H Y D R O G E S OVERPOTEKTIAL AND CORROSIOS O F I R O S
52i
H Y D R O G E S OT7ERPOTESTI.1L AND T H E PARTIAL I S H I B I T I O S OF T H E CORROSIOS OF I R O S J . 0’11.BOCKRIS
AYD
13. E:. COSW.11-
I m p e r i a l College, South Kensington, T,ondon, ,9.Tr77 . Englanii Received October 22, 1947 INTRODCCTIOY
During the hot \I orking of iron extensive oxidation of the metal occurs, causing formation of a hard brittle scale which must be removed before application of the uwal protective finishes. The acid-attacked scale is commonly removed by dipping t h e oxidized articles for the appropriate time in baths of dilute hydrochloric acid and hot dilute sulfuric acid. In the absence of an inhibitor, however, thi, processmay result in extensive attack on the base metal and in blistering due to occlusion of the evolved hydrogen gas in the metal Dissolution of the metal in the descaling pickling bath may be largely diminished by addition of certain organic compounds n-hich form nitrogen-containing cations, or oxonium ions in the rase of lietoneb, or by addition of salts of certain high overvoltage metals. T n o main mechanimi- of this inhibition of corrosion have h e n suggested: (i) It is due to an increase in the hydrogen activation overpotential on iron. ( 1 1 ) It i- due to merhanical protection by an adsorbed film, theincreaseinapparent overpotential following upon the addition of inhibitors to the acid solution being due t o the increase in ohmic overpotential caused by resistance of the film. T h e object of the piwent work has been to dibtinguish lietween these tn-o vien-2. P R E V I O T S WORK O S C O R R O b I O Y ISHIRITIOS
?’he earliest use of corrosion inhibitors appears to be that described by SIarclrlgoni and Stephanelli ( l G ) , who used extracts of bran, glue, and gelatin to diminish dissolution of iron in acids. The use of inorganic materials, e.g., arsenic in acid solution, as corrosion inhibitors, was reported by Hurge-s 13) and later by Watts ( 2 3 ) ,\rho used salts of tin, mercury, and arsenic. Polukarov and Polukarova (20), investigating the effect of arsenic in aqueous solutions of hydrochloric acidon rates of corrosion of steel pickled in dilute hydrochloric acid solution (0.1-2 .V) have found that arsenic can act both as a corrosion inhibitor and as an actiyator (ride inj”ra),depending upon the concentrations of ai’aenic and acid present. I n 0.1-2 .V aqueous hydrochloric acid solution small concentrations of arsenic increase the corrosion of steel in the acid medium; a maximum effect is observed in the range1.3-8 X 10-5g.-moles of arsenic per liter. At concentrationsof arsenic greater than8 X g.-mole.: per liter the corrosion activation decreases and is replaced by an inhibitive effect. These effects appear to he intimately related to changes of hydrogen overpotential at cathodic parts of local elements, brought about by the presence of arsenic. This is supported hy iome analogous results obtained by the authors ( 2 ) in which a study of the effect of arsenic present in S 10 aqueous hydrochloric : - t c d 3olution on the (bathodic
hydrogen overpotential at nicliel in the concentration range to 3 X g.moles per liter of aiwnic has shown that between and g.-moles per liter of arsenic a lowering of hydrogen overpotential occurb, whilst at higher concenI ration. :L considerablc inciwse is observed. The dependence of thc inhibiting pon er of a given siibctance upon it has been examined hy nieasiirements of the rate of dissolution of iron in aqueoiih normal sulfuric acid containing \-arious corrosion inhibitors. X a n n , Lauer. and Hultin (15) used a belie> 01 aliphatic and aromatic amines. For monoamine- of aliphatic hydrocarbons in the range ammonia to )L-amylamine inhibition is prugressively more effective as chain length is increased. Plots of inhibitor concclntration against effectiveness (measured by the decrease in evolution of hydrogen) are linear for ammonia and methylamine, ethylamine, and propylamine, hut the effectiveness s h o w a marked tendency to rise t o a limiting value for the two highest members. Similarly the effectiveness in a series RSH?,R*SH,and R 3 S increases as the number of radicals