A fundamental study of the influence of aluminium on the white rusting of galvanized steel

A FUNDAMENT AL STUDY OF THE INFLUENCE OF ALUMINIUM

ON THE WHITE RUSTING OF GALVANIZED STEEL

A FUNDAMENTAL STUDY

OF THE INFLUENCE OF ALUMINIUM

ON THE WHITE RUSTING

OF GALVANIZED STEEL

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT

OP GEZAG VAN DE RECTOR MAGNIFICUS IR. H.J .DE WIJS,

HOOGLERAAR IN DE AFDELING DER MIJNBOUWKUNDE,

VOOR EEN COMMISSIE UIT DE SENAAT

TE VERDEDIGEN OP WOENSDAG 11 DECEMBER 1963 DES NAMIDDAGS TE 2 UUR

DOOR

GERHARDUS JACOBUS ENGELBRECHT

DOCTORANDUS IN DE SCHEIKUNDE

GEBOREN TE PRETORIA, REPUBLIEK VAN SUID-AFRIKA

•

. , I

Dit proefschrift is goedgekeurd door de promotor prof. ir. P .Jongenburger

De s~hrijver wenst zijn dank te betuigen jegens de Suid-Afrikaanse Yster en Staal Industriële Korporasie voor de verleende bijstand bij het uitvoeren van dit onderzoek en voor de vergunning om de resulta-ten daarvan te mogen publiceren, alsmede voor de financiële steun, waardoor dit proefschrift mogelijk gemaakt werd.

Ook wil hij gaarne zijn dank betuigen aan The Department of Physics of the University of the Witwatersrand, Johannesburg, voor de onbaat-zuchtige hulp, verleend bij het uitvoeren van het electronen-micrösco-pische onderzoek aan dunne folies, alsmede de waardevolle steun, die gegeven werd door de staf van dit departement.

CONTENTS

page

CHAPTER 1. INTRODUCTION • 9

1.1 Definition and Description of White Rust • 9 1.2 The Basic Chemical Mechanism of White Rust Formation 10 1.3 Hot Dip Galvanizing and Personal Observations. 12 1.4 Historical Survey of the lnfluence of Aluminium on the

Corrosion Properties of Zinc • 14

CHAPTER 2. SURVEY OF EXPERIMENTAL METHODS 17

CHAPTER 3. CORROSION MEASUREMENTS 21

3.1 Humidity Cabinet Measurements. 21

3.2 Potentiostatic Measurements . 23

3.3 Dissolution Measurements . 24

3.4 Results • 2S

CHAPTER 4. MECHANICAL PROPERTIES • 31

4.1 Internal Friction . 31

4.2 Hardness Measurements 33

4.3 Flow Stress Measurements 34

4.4 Results • 34

CHAPTER S. ELECTRON MICROSCOPY 4S

5.1 Transmission Electron Microscopy. 5.2 Surface Electron Microscopy • 5.3 Results •

CHAPTER 6. DISCUSSION

j •

6.1 The Correlation between the Salient Features ofthe Results with Respect to the Time Af ter Quenching and the Alumi-nium Concentration •

6.2 Precipitation in Super Saturated Solid Solutions • 6.3 Discussion and lnterpretation of Results •

SUMMARY REFERENCES • 45 46 46 61 61 64 66 83 87

CHAPTER 1

Introduction

l.I. DEFINITION AND DESCRIPTION OF WHITE RUST

Zinc is a base metal appearing early in the electrochemical series of the elements, and is therefore a chemical reactive element suscep-tib1e to corrosion by water and aqueous solutions. Depending on the corrosive environment, the manner in which zinc corrodes in the pre-sence of water or water vapour can be divided into two clearly dis-tinguishable processes.

In the first of these, the nature of the corrosion products is such that they will rapidly stifle any further corrosion, thus protecting the re-active metal against continued attack. This is the normal slow atmos-pheric corrosion and it is on this mechanism th at the use of zinc as a protective coating metal for steel is based. In the second case the cor-rosion products offer very little or no protection against continued corrosive attack. The environmental conditions for this type of corro-sion to take place are rather specialized, but unfortunately are often met with in practice and are usually unavoidable. This type of corro-sion of zinc and galvanized articles is called white rusting or, in the United Stat es of America, wet storage stain. lt is recognizable by a white powdery deposit on the surface of the metal. In the case of only a slight attack the underlying metal may be unaltered in appearance or only slightly tarnished. In severe cases the underlying metal is badly stained and in the case of ga1vanized articles the coating may be com-plete1y perforated.

The conditions essential for the rapid corrosion process to become operative are stagnant water in contact with the metal and a differential concentration of oxygen in this stagnant water. Under these conditions bulky, fluffy, porous white corrosion products are formed (Gilbert and Hadden 1950, Bablik 1958). These conditions are met with in practice

whenever articles are closely packed and stored in an atmosphere with varying humidity and temperature, and limited ventilation. Under these conditions water vapour will condense between the articles and serious dainage may result. It is this rapid type of corrosion that will be dealt with in this investigation.

In many countries in Africa, America, Asia and Australia large quan-tities of flat and corrugated galvanized iron sheet and strip are used for roofing and constructional purposes. The use of these materials in these countries is governed by climatic conditions and economie rea-sons.

