Oct., 1965
KINETICS OF
THE
UNDERWATER CORROMON OF POWUEI~EU Miuxmu~i
1009
THE IWYETICS OF THE: UNDERWATER CORROSION OF POWDERED MAGNESIUM' BY ELI S. FREEMAN AND SAULGORDON Pyrotechnics Chemical Research Laboratory, Picatinny Arsenal, Dover, N . J . Receiued February 0, 1066
The kinetics of the underwater corrosion of atomized and ground magnesium povder was investigated in the presence of nitrogen, helium, carbon dioxide, hydrogen and oxygen. The course of the reaction was followed by observing changes in volume and pressure due to the hydrogen formed, as a function of time. Factors such as pH, nature of the film coating the particles, specific surface, gaseous atmosphere, pressure and temperature were found to affect the rate process. The kinetics appear to he determined by two rate-controlling mechanisms: (1) a non-diffusion controlled mechanism which may involve the neutralization of protons or the combination of hydrogen atoms on the surface of the metal to form molecular hydrogen, and (2) the diffusion of water through the hydrated magnesium hydroxide film coating the metal particles. The former mechanism predominates during th(: initial stages of the reaction; whereas the latter is important when films of sufficient t,hickness are formed. The kinetics of the initial stage of the reaction may be expressed by the logarithmic rate law: t = AeVk (see Appendix 1 for definition of symbols). The ensuing diffusion controlled stage of the reaction follows a general parabolic law, which in differential form may be expressed as dN/dt = BN+'. The activation energies based upon the logarithmic and parabolic rate laws owere calculated to be 7.48 kcal. per mole and 6.39 kcal. per mole, respectively, over the temperature range of 30.0 to 65.0
.
Introduction The reaction of magnesium and water in the absence of oxygen may be considered from the point of view of a galvanic cell having Mg, Mg(OH)2 and Hz, H30+ electrodes. The metal is oxidized and reacts with hydroxyl ions to form magnesium hydroxide, which coats each magnesium particle. The protons act as oxidizing agents aud are reduced to hydrogen. Since gas is evolved as a reaction product, a co1ivenient and simple method used to study the rate process involves measuring pressure or volume changes, at constant volume and pressure, respectively, as a function of time. The effect of factors such as surfitce treatjmeiit, specific surface, temperature, pressure and the nature of the gases in the system were studied, since they may affect t'he kinetics of the rea,ction and contribute to a n interpretation of the reaction mechanism. Experimental The magnesium powders used in t,lie experimental worlc were obt,ained from tthe Dow Chemical Company, the Golwynne Chemical Corp. and t)he .4merican hlagnesium Company, as indic,ated in Table I. The atomized samples were very closely screened fractions (95+%) and were found to be of uniform shape and size, by microscopic examinations. The particle sizes of the samples were ascertained by the air permeahility method using the Picatinny Arsenal particle size apparatus,2 and the chemical composition was determined by spectroscopic and chemical analyses. The results are presmted in Table I. The apparatus for measuring pressure changes at constant volume consists of a 100-ml. two-neck round-bottom Pyrex flask (reaction vessel) connected t o an open end manometer (2 mm. bore) containing dibutyl phthalate as the manometric fluid, and a 10-ml. buret. For measuring changes in volume a t constant pressure, the manometer is replaced by a gas buret and compensating tube, separated by a manometer. Two platinum wires are sealed in the intervening manometer and c:onnected to an electronic relay3 used to operate a solenoid valve.' An increase in pressure breaks the contact between the wire and the mercury on the buret side of the manometer, opening t.he solenoid valve. The mercury level in the gas buret (1) This i)sper was presented in p a r t a t t h e Meeting-in-Miniature of t h e Anirrican Cheuiical Society's New York Section before tlie Division of Physical and Inorganic Clieniistry. February, 145.1. (2) B. Dubrow a n d AI. Niemdka, A n a l . Chem.. 27, 303 (1455). (3) A thyratron operated relay siinilrtr t o the Eiiiil Greiner Co. illude1 E-2. (4) Autoinatic Switch Co. Valve #SZGZlZ.