attached to the nitrogen increases to three (tetraalkylammonium derivatives are less efficient, owing to an effective protection of the basic nitrogen center of adsorption). The general form of the plots of effectiveness and concentration is reminiscent of an adsorption isotherm, and by plotting logarithmic functions of both variables a typical log-log isotherm of adsorption is obtained. l17ith aromatic amines similar effects are observed, and inhibitor efficiency is foiind t o be dependent on the nature, size, and position of nuclear substituents. A striking conclwion resulting from the work of l l a n n , Lauer, and Hultin is that even at high concentration, i.e., under conditions of saturation adsorption, some inhibitors, e.g., ?I-amylamine, only reach 60 per cent effectiveness, whilst others at lower concentrations are more effective. This fact is probably connected with either different packing of the inhibitor in the surface layer at the cathode-solution interface or u-ith different magnitude of some specific effect upon one of the more important electrochemical reactions occurring during dissolution of the iron. - i n indirect examination of the mechanism of the inhibition of corrosion hy organic additives has been at tempted in 5ome previous polarization studies. Thus Chappell, Roetheli, and McCarthy (4) examined the apparent overpotential of hydrogen on iron and steel in acid solutions containing the inhibitor quinoline ethiodide. They also found a qualitative relation between the extent of inhihition and the concentration of the inhibitor,' and concluded that the inhibitor \\-as effective by causing a mechanical blockage or protective film a t the surface of the metal. The film formation I\ a- considered to be due to adsorption of the pohitire nitrogen babe ion on the cathodic areas of the iron electrode. The above experimental work was confirmed and extended by Jimeno, (;nf!fol, and Xorral ( 7 ) , 11-ho measured the apparent hydrogen overpotential on iron and steel in the presence of Tarions inhihitois. These ivorkers pointed out the parallelism between the time necessary after tlic addition of the inhibitor for the attain1
Contrast the ~ o r of k Pievcrts and Lucg (S), n ho found
110
such relation.
HTDROG1:S OVERPOTESTI.1L .kSD C O R R O S I O S O F I R O S
529
nient of the maximum overpotential and for the attainment of maximum effectiveness of inhibitor action. Upon the basis of this and other results these authors tend t o support the concept, that the inhibition is due t o an effect on the hytli*ogen overpotential rather than to mechanical protection by an adsorbed film. Previous studies of cathode polarization in the presence uf inhibitors possess the vommon disadvantage thnt they ivere carried out by the direct method of measurement,? and therefore no sure conclusion can be drawn from them as to whether the invrease in apparent hydrogen overpotential is c:iused hy an effect on the hydrogen activation overpotential or to the increase in resistance of the cathodesolLition interface due to the presence of an adsorbed film. Thus, no clear conclusion concerning the txvo types of mechanism put for\.r.:irtl for the action of corrosion inhibitors can he made. different approach iws made by Rhodes and IGihn (10). \vho attempted to evalunte the contrihution to the resistanc'e oT-erpotential in measurements of apparent overpotential by measuring the interfacd resistance between iron and sulfiiric acid in the presence of inhibitors, by means of a capacit:ince-resistance hridge. They found that the maximum surface resistance upon addition of a n inhihitor is not attained for some time; for example, in the case of phenylacridine and :icridine, it is reached after about 13 min. (cf. the observation on the time lag in the establishment of maximum inhibition and hydrogen overpotential reniaiked upon above). On the other hand, no parallelism \vas found hetn-een the resistivity of the adsorhed film and corrosion inhibition. S o definite conclusion :IS t o the mechanism of corrosion inhibition can be dran-n from this work, but it seems to indicate that nn increase in overpotential is a prime factor. From the results of studies on the