Flat or corrugated galvanized iron sheet is always transported in packs. Strip is shipped in the form of coils. Such packs or coils are exceptionally vulnerable to white rusting. According to the scientific and patent literature much time and effort has been spent in various countries during the past decade on methods to prevent or ameliorate the white rustingofthis type of material under adverse conditions. The majority of these efforts appear to have been directed towards the development of protective post-galvanizing treatments. These include treatments with chromates, nitrites, tungstates, molybdates, vanadates, carboxilic acids, amines, bensoates and organic resins. Unfortunately none of these methods, with the exception of chromates or chromic acid, have been very succesful. Acid chromates and chromic acid treatments have met with moderate success. Special packaging methods e.g. in prepared paper or feIt have also received attent ion but these methods are costly and also only offer limited protection.

1.2. THE BASIC CHEMICAL MECHANISM OF WHITE RUST FORMA-TION

Accordingto Gilbert and Hadden (1950) the mechanism of white rust formation at atmospheric temperatures can be explained by the follow-ing reactions:

Zn++ + 20H- + xH20 = Zn (OH)2 . xH20 Zn(OH)2

The reaction products are therefore Zn(OH)2 • xH20 and ZnO. Gilbert and Hadden found that Zn(OH)2 . xH20 predominated. These white pro-ducts are fluffy and offer no protection against continued attack. In the presence of water they remain physically and chemically unaltered.

During normal atmospheric corrosion, however, the hydroxide and oxide are converted to the basic carbonate, sulphide or sulphate by the gasses present in the atmosphere.

e.g.

5 Zn(OH)2 + 2C02 = 2 ZnC03 . 3Zn(OH)2 + 2 H20 or 5 ZnO + 2C02 + 3H20 = 2 ZnC03 . 3Zn(OH)2

It is this basic carbonate that forms the hard impervious layer that protects the reactive zinc against further corrosive attack when ex-posed to norm al atmospheric conditions.

Weast; Kotnik and Geehan (1961) have also extensively discussed the chemical reactions that occur during the formation ofwhite rust on zinco They point out the obvious fact that for corrosion to occur, zinc ions must leave the metal surface to enter into solution. This could happen by the hydration of zinc ions according to the reaction

Zn++(s) + 6H20 = Zn(H20)ë+ (aq.)

They give the heat of hydration for the Zn ions in this reaction a~ -492 kcal per mole at 250 C. The hydrated zinc ions can then subse-quently undergo various reactions, not involving large energies (5 to 7 kcals), all resulting in ZnO or Zn(OH)2 or hydrated forms of these compounds depending on the temperature at which they take place.

Whatever may be the exact reaction mechanisms that take place, it is clear that the first step involves the reaction of zinc ions from the metal lattice with ions or molecules from the corrosive medium, in this case water. Factors that influence this first reaction will have a pronounced effect on the complete process, às the subsequent reac-tions are low energy reacreac-tions.

1.3. HOT DIP GALVANIZING AND PERSONAL OBSERVATIONS

Until some years ago all galvanized sheet steel was manufactured by dipping each individual sheet into the molten spelter covered with a layer of a suitable flux. Since World War II the hand dip process has been gradually superseded by continuous processes. Apart from obvious economic advantages, galvanized sheet manufactured by means of these continuous processes is vastly superior to the old hand dipped sheets in many respects. The Sendzimir process was one of the earliest of the continuous processes and the main features of this process will be described briefly.

A continuous steel strip is first slightly oxydized and then reduced at approximately 8000_{C in a hydrogen atmosphere. It is claimed that the} pre-oxidation of the strip removes any residual oil from the cold rolling process and also gives rise to a slightly roughened surface with in-creased reactivity aft er th~ subsequent reducing treatment and thus facilitates the actual galvanizing operation. After the chemical reduc-tion of the surface, the strip is passed through the galvanizing bath without previous contact with the atmosphere. After emerging from the spelter, the strip is rapidly cooled by means of a st rong air blast" and coiled or cut into suitable lengths. The heat required to maintain the temperature of the molten zinc at approximately 4600_{C, is supplied}

by the hot incoming strip. No flux is used in this process.

The formation of intermediate iron -zinc alloy layers between the zinc coating and the steel base is an integral feature of all galvanizing pro-cesses. These alloy layers are brittle, and their composition and thick-ness determine the adherence of the coating to the steel base. In prime quality material it is essential that they are kept as thin as possible. The thickness and composition ofthe alloy lao/er can be controlled by the galvanizing temperature, the dipping time, the composition ofthe spelter and to some extent by the composition of the steel base.

In the Sendzimir process the dipping time i.e. the speed of the strip, is govemed by the heat requirements of the spelter and the temperature of the incoming strip. Thus, the alloy format ion can be controlled only by the composition of the spelter .

Aluminium has the property, when added in small amounts to the

spelter, to suppress the iron-zinc reaction initiaHy. After prolonged dipping times this inhibitive action is lost and the aHoy formation proceeds more rapidly than without aluminium. With increasing amounts of aluminium the inhibition period becomes longer. It is this inhibitive action by aluminium that use is made of in the Sendzimir process to control the iron-zinc reaction.Usually between 0.10 and 0.20% aluminium in the spelter is required for this purpose. In addition to limiting the aHoy formation to a thin layer of the less brittie 't" compound, the pre-sence of aluminium increases the brightness of the coating.

