TECHNICAL REVIEW
Measurement and Prevention by Electrochen S
DENNY A.. JONES is assistant professor in the Department of Mechanica1 Enrineering, Universitj7 of Hawaii, HonoZulu. He received the BS degree from the University of Nevada, i'he IMS degree from the Unitwsity of Arizona, and the PhD in materials enginemh g from Rasselaer Polytechnic Institute. Before joinin,7 the faculty at the University of Hawaii in September lgYO, he was associated with Battelle-Northwest Laboratmie:9 , Richland, Wash., and Kaiser Aluminum and Chemic:a1 COT., Spokane, Wash. He presently teaches courses in corrosion and materials science and has research interests in marine corrosion, electrochemistry, and hig&temperature metal oxidation.
are reviewed which have been, or are potentially, useful in solving or alleviating industrial corrosion problems. The treatment is designed for the practicing engineer concerned with selecting alloys for industrial service, measuring corrosion performance, and preventing corrosion in industrial installations. Theory
General Principles. Aqueous corrosion of most metals and alloys occurs by an electrochemical mechanism (16). T h a t is, electrons are exchanged duringthe reaction. Corrosion products are then charged particles (ions) in solution. (Conversely, d i p solution of a metal or alloy in another molten metal is not electrochemical, because only a physical process is involved; no electron transfer ensues.) Aqueous corrosion of metal M occurs by two partial processes: an anodic oxidation reaction in which matrix metal atoms are ionized and go into solution leaving behind an electron(s),
M (metal) + M + (solution)
+ e-
(metal)
(1)
and the cathodic reduction reaction in which a species in solntion is reduced by consuming an electron(s) from the metale.g., hydrogen ions reduced t o hydrogen gas, 2 H + (solution) T h e principles of electrochemistry and electrochemical kinetics have long been known, but Iinfortunately not well understood in application to aqueous corrosion of metals and alloys. As a result, many technological advances of recent years have not been fully utilized. Th is paper undertakes t o provide a discussion of electrochemical theory and how it applies t o aqueous corrosion processes. Pertinent laboratory methods are delScribed to demonstiate how electrochemical corrosion studies are conducted. With this as a background, appropriate ekxtrocbemical methods 12 Ind. Eng. Chem. Prod.
Res Develop., Vol.
ll.No.1.1972
+ 2e-
(metal) +. H, (gas)
(2)
As fast as electrons are liberated by the anodic reaction, they are consumed by the rednetion reaction, and charge neutrality is maintained in the metal. The corrosion potential Eoom is then defined by the potential a t which the rates of Reactions 1 and 2 are equal. The rate of any charge passage can be represented by a current-i.e., coulornb/second or amperes. Thus, the rates of the two partial processes can be represented by currents I,, and Izea, respectively. (Ioxand Iresare related to the actual amounts of metal dissolved and hydrogen liberated, respec-
The electrochemical theory of aqueous corrosion i s briefly reviewed, and the applications and limitations of laboratory electrochemical techniques are described. Examples of alloy evaluation by anodic polarization are discussed, particularly in environments simulating industrial service conditions. Applications and limitations of anodic protection are outlined together with references to the engineering of anodic protection systems. Cathodic protection is discussed with respect to the applied current and electrochemical potential required for adequate protection, controlled potential cathodic protection, and the performance of sacrificial anodes. Electrochemical ccrrosion rate determinations are reviewed. The polarization resistance (or linear polarization) method is described as a useful technique to monitor changes in corrosivity of certain industrial process streams. In general, the discussions are limited to those aspects of theory and practice which may b e of interest to the practicing engineer concerned with service corrosion problems and materials selection.
tiuely, by Faraday’s law.) Both theoretically (17) and experimentally ( 7 5 ) ,the rates of reaction of the two partial processes are exponentially related t o electrochemical potential E. Thus,
(3) and
(4) where q , called overvoltage, is the change in potential induced by current flow. Thus, 7 = E - Eo. Eo,, is the potential at which the anodic reaction is reversible-e.g., JI e M + e. Similarly, for the cathodic reaction 2H+ 2e S Hz,the reversible potential is Eo,,. These potentials are called oxidationieduction potentials or “redox” potentials. Each is related to the thermodynamic free energy, AG by the relationship, AG = -$’Eo, where 7 is the number of electrons exchanged in the reaction, and F is Faraday’s constant. The kinetics of rhe two reactions are indicated by the exchange currents I o ,and Io,,, respectively. At the reversible potentials, the equal rates of the forward and reverse reactions are 1 0 ,for ~ Reaction 1 and I o , , for Reaction 2-that is, for Reaction 1 the rate of J I -+ J I + e - is exactly equal to the rate of the reverse, X + e JP. This equilibration of rates defines Z O , ~Similar . statements apply t o the cathodic reaction. A, and A , are coilstants whose makeup need not coiicein us in this discussion, d detailed and readable derivation of Equations 3 aiid 4 in more fundamental form is presented by Potter (66). Equations 3 and 4 may be rearranged to give
+
+
+
+
-f
I n Equations 5 and 6 a plot of log I VS. E yields two linear curves, one of slope Pa, the other of slope -0,. Where the two curves intersect, I,, = I r e d = I,,,,, where I,,,, is the corrosion rate expressed in current (Figure 1). The steady-state corrosion potential E,,,, is defined by the potential at which I,, = I r e d . There are numerous conventions for plotting polarization curves. I n this paper, the convention recently adopted by the American Society for Testing Materials (9) and the National Association of Corrosion Engineers (48) is used. The noble direction is positive, and the active direction is negative, in agreement with the convention adopted by t h e International Union of Pure and Applied Chemistry in 1953. The currents Io,,, IO,,,I,,, I r e d , and I,,,, are often expressed in terms of current density (current per unit surface area), which is a specific property independent of surface area. When a polarizing current I,,, is applied to a corroding electrode, the electrode potential is altered t o a value E from the zero-current steady-state corrosion potential E,,,,. This change is defined as overvoltage, B = E - E,,,,. I n the present discussion both aiid e denote overvoltage. The former indicates a change from a redox potential, and B indicates a similar change of potential from the corrosion or mixed potential, E,,,,. Polarizing an electrode with a constant I,,, increases the rate of one of the two partial processes and suppresses the other (Figure 2 ) . The applied current must equal the difference between the rate of the acceleiated and suppressed partial processes to maintain charge neutrality in the electrode. For example, in Figure 2 during cathodic polarization, the specimen electrode is made negative by increasing the rate of Reaction 2 while suppressing the rate of Reaction 1.
(5)
and
where 2.3 RT A4 a
Pa = I
and
0.1
2.3 RT Ac
P e = __
I
cur:9nt,p.