adsorption of gelatin on iron in sulfuric ac*itlsolution and from direct measurements of the resistance of adsorbed layers of gelatin and other inhit)itors, Machu (11, 12, 13) has concluded that there is a direct relationship betn-een protection afforded by the inhibitor and the electrical resistance of the surface layer. It' has been found that inhibitors giving a surface Inycr resistance of 3 ohms give complete protection from corrosion, and this figure is suggested as a rriterion of the efficiency of an inhibitor. The surface resistance values found by 3Iachu are in substantinl agreement with those quoted by Rhodes and I h h n (21). In :in indirect determination of the total free pore space (e.g.. see 13) esisting 811 the surface layer of ;i number of inhihitors on iron, 1Inchu has found an average otnl f i w space of GG per cent and has conclutled from these results that the theory if inhihitor action 1)asecl upon a mechanical protective effect cannot be niainnined. Jlachu :iffirms that the adsorl~edlayer of cnpillnry-active inhibitor is 2 The direct method of measurement of overpotential involved potential measurement luring the pwsage of polarizing Current. If resistive films are present at the electrode urface, then the 1iorenti:il drop ncross this resistance during passage of current will be riclutld in the nicasuiwl ovcrpotentirll, t h u s rciicicring iinpossitile 3 distiriction betwccn iic*i,cnscdpo1nriz:ition 01' niccti:inicnl bloclcigc. t i o t l i of \rliich \voulcl. uritlcr tlicsc coriditiori,~f m ~ : i s u w n i e n t ,load 1 0 :iri incrcnst, i n tlic, app:ii.c'rit ~ ) r ( ' t ' ~ ) c i t ~ ' r i i i : i l .
530
J. O’M. ROCKRIS AND U . E. COXWAY
effective in diminishing corrosion by a process of retardation of ionic migration and diffusion in the vicinity of the surface of the metal, brought about by the smallness of the channels through the inhibitor layer. It appears that in this view, the origin of any increaLe in apparent hydrogen overpotential observed in the presence of an inhibitor ~\-ouldbe caused by concentration overpotential arising from the difference in concentration in the vicinity of the electrode surfaw caused by the postulated ionic retardation Lejeune and Jacquet (8) conqider that inhibition phenomena can be explained most satisfactorily by a combination of the mechanical protection theory and the theory that inhibitors are effectil-e by their modification of electrochemical reartions occurring at the metal surface during corrosion. Recent work by C‘h’iao and J‘Iann ( 5 ) . in which it has been confirmed that inhibitor efficiency and cathode potential increaw during cathodic polarization in the presence of inhibitors are related, is open to the objection previously raised that ohmic potential drops are newssarily included in the measured cathotlc potentials. It may be noted that Machu‘s result concerning the 66 per cent free space on the electrode in the presence of inhibitor is inconsistent with Ch’iao and JIann’s view because, as is clear, the decrease in free surface area reported by Jlachu can in no \yay explain the magnitude of the potential shift observed. The converse phenomenon of corrosion activation has been observed hy Perschke and \-inogrudova (18. 19), who showed that certain substances such as quinone and nitrobenzene increase the corrosion of iron in acid solution, although N a n n (14) finds that quinone is an inhibitor (hut see Lorch (9)). I n an electrochemical investigation of the effect of oxidizing compounds, e.&., quinone and nitro compounds, on the rates of corrosion of metals by acids, Onkin (17) has confirmed the observations of Perschke and T’inogradova (18, 19) that these types of compound facilitate the corrosion and has concluded that they are effective by a depolarizing action a t local cathodic points on the metal. It appears that kno\vledge concerning the mechanism of inhibition of acid corrosion is in an uncertain state, hecause no unambiguous measurements have in the past been made upon the relation of corrosion inhibition to the activation overpotential of hydrogen (free from spurious ohmic overpotential) at iron in a solut ion containing the inhibitors. EXPERIMEST.IL YECTIOS