drops until the original prcssure is restored, at, which poiiit the mercury-wire coiitact is again made, and the solenoid valve closes. The pressure of the nystem is controlled by varying the length of the wire. The response time for t8he apparatus is four milliliters per second. Changes in volume as small as 0.05 ml. are measured in this manner. A weighed sample of magnesium powder is placed in the reaction vessel, and immersed in a thermostatically controlled wat)er-bath in which the temperature is maintained to &0.2". The reaction vessel is then connected to the appropriate apparatus for measuring volume or pressure changes. Two and five giwn samples were used for the exeriments a t constnnt volumc and pressure, respectively. he desired gas or mixture of gases was passed t,lirough the system until all the air was displaced, whereupon 10 ml. of water (Raturated with magnesium hydroxide and the gas under study) was added to the flask by means of the buret. This solution, which was prepared by the reaction between ground magnesiuin and distilled water at the experimental t,emperature, had a pH of 10.7 (except in COZ)and was used in all the experirnentnl w o r k i n o d e r to mnintain a const,ant pH during the initial phase of tlie corrosion reaction. The gases used in this investigation ranged in purity from 99.6 tJo99.9% and were purchased from the AIatheson Company Inc. Prior to use the gases were passed through concent,rated sulfuric acid and a concentrated solution of sodium hydroxide. Blank determinations u w e carried out simultaneously with the experiments a t constant voluine in order to correct for variations in aml)ien t pressure and temperature during the course of the rewtions. The volumetric data obt,ained a t constant pressure were reduced to standard conditions. The values for the rate of change in volume and pressure were determined by dividing the difference in volume or pressure between t,wo successive tirnes, Ily the difference in the corresponding times. All experimentd determinations, with the esception of tbe temperature dependency series, were conducted nt 30.0 .
5
Results An indicat8ioiiof the effect that depolarization by oxygen5may have oil the kinetics of the reaction is obtained by obserriiig the rate of corrosion in an oxygen atmosphere. The results are shown in Fig. 1, which is a log-log graph of the rate of change i n pressure us. time, for the reaction between atomized magnesium powder (specific surface 176 per g.) arid water in oxygen and nitrogen environments. Also included in this figure is the reaction between ground magiiesiuni (specific surface 10,700 em. per g.) and water in an atmosphere of iiitrogen. The points fall along straight lilies indicating that ( 5 ) S. Glasstone, "Introduction t o Glcctt~ouliei~~istrJ.," D. Van Nostrand Co., Inc., N e w York, N. P.,1942, p. 501.
I
ELIIS.FREEMAN
1010
AND SAUL GORDON
TABLE I POWDERED MAGNESIUM SAAIPLES Screen fraction U.S.S. sieves
60/80 100/120 120/140
140/170
Particle size
yo coinpn.
S 1)ecific surface, Mge
(P)
19~5 124b 115' 904b 53. gc
176
278 209 360
09.1 00.8 90.5 98.5
AI
RIn
Ni. Cu, Bn
P h , Crl, Cr, Sn, Si, A g
b'e
0.02 .2-0.3 .2-0.3 1.5 0.02
0.09 .07 .07 .07 .09
0.02 .02 .02 .02 .02
0.03 .03 .03 .03 .03
0.05-0.0ti .05- 06 .05- OG .05- .OG 05- . O G
650 97.4 10,700d Dow (atoiiiized, disc iiiethod). b Golwynne Chemicals Corp. (atomized, jet method). American Magnesium Co. (ground). Specific surface as determined by krypton adsorption isotherms. e Metallic magnesium.
the rate of reaction may be expressed as bolic function of time
tt
hyper-
(dP/dt), = Ft-0
(1)
carhoii dioxide atmosphere, the initial pH of the solution is decreased, and hydrated magnesium carbonate rather than magiiesium hydroxide is formed 111 a
Mg
+ 2H30' + GO$' + XHzO =
MgCO:,.XH?O
+ 2H2O + H?
(2)
The existence of magnesium carbonate as well as the basic carbonate was confirmed by X-ray studies in this Laboratory and by others.6 A rapid de-
r-
partial COZpressure of unity. From 25 to 50 minutes there is a rapid iiicrease in pressure and the film coating the particles does not appear to exhibit inhibiting properties, since the rate of change in pressure is approximately constant. However, a parabolic type of corrosion follows. This is illustrated in Fig. 2 , a graph of the log of the rate of change in pressure a t constant volume us. the log of time, which shows that the rate of change in pressure is a hyperbolic function of time during the latter period. 3
.