100
I
1000
Figure 1 . Graphic representation of anodic and cathodic partial processes on surface of corroding metal M Ind. Eng. Chern. Prod. Res. Develop., Vol. 1 1 , No. 1 , 1972
13
I
curves of Figure 2. I r e d - I,, is then plotted vs. each potent'ial yielding the cathodic polarizat'ion curve. The same is done for I,, - I r e d at potentials inore positive than E',,,, to obtain the anodic polarization curve. iibove about 50 mV cathodic overvokage the oxidation reaction rate I,, becomes iiegligible with respect to I r e d . Thus, the polarization curve becomes essentially linear on a semilogarit'hmic plot. The same is t,rue anodically when I r e d becomes negligible with respect to Zox. Iinearity on an E-log I plot is termed Tafel behavior, and t,he slope of this linear curve is called the Tafel slope ( 7 5 ) . Cathodic and anodic polarization curves like those of Figure 3 are often realized experimentally. Tafel behavior is related t o an activation controlling step in the reaction sequence (7f). Deviat'ions are generally attributed to a diffusion coiltrolled step, solution resistance, or formation of passivating films (df, 69, 7 1 ) . Hist'orically, t'hese principles as they apply particularly to corrosion, were first disclosed by Wagner and Traud (77). Stern and Geary (71) reemphasized their usefulness; Steigerwald (69) as well as Fontana and Greene (21) have given still more recent reviews. The present discussion is by necessity brief and simplified. The reader is referred to the above authors for more thorough and lengthy treat'ments. Corrosion Rate Determination. One can obtain corrosion rate from a n experimental polarization curve (Figure 4). T h e extrapolation cf the Tafel portion of t'he curve back to the corrosion potential yields the coriosion rate, I,,,,, defined by the intersection. I n laboratory studies this has proved useful (4, SO, 49, 68). However, in many systems deviations from Tafel behavior are likely for the reasons mentioned, and accurate ext'rapolations may be impossible. The polaiization resistance or linear polarization method for measuring corrosion rate has exhibited usefulness in many situations. With this met'liod a series of small cathodic polarization currents is applied to a corroding specimen, and steadystate overvoltages below about 20 mV are plotted as a funct,ion of the applied current. Overvoltage is linearly related to current a t overvolt'ages less than about 10-15 mV. Furthermore, the slope of this linear polarization curve is inversely proport,ional to corrosion rate. The slope has units of resistance (ohms) and has thus been called polarization resist'ance (70). The validit,y of the linear polarization curve can be demonstrated theoretically. The applied polarizing current I,,, = I r e d - I,, = exp - A , q,/RT - I o , aexp A , qa/RT;the difference between. the exponential terms approximates a linear func,tion with potential E , as E +. E,,,, and I,,, +. 0 (7'f). This can also be tested graphically with t'he aid of Figure 2. Taking a series of potentials near E,,,, and plotting e = E - E,,,, vs. I,,, = I,, - I r e d , one obtains the linear polarization curve of Figure 5. S o t e that the curve begins to deviate from linearity after about e = 15 mV. Figure 5 represents the low overvolt'age portion on linear coordinates of the complete polarization curve already described in Figure 4. Stern (70,7'f ) has derived the relationship between polarization resist'ance and corrosion rate:
1
0.I
1.0
Current,
pa
100
1000
Figure 2. Suppression of anodic reaction and acceleration of cathodic reaction during cathodic polarization of metal M from E,,,, to €oath
e
c
P
/ , 0.1
10 Currant, pa
100
-
i
1000
Figure 3. Derived cathodic and anodic polarization curves (data points) for corroding metal M
a
\ 1 l1
E r p w i m e n t a l Cathodic
Polarlzotion CUI",
tI
I
0.1
1.0
io Current.
100
loo0
pa
Figure 4 , Extrapolation of Tafel portion of cathodic polarization curve to obtain corrosion rate I,,,, at corrosion potential E,,,,
The attained cathodic overvoltage E, is then defined by the potential a t which I,,, = I r e d - I,, (Figure 2). For anodic polarization (specimen electrode positive} the reverse is true; Reaction 1 is increased at the expense of Reaction 2, and Ispp
=
Io,
-
Ired.
From this information the cathodic or anodic polarization curve can be constructed. I n Figure 3 I r e d and I,, at given potentials negative to E,,,, are taken from the partial or local 14 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1, No. 1, 1972
where R, is polarization resistance, and the rest of the terms have already been defined. V i t h Pa = P , = 0.1 volt from Figure 1, and a measured R of 2.17 K Q from Figure 5, the calculated corrosion rate I,,,, is 10 j A which checks with the specified I,,,, of Figure 1. It has been argued (Sd) that the polarization resistance measurement is superfluous because one must know the Tafel
Anodic Current
pa 0.1
I .o
IO CURRENT,
pa
I lW0
too
Figure 7. Active-passive metal in active state a t passive state a t B
I
A and in
/
metals. In this method discontinuous changes in slopes (breaks) are assumed to occur on anodic and cathodic polarization curves. The currents a t t h e breaks are related t o corrosion rate as follows: Figure 5. Derived anodic and cathodic linear polarization curves for metal M
.lopes ( P ’ 9 in Equation 7 ) before corrosion rate can be calculated from R,. Of course, in measuring the Tafel slope, the corro61on rate can be obtained directly by extrapolation. Honever, accurate values of are not necessary to obtain useful values of corrosion rate. Stern and Keisert (72) she\\ ed previously that P falls within a predictable range foi iieaily all electrocheniical systems, and that the error owing to inaccurate P’s seldoin exceeds a factor of two. Also, accurate values of corrosion rate are often less important than compar~sonsbetween alloy.. or qolutions of the same type. Then, the P’s will be uiiiforni and comparisoiis quantitative. Furthermore, the polarization resistance measurement can be conducted niuch more rapidly and thus is easier to adapt to routine methods. The polarization break method (61-623) has gained some following for corrosion rate measurement, especially for buried
‘crit
, APPLIED
CURRENT,
pa
Figure 6. Anodic polarization curves of active-passive alloy obtained galvanostatically and potentiostotically
where I , and I , are, respectively, the currents a t the break of the anodic and cathodic polarization curves. However, Stern and Geary (71) showed some time ago that this interpretation is incorrect and outlined the now generally accepted theoiy described here. In a recent investigation ( S 7 ) , the polarization break and polarization resistance methods were applied to the same data on buried metal specimens. The polarization resistance method was less ambiguous although an experienced investigator can obtain usable results from the empiiical polarization break method. Passivity. For many metals-e.g., Fe, C r , Xi, Ti, a n d their alloys-oxide corrosion product films become thermodynamically stable (76) at noble (positive) potentials. If present, such passive films greatly suppress the anodic current over a range of potentials (Figure 6). From E,,,, up t o E,, (primary passive potential) the applied current increases as in Figure 4. Above E,, the passive film becomes stable, and the applied current drops t o a low value, I,,,,. Above Et, (transpassive potential) other reactions (oxidation to higher states and oxygen evolution) begin. The listed metals, which display active-passive behavior, form a majority of the structurally important alloys now in use. Thus, the study of passivity is quite important. The sample curve in Figure 6 is idealized to clearly show the significant features likely to appear. Anodic polarization curves for actual alloys may vary considerably from this model depending on the ease of formation and stability of the passive film. Specific examples are given later in this paper. I n a corrosive solution, a n active-passive metal can exist either in the active or the passive state (Figure 7 ) .A t point A the metal is active with a high corrosion rate, but at B the metal is passive with low corrosion rate. As mentioned, t h e stable corrosion state of a n alloy is determined b y the point a t which I,, = I r e d . The relative positions of the local anodic oxidation and cathodic reduction curves then determine whether state A or 13 is stable. The reduction curve moves vertically depending on the oxidizing power (E,,d) of the redox system and on the relative concentrations of the oxidized and reduced species of the redox system. The reduction curve Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 ,
