Alternating current corrosion

French W.H., Alternating current corrosion of aluminum, IEEE Transactions Power Apparatus and Systems, vol. 92, 1973, p. 2053-2062. 

W.H. Flench, Alternating Current Corrosion of Alu-mitmm, Trans. IEEE, Power Engineering Society wintermeeting. New York, Institute of Electrical and Electronics Engineos, Inc, 28 Jan to Feb 1973... 

The apphcation of an impressed alternating current on a metal specimen can generate information on the state of the surface of the specimen. The corrosion behavior of the surface of an electrode is related to the way in which that surface responds to this electrochemical circmt. The AC impedance technique involves the application of a small sinusoidal voltage across this circuit. The frequency of that alternating signal is varied. The voltage and current response of the system are measured. 

Choices of alternative materials. Corrosion probes are carefully chosen to be as close as possible to the alloy composition, heat treatment, and stress condition of the material that is being monitored. Care must be taken to ensure that the environment at the probe matches the service environment. Choices of other alloys or heat treatments and other conditions must be made by comparison. Laboratory testing or coupon testing in the process stream can be used to examine alternatives to the current material, but the probes and the monitors can only provide information about the conditions which are present during the test exposure and cannot extrapolate beyond those conditions. 

The effects of alternating currents are much less of a corrosion danger than those of direct currents. Experiments on steel have shown that during the positive half wave [34-37] only about 1 % contributes to the dissolution of iron according to Eq. (2-21). The remaining 99% is involved in the discharge of capacitances, of redox systems (e.g., Fe /Fe in surface films) or in the evolution of Oj by... 

Pick-up of stray current (a.c. or d.c.) (Section 10.5). Decreased use of d.c. in many areas has led to less possibilities of pick-up of direct current from utilities, mines, etc. The importance of grounded a.c. systems has been discounted, but Waters has shown that alternating currents can accelerate corrosion. Furthermore the rectifying effects of oxide films, clay minerals and other soil factors are not understood. 

Rhodium and iridium have a resistance to anodic corrosion comparable with that of platinum, and are more resistant to the influence of alternating currents. A platinum-iridium alloy, in the form of a coating on titanium, is preferred to pure platinum for the production of chlorine from brine , due to its improved corrosion resistance and lower overvoltage. 

In nearly all known cases of stray-current corrosion the damage is caused by direct currents, but leakages of alternating currents at industrial frequencies have been suspected of causing corrosion of buried metallic structures. The mechanism of corrosion caused by a.c. is not clearly understood and fresh studies are being made. However, the corrosion caused is much less severe than with stray d.c. and experiments indicate that stray a.c. at 50Hz will produce less than 1 % of the corrosion caused on most buried metals by an equivalent d.c. 

C.C.I.T.T. Study Group VI working party on corrosion by alternating currents (C. Cabrillac). A bibliography with over 60 references giving particular emphasis on buried lead, iron, aluminium and zinc structures, I.T.U., Geneva... 

The measured resistance of most cells varies with the applied alternating current, which is due to the polarization of the cell electrodes. Consequently, the use of large electrodes in cells with a high cell constant (capillary cells) will minimize the relative variation. In highly corrosive fluorides, however, most investigators use all-metal cells with low cell constants and thus considerable polarizations, so far. 

Electrochemical and Electrical Methods. Electrochemical and electrical methods for studying film properties and corrosion phenomena have been extensively reviewed (29-31). Comparisons of corrosion test results with direct current measurements of conductivity suggest that visible corrosion is associated with film resistance less than about 1 Mohm/cm, but this condition may well correspond with the occurrence of virtual pores in the film allowing development of local conductive pathways. In studies of the equivalent alternating current resistance as a function of frequency, Kendig and Leidheiser (44) found that the development of a region of slope -1 on a log permittivity versus log frequency plot... 

D. A. Jones, Effect of alternating current on corrosion of low alloy and carbon steels . Corrosion, 1978, 34, 428 433. 

G. Heim, Th. Heim, H. Heinzen, W. Schwenk, Research on corrosion of steel under cathodic protection due to alternate current (In German), 3R International, 1993, 32, 246 249. 

The causes and common means of detecting and mitigating stray current interference effects that result from direct current sources are reviewed in this Sect. [1, 40-44]. Alternating current, while creating a potential safety ha2ard, may contribute to corrosion of ferrous structure [42]. Extensive research is in progress in this regard. 

Electrochemical corrosion techniques are essential to predict service life in chemical and construction industries. The following direct current (dc) electrochemical methods are used in corrosion engineering practice linear polarization technique, Tafel extrapolation, and open circuit potential vs. time measurements. The alternating current (ac) technique is electrochemical impedance spectroscopy (EIS). This technique uses alternating current to measure frequency-dependent processes in corrosion and estimates the change of polarization resistance as a function of time. 

A. Schulte, S. Belger, M. Etienne, W. Schuhmann, Imaging localized corrosion ofNiTi shape memory alloys by means of alternating current scanning electrochemical microscopy. Mater. Sci. Eng. A378 (2002) 523-526. 

The presence of 5% bovine serum in lactated Ringer s solution (pH=6.5) increases the corrosion rate of Ti-6A1-4V alloy, as shown in the last two columns of Table 9.13 (Lewis and Daigle, 1993b). This table gives data obtained by direct current (d.c.) and alternating current (a.c.) methods. The difference between d.c. and a.c. corrosion rates found in this system is not unusual. The same table also shows that decreasing the pH of lactated Ringer s solution to 1 has a dramatic effect on corrosion rate. 

NACE. (1977). Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems. Recommended Practice RP-Ol-77. NACE, Houston, TX. 

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