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<* TECHNICAL NOTE No MET. 46

-W I • II \J t «_uJ

BRIHSH/Ü.S. RtSTRlCKO Ml Ho.?03t ROYAL AIRCRAFT ESTABLISHMENT

Farnborouöh. Hants.

FINAL NOTE ON THE RELATIVE SUITABILITIES OF D.T.D. 303 AND D.T.D. 327 RIVETS FOR

D.T.D. 118 MAGNESIUM ALLOY SHEET

by

S. ACKROYD, B.Sc.

H. C. COCKS, Ph.D., F.R.I.C. ^

ATTENTION IS CALLED TO THl PtNALTIFS ATTACHING

TO ANT iNFHINÜEMtNT OF THE OFFICIAL St. BEI ACT

THIS DOCUMENT 15 THE W*OF*.R»V QF HM OOVERNMFNT

* IT IS INTENDED IOR THE USE OF THE RECIPIENT ONLY, AND FOR COMMUNICATION TO SUCH OFFICERS UNDIR

HIM AS MAY REOUIRC TO 8E ACQUAINTED WITH THE CONTENTS Of TMt RfFORT IN THE COURSE Of TNtlH

DUTIES. 1HE OFFICERS EXERCISING THIS POWER Of COMMUNCATION WILL Bt HELD RESPONSIBLE THAT SUCH

INFORMATION IS IMPARTED WITH DUE CAUIION AND RESERVE

ANY PERSON OTHER THAN THE AUTHORISED MOLDER.UPON OBTAINING POSSESSION OF THIS DOCUMENT. BV

FINDING Oft OTHERWISE.SHOULD FORWARD H. TOGETHER WITH HIS NAME AND ADDRESS.IN A CLOSED ENVELOPE

THE SECRETARY MINISTRY OF SUPPLY.

THAMES HOUSE, MILLSANK, LONDON S.W.I.

LETTER POSTAGE NEED NOT Bt PREPAID!OTHER POSTAGE WILL BE REFUNDED.

ALL rtRSONS »F>E HEREBY WARNED THAI THE UNAUTHORISED RETENTION OR DESTRUCTION OF THIS

DOCUMENT IS AN OFFENCE AGAINST THE OFFICIAL SECRETS ACT I9II-I920.

^ •

A|RDGCl?M£"S0;V:S"V', MAC, Wa6Hl .

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RESTRICTED

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TECHNICAL NOTE No MET i6

— w i . * . _y i v_ u/

BRITISH/U.S. r\toT;> J,

ROYAL AIRCRAFT ESTABLISHMENT Farnborouöh. Hants.

FINAL NOTE ON THE RELATIVE SUITABILITIES OF D.T.D. 303 AND D.T.D. 327 RIVETS FOR

D.T.D. 118 MAGNESIUM ALLOY SHEET

bv

S. ACKROYD. B.Sc.

ind

H. C. COCKS, Ph.D.. F.R.I.C. ~

V* ['.' •• *UI H . • AM . • •• Av T

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NK'WMATON 'S MPARTEDWITH iit CAUTION ANC RESERVE

*-.* PtRS *• 3THER 'HAN THt »uTHOR.jtD HOLDE«.UPC*. OBTAININ M •

FINDING Oft ^THtHWl^.^OU-L1 ro«*ARD it, TOG» THtR «i'M Mlb N*Vt *',. V RESb.lN * Cl l IH

TO- THE SECRETARY. MINISTRY OF SUPPLY.

THAMES HOUSE . MIU.BAN* , LONDON S W.I.

LETTER PCX TA I v

A.. :•*• N- AKf HtHtb» **KNl THAI -••[. '.'. ' . •. • •. •••

*•, FFCNCE AGAINST TH) AL SEC «El A Ml '. .

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BRITISH »SSr^rTFO ?-iuali UNITED SIAM, •voliUCTED

SUITABLE

Class muter '321.8^.002.3 i «9.71p(U) i 669.721. 'A&) D.X.D.UB

Technical Kot» No. UeX.u6

Aiuuat, 19u£

ROIüL AL.SaAtT E5T» •T.Tsn>.rijfr| v^,-n.tCi\ :•-

i'inal Note on the illative üuitabilities of D.T.D.303 and D.T.D.327 Rivets for

D.T.D.118 wignojium Alloy Sheet

by 3. AcKroyi, B.'So.

and H.C Cooks, Ph.D., F.R.I.C.

H.A.E. Ref 1 Mat U5/o7itOA/ä*A71 KoS Ref s Rea,kat.l266/R.E.Mat (l»i)l

SUMMARY.