I
2
i
0
i OO \
4
" 0.01
100 10UO Time, minutes. Fig. 2.-The rate of corrosion of 2 6. of atoinized iiiagiicsiuin i n the presence of rar4;n dioxide. Specific surface l i B cm.2/g.; initial pressure (0.0 em.; 30.0"; log (dP/dt), os. log t . 10
0.01
IA -
10 100 1000 Tiiiie, ininutes. Fig. l.-l'Iie i,nte of reac,t,iori of t\vo gr':~insof atomized i ~ i i c l ground ningiiesiuin p o i ~ d ~ ? tat, ~ s constmalit volume; 30.0"; initial pressurc i5.0 ciii.; letrst square plot; log ( d P / d l ) , . vs. log 1. 1
1 0 2 A 3
;
{
01
1
nt80iiiized ni:tgiit~~~iuiii, specific surf:w 1 i G ciii.2/g., nitrogen ntiii.
4 @ \ :ttoinized magnesium, speci6c surface 176 cin.Z/g.,
oxygen atm. ground magnesium specific xurface 10,700 cm.Z/g., nitrogen atin.
crease ill pressure occurs during tlie initial 25 minutes of the reaction. This is attributed to the clissolution of GOz due to t,he addittioil of a sohition saturated ith CO, in air into a system having a ((;) J. E. R o g m wid bI. S. Silverstein, Frankfold Arsenal, Tech. lioii,since it is not thermodynamically feasihle under the esperimental conditions, biit rather to an increase in the extent of polarization due to the hydrogen environment8. This effectively causes an inci-easein hydrogen-hound surface and cotisequently a decrease in the amountJ of ma,giiesiiim available for renctioii. (!I)
R.
r , u s t l l ~ : l n ,?'I,III?.
E i ~ r t i ~ n ~ ~ /lSur., h r m . 8 3 , 3 1 3 (1042).
1 2 3 4 6 Volume (S.T.P.),ml. hydrogeii/cm.2 x 102. Fig. %-Log time us. volume of hydrogen evolved per cm.* of surfnce; same legend ns Fig. 6.
By increasing the pressure (43.0 to 71.9 cm.) the reaction rate is increased. This is attributed t'o an acceleratjionin the rate of diffusion of water. It is of interest to point out that iii the dry oxidation of metals, electrons and cations are thought t,o diffuse from the metal-metal oxide interface to the outer periphery of the protective film, where the met's1 oxide is formed. 10 In the underwater corrosion of magnesium, the outward diffusion of magnesium ioiis may t.ake place over short distances. However, since the cation is hydrated, this process is slow compared to the rate of diffusion of water into the film. Therefore, as a consequence of the greater size and weight of the hydrated cations the film growth probably occurs a t the metal-film interface. I n studying the effect of part.icle size, n differential form of equation 5 was used, i.e., equation 3. A broader understlandiiig of t8hesigiiificance of this relntionship may be obtained 1)y coiivertiiig t,lie eqiint8ioiito it8slogarithmic foim log ( d I - / d l ) P = log H - h. log 1'
(ti)
By clifferentiatiiig and solving for k , it is ei4dentf that although k is dimensionless it expresses a rate: the ratio of the change iii the log of the rat8eof change in volume tlo the change i i i t,he log of the increase i n volume d log (dV/dt)p = -k d log V
Since the volume is a measure of the increase in film thickness, X: may be looked upon as a rate parameter expressing the change in the log of the rate of reaction per unit change in the log of t,he increase are iiecesin film thickness. While both coi~st~aiit~s sary to define the system. the coiist'aiit li, a m e w lire of the effect, of film thickness, \\wuld appear t'n l e of great,er fuitdameiit,n,lsigiiificance t81iant,he co(10) w N. F. n i o t t , T ~ ,v a~r n d~a u, ,soI.., ~ ~36, 172 ( L R - L O I : ( 1 , ) c. n ' n g r ~ r rn t l d Iages of the react,ioii which deviate from parabolic behavior obey a logarithmic rate la^. From the logarithmic form of equation 4, which upon differentiation yields dv/d In t
=
C-l
&V/k’
(12)
The constant A has the dimensions of time and may perhaps be considered as a11induction period. The logarit,limic deviat,ion from parn,holic be(11) See .Appendis I1 for rletivntion.
