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ATMOSPHE RIC CORRQSION OFTIN A N D TIN ALLOYbY

M. E. Warwick

INTERNATIONALTIN RESEARCH INSTITUTEFraser Road, Perivale, Greenford, Middlesex

Tel: 01-997 4254( I .TA. . Publication No.602)

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CONTENTS PageIntroduction 3

1 TIN1 ? Allotropic transformation1.2 Tin whiskers1.3 Oxidation of tin1.41.5 Atmospheric corrosion of tin1.61.7 Surface treatments for tin

Reaction of tin with other gasesLong-term atmospheric corrosion of tin

2 P E W T E R 73 TIN-LEAD ALLOYS 74 TIN-COPPER ALLOYS 8

4.1 Oxidation of tin-copper alloys 84.2 Atmospheric corroSion of tin-copper alloys 94.3 Long-term atmospheric corrosion of tin-copper

alloys 105 TIN-ZINC ALLOYS 106 TIN-CADMIUM ALLOYS 117 TIN-NICKEL ALLOYS 11

7.1 Passivation of tin-nickel alloy 127.2 Atmospheric corrosion of tin-nickel alloys 12

8 TIN-COBALT ALLOYS 129 TIN-SILVER ALLOYS 13Acknowledgements 13References 13

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IntroductionTin has found an extremely wide range of applications of whichthe tw o most important are tinplate, steel wi th a thin coating oftin, and soft solder which is most commonly an alloy with lead.There are also a large number of other uses for which theatmospheric corrosion resistance of tin and tin alloys plays animportant role. These range from pewter, an alloy wi th a veryhigh tin content, to bronzes and include tin and alloy coatings ofboth ferrous and non-ferrous substrates. The general corrosionproperties of tin and tin alloys have been summarised in tworecent publications. '-*This review will concentrate on those alloys of tin whereatmospheric corrosion is relevant to the normal use of thematerial and where tin is present a t a significant level. Thus,white metal bearings are not included as they are not exposedto the atmosphere but to an oil film while aluminium-tinbearings, zircalloys, titanium alloys, dental amalgams andsuperconducting nobium-tin alloy wil l not be considered, as tinforms the minor constituent.

1 TINThe comments applied to the corrosion of bulk tin can usuallybe equally directed to tin when used as a coating if oneoverlooks any local corrosion at pore sites in the coating.Indeed some of the data referred to below were derived fromstudies of tin coatings. In a l l cases given, the information will beconcerned with performance of P-tin, which is the normalallotropic state in which the metal is used. However, since thecorrosion of tin in the atmosphere is occasionally confused wi ththe allotropic transition to cc-tin or wi th the growth of whiskers,some reference to factors governing the appearance of thesephenomena are given first.

1.lAllotropic TransformationThe transition temperature for the oc-p transformation of tin is13.2OC but, in practice, the p form is found to be stable evenbelow OC unless specific factors trigger the change. Aprerequisite for the transition to occur is that the tin is ofexceptional p ~ r i t y . ~t can be induced by inoculation of a cooledspecimen with crystals of oc-tin or by recrystallisation occurringat low temperature following deformation. Small quantities ofimpurity elements, such as those present in grades ofcommercial purity tin are sufficient to prevent a spontaneousallotropic transition OF to delay an induced change. Deliberateadditions to either bulk or electroplated coatings of elementssuch as bismuth (0.1%)or antimony (0.3%) an be made toprevent the transformation. The effects on the transformationtemperature of crystal orientation and impurities were studiedby Tammann and Dreyer!Tin coatings produced by hot-dipping seem to be virtuallyimmune from allotropic change, probably as a result ofimpurities present in the t in or because it is completely stress-free. The same is true of most electrodeposited coatingsalthough in the case of tin plated from an alkaline bath,codeposition of other materials may be negligible and thickercoatings may be susceptible to change.The transformation of p-tin tocc-tin is accompanied by volumechange since the densities of the t wo forms are 7.29g cm-' and5.779 cm-3 re~pe ctive ly.~he cc-tin forms in isolated areas asblue-grey "warts" on the white, metallic Etin and it crumblesaway as a coarse grey powder when disturbed. The damagedspecimen then appears as in Figure 1 and it can be readily seenhow the name "tin-pest" arose from this form of metal loss.The mechanism and kinetics of the process were studied byBurgers and Groen.6

Fig. 1Gray tin formation on pun, tin. Both samples were stored at -20% butthe top sample was bent at this tempemtun, end the other was leftundistul6ed.

It must be emphasised that genuine cases of failure because ofan allotropic transformation appear to be rare and the results ofatmospheric corrosion or other forms of attack have beenincorrectly described as tin pest.1.2 Tin WhiskersThe phenomenon of whisker gr owt h on pure tin coatings s onlymentioned insofar as it could be mistaken for atmosphericcorrosion in the true sense of the word. Like a number ofmetals, tin can undergo a recrystallisation at room temperaturewhich manifests itself as the growth of single crystal filamentsor whiskers, from the surface of tin coatings. The growthsappear to occur to relieve stresses in the coating includingthose induced by excessive codeposition of plating bathadditives,' migration through the coating of atoms from thesubstrate (e.g. zinc) and externally applied pressures.' A numberof studies have been carried out into the mechanism of theprocess. 9-10The potential problem of whisker growth need never materializesince a number of steps can be taken to give virtual immunity ofcoatings against their appearance. Thermal stress-relief is aneffective process and alloying, particularly with lead, is widelyused in the electronics industry where the problem could havethe most serious consequences?In those cases where whisker growth does occur, itcan vary inappearance from long (up to 1 cm) thin filaments to a mass ofvery small filaments. It is in this second form that a superficialexamination of the material may suggest the presence of asurface corrosion product but observation under a low powerviewing lens quickly reveals the true nature of the surfacechange.

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1.3 Oxidation of TinUnder most atmospheric exposure conditions, the corrosion oftin depends on the formation and stability of an oxide film andso it is pertinent to refer to laboratory studies of it s productionand chemistry.The most complete study of the interaction between tin andoxygen was reported by Boggs et. al. in a series of papers.They followed the oxidation of tin using a vacuum microbalancea t temperatures in the range 150-220C and a t oxygenpressures between 10-3mm o 500 mm Hg. Below an oxygenpressure of 1 mm Hg, dendritic a-SnO crystallites grew at acontinuously increasing rate over the duration of theexperiment with the rate-determining step apparently being thedissociation of oxygen. Above an oxygen pressure of 1 mm Hg,the oxide growth curves had a characteristic sigmoid shape.The initial stages of growth corresponded to the lateral spreadof oxide from numerous nuclei to form platelets of a-SnO.Subsequent growth followed a logarithmic law and wasconsistent with control by the diffusion of tin through an oxidefilm under a parabolic or cubic law while the formation ofcavities in the oxide film progressively reduced the area throughwhich diffusion ,could take place. Finally, for long oxidationtimes, the thick oxide film was subject to random fracture,giving rise to erratic results. Similar work was carried out byLuner l5 a t about the same time, using tin contaminated with0.17% Pb and 0.024% Sb over the temperature range 168 -21 1.5OC. His results could be described by a logarithmic ratelaw and were in broad agreement wit h those of Boggs et . al.A more systematic study of the effect of impurities on the rateof oxidation of t in was again carried out by Boggs et. usinga vacuum microbalance under similar conditions of pressureand temperature as their previous studies. Their results aresummarised in Table 1 and can be rationalised as follows. If theoxide of the alloying element i s less thermodynamically stablethan tin (11) oxide, the oxidation rate of the alloy remainsunchanged for additions whose ions have the same valency asthe tin. However, in those cases where the formal ionic chargeof the addit ion element is greater (Sb, Bi, Fe, TI) then the rate ofoxidation of tin increases. Those alloying elements forming onoxide more stable than tin ( 1 1 ) oxide undergo preferentialoxidation at the surface, thus inhibiting the oxidation of tin.Included in this group are alloying additions of zinc, indium,phosphorus and germanium.

