INDUSTRIAL A N D ENGINEERING CHEMISTRY
April, 1925
tective layers” was corroded completely through, and broke, while other sections of the wire were as bright as when first immersed. Hydrogen-Ion Concentration and Corrosion of Iron
In the absence of oxygen, the rate of corrosion of iron will be proportional to the acidity or hydrogen-ion concentration of the solution in contact with the cathode. The writers believe this holds not only in the acid region but down t o a pH of 9.4. Their results indicate that the H-ion concentration determines the rate and total acidity the continuance of the reaction. Consequently, above a pH of 9.4 any substances present that will hold the acidity constant or “buffer” the reaction will be the controlling factor in the corrosion. Substances such as gypsum, limestone, and carbon dioxide, when occurring together, give this buffer action a t a pH of 5.1, while carbon dioxide and water give a n acidity of pH 4.0. The continuance of the corrosion of iron embedded in clay is partially explained by the buffer action of the clay. Where the solution is stagnant, migration of H T to the cathode will be the controlling factor. The foregoing cpsiderations hold where conductivity and hydrogen overvoltage remain undisturbed. When oxygen is present the H-ion concentration is determined a t the cathode by the solubility of ferric hydroxide. At the anode it will be dependent upon the migration of
(OH)-.
The anodic reaction is Fe
and the cathodic 3/2 H20
+ 2 @+
+ 3/4+ + +
0 2
+.
Fe++ 3(OH)-
(1)
+3@
Fe++ @ + F e + + + F e + + + 3(OH)- + Fe(0H)a or summing (2), (3), and (4) 3/2 H10 3/4 00 F e + ++. Fe (OH)3
+
+2 @
(2) (3) (4) (5)
If hydrogen is evolved a t the cathode, there will be an excess of (OH)- over that necessary for the production of ferric hydroxide and these (OH)- will eventually migrate towards the anode producing a precipitate of ferrous hydroxide if the solubility product of this hydroxide is reached. The H - at the anode will therefore be that determined by the Fe-’. These considerations show that the H-ion concentration cannot alone determine the rate of the corrosion when ferric hydroxide is present. The rate of diffusion of oxygen to the cathode is, as Wilson and Whitman point out, the determining factor, while the production and precipitation of ferric hydroxide a t the cathode is the characteristic reaction. At an acidity where ferric hydroxide can no longer remain insoluble, the H-ion concentration again functions as the controlling factor and the reactions concerned in the corrosion are independent of the oxygen supply. Hydrogen Evolution in the Corrosion of Iron
In a corrosion cell the cathodic hydrogen overvoltage drops off with the current density; consequently, as the H-ion concentration diminishes and the rate of corrosion drops, as a sort of compensating measure, hydrogen evolution will continue a t a lower e. m. f. The presence of oxygen increases the current density and consequently the cathodic hydrogen overvoltage, so that the evolution of hydrogen will not extend down to so low a H-ion concentration as in its absence. Moreover, the oxidation potential for iron lies a little below that for the evolution of hydrogen, so that in low concentrations of H + a t the cathode, such as the solubility of ferric hydroxide produces, the oxidation of the iron will be the preferential reaction and no hydrogen will be .evolved. When the H-ion concentration increases beyond
385
the solubility of ferric hydroxide the possibility of hydrogen evolution obtains, and a t a concentration corresponding to a p H of 5.4, Whitman found hydrogen gas to be evolved in the presence of oxygen. The writers found the evolution of hydrogen in the absence of oxygen to continue down to a p H of 9.4 or a t a concentration in respect to H + 10,000 times more dilute.
Corrosion of Iron By W. R. Whitney GENERAL ELECTRIC Co., SCHENECTADY, N. Y.
The paper describes some experiments in which the wearing away, or cutting, of iron by water in motion in presence of air was followed. It is an attempt to show that in some cases, where i t has been customary to consider the erosion of metals in contact with moving water as due entirely to a mechanical effect of the water, it is quite probable that the removal of the metal is preceded by a chemical corrosion.