No. 1 , 1972
15
Experimental Methods
l---l---
Polarization measurements call be classified as either galvnnostatic (inteiisiostatic) iii which current is fixed aud specimen poteiitial allowed to reach steady state, or potentiostat~ic in which sl)eciineii potential is fixed and the polarizing current drifts to its stcady-state value. Galvanostatic techniques are simply and inexpensively conducted. A schematic of the circuitry is shown in Figure 8. ii battery or constant voltage power supply induces current through the high variable resistor, r, a i d auxiliary electrode (AUX) polarizing the specimen or working electrode (WE). 13ecause r is much higher than the variable cell resistance, the I)olarizing current is essentially constslit. Specimen potential is measured with respect to a reference electrode (REF) by use of a potentiometer in series with R high internal impedance electrometer. The potentiometer emf polarity is set to oppose bhe potential between the reference elect,rode aiid working elect'rode. When the potentiometer emf is exactly equal to the specimen potential, no current flows through the circuit as shown by the electrometer which acts a? a null detector. With the high impedance elcctrometer in the circuit, deviation from the null results in only a minute current through t'he circuit, not enough to polarize the reference electrode or affect the polarization of the working elect,rode. Ideally, the null detector impedance must be 109 Q or greater, beyond the capabilities of the ordinary vacuum tube voltmeter. The electrometer, used as a voltmeter, will also measure electrochemical potential directly but with a considerable sacrifice in sensitivity. The cell in Figure 8 is simplified to the extreme. Deaerating is usually necessary to eliminate uncontrolled effects of oxygen. The tip of the salt bridge is usually placed immediately adjacent to the working electrode surface to eliminate as much as possible diffusion and ir effects mentioned previously. The specimen must be mounted to expose a controlled surface area. Organic materials may contribute to the electrolyte oxidizable or reducible impurities which affect electrochemical measurements. Cell designs are innumerable, and there are many approaches to fulfilling these requirements. A particularly versatile widely used design made entirely of glass and Teflon is described by Greeiie (27);a diagram of the cell, associated salt bridge, and electrodes appears in Figure 9. A potentiostat is an electronic instrument designed to hold the potential between the reference and working electrodes a t some specified value while polarizing current varies. Several such devices on the market all consist of a voltage error sensor coiinected to a power supply. When any deviation between the specified and actual electrode potential occurs, t.he sensor transmits a signal to the power supply to increase or decrease
M H I I I Figure 8. Galvanostatic (constant current) electrical circuitry for electrochemical polarization
1a
&GAS
OUTLET
THERMOMETER -GAS INLET
LUGGIN- HABER
WORKING ELECTRODE Figure 9. Cell for electrochemical polarization studies [Greene (27)]
(and the exchange current IO,^^^) is moved to higher or lower currents-Le., right or left in Figure 7-depending on the surface properties of the corroding metal. A t the same time, the parameters lorit, E,,, aiid I,,,, for the anodic curve can all vary independently of one another depeiidiiig upon the metal, its surface properties, and the corrosive solution. The goal, of course, is to achieve the passive state 13 for corrosion protection. Inspection of Figures 6 and 7 shows that the passive state is easier to obtain and corrosion rate is lower for lower Icrit, more negative (active) E,,, arid lower I,,,, (21, 26). Iri fact, if E,, is quite active for a given metal, it is possible to achieve the passive state B in acid solutions. For titanium E,, is often more active than Eredfor the H+/Hz redox couple. Passivity has been obtained (73) by alloying titanium with a noble metal such as Pt or Pd. Reduction proceeds quite rapidly 011 the P t or Pd which accumulates 011 the alloy surface-Le., I o for H+ ion reduction (Reaction 2) is high on the surface Pt or Pd. Thus, the reduction curve for hydrogen intersects the dissolution curve in the passive potential range. Much can be gaiiiedfrom both anodic and cathodic polarization curves. The c:it,hodic curve gives the kinetics of the reduction process, and with Tafel behavior the corrosion rate can be obtained directly. Corrosion rate can often be obtained from polarization resistance measurements at low overvoltage. The anodic polarization curve shows the dissolution characteristics of the metal, and if of the active-passive type, the ease and stability of passiv a t'ion. 16 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , NO. 1, 1972
,--.
I
- 1 I
i
Polenlioslat
II /, 1
I
J
1
-
-
AUX
WE
Eleclrorneler
c
- -
4
1-I
I
Figure 1 0. Potentiostatic (constant potential) electrical circuitry for electrochemical polarization
the polarizing current, as is appropriate t o correct the deviation. A schematic of the circuitry for potentiostatic electrochemical studies is shown in Figure 10. T h e same potentialmeasuring circuitry described in Figure 8 also appears in Figure 10. Cell designs are equally suitable for either galvanostatic or potentiostatic studies. Poteiitiostatic anodic polarization is especially useful in the presence of passive films. Figure 11 shows a schematic experimental potentiostatic anodic polarization curve; note the close similarity to Figure 6. I n fact, the theoretical curve for the anodic dissolution process (Equation 3) in Figure 6 is deduced from experimental data, such a s in Figure 12. Galvanostatic polarization does not reveal the entire activepassive anodic curve because potential is not necessarily a single-valued function of current in this case. The corresponding galvanostatic curve is compared in Figure 11. U p to ierltthe galvanostatic and potentiostatic curves agree closely. At a current above iorlt the next stable potential is iii the transpassive region, and the passive part of the anodic curve is bypassed. Note in Figure 11 that current density (current per unit area) rather than current is used for the abscissa. The former, independent of total surface area, is used generally in expressing experimental results. Polarization measurements are useful when the corrosive medium is a conducting electrolyte. When conductivity is low-e.g., pule water-overvoltage measurements include a significant I,,,R potential through the solution. Here, R is t h e solution resistance between working and reference electrodes, and I,,, is the polarizing current through the auxiliaryworking electrode circuit. I,,,R can be partially eliminated b y reducing R. A salt bridge probe reduces R by reducing in effect the distance between working and reference electrodes. R can also be reduced b y adding a supporting electrolyte which increases the solution conductivity. Outside the laboratory, however, i t may not be possible t o use a Luggin probe arrangement; for example, polarization measurements in soils, chemical process streams, and nuclear reactor coolant water. I n dilute solutions, added supporting electrolyte may have a n undefinable effect on the mechanism of the corrosion reaction. Moreover, in low conductivity solutions-e.g., pure water-R may still be appreciable even when using a probe.