The rate of contact oorrosion between D.T.D.118 magnesium alloy sheet and D. i.D. 327 aluminium alloy rivets was found to be. greater thc-n that between D.T.D.116 sheet and. D.T.D. 503 aluminium alloy rivets v;hjn subjected to a sea-water spray test. This -«as so with "both stressed am. unstressed spooimens; both in conjunction with various protective paint ing schemes, and without protective paint.

Visual examination w\a confirmed by strength tests on the corroded specimens and "by measurement of the depth of corrosion on the magnesium alloy sheet.

The greater rate of corrobion with the D.T.D.327 riveta is consider, to be due to factors such as:-

(a) lower hydrogen overvoltago with D.T.D.327 rivets than with D.T.D. 303 rivets

(b) a greater difference of potential between D.T.D.327 and D.T.D.113 than between D.T.D.303 and D.T.D.118.

It is concluded that D.T.D.32? riveta are unsuitable for replacing D.T.D.303 rivets where t'uoy are used fox joining D.T.D.118 sheet.

LIST OP OOtlTSUTS

1. Introduction 2* Preparation and Corrosion oi' Specimens 3, Teats and Observations

3.1 Visual Examination 3.2 Strength Test» 3.3 Measurement of Corrosion 3.1(. Microscopical Examination of Riveta 3.5 Determination of Corrobion Current, Klectrode Potential

and Overvoltage 1*. Conclusions

References Circulation

3 3 3 '

:-

•

i r

7-.cK.Hot. i.o. U.t.i~>

X.IJ'1 Qy 'iVi-LES

Details of i-rotective Schemes aivlIXxd.gna.tion 01' JiJtcim_nö

heaults of Strength Teats on Coi-nxled Specimens

Current aid Potential Measurements on Magnesium Alloy Sheet in contact with Aluminiuv. Alloy Rivets

7

I

El

HI

LIST Or1 ILLUogHATIOMS

Appearance 01' Unatres.ed Single Riveted Joints after 1 eiiy, aubjooted to Seawatev ;ipray TeHt for 25 days.

Appearance of Unutressed Single «iveted Joint:; after being subjected to äuawater apruy Testa for 8 weeks • •

Appearance of Stre^ei single Riveted Joints after beinc subjected to Jeawater Spray Testa for ii weeks

Appearance of Unstressed Single How Lap Joints after 'being subjected to üeawvter Spray Tust for 8 weeks

Fig.

-

.

;••*•

taoh.**« *<o. Uet,ir6

1 Introduction After iXposure to high tr*>pical teiap«ratures, rivets to specilio.vtior. U.T.E.303» wbich-are made from -n aluminium alloy containing % magnesium, are Habit to f di vy stress corrosion cracking. It «as ausgested that those rivets which have Y.:en used for joining magnesium alloy sheet to specification D.T.D.11Ö, as viull as for joining aluminium \lloys, should bo replaced by rivetj rayde in a low copper dur.ui:..in typo alloy to D.T.0.3^7. It -«as suspected, however, that the contact corrosion between D.T.D.327 and D.T.D.118 would b^ greater ttian that between D.T.D.303 and D.T.D.118,

and tests were made to obtain evidence aa to whether this suspicion was

justified. 2 tTot-aration anä CorroAon oi' äeecirau-u-

Each of 120 of the 132 apeoiuKno usea in the investigation consisted Of a pair of 3-4" * V * 16 S.Y.G. D. T.D.11Ö magnesium .aioy strips riveted end to Jtvl with either a D.T.D.303 or ü.T.D.327 rivet, and having a |" overlap. In all case.- the htaijnesinn alloy sheet «es chromato treated to .Specification D.I.D.911 in ^at'u (iii) (hot half hour bath), and one of the

follovdn.-; materials was uswi as a jointing coni>ounü. D.T.D.279B temporary metal protv-ctiv... (Lanolin pi.juented with

zinc chrome^

o.

