Pitting factor

Figure 4-426. Sketch of deepest pit with relation to average metal penetration and the pitting factor. (From Ref. [186].)...

Discontinuities in the corrosion product film result in differential aeration cells, leading to pitting corrosion. A pitting rate of 0.25-0.38 mm/yr on bare steel and 0.5 mm/yr on steel with mill scale has been observed. Assuming the average corrosion rate of 0.125 mm/yr, the pitting factor works out to be 2 to 3 for... 

Sample Time of exposure (yr) Pitting factor Reference... 

Figure 6.18 shows the corrosion of iron. It represents a section of the pit and a growing pit inside the metal. The pitting factor = P/d considers the deepest pit compared with the uniform corrosion loss (Figure 6.19). However to characterise the pitting phenomenon statistically, it is recommended to take the average of the deepest 10 pits as recommended in ASTM G48 4...

Figure 1.10 Deepest pit in relation with penetration in metal and pitting factor. (Reproduced by permission, John Wiley and Sons (8).)...

If appreciable attack is confined to a small area of metal acting as an anode, the developed pits are described as deep. If the area of attack is relatively large, the pits are called shallow. The ratio of deepest metal penetration to average metal penetration, as determined by weight loss of the specimen, is known as the pitting factor. A pitting factor of 1 represents uniform corrosion. 

Type Corrosion rate (mm/year) Pitting factor... 

Pitting factor The depth of deepest pit divided by the average penetration. 

Table 10.4 Corrosion rate (mm/yr) based on the maximum pit depth and based on the mean of the pit depths that were calculated from pit measurements on SRpCT tomograms. The pitting factors demonstrate that pitting is the prominent corrosion form in MgF2 coated LAE442 after 6 and 12 weeks of implantation. With kind permission from Elsevier (Witte et a ., 2010).

Evaluation of Pitting. Pitting can be described in several ways. ASTM G 46 includes procedures for the use of standard charts, metal penetration, statistical analysis, and loss in mechanical properties to quantify the severity of pitting damage. Mme than one of these methods can be used, hr fact, it is often found that no one method is sufficient by itself Because pitting tends to be nonuniform, the pitting factor (the ratio of maximum to average penetration) described in ASTM G 46 is not useful for aluminum. 

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