Corrosion Studi es or Magnesium and Its Alloys

HY J. l ) , l [ A N A 1\'AI.T, * M I; ~IJ3ER A.I.M.E. , c. E. NELSON* AN D J. A. P io: LO LTBET*

( Phi ladelphia M eeting, October l94 r)

T11 E subject o[ th e salt-water corrosion resista nce o[ magnesium a ll oys is some­what like that of th e pitt in g of stainless steels in that it in volves a relat ively small perce ;1tage of the a pplications but recei_ves a mu ch greate r proportion of the pubhe~ty . Actuall y ma gnesium a ll oys arc in usc in man y ex te rio r a ppli cati ons as well as inte ri o r with no troubl e fro m co rrosion. ·Magnc; ium a nd its a lloys have a strikin g stabi lity against atm ospheric a ttack, bci ng far superior to iron. Specimens exposed unpro tected to a tmo:phcric weat herin g for

10 years show only a superrtcia l surface­

film formati on. The basic nature of magnesium and of

its importan t alloys is tha t they a re also resistant to th e attack of salt water even in an unprotected state, giving substa ntia lly no loss in weight after one year of a lternate immersio n in 3 pe r cent NaCI solution (Fig. r). Th is basic behavior is. ~f ~c n masked because of the extreme sc nsttt vtty of th ese all oys to certa in impuri t ies a nd combinations of im pu ri ti cs. It is known, and the subject o[ inc reasin g emphasis, that higher purity leads genera ll y to greater s tability for man y materials, non­meta lli c as well as metalli c . ll o\\'cve r, a rev iew of th e lite rature 011 th e co rrosion factors in magnesium alloys shows that a lmost a ll resea rchers wo rked with base metal in wh ich metallic impurities va ried in un co ntrolled a mounts. The major fac tor, that of metalli c impurities, probabl y

M an usc ript received at t he o fli cc of t he Inst itute D ec . I .L 11)-.\0: re v ised AtJril q, '.9 ~1' · l ssul'd :l S

T . P. I J .~ . ~ in MET ALS TI~C II N ( tU!<;v, St.: ptl'm lK·!· . ' '>·1' · * T h e Dow C' hcnncal Comp:~ny, M1 uland,

Michi gan.

greatl y overshadowed t he e ffects of minor

ones such as flu x inclusions, nonmeta lli c

impurities, po rosity, meta llographic state ,

whi ch th ese resea rchers pres um ed to study .

In th e prese nt in ves tigati on th e rtrst

observat io n o f interes t is th a t whereas mag nesium of comm ercial purity (99 ·9 per ce nt pure) ha s a co rrosion rate in altern a te immersion in .) per ce nt NaC I solu tion at roo m tempera ture of a ny where from 5 to roo mg . pe r sq . em. pe r day, th e corrosion rate of hi gh-purity magnesium is o. r s ± o.os mg. per sq . cm. pe r day . The principle followed in the experiments was to add to such base magnesium oth er elements, singly and in co mbin a tion, in co n trolled <tnd kn ow n amounts and to observe th e co rrosion behavior of the resu ltin g a lloy.

These ex perim ents sho w th e exi stence of what may be ca lled " to lerance limits" o[ magnesium a nd some of its alloys for ce rta in impuriti es. The ext re mely small pe rce ntages of impurities in vo lved and th e sharpn ess of th e corrosion di sco ntinuiti es appea r to be qu ite unpredictab le.

Jn o rd er to ca rry on this work , it was necessa ry to de velop a n a nalysi · technique suitabl e to t he problem. In some cases it was necessary quantitatively to a na lyze im pu ri t ies to less than o.oo 1· pe r ce nt. The ex perim enta l da ta to be prese nted arc based o n obsc rv<ttion of a bout sooo a lloy specimens and on more than so,ooo quantitat ive a na lyses.

273

Whil e th e prim a ry purpose of th is paper is to present t he experim entall y obtained data on corrosion rates as a fun cti on of co mposition, a nd parti cularly to demon -

274 COHROSION STUDIES OF MAGNESIUM AND ITS ALLOYS

~tra te t he existe nce of sha rp t ole ra nce limit s fo r certa in impurities, it would

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'" ~ "' "' ~ Y. 0 <f)

"' ~ ,..,

natura Uy be of inte rest to ascerta in whether t he results can be explained in terms of any

gcnera.lly accepted theories of corro~to t l

behavior o f meta ls. T o th is end , p relimi ­nary result a re included fro m experiments on spot s pectrograph ic ana lysis and me tal ­lography , o n solu tion potentia l and over­voltage mea surements, and also o n electron ditTract ion st udies of the al l-im portant rdm formations .

T he g reat majori ty of the a lloy composi­tions stu d ied have been made o n 10o-g ram melts, which a re just la rge e noug h for a na lysis a nd corrosion samples. In a la te r section of this pape r, howe ver, a re repo rted resu lts on materia l made in fu ll prod uctiotl­scale melts and fabricated into standard lest ba rs, from which the eiTect o n mechan­ical p roperties o f exposure to sa lt solution ha s bee n obta ined.

COl{HOS IO. i\S ,\ FUi\CTlON OF ALLOY COM PO 'ITION

E XPERIM ENTAL l\1ETHODS

A1! agnesiu.m. Pw·ificat·ion T echnique

J\Ietallic m agnesium as available com­mercially has t he following ty p ical analysis

P ER P ER CENT C E N T Fe . O . O,SO AI . 0 . 0 1 i\ i . < o .ooos M n. 0 .00 2 Cu. < o . oos l'b .. <o . oo t

~i 0 . 0 0 .1 Nn. <o .oos

and is th erefore un suitable for t he study of corrosio n be havio r o f mag nesiu m because of t he presence o f several elements in amount surt'lcient g reatly to influe nce the corro io n rate. ] ligh-purity magnesium was produced by a special va porizatio n tech­nique, sin ce it was found that m a gnesium nonun iform in iron content was obtained by simple vaporization . Good purity magnesium could be produced at a satis­factory rate by d istilling the metal from a magnesium-lead a Uoy and passing the vapors t h rough a fll ter. After p uri ftcation the magnesium is stripped fro m the con­denser a nd representative sam ples are taken for routine analyses a nd control corro ion tests. Crystals of t his purified

J. D. H ANAWALT, C. E . NELSON AND J. A . PELOUBE T 2 75

magnesium are only superficia lly co rroded even after exposure of a year o r more to 3 ;per cent NaCL soluti on . T he origin al technical magnesium would be entirely corroded away in severa l clays if exposed to 3 per cent l'\a Cl solutio n. A typical a na lysis of th e purifted magnesium prod uct follows :

Fe. Ni . AI. . Ca. Cu.

P ER CE NT

< 0.001 < 0 .000 1 < o . OOI < 0 . 00 1 < o . OOO l

Na. Mn .. l'b . Si ...

P ER CENT 0.00 2

< o. ooos < 0.0 1 < o . oo r

Pure A ttoy Produc! ion 'f"ecllll iquc

The melting a nd a lloying of th e super­p urity mag nesium a lso req uired th e devel­op ment of a. specia l techni q ue a nd a careful selection of ;tlloy in g materi a ls, so tha t the desired com positions coul d be mad e with ­out co ntamina tion by oth er undesirable e leme nts.

It was discove red th a t superpurity magnesium could not be melted in a n iron or steel crucible a nd th e molte n meta l could not be contacted with tongs o r ot her apparat us mad e of these ma te rials because of the t ende ncy of th e a lloy to pick up iron. Recourse was necessary, th e refo re, to materi a ls of a refracto ry t y pe, of whi ch solid graphi te , plumbago, carbon-lined silico n carbide (Tercod ) and sillimanite were most sati sfac to ry . T he g reat bulk of the a ll oys for the prese nt ex perime nts were made in sillima nite o r plumbago and spectroscopically pure graphite was used fo r st irrin g rods, sa mplin g devices a nd th ermoco uple well s. JVfa.gnesium may not be melted in the plumbago or sillima nite c rucibles without the use of a nu x because the metal will a ttack th e c ru cibl e and contamina tion of th e melt wit h a luminum , iron a nd silicon will result. T he usc of t he co mmon magnesium melting flux es, how­ever , la rgely preve nts thi s attack a nd a t the same tim e protect s th e a lloy from oxida ­ti on. Specially puri fied nu x was used in the a mount of ro to 30 per cent of t he weight of the metal. The bulk o f the flu x was

melted in the cru cible a nd when hot the slightly p reheated pieces of the weigh ed magnesium a nd necessary a lloyin g con­stituen ts were added to the crucible with a careful sprinkling of powdered flux to inhibit burning. When t he e ntire charge was in a nd thoroughl y molten, the a lloy was stirred carefully wit h a graphi te rod severa l t imes at intervals of a few minutes. Quiet standing fo r 5 to I 5 min . was th en a llowed, a nd fi nally the cru cible was removed from the furn ace, afte r whi ch the charge froze and was the n c racked out of th e c rucible, and the froze n flu x washed from the metal. Thi s simple technique avoided po uring of the metal and the conseq uen t diff1 culti es due to very sma ll batches a nd danger of conta min a.tion. The ftni shed a lloys we re th en cut into samples fo r a nalysis and corrosion, as will be dis­cussed la ter.

It has bee n stated in th e lite rature that such substances as :MgO , iVf"g ;~N2, mag­nesium oxychlorid e or chlo rine a re a ppre­ciable fac tors in th e co rrosion of magn esium a nd its a ll oys. In order to check the p rese nt ex perim enta l proced ure, a nd because of t he possibility of th e occurrence of these co m­poun d s in t he product io n o r fabr ication of mag nesium , it was co nsidered ad visabl e to in vest igate the behav ior of such substances. rt was foun d , provid in g harmful meta llic impuriti es were not sim ultaneously intro­duced , that th e foll ow ing ma terials have no signi ftcant e ffect on t he co rrosion of high ­purity magn esium a nd its a lloys: CaC 2 ,

N l-r. CI , N I f4HF. , S, iVIg,. N ,, MgO , MgOC I,, C, li e , N,, CS ,, C2ll ,, CO , C02, II 2 , II 2S, li Cl, C l2, C l-1 .,, SO,, 0 2, natura l gas an d a ir. It was a lso found that flu x in cl usions d id no t accele ra te th e co rrosion ra te of superpure a lloys in 3 per cent NaC l solution.

The process and eq uipm ent for ma king the p ure a ll oys havin g now been brieny outlin ed , so me further co nsiderat ion will be given to the select ion a nd prepa ration of th e a lloy ing ma te ria ls used .