From the above it is therefore clear that by means of the Sendzimir process it is possible to produce a galvanized sheet with an extremely tight coating (in a cupping test the steel wilI tear before the coating flakes) having a very attractive and bright appearance. Furthermore, this can be achieved at high production speeds (speeds in excess of 200 ftjmin.i.e. approximately 1 mjsec., are possible).

Two Sendzimir continuous galvanizing lines have been in operation since 1949 at the Vanderbijlpark Works of the South African Iron and Steel Industrial Corporation. Unfortunately the impression has been gained during the last few years that, apart from the desirabie pro-perties of this type of sheet, the Sendzimir sheet is more susceptible to white rusting than sheet made by means of the old hand dip process which did not contain any aluminium. Accelerated laboratory corrosion tests and field tests have confirmed this to be true.

It was during experiments with post-galvanizing anti-corrosion treat-ments with chemical conversion coatings on these galvanizing lines that certain observations were made which lead to the conclusion that it could possibly be the presence of aluminium that was affecting the cor-ros ion propertiès of the coating adversely.

The first step in the chemical treatment of the strip consisted of a dip in a dilute solution of sulphuric acid. On the galvanizing lines this treatment was applied 40 to 60 seconds after solidification ofthe coating. The laboratory experiments were always conducted on sheets at least 3 to 4 days old._It was found that, whereas a sulphuric acid concentra-tion of 0.5% w jv1) gave the desired affect in the laboratory, a

tration of approximately 2.0% w

I

v

was required to give the same effect on the Sendzimir lines at the same temperature and dipping time. After the sulphuric acid dip achromate treatment was applied. lt was invari-ably found that the chromate treatment applied in the laboratory was more effective against white rust than when applied on the line. In later years a single step treatment with chromic acid showed the same phe-nomenon i.e. that treatmentin the laboratory was more effective in preventing white rusting than treatment on the line.

From the above observations it was concluded that af ter a few days the zinc layer was chemically more reactive and thus ,gave r~se to a more complete conversion coating than immediately af ter galvanizing, It seemed reasonable to assume that this change in the chemical reac-tivity could possibly also affect the corrosion properties of the sheet and that it might be related to incomplete precipitation of the aluminium in the coating. If this was correct an investigation into the behaviour of the aluminium was indicated.

1.4. HISTORICAL SURVEY OF THE INFLUENCE OF ALUMINIUM ON

THE CORROSION PROPERTIES OF ZINC

It is common knowledge that small additions of foreign elements to a metal or alloy can affect the properties of the base metal profoundly. The effect of aluminium additions on the corrosion properties of zinc has been the subject of many investigations in the past. However, in spite of the large amount of work on this subject the position with re-gard to the effect of aluminium additions on the corrosion properties of zinc still appears to be confusing.

Richards (1891) claimed that aluminium additions up to 0.01% Al increased the corrosion resistance of galvanized coatings and also improved the brightness. Burkhart (1937) found that the corrosion resistance of zinc was lowered by aluminium. Bablik (1942) stated that aluminium additions do not affect the atmospheric corrosion resistance ofzinc, but do lower the corrosion resistance in liquids. Gilbert (1946, 1953) concluded from his experiments that aluminium did not affect the corrosion properties of zinc in concentrations up to 3.0% Al. Hughes

(1950) expressed the opinion that the corrosion resistance of zinc was improvedbyadditionsofuptoO.05% Al. Gilbert and Hadden (1950) con-cluded from distilled water spray tests that with an addition of 0.05% Al the maximum corrosion resistance was obtained and that with 0.15% Al the corrosion resistance of the alloy was lower than that of pure zinco Bablik (1958) added to his original opinion that galvanized sheets manu-factured by means of a continuous process (and such sheets always contain aluminium in the coating) tamish more rapidly during atmosphe-ric exposure than sheets made by means of conventional hot dip pro-cesses which usually do not contain aluminium in the coating. Wilcox and Dismukes (1961) showed by means of distilled water immersion tests that galvanized sheet manufactured by means of a continuous process corroded about twice as fast as sheet without aluminium in the coating. From curves published by them it can be seen that aft er 42 days immer-sion the aluminium bearing coating showed a zinc weight loss of approx-imately 8.1 mg/cm2, whereas an aluminium-free coating showed a weight 10ss of only 3.7 mg/cm 2 for the ,same period. From humidity cabinet and water film tests, Sebisty and Palmer (1961) also concluded that the white rustingofzinc coatings was increased to a major degree by alu-minium additions. Although the method of evaluation of the amount of corrosion th at had occurred was admitted to be inexact, Rädeker, Peters and Friehe (1961) arrived at the same contlusion.