The hydrogen overpotential has been meahured on iron electrodes of area between 0.1 and 0.4 sq. cm. in 10 hydrochloric acid solution, using the direct and indirect methods. t\vo-compartment cell suitable for measurements at high current densities (1) and having a hydrogen reference electrode was used. FIytlrochloric acid ~olution-\\ ere pr~paretlhy diluting Analar 36 per rent hydrochloric acid \\ ith distilled I\ ater. The electrolytc 11 ab added to the cell in the presenre of air ( L ~ iufra). E The iron electrode and connecting bridge to the hydrogen electrode \yere hupported in a rubber bung, fixed into the top of the cathode compartnient. I n order to maintain experimental conditions near to those of a commercial pickling bath, n o deoxygenation of the electrolyte was
HYDROGET OVERPOTESTIAL h S D C'ORROSIOS O F IROX
53 1
carried o u t . Hydrogen gas, purified hy p age through liquitl nitrogen traps. was passed into the hyclrogen electrode vessel and also h b b l e t l over the iron clcctrode in the cathode compartment, to rcducc conccntrntion ol-eiyotential t o ncgligihle proportions. The iron electrodes i v w e prepared hy cutting strips of the metal from 21 larger strip of Hilger pure iron. These strips ivere seded into glass tiihes in a stream of hydrogen gas. 'The electrodes thus made \\.ere bright grey in color and h:td it high luster-. The metal \vas pretreated hefore sealing by Irashing Lvith witer and wiping with filter paper. The static potential of the iron electrode in the given solution !vas measured at 15-min. intervals by means of a Cambridge portable potentiometer or a lTiill~trr1 electronic potentiometer with visual tuning indicator until i t hecame approximately constant, this being taken to mean that the rate of change of potential ivith time was less than 1 cv. (cy. = centivolt) in 30 min. -1polarizing current (equivalent t o a current density of IO-" amp. sq.cm.) was then applied from accumulators and the hydrogen overpotential measured a t 15-min. intervals for L lir., at which time an inhibitor solution in +I7'10 hydrochloric acid was added in air to the catholyte to give the desired concentration. The change of overpotential during the nest hour was then followed by making measurements at first very frequently during the initial rapid r1tang.e of cat'hode potential and then at 1.5min. intervals. .It the end of the second hour of polarization at a given current density, the latt.er \vas increased to ten times its former value. and u potential measurement made immediately; further polarizations at 5 x IO-' amp q . c m . and 10-1 amp. sq.cni. \vwe then carried out in rapid succession. measuremcnts of cathode potential at these current densities also being made directly d t e r the commencement of polarization at each current density. The measurement of overpotential \vas also made hy an indirect method, using an electronic interrupter of the type described 1)y I-Iic*kling(6). 'The polarizing current applied to the cell n-as interrupted 50 times per second and potential measurement at various period> of interriiption \\wmade by means of a thyraton potentiometer (6). 'The inhibitors ubed n-ere aniline, o-toluicline, quinoline, pyridine, acridincb, 3naphthylamine, and morphine, ivherc possible in concentrations of 10-'4,1 O F , and IO-' moles per liter. Solutions which increase thecorrosion of iron (17, 18),such n.; nitrobenzene, picric acid, ant1 benzoquinone, were also tested. -1s ii subsidiary test substance ter'f-butyl alcohol was investigated. Inhibitor solutions \vert in general p r e p a r d t)y dissolving a iveighetl quantity of either thc distillrd 01' rec~i~ystallizecl organic material in S 10 hyclrochloric acid .+elution. I n the c : t s ~of iicaritline n solution \\.as made in 4 -IThydrochloric. acid and :t f c t v clrops pipctted into -1-10 hydrochloric acid. Acridine was precipitated and filtered off. Tlie saturated solution \vas then used for the test>. 111 the ('ase of ;j-nuptithylamine purification of the base \vas carried out by preparation and crystallization of tlir hydrochloride, i\-hicli \vas dissolved in -\- I O hytlrochloric :tricl f o r use during thc csperiment;.