Table 1The effect of alloying additions (0.1 atomic %) on the rate ofoxidation of tin at 190C and 10 torr oxygen pressure( Da t a abstracted from ref. 16).

Alloying elementManganeseAntimonyThalliumBismuthIronLeadN o alloying additionCadmiumPhosphorus (0.5%)Zinclndiurn

Increase in weightafter 1000minutespg cm-22.72.52.11.71.61.31 .o1 .o0.30.20.1

10 20 30 40Time, days

Fig 2The effect of relative humidity on the rate of oxidation of tin. ( 0 )Samples stored in a cupboard. (0 )Samples stored in a desiccator.

Other laboratory studies of the oxidation of tin have beenconcerned with the metal in contact with air. Kutzelnigg 17-18demonstrated the formation of an oxide film in air andMacnaughton and Hedgesi9 produced weight-incrementcurves. Kenworthy followed the weight gain of tin duringindoor exposure and decided that growth of the oxide film waslinear after the first few days and it was of the non-protectivetype. Britton and Bright2, then Britton and Sherlock,22studiedthe oxidation of tin and tinplate and followed the growth offilms by coulometric and X-ray analysis. They decided that thegrowth of the oxide film followed a logarithmic rate law up to13O0C, but above that temperature, growth tended to beparabolic. It was not possible unequivocally to identify the oxidespecies present but the results could be explained on the basisthat a t room temperature an amorphous film is formed, while a thigher temperatures it is apparently crystalline aSnO, possiblywith some SnO,.Jenk in~,~ound t h a t SnO films were formed on tin just aboveits mel ting point but that SnO, was formed at highertemperatures. Hart24 emonstrated the effect by spot-heating apiece of foil. He found SnO, a t the centre surrounded by SnOwhich in turn was surrounded by amorphous oxide. In fact, SnOis unstable wi th respect to SnO, but Platteeuw and MeyerZ5have demonstrated that the disproportionation of SnO to tinand SnO, is a slow process even a t 3OOOC. Again, a recentE.S.C.A. study has shown 26 that tin exposed to oxygen a t100C produces an oxide film of both SnO and SnO,. Closeinterpretation and comparison of results achieved in the studyof the oxidation of tin in the laboratory should not beoveremphasised since it has been found that the surfacepreparation of tin plays an important role in determiningoxidation rates. Davis and Shah showed that cathodiccleaning reduced the subsequent rate of oxidation of tin. Inaddition, the same authors demonstrated that the relativehumidity under which experiments are carried out can have aprofound effect on oxidation rates as shown in Fig. 2. Otherworkers showed that minor impurities could affect the rate ofreaction of tin exposed to air. Bri tton and Bright2 confirmed theresults of Boggs et. al., whose data were collected in a closedsystem, when they found that small amounts of indium,phosphorus or zinc slowed down the oxidation of tin. However,0.1% aluminium or 0.1% magnesium have been shown toreduce the resistance to attack of tin in moist conditions.Rawdon2* found that traces of alumin ium can causeembiittlement as a result of intercrystalline attack. This effectmay, however, be counteracted by an addition of antimony.

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1.4 Reaction of Tin with other GasesMantellZ9 arried out a comprehensive survey of the interactionbetween tin and H,S and SO,. Under normal atmosphericconditions, both have littl e effect but above 100C, SnS i sformed. High concentrations do however produce darkening ortarnish, while a t low temperatures, Cooke30 ound that liquid orgaseous SO, produced a crystalline solid wi th tin. Skorchellettiand Tukachinskii3' monitored the thickness of adsorbedmoisture films on tin a t relative humidities be low 100% using atorsion microbalance. They found that SO, at different levelshad little influence on their results.The halogens attack tin readily at room temperature except forfluorine which only reacts a t a significant rate above 100C.Similarly, fumes from concentrated acids attack tin.Tin resists corrosion by the vapours of organic acids and otherorganic compounds such as might be emitted from wood orother packing materials. Rance and C ~ l e ~ ~nd, more recently,Donovan and Stringer33 have been among those who havestudied this aspect of tin corrosion.

1.5 Atmospheric Corrosion of TinThis subject has been studied by a number of workers using arange of exposure sites and wit h either tin or tin coatings undertest.WhittakeP4 surveyed available data in 1924 but K e n ~ o r t h y ~ ~was the first to carry out a comprehensive test on tin. Heshowed that in normal indoor environments, tin remains brightindefinitely, especially if regularly washed wi th soap and water.In outdoor exposure, without exceptional pollution, tin does nottarnish but gradually becomes dull with the eventual formationof some white corrosion product when the surface is shelteredfrom the rain.The most comprehensive study of the atmospheric corrosion ofbu lk tin was reported over a period of 20 years bysubcommittee VI of the ASTM Committee B-3 on Corrosion ofNon-Ferrous Metals and alloy^.^^-^^ Commercial tin of99.85+% purity was exposed as 9 x 12in. (23/30cm) sheetsand corrosion rates were determined from weight lossmeasurements. In addition, tensile properties and surfaceappearance were recorded. The tests were conducted at sevensites in the U.S.A. and included industrial, sea coast and ruralatmospheres and results were reported after 1, 3, 5, 10 and 20years exposure. The most valuable data are probably thosebased on weight loss measurements and these are summarisedin Table 2. The change in strength of some of the test sampleswas due to inadequate support during exposure while themajority probably experienced normal self-annealing and theeffect measured did not result from corrosion.

Table 2Average weight loss of tin exposed to different environmentsover a period of 10 and 20 years (Data abstracted from ref. 40).

Corrosion Rate, mg dm-* day-'Site Description

Heavy IndustrialMarine Heavy IndustrialMarine (N ew Jersey)Marine (Florida)Marine (California)Semi-aridRural

1Oy test 20y test- 0.34- 0.260.38 -

0.46 -- 0.57__ 0.088-.097

More recently, Speddin very briefly reported4I he results of aten-year exposure test for tin in industrial, marine and ruralatmospheres. The added costs as a result of SO, pollution ofatmospheres were estimated for a number of metals.Clarke and Longhurst4, have carried out atmospheric exposuret e s t s on tin in a variety of tropical environments over a period of2 years. Test samples 1 5 x 10cm were exposed in open orsheltered conditions in three sites in Nigeria. The first w a s hot-damp inland, the second hot-damp town and the third hot-damp sea coast. The specimens for full exposure were mounted30 o the horizontal, approximately 1.2m above ground andfacing south. The sheltered exposure samples were supportedby insulated slots vertically inside a ventilated box. The resultsof both tests are summarised in Table 3.

Table 3Corrosion of coatings of tin, 80% in/20?? inc and zinc (alI25-pmthick) on steel at three tropical sites. Full exposure weight losswas measured after removing the corrosion productscathodically (Data abstracted from ref. 42).