From the literature survey presented above, confusion appears to

exist as regards the influence of aluminium additions on the corrosion properties of zinco If however, a distinction is made between atmos-pheric corrosion and corrosive conditions under which no protective la yer is formed, the position becomes clear. It has already been stated that during atmOspheric corrosion a protective layer of basic zinc carbonate is formed and thus, theprotective nature of this layer is the rate controlling factor under these conditions and not the chemical reactivity of the metal. Thus, the results of Bablik (1942) and Gilbert (1946, 1953), are not applicable in determining the influence of alu-minium on the chemical reactivity of zinco The results obtained by Gilbert and Hadden (1950), Wilcox and Dismukes (1961), Sebisty and Palmer (1961) and Rädeker, Peters and Friehe (1961) do, however, indicate that under conditions where no protective layer is formed and

thechemicalreactivityof the base metal is the rate controlling factor, aluminium does increase the corrosion rate of zinco The observation by Bablik (1958) that continuously galvanized sheet tarnishes more rapidly than hand dipped sheet during atmospheric exposure is in agree-ment with this. Aluminium - bearing coatings initially tarnish rapidly due to their increased chemical reactivity but as a protective layer is formed, the rate of corrosion slows down. The observations by Richards (1891), Hughes (1950)andGilbertand Hadden (1950) all seem to indicate that up to 0.05% Al the corrosion resistançe is increased and that it is only with further aluminium additions that the zinc becomes chemi-cally more reactive.

Thus, it is finally concluded that the rate of white rust formation on zinc is lowered by aluminium with additions up to 0.05%, but with further increases in the aluminium concentration the rate is increased. lf this conclusion is correct, the logical question that arises is what is the basic mechanism through which aluminium exerts this influence on the corrosion properties of zinc?

This investigation was undertaken to find an answer to this question.

CHAPTER2

Survey of Experimental Methods

In the liquid phase zinc and aluminium are completely solubie in each other. At room temperature the solid solubility of aluminium in zinc is oftheorderofO.05% by weight (Hansen 1958). A comprehensive optical microscopical examination of aluminium-bearing galvanized coatings failed to reveal any visible precipitates although the aluminium content of the coatings varied between 0.12 and 0.17% Al. This indicated that it was possible that the aluminium in these coatings was held in super saturated solid solution.

It was therefore decided that, in order to obtain more information on the manner in which aluminium affects the corrosion properties of zinc, . experimental methods would have to be employed which could supply

in-formation on the process ofprecipitation in these alloys. Broadly speak-ing the investigation carried out can be divided into three main cate-gories viz. controlled corrosion measurements, the measurement of mechanical properties and a surface and thin foil electron microsco-scopical investigation.

Before reviewing the actual experimental work th at was carried out, the preparation of the alloys used for this investigation will be described. All the experiments were carried out on alloys prepared and heat treated in the same way, except for the technical corrosion tests. The prepara-tion of the specimens for these tests will be described separately.

Aluminium-zinc alloys which contained between zero and 1.40% Al were prepared from 99.99% Zn and 99.99% Al by melting in silica-free alumina crucibles heated in a resistance fumace. After a holding time of about one hour. during which the metal was frequently stirred, the melt was cast into graphite moulds. The moulds we,re heated at the top to ensure sound castings. When cooled, the casts were mechanically shaped into the form required for any particular experiment. The shaping usually consisted ofmachining or rolling, or both. Having been

shaped, the specimens were solution annealed at 3500C for 7 days in a purified argon atmosphere, aft er which they were quenched in water. Depending on the size of the specimens the average 'quenching rate varied from approximately 100 to several 1000C/sec. The. reason for the rather long annealing time will be discussed in para. 4.4.

Some of the measurements were commenced as soon as possible after quenching and continued, in some cases, for periods up to 1,000 hours or more. In other cases the experiments were carried out 24 or 48 hours aft er quenching. All measurements were performed on at least three specimens from each melt and each experiment was repeated on replicate melts prepared ab initio.

For the technical corrosion tests, galvanized steel strips were used. The galvanizing quality mild steel strips were first pickled in hydro-chloric acid and then dry fluxed in an aqueous solution of 10% ZnCl2 and 10% NH4Cl. The strips were then dipped into the molten zinc-aluminium alloys for about 15 seconds. At the higher aluminium concentrations the dipping time had to be increased due to the inhibitive effect of the

alumi-\ .

nium on the iron zinc reaction already described. Af ter dipping the gal-vanized strips were rapidly cooled in a strong air blast~ The zinc and the aluminium metals used for galvanizing of the test strips were the same as used for the preparation of the alloys for the other experiments.

The corrosion measurements were carried out in three different ways. The first test consisted of the exposure of packs of the galvanized strips in a humidity cabinet. This test was commenced 48 hours aft er galvanizing and was continued for 336 hours. In the second test the cor-rosion current of zinc alloys containing various amounts of aluminium was determined potentiostatically. This test was carried out 48 hours after quenching. In the third test the rateof dissolution of the zinc-aluminiumalloys in a buffered alkaline solution was determined. This test was carried out 24 hours aft er quenching and repeated 48 hours after quenching. The rate of dissolution of an alloy containing 0.14% Al as a function of time was also measured.

The mechanical properties that were measured were the intemal friction, the Brinell hardeness and the flow stress 1) of alloys with 1) In this case the elastic limit as determined graphically from the stress-strain

aluminium concentrations ranging from zero to 0.90%. The intemal friction measurements were commenced as soon as possible af ter quenching (usually a few minutes thereafter) and were carried out con-tinuously for periods, in some cases, up to 1,000 hours or more. The hardeness measurements were carried out 24 and 48 hours af ter quench-ing and aft er prolonged aging at room temperature. The flow stress measurements were carried out af ter aging for 48 hours at room tem-perature.

The thin foi! and surface electron microscopical investigation was also carried out on alloys containing various amounts of aluminium up to 0.90%. In this case too, the examination was executed 48 hours after quenching.