CO ilH OSION STUDIES OF Mi\GNE SI UM AND ITS ALLOYS

If lnminnm.- Comm ercia l alumintllll con­ta in s far too mu ch iron, ni ckel a nd sili con fo r use in th ese ex periments , so a special grad e of aluminum , which co ntain ed o nly about o .oos per cen t iron and o the r im­purities in ve ry sma ll a mounts, was u eel.

Zinc.- F ortun a.tely, in this case the regular co mm ercial g rade of ll o rse head zin c is of sati sfac tory purity.

JIJ! nnga il ese .- Co mmercial meta lli c ma n­gan ese or ha rdener :1lloys of aluminum and manga nese have until ve ry rece ntly co n­tained fa r t oo mu ch iron and nickel to be of usc in th e production of pure a lloys. J t had , howeve r, been previously kn o wn tha t ma ngan ese c;tn be added to mag nesium a ll oys by th e redu ctio n of M nC 12, 1 a nd since the la tter salt is a va ila bl e in a ve ry pure sta te thi s a fford s a good so urce of pure man ga nese . For th e mos t r a rt. , ha rdene rs o f pure mag nesium with 2 .0 pe r cent ma ngan ese were mad e in la rge batches, and t hen pa rts of this pre-a lloy we re added in t he co rrect proportio ns to smalle r melts as required .

h on .- No rm ally iron was added to t he pure a ll oys, when desired , through the use of a pre-all oy made by redu cing pure l:'eC I:. with pure magnesium or a pure ma.gnesi um -base a lloy . It was ncccssa ry to mix th e F eCI:l to be reduced \\' ith a. propo r­tio n of th e regular meltin g nu x before adding it t o t he molten a lloy, in o rde r to preve nt th e loss of the FeCI:. by excessive vaporizatio n. I ron co nte nts of o.o3 per ce nt or slightly ove r may be obta i ned in t he binary magnesium-iron a lloy by the fore­going tec hni que. Iron can be added by direct contac t wi t h molten magnesium ;t t eleva t ed te mpera tures, but such a. pro­cedure is no t dependa ble fo r unifo rm or predicta ble co mpositions.

Nickel , Copper and L ead.-Sin ce ni ckel, copper a nd lead are available in a high deg ree of purity , a nd a lloy readily with magn esium , th ey may be add ed directl y to the ma gnesium aLl oy . Owing to th e

1 R eferences are at the e nd of t he paper.

extremely sma ll a mount required a nd th e f'tn e co ntrol of unifo rmity ;wd qu a ntit y desired , th e general prac tice was to ma ke pure pre-alloys of magnesium o r mag­nesium -base co nta inin g o .o r or 1.0 pe r cent of th e metals a nd then add a li q uo t po rtions of these to t he interm edia te a lloys.

Silicon.- Sili co n was pe rhaps th e most troublesome of a ll the elements to add becau se of th e lack of a. source of high­purity sili co n and th e consequen t te ndency to introduce simultaneo usly into th e alloy cxcessi ve am ounts of oth er impurities . Afte r man y more or less successful a t tempts to redu ce va rious pure sili co n co mpound s, it was ftn a ll y found that the best technique was to fo rm a pre-alloy by using com­merci;d 97 pe r cent ·ili co n (fe rrosili co n o r 1\o pe r ce nt aluminum , 2 0 per cent silicon ha rde ne rs), a ddin g th ese to very pure mag­nesium o r mag nesium all oys in a proportion suf1'1 cient to produce a fin al sili co n content of 1 .o to t o.o per ce n t, and then removing a ny iron thus added by a precipita tion treatm ent, so th a t the end product would be a pure magnesium-base a lloy containing appreciable sili co n b ut ve ry little iron. This pre-a lloy co uld th en be a na.lyzed and used in making o the r sili co n-containing a lloys.

M ore Co mjJlin llcd If /loy !l ddit ions .- The usc of ha rd e ncrs or p re-a lloys, a s has been prev iously mention ed , made th e production of th e more co mpli cated co mbin a tions of elements possibl e. In prac tice, la rge ba tches o f the various classes of pure base a lloys we re made up with no impurity additions an d a lso with indi vidu a l and v a rying am ount s o f the different elements to be studi ed. In t his way, through the use of so me so to too base alloys, alm ost a ny desired a lloy could be made by mixing the proper g roup of base alloys in calculated proportions, with out fea r o f a ltering th e ge neral a lloy co mpositions by such mixing.

M et!tods of Analysis (Sec If ppendix A)

i\l[os t o f: the a na lyses were mad e by a spectrochemi cal me thod. Occasiona l check

J. D. HANAWALT, C. E. NELSON AND J. A. PELOUBET 277

determination ~ were mafic by dependable chemical procedures.

In order to compare the composition of small corroded a reas on a particular speci­men with the composition of the non­corroded areas, it was neces a ry to use a procedure somewhat different from that described in Appendix A.

Samples of the areas to be studied , weighing ro to 20 mg. each, were taken by use of a small drill, the dia meter of which was less tha n the diitmeter of the area to be sampled. This materia l was dissolved in d ilu te sulphuric acid and the solu tions were brought to a concentration of 5 per cent ba.-cd on the magnesium. On each one of a. pa ir of spectroscopically pure graph ite support i11g electrodes, o.o.; mi. of this solu tion was phtcccl a nd. then dried. The resulting salt was excited by means of a 22oo-volt a.c. a rc. From this point on the analytical p roced ure was identical to that described in Appendix A.

Specimens selected for this type of analysis were treated in several different ways prior to sampling. '.!.'he first group of specimen was subjected to the regula r corrosion test and then samples were taken from the corroded and noncorroded a reas. 1\trany of the corroded a reas showed an iron content as high as o .o5 rcr cent while the noncorrodcd areas conta ined less than o.oo2 per cent. I fowcvcr, t here were cases where there was no difTercnce in the iron content of the two a reas after corrosion. La ter tests indicated that the rca · on for this probably was that most of the iron had already left the corroded a rea either by dis­solving in the corroding medium or by being mecha nically removed as the magnesium a round the iron particles dissolved .

T he next group of specimens was sub­jected to the corroding medium only until the bad a reas began to appea r. i\ll samples taken from these specimens showed greater iron concentrations in t he corroded areas. E leven different specimens analyzed in this ma nner showed iron concentrations in the

corroded areas vary ing from s to so times greater than the iron concentration of the noncorroded areas of the same specimen. A specimen treated in this same manner was analyzed by microchemical methods using t he aa' dipyridyl reagent. T his analysis showed o.oor 5 per cent iron in the good areas and o.oi 5 per cent iron in the cor­roded or bad areas.

A third series of samples was taken in the following manner. A specimen was cleaned and polished on one surface. This surface was carefull y mapped and approxi­mately roo samples were taken from areas over the entire surf ace. The specimen was then immersed in the corroding medium u nlil corrosion spots appeared. The small samples previously taken were then divided into two groups, one group being taken from the areas that corroded when placed in the corroding medium and the other group from a reas that did not show attack . T he iron content of the ftrst group of sam­ples was o.ooS per cent. The iron content of the second group was o.o014 per cent.

In a fourth method a specimen was mounted under the microscope a nd a drop of 3 per cent NaCl solu tion was placed on it. The action of this solution was observed just long enough to locate deftnitely the source of a llack, indicated by the evolution of bubbles of hydrogen. T hi a rea was then marked and "'ashed with distilled water. The sample taken a t this area conta ined o.os per cent iron. T he iron content of a sample taken from the unaltacked a rea contained o.oo i per cent iron.

In addition to analysis for iron, all of the <tbove-mcntioned samples were an a­lyzed for nickel and chromium, two other metals that could cause increased corro­sion. ln no case was nickel or chromium detectable by either the spectrochemica l or microchemica l techni ques.

Corrosion T esting

T he specimens produced by the alloying technique described were subjected to a

CORROSION STUDIES Ol' MAGNESI UM AND ITS ALLOYS

sta nda rdized corrosion testing procedure. r\ t the same time an accura te spectroscopic ana lysis of the im purity content was obtained. The actua l specimens for cot-ro­sio n testing <tnd a nalysis were carefully cut fro m sections of t he a lloy that were known to be uniform a nd free from inclusions of nu x. 'J.'he corrosion sample was cut to a uniform size of ;)1 by I by 1 )>~ in . a long with the two a na lysis electrodes 78 by 7.Z by % in. each .

F or testing purposes, the specimens were ftrst drilled near one end to accommodate a 78"-in . glass hook. Subsequently the sam pies were g round all over on ~L clean Ko. so grit ro ta ting a brasive cloth . T his abrasive was of a n a luminum ox ide type, which did not harmfully contamina te the meta l being g round . Finer ftnishes were of no :tdditiomt l value . T he grind ing was ca rried deep enough to eliminate all saw ma rks and o riginal surfaces. Careful ha nd ling permitted the omission of a degreasing operation before t he corrosion test.

Hef ore the corrosio n test, the samples were weighed to the nearest milligra m and measured to the nearest miLlimeter. From the la tter ftgures the origina l a reas were ca lculated .

The standard corrosion test is essentially an a lternate immersio n-emersion in 3 per cen t NaCl solution . The cycle consist s of 7~ min. in the solu t io n fo llowed by 2 min. in air. During the la t te r time interval the specimens do not com pletely dry.

The specimens were dipped into indi­vidual glass co ntaine rs as a means of avoiding the contaminatio n of one speci­men with corrosion product f rom another. At frequent intervals, distilled wa ter was added to the conta iners to ma ke up for loss by evapora tion . T his not only provides a mo re uniform concentra tion of NaCl but ensures a complete wetting of the samples on each cycle. Every t wo or three weeks (or oftener if necessary) the test solutio ns were renewed in order to prevent undue accumula tion of corrosion p roduct.

T he tempera ture of the a.tr and salt solution was that of the surrounding room, a nd was within the range o f 6o0 to JOo°F. lt should be pointed out here that impure specimens corrode more rapidly than pure a lloys with increase in tempera ture, while the corrosion ra te of the latter is independ­ent o( the room-tempera ture cha nges. T hus, while working wi t h specimens o( low co t-ro ion ra te in determining impurity tolera nce limits, t he norma l temperature varia tion is not an essential factor.

The corrosion test was continued for a period of 16 weeks. Specime n that did not survive the entire time were noted as destroyed . At the end of t he test period , the sample· were prepa red for the final weighing by a comple te removal o[ all corrosion products. This was accomplished by means o[ a r -min . immersion in boiling 20 per cent CrO, solut ion to which has been added I per cent AgN03.3 A Cr03 solution alone does not appreciably attack magnesium or its alloys, but a considerable a mount of sodium chloride is carried over to the cleaning ba th within the corrosion product. If this were a llowed to remain in t he cleaning solu t ion, the resultant hydro­chloric acid would a ttack the specimens a nd produce er roneous weight losses. T herefore, a concentra ted AgN 0 3 solution is added to t he 2o per cent Cr03, yielding a fine precipita te o[ Ag2C r0 4 • T his Ag2CrO. reacts in turn to precipitate any chlorides that may be brought into the cleaning solu t ion.