Coatings 25pm thick on steelTest I i te

Tin 80%Sn/2W/obn ZnMaterial loss, Jungle 0.18 0.46 0.53pmlyearafter 2 year Town 1.02 1.35 1.45fu l l exposure Coast 3.02 2.87 2.90Weight loss, Jungle 1.6 9.3 16.5mg/dm2after 2 year Town 11.3 15.5 10.7shelteredexposure Coast 40 23 18.1

Tin does not have great structural strength and so it finds mostapplication as a coating for both ferrous and non-ferroussubstrates. Use is made of the corrosion resistance, solderingcharacteristics or ability to prevent galvanic corrosion effectsbetween other metals. During atmospheric exposure, thepresence of a substrate, exposed through pores in the coating,has little effect on the corrosion of tin.Tin on steel is widely used as a packaging material and for thefabrication of processing and storage equipment, particularly inthe field of food handling although other materials such asstainless steel are increasingly favoured for these applications.Under atmospheric exposure conditions, tin offers no galvanicprotection to the underlying steel and so rusting at pores in thecoating is the form of attack always found. The presence ofexposed steel seems to have little or no effect on theatmospheric corrosion of tin. Despite these apparent limi ta-tions to the protection afforded to steel by tin coatings, anumber of studies have been carried out. C ~ m p t o n ~ ~rieflycommented on the performance of bin coatings on steel but amore complete study was reported for the Protective Coatings(Corrosion) Sub-Committee of the Corrosion Committee of theBritish Iron and Steel Research Association. In that workHudson et. al. r e p ~ r t e d ~ ~ - ~ ~n exposure tests carried out for upto twelve years in an industrial atmosphere (Sheffield), ormarine atmosphere (Calshot and Congella, S. frica) and a ruralatmosphere with heavy rainfall. The production of protective

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corrosion residues in the pores of the coating was cited inexplanation of the superior performance of samples in theindustrial atmosphere. Britton46 has made similar commentsabout his own results. Dettner4' carried out a similarprogramme using accelerated corrosion tests and outdoorexposure in the urban atmosphere of Berlin. Testing was onlycarried out for one year but a number of conclusions weredrawn from the work. Reflowing (fusion) of tin coatings wasclaimed to improve their corrosion resistance except to a saltspray test and deposits from an acid electrolyte were said to bebetter than those from a stannate bath. Again, in a similar srudyconducted at about the same time, Biestek r e p ~ r t e d ~ ~ - ~ ~esultsfor the exposure of tinned steel to urban, urban-industrial,seacoast industrial, industrial and rural atmospheres.A separate trial was also organised by Biestek50 n China duringwhich tin plated steel was exposed to a tropical atmosphere.Samples of plated mild steel were tested in two environments.In the first, they were mounted a t 45' in the open and facingsouth. In the second condition, the racks were placed in amatting shed so that solar radiation, wind and rain were keptaway from the samples. The test was continued for 58 monthsbut it was found that coatings of 25pm of matte tin should notbe exposed in either environment for mor-e than one year andthat similar coatings 32pm thick should not be exposed for twoor more years.In general, the conclusion of all these studies was that thecorrosion resistance of tin coatings, meaning their protection ofsteel, was not very high. This is reflected in the internationalstandard covering tin coatings ( IS0 2093-1 973) whichrequires 30km thick deposits for exceptionally severeconditions, 20pm for severe service, 42pm for moderateservice and 4pm for mild service where the main requirement i sfor good soiderability of the material.Tin coatings are also widely used on non-ferrous substrates,usually for one of four reasons. The solderability of materialscan be improved and retained over a long period by the use oftin coatings. As with steel, food contact applications still makeuse of tinned articles as does some non-food processing equip-ment. Finally, tin coatings are used to prevent galvanic effectsbetween dissimilar metals and to provide low resistancecontacts in some electronic applications. In view of theseapplications, it is not surprising to find that copper and copperalloys are the most frequently tinned non-ferrous materials. Tintends to be anodic to copper and copper alloys and to the inter-metallic compounds formed between tin and copper in mostaqueous environments and hence accelerated corrosion of thecoating might be expected. This can show itself as theappearance of "black spots" on a tin coating. These are amanifestation of local corrosion of tin around discontinuities inthe coating and while normally associated with total aqueousimmersion, their appearance can follow outdoor exposureinvolving cyclic condensation. However, since tin and copperboth show good atmospheric corrosion resistance, referencesin the literature to studies of their performance tend to berelated to changes relevant to specific end-uses.The deterioration of the solderability of tinned copper duringageing has been the subject of many studies and acceleratedtests have been developed to simulate the e f f e ~ t . ~ l - ~ ~imilarly,changes in the contact resistance54of tin coatings have beenmonitored and the increase found can be related to th e growthof the oxide film on tin.Special mention should be made of the atmospheric corrosionof tin coatings on brass. It has been demonstrated that zinc canrapidly diffuse through tin and that its appearance a t thesurface of the coating is the cause of a number of problems.*The surface of the tin becomes covered with white zinc

corrosion products which adversely affect its solderability andcontact resistance and increase the susceptibility of the tin towhisker growthg. The problem is easily overcome by ensuringthat a barrier metal is deposited between the coating andsubstrate and 2.5ym of copper or nickel have been found to beeffective materiak8The specificat ions for tin coatings on non-ferrous substrateswith the exception of brass are similar to those for steel. IS02093-1973 suggests 30pm of tin for applications in whichsubstrate exposure cannot be tolerated and 15pm for severeservice. Moderate service environments require 8pm of t inwhile 4pm is sufficient for solderability purposes. Qther authorsrecommend that to guarantee an acceptable shelf-life forsolderable materials, a coating thickness of 5pm should besed.^'-^^.1.6 Long-Term Atmospheric Corrosion of BinA number of studies of artefacts of archaeological interest havebeen carried out and although the exposure conditions cannotbe accurately described, it is interesting that the corrosionproducts found are consistent with all of the precedingcomments.Bannisters6 tudied a mediaeval or Roman tin scabbard found ina gutter in Roman Watling street in England. The article wascovered by an adherent layer 2.5mm thick with thecomposition shown in Table 4. The protected metai was 2mmthick and contained 99.98% tin and a trace of copper. Severalblocks of t in have been discovered and the results of an analysisof rhe corros ion products present have been reported. Smythe"studied a 72kg biock of tin, dredged from Falmouth Harbour in18 23 among a number of other specimens. The data availablesuggests that it was weathered for six years before beingplaced in a museum. The surface of the sample was covered ina thick, light grey-brown scale and its composition was asshown in Table 5. A smaller block, found at Treireife in 1841,was covered in a brown scale which was found to be 44.7%hydrated SnO.

Table 4Analysis of the scale of corrosion product on an ancient tinscabbard (Data abstracted from ref. 56).

CompoundHydrotis tin(ll)oxideTin(lV) oxideCopper oxideSulphuric anhydrideIron oxide etc.Lime

Wt. % of total43.3554.680.8 10.850.120.10

Table 5Analysis of the scale of corrosion product on an ancient blockof tin (Da t a abstracted from ref. 5 7 ) .Compound Wt. %of total

Tin(lV) oxide 84.93Hydrous tin (ll )oxide 2.61Tin(ll) chloride dihydrate 5.03Silica 1.67Water 5.89