In addition to the above measurements, a few specially selected ex-periments were performed. The intemal friction of two alloys was measured continuously as a function of time at 3500_{C to determine the} time required for effecting the complete solution of the aluminium. The two aluminium concentrations at which this was done were 0.23% and 0.90%. Corrosion measurements were carried out on an aHoy with 0.14% Al which was quenched, aged for48 hours at room temperature and then re-annealed ai: temperatures between 50 and 1200_{C for 24 hours.} FinaIly, thin foils prepared from an alloy containing 0.6% Al which were quenched, aged for 48 hours at room temperature and re-annealed at 1200_{C for 24 hours, as weIl as foils from the same alloy which were} solution annealed and fumace cooled, were examined electron micro-scopically.

To facilitate a better comprehension of the experimental work that has been carried out, a scheme of the experiments is presented in table Il.I.

In the further description of this investigation frequent reference will be made to the aluminium concentration of the zinc-aluminium alloys as weIl as the time after quenching. To avoid laborious reference each time to these two variables it is suggested that the following no-tation is used: <x/y> where x refers to the aluminium concentration in percentage and y refers to the time aft er quenching.

N o Type of measurement Corrosion (humidity cabinet) Corrosion (titration) Corrosion (titration) Corrosion (titration)

Internal friction lnternal friction Hardness Flow stress microscope (thin foil) Electron microscope (thin foil) Electron Electron microscope (surface) Percentage aluminium 0.00 to 0.90 0.00 to 0.90 0.00 to 1.40 0.14 0.14 0.00 to 0.90 0.23 and 0.90 0.00 to 1.40 0.00 to 0.90 0.00 to 0.90 0.60 0.60 0.14, 0.60 and 0.46 TABLE 11.1

Time at which measurements

conti- 24

nuous

Remarks

Test carried out on galvanized strips. Exposure commenced 48 hours

af ter galvanizing.

Alloys were solution annealed, quenched and aged at room temperature

for 48 hours and re-annealed between 50 and 1200_{C for 24 hours.}

Measurements carried out at 350oC.

Alloy was solution annealed, quenched and aged at room temperature

for 48 hours and re-annealed at 1200_{C for 24 hours.} AHoy was solution annealed and furnace cooled.

CHAPTER3

Corrosion Measurements

The correlation between corrosion results obtained by means of accelerated tests and actual field tests is often difficult due to the un-predictabie effect of the corrosion products. In the case of this inves-tigation, matters were simplified due to the fact th at it was the che-mical reactivity ofthe alloys that was of interest and not the nett effect ofthe chemical reactivity plus corrosion products. However, to ensure that the conclusions derived from the results would be dependable, three completely different methods in different environments were used to measure the chemical reactivity. The first was in the nature of a technological test in which the conditions in practice were simulated as closely as possible, the second was an electrochemical method where-as the third wwhere-as a purely chemical determination. These three methods will be described separately.

3.1. HUMIDITY CABINET MEASUREMENTS

Strips 5 x 12.5 cm of galvanizing quality mild steel were galvanized with zinc alloys containing between zero and 0.90% Al as described in chapter 2. Microscopic examination of the coatings showed that they were similin to the coatings produced on the Sendzimir galvanizing lines except for the thickness of the coating and the size of the spangle i.e. the crystalline structure of the surface. The coating thickness varied from specimen to specimen but was always larger than that pro-duced-on the Sendzimir continuous galvanizing lines. This was not con-sidered to be of any consequence because the experiments were termi-nated long before the coatings were perforated. The spangle was smaller than that of sheet produced on the production lines. The flat strips were weIl washed with water to remove any flux residues and then dried and

-weighed. Fo.rty-eight ho.urs after galvanizing six o.f these strips were assembied into. a pack with two. similar but unweighed strips o.n the o.utsides. Fo.r each co.mpo.sitio.n o.f spelter three separate packs were assembied. The packs were held to.gether with rubber bands. These packs were then expo.sed in a humidity cabinet.

The humidity cabinet which was used fo.r the purpo.se o.f these tests is a 1000 liter capacity apparatus o.f Swiss manufacture. The cabinet is cylindrical in shape and made o.f transparent plastic material. A generato.r in the base o.f the cabinet pro.duces a fine co.rro.sive mist, fro.m any desired so.lutio.n, which is slo.wly swept upwards thro.ugh the

cabinet by a fo.rced air flo.w o.f abo.ut 30 liters per. minute. The specimens

are suspended high up in the cabinet to. ensure a ho.mo.geneo.us atmo.s-phere aro.und them. The tests can be carried o.ut at any auto.matically

. co.ntro.lled temperature up to. 55o.C o.r varied acco.rding to. a

predeter-mined pro.gram. The aero.so.lmist can be turned o.n and o.ff manually o.r also. acco.rding to. a preset pro.gram.