After the cleaning opera tion, washing in water a nd d rying, the specimens were reweighed . F rom the determined weight loss, original a reas a nd time of test , the corrosion rate was calculated as milligrams loss per squa re centimeter per clay.

All specimens were examined visua lly not only to obtain informa tion on the t y pe of attack b ut to locate a ny extraordina ry defects that might cause the quantitative results to be unreliable.

Fig. 2 is included to aid readers in obtaining a visual picture of the corrosion

J. D. HA NA WALT, C. E . NELSON AND J. A. PELOUBET 279

corresponding to a certain numerical rate in milli gra ms per sq uare centim eter per day.

.I .5

corrosion rate. T his phenomeno n is de­scribed by saying tha t t he tolera nce limi t of magnesium fo r iron is o .oq per cent.

I.

DISINTEGRATED IN

35 DAYS

10. CO I<J<O S IO N J{i\TE, MC: . I" F. H SQ. CM. PER Oi\V.

F IG. 2 .- 1\l' l' l•: i\H i\ NC' I•: I IF S I"EC IM '" NS ll i\V I NC: D I FF I·: HE NT COilHOS IO N l( i\ TI ·:S i\FT I•: I( 4 MO NT II S

J\ LT I•:H N ATE IM!\f i•:RSI O N JN 3 I'I•: R C E ~T SO D I U M C II LO I~ ID !o: .

R I·:S U I.T S

Graphica l prese nta tion of the resul ts obtained in this wo rk is used as an aid to an easy visualizatio n of th e co rrosion da ta. As has been m entioned, co mmercia l purity magnesium has widely varying co rrosio n ra tes within t he ra nge from 5 to roo mg. per sq. em. per day while high-purity magnesium has a ra te of o.I5 ± o .os mg . per sq. em. per day.

F ig. 3 gives a broad view of the co rrosio n of magnesium binary alloys wi th va rious comm on elements. While some clements a re not ha rmful in la rge proportion, o th e rs a re de trimenta l even when present in minute amounts. T he present importa nt a lloy compositi ons co me from combin a tio ns of th e eleme nts tha t arc not ha rmful to corrosiOn. T he common impuriti es in commercial m agnesi um a lloys a re iro n, ni ckel, copper , lead a nd silicon . Mo re detailed co nsiderat ion of these impuriti es in the imrorta nt magnesium alloys co n­taining a luminum , ma nga ne c and zin c is then necessa ry. O ne begins with the st udy of iron, nickel and copper in magnesium . The results are show n in Figs. 4, 5 a nd 6 for iron, nickel a nd coppe r, respectively.

In a moun ts up to o .oq per cent , iro n has no effect o n th e co rrosion ra te of magnesium . Any excess of this q ua ntity, ho wever, causes a ma rked increase in

ln Figs. 5 a nd 6, simila rly, th ere is a sharp tolerance limit for ni ckel at less th a n o .ooo5 per cent a nd for copper ncar o . T per cent.

T he effect of th e a lloy elements ma n­ga nese a nd zinc o n the im purities iron , nickel a nd copper is see n in Figs. 4, 5 a nd 6, respect ively, whi ch show th :tt in th e cases of copper and iron the tolerance limi t is un cha nged th o ugh the in crease in co r­rosion ra te due to copper or iron above t hci r respecti ve toleran ce limi ts is consid ­erably red uced by th e presence vf about J.O per cent of ma ngan ese or zin c.

Fig. 5 shows that th e tolcra nce of mag­nesium for nickel is g reatl y chan ged by mangan ese an d is ra ised wit h in creasin g man ga nese conte nt , be in g a bou t o.oo J pe r cent ni ckel for 0. 2 per cent man ga nese a nd abo ut o .or 5 per ce nt ni ckel for 2 .0 per cent manganese. It a lso shows tha t zin c serves to ra ise th e to lerance li mit of magnesium fo r ni ckel.

While th e usc of 0 . 2 to 2 .0 per cent man ganese and of r.o to r. 5 per ccn t zinc appea rs a rbi tra ry, these choices arc i ncli ­catccl bcca.usc t hese amounts correspond to t hose in co mm on mngnesium a ll oys.

A logical co ntinuat ion of the ex per i­ments wo uld be to show th e e ffect of th e presence of aluminum in magnesium on t he toleran ce limi ts for iron, nickel and cop pe r. When one a ttempts to do this for iron he

280 CORROSION STUDIES OF i'v!J\GNESIUM i\ D ITS ALLOYS

gets results as shown in Fig. 7, in " hich it is shown that with even a very small content of a luminum the tole rance li mit fo r iron has d roppecl from o.oq r er cent to

FE

cu 15

NI , Co

10 1-!------1--

5 Hl-------1

2

which g ives a co rnpa ri o n of the results with o.o per cent ma nganese a nd 0. 2 per cent ma nganese. vVith 0 .2 pe r cent manga­nese the tolerance limit for iron does not

3 4 5 I PERCENT OF ADDED CONSTITUENT I

F1 c . 3· CO I~ I~OS JON O F MAC N E SI U M BJ ~,, 1~v ,,J.LOvs.

a few thousandths pe r cent a nd that wit h 7.0 per cent a luminum it is about o.ooos per cent. With Jo.o per cent a lu mi num , the limi t is too low to be determined even t hough prod uction a nd a nalysis ca n wit h s ufT1cient care be carried to about o.ooo2 per cent iron. Therefore, study o[ the toler­ance limits of other impurities, such as copper and nickel, in the system mag ne­sium-aluminum would be very dillicult , since it would require p ractically nil iron .

I t is only possible to proceed in a satis­factory ma nner by working with t he system magnesium-alu minum plus a small amou nt of ma nganese. T he action of this small amount of m a nganese is shown in F ig. 8,

drop below o.oo 2 per cent, but instead holds very co nsL1nt over a wide ra nge o f alu mi­num content. A few hundredth per cent manganese is suffi cient to give this effect.

Next comes the st udy of the impurities iron , nickel, copper in the magnesiu m­alum in um-o.2 per cent ma nganese system instead of in t he magnesium-aluminum system.

The en·ect of a luminum wit h o.2 per cent manganese on the impurities iron, nickel and copper in magnesium is shown in Figs. 9, 10 and u, respectively . T he aluminum content in the specimens indicated in these g raphs varied in the range 2 to 12 per cent, with t he great majority of the specimens

J. D. HANAWALT, C. E . NELSON AND J. A. PE LOUBET

ly ing be t ween 4 and 8 pe r cent. While the that of the nickel probably not a t a ll , since a luminum changes g reatly, a nd in fact , is the o. z per cent manganese would account

_,._ " I CORROSION RATE

/ MG/ CM2/DAY

" 90

I X MG

" 80 0 MG 1.0 MN I " .c. MG 1.0 ZN I 70

"t! 60

I 50 X I

4 0

30 X I I 20

I 10 "

0

I I " 1.0

0 .5

I ~..:o~" ..

0 Q ~ .... ~ v 1-W~ ""J'

... ·x ;:" "' 0 •• 0 .004 .008 .012 .016 .020 .024 .028 .032 .036

I PERCENT FE I ftC . 4.-E~' F I~Cl' 0!' lRO~ O N ~I AC:~ J ·:S IU~l, MAC: ;\ESI UM- Mi\ NGA I\ IcS I•: A ~ IJ MAG~ ES I C J\1 -Z I NC.

the ma in determining fac tor of the position of the iron tole ra nce li m it , it changes the positio n of the copper limit very little a nd

fo r the s mal l shift from < o.oo 1 per cent to o.oo 1 per cent nickel , whi ch d ocs t:1ke place. Since these la rge amounts of a luminum

COHROSION STUDIES O.F MAGNESIUM AND iTS ALLOYS

have a. negligibl e cfTecl on t he copper a nd ni ckel lu le rance li mits , it is unnecessary to

I CORROSION RATE I MG/CM1 / DAY

90

80 X

X

70

Since magnesium co ntaining a luminum in th e order uf several per cen t docs not

--1.----v

MG /MG0.2 MN

60

I 50

1/ X MG

0 MG 0.2 MN

40 0 MG 1.0 MN X

.Cl. MG 1.0 ZN

30

20 X

10 X

I I ,.j

1.0 lj Vfo" 0 .5 X v.

~''·or z v ~ ~CI 0

0 .002 .004 .006 .008 .010 .0 12 .014 .016 018

I PERCENT N I I FI G. s.- EFFECT OF - IC KEL ON" M .\ La\ES LUM, MAGNES rt l ~l- ~(A NG A N I ·:S I ·: AN D M.A GNES l UM- llKC.

sludy th e eiTccts over a wide range of aluminum , as was necessa ry in co nnection with iron.

di sso lve more t han a few tenths per cen t manganese at t he freez in g point, one need not study the eiTects of more t han this

] . D. H ANAWALT, C. E. KE LSON AND J. A . PEJ"OUBET

percentage of m anga nese on magnesium ­alumi num a lloys .

Nex t , sin ce zinc was effecti ve in red ucin g

I CORROSION RATE MG/CM2/ DAY

90

X MG

80 0 MG 1.0 MN

L:;. MG 1.0 ZN

70

60 )( /

50

/ X

40 /

30

20 MG MN

10 v---I )(

1.0

0 .5 / MGZN -~

X l(

x MG MN

0 v~ I

0 .I .2 .3

copper in th e magnesium -alumi n um-o. 2 per cen t ma nganese system must be studied.