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Examples of ancient tin coins from Malaysia were examined byPlenderleith and OrganJ8who found them to be covered bysuccessive layers of brown and grey scale. Analysis showed thescale to be principally SnO, wi th about 7 % ti n ( 11 ) compound,probably the oxide. Sulphate and traces of silica and iron werefound but significant levels of chloride ion were not detected.LihEsy reported a study of seventeenth-eighteenth century tinsarcophagi in Vienna in which tin pest had been suspected asthe cause of th e visible deterioration of the sample. However,he found that the casting was porous and that air and moisturehad produced oxide films within the casting. The resultantvolume change then caused sweiling, blistering and cracking.As in the previously mentioned examples, the principalcorrosion products were SnO and SnO, and in this case, somebasic copper carbonate was also found.1.7 Surface Treatmentsfor TinSurface treatments are not applied to t in for the majority of itsapplications although, as has been mentioned above, cathodiccleaning in dilute sodium carbonate solution can reduceoxidation rates of the metai. This is presumably related to theremoval of residues which might otherwise help to break downthe oxide and so prevent a continuous film from growing andstifling the reaction, The deliberate growth of tin oxide or mixedoxide films in both acid and alkaline solution has beenreviewed6 recently and will not be discussed further here.The exception to the above is electrolytic tinplate whichreceives a surface treatment during its production, which isreferred to loosely as passivation. The purpose of such treat-ments is to produce a surface more amenable to can-makingoperations such as lacquering and soldering and more resistantto sulphide staining by natural protein present in foodstuffs.These aims are achieved by modifying or removing the oxidesproduced on the surface of tinplate during manufacture anddepositing a stable protective film. The most common treat-ments involve either simple immersion or cathodic polarizationin sodium dichromate solution, although a number ofalternatives exist.A great deal of discussion surrounds the identification of t h especies produced on the surface of tinplate during passivationand the consensus view is that a mixture of tin oxides andchromium oxides with various levels of hydrat ion are present. Inaddition, the majority of workers consider that significantamounts of metallic chromium exist in the film producedcathodically and they cite both electrochemical 61,62 andE.S.C.A.h-h4 data as evidence.Despite the universal use of passivation films for tinplate, littlehas been published concerning their effect on the atmosphericcorrosion of the material. Those paper^"^-^^ which haveappeared show that rusting resistance is increased and this ofcourse probably results from the effect the treatment has on theiron exposed through pores in the tin coating of tinplate. Anumber of papers however, have been concerned with theincreased resistance to oxidation of the tin coating on tinplateas a result of the passivation treatment, but these largely relateto elevated temperature treatments such as would be carriedout during the lacquering of the material.hx-7

2 P E W T E RModern pewter is an alloy of tin containing up to about 7.5%antimony and about 2.5% copper. The material behaves invirtually the same way as pure tin and in indoor environments,to which it is normally exposed, a bright, white lustre is

retained. Surfaces contaminated by particles of foreign matter,whether it is a fabrication residue or household dust, candeteriorate as a result of local breakdown of the protectiveoxide film but regular simple washing ensures that the surfaceremains in good conditi~n.~-~In the past, pewter alloys contained lead and this played asignificant role in the corrosionof the material, producing a darkpatina during atmospheric exposure. Mention will be made ofthis below in dealing wi th tin-lead alloys bu t it should be notedthat some modern pewter is chemically treated to reproducethis patina. A number of proprietary processes exist includingthose based on immersion in iron chloride or sodium nitratesolutions or acidic solutions of copper and ar ~e ni c. ~ -~ ~lectro-chemical treatments such as anodizing in phosphate solutionwi ll also produce the patina.67 Once formed, the colouredsurface film on pewter appears to have the same stabilityagainst atmospheric corrosion as the normal oxide film on tin.The stability of pewter against tarnishing in the atmosphere hasbeen reported by Kenworthy and Wa ld ra n~ ~ ut furtherdetailed studies do not appear to have been carried out.

3 TIN-LEAD ALLOYSThe second largest industrial use of tin is as a constituent ofsoft solder where it is most commonly alloyed with lead. 1addition, both ferrous and non-ferrous substrates are coatedwith tin-lead alloys to confer solderability on those materials.However, a coating of tin-lead is also used to give corrosionresistance to some materials and the most important exampleof this is the coated steel material called ternepiate. The nameis applied to a range of products made from cold rolled mildsteel coated with an alloy of 2-20% Sn balance Pb. The uses ofthis material have been recently reviewed by M a ~ k a y . ~Even very small additions of lead to tin cause tarnishing of thematerial in full exposure to the atmosphere. Much longer timesor higher lead contents are required to produce the same effectin indoor environments. As the lead content increases, thesurface of the alloy takes on an increasingly dull appearance butdestructive corrosion is rare. Occasionally, condensed watermay extract lead from the alloy bu t the most common cause offailure in the atmosphere is contact with organic acid vapours.Unlike tin, lead does not resist attack by these and contact withmaterials which are sources of formic and acetic acids, amongothers, should be avoided. In most atmospheric exposureconditions, tin-lead alloys become covered by corrosionproducts, such as lead sulphate, which offer protection againstfurther metal loss. However, in marine environments or whereresidues, such as fluxes from a soldering operation, are present,then this film may break down.Tompkins 77 has reported a study of the interaction of O,,SO,,NOz,H,S, Cl,, CO and NH, wi th a 50% Sn/50% Pb alloy and thechanges found at different relative humidities. He found thathigh humidity enhanced the reactions w ith low concentrationsof CI2 which produced PbCI, and SnCI,. On the other hand theactivity of low concentrations of NO, in producing Pb(NO,), wasinhibited by high relative humidity. All of the other gases hadsome interaction with the alloy when they were present at highconcentrations, except CO and NH,, bu t only CI, and NO, wereactive below a concentration of 100ppm.Bird7nhas recently carried out an E.S.C.A. study o f a tin-leadalloy, the surface of which was exposed to a laboratoryatmosphere for only a few minutes. He concluded that the tinwas markedly segregated at the surface of the alloy and that itpromoted the formation of a form of lead, thought to be PbO,,more highly oxidised than that obtained on the surface of purelead under the same conditions,

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K e n ~ o r t h y ~ ~ppears to be the only researcher to have carriedout a systematic atmospheric corrosion study of a bulk tin-leadalloy and his work led him to the conclusion that a 3% Sn/97%Pb alloy had indoor corrosion resistance superior to that of purelead. Both the surface appearance and weight increment dataafter 1 4 months testing supported this. In full outdoorexposure, there was litt le difference between the performanceof this alloy and pure lead.In addition to a study of ancient tin blocks which wasmentioned earlier, Smythe5' investigated the corrosionproducts found on two tin-lead alloy blocks. The first was ofRoman origin and was found at Benwall in Northumberland.Analysis showed it to be a 96.7% Sn/2.73% Pb alloy and thescale had the composition given in Table 6. Also in that Tableare the results of the analysis of the hard and brittle off-whitescale found on a block of 94.7% Sn/5.3% Pb alloy. This wasalso Roman in origin and was found at Cambridge.

Table 6Analysis of the scales of corrosion product on two Romanblocks of tin-lead alloy (Data abstracted from ref. 57).

Wt. % of totalCompound

Tin ( IV )oxideHydrous in(ll)oxideTin(ll)chlorideSilicaLead sulphateBasic lead carbonateIron carbonateWater

96.YO%Sd2.73%Pb 94.79Sd5.3%Pb68.8 57.327.9 34.1- 4.40.5- 2.72.7 1.50.1--- -

Most outdoor atmospheric corrosion studies of tin-lead alloyshave been carried out on coatings on steel. As wit h r in coatings,deposition of the alloy can be electrolytic or by hot-dipping ormetal spraying. Generally speaking, the higher the tin contentof the coating on terneplate, the lower its porosity and hencethe greater the protection afforded to the substrate. Again, liketin coatings, tin-lead does no t offer any galvanic pro tection tosteel in the atmosphere and hence protection against rustinglargely depends on coating continuity and the formation ofprotective corrosion products in the pores that are present.Hudson and 5anfield79 tudied tin-lead coatings on steel in anindustrial atmosphere. They compared 12% Sn/88% Pb withpure tin and lead and found that the lead-containing coatingsdeveloped a white fi lm which they believed to be lead sulphate.At about the same time, Durose" looked at the corrosionprotection afforded to steel by electrodeposited tin-leadcoatings, using a range of coating weights and different platingbath additives. He concluded that a 5.5% Sn/94.5% Pb alloywas superior to a pure lead coating and also coatingscontaining 7, 10 or 15% tin. The 10 and 15% tinhead alloyswere found to be comparable with pure tin coatings. This resultwas in part confirmed by Graham and Pinkerton" who reportedatmospheric exposure results at three sites for lead, 5%Sn/95% Pb and 14% Sn/86% Pb electrolytic coatings on steel.Protection of the panels at al l the sites (severe industrial, ruraland marine) by the tin containing alloys was superior to thatproduced by the lead alloy, particularly on the backs of the testpanels.