532
J.
o'Ar.
B O C K R I ~.
nu
B. E. COSKAT
RESULTS
Typical results arc expressed graphically in figures 1-6; the numerical results are given in table:: 1-4. TABLE 1 Inhibitors
o-Toluidine
0.65 0 67 0.53
0.67 0.6s 0 . 34
0.55
10-3
0 . 4 9 0.5s 0.62 0 . 5 2 0.60 0.61 0.33 0.37 0 . 3 i
0.98 0.64 0.35
1.30 0.88 0.39
Tending t u maximum Still rising after 1 hr. Still rising after 1 hr.
lo-? 10-3 10-4
0.33 0.34 0.40
0.35 0.38 0.44
0.40 0.40 0.45
0.65 0.43 0.50
1.57 0.s0 0.53
Tending t o maximum Tending t o niaximum Still rising after 1 hr.
lo-'
0.34 0.36 0.34
0.37 0.42 0.36
0.45 0.44 0.37
0.46 0.44
0.4i
0.4s 0.40
Slowly rising after 1 hr. 810n.ly rising after 1 h r . Constant value after 1 hr
0.34
lo-* 0.36
0.38 0.3s
0.34
0.3s
0.41 0.40 0.39
0.44 0.41 0.30
0.50 0.44 0.41
Coiitiiiuing t o rise after 1 hr. Continuing t o rise after 1 hr. Tending t o constant value
0.44 0.37 0.39
1
0.46 0.39 0.41
Slowly rising after 1 hr. Tending t o constant value Tending t o constant value
0.43
1
0.49
Slowly rising
10-'
IO-? 10-j Quinoline
-4cr i d i n e
Pyridine .
@ - S a ph t hyI amine
10lo-?
lo-'
0 49 0.53 0.46
0.61 0 66 0 52
0.84 0 . i 3 /Continuing t o rise after 1 hr.
0.3s
I I
Morphine
lo-? 0 35 0.40 10-3
0.34 0.34
0.36 0.38
0.42 0.37 0.3s
tert-Butyl dcohol 10-1
0.3s
0.41
0.42 -
-
____
-
-
~
_
- _
__
__
For the organic bases and tertiary butyl alcohol, a sharp rise of orerpotential upon addition of inhibitor occurs w e n in the most dilute solutions. For aniline and o-toluidine the slope of the oyerpotential-time curye is slightly raised after addition of inhibitor. In the cases of acridine, quinoline, etc. the rise is much more marked and the variation of overpotential after addition of inhibitor is much
TSBLE 2 Activators
'
ACTIVATOR
I ~
cosCET-
DITIOS
RCMARKS
OF .A< -
TIOX
TIVATOR SOLU-
......
moles,' lilcr
:'n//b
:~!is
'
w!/.7
:riii,
;d::,
Quinone. . . , . . . . . 10-2
0.34
0.38
0.14
0.14* 0.17
10-3 lo-'
0.34 0.35
0.36 0.40
0.32 0.40
0.32 0.40
0.35 0.39
"0.14 after rise t o temporary niasiniuni nt 0 . I 6 Slonly rising Slowl!. decreuing
Sitrobenzene. . . . , lo-?
0.35
0.36
0.14
O . l c i * 0.17
* O . 16 d t e r rise t o teinporary
10+ lo-'
0.34 0.34
0.37 0.40
0.33 0.40
0.33 0.40
niclsiniuni for 1 min. S l o ~ l yrising Constant vnlue
Picric acid . . . . . . . lo-?