Figs. Q, 10, and r 1 show th e results of

X

/ //

v /

/ X

/-G

-MG ZN

.4 .5 .6 .7 .8 .9

I PERCENT cu I F 1c . 6.- l ·: FF J·:cT OF cO PPER ON ,\J A<:N~<:S LUM , ~ 1 ACNJ<: S i li~ 1 - ~L\ N t:JI N J<: S 1·: ,\ N D ,, , A< : NJCS 1 11 ~ 1 - Z 1 NC .

t he ha rmfu l effects of iron, nickel a nd copper addin g o.s and 3 per cent zinc to th e mag­in mag nesium , its act ion on iron, nickel and nesium-a lum in um-0. 2 per ce nt ma nganese

COR ROSIO 1 STUDIES OF J\f AG J:SI UM AND ITS ALLOYS

a ll oy:;. l' ig. o s hows that o.s per ce nt zin c docs not shift th e positi on o[ th e iron toler­an ce li mit at 0.00 2 pe r cenl. but somew hat red uces t he magnit ude o l t he corrosion rates

I CORROSION RATE I MG/ CM2/ DAY

15 /10 AL

mag nesium - a lum inum - m:L ngancsc -ni cke l a lloy ·, 3 per cent zin c ha s shifted the to l­e ra nce li mit from o.oo 1 to o.oo 2 per cent ni ckel :wd greatly reduced the co rrosion

0.~/ - - · -

I / /

/ / / I 10

I - / - .5 - 4 AL ----" --- - -;J"" /

/

C AL

/ / I / / /

~ L. / /

~ ---

~ /

/ / ...- /

/

----- -- ---0 0 002 004 006 ooe 0 10 01 2 0 14 0 16 ore 020

I PERCENT FE I 1' 1< :. 7. Cu i~HO~ I o N (IF ~1 , \ t: N I ·: :-; t t ' i\ 1 - AI.t l i\ II N t J i\ 1 co NT t\1 ~ 1 :-J<: II~O N .

for hi ghe r percentages of irun. ll owevc r, additio n o( 3 per cent zinc gi ves a n inter­esting result in t hat t he to le ran ce limit is ra ised to o.oo3 per cent iron and t he co r­rosion rate grea.Li y red uced fo r highe r per-

l:::~

rat e at higher percentages of ni ckel. Fig. 11 sho ws t hat o. s per ce nt zin c raises the cop­per to le ra nce li mit onl y sli gh tly whereas 3 pe r ce nt zin c raises it to at least o. s per cent copper.

t:_;C1 9

- '-- 9~

8 8~ 0 .0 PERCENT MN 0.2 PERCENT M N

Sf>'\OEO AREA -t . 2 MG/CM'IOAY 'SHADED AREA ~ .2 MG/CM2/0AY 7

7 ~ 6 - 1- - - ,_

:I - f--

~ 5 - - 1-

~ 4 -

3 ~ f--3~

2

I ~a 2~ ~ ~ ~ I~ ~ 0

0 004 008 .012 016 0 004 .OOB .012 .016 !PERCENT FE I I PERCE NT FE I

FIG. 8 .- El'F ECT OF M A NGAl\'ES I·: 0 TilE IRON TO LE RANC E OF M'A GNESlUM-AL UMl NL:M.

cen tages of iron up to o.or z, at which poin t a gr~Ldual increase of corrosion rate begins. Fig. 10 shows that while o.s per cent zinc has litt le e fTect on t he corrosion behavior o[

T hese data serve to show that the magn esium a lloys stud ied are characterized by tolera nce limits fo r iron, nickel and copper when present singly in the alloys.

J. D. HANAWALT, C. E. NELSON AND J. A. PELOUBET

I t remains to show the results when the on the contra ry, there exist s a n interaction impurities are simulta neously present. of some or a ll of them, which wou ld resu lt

'

CORROSION RATE I MG/CM2/DAY

.'ALMN

20~----+-----~----~~---+----~~--~ x MG AL MN

o MG AL MN .5ZN

D. MG AL MN 3ZN '/ 10~-----r----~~----~----+------+----~------+-----~----~

~ / MG AL MN .5ZN

~~~·~+-~0~+---~1 ' I '

1.0 I

I ~~t< _;

~/.Gr>.\..~~ ~---- . ,5 ~~~~--;rt-~--t------+------+-------t-----~~~~-+------4-----~

' • J~~~~------1---~--t-~.--

~~~~· ' /. rwr.r. ~· . 0~~--~--~~--~----~----~-----L----~----~----J

0 002 004 006 008 0 10 012 014 .016 .018

I PERCENT FE I FI G. 9.- I•: FrECT OF JJ~ OK O t\' i\t.\t: N ES J L' ~t -A t. lli\t l NU M -~I A N l:A N J ·: S J ·: .\ N D M .\C N E:-i J UM-A LU MI NlJ ~I ­

i\t i\ N"GA N E SE- Z l NC.

I CORROSION RATE I MG/CM2/DAY

X

20~--~-----4-----+----------~~-----t<~~G~\..

I~ '~~~--,;-+9~---l------l------1 1,0 A I

X MG AL MN

.5 0 MG AL MN .5ZN I D. MG AL MN 3ZN

A

I ~/ 0 ~~--~----~----~----~----~----~----~----~--~

0 .002 .004 006 .008 .0 10 012 .014 .016 .018

I PERCENT Nl I f-'l t:. I O.- f<:FFECT OF N IC "EI. ON MACK J·: S I UM.-ALUMI NU ~I - i\I AI"CA r\ I•:S E t\ N IJ l t\C N ES! Ui\1- AI.UMI NU.\l -

Mi\N L:i\NI::SE- Z INC.

Without experimental work it could not be known whether some of t he elements counteract the effects of others or whethe r,

in a lowerin g of the tolerance amounts as determined for the ind ividua l elements. As evidence of these possibilities, it is

CO l~ ROSTON STUDIES OF M 1\ GNESI UM 1\ N D lTS A LLOYS

poi nted out t hat 0.0 1 per cent aluminu m grea ll y increa::;ed the sensiti vity to iron while o.o 2 per cent ma ng;tne::;c , on the other ha nd , was very efTeclive in counteracting

l CORROSION RATE MG/CMz/ DAY

20

I 10

VMG l MN

I

a I loy:; in which the i mpu ri lies ::;omewha t excccdccl their tole rance a mounts.

!\ ::;tudy of this second group of a lloys ::;howed tha t there were no cases having

X MG AL MN

0 MG AL MN .5ZN

A MG AL MN 3ZN

1.0

; t MN .!,. " .5

I

rf ~ " ()')<, MG AL MN 3ZN

• X "" X X

0 0 .05 10 15 20 .25 .30 35 .40 .45

I PERCENT cu I l'rG. 11 .- EFI' I':CT O F COPPI•: I{ ON i\1 /\C: N I·:S J U M -!\L li M I NlJ M -MA ~<~t\. N I-: S I•: /\ i\" 1) ~I AC NES r U M -A LC MlN UM-

Mi\ N GA N I·:S t·>Z I :--.rc.

t he corrosi vc cfiecl of iron . Th u::; even ex­tremely small amounts o f certain clements may result in s l ri king changes in t he cor­rosive e ffects of other clements.

Further, in order lo make t he results of practica l importance, one must st udy no t on ly the combina tions of the known ha rm ­ful im purit ies, iron , n ickcl and copper, but m ust include as well Lhe common im­purities silicon a nd lead , even though separately silicon and lead ca u e no corro­sion, though present in a mounts 10 l imes as great as t heir normal a mou nts in com­mercia l a lloys.

I n order to answer t hese questions, t he corrosion rates were studied for a group of magnesium-aluminum-m anganese a lloys in which t wo or more of the elements iron , nickel, copper, silicon a nd lead were present - none, however, being presen t in excess of its tole ran ce limit as determined indi ­vidua lly- and a lso fo r a second group of

obse rvable coun te racting effect on the corrosive action of a ny of the impurities.

F rom the study of t he f1rst g roup, how­ever , it wa s found th a t dc f1n ite interaction exis ted fo r combinatio ns of iron with copper, lead or silico n. These combinations were then s tud ied in detail, wit h t he results as show n in Figs. 12, tJ a nd 14.

The signiftca nce of t he boundary lines d rawn on Figs. 12, 13 a nd 14 is that wit hin these lines corrosion rate does no t exceed o. 2 mg. per sq. em. per clay whi le outside of these lines t he rate is greater than o. 2

a nd the ref o re greater than the ra te fo r high-purity magnesium . T his means that t he compositions outside the lines have a detrimenta l efTect o n the corrosion resist ­a nce. D ifTtcult ies due to a nalysis a nd uniformity of composition of these speci­mens were greater than when only one impurity was being studied , and this accoun ts probably for the failure of the

J. D. H A AWAJ~T, C. E . NELSON AND J. A. PELOUBET

points to determine a more precise location o( the bounda ry line. N evertheless the points give strong evidence of the exist ence of the a reas substa ntia lly as shown.

1 PERC~ENT r 0 0 0

.14

.12 0 0

.10

0 0 0

.08 1\ 0

\ · 0

.0 6

.~· < 0

.0 4 0

~ \. 0 . . .02

0

oA\ 0 )( 0)( )(. 0

technique. i\ lloys of m agnesium, mag­ne iu m p lus 3 per cent aluminum and mag­nesiu m plus 3 per cent aluminum plus o.o5 per cent ma nganese were made with vary-

X .Z .20 MG/CM2/DAY

0 ) .20 MG/CM 2/ DAY

---- - -

0 0 .002 .004 .0 06 .008

I PERCENT FE I .010 .012 .014

1-"IG. 1 2 .- C O RROS IOI\" OF ~I M:N I•:S II J.\1 - i\ L U MI NUM-MA N( : i\ l·:SE COKT A I N I NC: IRO N A N D CO Pl' io:R .

When 3 per cent zinc is a.dded to mag­nesium-aluminum-ma nganese alloys, no evidence fo r interac tions of t he ty pe show n in Figs. 1 2 , 13 a nd 14 can be detec ted .

1\l i·:T i\ I.LO<: Ri\ PI ll C 1·: \: A i\11 N ,\TI O N

MI·:T II OD

The a lloys fo r meta llogra phic study were prepared from high-puri ty magnesium a nd pure a lloying materials in a unifo rm ma nner as rega rds a lloying and pouring

ing a mounts of the single impurities iron , nickel and copper in such proportions as to cover a ra nge from subs ta ntia lly zero to well a bove the to le rance limi t fo r t ha t pecilic impurity in each o f t he basic compositions mentioned . T wo types of castings we re made, one sand-cas t in a s tandard tensile mold , the other chill -cast in a graphite mold of }:l by ~~-in. cross section. Samples fo r s tudy were cut from the sa me place on each casting in such a way as to present ·a n average cross section. Most of the com -

CORROSION STUDIES OF "MAG NESiliM AN D I TS AU, OYS

p<t rati ve da ta were taken from observations on the chill-cas t samples. ll owever, cxil mi ­nations were also made of the ~and -cas t

I PERPC:NT I 1.00

0

.90

0 0

.80

0 " 000 00 0

.70

.60 \ _\

.50

" x_\ 0

0

.40 X \ 0

X

" ~ 1\ .30 X

\ X xx X

0

.20 0\ 0 < 0

oox XX

xx

.10 ,..)( )( )()( )(

~x;:;x

0~ \ X X

~M 2:)()( ot, 0 0

XX X 0 0 0

lx lo

0

0

.) . Fina l wet poli shing on a simil a r II' heel covered wi th velve t , usin g a disti ll ed ­water ~ u spe n s i o n of relev igated a lumina .

