Corrosion features pitting

Finally, pitting may be viewed as a special form of concentration cell corrosion. Most alloys that are susceptible to crevice corrosion also pit. However, many metals may pit but not show crevice attack. Further, although sharing many common features with concentration cell corrosion, pitting is sufficiently different to warrant a separate categorization. 

Crevice corrosion and pitting have a number of features in common, and it has been stated that pitting may be regarded as crevice corrosion in which the pit forms its own crevice however, whereas a macroscopic heterogeneity determines the site of attack in crevice corrosion, the sites of attack in pitting are determined by microscopic or sub-microscopic features in the passive film (5 Sections 1.3 and 1.5). 

Interconnecting pits were observed inside the pipe in the ruptured area. The pits showed striations and undercutting features that are associated with microbial corrosion. A pit profile showed the chloride in the pits increased steadily from top to bottom. Sulfate-reducing, acid producing, general aerobic, and anaerobic bacteria were present in the deposits taken from two pit areas in the piece of line where internal corrosion was noted after the accident 630 m downstream of the rupture site. 

One of the earliest NDT applications for laser-based profilometry systems was for the inspection of marine boiler tubes. Under funding from the U.S. Navy", several systems were developed that were capable of rapidly and accurately mapping the inside surface of 25 mm and 50 mm diameter boiler tubing. Features such as internal pitting and corrosion can be located and quantitatively assessed using computer-graphic analysis. The system employs an... 

Previous considerations of pitting have been largely confined to metals and alloys that have a strong tendency to passivate, but since the pitting of copper has a number of unusual features it is appropriate to consider it in some detail. Reference to the potential-pH diagram for the Cu-H O (Section 4.2) system shows that in neutral solutions at the potentials encountered in oxygenated waters the stable form of copper is Cu O, and the corrosion resistance of copper thus depends upon whether or not the CU2O forms a protective film. 

One of the main factors which establishes the corrosivity of water to stainless steel is the chloride content. Also significant are oxygen content and pH, and it is also probable that other features such as hardness and the nature and concentration of other anions and cations have effects. Water temperature and flow velocity can also be important. Any corrosion takes the form of pitting or, if crevices are available, larger areas of attack within the crevice. 

Fig. 8. Scanning electron micrographs (SEM in secondary electrons mode) of the surface features of HT materials ground to 100-125 pm prior to (small micrographs) and after 10-day corrosion (large micrographs). Corroded sample P7 exhibits pits but almost no secondary mineral phases corroded sample P16 shows a dense cover of secondary minerals.

Some attractive features of radiography are access to interior difficult-to-reach portion, such as valves and pipes, the objects or sample covered with insulation can be tested, reference radiographs can be accessed, the nature and severity of the attack can be assessed with ease, in principle, pit depths can be obtained although laser profilometry is the preferred method in this regard. The main drawback of the method is the whole sample needs to be exposed. Some applications of the method are determination of flaws in weldments and castings to identify sites of initiation of crevice corrosion, valve... 

The ability of the ferritic alloys to resist chloride see is one of their most useful features in terms of corrosion resistance. During the 1970s, developmental efforts were directed at producing ferritic materials that could also exhibit a high level of general and localized pitting resistance as well. 

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... 

This form of corrosion is characterized by narrow pits with a radius of the same order of magnitude as, or less than, the depth. The pits may be of different shape, but a common feature is the sharp boundary (Figure 7.28). Pitting is a dangerous form of corrosion since the material in many cases may be penetrated without a clear warning (because the pits often are narrow and covered) and the pit growth is difficult to predict. This is connected to the fact that the extent and the intensity of... 

The cause of nonuniform corrosion is the presence of corrosion elements, that is, heterogeneous mixed electrodes. The common feature of both types of corrosion is the geometry of the damaged area. The surface spread of these areas is generally greater than the depth. Accordingly, compared with localized types of corrosion such as pitting, these types of corrosion are limited in extent. 

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