Corrosion from factors affecting

FIGURE 38 Lead coffin. Lead coffin (first—third centuries c.E.) from Jerusalem, Israel. Lead, widely used in many ancient civilizations, was one of the first metals to be recovered from its ores. Lead objects date back from as early as the seventh century b.c.e. In Mesopotamia molten lead was used to fasten bolts and shafts into masonry. In Syria it was made into rods used as currency, and in Greece it was cast into coins. During the Roman Empire the use of lead become so widespread that the health hazards caused by lead exposure are suspected to have been one of the factors affecting the fall of the Roman Empire. Since it is very resistant to corrosion, lead was also used by the Romans, for making coffins as the one illustrated. 

Most of the factors affecting the rate of corrosion can be understood from a graphical superposition of the current-potential curves for the metal-dissolution and electronation reactions. The principle of the graphical superposition method is straightforward. 

Factors affecting the lifetime of materials exposed to sea water include dissolved oxygen, temperature, pH, salinity, sulphates, pressure, and marine life (biofouling). Heiser and Soo [23] provide a detailed review of the influence of these factors on a range of materials, with data derived from analysis of dump sites in the Atlantic and Pacific. However, little data is available on the behavior of materials in relatively shallow Arctic waters. To derive corrosion rates for the lASAP, comprehensive reviews of available source references were made in support of this work, with notable contributions from Carter [22] and Heiser and Soo [23]. 

Apart from the effects of alloying and impurities, there are some other factors affecting corrosion, for example, changes in microstructure by thermal or mechanical treatments and ensuing changes of the surface condition. 

Factors Affecting Corrosion in Soils Since corrosion is an electrochemical process, it requires a potential difference between two points electrically connected and immersed in an electrolyte. Electrons flow from the anodic area through the metallic path to the cathodic area to complete the circuit. The anodic area is the most negative in potential sind is the area that corrodes through the loss of metal ions to the electrolyte. 

Another factor affecting the relative corrosive rate resulting from rain is the orientation of the metal surface. In areas of heavy industrial pollution, skyward-facing metallic surfaces benefit from rain. In those areas where dry deposition is considerably greater than wet deposition of sulfur pollutants, the washing effect of rain predominates, and the corrosion rate is reduced. In areas having less pollution the situation is reversed and the corrosive action of the rain predominates. 

Protection of galvanized surfaces from atmospheric corrosion is due to the formation of films of basic salts, primarily carbonate. The most widely accepted formula is Zn.j(0H)(,(C03) . The galvanized surface of zinc is rapidly attacked when environmental conditions lead to the formation of soluble films. One of the most important factors affecting the corrosion of zinc in the atmosphere is the duration and frequency of moisture contact. 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

For both heat-affected zone corrosion (intergranular attack) and knifeline attack the heat flux during welding and the time at temperature can critically affect the severity of the attack. Both these factors may vary from one welder to another, and when preparing pieces for corrosion testing not only should fabrication welding conditions be accurately reproduced, but the work of more than one welder should be evaluated. 

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