~ .20 MG/CM2/DAY I > .20 MG /CM2/DAY I

0

0 0

0 0

0 .001 .002 .003 .004 .005 .006 .007 .008 .009

I PERCENT FE I Jo'rG. 13.- CO RR OSION OF M.AG NES IUM· A LUMrNU M"· MM\C:A NE S ic CONTAI NI NG l R0:-1 AN D LEAD.

sa mples, in order to check the effect of t he ra te of chi ll.

The specimens for metaliog rap hic exam­ination were prepared by th e followi ng steps:

1. R ough grinding up t hrough ooo French e mery paper.

2 . Wet polishing on a horizo ntal rotati ng wheel covered with Vel-Cha rn ee clo th , us ing a di sti lled -water suspension of No. 6oo alundum.

4· Etchin g to bring out th e various constituents, as will be discussed in more detail later.

RESULTS

While examination of t he samples in th e as-polished sta te indicated the presence of some of t he impurities, it was found neces­sary to evolve a n etching tech niq ue for each element in order to obtain positive identiftcation. In samples containing iron ,

J. D. HA NAWALT, C. E. NELSON AND J. /\. l'ELOUBET

particles of the iron phase ca n be seen in the as-polished co ndition, but m ore positi ve ide n ti ftcation depe nd s on t he fact t hat exposure to ordin a ry tap water for a few

I PERS~ENT I

.9 "

>(

0

.8

• . 7

• 0

0

.6

0 0

.5 0 • " X 0 0

0 0

.4

0

.3 f----0

X X X 0

.2 .. X 0

X

00 0

0 X 0 O 0 0

. I I" " 0

~Q:') x Ox )( 0 0 0 X 0

axxc~)( X X 0 0

X c31\ X XxX.X i X X X )()()( *,px 0 0 1:\x

0 0 0 .001 .002 .003

0

0

be allowed to take place while the speci­men is und er observa tion with the mi cro­scope. Gas can be seen to be evolved slowly from th e particles, acco mpa nied by

X .z .20 MG / CM2/ DAY

0 > .20 MG /CM'I DAY

0

oP 0

0

0 0

0 0

0

0 j .004 .005 .006 .007

I PERCENT FE I FI G. 1 4.-C0 1~ROS I O N O F ~ 1 /\C N I·:S it l ~ l -t\Ll i ~I I N I I ~ I - ~ 1 ~\ N t : A N I ·:S J·: ('U NTA I :-.J i i': ( : IR O N 1\ N D S I I. I CON.

seco nds causes a di sso lutio n of th e mag­nesium -ri ch matri x a round th e iron parti ­cl es, as is indicated by the appeara nce of broad circles around th e inclusions. Longe r attack by tap water or salt solution may

g rad ua l broade nin g of th e anod ic areas surroun d in g t hem. ;\ ctual counts of th e numbe r of iron particles in a s t a ndard a rea of t he va rious specimens ind icate th e num­ber of particles to be approximately

CORROSION STUDIES OF MAGNESI UM AND ITS ALLOYS

proportional to t he iron content. Some of th ese fac ts ha ve a lso been noted by \' osski.ihl cr. '1 In co mparing th e number of particles in a magnesium-iron a lloy with that fo r a mag nesium-.) per cent a luminum­iron, one fi nels that at o.oo r per cent iro n no pa rticles can be see n in the magn esium ­iron whereas a number ca n be see n in the magncsium-3 pe r cent a luminum-iro n. At their respecti ve iron tolerance limits, 0 .0 1 7

a nd o.oo 2 per cent iron, the two a ll oys ap­pea r to conta in approximately the same number of pa rticles, but the p~lr ti c l es in the magnesium-iron a rc so mewhat small er. One f u rth cr obse rvation of in tcrcst is th a t the pa rticles have a random di stribution and lend to seg regate in varyin g degree through I he samples. Although a determination with any degree o f acc uracy is diflicu lt , it is roughl y es timated th a t the particle diame­ter of th e iron co nstitu en t a t the tolera nce li mit is of th e order of o.oooo 1. in. , whi le the ave rage dista nce between particles is about o.ooo t to o.ooos in ch.

In copper-co ntaining magnesium , the compound lVfg2C u can be detected by etchin g in a solution of 74 per cent di­et hy lene glycol, 2 s per cent water a nd r per cent 1-11'\0,.. In a specimen conta ining o.os per cent copper the Mg2Cu co mpo und is almost entirely in the grain boundaries, whereas if o.to per cent copper or ove r is prese nt the Mg2C u co mpound is not on ly in th e g ra in boundaries but also a great number of particles appea r throughout the mat ri x.

For magnesium and its a lloys containing ni ckel, it is possible to use the glycol etch to reveal the ni ckel phase. Although only a li ttle work has bee n clone thus far , it is wo rth y of note that a magnesi um -nickel co mpound ca n be seen in th e grain bo und ­a ries a t nickel contents as low as o.ooo s per ce nt.

SOLUTION POTENTIAL AND HYDRO­CE:'\ OVERVOLTACE MEA SU RE.YIENTS

In order to a id in the test of the applica­bility to the corrosion results of any

accepted electrochemi cal th eo ries, so me measurements of solution potential and hydrogen ove rvoltagc are included. The measureme nts we re made in 3 per cent NaCI under the conditi ons used for the co rrosion tes ts, with the hope that some co rrela tion of results would be possible. Unfortun ately , th e electrochemical meas­urements a re not co mplete enough at the moment to cove r th e behavior of mag­nesium-base a lloys as a fun ction of varying im purity content , and therefore do not aid in an explanation of the existence of the sha rp di sco ntinuities or toleran ce limits. ll owcvc r, da ta arc prese nted to show why ce rtain clements a rc de trimental to the corrosion resis tan ce of magn esium .

M I•:TIIOD

f P1·epm·ntion of Sam ples

Magnesium -alloy elect rodes were ground to a size of o. s by o. s by 5 em. on a luminum oxide abrasive paper, the ftn al cut being made with No. 3 2 0 grit. The samples were not handled after grind ing, since grease delayed the attainment of a steady poten ­tial. The sha pe of other metal electrodes used in the tests depended somewhat on the samples availa ble. The surface prepara­tion was t he same as for magnesium alloys.

M easn1'C11lent of li.lcc/rode Potentials

The electrode potentia ls were measured with a Leeds a nd No rthru p special wide­range type K potentiometer, using a Weston unsatura ted cell as standard. T he potentials were measured in the cell metal : 3 per cent NaC I; saturated calomel electrode at 25 .o°C. Precautions were taken to prevent d iffusion at the liquid boundary, and frequent checks of the wo rking calomel electrode against a refer­ence gave differences never exceeding o.ooo2 volt. Each elect rode was observed for a period of at least 24 hr. In all cases a steady potentia l was achieved af ter 1 2 hr. Once a steady potential had been achieved, it remained constant in most cases to

J. D. HA AWALT, C. E. NELSON AND J. A. PELOUIIET

within ± o.oos volts as long as observed (maximum 48 hr.). A few electrodes varied by ± o.or volt in 48 hours.

M ea.sure·ment of 1/ ydrogen Overvoltages

Hydrogen overvoltage, defined as the negative potential that must be applied to an electrode to liberate hydrogen a t a specified current density, was measured by passing a current through the electrolytic cell: metal cathode: 3 per cent NaCl pla tinum anode and measuring the in­crea e in potential of the metal electrode above the zero current value, with respect to the satura ted calomel electrode. The ovcrvoltages used in the calcu lations were measu reel a t a cu rrcn t dcnsi ty of 5 ma. per sq . em. Since the relati ve values a rc essentia lly independent of current density, t his choice i somewhat a rbitrary.

R ESULTS

Some results that seem to be of particular interest a re given in T able 1.

EI.I·:CTRON DTFFR1\ CTIO'J P,\'I."''I·:RNS O F SU RFAC ES

'.l.'hc na ture of the protective Ji.lm fo rma­tion on the surf;tces of metals is recognized to be an important facto r in metallic corrosion. The possibility ex ists of obtain­ing some informatio n about the structure of such li lms directly by mea ns of electron diffraction if the ftlms happen to be in a su fli cienlly crystalline sta le.

The compositions studied included Ig, .Mg-6AI, ·Mg-1Mn, Mg-6A I-o.2Mn, Mg-4AI-.3Zn-o.2Mn . The specimens had pre­viously been ex posed to d ry air, d isti lled water, 3 per cent NaCI solu tion, a nd other treatment. J\ specia lly high dispersion and resolu tion was obtained in these patterns by using a distance from specimen to photographic pla te of 72.3 ± o.r em. and by con trolling the high potential supply of minus 40 kv. to within ±3 volts. The

electron-beam pinhole diaphragm was 0. 25

mm. a nd the pinhole to lens distance was 7 5 em. The surface of the specimen covered by the convergent electron beam was 0.7 by I.o centimeter.

TABLE I.- Val·ues of Solu.t-ion Potential and

H ydrogen Overvoltage Determined in 3 P er Cent Sod·ium C!tlor·ide

Composition"

M g ... . M g-o.oz Fc . . M~-JA I. . . ... M g-4sA I ( M g.,i\ L,) Mg-6JAI ( Mg, A !,) . AI. .......... . M~- 1 . 1 1n . Mn ... M g - 1. 2 Zn . .. M K-7J Zn ( M gZn) . 1 g-~4Zn ( MgZn ·,).

Zn .. ... ... . . M o<- s7Cu ( M g,Cu) .. '' tl-84Cu ( M gCu,).

Fe .... ... . . , ... . Fc-soAI (FcAb) . Co .. . . M g-ss K i ( M g, t\i) . N i ..... . . . ..... .... . . . . Mg- 2A~ ...... . ... . M ~;:- 8zAg ( M g Ag).

Solution Poten tial

I . 70 1.57 I . 7 (1

l . 24 I . 2 J

. a(>

. 70

.27

. 7J I . 27

I .09 I . 06 I . 0 1 0.9 1 0 . 73 0.7 2 o . so 0.92 0. I 5 I . 64 0 . 25

Hydrogen Over-

voltage

0 . 6.~ 0 . 0 2 0 . 66 o . s z 0 . 7 0 0 79 o . J S 0 . 39 0 . 2.)

o . 34 o . SJ o . 39 0 . I J 0 . . ) 4 o . s6 o . J s o. 7 1 0 . 0 5 I . 02 0 . 17 I . 19

" Magnesium it nd m:q~ncsiurn-basc a llo ys lis lcd arc o f h i ,~h purily.

