Pitting electrochemical tests

Wilde, B. E. and Williams, E., On the Correspondence Between Electrochemical and Chemical Accelerated Pitting Corrosion Tests , J. Electrochem. Soc., 117, 775 (1970)... 

Takenori, N., Norio, S., Tatsuo, 1. and Okamoto, G., Electrochemical Test for Pitting Corrosion in Stainless Steels , Hakkaido Daigaku Kogakubu Kenkyu Hokoku, 44, 1 (1967) C.A., 70, 16534h... 

Ringas and Robinson performed electrochemical tests on stainless steels and mild steels in three cultures of SRB. In all cases pitting resistance was lower in cultures of SRB. Potentiodynamic polarization... 

Electrochemical Testing. Potentlodynamlc polarization measurements provided a sensitive means of evaluating the inhibitors with respect to environmental (Cl ) corrosion protection. The results obtained from anodlcally polarizing polished 7075-T6 A1 samples are presented in Fig. 9. For the control electrolyte (O.IN Na2S0, 0.002N KCl, no inhibitor), pitting was observed almost immediately on the surface, and the aluminum showed no evidence of passivation. The addition of NTMP to the solution did not appear to protect the metal... 

The most common electrochemical test for localized corrosion susceptibility is cyclic potentiodynamic polarization. As was discussed briefly in the section on the electrochemical phenomenology of localized corrosion, this test involves polarizing the material from its open circuit potential (or slightly below) anodically until a predetermined current density (known as the vertex current density) is achieved, at which point the potential is scanned back until the current reverses polarity, as shown in Fig. 42. The curve is generally analyzed in terms of the breakdown (Ebi) and repassivation potentials (Elf). Very often, metastable pits are apparent by transient bursts of anodic current. The peaks in current shown in Fig. 42 for a potentiodynamic scan are due to the same processes as those shown in Fig. 25 for a potentiostatic hold. 

Crolet remarked that causality does not always follow from correlation of results from electrochemical tests and results from surface analysis (19). Strictly speaking, the effect of inclusion removal on pitting potential cannot be distinguished from effects due to Cr surface enrichment. 

With the aid of electrochemical tests [DIN 50918] quantiative statements can be made about the corrosion mechanism. The experimental determination of the current-potential relationship provides the most important information. It can be used to determine the limiting potentials for the occurrence of, for example, pitting corrosion and stress corrosion cracking. 

B.E. WHde, E. Williams, On the correspondence between electrochemical and chemical accelerated pitting corrosion tests, J. Electrochem. Soc. 117 (1970) 775—779. 

R. Qvarfort, New Electrochemical Cell for Pitting Corrosion Testing, Corrosion Science 28 (1988), pp. 135-140. 

Pitting and crevice corrosion are associated with the breakdown of passivity [SO]. Electrochemical tests for evaluating the susceptibility of a material to pitting and to crevice corrosion include potentiodynamic, potentiostatic, scratch potentiostatic, potentiostaircase, tribo-eUipsometric methods, pit-propagation rate curves, galvanostatic, and electrochemical noise measurements [80-S2]. 

ASTM Standard G 150, Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels. 

ASTM G 150 (Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels) describes in detail how to perform such experiments. The solution prescribed is 1 M NaCl, the suggested applied potential is +700 mV(SCE) and the starting temperature is deemed to be 0°C, with the temperature being increased at l°C/min. The CPT is defined as the temperature at which the current exceeds 100 pA/cm and remains so for greater than 1 min. The test method clearly states that alternative potentials can be used if they are within the range for which the CPT is potential independent. 

Postlethwaite, J., Electrochemical Tests for Pitting and Crevice Corrosion Susceptibility, Canadian Metallurgical Quarterly, Vol. 22, No. 1, 1983, pp. 133-141. 

Localized corrosion propagation rates are highly variable and not normally measured unless the time to initiation of attack is determined visually or by electrochemical monitoring. Pits in nickel alloys such as alloy 400 (UNS N04400), which are usually of the broad and shallow variety, tend to propagate much more slowly than the narrow undercut types found in chromium-bearing nickel alloys. Electrochemical tests are useful to predict pit initiation, while immersion tests evaluate initiation and also provide some indication of the possibility of severe pit propagation. 

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