W m -I
g
.
/I/
POTENTIOSTATIC ___. GALVANOSTATIC
, I
-
I
VI
1I
0
I I
-I
5 -
I
t ,
Iw I-
2.
I
I
,
1
t
I L -
W
2 . I4 o
Emrr
i-_--------’
i
1 TIME
Figure 12.
Schematic potential growth and decay curves AMMETER
n
I
REF
WE.
200 M R Figure 13. Bridge circuitry to compensate ohmic ir interferences out of polarization potentials [Jones ( 3 3 ) ]
When applied current is suddenly interrupted, the I,,,R contribution t o polarization disappears almost instantaneously (Figure 12). Activation and concentration polarization decay more slowly and thus can be differentiated from IsPpR. Sophisticated electronic instrumentation is often needed to effect periodic current interruption and to monitor the rapid potential transients which result. A bridge circuit (33) (Figure 13) will offset Z,,,R potentials with considerably simplified circuitry. When the adjustable resistance X (Figure 12) is set equal t o R , the measured output P of the potentiometer is equal t o E , where E is the actual potential between REF and WE. Theory and operational details of the circuit have been given (33, 81). Applications
The following examples are listed to show how laboratory electrochemical methods have been used t o predict the corrosion performance of materials and structures in service. These examples have been selected as especially appropriate and of interest t o the practicing engineer. However, the selection is b y no means complete, and a recent comprehensive review of electrochemical techniques for testing metals and alloys is recommended ( 1 2 ) . Galvanic Corrosion. Galvanic corrosion is especially important because of t h e disastrous corrosion often experienced when dissimilar metals in contact are exposed t o a corrosive environment. Often, solution potential measurements are used to predict the galvanic attack t o be expected between a pair of dissimilar alloys. T h e Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1, 1972
17
-.zoo ~”
- 300 -.400
-
-.SO0
-
-600
-
-.700
-
-.a00
-
-900
-
W
u
I
Y)
2
0
J
5 z
0
-1000 -
W
-1.100
- *
-1.200
-
ANODIC
POLARIZATION OF C 3 N G UIT
ALUM IN U I4
CATHOCIC POLAR:ZATION OF STEEL R O D S
-1.300 A “ “ “ ‘ (
IO-^
10-6
10-4
10-5
CLIRRENT,
10-3
crnp
Figure 14. Polarization curves for stainless steel (cathode) a n d aluminum (anode) forming galvanic couple in concrete
(34) difference in solution potential between two alloys has not proved a n absolute measure of t h e resulting galvanic corrosion current, undoubtedly because of different polarization in specific cases. I n a galvanic couple the noble half of the couple is polarized cathodically and the active half anodically until both have the same electrochemical potential, E,,, at short circuit. The corrosion rate of the active metal is increased to I,,, the current dowing in the shorted couple. The anodic and cathodic polarization curves on the active andnoble halves, respectively, of a galvanic couple can be used to predict the galvanic behavior of the couple in a particular environment. An example of this procedure is given in Figure 14 (34). Here, anodic and cathodic polarization curves were obtained on AA6063 aluminum and stainless steel, respectively, in concrete. The cathodic polarization of stainless steel followed well-defined Tafel behavior, and the current in the couple was limited by polarization of the aluminum alloy anode (84). Effect of metallurgical, alloy composition, and solution composition variables on galvanic corrosion can be anticipated from their effect on the individual anodic and cathodic polarization curves. Polarization Resistance. T h e polarization resistance method has found application in several areas. T h e corrosion of aluminum containers b y numerous food products has been studied by Evans and Koehler (15). Skold and Larson (66) used the polarization resistance method to monitor the corrosivity of various domestic waters on iron. Butler and Carter have used the method as a rapid corrosion test for stainless steel (10) and as an evaluation test for tinplate 111 various food products (9). LeGault and Walker (40) and Simmons (65) have evaluated inhibitor efficiency with polarization resistance. Colangelo et al. ( 1 1 ) measured the corrosion of iron slugs implanted in the muscle tissue of clinical animals by use of needle probes and skin electrodes. The method has been adapted t o measure corrosion late d o w i to A/cm2 (36)* 18
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1, 1972
The usefulness of the polarization resistance method is reflected by the listed applications. The primary advantages of the method are that it is rapid, remote, continuous, and nondestructive. Thus, the method can be used as a n evaluational test more rapid than the standard weight loss test. It gives a value for the instantaneous corrosion rate a t a given time whereas other corrosion rate measurements such as weight loss give an integrated or average corrosion rate over a period of time-eg., the t’ime necessary to generate measurable weight loss. This is of value when the time variance of corrosion rate is being studied and a i l 1 indicate immediately the effect of changes in the corrosive conditions. The polarization resisbaiice method is finding increased industrial use. Commercially available two-electrode or Dhreeelectrode probes have been inserted into process streams to monitor corrosivity. I n the three-electrode setup, the three electrodes are the working (RE), auxiliary (AUX), and reference (REF) electrodes in Figure 8. Changes in process stream corrosivity are reflected in changes of the corrosion rat,e of the working electrode as measured by polarization resistance. Three-electrode probes have been used recently t Q monitor corrosion a t various positions in an oil refinery (52). The theoretical foundation of the two-electrode modification (44) is doubtful, and the three-electrode arrangement i s generally more reliable ( 3 ) . -4pparently, as a result, t,he threeelectrode method is supplanting the earlier two-electrode method. Such probes could be used as the sensing element for control circuitry. The first attempt a t this has been a control instrument to regulate inhibitor additions, pH, and bleed-off rate for the cooling water in cooling tovers (18),The bleed-off rate is controlled to maintain a selected mater conductivity (a measure of total dissolved solids). The pH is measured with a two-electrode probe, and inhibitor additions are controlled by measuring the pitting susceptibility of a second two-electrode probe. Inhibitor additions are reported to reduce the pitting susceptibility (18). However, valid ineasurement of pH and pitting with polarization resiatance probes have never received a satisfactory confirmation in the literature. Without more favorable evidence, the use of such a controller must be viewed with caution. Nevertheless, the principle has merit, aqd some qualitative evidence (59) indicates that general corrosion rate as measured b y polarization resistance sensors is sensitive to such parameters as pH, dissolved oxygen, and temperature of the corrosive. Galvanic currelit in a shorted galvanic couple can be measured without inserting conventional instrumentation between the two halves of the couple (34) by applying the polarization resistance method to a shorted galvanic couple. The current in a shorted couple is analogous to the corrosion current-Le., corrosion rate-on a corroding metal because corrosion proceeds via numerous microgalvanic couples on the surface. Thus, galvanic short-circuit current can be measured by the polarization resistance method, based on the same principles for measuring corrosion rate on a corroding metal. The method was applied to aluminum-steel couples in concrete (84), and reasonably good agreement was obtained between galvanic current measured by polarization resistance and more conventional techniques. Still more recently, Schwerdtfeger (60) found that the corrosion of the anode in galvanic couples (proport’ional to the galvanic current) could be measured by electrochemical methods. Anodic Polarization. One of the major uses of anodic polarization is alloy evaluation. The following discussion illustrates how alloy selection can be aided b y reference