RESULTS

T he results of the electron clifTracliun study were negative in tha t they failed to show a ny substantial d ifferen ce in the patte rns as a function of the composition of the a Lloy. I n dry air at room te mpera ture the result was a difTu ·e :MgO pattern . In aqueous media at room temperature the result was t he pattern of Mg(OHh, though with rela ti ve in tensities of the lines d iffering from those of bulk Mg(Oll )z, and with indications of slight varia tions in li ne positions. These intensity variations a re probably related to orienta tion effects, bul will require furth er study, as will a lso the other differences observed. The published literature on electron diffraction studies of magnesium structures a lso gives evidence for anomalous structures. ~ · 6

Visual observations of the protective film formed in aqueous media on mag­nesium alloys with aluminum in the range

CORROSION STUIYI ES 01'' MAGNE SIUM AND ITS ALLO YS

up to 10 per cent plainly show tha t t he ftlm is thinner t he higher the a luminum content of the alloy, and therefore strongly indicate tha t t he chemical or physical na ture of t he ftlm is difierent , depend ing o n t he percentage of aluminum in t he a lloy.

It has bee n ·taled in t he lilerature7 t hat the protective coat on manganese-con­tain i ng magnesium alloys is ma nganous hydroxide at least in su bsl~ln Lia l pa rl.

The electro n d iffraction results reported do mean t hat .\1g(Oll h or .:\ lg (O II h modified slightly, is a major part of the fil m, independent of the a lloy composit ion within the ra nge of composition cove red . T hey should not be in Lcrprclcd to mean tha t the ftlm action is not grca.Lly de ter­mined by the p resence of a lu mi nu m or manga nese compounds, since these may be a morphous or for o ther reasons m<LY not be detected by e lectron di ffraction.

DISCUSSIO. OF R ~SU LTS

Probably t he most extensive discussions in t he li terature on the corrosion behavior of magnesium a re by W. Schmidt a nd W. Schultze8 and by W. Schull7.c. 9 T hese a rc excellent articles and devote consid era ble space to the effects of meta llic impurities in magnesium . T hey do not , however, ma ke a study in suflicicnl deta il to bring out the critica l relationships presented in the present paper, so no very direct com­pa ri on of result s is possible.

It is commonly accepted 10- 12 tha t the corrosion of magnesium in salt solu tions is entirely electrochemical in nature, the sole cathodic rea ctio n being t he evolu tion of hydrogen gas, the anodic reaction consist­ing of t he passage of M g++ ions into solution wit h subsequent reaction with OH- ions to form Mg(OH) 2 or perhaps the direct forma tion of Mg(OHh on t he surface of the metal by t he discha rge of OH- ions. Constituents nobler tha n magnesium pres­ent as a second p hase in the magnesium should act as cathodes for local corrosion

cells . One might. then logica lly look for a n explanation for t he tolera nce limits in terms of the limi ts of solubility in t he a lloy for the constituent concerned . T his may possibly be the explana tion fo r t he tolerance limit of magnesium for copper al a bout o. r per cent. Mcta llographic exil mina tion shows that copper is solub le lo a small degree in magnesium .13 - 16 It is solu ble Lo about o .2 per cent a t 400°C . a nd the solubi lity d rops with tempera ture to something less tha n o .os per cent at room temperature. In the as-cast meta l, which is th e sta te used for t he corrosion test s, t here appears a precipitate of the copper phase in large quant it ies for copper con­tents of o. r per cent or greater.

Examining the ca se of iron, which in magnesium has a tole ra nce limit a t o.o17 per cent, it is found tha t t he li mit of solubility in molten magnesium a t t he freezing point is slightly under o.o2 per cen t. ·ll owever, in the solid state, metal­lograph ic exam inatio n shows i ron to be insolu ble dow n to a t least a few thou­sandths per cent. T he numbers of iron­phase particles (which occur within the grains) that can be cou nlecl in the micro­scope are proport ional to the iron content of the magnesium . Tlut s it is certain that tlte

solid solnhili:!y li 111 it alllllltc tolerance l imi t

rio not corrcs /Jiintl i n lilis case . Further studies may sho w whether the liquid mag­nesium solubil it y limit is of a ny significance in re la t ion to t he tolerance limi t.

For magnesium-a luminum alloys a nd magnesi u m-a lumi n um-ma.nga nese alloys, the solu bili ty of iron in t he liquid state is a pprccia ble a t high temperatures but is less than o.oo r per cent iron at t he freezing point. T he iron solubi li ty in solid m agne-iurn-alum in um o r magnesium-aluminum­

manganese is a lso very low. Metallographic examina tion shows the particles of the iron phase to be present a t as low as o.ooos per cent iron . T!terefore the tolerance limi t a.t

o.oo 2 fNr ceut iron does not correspoud to a.n y solu bility lim i f in these alloys.

J. D . HAN AWALT, C. E . NE LSON AND J. A. P E LOUBET 293

The iron pa rticles appear to be in ra ndo m distribution, the concentration of particles being higher at some places a nd lower at others. T his agrees wi t h t he results of chemical and spectrographic spot a nalyses, which also were a ble to sho w segregat ion of iron in t hese a lloys. It is of interest to note that on the a verage the number of pa rticles of iron p hase per unit o f surface observed in magnesium a t o .o q per cent iron or in magnesium-alumin um at o.oo2 per cent iron is a pproxima tely the same, while the number of iron-phase particles in magne­siu m a t o.oo 2 per cent iron appears to be only a very small fra ction of t he num ber in magnesium-alu minum at o.oo2 per cent iron . T he pa rticlcs arc very sma II and, at t he percentage corresponding to the tolcr­~lllCC limit , a rc separated at distances esti ­mated to be of t he o rder of ma gnitude of o .ooo1 in . Presum ably the composilion of the p articles in m;tgnesium is iron, while in magnesium-aluminum or m<tgnesiu m-a lu­minum-ma ngancsc t he composition is FeAb or some more complicated substa nce.

An explanation mig ht be found for t he great increase in corrosion rcsult i ng from only a small increase in the number of pa rticles beyond a certa in critical number in tha t up to this c ri t ica l number the corroding spots arc isolated and do not contact or expose new particles, but t hat beyond this number t he particles arc close e nough together so th:tt the corrosion ca used by t he pa rti clcs ex poses new pa rli­cles to t he corroding medium . T his t heory assumes tha t each pa rt icle causes a. dcftnite a mount of dissolutio n of magnesium but does not need to make a ny assumption about the mecha nism by which t he corro­sion due to the particle is s lopped after a defmite a mount of corrosion has taken place. For specimens with i run below t he tolera nce limit, t he fact mentioned pre­viously , t ha t there are random segregations or regions of higher concentration of pa rticles, accounts for the observation tlmt there may be localized a reas of visible

corrosion in such specimens. The iron a t such places was found by special analysis to be above t he tolera nce limit. T he con­ception of a. specimen with im purity above it s tolerance limi t is, then, that for such a specimen the regions of higher concentra­tion a re so numerous t ha t they touch each o t her and cover t he surface, or perhaps, said more simply, the impurity concentra­t ion is such tha t the tolera nce li mit is ex­ceeded a t every point. Specimens wi th average impuri ty below the tolerance limit may, a nd do, ha ve isolated a reas in which the average impurity content for these :trcas exceed the tolernnce limi t.

Fu rther hy potheses wo uld be necessary to predict t he absolu te mag ni tude of the tolera nce limi t for a ny particula r case. Since t he particles do not touch each other (as do the base a toms in the "critical resistance limits" of T a m ma nn) but rather, fo r example, in iron inclusions, appear to be o f the order of 10 to 100 times their diam­eter apart in the ncigh borhoou uf the critica l concentra tion , the ab olu te magni­tude of the critica l concentration depends upon the ex tent o f corrosion caused about each pa rt iclc. l'roba bly corrosion conti nucs until the particle is removed by under­mining itself. T his is the sim plest a nd most plausible theory and has conftrma tion in the experiments on metallography and spot. a nalysis of corroded a rcas.

J\s has been show n by t he measurements of solution potentia ls and hydrogen over­voltage, the particles of impuri ty serve as cathodic points, wh ich perm it the evolu­t ion of hydrogen gas. lt is possible that t here exists a critica l anod ic cu rrent density below which protecti ve f-i.l m forma t ion ta kes place but a bove which corrosion can continue. T he na ture of the fdm that forms, and the critica I value of the a nodic current density that will pe rmit the ftlm lo form , should both be functions of t he composit ion of the a lloy. T he magnitude of t he anodic current density is governed by the amount of hydrogen gas tha t can be evolved a t

2 94 CORROSION STUDIES OF MAGNESI UM AND ITS ALLOYS

cathodic areas. The la rge r the ra tio of ca lhodic area to a nod ic a rea, th e smaller the di stan ce between these a reas a nd the higher the conductivity of the electrolyte a nd of the surface ftlm , the greater will be th e a mount of hyd rogen evolved. i\s d is­cussed later, it should be a fun ction o[ the "d ri ving force" measured by the excess of the d ifTerence in solution po tenti a ls between a nod ic and cathodi c a reas over th e hydroge n overvoltage of the cath od ic a rea.

If co rrosion is stopped by fum formation, it may be necessary not on ly for the film to form ove r the a nodic area but to be able to grow over a nd shu t ofT or " stra ngle " th e ca th odic a reas. It is on ly when polar iza­tion a t th e cathodi c a rea i: acco mpli shed at negligible current that co rrosion ca n be said to ha ve ceased. Thus one may be con­cerned not only with a critical anodic current den sity that allow ftlm [ormation but with a cri tical size of cathode a reas, whi ch the a nod ic ftlm is capable o[ shutting off.

The following experiment can be inter­preted as evidence [o r the existence of a critical value o[ current density as discussed above. If a magnesium-a lloy specimen with a ll impurities below thei r to lerance limits is placed in salt solu t ion, there is a n evolu­t ion of hydrogen from the specimen while film formation is laking place . Visibl e gas evo lu t ion soo n slops a nd ftlm formation is presu mably co mp lete. If a new cathod ic a rea that can evolve hyd rogen is connected to the specimen, a hi gh current fl ows, but not only to the new cathode . Hydrogen also begins to evolve fro m the specimen again and will co ntinue to do so until t he external cathode is removed; that is, protective fLlm formation makes no head­way as long as the additional current is fl owing. As soon as the external cathode is removed, protective ftlm formation begins again a nd soon stops the hyd rogen evolu­tion from the local cathodic areas. This experiment serves to emph asize that the fi lm on the anodic areas d id not prevent

corrosion whiJe there ex isted a ny a reas on which hydrogen cou ld evolve. Boye r17 and \'yskocil' 8 have discu ssed the cause of the breakdown of the anodic film in terms of a "cata lytic" effect of chloride ions.

