The negative difference effect in the case of steel can be explained in terms of corrosion by removal of blocks or "chunks" of metal containing perhaps only a few atoms. The "chunk effect" also explains the observed fact that, under conditions of high corrosion rate, minimum protective current can be much less than the calculated corrosion current. It is suggested that the chunk effect and anodic polarization occur on the same piece of metal. Either the positive or the negative difference effect may be observed depending on the corrodent. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.38.0.53 Downloaded on 2015-06-21 to IP Vol. 107, No. 12 DIFFERENCE EFFECT AND CHUNK EFFECT 961 J=o ~=1 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.38.0.53 Downloaded on 2015-06-21 to IP

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The standard Winkler method for determining dissalved oxygen yields unreliable results with many oil field brines because of the presence of high concentrations of magnesium, calcium, and ferrous salts. A method was desired which would p e r m i t rapid and convenient determination of dissolved oxygen in the field, principally to cheek the operation of brinetreating plants. A simple portable polarographio < NSITIVE chemical method for determining dissolved A"" oxygen was given by Winkler in 1888 (5). With modifications, this method will work in most aqueous solutions, but is unreliable when appreciable amounts of iron, calcium, or magnesium salts are present, as in many ail-field brines. A directreading method w a desired which would approach the socursey of the classical method and which would operate satisfactorily in these naturally ooourring brines. The instrument as described provides this operation with an accuracy of nbont +5%, and a t the same time is oampletely portable.The most practical solution to the problem of designing a directreading instrument for determining dissolved oxygen in brine appeared to be the polarographic method. The method in this case consists of the measurement of diffusion current as dissolved oxygen is reduced at a platinum or meroury cathode. Several studies (2, S), indicated that of the commonly used cathodes, the ratsting platinum microelectrode would yield the highest ratio of change in diffusion current to change in dissolved oxygen concentration. Experimentation led to the development of an apparatus which is capable of making a dissolved oxygen determination in less than 1 minute Kith the desired accuracy.The theory of dissolved oxygen analysis with the rotating platinum microelectrode is similar to that of oonventianal dropping mercury polarography. As a small electrode of platinum is rotated through a solution, a negative potential is applied to it from an external source. The negative potential is referred to a nonpoladzing reference electrode--o.g., a saturated calomel halfcell-which is also in cont.act with the solution. Neutral oxygen molecules are reduced a t a negative potential between about -0.4 and -0.8 volt, while other reducible ions likely to be encountered in brine, such BS ferrous, ferric, odoium, and magnesium, are reduced a t more negative potentials and have been found experimentally not to interfere over the range of pH from 5.0 to 9.0.The applied potential used in the original apparatus is -0.73 volt us. saturated cdomel. The change in diffusion current with ohange in dissolved oxygen concentration was found to be approximately linear for any potential in the range of from about -0.50 to -0.75 volt. Also diffusion current is practically a linear funation of applied potential over this range. It would appear that any potential more negative than about -0.45 volt could be used, but beyond -0.8 volt there is increasing probahility of interference; hence a potential of around -0.7 volt was chosen as about the optimum. APPARATUSThe apparatus described here ...

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The interrelationship of corrosion rate of steel, dissolved oxygen concentration, agitation, electrode potential, and cathodic protection current density were studied under controlled laboratory conditions. While the corrosion rate of steel is essentially linear with amount of dissolved oxygen, the rate appears to fall almost to zero near 1.0 ppm dissolved oxygen rather than at zero ppm dissolved oxygen as commonly assumed. In the region below 2 ppm dissolved oxygen, there is almost no effect of agitation on corrosion rate; above 2 ppm there is a large effect. By measuring the corrosion rate of an unprotected specimen, it should be possible to compute the minimum current density needed for cathodic protection in the field. In acidic environments, less current is needed for protection than is predicted by analysis of the mechanism of cathodic protection. This fact suggests that the anodic reaction in low pH corrosion proceeds by the removal of “chunks” of metal which contain several atoms. The “chunk effect” also appears to operate in the neutral range, but only when the corrosion rate is very high. The effect of differential aeration on corrosion rates is discussed in detail. 5.2.4