0.34 0.36 0.35
0.34 0.41 0.40
~
IO-'
,
,
0.31 0.27 0.39
0.34 0.40
0.12 0.12 0.33 0.39 0.39 , 0.41
81ow1y rising Slorvlyrising Slowly rising
TABLE 3 Inhibitors Overpotential-log current density data Ih'EIBITiIR
COSCESTR.\TIOS
__~_
0I'ERPOTESTI.AL AT C U R B E S T D E S S I T P (UP.,'SQ. CU.: OF
5
10-2
10~
__
.
x lo-?
___10-1
~~
moieslliter
P y r i d i n e , .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10-1 10-2
0.77 0.67 0.60
1.24 0.92 0.85
10-3
1.21 0.70 0.39
2 2 1.45 1.55
2.00 0.75
10-2 10-3 10-
2.1 1.00 0.55
2.3 2.2 1.42
2.2
lo-:
0.49 0.44 0.41
0.53 0.63 0.57
1.28 0.93
0.44 0.43 0.39
0.54 0.59 0.60
0.7s O.T4 0.SO
0.96
10-3 10-1 lo-'
0.47
0.80
1.06
1.20
10-s
Juinoline
10-1 10-2
icricliiie
.........................
:-Snphthylamine . . . . . . . . . . . . . . . . . . . .
10-2 10-3 Iorphine
~ ~ t - B unlcohol t~l
1.80 1.45 1.03
0.48 0.46 0.41
10-2
* This and succeeding blank spaces in the table iritlic:ite )o high t o be measured on the available poteiitionit1tcxrs
533
I
~
*
0.81
2.2 1.04
1.94 1.20 0.98
0.53 1.0;
ttiar the overpotential was i.e 2 . 2 v. .
.
534 TABLE 4 dctifiatora .__~
. . . . .
_
~.
.............
IhHIBlTOB
CONCESTBAIIOS
,
---
~~-
~~~~~
O\'EBPOTENTIAL AT CUPPENT DENSITY (AMP. 'SQ. CM.) OP
-~ 10-8 -~ ~-
10-2
.
5
x
10-2
10-1
molcslliier~
Quinone
10-2 10-3
Sitrobenzene . . . . . . . .. . . . . . . . . . . . . . . . . .
Picric acid... . . . . . . . .. . . . . . . . . . . . . . . . . .
1.3i 0.88
0.68
0.82 0.77 0.76
0.48 0.64 0.57
0.93 0.96 0.74
1.30 1.30 0.86
0.31 0.58 0.63
0.50
0.65 1.04 1.11
0 34 0.58
10-4
0.18 0.35 0.42
10-2 10-3 10-4
0.17 0.44 0.41
10-2 10-3 10-4
0.13 0.35 0.39
'
0.72 0.94
0.93
0.5.
o,2A
io 2b
30
40
so
i o SO
$0
100
iio 126
TIME IN MINUTES AFTER START OF POLARIZATION FIG.I . Overpot entia1 Ixhavior o f iron i n S!10 hytlrorhloric acid containiiig !)yri(line. l'ol~rizing current drnsity = 10P anip.,/aq. r m .
greater than for the above-mentioned inhibitors. Foi wliitions containing acridine, overpotential continues to increase considerably for more than 30 min. after addition of the hase. In general, there appears to tie N parallelism hetween tlic
HYDROGES OVERPOTESTIhL .4SD CORROSIOS O F IROX
535
increase in hydrogen overpotential caused by a given inhibitor and the effectiveness of the inhibitor (as determined by the estent of hydrogen evolution or rate of loss of weight of metal). The increase in overpotential, and therefore probably the effectiveness of corrosion inhibition, is in most cases greater with the increasing complexity and molecular size of the inhibitor. Increase of inhibitor concentration also brings about a greater effect on the corrosion inhibition (see figures 1, 2. 4).
1.2. I. I 1.0. fi 0.9.
5 0
> O.8r
- _ - -_ _ _ _ _ - - IR ADDED 100 110 120 TIME IN MINUTES AFTER START OF POLARIZATION FIG.2. Overpotential behavior of iron in S/10 hvdrochloric acid containing acridine. Polarizing current density = amp./sq.cm.
