Corrosion prevention electrochemical polarization

Figure 5.14 a Anodic and cathodic Tafel curves for corrosion of metal, M, showing the principle of cathodic protection. (From Jones, D.A. (1981). Principles of Measurement and Prevention of Buried Metal Corrosion by Electrochemical Polarization. In Undeiground Corrosion, STP 741, ASTM. Reproduced by the kind permission of ASTM, PhUadelphia, PA, USA)... 

Localized corrosion and stress corrosion may often be observed. Stress corrosion cracks usually initiate at pits in many systems. The role of pitting is to disrupt films that otherwise prevent the ingress of hydrogen (118, 119). Electrochemical polarization technique may be used to distinguish between SCC and HE mechanisms in high-strength steels in sodium chloride solutions (120). 

Corrosion tests for hydraulic brake line fluid, as well as for motor oil, grease, and lubricants, are listed in Table 12. In addition, the effects of various fuels on materials have been of importance due to the effect of small amounts of water, which can become trapped in eureas such as the fuel tank [57]. Various additives to fuels can prevent corrosion effects and include inhibitor packages and small amounts of alcohol and aromatics. Recently, corrosion testing of ethanol fuels has been of interest because small amounts of water as well as other contaminants (chlorides) cause severe corrosion to occur [55]. Tests for fuels include electrochemical polarization and immersion tests (Table 12). 

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

In fact, anodic polarization generally increases the rate of the crevice attack. The only situation for which anodic polarization would be useful in preventing crevice corrosion is for systems that can be completely described by the ohmic drop model and for which no changes occur in either the occluded solution composition or the electrochemical behavior of the material in the occluded region. 

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