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Smith" compared the performance of a number of commercialterneplates in both accelerated corrosion tests, sulphur dioxide,humidity and salt-spray, and in full outdoor exposure to eitheran industrial or marine environment. Exposure was for 12months only and interpretation of the results was complicatedby the fact tha t samples were not prepared to standard coatingthicknesses. Assessment was on the basis of the degree ofrusting of the substrate steel, little mention being made of anyattack on the coating. However, it was noted that lead-tinalloys showed greater resistance to chloride attack than lead-antimony alloys. At the same time, it was said that coverage ofthe steei increased with tin content of the alloy and that theresistance of the coating to attack in both chloride-rich andhumid conditions aiso appeared to increase.The use of tin-lead coatings on copper alloy substrates tomaintain the solderability of such materials for the electronicsindustry has led a number of workers to comment on theimportance of surface changes on performance withoutsystematic studies of those changes having been carried out.Examples of the effect of storage, in part related to atmosphericcorrosion are summarised by Thwaitess5 and have beendiscussed briefly by Ti~ si er .* ~s a result of the importance ofthe degradation of solderability during storage, a number ofaccelerated ageing techniques have been developed andAckroydsz has recently reviewed these procedures. Increasinginterest is now being shown in the use of tin-lead coatings ascontact materials. One of the potential difficulties faced by tin-lead materials as low energy contacts would be the formationof films of corrosion product which migh t lead to poor electricalconductivity ,for the system. A number of a ~ t h o r s ~ ~ - ~ ~avecommented on the atmospheric corrosion of these contactmaterials although systematic controlled studies have not reallybeen carried out. One of the reasons for this appears to be tha tthe films which are normally formed are readily mechanicallydisturbed when contacts are mated and hence their presencedoes not normally create a problem.86

4. %!RI-COPPER ALLOYSThis group of materials, consisting mainly of bronze, gunmeta land brass with t in additions, represent the first to be consideredin this paper in which tin is not normally a major alloyingaddition although in exceptional cases it may be present atconcentrations up to 25%. Thus the corrosion of bronze in theatmosphere is really like that of copper and is usuallyaccompanied by the formation of a layer of green basic coppersalts, mainly suiphate, on the surface. The layer is adherent andprotective and can have a pleasant appearance.

xidation 0f Tin-Copper AlleysEarly studies of the effect of tin additions on the rate ofoxidation of copper include those of J ~ r d i s , * ~ho sealedsamples into tubes with air or oxygen and then tleated them.However, a more thorough study has been carried out byGesmundo er who investigated the weight gain of Cu-Snalloys containing up to 13% Sn in oxygen ( 1 atm.) in thetemperature range 550-800OC. They found that the oxidationrate of a 3% Sn alloy was lower than pure copper at 550 andSOOOC but higher above 60O D C .The rate of oxidation of moreconcentrated alloys however was always lower than purecopper. They argued tha t the formation of an SnO, layer at thebase of the scale, although thin and usually discontinuous,slowed down the outward diffusion of copper. The highest tinconcentration allowed by its high temperature solubility incopper was not sufficient to produce a continuous thick healinglayer of SnO, at the alloy surface but nevertheless, it produced apronounced decrease of the oxidation rate.

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4.2 Atmospheric Corrosion of Tin-CopperIn 1929, Hudsons9 reported the results from atmosphericcorrosion tests carried out on a number of copper alloys.I n c l u d e d i n t h e p r o g r a m m e w a s a b ro n ze w i re(6.3%Sn/O.O3%P/Cu) and sheet (6.3%Sn/0.08%P/0.5%Zn/Cu)and exposure for one year in rural, suburban, urban, industrialand marine environments was monitored. Corrosion in bothsheltered and full exposure was measured by following weightgain, electrical resistance and tensile strength changes. All ofthe materials showed performance which varied with the levelof atmospheric pollution and the bronze samples were alwaysamong the better specimens. This was attributed to the facttrhat they appeared not to form deliquescent corrosion productswhen submitted to accelerated corrosion tests. Table 7 showsthe loss of tensile strength (mean for all five locations) derivedfrom Hudsons work and the superiority of the bronze tobrasses can easily be seen.

Alloys

Table 7Mean values for the loss of strength of a number of copperalloys exposed at five d ifferent sites (Data abstracted from ref.80).

Lossof Strength after 1 yearAlloy %6% Tin bronzeH. C. Copper3.5% Aluminiilm bronze70 Nickel/3O Copper60 Copper/40 Zinc70 CopperJBOZinc

1.212.402.103.2418.48.58

In a major study extending over 20 years and conducted at 7different sites, Tracygo reported the atmospheric corrosionresistance of a number of copper alloys. The samples includedphosphor bronze (7.85%Sn/O.O3%P/Cu), Admiralty metal(1.22% .Sn/29.01%2n/69.75% Cu) and a nickel tin bronze( 1.04%Sn/28,64%Ni/O.55%Zn/69.1%CuL As with Hudsonsstudy, corrosion was monitored by following weight, electricalresistance and tensile strength changes and in agreement withhis work, atmospheric pollution was a major factor In decidingthe rate of corrosion. industrial atmospheres were morecorrosive than marine which in turn were more aggressive thanrural environments. in general, the extent of pit ti ng was foundto be negligible for all. of the alloys studied. Somerepresentative data from the study are shown in Table 8. Theability of tin additions to restrict dezincification of brass wasalso apparent.Another study was conducted in the U S A . by Quickv1 whoexposed screen wire cloth a t four different sites for up to nineyears. He inc luded brasses, a luminium bronze, nickel/copperand a 2% tin bronze. The brasses failed rapidly but the tinbronze showed the lowest percentage loss of strength at all thesites.Brittong2 confirmed the effects of pollution, particularly bysulphur containing compounds and showed that in railwaytunnels used by steam locomotives the rate of corrosion of anumber of copper alloys was about an order of magnitudefaster than in the open air. Despite this, the projected lifetimesfor the materials, including a 2%Sn bronze wire, were good. The

Table 8Loss of strength of a number of copper alloys after exposure tofour different environments for ten years (Data abstracted fromref. 90).

Loss of Strength after 10 years, %Exposure

Environment

Heavy IndustrialMarine HeavyIndustrialSevere MarineRural

Copper 8%Tin 7o%Gopper 7WCopperBronze 30%Zinc 29%Zinc1%Tin5.9 7.2 30.9 9.06.3 8.0 28.2 7.97.6 5.7 8.0 2.53.1 3.1 3.2 2.2