Resulting fro.m many years o.f experience with two. o.f these cabinets the fo.Ilo.wing test co.nditio.ns were fo.und to. give results o.n galvanized steel sheets that co.rrelated weIl with field tests o.n the same material and were therefo.re ado.pted fo.r the purpo.se o.f this investigatio.n. The aero.so.l co.nsisted o.f a so.lutio.n o.f 0.5% ZnS04 in water. The purpo.se o.f the zinc sulphate was o.nly to. increase the aggressiveness o.fthe en-viro.nment. Fro.m the previo.us experience the zinc sulphate was no.t fo.und to. affect the relative co.rro.sio.n rate betweèn galvanized co.atings o.f different o.rigin o.r co.mpo.sitio.n as co.mpared with the results o.b-tained from field tests. The aero.so.l was turned o.n fo.r 45 minutes and then o.fffo.r 15 minutes. The temperature was maintained at 35o.C thro.ugh-o.ut the test as weIl as the additio.nal airflo.w. Each test was co.ntinued

fo.r between 300 and 400 ho.urs. In the case o.f the results to. be presented

in this investigatio.n the testing time was 336 ho.urs.

After expo.sure, the packs were o.pened and each strip cleaned by

dipping in a

1.0%

citric acid so.lutio.n, gently rubbing with co.tto.n-wo.o.l,

rinsing in water and alco.~o.l, fo.llo.wed by drying with ho.t air. The strips were then re-weighed and the average weight lo.ss o.f zinc in milli-grams per square centimeter o.f sheet determined.

3.2. POTENTlOST ATlC MEASUREMENTS

For these measurements, round bars of zinc with various amounts of aluminium prepared, as described in chapter 2, were cast in heated graphite moulds. From each bar three short sections were cut off. The one face ofthe cut section was ground and polished. The specimens were then given the customary annealing and quenching treatment. After aging for 48 hours each section was mounted in Stellon R cold setting dental re sin in such a manner that only the polished surface was exposed.

Im-mediately before mounting in the electrode cell, the surface was lightly cleaned with a 1% hydrochloric acid solution to remove surface oxides. The potentiostat used for this investigation was a "Teclement" of French manufacture. As reference electrode a tungsten electrode was used whereas the auxiliary electrode was a platinum disco The electro-lyte was a deci-normal potasium chloride solution. Various other elec-trolytes were initiallytried, but it was found that this solution gave the best resolution. The scanning speed was 150 mV jhour. An extern al micro-ampere meter w'ith adjustable sensitivity ranges was used for greater accuracy. The current was read at 5 mV intervals.

The cell potential i.e. zinc versus tungsten, was plotted against the logarithm ofthe cell current. At high cell currents this curve becomes a straight line when the zinc is anodic with respect to the tungsten elec-trode. ln this region the cell current is determined by the electrode

potentialof the zinc only, according to the Tafel equation:

f1 V

=

a + b log i

where f1 V = the difference between the electrode potentialof the zinc and the rest potential

i = cell current

By extrapolation of the straight part of the curve to the rest potential i.e. th at potential at which the applied current becomes zero, the

cor-rosion current io ofthe zinc electrode in the particular electrolyte can

be determined. By measuring the exposed surface area of the zinc electrode the corrosion current per square centimeter could be cal-culated. In this manner the specific corrosion current for zinc alloys with various amounts of aluminium was determined.

Although it is probably already clear from what has been said above, it may be useful to drawattention to the fact that with potentiostatic measurements the surface of the specimen is kept clean and free from

corrosion products at all times due to the current through the cello

Thus, this type of measurement gives information about the corrosion activity of a metal without the disturbing influence of protective oxide layers. From what was been said in paragraph 1.3 it is this corrosion activity of the bare metal that is important in determining the rate of white rusting.

3.3. DISSOLUTION MEASUREMENTS

The third method used to determine the chemical reactivity of the

zinc-aluminium alloys was based on the rate of chemica I solution of

the specimens in an alkaline solution buffered at a pH = 10.

The cast alloys were rolled into strips approximately 0.5 mm thick. Specimens of about 5 sq.cm were cut from these strips and cleaned in dilute hydrochloric acid. They were then annealed and quenched in the usual way.

The measurement of the rate of dissolution in the manner to be de-scribed below, was carried out 24 hours af ter quenching and repeated

on the same specimens 48 hours after quenching. In another experiment

the dissolution rate was determined of specimens of an alloy containing 0.14% Al which were first aged at room temperature for 48 hours after quenching and then re-annealed for 24 hours at 50, 60, 70, 80, 90 and 100oC. One set was not re-annealed but just left to age further at room temperature during this time. Furthermore, the dissolution rate of specimens ofthis same alloy was determined at intervals as a function of time af ter quenching up to 50 hours thereafter.

For the actual determination the specimens were mounted vertically in a plastic frame and placed in 100 mI of an ammonia-ammonium

chloride buffer solution with pH = 10. The specimens corroded in this

solution and the zinc was dissolved to form complÈ~xes of the type

(NH3)3 ZnCl5 and (NH3)2ZnCI2. The amount of zinc dissolved was de-termined every 4 minutes by titration with O.OlN E.D.T.A. (ethylene

diamine tetra-acetic acid disodium salt). This titration required approx-Jimately 5 to 10 seconds. Eriochrome black T was used as indicator. E.D.T .A. forms a stable complex with zinc which does not dissociate at the chosen pH of the buffer solution. Aluminium does not form a com-plex with E.D.T.A. at this pH. Thus, the zinc dissolved was determined and simultaneously removed from the solution at regular intervals while the dissolved aluminium had no effect on the determination. For repro-ducible results it was found that the solution had to be stirred and that the speed of stirring was critical, The magnetic type stirrer used was therefore run from a constant voltage transformer and adjusted to 9 r.p.m. bymeans of a stroboscope. With these precautions the reprodu-cibility obtained was satisfactory. After repetitive titrations at four minutes intervals the average rate of solutioh ofthe zinc was calculated in milligrams per minute per square centimeter. In the case of the de-termination of the rate of solution as a function of time after quench-ing, only duplicate titrations were made during the first period, be-cause at this stage the time involved for each determination was still comparable with the time of aging.

