Potentials for pitting

Generally, pitting corrosion only occurs on passivated metals when the passive film is destroyed locally. In most cases chloride ions cause this local attack at potentials U > U q. Bromide ions also act in the same way [51], The critical potential for pitting corrosion UpQ is called the pitting potential. It has the same significance as in Eqs. (2-39) and (2-48). 

Horvath, J. and Uhlig, H., Metallurgical Factors Affecting the Critical Potential for Pitting Corrosion of Cr-Fe-Ni Alloys , J. Electrochem. Soc., 114, 201c (1%7)... 

Broli, A. and Holtan, H., Use of Potentiokinetic Methods for the Determination of Characteristic Potentials for Pitting Corrosion of Aluminium in a Deaerated Solution of 3< o NaCl , Corros. Sci., 13, 237 (1973)... 

Fig. 19.42 Effect of pH on critical potential for pitting in 0-1 mol dm NaCl 25°C (after...

Protection Potential for Pitting potential below which new pits cannot be initiated nor pre-existing pits continue to propagate. 

A chloride-based DA will not reduce the TDS of the water, and therefore the tangible benefit of continuous cost savings through lower boiler BD rates is never realized. Also, the introduction of additional chlorides into a boiler may provide the potential for pitting corrosion to take place. 

Figure 44b Effect of charge density on the repassivation potential for pitting and crevice corrosion.

From an engineering perspective, the repassivation potential is a more important parameter than the potential for pit nucleation. We want to know the potential below which pits will not grow. This is analogous in theory to measuring KiC or Kiscc in mechanical and SCC testing. One way to test this is to produce a completely bare surface that is dissolving rapidly, and determine at what potential it can repassivate. An easy way to do this is what may be termed an electrochemical scratch. ... 

The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

M.J. Johnson, Relative Critical Potentials for Pitting Corrosion of Some Stainless Steels, Localized Corrosion, STP 516, ASTM, 1972, p 262-272... 

J. Horvath and H.H. Uhlig, Critical Potentials for Pitting Corrosion of Nickel, Chromium-Nickel, Chromium-Iron, and Related Stainless Steels, J. Electrochem. Soc., Vol 115, 1968, p 791... 

Pitting corrosion Austenitic staiidess steels owe their corrosion resistance to the formation, from the chrome (Cr) content of the steel, of a passive layer of chromium oxide on the exposed surface. Pitting occurs when the protective oxide film breaks down in small, isolated spots. The rate of attack tends to increase because of the differences in electric potential between the large surrounding passive surface and the active pit. This action is accentuated by the presence of saline solutions. A smooth surface, free of sensitive local minute pits or small depressions, reduces the potential for pitting to commence. The most appropriate quality of stainless for such duties should be selected. 

Evaporation Corrosion Test Manufocturing aids evaluation (lubricants/coolant) Potential for pitting caused by fluid left to evaporate on machined alloy after 7 days Currently qualified fluid... 

Upp = Breakthrough potential for pitting corrosion (pitting potential)... 

Figure 23 Repassivation time vs. pitting time for Fe 04 = 0.13 cm ) and Ni 04 = 0.2 cm ) at different potentials for pitting as indicated, ij. leading to deviations of continuous current increase for one potential sequence. ( ) Fe 0.01 M Cl (O) Fe 0.01 M Cl , ( ) Ni 0.1 M Cl A Fe 0.01 M Cl leading to continuous increase of current for many subsequent potential cycles, phathalate buffer, pH 5.0 (From Ref 4.)...

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