Localized corrosion pitting

As mentioned, corrosion is complexly affected by the material itself and the environment, producing various kinds of surface films, e.g., oxide or hydroxide film. In the above reactions, both active sites for anodic and cathodic reactions are uniformly distributed over the metal surface, so that corrosion proceeds homogeneously on the surface. On the other hand, if those reaction sites are localized at particular places, metal dissolution does not take place uniformly, but develops only at specialized places. This is called local corrosion, pitting corrosion through passive-film breakdown on a metal surface is a typical example. 

Figure 26 Illustration of the period of propagation of localized corrosion (pitting) as defined by the relative values of EC0RR and the breakdown (EB) and repassivation potentials (Er). The shaded areas associated with EB and ER illustrate the uncertainties in the values of these two parameters.

Nonmetallic conductors and corrosion products. Carbon brick in vessels is strongly cathodic to the common structural alloys. Impervious graphite, especially in heat-exchangers, is cathodic to structural steel. Carbon-filled polymers can act as active cathodes. Some oxides or sulfates are conductors, such as mill scale (magnetite Fe304), iron sulfides on steel, lead sulfate on lead can act as effective cathodes with an important area to that of the anodes. Very frequently, the pores of the conductive film are the preferable anodic sites that leads to localized corrosion (pitting).5... 

The model described here has been developed from a metallurgical standpoint, rather than an electrochemical one and originated as a cellular automata (CA) finite difference model. This approach dealt with the evolution of a representative concentration and electrical potential throughout the electrolyte only [1], The model was able to predict morphological features such as localized corrosion pits and capping but was limited to qualitative simulation. However the CA method has found alternative applications in the growth of corrosion pits... 

Fig. 5.35 Schematic polarization curve for an active-passive alloy having susceptibility to localized corrosion (pitting) due to chloride ions. Pitting initiates at Eb,pit- Small-dashed section is observed when chloride ion concentration initiates penetration of the passive film.

Fig. 7.44 Schematic polarization curve for an alloy susceptible to localized corrosion. Pitting is initiated at Eb pit and stops at Eprotpjt. Crevice corrosion starts at Eb crevice and stops at Eprot crevice. P ...

Localized corrosion (pitting, dewpoint corrosion, deposit 15.9 ... 

It is also instructive to see that methods such as circumferential MFL and axial MFL can be used in detection of general and localized corrosion (pitting) but not pinholes (see Figure... 

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. 

ASTM G 61, Test Method for Conducting Cyclic Potentio-dynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys— This standard presents a procedure for performing cyclic poten-tiodynamic polarization testing to determine the relative susceptibility of iron-, nickel-, and cobalt-based alloys to localized corrosion (pitting or crevice corrosion). It illustrates and presents test apparatus, reagents, and materials, test procedures, and interpretation of results. 

Molybdenum, and sometimes tungsten, are also added to Ni-Cr-Fe allo3rs for improved localized corrosion (pitting and crevice corrosion) resistance. An example is alloy C-276 (UNS N10276), which contains about 57Ni-15.5Cr-15.5Mo-4W-5Fe and is one of the most pit-resistant nickel-base alloys available. 

The effect of the environment on crack initiation is to decrease or even eliminate the fatigue limit. In Fig. 5-9 a comparison of results obtained in NaCl and H2SO4 solutions shows how the fatigue strength of an austenitic stainless steel decreases as the dissolution rate increases in an aqueous environment. Localized corrosion (pitting) also strongly favors fatigue crack initiation, both due to stress concentration and local acidic environment. 

The first three forms of corrosion uniform or quasi-uniform general corrosion, galvanic corrosion and localized corrosion (pitting, crevice and filiform) have no clear separation. The oxide-hydroxide passive layer can play the... 

Describes a procedure for conducting cyclic galvanostaircase polarization (GSCP) to determine relative susceptibilily to localized corrosion (pitting and crevice corrosion) for aluminum alloy 3003-H14... 

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