The effect of addition of inhibitor upon the relations between overpotential and current density (figures 3 and 3 ) is to vitiate the usual logarithmic relation (the Tafel equation) and cause positive deviations. I n some systems, e.g., for solutions containing quinoline or acridine, apparent orerpotentials greater than 2 v. were observed, the &value of the Tafel equation rising far abore the normal value for iron (0.12) in pure acid solutions. The most important of the preient results are thoqe (hguie (i)in n-hich the o i elpotential \\’its measured both by the direct method (in IT hich the apparent overpotential \vould include, In addition to the true activation ovripotential, a ceontrihution due to any possible ohmic overpotential caused by the resistance of an Idtorbed hlm) and by the indirect method (which aroids such ohmic contribution.), I’riti of the intlirert mcthod 1 1 ~ 1ere made by inti*oducinga linen-n ohmic, o j ci -
2.01
LOG,,
C.D. I N AME/SQ.CM.
FIG.3. Ovcrpotential oersus log of current density curves for polarization of iron in N/10 hydrochloric acid containing pyridine as inhibitor.
z F 0.2. ,
,
LTU,,
/
,6z MOLES/L.
NITROBENZENE CORROSlON ACTIVATOR ADDED
,
,
potential by m0\-1ng the Luggin capillarj- ai\ ay from the cathode. Satisfactory elimination of the resistance contribution t o the overpotential was achieved. Experiments in 11 hich direct and indirect methods were used were carried out in solutions containing the inhibitors pyridine and acridine, xhich cause relatil-ely large increases of orelyotentid. I t \\-as found that the overpotential measured by both direct and indirect methods TI as ~ u b ~ t m t i a l the l y same in the solutions containing either of these inhibitors up to a current tleiisity of lo-? amp. "q cni. It may be concluded, therefore, that the prescnce o f f h e irzhzbitors pyridine -
J1.41 AT IO" AMP/
I . I1
1.0:
I
Ln
0.7:
50
> 0.6i
04
!
/ -I
-2 LOG,,
C.D. IN AMP/SQ
I
CM
FIG.5 . Overpotentin1 vei'sics log of current clensity curves foi polarization of iron -1-'10 hydrochloric acid cont:iining quinone R S corrosion xctiv:itoi'.
iri
:ind acridine in aqueous acid solutions C U Z L S ~ (SI consitlcrcible increasc the true ii,ydrogoi ncticatioiz overpotoitial 011 iron and that the apparent increase is substantially equal to the true increase (i,e., ohmic errors are negligible). This conclusion can pre,wmably be extended to all the inhibitors mentioned shove, so that mcvhanical protection hy an aclsorbed film is not of esential importance in corrosion inliibi tion. The rewlts of the present investigation a1>0 provide strong evidence against a view based on retai.cied ionic diffusion as a cause of inliibition. In some cases, increases of overpotential of the order of 1 T-. w e observed and if the retarded :iiffusion vielv were true. this would imply that the ratio of the ionic concentration i t the electrode surface to that on the solution d e of the layer of inhihitor is of
538
J . O'hf. HOCKRIS AND B. E. COXWAY
the order of lo?". l'his appears t o be an impo\siblc result under conditions of steady electrolysia present in the experiments ieported here, so that Machu's theory seems a very improbable one. I t appears probable, therefore, that the effect of organic inhihitors of acid corrosion is brought about tliiwigh the resultant increase in the hydrogen overpotential on iron. The increaw oi overpotential Cau>etl by the inhibitors appears to resemble that brought ahout by catalytic poihoni, w c h a5 arsenious oside (2). The mechanism of thih poisoning effect ha, not yet been fully elucidated. For iron it appears likely, however, t o be due to the deactivation of the active Centers
0.2' 0
IO
20
30
40
50
60
70
PERIOD OF INTERRUPTION, SEC.x IO5 FIG.6 . L>cc:iy curvcs for c a t horiic po1:iriz:ttioii of iron i ti .\-/lo hydrochloric acid solution :ind i n S i l o hydrochloric acid contninirig 10 -1 uloles per liter of pyriciinr. Curvc I , 10-1 Jf pytitlitLc. solution. current, tlciisity = 1W.2 n m p . / s q . c m . ; PU!~W 11. IO-! df pyridine solution, curt'cnt density = 10F :mip./sq.cni.; rut'vc 111. pur(- S/lO hydrochloric :icitl solutioit, currrrit tlrnsity = :tinp!'sq.(~in.