Tn.569A "»^" ^TV^ (Loivoili^-l-^

iuia chroMftto.; zixi0

/*•» "solac" UBt- ^Sj \

^nations of the ^a thr

Thu corrosion test £-,. £ ^ m^fggSL by daily with -Xv^rbotw-n fc^jTight «*. «- !*£«» during the tost var ^ a porxoa six aerie8 or jj, 1 Stressing 3,Xty^P » nw^-^'J^ &e other end,^^ of the

linka «^o^eries *• ^'V I*** ^£tTfor oh **>" °£ "" One end °* °^ghted lever. £ ^ j00 lb. f« Vy means oi **""*£ tno join*. ttllr «msisting o1 " S* *•* •art"' .... for observatio»Sii-^6

0|^G.riv. ted ^Ove larger test £%$£ ^eta 9» * *,££ edg* w«re f£

«ith every 1.^•;;l ^th on the ri

prom the hogM»- " ^ vith the u^j w thc sne». -^ ^

rivet, •*1B . t 'Ehi" 1^.1-w- D.T.D.303 ".v.to. ^

Teoh.«c*e I<o. Uet.<*6

Fig. 1 shows a representative l'rora ;iv rou; o!" tlv-. sin.-le riveted unstressed specimens after 2j> day» corrosion.

Fig.2 shows the (visually) most oorroded and least corrodci of each group of five single riveted unstressed specimens after ej ht i kfl corrosion.

Fig. 3 ahoi-.s the same as Pig.2, 1 ut fOr tile stressed specimen?. (The marking at each end of those specimens are duo to paint I'rom the magnesium alloy links, and not to corrosion.)

Fig. 4 shows four oi" the large painted specimens after eight weeks corrosion.

3.2 Strength tests

Afto-r eight weeks corrosion all the single riveted joints were test 1 to destruction in tension (i.e. with the rivet in shear) wd the. results tabulated in Table II. Three failur. a were due to the rivet shearing, 2 to the sheet breaking at the pins and 115 to the sheet failing near t! . rivet.

An analysis of variance was made to determine if observed stronj tl differences between .single factors were significant« As was 3usp<. • , the D.T.D.303 riveted joints were found to be stronger than the D.T.D. 327 riveted joint3, the difference in strength beirw highly .aigtuficunt. It was also found that the difference in strength between painted ai»; un- pointed specimens was bighjrsignificant, the painted ones being the stron • and that the stressed specimens were significantly stronger than the un- stressed one3. There was no significant difference in the strength of specimens treated with different jointing compounds. That the stn ssed s^eoimens should be significantly stronger than the unstressed on^.; seems to indicate that the stressing caused v.-rk hardening of the magnesium alloy sheet.

3.3 faeasureriient of Corrosion

To verify the strength results, the maximum depth of corrosion on the magnesium alloy sheet was measured on the following specinie-ns:-

Spocimena Jäxoioined

CD 22, 23, 24 GH 22, 23, 24

CD 32, 34 iF21 GH 33, 3k *F27

CD 8, 9, 10 GH 6, 8, 9 AB 17, 16, 20 SP 1?, 18, 20

iiivet

D.T.D.303) D. I.D. 327)

D.T.D.303) D.T.D.327)

D.T.D.303) D.T.D.327) D.T.D.303) D.-x'.D.327)

Point

Unpointed

lnintud

The fractured specimens were cut longitudinally down the middle- of the strip, the- rivet reiuoved and the out face of each strip i^'H«!»'I .-.»1 projected on to the screen of a Vickers Projection liiorosoope. The maximum depth of corrosion of each strip was measured. This depth was found to be statistically significantly greater for the D.T.D.327 than for the D.T.D.303 riveted specimens, the average depths being 29$ and 12^ of the unoorroded thickness respectively.

3,4 Miorosoopical Extimination of Rivets

The rivet from eaoh of- the following sjponLmens was eot-mine-d .

toeh.Mota lo. kct.46

nioroocopicvLly to identify the type of corrosion«

Uicro-Speciosns

CD 22, 23, 24, 5 GH 22, 23, 24

CD 32, 24 EF21 GH 33. 34 EP27 EP 29

Rivet

D.T.D.303) D.T.D.327)

D.T.D.303) D.T.D.327) D.T.D.327

Stress

Unstressed

Stressed

Stressed

ialnt

Unpainted

Painted

All the rivets showed some signs of picting corrosion, the D.T.D.327 rivets slightly more so than the D.T.D.303 rivets, but in neither type was there any susceptibility to intercrystalline corrosion, altiAugh there was some corrosion penetration at isolated places where the rivet head had been cold worked at formed and preformed heads. At many of the- corners between head and shank large craoka were found, but these were attributed to either the riveting or strength testing. CD5 and CD22 wort, two of the three rivets which sheared during the strength tests and SP29 was the specimen which had the lowest strength. None of these three showed corrosion characteristics different from the otüer rive;. (ihe low strength value of JS'29 and possibly one or two others was attributed to a possible crack in the sheet due to the specimen being bent during riveting.)