t o t8he appropriate anodic polarization curves. Anodic polarization of t,hree nickel alloys in sulfuric acid is shown in Figure 15. Hastelloy 13 (r\ri-%yO N o ) shows oiily a hint of active-passive behavior. Hastelloy C (Ni-15% Cr-15010 hIo-5y0 Fe) shows active-passive behavior with a relatively low I,, but with a continuously increasing passive current at higher (more noble) potentials. On the other hand, for Hastelloy (2-276 (low C, low Si modification of Hastelloy C) the passive current density remains low a t high potentials as well. I,ow Si and C in Hastelloy C-276 permit use in the aswelded state where precipitates would normally lower the corrosion resistance of Hastelloy C. Figure 13 shows t'hat anodic polarizat~ioiiis sensitive to the presence of these elements even though both alloys are in the solution annealed condition (precipitates dissolved a t high temperature and maintained in solution by rapid cooling). For reducing conditions (act'ive state), there is little to choose from between the three alloys from the standpoint, of corrosion resistance. At moderately oxidizing condit'ions, Hastelloy C is superior because of its low I , and active Ifp,,. However, in highly oxidizing solutions, Hastelloy C-27G seeins better because of its lower passive current density at higher oxidizing potentials. The corrosivity of part'icular solution species can be judged by their effect on anodic polarization curves. Figure 16 shows t,he effect of chloride on the anodic polarization curves for 304 stainless steel and Hastelloy C (29).The stainless steel shows a large effect owing to chlorides with the breakdown potential (39) typical of rapid pitting attack. Chloride has much less effect on the anodic currents for Hastelloy C, reflecting higher resistance t o such solutions. On the other hand, 304 stainless steel is more suitable (and economical) in chloride-free solutions. I n the same study (29) chloride had 110 effect on the polarization curve for titanium. The polarization curve shows graphically the corrosion behavior of a n alloy in a particular corrosive solution. A series of such curves gives the behavior of similar or different alloys in one or a number of corrosives. For example, Leonard (42) has extended Greene's previous work (28) on nickel base alloys to other nickel alloys and acid electrolytes a t higher temperature. Taken collectively, these curves give a broad picture of the corrosion resistance of numerous nickel alloys in a variety of acid corrosives. They do not replace conventional corrosion testing but do provide a n additional tool in alloy evaluation and selection. Comparison between anodic polarization curves cannot be meaningful unless care is taken to duplicate experimental procedures (especially the time rate a t which potential is changed) for each curve. Thus, comparable polarization curves for most alloys are not available. Some progress should be made on this problem with adoption of standard anodic polarization procedures now under consideration ( 2 ) . Polarization curves are obtained in a few hours whereas weeks or months may be necessary to obtain the equivalent by conventional testing. This, of course, saves time when developing new alloys or testing available alloys for new applications. For example, selection of materials becomes more difficult in new chemical process plants because recent. technology has shortened or eliniinated pilot plant studies ( S I ) . As a result, actual process conditions ale not known until shortly before plant start-up. Polarization studies conducted in simulated process environments or in similar plant environment's can give rapid and valuable indications of service performance in such situations. An example of electrochemical polarization studies in simu-
4
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Figure 16. Effect of chloride on potentiostatic anodic polarization of AIS1 304 stainless steel and Hastelloy C [Greene and Judd ( 2 9 ) ]
lated service conditions is the work of Bohlmaiin and Posey (7) and Posey and Bohlmann (55).They conducted polarization measurements on aluminum and titanium in flowing salt solutions a t elevated temperatures to simulate conditions in a desalting plant. Similarly, Wilde (89) conducted anodic (and cathodic) polarization experiments in high-temperature (289°C) , high-pressure water containing 02,€I2,and XH3 gas, all variously reported as being beneficid t o the corrosion resistance of plain carbon steel in boiling water iiuclear reactors. His polarization measurements showed that u p to 50 ppm O2 corrosion was increased, but at 100 ppm O Lan oxide film formed on the surface and inhibited corrosion. Further polarization measurements showed that ?;Hs improved corrosion by altering the pH of the water. Walker and France (78) conducted polarization measurements on several alloys in a simulated automotive cooling system and observed the effects of various inhibitor additions. X o r e polarization studies under simulated service conditions can be expected in the future. Anodic Protection. If a n active-passive metal is maintained in t h e passive region with a potentiostat, its Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1, 1972
19
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Figure 17. Potential acid concentration diagram showing specific regions of intergranular attack for CF-8 stainless steel (similar to AIS1 304) [France and Greene ( 2 4 ) ]