I
T MAY be a question to some whether enough, and much more, has not already been written on this subject; but it will probably prove true that as new viewpoints arise much of the theory of corrosion will be rewritten, and even many of the old experiments will have to be repeated under influence of modern knowledge. For example, possibly many experiments on corrosion which, in the hands of different investigators, failed to give identical results, may be now explained, not only by differences in chemical purity or conditions of local stresses, but by differences in surface atomic orientations of the crystal structure as disclosed by X-ray crystal analysis. A sand-papered surface of pure iron may well act “energetically” in quite a different way from the normally oriented or annealed crystalline material. It is not the purpose of this paper to add to the quantity of theoretical matter which has so generally permeated the subject of corrosion of iron, nor yet, on the other hand, to deprecate or depreciate such theoretical work. Iron is our most generally useful element, and the value of even the crudest guess may prove to be a positive quantity. But a few experiments that have been made in this laboratory and which fit into the general subject may be useful and suggestive, and ought to be recorded. Corrosion of Soft Iron in Presence of Distilled Water
Mr. Fuller, of this laboratory, has shown that ordinary soft iron, when cleaned, as by surface grinding or by the use of emery paper, starts corroding very rapidly in the air where a drop of distilled water is in contact with the metal. Within less than 2 minutes a greenish color appears, and in 4 minutes a suspension of rust becomes visible throughout the drop of water. This indicates an exceedingly rapid attack of iron by water and air, which in general must be greatly slowed down by some cause in a short time; otherwise, no iron mould last as long as it does. This reduction of speed may he the effect of the covering when this is not removed. This coating is not easily and simply removed when dry, though it may possibly be removed before it dries. According to experiments reported by Fuller, as above, successive applications of the same size water drop to the identical spot on iron, resulting rust being removed by filter paper after each water application, showed that the quantity of corrosion rapidly decreased with successive applications. However, since the iron area was rougheningand therefore increasing, it seems as though some rptardirig wrface was produced.
I.\'DUSTRIA L AA'D BlVGINEERING CIIB.IIISTRY
386
Tlre fwt that the oxide or hydroxide so quickly appeared in t,hr water, instead of being visible on t,iiemetal, suggested that the rate of corrosion might greatly depend on the rate of effectiveremoval of a loose mat of hydroxide. Corrosion on Steam Turbine Buckets 111 stcam turhines there are several kinds of corrosion, quite different among themselves. This description will he rirniiiied to corrosion of the buc!kets.
VOI.
17, s o . 4
The asmnption was made that the water, or, more particularly, the water and such oxygen as gets into the steam from the boiler, acted together and, superinduced by the rapid motion of the water, caused the formation of flocculent yellow oxide of iron characteristic of this temperature. This iron oxide is continually removed by the erosive effect of tlre water. The import.ance of the atmospheric oxygeii was of chief interest in this case, because it was not believed that this erosion could occur without it. The qnestion became tlien-
Fieurc I
FMure 2
It! tlic lrighcr teml?ci.ature and pressure stages tho buckets to the action of dry and superheated strum, rind they are wetted only seldom, by accideiit. Here the red : i d i,lnr:k oxides of iroii very slow-lg S u m and usually remain :is ~t fairly good proteat,ing coat to the iron in upper stages of tilrbines. But corrosion of steel t,urhine blades is marked :xiid clraracteristic in appeamnre at somewhere ne8.r the middlc portion of the turbine. For example, it may begin on the 1 Ith stage or disk of buckets in a 23-stage turbine. This wrmsion exhibits characteristic pits and stream lines, showing plainly that the friction of the waber which ha.8 condensed from the steam a t that part of the turbine is responsible, in part a t least, for the location and direction of the erosive artion. At this middle portion of the turbine thc temperat,ure is not far from 100" C. Xear the exhaust,end of the turbine the t.emperature reaches more nearly room temperature, a n d the degree of vacuum under which this endof the rotor isoperatingmeans thatlessliquid witer is being condensed on and floured over a unit surface of h k e t than in the middle portions of the rotor. For tbis r~asmrthe last, fmv di. of buckets i n n turbine are seldom
will iron, snl~mittetlto t.he rnpid motion uf water uver it f : m y , or tile Ix~mhnrdment by condensed water drops, pmduee
~ I C siibnritted
Pisure 3 size
s/m actual
~,orrudcdat all. They are, however, deeply corroded if liquid water, candenscd iry previous wheels near the middle of the turbine, is allowed to collect and to pass along the turbine shell walls and drop on the buckets, as it sometimes does, in spray or a small stream near the periplieral ends of the buckets. F'or this reason, among others, it is customary to drain off the water from a turbine as near the points of its condensation ns possible. The peculiar surface appearance of the corroded and eroded surface of the middle stage, and the occasional peculiarity in the last stage of erodrd buckets, led to the following experiments.
this erosion if free oxygen liss bmn eliminated? It has hrcri frc