A Ir‘ B F a n b k k‘
a constant a constant a constant a constant = a constant = a constant, = a constant = rate constant = rate constJnnt = = = =
1
= time
P = pressure V = volunie N = moles of hj.drogen foi,med P, = change in pressuye a t constant vol. (dP/dt), = rate of change in pressure a t constant vol. (dT’/di)p = rate of chnnge i n vol. nt,constant pressure
Appendix I1 Evaluation of the Constants IC and B in Terms of a and F.-The rate of corrosion obeys the general parabolic rate law N“
=
ICt
(1)
where N may equal pressure, volume or number of moles of hydrogen formed per unit surface area. Differentiating implicitly and explicitly
The experimental rate equations are (4)
(11)
Incorporating the rate coiistmt into equation 4, the final form of the rate equation is obtained t
havior is of interest since it indicates the possibility of an additional rate-determining mechanism prior to the formation of a protective film. Another iiidication of two rate mechanisms may be seen in Fig. 5 from which it is evident that an additional rate mechanism is operative prior to the attainment of a certain minimum film thickness, beyond which the reaction rate is diffusion controlled. An analogy may be drawn between the underwater corrosion of magnesium powder and the electrolysis of many aqueous solutions, for which a rate mechanism involving the discharge of protons by electrons or the recombination of hydrogen atoms on the surface of the metal to form molecular hydrogen is The activation energy during this initial period, based upon logarithmic behavior (7.48 kcal. per mole), is in the range expected for this mechanism. Acknowledgment.-The authors wish to express their appreciation to J. J. Campisis and S. Weisberger for the X-ray diffraction and spectroscopic analyses reported in this study, and to B. Dubrow, M. Nieradka and A. Tomichek for the particle size analyses and preparation of the powdered metal samples. We also .r\.ish to thank M. Katz of the Chemical Physics Branch, Squier Signal Laboratory, Fort Monmouth, New Jersey for the krypton adsorption studies. In addition, the authors express their gratitude to D . Hart, H. Cohen, G. Weingarten and B. Werbel for their discussions and review of this manuscript. Appendix I
(10)
it is evident that C-l is a rate constant referring to the change i i i volume per unit change in the logarithm of time rate constant, k’ = C-1
TTol.59
GORDOK
(5)
By comparing equations 2 with 4,and 3 with 5 the following relationships are obtained B = IC/n
(6)
(12) S. Glmntone. K. J. Lnidler n n d H. Eyrirlg, “Tlie Theory of Rat,c Prorpsses,” JlcCi,nw-Hill Book Po., Ne\\, York, N . Y , , 1941, p.
583.
(7) ( l / n ) - 1 = -a;
n = (1 -- a ) - l
Substituting equation 8 in equation 10 results in the final relationship
(8)
B = ( 1 - a ) (1__a>cl-ai-'
Solving for K in equation 7 I\: = (Fn)"
(9)
By comparing the exponents of equations 2 and 4 X:=n-l
K and B may be related by substit,uf,ingequatJion 6 in the above expression B =
(11)
(12
Substituting equation 8 in equation 12 (Fn)"/jl
(10)
k = a(1
- a)-1
(I3
SPECIFIC HEAT OF SYNTHETIC HIGH POLYRIRRS: I\r. POLYCAPROLA4CTARI BY PAULMARX,C. 117. SMITH,A. E. WORTHINGTON AND MALCOLM DOLE Contribution from the Cheinical Laboratory of Northwestern University, Evanston, Illinois Received March 11, 1965
Polycaprolactam, or 6 Nylon, has been studied with respect to its specific heat using a new calorimetric system over the temperature range -20 t o 280" in flake, drawn, undrawn filament and annealed forms. The results are compared with similar measurementsoon 6-6 Nylon. Apparently, depolymerization in the melt on annealing lowers the glass transition temperature about 15 . The undrawn filaments exhibit irreversible recrystallization effects from 90 to 160". The crystallinity of 6 Nylon is slightly less than that of 6-6 Xylon, but exhibits a maximum in the case of the drawn and undcawn fibers a t 190". At room temperature the crystallinity of the drawn fibers is less than that of the undrawn.