corrosion films were found t o be basic chlorides and sulphatesof copper although copper metal and cuprous oxide were alsofound together with traces of the alloying elements. A similarcomment concerning the ecomomic results of SO2 pollution onthe corrosion of a number of materials, including an 8%Snbronze was recently made by Speddinn4A number of authors have been concerned with theperformance of tin-copper alloys in tropical environments.Ambler and 5aing3compared the corrosion resistance of somecommon metals, including a tin-containing brass, at more than2 0 sites in Nigeria. In another study, Southwell et . al.compared the performance of five stainless steels withphosphor bronze (4.38%Sn/O.358%P/Cu) and a low alloy steela t tropical inland and sea-coast sites over a period of eightyears. The marine atmosphere was more aggressive towardsthe bronze and a t both sites the stainiess steels showed lowerweight losses while the low alloy steel was severely attacked.However, the bronze did not suffer pitting attack or a loss intensile strength while this did occur for some of the stainlesssteels. In a subsequent gaper,95 he same authors reported th eresults of 1 2, 4, 8 and 7 6 ears tests on a number of copperbased alloys. Two tropical sites, marine and inland semi-rural,were used and the tin containing alloys were phosphorb r o n z e ( 4 . 3 8 % S n / 0 . 3 6 % P / C u ) N a v a l b r as s(0.84%Sn / 39.02%Zn / 60.08% CUI and manganese bronze(0.65%Sn/l.o4%Fe/40.25%Zn/Cu). Generally, all of the copperalloys tested were resistant to the tropical atmospheres, againcorroding more a t the coastal site than inland. The tin-containing alloysawere as good as, or slightly superior to theother alloys. More recently,96 he same group of workers hassummarised their results and drawn together comparative datafrom different programmes conducted at the same tropicalsites. Results for four cast bronzes, (5%Sn/5%&/5%Pb/Cu;6 %Sn/3%Zn/2% P b/ 1%N /Cu ; 9 % S n/3%i!n/ 1% N /C u ;3%Sn/2%Zn/6%Ni/Cu) were added to the previously reporteddata but the conclusions were broadly the same as before. Themost interesting additional data presented in this final paper inthe series was for the effect of coupling phosphor bronze toequal areas of a wide range of other metals. In this test it wasfound that the coastal environment was 4-8 times moreaggressive than the inland atmosphere.Mention should also be made here of the generally superiorresistance of tin-copper alloys to corrosion fatigue. Gough andSopwithg7 compared a 4% tin phosphor bronze with othercopper alloys and stainless steels in a salt-spray and found it tobe superior to all the materials. The second best sample was aberyllium-copper alloy. The subject of stress corrosion cracking

9

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is really best considered in the context of copper alloys but it isgenerally agreed that phosphor bronzes show high resistance tothis form of attack while Admiralty brass in common with70/30brass is not very resistant.Occasional references have appeared in the literature to studiescarried out on the atmospheric corrosion of complex bronzealloys. A P%Sn/S%Ni/Cu alloy was developed by the BellTelephone Laboratories in collaboration w ith others and it wasclaimed to have tarnish resistance superior to that of otherbronzes used in the electronics industry as connector materials.Ward and Lovettg8 have summarised the principal propertiesclaimed for the alloy, including the effect of atmospheric ageingon solderability. This is an index of the amount and nature ofthe corrosion film on the alloy and it is clear that the alloy issuperior in performance to both nickel-silver and an 8% tinphosphor bronze. A more general study of th e Cu-Ni-Sn andCu-Ni-Zn-Sn systems was made by Mori and Nakojimag9butl i t t le reference was made to atmospheric corrosion except tosuggest that the zinc containing alloys were found to be morestable against oxidation. Habrakenlooet. al. studied some alloysin the Cu-Sn-AI system and decided that those containing a tleast 5% each of tin and aluminium had promising tarnishingresistance. Alloys were exposed to rural, urban and industrialatmospheres and the mosr promising material was found to be5%Sn/7%AI/Cu. In a later paper, AhmadlO1 pointed out thatsuch alloys could be brittle but that further additions of 1% Feand 1% Mn overcame this difficulty while the presence of tinstill improved the corrosion properties of the alloy.Electrodeposited copper-tin alloy coatings find some industria lapplications. Bronze deposits containing 10-20% t in have beenused as undercoatings102 or nickel-chromium or tin-nickeldeposits for decorative applications. They have also been usedin their own right a t a thickness of 5 0 ~ mn steel componentsof hydraulic mine equipment. This has been because of theirsuperior hardness and slightly better corrosion resistance tomarine atmospheres than copper. The alloy containing 42% t inwhich at its correct composition is actually the intermetalliccompound Cu,Sn 103-104 was once used as a material formirrors, hence the name speculum. When polished, a coating ofspeculum has a similar appearance to silver and it shows agood resistance to sulphur compounds and other tarnishingagents. It normally remains bright indoors but rapidly turns dulland grey when exposed outdoors if it is not frequently cleaned.Early work by Faust et. al.105 escribes some of the properties ofbronze and speculum coatings and more recently, Laub106 hasreported the corrosion resistance of tin-copper alloys platedonto steel in both industrial and marine environments.

4.3 Long-Term Atmospheric Corrosion of Tin-Copper AlloysIn view of the long history of the use of bronze, it is notsurprising that a number of workers have been concerned withcorrosion damage and repair of the material. Fink andEldridge'O' reported a method for reducing the basic saltspresent in the crust of corrosion products on ancient bronzes byelectrolytic reduction in a 2% NaOH electrolyte. A series ofelectrolytic treatments were also used by Evans'O* to recoverartefacts which had suffered "bronze disease." This took theform of the production of soluble copper acetate beneath theporous original patina. Acetic acid was thought to have derivedfrom the packing materials used to store the articles. Theacetate was converted to basic carbonate or sulphate, releasingthe acetic acid for further attack on the bronze.Another case of bronze disease was investigated by Organlogwho suggested that the formation of unstable CuCl in the lowerlayers of the patina was the cause of the problem. He

suggested a number of remedies to nullify the effect of CuCl orto remove it completely.Collins"' examined a number of Chinese bronzes which hadbeen buried for 1,000 to 4 ,000 years. The alloys covered awide range of compositions and slight differences in theircorrosion films were found. On some of the bronzes, a layer ofbotryoidal malachite (CuCQ,.Cu(OH),) 1.2 cm thick was found.This was accompanied by crystals of cuprite (Cu,O) and thickcrusts of azurite (2CuC03.Cu(OH),). Leaded bronze(21%Sn/4%Pb/Cu) was examined by Gettens"' after it hadprobably been buried for about 2,500 years. He described thesurface as having an outer, completely mineralized zone and aninner partially mineralized region. This contained CuCl coveredby Cu,O interspersed with disconnected seams of SnO,.Summarising the work of others, Mantel1112 lists thosecompounds found in the patina of bronze as malachite(CuCO,.Cu(OH),). atacamite (CuCl,.3Cu(OH),). and azurite(2CuCO,.Cu(OH),). These can be mixed with cupr ic chloridesand salts of alloying elements such as tin oxide and leadchloride.5 TIN-ZINC ALLOYSAlloys of tin and zinc, normally in the range of 15-30% Zn, findlimited applications as a corrosion protection coating for steelwith good to moderate solderability characteristics. Over theyears, spasmodic interest has been shown in such coatings assubstitutes for cadmium plating and it appears that for certainapplications they offer a viable alternative. The literature on theatmospheric corrosion of these alloys reflects this interestalthough Lashko and La~hko-Avakyan"~eported the results ofa study of tin-zinc alloys used as solder to join aluminium. Theyfound that the corrosion resistance of the solder to a tropicalatmosphere during up to 9 months exposure was a function ofthe zinc content. The alloy 80%Sn/20%Zn was found to be themost resistant to attack and to have the best strength retention.Tin-zinc alloy coatings behave as a simple mixture of the twometals and may be deposited from a number of bathformulations over virtually the entire composition range.114Thezinc in the coating can provide protection to any substrate steelexposed a t pores but in full inland exposure to pollutedatmospheres, the depletion of zinc is too rapid so that rustappears before it would on a zinc coating of comparablethickness. However, in sea-spray or similar marineenvironments, coatings containing 70-85%Sn can performbetter than pure zinc because of the apparent formation of aprotective corrosion product. The main uses of the coating havebeen in sheltered environments where its ease of solderabilityand protection of steel exposed a t pores are an advantage.Under these conditions, it is less likely than zinc to produceloose corrosion product and it is also less affected by exposureto acid vapours from wood. It is also possible to improve thesolderability and long-term cleanliness of the coating bydepositing tin separately on top of a layer of zinc which is thenstill able to offer protection a t pores in the duplex coating.Angles and Kerr'" appear to have made the first systematicstudy of tin-zinc coatings on steel. They examined theperformance of alloys in the range 28-92% Sn and comparedthem with coatings of tin, zinc and cadmium. The conclusionfrom both accelerated tests and atmospheric exposure in anurban environment for one year was that coatings of80%Sn/20%Zn offered superior protection to steel and that theappearance of white zinc corrosion product increased wi th thezinc content of the alloy. A suggestion was also made thatchromate passivation treatments improved the overallperformance of the coating, making it less susceptible tostaining by finger or grease marks. This study was followed by a