Considerably more work will be neces­sa ry to a iel in choosing between th e various specu lations on the explanatio n of the cha racteristics of th e to leran ce-limi t phe­nome na. T he simpler question of why some elements ac t in the direction of accelerating the co rrosion o[ magnesium in salt solution while others do not can be explained in terms of elect rochemica l properties. T he ligures in Table 2 have been derived fro m th e ex perimentally measured values given in Table ' by taking the difference bet wee n the solution pote ntial s of magnesium a nd of the material in question and then sub­t racti ng the hydrogen overvollage of t he mate ri al in questi on. T hi s is cl one on t he assum ption tha t thi s quantity is some measure of the dr iving force tending to evolve gaseous hydrogen at t he cathodic spot. For elements that have solid solu­bility in magnesium , the solu tion potential of th e phase that wo uld be precipitated shou ld be subtracted from the solution potential of the soli d solu t ion phase instead of fro m the solution polen ti al of magnesium.

T able 2 shows that for aluminum , ma n­ganese an d 7. in c there is either no resultant dri ving force or o nl y one of a bout o .I volt , while for iron , cobalt , copper a nd nickel th e d ri ving force is greater than 0-4 volt. Sil ver shows a n intermediate position at 0. 2 1 volt . Thus t he value of the q uantity P .D .-0. V. serves to co rrectly classify each metal observed accord ing to its corrosive action on magnesium. It is to be noted that there is no co rrelation of corrosion effect with hydrogen overvoltage.

It is stated by Gatty a nd Spooner19 t hat the hydrogen overvoltage of magnesium is very low, a nd a part of their discussion is based upon this cla im. 'Ltble 1 shows that t he overvoltage of pure magnesium is high but is greatly lowered by iron. F rom the

J. D. HANAWALT, C. E. NELSON AND J. A. PELOUJlET 295

a nalysis for magnesium given in their paper, it can be see n that t heir determined overvoltage for magnesium was in error because of the presence of iron in t heir specimen.

TABLE 2 .-D1~[ference ·in Sol ntion Potentials and OvervoUage V alnes for Vario ns T y pes

of Cells

A nod ic Materia l

Mg-3AI. M g-3AI. . .. Mg~ 1 .. 1 Mn . M g· I. 2Zn. M g- 1. 2Zn . M g .. .. Mg .. . . M g .... . Mg-3A I. M g .... .. M g .... . M g-2Ag .

Cat hodic Mate-

ria l

--M ~4Ab M ~,Ah M n MgZn M!{Zn, MI{:!C u MgC u 2 Fe Fe A!, Co M g:!N i Mg Ag

Differ-cncc in Solu-

t. io n Pot cn-tials

( P. O.) , Vol ts

---0 . .)2

u . SJ 0 . 4J O. t\6 0.04 0 . 69 0 . 79 0 . 97 I . 0 11 I . 2 0

0 .78 I . Jy

Hyd ro-gen

Over-voltage P.O.-of o.v .. Cathodic M ate- Volts

r ia l (O .V.) . Volts

--- --o . Sl 0 o . 70 - u . 17 O . J9 0 . 0 .1 O.J4 0 . 1 2

o. SJ 0 . I I

0. I J o . s6 0 .34 o . 45 o. s6 0 . 4 1 0. 35 0 . 69 0 . 71 0 -49 o . os 0. 7J I . 19 0 .2 0

T he results published by Boye r 17 show that whil e his magnesium specimens were pure enough for studies of mag nesium metal, they were not of sufftcient puri ty to use as a base for a lu minum -containing alloys , a nd this acco unts for ma ny of the di screpancies between hi · results and t ho ·e of the present pa pe r. Schu ltzc 9 has made valu able co ntributio ns to th e subject of th e co rrosion behav ior of magnesiu m a nd has give n carefu l co nside ra tion to t he effects of small a mounts o f impurities in mag­nesium. However, hi s p ubli cation s co ntain corrosion results th at arc presumed to be characterist ic of mag nesium and a lloys but t ha t obviously arc inllucnccd by im puriti es.

lt see ms fair to say t hat man y of t he results reported in the li terature on t he co rrosion o[ magnesium an d its a lloys a re not represe ntat ive of th e basic nature of the metal but a re determi ned by t he prese nce o[ un controlled or in suft·iciently con troll ed meta llic impurities, which we re regarded as negligible . It wou ld be well if

all published results were critically exam ­ined from this point of view.

PRACTICAL ASPECTS

While for purposes of establishing th e funda mental factors governing corrosion behavior of magnesium and its common alloys it is necessa ry to begin with very pure or sublimed magn esium and to keep every element nil except the ones un der investigation, this is not necessary for purposes of producin g good corrosion­resistant magn esium a lloys in practice. The prese nt high grade of ingot magn esium avai lable comm ercia lly in th e United States can be used as a base for such corrosion­resistant <t lloys simply by making use of the insolu bility of iron as previously de­scribed, giving due rega rd to selection of materia ls a nd exercising adequa te pre­cautions in ha nd ling.

It is in terest ing a nd im porta nt to note that in the past, for certain alloys, the e ffects of some impuriti es have been over­emphasized , whereas t he data presented show that these im purities may really be present in amounts in excess of t he usual specifications without det rim ental effect on co rrosion stability. F or example, t he co m­me rcial speciftcations on :Mg-6Al-M n-3Z n limi t copper to o.os per ce nt while o.s pe r ce nt would be a llowable as fa r as co rrosion is concern ed.

Alloys with impurities below the to ler­a nce limi ts have been p roduced on co m­mercia l scale in the form of sand castings, d ie casti ngs a nd wrought shapes. T he corro­sion ra tes on th ese a lloys a re less than o. 2

mg. per sq. em. per day. Tests o[ th e loss in tensil e properti es due to co rrosion have been ca rried out o n s ta nda rd lest bars of so me co mmon Mg-Al-Mn-Zn a lloys with the results as show n in .Fig. 15. T here a re only mino r losses in properties over a period of 1 80 days in salt solu tion for cast a lloys, though a lloys in the wrough t fo rm a re somewhat more sensitive .

CORROSION STUDIES O:F MAGNESIUM AND ITS ALLOYS

S UNIMARY O F R I~S lJLTS

1.. T he basic na lurc o[ magnesium a nd o f il. important a lloys is t hat they are very rc islant lo salt -water corrosion bul a rc

' STRENGTH ' 1000 PS I

~

t he hydrogen ovcrvollagc of the cathod ic materia l.

S· On l he basis of melallogra phic a nd other l ypes of exami nation, a n assumption

28 - MG-6AL· .2MN- 3ZN

MG_I9AL· .2MJ.7ZN

ULTIMATE -- I 24 ULTIMATE

20 - ---

16 f-. MG- 9AL-.2 MN- .7ZN YIELD

MG~6AL- .2M~- 3ZN Y11ELD

12 1-

I •;. ELONG I IN 2 IN .

8

MG- 6AL- .2MN- 3ZN E LONGATION 4

MG~9AL-.2JN-.7ZN IELONGA~ION

0 0 20 4 0 60 80 100 120 140 160 180

I DAYS I F1 <: . I 5.- CORROSION- T E NSI I.E PROPERT I E S O F CAST :MA.(: l\' t•:Si l "i\1 - A I. l 'J\1 1 NliM_- l\I AN GJ\t\E.SE - ZINC.

cxlrcmcly sensiti ve lo certain clcmcnls and com binalions of clcmcn ls.

2 . The speci rtc effect on the sall-waler corrosion resistance of magnesium clue lo the elemen ls a lum inum , manganese, zinc, iron , nickel, copper, silicon and lead singly and in a ll combinations has been determined.

3· The existence o[ " tole rance limits" or d iscontinuities in the ra te o f corrosion as a function of the percentage present of cer­tain elements has been demonstrated.

4 · It is shown that the element studied can be classified in t heir gross effect on t he corrosion resista nce of magnesium accord­ing to the excess of t he eli fierence in solution potentials of the magnesium or anodic material a nd of the cathod ic material over

of a critical co ncen lralion of cathodic particles is used lo expla in l he origin o[ t he corrosion d isconli nui lies.

6. i\ lloy composit ion l hal meet t he purity re(] ui remenls for good corrosion

stabili ty, and which rcpre cnt the range of com mercia ] usage, ha ve been prepared on

product ion scale in the form of ingots, sand cast ings, d ie cast ings anrl wrough t forms.

7· The sa lt-water corrosio n resistance of these alloy composit ions is [rom 10 to roo times greater than t ha t of com mon mag­nesium alloys. Strengt h tests on some high­

purity casting a lloys show negligible loss in properties after 6 mon t hs' alternate

immersion in 3 per cent N a CJ.

J. D. HANAWALT, C. E . NELSON AND J. A. PELOUBET 2 97

APPENDIX A.- Analysis Technique

In the a nalysis two pairs of solid metal electrodes II by 3·5 mm. were cut fro m the specimen to be a nalyzed. T he surfaces from which the discharge was to take place were polished on an Aloxite sanding wheel and carefully cleaned by allowi ng 3 N hyd rochloric acid to flow over the area for IO to I 5 sec . T he electrodes were t hen rinsed in dist ill e l water a nd dried on a clea n cloth . T hi s clean in g treatment was given the elect rodes immed iately before they were placed in t he electrode holder, in order to reduce cha nces of contaminati on to a minimum. T he elec trodes were placed in a water-cooled electrode holder with a gap of 5 mm . between th e electrodes. In a nalyzing magnesium and its a lloys for iron, nickel a nd lead at co ncent ration· below o. r pe r cent a nd silicon at concentra­t ions below o.or per cent , a 2.5-amp . d .c. a rc source was used. T he a rc was st ruck with a pure graphite rod. No lens was used between the sou rce a nd the spectrograph sli t, t he source bein g placed at a su fTtcien t distan ce fro m the sli t to make sure that all ligh t t ha t passed through th e sli t a lso passed through the spectrograph lens. The spect ra we re recorded on Eastman poly­chrome plates by means of a medium quartz spec trog raph .

In the de terminatio ns a bl ackening­logarithm-of -inte nsity cali bration pattern was placed o n each pla te in add ition to the spectra of the specimens. Calibrat ion was accomplished by photographin g a co n­t inuous light sou rce through a "Il a nse n step-d iaphragm acco rd in g to th e method of Thomso n and Duffendack. 2 The blacke n­ings (defined as the diffe rence betwee n the peak of the galvanometer deflection of a microphotometer for the spect ra.! line a nd the defl ection fo r t he transparent plate) o[ th e selected spect ra l lines and of the steps of the intensity calibration pattern were obtained by means of a nonrecording microphotometer. F rom the blackenings of the steps of this pattern , a cha rac teristic

curve for each plate was drawn and was then used to determine the logarithm of the relative intensity of the selected line pair. This procedu re, carried out for a series of specimens of kn own composition in which the element under analysis varied over the desired concentratio n ra nge, yielded a n analytical curve for the a nalysis of thi s element. T he analysis of a specimen of unknown co mposition was made, apply in g the logarithms of t he relative intensities of the selec ted spectral line pairs deter­mined in th e ma nner desc ribed above to the appropriate analy ti cal cu rve a nd by read­ing therefrom the pe rcentages of the ele­ments under test.

