_STP_727-1981.pdf

ELECTROCHEMICAL CORROSION TESTING A symposium sponsored by ASTM Committee G-1 on Corrosion of Metals AMERICAN SOCIETY FOR TESTING AND MATERIALS San Francisco,Calif.21-23 May 1979 ASTM SPECIAL TECHNICAL PUBLICATION 727 Florian Mansfeld,Rockwell International Science Center,and Ugo Bertocci,National Bureau of Standards,editors 1,B,and C are separated from each other and have the respective Evans-type diagrams shown in Fig.2a.Each area is at its open circuit potential;measurements of the potential as a function of the position in the solution can be represented by Fig.2c.If these separations are removed,two conditions can arise de-pending on the anodic behavior of area 5.If the lower polarization is consid-ered,the polarization of the three areas would be close to potential Ey in Fig.(a)Polarization diagrams for each exposed area.A Passive surface supporting anodic and cathodic reactions.fiLocal pitting anode with either high(I)or low(II)polarization.CMetal-coated area supporting only a cathodic reaction.(b)Areas exposed to separate solutions.(c)Open-circuit potentials in solutions between iron and the reference electrodes.fd)Equipotential lines in solution when area B has low polarization(II).(e)Potential variations on scanning across the sample for case(dK ff)Equipotential lines in solution when area B has high polarization(I).(g)Potential variations on scanning across the sample for case(f).FIG.2Schematic variations of the potential in a solution above a partially coated iron surface.Copyright by ASTM Intl(all rights reserved);Sun Dec 27 13:39:07 EST 2015Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.8 ELECTROCHEMICAL CORROSION TESTING 2a.The currents from the anodic area B would flow to both A and C;the potentials developed in the solution are shown by equipotential lines in Fig.2d.If the potential were measured at a fixed distance from the metal surface above the coating using the SRET,the potential variations would be similar to the curve shown in Fig.2e.The magnitude of these potential variations cover a range AE,which is much smaller than the potential range shown in Fig.2c.Under conditions where the anodic area B has the larger polarization(I),the potential of the three areas would be close to 2 in Fig.2a.This potential is equal to E,the open circuit potential of area A;under this condition no currents will flow to or from A.The equipotential lines for this condition are shown in Fig.2/.The shape of the potential variations from the SRET is shown in Fig.2g.It should be noted that no potential variations are observed when scanning above area A;the potential measurements would not be ca-pable of distinguishing whether the probe is directly over the area A or the coatings adjacent to it.A similar condition could be achieved if area A is placed between B and C.If the coating were thin,it would once again not distinguish between the presence of a coating or the passive metal surface under the scanning reference electrode from the potential variations.This analysis demonstrates clearly that the SRET only responds to potential varia-tions in the solution,which are associated with the flow of current,and does not relate directly to the potentials of the metal surfaces or the coating.Experimental Technique The potential fields generated in the electrolyte due to local corrosion sites can be measured by scanning a microtip reference electrode over a horizon-tally exposed surface facing up.The equipment built at Brookhaven National Laboratory is shown schematically in Fig.3 20,21.The microtip reference electrode is held by a mechanical stage attached to low-friction,linear bear-ings for smooth motion in the A-and y-direction,and driven by two stepping motors.The mechanical stage can be automatically programmed to scan both in theA-and y-directions parallel to the specimen surface.The length of the X-direction can be varied up to 26 mm and,at each end of the X-scan,the y-direction can be shifted to a set value(from 30 to 200 fim).An area of the surface is thus scanned by a rectangular wave.The linear speed of the scan in the X-direction can be varied from 0.1 to 300 mm/s.Alternatively,potential fields on cylindrical samples can be obtained by keeping the probe stationary while rotating the sample 22,23.The microtip reference electrode measures the potential variations along the circumference of the sample as the potential field around the sample rotates with the sam-ple.A schematic of such an instrument is shown in Fig.4 23,A motor is used to rotate a cylindrical metal specimen in the electrolyte so that the rota-tional motion of the sample is synchronized to produce a signal that is pro-Copyright by ASTM Intl(all rights reserved);Sun Dec 27 13:39:07 EST 2015Downloaded/printed byUniversity of Washington(University of Washington)pursuant to License Agreement.No further reproductions authorized.ISAACS AND WAS ON TECHNIQUES IN