corrosion rate will be low at I,,,, (Figure 6). Edeleanu (IS) first suggested that large industrial systems could be maintained in the passive state electrically and thereby be protected from corrosion. This method, called anodic protection, has since gained considerable application in the chemical industry. T o initiate anodic protection, a metal must be polarized from the active state through E,, where current is a maximum a t Ierit(Figure 11). Consequently, power requirements are high for the electrical components of an anodic protection system. Current supply is much lower once the passive region is attained, but extra current capabilities are necessary (14) in case the metal should lose its passivity for any reason-e.g., brief power losses to the potentiostat, changes in process stream composition. One cannot polarize rapidly through E,, because icntbecomes impossibly high as polarization rate increases. A low polarization rate lowers &it and consequent power requirements, but metal dissolution is still highly accelerated near E,,. Thus, a compromise must be reached between high power requirements a t high polarization rate and excessive anodic dissolution a t low polarization rate. Another design requirement of potentiostats for anodic protection is the voltage available across the cathode (auxiliary electrode) and the anode (protected structure). The required voltage is the sum of overvoltages a t the cathode and anode as well as ohmic potentials through the solution, the metal-electrolyte interfaces, and the external circuitry. Overvoltage at the cathode can be controlled by the size of the cathode relative to the protected structure. These and other aspects of anodic protection system design are discussed by Foroulis (22) and by Stammen and Townsend (67). I n the past anodic protection has been used primarily to alleviate corrosion in steel tanks for acid storage (6, 10,53, 57, 64). Active-passive behavior of ferrous alloys in acids is well defined and has been widely studied. Furthermore, the geometry of a tank is probably the least complex of any piece of process equipment, simplifying the design of an anodic protection system. 20 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1, 1972
For more complex shapes the problem becomes the “throwing power” of anodic protection-that is, the ability of aiiodic protection to be transmitted long distances or into partially shielded areas through the corrosive electrolyte. Several experimenters have shown that anodic protection can be thrown down long lengths of tubing (6, 14, 47). Nevert’lieless, few applications are recorded for aiiodic protection of tubing or pipe. This is probably because either can be replaced periodically without excessive capital outlay or can be made of a highly resistant alloy which would be too expensive for a tank. A special problem of throwing power is anodic protection in the presence of crevices. France and Greene (25) have shown t h a t because of ohmic losses down the length of a constricted crevice, the metal a t the bottom of the crevice may remain in the unprotected active state while the surface is fully protected. T h a t anodic current which does flow a t the bottom of a crevice accelerates active metal dissolution, resulting in severe localized corrosion. Complicated systems containing crevices are thus quite difficult to anodically protect. Morgan and Evans (46) reported an attempt to anodically protect a titanium heat exchanger. By insulating the heat exchanger tubes from the graphite tube sheet, the latter was used as the auxiliary electrode. Efforts were unsuccessful because of recurrent leaking at the crevices between the titanium heat exchanger tube and the insulating gasket. The probable cause was the accelerated attack in crevices during anodic protection. An insulation-anode crevice must be present somewhere in a closed system such as this. If possible this crevice should not be immersed in the corrosive electrolyte. I n applying anodic protection to complicated structures, it seems best to avoid all crevices by using flush-welded seams and joints as much as possible. Also, alloys with low I o r i t are easier to passivate in crevices (93). Morgan and Evans (46) brought out another point which must not be overlooked. Anodic protection affects only the immersed area. If the vapors are corrosive, any areas above the electrolyte can still be attacked even though anodic protection is functioning effectively on the immersed area. There is presently some conflict in the literature over whether anodic protection can or cannot protect against intergranular attack (IGA) . Normally corrosion-resistant austenitic stainless steel often fails in acid solutions by grain boundary attack because of welding or other faulty heat treatments which cause precipitation of chromium carbides a t the grain boundaries, “sensitizing” the material to IGA. Juchniewicz et al. (38)used anodic protection to prevent IGA in sensitized austenitic stainless steel exposed to 30% H2S04 1% NaCl a t 25OC. Higher anodic currents were required for protection of sensitized as compared with nonsensitized specimens. On the other hand, Osozawa et al. (50) experienced IGA in sensitized austenitic stainless steel a t all passive potentials in 90°C, 2N H2SOa. These contradictory data suggest that IGA in austenitic stainless steel may depend on the particular experimental conditions. I n support of this, France and Greene (14)demonstrated that IGA occurs in particular regions on maps of anodic potential vs. acid concentration. The regions of IGA then become more extensive at higher temperature as illustrated in Figure 17. Thus, a t 25OC IGA was observed only in the active and not in the passive region in all acid concentrations tested (10N maximum). This is consistent with the results of Juchniewicz et al. (38). However, at 90°C the regions of IGA were broadened much
+
into the passive region and were present at all potentials in acid concentrations above SAr. A t lower coiiceiitrations no I G d appeared in the region near 0.6 volts. This contradicts to some extent Osozawa et al. (50) who observed fine IGA at 0.7 volts in 21%’ I-I2SO.i. Both investigations, however, are consistent in showing more extensive IGA in the passive region a t higher solution temperature. The differences might be attributed to the different alloy histories (Le., cast alloys used by France, wrought by Osozawa), alloy compositions, or IGA detection methods used ill the two investigations. It is also possible, as pointed out by Streicher (74), that sensitized austenitic stainless steel will develop IGA under any contiitions if exposed long enough to sulfuric acid. I n reply France and Greeiie (25) cite tests in which sensitized austenitic stainless steel showed no IGA after exposures as long as eight niont’hs in various sulfuric acid media. I n view of this controversy, considerably more long-term testing is necessary before one can confidently expect to anodically protect sensitized-e.g., welded-austenitic stainless steel in media which could produce IGA. Anodic protection is usually used only in the absence of chlorides. However, anodic protection can be effective in acid chloride solutions (5) and will suppress stress-corrosion cracking ( 1 ) . Since chloride increases I,,,, (I)! anodic polarization is more likely to cause accelerated corrosion in crevices. Further, for nearly any metal, pitting is drastically accelerated above (more noble than) a certain “critical” potential (39). Thus, localized crevice or pitting corrosion is a real danger during anodic protection with chlorides present. Watson (7‘9, 80) has described an interesting and novel application of anodic protect’ioii. Wood chips are batch reduced to pulp for paper making in caustic solutions a t elevated temperat’ure and pressure. Tlie steel digesters are subject to severe corrosion and must be replaced every few years a t considerable expense. Watson applied anodic protection to the digesters while they were subject to maximum corrosion