows uncoated and hot-dipped galvanized ferrous materials in domestic installations to be protected from corrosion. If impressed current aluminum anodes are installed in water tanks, the pipework is protected by the formation of a film without affecting the potability of the water. With domestic galvanized steel pipes, a marked retardation of the cathodic partial reaction occurs [15]. Electrolytic treatment alters the electrolytic characteristics of the water, as well as internal cathodic protection of the tank and its inserts (e.g., heating elements). The pipe protection relies on colloidal chemical processes and is applied only to new installations and not to old ones already attacked by corrosion. 

This is used widely for chemical and process plant applications where its resistance to corrosion and high thermal conductivity are desirable characteristics. The strength of aluminum can be increased by cold working, as shown in Table 3.29. 

In safety applications, the corrosion resistance of the duct materials deserv es special consideration. Since material costs generally increase along with corrosion resistance, the selection of material must be determined by the desired life span in the anticipated environment this environment is a function of the characteristics of the chemical being processed and the operating conditions of the reactor. For maximum resistance to moisture or corrosive gases, stainless steel and copper are used where their cost can be justified. Aluminum sheet is used where lighter veight and superior resistance to moisture are needed. 

This is the exchanger where heat flows from the room return or mixed air to cold refrigerant or to chilled water. It is an arrangement of finned tubes normally of aluminum fins on copper tubes, but copper fins can be specified for corrosive atmosphere. Performance characteristics are controlled by fin and tube spacing. If the room rh is high, dehumidification may be brought into use by operating the coil or one of a number of parallel coils at a low temperature. If the room s sensible heat load is low reheat must be allowed to operate at the same time. 

Aluminum has many of the characteristics and qualities required for fluid power lines. Is has high resistance to corrosion and is easily drawn or bent. In addition, it has the outstanding characteristic of lightweight. Since weight elimination is a vital factor in the design of aircraft, aluminum alloy tubing is used in the majority of aircraft fluid power systems. 

It is important to recognize that all materials will have problems in certain environments, whether they are plastics, metals, aluminum, or something else. For example, the chemical effect and/or corrosion of metal surfaces has a damaging effect on both the static and dynamic strength properties of metals because it ultimately creates a reduced cross-section that can lead to eventual failure. The combined effect of corrosion and stress on strength characteristics is called stress corrosion. When the load is variable, the com-... 

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 ... 

Table I shows the chemical composition limits of various aluminum alloys presently used for packaging applications (3). In general, these alloys have good corrosion resistance with most foods. However, almost without exception, processed foods require inside enameled containers to maintain an acceptable shelf life (4, 5). Moreover, when flexible foil packages are used for thermally processed foods, the foil is laminated to plastic materials that protect it from direct contact with the food and also provide heat sealability as well as other physical characteristics (6,7).

Metals such as aluminium, steel, and titanium are the primary adherends used for adhesively bonded structure. They are never bonded directly to a polymeric adhesive, however. A protective oxide, either naturally occurring or created on the metal surface either through a chemical etching or anodization technique is provided for corrosion protection. The resultant oxide has a morphology distinct from the bulk and a surface chemistry dependent on the conditions used to form the oxide 39). Studies on various aluminum alloy compositions show that while the oxide composition is invariant with bulk composition, the oxide surface contains chemical species that are characteristic of the base alloy and the anodization bath40 42). 

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