In the analysis of magnesium for alu ­minum , calcium , copper , mangan ese, sili­co n, zin c and cadmium , lead and tin a t co ncentrations greater than o.I per ce nt , t he procedure used was similar to that out lined above except t hat a conde nsed spark source was used in place o[ the direct current arc.

T he procuring of satisfacto ry standard specimens to be used in co nst ru cting t he a nalyt ical curves refe rred to above ofiered no diff1cu lty in the case o[ metal co ncentra ­ti ons of o . r pe r cent or g reater. The oblain ­i ng o[ sa t isf aclory sta nda rds in t he concentrat ion range below o. r per cent d id , howeve r, present a real problem . /\ II standard specimens used in the latter co n­cent rat ion ra nge we re made by adding kn ow n a moun ts of the various co nstitu ents to sublimed magnesium. These specimens were then carefull y a na ly;,cd by th e foll ow­ing chem ica l procedures:

T he thiocya nate and th e aa' dipy ridy l met hods were bot h used t.o aid in t he sta nd ardi zati on of the iron ana lysis. T he aa' dipyr idyl meth od was less critical to slight cha nges in th e a nalyt ical techni que and , sin ce bot h methods gave the same resu lt , it was the method used in t he analysis of most o[ th e standard samples.

J\t the beginning or this in vest igation no chemical method was developed that

CORROSION STUDIES OF MAGNESIUM AND ITS AJ. LOYS

wou ld yield a satisfactory a nalysis for nickel in magnesium in the order of o.ooi pe r cent. There were satisfactory methods, however, for the analysis of nickel in the range o.r to I.o pe r cen t concentra tion. To obtain the standard specimens, a lloys with nickel contents of o.r to r.o per cent were ftrst prepared. After satisfacto ry chemical a nalyses of these specimens were obtained , the specimens were d iluted by known a mounts with nickel-free magnesium . Since nickel is completely solu ble in magn esium in t his concentra tion mnge it was possible to prepa re satisfactory standards in this manner covering the concentration ran~:::e

of o.ooo25 to o.os per cent. A modif1ed furil dioxime method for this determination has s ince been developed . J<esults obtained by this chemical method :tnd by the spectro­chemical method based o n the standards prepared in the ma nner described above a rc in very good agreement. The average error of the spectrochemical a nalysis in the range of o.oo2 per cent iron and nickel based on triplicate determination is ± o.ooos per cent.

T he standard specimens for th e analysis of lead in magnesium were a nalyzed by the colorimetric dith iazone method.

The chemical silicon determinations in concentrations below o .os per cent were made by the molybdate colo rimetric procedure.

The elect roly tic, colo rimetric a nd iod ide m ethods were used fo r the chemica l determ inatio n of copper. The e lectroly tic method was most satisfactory at concen­trations of the order of o.r per cent and higher. Below this figure the colorimetric and iodide methods were more sensit ive and accurate.

REFE RENCES

T. W. R. Veazey: U. S. P atent 1377374. 2. K. B. Thomson a nd 0. S. Duffendack: Jnl. Op­

tical Soc. A mcr. (1933) 23, IO I- I04. 3 . L. Whitby: Jnl. Chem. and Ind. (I93 I) To, 83- 85 . 4· H. Vosskf1hler: Magnesiu m and its A lloys, 43·

B eck, 1939. 5· G. I. Finch and A. G. Quarrel: Proc. R oy. Soc.

( 1933) 141-A , 398.

6. G. D. Preston and L. L. Bircumshaw: fJIIil. A~fag. ( 1935) 20, 706.

7. W . 0. Kroening and S. l~. Pawlo w: K orrosion 1wd Metallschlllz ( 1934) 10, 254.

8. W. Schmidt and W. Schultze : Die Korrosion M ctallische r WcrkstofTe , 445- 4 77· l,eipzig, Hi rzel , 1938, 0. Bauer.

9. VV. Schultze : Magnesium and its Atloys. 272-312. B eck, 1939.

1 0. G. D. Bengough: Tra.n s. Tnst. Chern. Engrs. ( 1933) II, 176.

11 . T. P . H oar: Mel~tl hul. (Aug . 19J6) 177. 12. L. Whitby : Trans. Fara d ay Soc. ( 19J3) 29, 853,

13 18. 13. M. H ansen: Jnl. Inst. M etals ( 1927) 37, 93- 100. 14. N . J. Ste panow and j. J. K orni!off: Trudi

Nauchno Jssledovatclskego Inst ituta Legkhi Metallov ( 1932) H . 57.

15. J. W. Jenkin : Jnl. I nst. M etals (1927) 37, 93. 16. J. A. Gann: Trans. A. LM.E. (t929) 83, 309. J7. j. A. Boye r: Nat. Ad vis. Comm. fo r Aero. , Rcpt.

248 ( 1926). 18. Vys koci l: Call. Czech. Chcm. Comm. ( 1934)

6, [. 19. Catty ~nd Spooner: T he Electrod e Potential Be­

havio r of Corroding Metals in Aqueous Solu­tions. Oxford Univ. Press, 1938.

DISCUSSION

(P . 1". StroufJ fncsiding)

1'. T . STRO UP,* New Kensington, Pa.- This is a significant contribution in the field of corrosion studies of magnesium. I think it rather strikingly illustrates the extensive damage caused by new tr~tccs of iron and magnesium and the remarkable effect in counteracting this that is caused by manganese.

E . i\. ANOEllSON, t Palmerton, Pa.- This paper is so beautifu lly and comprehensively written as to be readi ly understandable without hard work, yet it encompasses a great deal o[ really good research.

The paper illustrates something that we do not :til think about enough ; that is, we do not really know the properties of an clement. vVc arc only beginning to grope nearer to th~tt

knowledge. We have been talking about mag­nesium as nonresistant to chloride, with all the qualifications these authors have put on that statement. Now we fmd that when we approach the elemental form it is a n entirely different product.

Workers in industria l research laboratories ought to pay more attention to this basic problem of trying to produce a metal that is really pure, and try to find out what it really is.

L. W. K E M:PF,t Cleveland, Ohio.- It would be interesting to know the actual concentra-

* Research Metallurgis t, Alumi num Company of America .

t Chief of Metal Sectio n. R esea rch Division, N ew Jersey Zinc Co.

t Metallu rgist, Alurninum Research Laboratories.

DISCUSSION 299

tions of iron and perhaps copper a nd nickel in the specimens.

J.D. H ANAWALT (author's rep ly) .- 1 should like to further emphasize Mr. Anderson's com­ments about the importance of knowing t he inherent characteristics of pure materials . Without knowing the drast ic effects of trace impurities on corrosion resistance in aqueous media, one is like ly to place magnesium in a class with calcium instead of in a class with a luminum, and, further, to assume that the solubility or the density or some other c ha r­acteristics of its naturally forming film is responsible for its behavior and that nothing can be done about it. i\ctually, its protective fi lm characteris t ics arc q uite adequate in the absence of cathode elec trodes wit h large driving poten tials with respect to mag nesium, and the inherent cha rac te ri stic of magnesium that is d ifferent from t hat of other corrosion- resis tant metals is its high anodic solu tio n poten tial. The problem then becomes one of contact corros ion.

In answer to Mr. Kempf's q uestion on the composition of t he ·pccimens , the data for which arc show n in Fig. •s, the concentrations of impurities in these specimens fa ll in t he ra nge of less than one thousandth iron, less t ha n one hund redth copper, a nd less than one thousandth nickel.

Since the present paper was submitted for publication, a n a rticle by /\. Beerwald has appeared in z,•itsrlirijt fiir iltttaltkundc f( ' 94 ' ) 33, 28] wh ich covers so me of the same subjec t matter as t he present pape r and requires d iscussion, part icula rly in connec tion wit h the conclusions of the two papers. The titl e of the paper is " The I nflucnce of I mpuritics on the Corrosion Behav ior of M agncsium and Tts Alloys with Manganese and Aluminum."

The pa pe r presents the analysis of cell

magnesium and of distilled magnesium , notes the extreme difference in salt-water stabi li ty between them and concludes that t he d ifference must be ctue to Fe or Cl. The specime ns were prepared from t he distilled magnesium as base, a nd the rate of hydrogen evolu t ion during the frrst few days exposure to 3 per cent ' hCI was measured. In agreement with the present authors, Beerwald found no detrimenta l e iTect s due to Cl. However, because of t he way in which his experiments were carried out, he states his conc lusion on the effect of iron so that it sounds in absolute cont rad iction to ours, whereas in reali ty, the two paper a re in com­plete agreement insofar as they :tre related. For this reason it seems desi rable to discuss Hecr­wald's paper at t his time. With respect to iron , Bccrwald stales in his concl usion that t he co r­rosion rate is ~ really increased hy a trace in t he order of thousandths per cent and increases with increasing iron content unt il t he per­centage of iro n reaches about o.o3 a nd t hat higher percentages of iron over t his amount arc only of small sign itica ncc. Bccnv:tld's obser­vations a rc limited to the early stages of coatin~

formation and his concl usions, therefore, refer to the ini t ia l rates of corrosion , whereas our rates represent a s teady-stale co nd ition .

Our hy pothesis, t hat the in itia l pe riod of corrosion is one in which cathod ic impurities arc being undermined and removed or arc being covered over hy spread nf t he anod ic fi lm, would predic t results similar lo those of

Hccrwald 's; tho ugh with the additiona l condi­

tion t hat, if impurities arc below the tolerance

lim it, a fi na l st ead y stale of low corrosion ra te

is reached after about e i ~ ht days. T hese resul ts arc given mon; fu lly in a recent publication h,·

R . I·:. i\'l cN ully a nd j . IJ. ll anawalt [Preprin t

S r- 12, I·: lec troc he m. Snc. (:\pril 1942) ].

TRANSACTIONS OF THE

AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS

Volum.e 147

l NS'TI_'TU'TE OF ME'TALS DIVISION

1942

P ,\I'EI~S .\t\D DI SCHSS I ONS PRESI•: XT I•: D BJo:F( JI~E ' I !I E 1> 1\' I SIO~ ,\T i\I J•: J·;T I N t :S III•: I.IJ .\T

I' IIIL.\01-:I.PIII.\ 1 O CTOBER 20 22, 194- 1 , .\ N D :\'E\\" \'OI(h: ,

FI•:BI(l f, \1!\' 9 12 , 1942

liBRA~Y £RSIIY MICHtGAN TECHNOLOG,CAL UNIV

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