Pitting corrosion alloy composition effect

Effects of Alloy Composition on Pitting Corrosion Inhibition of Pitting Corrosion Crevice Corrosion... 

These measurements lead, in particular, to an understanding that there is a critical solution composition that must be maintained within the pit, and if this does not occur then the pit will die. As a consequence, there is an important effect of the kinetics of active dissolution of the alloy within this critical solution. Measurements of the characteristics of metastable pits, coupled with studies of the dissolution kinetics of steel within the aggressive solutions that characterize the local pit environment, have provided an explanation as to why certain alloy additions (specifically Mo) act to inhibit pitting corrosion by making the maintenance of metastable pits more difficult. Other important effects include those of salt films, which... 

Figure 12 Effect of composition on the resistance to crevice and pitting corrosion of stainless alloys in chloride enviromnents—the PREN index. (From Ref. 15.)...

Effect of Exposure Time. A very important characteristic of weathering of aluminum and of corrosion of aluminum under many other environmental conditions is that corrosion rate decreases with time to a relatively low, steady-state rate (Ref 8). This deceleration of corrosion, often referred to as self-stopping or self-limiting corrosion, occurs regardless of alloy composition, type of environment, or the parameter by which the corrosion is measured (see, for example. Fig. 2, 3, 5, 6, 8, and 9). However, loss in tensile strength, which is influenced somewhat by pit acui and distribution but is basically a result of loss of effective cross section, decelerates more gradually than depth of attadc (Fig. 8). 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

Some understanding of the corrosion of aluminium alloys used as cladding on research and test reactor fuel has been obtained from the CRP. Aluminium corrosion is extremely complex and the variables affecting localized corrosion (pitting and crevice corrosion) act both independently and synergistically. Additional information about the effects of deposited particle composition on the corrosion behaviour of aluminium alloys is needed. Surface finish affects the corrosion of aluminium alloys, and more information is required with respect to this parameter. Additional data on the effects of certain impurity ions in basin water on localized corrosion behaviour are necessary to better identify the ions that cause corrosion. A goal would be to develop an equation for pitting as a function of water chemistry parameters. 

The details of the mechanisms also depend on the metal or the composition of the alloys as well as on the electrolyte and other environmental conditions. Metallic and norunetallic inclusions often play a decisive role in the start of a corrosion pit. In most cases the presence of aggressive anions is necessary for breakdown of passivity and stable pit growth. The discussion in this chapter explains the effect of these anions by their tendency to form complexes with metal ions. It concentrates on the behavior of some pure metals, such as pure Fe and Ni, in simple electrolyte solutions. Some basic concepts are the center of interest, although it is known fi om the technical applications of the difieient materials that the appearance and the... 

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