3«5 Determination of Corrosion Current, aleotrode Potential and Over- voltage

A circuit was arranged whereby the current flowing between a D.T.D.118 sheet and a rivet, placed in seawater, could be measured. ••or this test all protective coatings were removed from the rivet and sheet, which were then placed about I3 inches apart in seawater. The current flowing was measured over a period of hours by means of a record- ing milliammoter placed in the external circuit, the latter being of low resistance (0.055 ohms). Shu large difference of current obtained using the two kinds of rivets (see Table III) caused determinations of eleotrode potential and overvoltage of the rivets to be made.

The uleotrode potential in seawater of each type of rivet and of the sheet was measured by means of a valve potentiometer, the potentials being measured against a saturated calomel electrode with free access of air at 20°C. The difference of potential of the D.T.D.327-D.T.D.118 pair was found to be slightly largerthan that of the D.T.D.303-D.T.D. 118 pair (see Table Hi) but this only partly explains the large difference of corrosion current.

Hydrogen overvoltage is a factor which opposes the flew of corrosion current« Hence, if the overvoltage with the D.T.D.327 rivets was nox aa high as that with the D.T.D. 303 rivets, this would also be a factor in explaining the much higher oorrosion ourrent vdth the D.T.D.327 rivets.

The measurements of hydrogen overvoltage were carried out (at inter- vals during 24 hours) by the "direct" Method, i.e. with the polarising current flowing continuously and maintained at a constant value. Aotual rivets and magnesium alloy sheet with protective costings removed were used as electrodes. The seawater in whioh the metals wore immersed had been previously saturated with magnesium hydroxide by electrolysis using a. platinum cathode and magnesium anode. The pM of the seawater \/as measured by means of the glass eleotrode before and after overvoltage measurements. The overvoltagea were obtained by subtracting the

r

Vech.Kotü iio. . '.....

potentials of the reversible hydrogen electrode in the same solution from the measured c&thodic potentials« while the hydrogen electrode potentials were obtained from the Observed pH values (see Table III).

Aa was suspected the overvoltage with the D.T.D. 327 rlveta was found to be lower than with the D.T.D.303 rivets. It was also observed that with the D.T.D.303 rivet the overvoltage rises with time while with the D.T.D.327 rivet it falls with time. In addition, the overvoltage at the higher current dimity with the D.T.D. 327 rivets is, after 2U hours, not only lower than with the D.T.D.303 rivet, but also lower than with the D.T.D. 327 rivet at the lower current density. This suggests that the effect of overvoltage is cumulative.

k Conclusions

The rate of corrosion in seawater between D.T.D.118 sheet and D.T.D.327 rivets was found to be greater than that between D.T.D.116 sheet and D.T.D.303 rivets, the corrosion current with the D.T.D.327 rivet being about 30 times that with the D.T.D.303 rivet ..hen in contact with the same magnesium alloy sheet.

Experimental evidence has been obtained showing that this is duo toi-

(a) Lower hydrogen overvoltage with the D.T.E. 327 rivt.ts than -..ith the D.T.D.303 rivets.

(b) Greater difference of potential between D.T.D.327 rivota and D.T.D.118 sheet than between D.T.D.303 rivets and D.T.D.118 sheet.

Factor (a) appears to be more important than factor (b).

Microscopical examination revealed no tendency to intorcrystalline corrosion in either rivet, althoutfihad the D.T.D.303 rivets boon previously subjected to tropical temperatures this type of corrosion might have occurred.

It is concluded that D.T.D.327 rivets are unsuitable for replacing D.T.D.303 ri.vet3 where they arj used for joining D.T.D.118 aheet.

RHTERENCK

R-.-f.Ko. Author

1 Metoalfo, G.J.

Title, etc.

Tho effect of tropi-al temperature on the miorostructur«. and corrosion resistance oC y/i mufrvisium aluminium alloy rivets to Specification D.T.D. 303. R.A.K. Report Ho. M.7925A, June, 1%!).

Attached: Tilles I - III Vigs.l - if = Ncgs. M.87WA.

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TAK..-' IH Current ani rotential Measurement? on ii-iitnesium Alloy öhefct

j.1. cor.t'-ft -if: Aiu£J.:;iui,. .—loy ..ivet.

(a) H»aaurement of Jor.-oai.on Current

Ihe current flowing between a D.T.D.327 rivet and D.T.D.118 sheet wh< placed about la" apart in seawater and connected externally \r\a about 30 times as grout as the current flowing between a D.T.D.303 rivet \nd a D.T.D.11B sheet under similar conditions.