Just as in the case of the potentiostatic maesurements, it is clear that in the case of these dissolution measurements a clean metal sur-face was automatically maintained at all times.

3.4. RESULTS

The results of the humidity cabinet experiments are presented in figure lIl. I and the results of the potentiostatic measurements in figure 111.2. The equivalent results for the dissolution method are given in figure III.3. The results of the amount of zinc dissolved per minute per square centimeter as a continuous function of time aft er quenching are found in figure 1II.4. and finally the results for the 0.14% Al alloy re-annealed at various temperatures, are found in figure 111.5.

~ Ó> E .E ~ .Q :E 0> .~ ~ <'I. E ~

t

"-.E

l

~ c

a

0 ~

26

FIG. III.I 430 )(10-: 3 390 350 310

(

_r\

\

_\

270

i\

\

V

~

I--....

I--lA percentage aluminium

Curve ofweight loss of zinc as determined in a 336 hour humidity cabinet experiment vs. percentage aluminium.

FIG. IIl.2 20 1\

/

~

~

/

---

t--o

V

15 12 8 4 percentage aluminium

Curve of corrosion current as determined potentiostatically 48 hoursafter quenching vs. percentage aluminium.

Ol E ~ ! _c N ti> E N E .!! .~ C N d> E FIG. 1II.3 xl0 -18

~

~~

_,

...

-

-

t

-"Y

._-

1-- ___

_~

--1'6

I

_ _ 24 hours aft ... q..oencting

--._e_

48 ,. .. ..

I

I

02 04 06 12

Curve of weight loss of zinc as detennined by dissolutionmethod vs. percentage aluminium. FIG. III.4 2,20 .1er 2'12 204

~

\

\

1<l6 168

\ I

V

V

50 100 500 1000 hOLrS of ter ~g

Curve ofweight loss of zinc for a 0.14% Al aHoy as determined by the dissolution method vs. hours-after quenching.

"'_E . -"!

"

:€ _c N

'"

E FIG. UI.S 1 xl0-2 2·0 1·9 i-o-

-r---

_~

""

,.8

"

40 60 BO 100 120 140 160 re -crnealing temperature in oe

Curve of weight loss of zinc for a 0.14% Al aHoy quenched from 3500C and re-annealed for 24 hours as determined by the disso-luÜon method vs. re-annealing temperature.

Figurés lIl. I , III.2 and IlI.3 all three show a lowering of the reac-tivity up to approximately 0.05% Al. When the aluminium content is increased above this value the reactivity rises sharply to a maximum. This maximum is reached at about 0.20% Al on the humidity cabinet and the potentiostatic curves. The dissolution curve shows this maximum at about 0.25%. The somewhat higher value for the aluminium content at which themaximum occurs in this latter case is probablydue to the more effective quenching of the small thin specimens used for these measurements.

After the maximum has been reached, the reactivity drops again, first rather rapidly and then more gradually. The transition from the rapid to the more gradual decrease occurs at about 0.35 - 0.40% Al. Thus, the reactivity curves can be divided into four stag~s with the transitions from the one stage to the next occurring at approxiinately 0.05, 0.20 and 0.40% Al. The transition from a sharp drop to a more

gradual drop in the corrosion rate at about 0.40% Al. is not as clearly

defined with respect to the aluminium concentration at which this change occurs as the other transitions.

The reactivity of the 0.14% Al alloy as a continuous function of time aft er quenching (figure III.4) shows a weU defined minimum at about 4 hours af ter quenching, af ter which it rises again to a value which re-mains steady af ter 10 hours. This curve clearly indicates that during the first few hours aft er quenching, changes take place in the alloy which also exert their influence on the corrosion rate.

From the curve of the reactivity vs. annealing temperature for the 0.14% Al alloy (see figure IIl.S) it is evidentthatthe reactivity is lowered as the annealirig temperature is increased. From the shape of this curve it appears as if the reactivity would be lowered even further with a

CHAPTER4

Mechanical Properties

The mechanical properties that we re measured were the internal friction, the hardness and the flow stress. These properties were either determined as a function of the aluminium concentration at a fixed time aft er quenching or as a function of time after quenching at a fixed aluminium concentration, or both.

4.1. INTERNAL FRICTION

The intemal friction and resonant frequency was measured conti-nuously, in some cases for periods up to 1,000 hours or more aft er quenching, for alloys with various amounts of aluminium. In addition to the above, the intemal friction oftwo alloys containing 0.23 and 0.90% Al was determined continuously at 3S00_{C during the solution annealing} of these alloys. They were then quenched, aged for 48 hours and the experiment repeated on the same specimens.

