Uniform corrosion Subject

Ail homogeneous metals without differences in potential between any points on their surfaces are subject to this type of general attack under some conditions. Uniform corrosion is usually characterized by a chemical or electrochemical attack over the entire exposed surface, Figure 4-423. Metal corrodes in an even... 

The law (2) represents the behaviour of a system containing only two elementary processes controlled by the activation energy [21]. Its importance, however, is connected with the experimental observation that it satisfactorily describes the kinetics of a very large class of electrochemical systems. This is particularly true of metals and alloys of technological interest that are subject to uniform corrosion in a great number of aggressive environments. 

A coating that is cathodic, compared to the metallic substrate subject to general uniform corrosion, may be inherently dangerous, because the presence of defects in the coating would promote the localized dissolution of the base material with a penetration rate that is higher when the exposed surface area is small, that is, when the density of defects is low. 

Uniform corrosion is a loss of material distributed uniformly over the entire surface exposed to the corrosive environment. Metals in contact with strong acids are sometimes subject to uniform corrosion. 

Testing for uniform corrosion is one of the most common corrosion tests employed to determine material suitability in a wide variety of applications. The following is intended to be an introduction into the subject of testing for uniform corrosion, its applications, basic methodology, and limitations. The reader is referred to applicable standards fully detailing the procedures. 

In soil, stainless steel is fairly resistant to uniform corrosion, which occurs over the entire surface however, it may be subject to pitting corrosion. Stainless steel is most often used in situations where contamination of the material carried in the pipe is the prime concern. However, as pitting of the buried structures might occur, where soil conditions surrounding the pipe vary, it would be prudent to install stainless steel pipe with a uniform, well-installed backfill where differential oxygen corrosion cells will not occur. Coatings and cathodic protection of buried pipehnes in corrosive soils should be considered. In noncorrosive soils, coatings for stainless steel are recommended. 

While carbon steel Is not so reslsteuit to uniform corrosion attack or some types of pitting attack as stainless steely Its resistance Is acceptable. Further/ carbon steel Is superior to stainless steel In some aspects> e.g. It Is not subject to chloride stress corrosion cracking ... 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

It is notable that while it is possible to produce maraging steels with consistently uniform mechanical properties, the stress corrosion properties are subject to scatter, as indicated in Fig. 3.34. To a large extent this scatter is an indication of the greater sensitivity of s.c.c. resistance to metallurgical variables. Although the variation in cracking resistance is not well understood, and the reaction to certain treatments not always consistent, certain observations may be used to indicate guidelines for improved properties. 

Uniform microstractuie is cracial to the superior performance of advanced ceramics. In a cerantic material, atoms are held in place by strong chentical bonds that ate impervious to attack by corrosive materials or heat. At the same time, these bonds are not capable of much "give." When a ceramic material is subjected to mechanical stresses, these stresses concentrate at minute imperfections in the microstmcture, initiating a crack. The stresses at the top of the crack exceed the threshold for breaking the adjacent atomic bonds, and the crack propagates throughout the material causing a catastrophic brittle failure of the ceramic body. The rehability of a ceramic component is directly related to the number and type of imperfections in its microstmcture. 

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

The two basic types of corrosion discussed above form the general background to the subject. How, and to what extent, any particular object or structure corrodes also depends on other factors, in particular, on whether corrosion is uniform or not and on the effects of mechanical strain. These factors are interactive and in combination, their individual effects can be enhanced. 

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

Guide Guides the stem in order to have a perfect and uniform alignment to transmit the spring force on the disc. It is a very critical component for the correct operation of the SRV and also a vulnerable one because it is easily subject to galling, corrosion and so forth. 

Although corrosion is favored by a large difference of potential between the anodic and cathodic portions of a system, even the smallest of such differences is sufficient to stimulate corrosion in the presence of a depolarizer. In an apparently uniform piece of metal, any portion which has been subjected to strain is less noble than an unstrained portion and small crystals are less noble than large ones further, minute inclusions of noble material are often found in relatively pure metals. These differences permit local voltaic cells to be set up and, in the presence of a depolarizer, corrosion of the baser (anodic) regions will occur. 

Brass alloys of copper can be subject to deallo)dng (dezincification), a type of corrosion in which the brass dissolves as an alloy and the copper constituent redeposits from solution onto the surface of the brass as a metal in a porous form. The zinc constituent may be deposited in place as an insoluble compound or carried away from the brass as a soluble salt. The corrosion can take place uniformly or locally. The latter, called plug-type... 

General corrosion is characterized by a uniform attack over the entire exposed surface of the metal. The severity of this kind of attack can be expressed by a corrosion rate. With titanium, this type of corrosion is most frequently encoimtered in hot, reducing acid solutions. In environments where titanium would be subject to this type of corrosion, oxidizing agents and certain multivalent metal ions have the ability to passivate the titanium. Many process streams, particularly sulfuric and hydrochloric acid solutions, contain enough impurities in the form of ferric ions, cupric ions, etc., to passivate titanium and give trouble-free service. Refer to Table 20.6 for compatibility of titanium with selected corrodents. 

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