during the high-temperature high-pressure cook. This is a rare instance in which anodic protection was successfully extended from storage tanks t o an operating chemical process component. Cathodic Protection. T h e theory of cathodic protection has been described recently in some detail (35). Therefore, it is only necessary to review the pertinent conclusions at this time. The priiiciple of cathodic protection is simple and can be understood by exaniination of Equations 1 and 2. A metal surface is cathodically protected by polarizing it in a n active (electronegative) direction with a n electrical circuit, such as in Figure 8. With the excess of electrons a t a negative surface, the dissolution Reaction 1 is suppressed, thereby lowering the corrosion rate. It follows then that the rate of the anodic process (Reaction I)-i.e., the anodic polarization curve-is critical in the intelligent application of cathodic protection. The anodic polarization curve gives the corrosion rate a t a given level of cathodic protection (36).The cathodic polarization curve gives the current necessary t o cathodically polarize t’o a given level and is used to design or specify the electrical equipment of the cathodic protection system. Cathodic current is usually supplied through an external circuit with a high constant voltage de rectifier acting as the battery or power supply. The engineering of such systems has been frequently described-e.g., References 45 and 51. Tlie application of constaiit voltage results in a relatively constant current through the circuit, which consists of the protected structure, the auxiliary electrode(s) , and the rectifier. If the reduction process in a corrosive is rather change-
able-e.g., variable diffusion of dissolved oxygen to steel surfaces in seawater-the structure may be underprotected or uneconomically overprotected for a large part of the time when using a constant current protection systeni (35).X new system, which utilizes automatic potential control (Al’C) rectifiers (19), has become available recently. This equipment uses electrical control circuitry t o maintain electrochemical potential at a constant value, permitting the applied current to vary and compensate for changes in the rate of t’he reduction process. It should be noted that the APC rectifier operates identically in principle to the potentiostat discussed above. If for a n y reason the anodic partial process is variable, constant potential cathodic protection is of no special advantage (35). The bridge circuit discussed previously was originally used by Schwerdtfeger (61) in cathodic protection studies of buried steel. The circuit is useful in compensating resistance, I,,,R, polarization out of measured polarization potentials, especially in a high resistivity electrolyte such as soil (43).Resistance polarization hides the true surface potential needed to determine the degree of applied cathodic protection. Current interruption methods previously discussed usually require cornplex electronic equipment suitable only for laboratory studies. However, a t least one commercial cathodic protection syst’em has been reported (8) which utilizes current interruption t o measure and compensate for Ia,,R. Cathodic protective current also results when a metal is coupled to an alloy which has a more active (electronegative) electrochemical potential. As a result of coupling, galvanic current floms with the protected metal acting as cathode. The active “sacrificial anode” is consumed by anodic dissolution. Sacrificial anodes are characterized by four electrochemical properties (35): The elect’rochemical potential of the alloy, which must be quite active to maintain a driving force for galvanic (protect’ive) current through the electrolyte The degree of anode polarization, which can limit the galvanic current to low values The electrochemical equivalent (output) of the alloy, which is the charge theoretically available to provide galvanic current per unit weight of the alloy The efficiency of the alloy, which is the percent of the theoretical output actually obtained in practice The application of these factors is illustrated b y the present’ly used anode alloys. Three metals have been found suitable for use as sacrificial anodes: zinc, magnesium, and aluminum. All generally perform best with controlled alloying additions. However, many trace contaminants are detriniental t o performance. Zinc, probably the most widely used of anode metals, has a fairly low theoretical potential of about -1.0 volt vs. SCE and is not polarized a great deal by anodic current. Further, it has a high efficiency of usually 95% or more (41). llagnesium has the particular advantage of having a low potential of about -2.6 volts vs. SCE. Nor does magnesium polarize much on passage of anodic current. In fact, the low potential and attendant high currents result in short anode life. Thus, pure magnesium is most often alloyed with aluminum and zinc to attenuate these properties and thereby make a longer life alloy (45). Aluminum has attractive possibilities as an anode alloy. It has a theoretical potential (-1.9 volts vs. SCE) intermediate between zinc and magnesium. Further, it has a theoretical maximum output higher than either of the other two (Al: 1352, Zn: 372, M g : 1000 amp-hr/lb). However, Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1 , 1972
21
until relatively recently, aluminum has not’ been widely used in sacrificial anodes because of prohibitive polarization and low efficiency on passage of anodic current. Polarization is caused by a protective adherent oxide film on the surface. Certain alloy combinations-namely, Al-Zn-Sn (54) and AI-Zn-Hg (58)-have a t least partially overcome these difficulties, and aluminum is becoming more widely used. A system utilizing sacrificial anodes has the advantage that once installed, 110 maintenance or att’eiition is required for a considerable period of time. However, for some structures such as buried pipelines, it may be difficult to replace the sacrificial anodes after t’hey have been expended. In such a case an externally applied cathodic current through an ausiliary electrode may be preferable. Furthermore, with an external source a relatively unlimited voltage is available to drive current through a high resistivity electrolyte such as soil. Also, higher volt’age permits a larger area of t’he structure t o be covered from a single auxiliary electrode. I n seawater applications (ship hulls, off-shore drilling rigs) , where the electrolyte resistivity is low and the structure is accessible, sacrificial anodes are often used. Summary