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number of authors who largely came to the same conclusionsconcerning the protection of steel against atmosphericcorrosion by tin-zinc a I oy coa ings. 42,46,One of the more complete studies, again by Angles119 but incollaboration with Britton, compared coatings in urban,suburban and marine environments. They decided that theorder of merit in urban exposure was zinc, 50%Sn/50%Zn,80%Sn/20%Zn, the n cadmium. in marine exposure,50%Sn/50%Zn was better than zinc and cadmium but$O%Sn/20%Zn was inferior to zinc and no better thancadmium. These results have been summarised in Table 9.

I '

Table 9The reiative ability of different coatings to prevent the rustingof steel in a marine atmosphere (Data abstracted from ref. 119) .

Mon ths to the first appearanceof rustCoating Me tal

ZincPassivated zinc50% tin/50% zincPassivated50% :in/50% zinc80% t i n /20% zincPassivated80% tin/20% zincCadmiumPassivatedcadmiumTin

7.5pm181825299138131

12.5~m33183535182121211

25wm3636)48

)48363634251

The most recent contribution on the subject was published byRaub et. al.Izo who also included information concerning thelatest developments in the nature of the electroiyte used indepositing the alloys.Bri tton and de Vere Stacpoole121 nvestigated the properties oftin-zinc alloy coatings on steel nuts and screws exposed tosuburban, industrial and marine atmospheres in contact withaluminium plates. They found 80%Sn/20%Zn coatings to begenerally superior to zinc and cadmium when considering a i lthe exposure conditions in that, while failure of the coatingresulted in rusting of the steei more quickly than with zinc andcadmium in particular environments, it did not result in rapidattack of the aluminium. An additional advantage was theabsence of hvgroscopic corrosion products on the tin-zinc alloyso that a ring of moisture did not form around the nuts andscrews; this was a source of po tential problems with the zincand cadmium finishes.The resistance of tin-zinc alloy coatings to the production ofcopious amounts of corrosion product when exposed to theatmosphere was illustrated in work by Phillips and Johnson.122They measured the contact resistance of a number of coatingson steel after exposure to a rural outdoor atmosphere. Theyfound that tin-zinc and tin-cadmium alloy deposits maintained alower contact resistance than d id equal thickness of tin, tin-leador tin -antimony alloys. The samples were exposed for 2 monthsunder a constant mating load and while the contact resistanceof a tin-lead alloy coating was IOW, it failed to give as goodcorrosion protection to the substrate during the test.

6 TIN-CADMIUM ALLOYSThe atmospheric corrosion studies carried out on these alloysrefer to their performance as coatings for the protection ofsteel. However, there has been litt le practical use made of themalthough interest is occasionally renewed as a result of some ofthe test results previously reported.The range of tin-cadmium alloys may be electrodeposited ormechanically plated and in appearance and behaviour they arelike tin-zinc coatings. Since cadmium is less effective atsacrifically protecting steel exposed at pores, the optimumconcentration in the coating is in the range 25-50%. As withtin-zinc deposits, the merits of using a duplex coating of tin ontop of cadmium have been explored and have proved to bevaluable where exposure to organic vapour might be expected.Bennett,Iz3 then Scott and Gray"4 developed the electrolytesfor the deposition of tin-cadmium alloys and reported somework on the protec tion afforded to steel by the coatings. Brittonand de Vere St a~ po ol e' ~ ~arried out a thorough investigation ofthe coatings, in particular suggesting that the alloys performedbetter than cadmium alone in marine environments. This wasfollowed by a study by CohenIz6who reinforced the point madeby Britton that the attack on the coating by organic vapourswas less than for pure cadmium. Burneri2' wen t further thanthese authors in suggesting that tin-cadmium alloy coatings,particularly with a chromate surface treatment, performedbetter than cadmium coatings of the same thickness.Recently, tw o other papers have appeared which again came tothe same conclusions as those referred to above. Beck and3anowsky"8 reported that the protection afforded to steel byalloy deposits was greater than that given by duplex coatings oftin and cadmium. Cooke et. al.'29 showed that zinc or tin-zinccoatings gave greater protection to steel in industrialenvironments than tin on cadmium or cadmium on tin but thatthe order of their relative performance was reversed in a marineatmosphere.a TIN-NICKEL ALLOYSAs with tin-zinc and tin-cadmium alloys, the use of tin-nickeialloys is restricted to that of a coating on both ferrous and non-ferrous substrates. Alloys in the range 18-25% nickel may bedeposited from a mixed cyanide-stannate bath as hard, brightcoatings with good resistance to nitric acid. However, nointerest has been shown in these finishes, mainly because ofthe brittleness of the product of this particular platingprocess.i30 Similar wo rk wi th complex pyrophosphateelectrolyte was reported by Rama Char.'31 Application of tin-nickel alloy deposits has centred on the in termetallic compoundNiSn which may be readily plated from mixed acidchloride/fluoride ele~trolytes.'~~-'~~The deposit is metastable but does not transform to a mixtureof other intermetall ic compounds unless thermally aged.'34 It ishard, nearly bright and reasonably solderable, has remarkableresistance to attack by a wide range of solutions and displaysgood wear re~istance.'~~ecause of this, the coating has beenused as a decorative corrosion resistant finish for balanceweights, drawing instruments and pistons on automobile brakesystems. It has recently gained popularity in food contactapplications, being specified as a finish in beer dispensingequipment. Despite these applications, the full potential of thecoating does not seem to have been realised and this might bein part due to i t s limited d ~ c t i l i t y . ' ~ ~ - l ~ ~his means that impactdamage could lead to damage of the substrate since thiscannot receive any galvanic protect ion from the very noblecoating.Coating thicknesses for various duties are specified in IS0

