Metals, corrosion environment

Before examining in detail the theories of aqueous corrosion processes and the bases for making quantitative calculations of corrosion rates, it will be useful to develop qualitatively the major phenomena involved. The following sections review several general types of metal/corrosive-environment combinations, the chemical reactions involved, idealized mechanisms for the transfer of metal ions to the environment, and the electrochemical processes occurring at the interface between the metal and the aqueous environment. 

FIGURE 20.69 Position of a cathodic half-cell plot can affect the nature of metallic corrosion. Environment A intersects the anodic polarization curve by the active region, and environment P intersects in the passive region. 

Metal Corrosive Environment Corrosion Rate (Mils Penetration per Year)... 

The presence of a chemical compound in an environment, even in small concentrations, can lead to significant changes in speed and form of corrosion of a metallic material in contad with the environment. The acceleration or inhibition of corrosion processes are spedfic methods, dependent of metal-corrosive environment charaderistics. 

Both processes of oxidation and reduction simultaneously occur at the metal-corrosive environment interface during an electrochemical corrosion. The basic processes occurring during the electrochemical corrosion of metallic materials are ... 

Oxidation and reduction processes are accompanied by the flow of electric charge through the interface metal-corrosive environment. In metals the charge carriers are electrons while in the corrosive environment charge flow is due to ions. Thus an active assessment of electrochemical corrosion processes can be achieved by assessing the electrical charge transfer process. In the reactions of corrosion that are controlled by the rate of charge transfer, the current - potential relationship can be described by the Butler-Vokner equation ... 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

Despite the benefits of lead and lead ahoys, the use of these materials is declining rapidly, owing primarily to environmental health and safety factors. Eor many years, lead ahoys were the materials of choice for many corrosive environments, but are now being replaced by stainless steel, plastics, and exotic metals. The toxic nature of lead requires special precautions and handling not necessary with other materials. These requirements have reduced the usage of lead ahoys. 

The first commercial use of tantalum was as filaments ia iacandescent lamps but it was soon displaced by tungsten. Tantalum is used ia chemical iadustry equipment for reaction vessels and heat exchangers ia corrosive environments. It is usually the metal of choice for heating elements and shields ia high temperature vacuum sintering furnaces. In 1994, over 72% of the tantalum produced ia the world went iato the manufacturiag of over 10 x 10 soHd tantalum capacitors for use ia the most demanding electronic appHcations. 

A uniform coating of calcium carbonate deposited on the metal surfaces physically segregates the metal from the corrosive environment. To develop the positive LSI required to deposit calcium carbonate, it is usually necessary to adjust the pH or calcium content of the water. Soda ash, caustic soda, or lime (calcium hydroxide) may be used for this adjustment. Lime is usually the most economical alkaH because it raises the calcium content as weU as the alkalinity. 

The gating and riseting system for cast aluminum bron2e is extremely important and must be arranged to iatroduce the metal quietly at the lowest portion of the mold. The alloys shrink well hence the gating and riseting must be well adapted to the particular casting. See Table 12 for properties of these alloys. Alloys C 95300, C 95400, and C 95500 are heat-treatable for iacreased mechanical properties and the last two should be temper-aimealed if used ia a corrosive environment. 

Corrosion also occurs as a result of the conjoint action of physical processes and chemical or electrochemical reactions (1 3). The specific manifestation of corrosion is deterrnined by the physical processes involved. Environmentally induced cracking (EIC) is the failure of a metal in a corrosive environment and under a mechanical stress. The observed cracking and subsequent failure would not occur from either the mechanical stress or the corrosive environment alone. Specific chemical agents cause particular metals to undergo EIC, and mechanical failure occurs below the normal strength (5aeld stress) of the metal. Examples are the failure of brasses in ammonia environments and stainless steels in chloride or caustic environments. 

Biological Corrosion The metabohc activity of microorganisms can either directly or indirectly cause deterioration of a metal by corrosion processes. Such activity can (1) produce a corrosive environment, (2) create electrolytic-concentration cells on the metal surface, (3) alter the resistance of surface films, (4) have an influence on the rate of anodic or cathodic reaction, and (5) alter the environment composition. 

Inhibitors The use of various substances or inhibitors as additives to corrosive environments to decrease corrosion of metals in the environment is an important means of combating corrosion. This is generally most attractive in closed or recirculating systems in which the annual cost of inhibitor is low. However, it has also proved to be economicaUv attrac tive for many once-through systems, such as those encountered in petroleum-processing operations. Inhibitors are effective as the result of their controlling influence on the cathode- or anode-area reactions. 

Electrochemical techniques have been used for years to study fundamental phenomenological corrosion reactions of metals in corrosive environments. Unfortunately, the learning curve in the reduction of these elec trochemical theories to practice has been painfully slow. However, a recent survey has shown that many organizations in the... 

FIG. 28-19 Equipment for measuring internal friction (modulus) changes during in situ tensile exposure of a metal in a corrosive environment. 

Erosion-corrosion is a fairly complex failure mode influenced by both environmental factors and metal characteristics. Perhaps the most important environmental factor is velocity. A threshold velocity is often observed below which metal loss is negligible and above which metal loss increases as velocity increases. The threshold velocity varies with metal and environment combinations and other factors. 

Nascent atomic hydrogen released at metal surfaces by chemical reactions between the process environment and the metal (corrosion or cathodic protection reactions)... 

Figure 7-81. Flat disk used for low pressure and for isolation of corrosive environments. Usual pressure range is 2 to 15 psig with 1 psi tolerance. Stainless steel disk with Teflon seal is usually standard. By permission, Flke Metal Products Div., Fike Corporation. Catalog 73877-1, p. 35.

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