Electrochemical theory, as developed in recent years, provides a sound basis for studying aqueous corrosion of metals and alloys. Of still more importance to the practicing engineer, the theory has resulted in new methods for measuring and protecting against corrosion, as well as a better fundamental iinderst’andiiig of the older electrochemical prot,ection methods. The linear polarization or polarization resistance method permit’s remote, noiidestructive monitoring of corrosion. This is being used increasingly to monitor in-plant operating conditions. However, in many of its forms the method gives only a qualitative indication of corrosion, and further claims that the method can detect pitting (localized corrosion) susceptibility and solution pH must be viewed with caution. Cathodic protection has been utilized for many gears and can be effected b y either impressed current or sacrificial anodes. The method has been used extensively for buried pipeline and undersea structures. The introduction of constant potential cathodic protection should prove useful particularly for undersea structures. dnodic protection results when an active-passive alloy (usually iron-base) is maiiitained in the passive (low corrosion) state with electrical control circuitry. The method has been used chiefly t o protect chloride-free acid storage tanks a t ambient temperature. Anodic protection is possible for more complicated structures, but throwing power becomes a problem, especially in constricted areas such as crevices. Anodic protection must be closely controlled in the presence of chlorides because pitting will ensue above certain critical potentials. Chlorides can accumulate in crevices during anodic protection, resulting in further accelerated attack in these areas. Anodic protection is not necessarily effective in preventing intergranular attack in sensitized (e.g., welded) stainless steel. Anodic polarization has proved to be a sensitive, rapid method of alloy evaluation. Alloys of varying composition and heat treatment can be compared under uniform, controlled conditions in a short time. Corrosive conditions favoring passive (low corrosion rate) behavior can be readily determined for an alloy or number of alloys. Polarization studies in corrosives and under conditions of practical interest should 22
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1 , 1972
provide a useful tool in alloy selection for part’icular plant applications. Experimental met’hodshave been reviewed as a background for the discussions of this paper. Potentiostatic (constant potential) control is used predominately in anodic polarization evaluations of alloys. Galvanostatic (constant current) methods are the simplest and are useful for cathodic polarization and polarization resistance measurements. Bridge circuit’ry to compensate ohmic potential interferences should be useful for both cathodic protection systems and polarization resistance measurements in lorn conductivity electrolytes such as soil arid relatively pure water. Jlaiiy other electrochemical met hods are available to evaluate materials and study elect’rolyte and surface reaction mechanisms (12). The present discussions have been limited to t’hose methods most commonly used and of direct benefit t o the pract’icingengineer. Acknowledgment
Initial parts of this paper were conceived while the author was affiliated with the Kaiser dlumiiium and Chemical Corp., Spokane, Wash.; their permission t o publish is gratefully acknowledged. The author is indebted to F. G. Hodge, Stellite Division, Clabot Corp., for supplying the data of Figure 15. Literature Cited
(1) Acello, S. J., Greene, N. I>., Corrosion, 18, 286t-90t’ (1962). (2) Amer. Soo. Test. Xater. Ilecommended Practice G.7-69 ASTJI Standards, Part 31, 1969. (3) Annand, It., Corrosion, 22, 215-28 (1966). (4) Araniaki, K., Hackerman, N., J . Electrochem. SOC., 115, 1007-13 (1968). (,5) Banks, W.P., Hutchison, AI., X at ar. Prot., 7, 37-9 (Sept. 1968). (6) Banks, FY, P., Sudbiiry, J. I)., Corrosion, 19, 300t-07t (1963). (7) Bohlmann, E. G., Pose?., F.A., Proc., 1st Int. Symposium on Water Ilesalination, Vol I, 306-25, U.S. Gov’t Printing Office, Washington, D.C., 1968. (8) Bushman, J. B., Paper Yo. HC-TASC, Harco Corp., 4600 E . 71st St., Cleveland, Ohio 44216. (9) Butler, T. J., Carter, P. R.,Elrctrochem Tcchnol., 1, 22-7 ( 1963 ) . (10) Butler, T. J., Carter, P. It., Electrochem Technol., 3, ln7-61 (196.7). (11) Colangelo, 1’. J., Greene, K. I)., Kettelkamp, D. B., Alexander. 11.. Carnubell. . 1, . I , C. J.. J . Biomad. J f a f e r . Res., 40t7-14 (1667): (12) Dean, S. Q’., France, W. I)., Ketchiim, S.J., “Handbook on Corrosion Testing and Evaluation,” W. H. Ailor, Ed., WileyInterscience, New York, N.Y., 1971. (13) Edeleanu, C., dletallurgica, 50, 113-16 (1934). 114) Edeleanu., C.., Gibson. J. G., Chem. Znrl. 301-08 (Alarch 11, 1961). (15) Evans, S., Koehler, E. I,., J . Electrochcm. Soc., 108, 309-14 (1961). (16) Evans, TJ. R., “The Corrosion and Oxidation of Jfetals,” Arnold, London, England, 1960. 117) Evrine. H., Glasstone, S.,Laidler, K. J., J . Chem. Phys., 7, 1053-65 (1939). (18) Feitler, H., Townsend, C. R., >later. Prot., 8, 19-22 (March 1969). (19) Ferry, R . E., Jfater. Prot., 7, 27-9 (August 1968). (20) Fisher, .4,, Brady, J. F., Corrosion, 19, 37t-44t (1963). (21) Fontana, RI. G., Greene, N. D., “Corrosion Engineering,” XcGraw-Hill, New York, N.Y., 1968. (22) Foroulis, Z. A., Corros. Sci., 5 , 383-91 (1963). (23) France, W. D , , Greene, N . I)., Corrosion,,24, 247-31 (1968). (24) France, W.I)., Greene, S . I)., Corros. SCZ.,8 , 9-18 (1968). ( 2 5 ) France, W. D., Greene, S . D., Corros. Sci., 10, 379-82 (1970). (26) Greene, X. D., Corrosion, 18, 136t-42t (1962). (27) Greene, N. D., "Experimental Electrode Klnetics,” Rensselaer Polytechnic Institute, Troy, N.Y., 1965, pp 3-6. ( 2 8 ) Greene, N. I]., Proc. 1st Int. Cong. Met. Corrosion, 113-9, Bntterworths, London, England, 1862. (29) Greene, N . I)., Judd, G., Corrosion, 21, 13-9 (196.7). (30) Greene, K. l)., Saltzman, G. A., Corroszon, 20, 293t-8t (1964). (31) Hines, J. G., T r a n s . I n s t . Chcm. Eng., 47, T172-6 (1969).
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(34) Jone., 1). A.,Ekctrochem. 1‘(chnoi., 6, 241-51 (1968). (3.5) Jones, I). A,,’Corros. Sci., 11, 439-51 (1971) (36) Jones, I). A., Greene, N . I)., Corrosion, 22, 198-20.7 (1966). (37) Joiieb, 1). A,, Lowe, T. A., J . .?later., .4ST,\L, 600-17 (1969). (38) Juchniewicz, lt.) Pompowski, T., Walaszkowski, J., Corros. Scl‘., 6, 23-31 (1966). (39) Kolotyrkin, J. 31.,Corrosion, 19, 261t-8t (1963). (40) LeGault, It. A . , Walker, 31. S., Corrosion, 19, 222t-6t
i 196‘3 ). (41) Leimox, T. J., Jlatcr. Prot., 1, 37-45 (Sept. 1962). (42) Leonard, I t , B., Corrosl’on, 24, 301-07 (1968). (43)Lindheig, 1:. A, Reynolds Metals Co., Richmond,. Va.,. .private coiniiiuiiicatioii, 1967. (44) lIarsh> C;. A,, Proc. Second Znt. Cong. J l c t . Corrosion, 936-41, National Association of Corrosion Engineers. Houston, Texa., 1966. (4,j) 11orgai1, J. H., “Cathodic Protection,” ~lac3lilla11,S e w Tork. N . l - , . 1959. (46) 3\lbrgan,’P. E., Evans, L. S., J l a t c r . Prot., 4, 60-2 (Jan. 196.)i. (47)Muellei, JT.A, Corrosion, 18, 359t-67t (1962). (48) National Association of Corrosion Engineers, Corrosion, 24, 308-11 (1969). (49) Alurakaaa, T., Hackerinan, N., Corros. Sei., 4 , 387-96 i 1964 ). (tiO) Oyozawa, K., Bohiienkanip, K., Engell, H . J., Corros. Sci., 6, 421-83 11966). (.;I) Parker, AI. E., “Pipe Line Corrosion aiid Cathodic Protection,” 2nd ed., Gulf Publishing Co.. Houston. Texas. 1962. (.i2) Paul, It., -\inter. Prot., 8, 2;-9 (Jan. 1969j. (53) Perrigo, L. I)., M a t e r . Prot., 5, 73-6 (AIarch 1966). (.i4)Ponchel, B. AI.) Horst, R. L., X a t e r . Prot., 7, 38-41 (llarch
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