caused by the adsorpt ion of thc poison, ivliich t hcrefore retards the comt)inat ion reaction probably i q x m s i h l e for the hydrogen overpotential on iron. Corrosion activators are seen from figure 3 to cause a mnrked lon-wing of the hydrogen overpotential. This lowering is prohably chic t o depolarization process n-hich effectively inrreases the facilit'y of removal of hydrogen from the cathodesolution interface: thus decreasing the polarization and increaing the corrosion. STIIJIhRT
1. The magnitude of the increase in hydrogen overpotential on iron at high with increwing concencurrent density caused hy corrosion inhibitors increa tration and complexity of the inhibitor.
H T D R O G C S 0VCRPOTCSTI.lL .‘AD
CORROSIOS OF IROH
530
2 . Tllic degree of inhibition of corrosion and simultaneous increase in OT-PZvoltage ~ u approximately n parallel t o each other. Conversely, :tc.tiI ator-: clcc3rea.e overpotential. 3. 3Ieasurement of true actiration overpotential with an electronic con-mii:tator &owed that the ohmic 01erpotential I\ a- negligible in solution, showing tlw largeyt increase of overvoltage on addition of the inhibitor. 4. The mechanism of the inhibition of corrosion is by mean> of a direct effect on the hydrogen activation overpotential and not by mechanical protection through an adsorbed film. 3 h n y thanks are clue to Dr. J. F. Herringsham for his helpful comments upon this p:tpci~. REFEREKCES (11 UOCSRIS: Trans. Faraday SOC.43, 41i (1947). ( 2 ) BOCKRIS. i n C o s w a r : I n course of publication. (3) BKRGESS:Trans. Electrochem. Soc. 8, 165 (1905). (4) CHAPPFXI., ROETHELI, AND C CAR THY: Ind. Eng. Chem. 20, 582 (19281. ( 5 ) CH’IAOASD MANX:Ind. Eng. Chem. 39, 912 (194ii. (6) HICKLISG : Trans. Faraday Soc. 43, 1540 (1937). (7) J r m s o , GRIFFOLL,.wu M O R R A LTrans. : Electrochem. SOC. 60, 36i (1936). Bull. soc. chim. 3, 2137 (1936). (8) LZJECSE . ~ N DJACQCET: ( 9 ) LORCH:Electrochem. Soc. Preprint, 70 (-4pri1,1936). (10) X~CHI:: Oesterr. Chcm-Ztg. 39, 144, 152 (1936). (111 MACHC:ringew. Chem. 61,853 (1938). (12) MACH^: Korrosion u. Metallschutz 13, 1-20, 20-23 (193i). 13) 1 1 . 4 ~ ~Korrosion ~ : u. Metallschutz 20, 6 (1914). (14) hI.4h-x: Electrochem. SOC.Preprint 69,354 (April 1936). (15) 1 I a s s , L.ICER,.4su HL-LTIN:Incl. Eng. Chem. 28, 159 (1936). (16) ~ ~ a ~ a sA N ~DoSTEPHASELLI: s r J . Cheni. Soc. 26, 116 (1872). (17) OSSIS: J . Phys. Chem. (U.S.S.R.) 13, 631 (1939). (18) PERSCHKE .ISU VISOGRADOI-.