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2179:1972 and this requires a deposit of 25pm for severeservice, 15pm for moderate environments and 8pm for mild lycorrosive conditions. For coatings on steel intended formoderate or severe service, an undercoat of copper, tin orbronze wit h a minimum thickness of 8ym is also specified andporosity tests are required.7.1 Passivation of Tin-Nickel AlloyRenewed interest in the properties of the equiatomic alloy of tinand nickel as a contact material, on its own or with an over-plate of gold, has been partly responsible for a number of.studies of the growth and composition of the air-formedpassivation film on the alloy. This film provides the basis for thechemical stability of the alloy but a t the same time it produceshigh and unpredictable electrical resistance which can inter-fere wi th its operation as a low voltage contact material. Antlerhas been very active in this field of studies and has recentlyproduced a comprehensive review of the subje~t.' ,~ arly workwas necessarily electrochemical in nature dnd led to thesuggestion, supported by several worker^,'^*-'^^ that thepassive film was environment sensitive but that it probably con-tained oxides and hydroxides of tin and nickel in the sameproportion as the metals appeared in the bulk material. Thepossibility of specific mixed nickel-tin oxides was alsop o s t ~ l a t e d . ~ ~ ' - ~ ~ ~More recently, physical techniques such as Auger spectro-scopy and E.S.C.A. have been turned to the problem and a morecomplicated picture emerges. Hoar et . al.143 suggested thatthe air-formed film contained tin and nickel in the ratio 3:l withan oxygen to total metal ratio of 0.7. Tompkins and Bennett'44questioned some of these findings and from their own workconcluded that the surface of NiSn in air was rich in tin as anoxide film several atoms thick. The same authors followed thiswork wi th another similar in which they also suggestedthat the other intermetal lic tin-nickel compounds (Ni,Sn,,Ni,Sn, and Ni,Sn) probably carried a comparable passive film.Other authors, most notably Thomas and Sharma, 1 4 6 ,1 4 7 haverecently produced a series of papers supporting the conclu-sions of Tompkins and Bennett and providing more quantitativedetail. Tompkins, Wertheim and Sharma'4x ollowed the growthand composition of an air-formed passivation film over a periodof up to 62 days. Some of the samples were aged straight fromthe plating electrolyte while others were given a hydrochloricacid etch before ageing. As-plated films of any age werecomposed of amorphous hydrous tin oxide n i t h an oxygen'tinration of about 3. They were a l l about 2nm thick over theageing range of 2 minutes to 62 days but during that time, thenickel:tin ratio of h e ilm increased f r o m 0 to 0.1 1. Thecrystallin ity of the fi lm was also seen to increase with time. Thesamples which had been etched before ageing showed adifferent pattern of film growth. They increased in thicknessfrom 1.2 to 2nm over the period 2 minutes to 52 days and the0xygen:tin ratio was much higher, changing from 5.5 to 6.5.However, the nickel:tin ratio was comparable with that forunetched samples, increasing from 0 to 0.13. In both theetched and unetched samples, the nickel in the passive filmwas thought to be an hydroxide.It is generally agreed from the work described that tin concen-trates a t the surface of tin-nickel alloy electroplates but noadequate explanation of this phenomenon has yet been putforward. Antler'" reports that Lo has suggested in a privatecommunication that the surface is actually composed of Ni,Sn,grains wi th tin concentrated a t the grain boundaries. Thus thebulk analysis would still yield a composition corresponding toNiSn but the explanation assumes that tin would be present onthe surface of a deposit in the as-plated cond ition and that after

12

etching, tin could diffuse from the grain boundaries to cover thesurface again. This explanation does not however seem to becompatible with al l of the observations made on the long-termstability of this alloy.

7.2 Atmospheric Corrosion of Tin-Nickel AlloysStudies of the atmospheric corrosion of tin-nickel alloy electro-deposits can be separated into two clear phases. The early workwas concerned with the potential use of these coatings indecorative applications where corrosion resistance was alsorequired. Performance under these conditions was oftencompared with nickel-chromium systems and frequentlyencouraging results were obtained.In a series of paper^,'^^-'^^ Britton established that electro-plated tin-nickel alloy is untarnished by atmospheres grosslypolluted wit h SO, and H,S. Comparisons made wi th nickel-chromium deposits by the same author showed that inpositions sheltered from the rain, tin-nickel was better able toretain i ts brilliance. Chadwick15* hen L~wenheim"~ame tosimilar conclusions, a t the same time considering the composi-tion of undercoats used for both alloys on steel. Nickel-chromium deposits were found by Angles154 o be superior totin-nickel plate in marine environments.More recent studies of the atmospheric corrosion of tin-nickelalloy electrodeposits have been concerned with the effect thatsurface changes have on the electrical contact resistance of thematerial, either alone or with a thin cover of gold. Contact resis-tance is adequately low for moderate voltage (e.g. 50V )applications, but is too high for low voltage uses. This is largelyas a result of the insulating passive film which forms and thehigh hardness of the material. Mills'55briefly summarised theproperties of NiSn as a contact material but more recently,Antler has been much involved with such studies. HereportedlS6 r: the interaction of SO, H,S, NH,, NO,, sulphurvapour, salt-spray and synthetic dust with NiSn, Ni,Sn, andNi,Sn,. The behaviour of all three phases was found to besimilar and stability against attack and comparatively smallchanges in contact resistance were found after exposure toeach of the atmospheres. In a later study, Antler '57 ollowed thechange in contact resistance as a function of time in differentatmospheres. Changes were relatea to the growth of thesurface oxide film under different conditions of temperature andrelative humidity. These results have since been rati~nalized"~by that author in the light of studies of the passivation filmcornposition which were discussed above.

8 TI N-COBALT ALLOYSInterest has recently been aroused in electrodeposits of tin-cobalt alloys which in many ways resemble those of tin-nickel.Deposits of SnCo'5R-'59r SnCo mixed wi th Sn,Co'60 have beenobtained and proprietary systems have been patented. Thedeposits are bright and have a colour very similar to decorativechromium plate and advantages in the deposition process havesuggested their use in place of that element in nickel-chromiumsystems. Most studies have therefore been concerned with theperformance of systems consisting of steel coated by nickelwith a very thin film of tin-cobalt alloy to provide a pleasingfinish.Miyashita and Kurihara,I6' in reporting details of the electro-deposition of tin-coba lt alloys, commented on the resistance ofthe coatings to salt-spray and CASS tests and exposure toammonia. They found that the deposit resisted attack in all ofthese and that furthermore. i t s ductility was higher than that ofa tin-nickel deposit.The results of CASS tests and outdoor exposure tests on plated

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systems consisting of nickel plus tin-cobalt alloy were reportedby Hyner'62and then Hemsley and R ~ p e r . ' ~ ~hey each used adifferent bath and minor differences in the deposits wereproduced but their conclusions were very similar. The perfor-mance of the system was claimed to be comparable with thatof a nickel-chromium system in all but the more severeexposure conditions.As might be expected by comparison with tin-nickel alloycoatings, tin-cobalt owes its stability and corrosion resistanceto the formation of an adherent protective passive film. Thiswas demonstrated by Tsuji and I ~ h i k a w a ' ~ ~or a number ofelectrolytes, then Thomas and Shar r r~a '~ ~arried out an Augerand an E.S.C.A. study of the alloy surface. They concluded thatplated coatings of 80%Sn/20%Co alloy exposed to oxygengrow a film similar to that found on 65%Sn/35%Ni atby. That isto say, the surface layer which varies from 6.5-10.5 A consistsprimarily of hydrated tin(lV) oxide wi th some cobalt ( 1 1 ) oxide.Similar results were found for layers of sputtered tin-cobaltalloy exposed to oxygen unless the partial pressure of tha t gaswas low. Under those conditions, the film was primarily tin ( 1 1 )oxide.

9 TIN-SILVERSome years ago, tin-silver alloys were examined as potentialreplacements for sterling silver in decorative applications. Priceand Thomas166compared the corrosion resistance of silver withthe 7.5%Sn/92.5%Ag alloy and found it to have a number ofadvantages. Duckett, Robins and BrittonI6' followed this with astudy of the binary system between tin and silver for alloyscontaining up to 10%tin. The tarnishing of alloys in hydrogensulphide, sulphur dioxide and indoor atmospheres was followedand the resistance to oxidation on heating in air and to salt wascompared wi th that of sterling silver. The tin containing alloyswere found to be at least as good as the copper containingalloys and in some cases they were even better. In particularthe resistance to chloride attack was considerably better anddiscolouration during heating in air was less. The pre-oxidationof tin-silver alloys was found to improve their resistance toattack by sulphur-containing atmospheres.AC KNOWL EDGMENTSThe assistance of Mr. G. O'Boyle in assembling many of thereferences contained herein and the permission of the Inter-national Tin Research Council to publish the paper aregratefully acknowledged.References1 .2.3.4.5.6.7.8.9.10.1 1 .12.13.

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