I. Introduction Polycaprolactam or G Nylon is chemically identical with po1:vhesamethylene diamine, G-G Xylon, escept for 1ie:id-to-tail arrangement of its imine and cnrlionyl groups as compared to the head to head arrangement of the imine or carbonyl groups i i i G-G Nylon. For this reason it appeared to be of iiiterest to measure tlie specific heat of G Nylon for comparison with previous measurements on 6-6 Nylon.' Such nieasiirements include the mmsurement of heats of fusion, heats of transition, heats of crystallizatioii and heats associated with second order or glass? trmisitmions. -4s far as we ara aware there have beeii n o previously published data for the specific lieat of G Njrloii. Brill3 made an X-ray study of G ant1 G-G Nylon as a function of temperature. He demonstrated that the 020 mid 220 lattice spacings increased more rapidly with temperature in the case of the G-G hr.vlon than in tlie case of G Nylon and equalled the 200 spacing a t IG5" giviiig to the G-G Yylon a pseudo hexagonal st,racture a t that temperat'iire. This hesagoiial structure esists in cross-secl,ion only. In the case of G-G Nylon the breaking: of hydrogeu boiids followed by the sliiftiiig of adjacent chains and :t rotational oscillat'ion of aljoiit GO" of the chain segments permits the formation of new hydrogen bonds aucl the estahlishnieiit of tjhe pseudo hexagoiial st,ructure. In G Nylon the segmental rotation does not result in coiitlitions fai.or,zble for hydrogen bond formation, heiice G Nylon does not make tlie trniisition to pseudo hesagolid symmet,ry. Mikhailov and Klesmaii4 strudied tlie effect of ex(1) R. C. Willloit and Rf. Dole, THISJ O U R N A L57, , 1 4 (1953). (2) P. J. Flor,y, "Principles of Polymer Chemistry," Cornell Univcrsity Press, Itliaca, N. Y., 1953, p. 5G; a n d W. Raustnonn, Chem. Reus., 43, 219 (1948), hove reviewed t h e subject of second order and class transitions. ( 3 ) R . Brill, J . pralil. C h e m . , 161, 44 (1042). ( 4 ) N . 1'. llikliailox. and 1'. 0. Klesnmn, DoHadu A k o d . Nnirb.. TJ.S.S.R.. 91,$19 (l$l.X3): C . A . , 48, 20 (l:l54); 48, I3RB!l ( l f l , i 4 ) .
tent of crystallization on the thermal properties of G Nylon. Making a thermographic and X-ray analysis of samples prepared from an e\.aporation of a formic acid solution or by slow cooling and samples prepared hy rapid cooling, they concluded that 6 Nylon could exist, in two forms, an imperfect, unstable crystalline form, and a glassy amorphoiis form. The sloivly cooled form exhihit,ed a single endothermic effect a t 200-216", while the amorphons form showed t,wo endothermic effects, at, 120-158' and a t 216-222'. The 120-150" range is also referred to as n range of temperat>ureswhere \.itrificntion occurs. They speak of a ''1ie:Lt of fusion" of t'he crystalline form as 12.4 cal./g., and of t8heglassy modification as 9.4 cal./g.
11. Experimental Details The following samples of 6 Nylo115 were studied: 1. Estracted 6 Nylon flake containing npprosimately 1.7.% water estractables having a number average inolecular weight, on an extimacted basis of about 20,000. 2. Undrawn G Nylon yarn (1050 denier, 34 filaments) containing about 4% water estractables and having a number average molenul:u weight on :in extracted Imsis of about 18,500. 3. Di,an-n 6 Nylon yarn (210 denier, 34 filaments) containing about 4% water ext,ractnbles, and having a number average molecular weight on an estmcted basis of about, 18,500. The above samples were used as received from the du Pont Company except that the samples were dried t o constant weight by evacurtt,ion before each specific heat measurement. "Melt annealed" samples were prepared by slowly cooling the G Nylon while still in t,he calorimeter after the 6 Nylon had been heated above the melting point. It was assumed that due t,o depolymerization in the melt the melt annealed samples contained llyo of monoinri, (caprolactam) wlien heated5 to 282", and 7y0 non no me^^ i f heated6 t o 235'. Other annealed samples ~vet'ep r c ~ ~ i : i ~ ~ t ~ I by slow cooling from a temperature somewhat I)clow the melting point,. These were called "210 or 190' a n i i ~ : d ~ ~ I " Nylon. (5) Generously forwarded t o 11s hy Dr. ,I. Ziiiiiiierninii of t l i p Nylon Research Division, E. I. ( l i i Pont de N e i i ~ o i i r sand Clo. (G) Information received froni Dr. R . E. Wilfong, Nylon Rrsewrli Dii.isioii, E. I. dii Pont de N e i i i o i i r s and Co.