(b) measurement of Potential Differences

The potentials, measured in soawater with free access of air at 20 C., against a saturated calomel uleotrode were:-"

D.T.D.303 rivet D.T.D.327 rivet D.T.D.118 sheet

Potential -0.77 volt -0.6/f volt -1.65 volt

Hence P.D. between D.T.D.303 rivet and D.T.D.llfl 3heet • O.oC volts P.D. between D.T.D.327 rivet and D.T.D.118 sheet n 1.01 volts.

(o) Mearuraimnt of OvervoltuKea

Kivet D.T.D.303 D.T.D.327

Experiment

Current Density at Rivet (tilillirmp s/sq.. at)

' ,0

pH at beginning of experiment 9.70 9.70 9.70 | 9.80 ' 9.70

pH at end of experiment 9.52' 9.54 I 9.52'. 9.53 i 9.52 _ 1 ; 1 \

Overvoltage at beginning of experiment (voltö) 0.82 0.835' 0.80 0.80 , 0.93

0v«rvolt-\ge at end of experiment (volta)

0.92 0.925iO,63i0.i.35 O.Sk

.

1 •Man

•

FIG. 3

I /3-

"KEaTTKIWl CD

RESTRICTED TITLE: Final Note on the Relative Suitabilities oJ D.T.D. 303 and D.T.D. 327 Rivets for

D.T.D. 118 Magnesium Alloy Sheet AUTHORJS) : Ackroyd, S.; Cocks, H. C. ORIS. AGENCY : Royal Aircraft Establishment, Farnborough, Hants PUBLISHED BY : (Same)

ATI- 9031

(None)

MET-46 nJSUSMINO AMMCT MO.

(Same) DATS

Aug ; 46 DOC CUSS.

Restr. COUMTHT

Gt. Brit. UUtOUAOl

English PACtS IUUSTBAHONS

14 I photos, table ABSTRACT:

Rate of contact corrosion between DTD 118 magnesium alloy sheet and DTD 327 aluminum alloy rivets was found to be greater than that between DTD 118 sheet and DTD 303 aluminum alloy rivets when subjected to sea-water spray test. This was so with both stressed and un- stressed specimens both in conjunction with various protective painting schemes and without protective paint. Visual examination was confirmed by strength tests on corroded specimens and by measurement of depth of corrosion on magnesium alloy sheet. DTD 327 rivets are unsuitable for replacing DTD 303 rivets when used for joining DTD 118 sheet.

DISTRIBUTION: Copies of this report obtainable from CAPO. DIVISION: Materials (8) SECTION: Magnesium and Alloys (11)

ATI SHEET NO.: R-8-U-27

SUBJECT HEADINGS: Magnesium alloys - Corrosion (58421.08); Aluminum alloys - Corrosion (10S77.7)

Central Air Documonts Offlco I Wright-Potlonon Air Forco Baso, Dayton, Ohio

AIR TECHNICAL INDEX

nrrTnif-rrr*

TITLE: Final Note on the Relative Suitabilities of D.T.D. 303 and D.T.D. 327 Rivets for D.T.D. 118 Magnesium Alloy Sheet ,

AUTHOR(S) : Ackroyd, S.; Cocks, H. C. A ORIS. ASENCY : Royal Aircraft Establishment, Farnborough, Hants \, PUBLISHED BY : (Same) •»

ATI- 9031

(None)

MET-46 PUQUSHIKO AGENCY NO.

(Same)

Aug ' 46 Restr. COUNTOT

Gt. Brit. English PAOCS IIUKTSATIOMS

14 I photos, table ABSTRACT:

Hate of contact corrosion between DTD 118 magnesium alloy sheet and DTD 327 aluminum alloy rivets was found to be greater than that between DTD 118 sheet and DTD 303 aluminum alloy rivets when subjected to sea-water spray test. This was so with both stressed and un- stressed specimens both in conjunction with various protective painting schemes and without protective paint. Visual examination was confirmed by strength tests on corroded specimens and by measurement of depth of corrosion on magnesium alloy sheet. DTD 327 rivets are unsuitable for replacing DTD 303 rivets when usedjqf Jobjing DWPAt*££?et.

P(S) »M <&& DISTRIBUTION: Copies of this report obtainable from CAPO. DIVISION: Materials fa) $t, SECTION: Magnesium and Alloys (V) 7

ATI SHEET NO.: R-8-11-27

SUBJECT HEADINGS: Magnesium alloys - Corrosion (58421.08); Aluminum alloys..- Corrosion (10577.7)

Control Air Oacumonfi Office I Wright-Patterson Air Force- Baso, Dayton, Ohio

AIR TECHNICAL INDEX

Ke. mr— Q