Environmental Degradation of Metals

of technology. However, a of metallic materials (and of other materials too!) is the deterio-

2.1 ELECTROCHEMICAL NATURE OF AQUEOUS CORROSION of all corrosion pro-

of the metal, and 2.3.1 Exchange Current Density

of polarization, i.e., activation polarization and of a slow step in the electrode reaction for of the order of 0.1 V of the above equation, as applied to hydrogen evo- of 0.1 Vis shown in Fig. 2.12. It can be noted

metal exhibiting passivity, e.g., iron in l N H2so •• is anodically polarized of the curve shows an increase in potential in the positive

3.1 CLASSIFICATION OF AQUEOUS CORROSION of metaJs and alloys takes place in a variety of environments

Figure 3.20 (a) Step structure from electrolytic-oxalic acid etch test in solution-an- of sensitization. No such peak is observed for unsensitized 3.6.4 Other Alloys

of vacancies into the interior of the alloy, thereby aiding in diffu-

•• E-C Figure 3.29 Erosion corrosion of lead by 10% sulfuric acid flowing at 39 ft/s. Cross- 3.8.2 Metallurgical Factors

t: I I

of bacteria have been identified that aggravate corrosion of metals of air. They survive under a wide range of (0-50°C) and pH (3-10). The important microorganisms contribut-

of corrosion have been pre- of environment

of materials for corrosion applications. The most of its of their excel-

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of the reaction behavior can be achieved only

OXIDANT SYSTEMS of a metallic component exposed of affinity of the metal to oxidizing gases, which leads to its degradation of some compounds with the oxidant. For judging of Gibbs' free energy change for the total reac- of metal oxide (M0), whereas if is positive, of the oxidant, thereby leading of the oxide. But thermodynamics deals with the

M/M0and M02/02 can estimated. of single

of defect formation reaction, it is essential to have mass of the law of mass action. Use of this law implies the of Boltzmann statistics, i.e., random distribution of defects are con- Null= of unit activity. Under the situation of 0 << 1, Henry's law is applicable. However, it is convenient to use concentration of number of defect per cubic meter of the crystal represented by exp( (5.8) exp( A{')exp( = = + +

Oz(g) Zr(g) [0"] pill = + P;i Examples:

= [c;DD;

of vacancy concentration at this interface is = Cexp of vacancies through the stressed oxide, the following stress-depen- _ D~

of metallic components in a variety of industrial installations are of such

of such oxides involves excess cations at interstitial of the oxide lattice with excess electrons in its conduction band. As of interstitial zinc ions and excess

zn•• zn•• o= zn•• zn•• zn•• zn•• zn•• Zn·· zn•• zn•• zn•• cr••• zn•• cr••• zn•• zn•• zn•• o• zn•• zn•• u• zn•• o" o"' zn•• zn•• Figure 6.1 u+ additions. of dissolving cations of higher and lower valences than the

Ni•• cr••• cr···

Cr acts as an oxygen getter, inhibiting internal oxidation. 6.5 SCALING OF BINARY AND TERNARY ALLOYS of alloys involves the same general phenomena as described for of the following:

Type 02lg)

A.E.OXU DISPERSION

of the sulfate of NiO corresponding to of Na0 as 10- ·the solubility goes on increasing with the increase of

of all susceptible alloys and of reaction, possibly with the alloy being protected by a stable Cr0

rtoetion product

Salt PROPAGATION STAGE

.,+=

Si0[2]. of diffusion due to their high concentrations of point defects. of oxygen point defects

Eltetrodes

of solid metals by liquid metals may be manifested in the following of a solid metal under applied or residual stresses when

Effect of Grain Size of a metallic material nonnally bear a of grain diameter, which is known or fracture stress of fracture

of testing, but failure in a brittle manner was observed when of magnitude. The effect of strain of Alloying

8.1 INTRODUCTION of physical and mechanical properties of metals resulting from the action of hydrogen, which may be initially present

of damage that occurs in carbon and low-alloy steels of the alloy or formation of

HSC produced fracture surfaces show- of shallow of plastic How associated with the propagation of fracture by making dis- or ftow stress due to dissolved hydrogen of crack tips, then embrittlement due to of the model [6], the adsorption of hydrogen has been of dislocations at the crack tip which, in tum,

of metals and alloys with energetic particles generates point defect of the materials and dimensional changes in the metallic components. Radiation

..... '"i-t.-