The intemal friction measurements were carried out at high fre-quencies with the aid of an "Elastomat", an instrument designed and manufactured by Dr.Förster, Reutlingen, Germany. In this apparatus the specimen, in the shape of a round or square bar, is suspended on two thin tightly stretched steel wires at two acoustical nodes. The bar is brought into vibration by means of a crystal transducer connected to one end of the bar by means of a very thin steel wire. The vibration of the bar is picked up at the other end by a similar transducer. By adjustment of the driving oscillator, the specimen can be made to vi-brate in either the transverse, torsional or longitudinal basic modes of vibration, or harmonies thereof. The instrument is equipped with a small oscilloscope by means of which can be determined whether the bar is vibrating in one of the three basic modes or harmonies of these

frequencies. The resonant frequency is determined by counting the number ofoscillations for a 10 second periode The counting is done by meansof a crystal controlled scaler and the counting time is auto-matically controlled. To determine theinternal friction the excitation of the specimen is interrupted and the number of oscillations counted while the amplitude of vibration of the specimen decays from a value ao to ao/e. The opening and closing of the gates at values ao and ao/e is automatically controlled by the instrument. The logarlthmic decay (internal friction) is given by

1 _ao

D = - l n

-n _an

where n = number ofcycles during which the amplitude decays from ao

to an. If _{an =ao/e then}

1

D=-n

The above equation for the logarithmic decrement is only va lid if the iriternal friction is amplitude independent. To determine whether this was the case within the amplitude range of the experiments for the alloys 'that were investigated, the logarithmic decrement was deter-mined with a high exciter amplitude and low receiver sensitivity and vice versa. No signifIcant differences could be determined and it was therefore concluded that in this case, the internal friction was ampli-tude independent. Thus, the internal friction was found simply by taking the reciprocal of the reading on the scaler. Rapid repetitive readings are possible with this instrument.

All the measurements except the high temperature determinations, were made in the longitudinal mode of vibration. This was done because it was found that this mode of vibration was less affected by extraneous acoustical noise. The resonant frequency varied between 14,000 and 18,000 c.p.s.

The specimens, with various amounts of aluminium, were melted and cast in the usual manner in heated graphite moulds. Round bars about 1 cm diameter and 13 cm long with a very fine surface finish, were turned from these casts. The bars were then annealed and quenched in thenormal way. Immediately after quenching they were mounted in the

Elastomat and as soon as the resonant frequency of the longitudinal vibration had been found, the measurements were commenced. This was usually approximately 10 minutes af ter quenching. The measure-ments were made at frequent intervals at first and as aging proceeded, at longer intervals.

The purpose of the intemal friction determinations at 3500_{C on the}

two aUoys containing 0.23 and 0.90% Al was to determine the time

re-quired to accomplish complete solution of the aluminium. In this case the torsional mode of vibration gave the best results. Although this experiment is described in this chapter, it was actually the first

ex-periment to be carried out, because these results were required to

determine the standard solution annealing time.

It was reasoned that the internal friction would change as long as the aluminium went into solution and when complete solution was achiev-ed the intemal friction would remain constant. It was realisachiev-ed of course, that stress relieving and recrystallization would also affect the intemal friction, but nevertheless when a constant value for the internal friction was obtained, this would indicate that a steady equilibrium condition had been reached at the annealing temperature. The experiment was carried out in a high temperature fumace (supplied with the elastomat) in a protective argon atmosphere.

4.2. HARDNESS MEASUREMENTS

The hardness measurements on zinc alloys with increasing amounts of aluminium were carried out on cast blocks, surface ground and po-lished before annealing and quenching. A Franck BrineU hardness tester was used. The conditions for these measurements were a 5 mm dia-meter balI, a load of 125 kg and a loading time of 30 seconds. The loading time was automatically controlled by the instrument. The hard-ness was determined 24, 48 and 34,000 hours af ter quenching.

4.3. FLOW STRESS MEASUREMENTS

The flow stress of the zinc alloys with various amounts of aluminium

was determined on strips by means of a continuous tensile test. The

strips were rolled from the cast alloys to a thickness of I mm and then accurately dimensioned and smoothly finished to a width of 15 mm and a length of 200 mm, after which they were annealed and quenched. The tests were carried out on a modified Denison mechanical testing ma-chine. The modifications related to a reduction in the speed of straining, which was lowered to 5 mm per minute and the addition of a sensitive stress-strain recording device. The flow stress was determined from

the stress- strain curve .. All measurements were carried out at 48 hours after quenching.

4.4. RESUL TS

The results for the internal friction measurements on the 0.23 and 0.90% Al alloys at the solution annealing temperature are given in figures IV.l and IV.2 respectively.

. 6 1Î ~ II t ~ FIG. IV.l 250 xl0 -4 200 ~

_.

... L _ _

-,

( r · / \

J1

o 0 I

,

I

.

150 100

~~~

",'

f/

,

'--*""

50 o o 20 40 60 80 onrleoling Ü'ne-ho..rs _ _ _ _ 1st anneal -- . . -- -- 2 n d "

I

I

100 120 140

Intemal friction at 3500_{C of a 0.23% Al aHoy as a function of}

250 x1<T4 200 150 100

.,J.---,,0

/ " 50 !Y o o 20

...

_-""

FIG. IV.2 .,,),e;-...

:/'

.--....

..".. 0 ~lstCJ"'lneQI -~

_._._2nd

..

I

I

40 60 80 100 120 140 antWoiW1g time-hours

Intemal friction of a 0.90% Al aHoy at 3500C as a function of an-nealing time.

From these figures it will be seen that the internal friction of the .

0.23% Al alloy reaches a steady value after about 60 hours annealing

at 3500_{C. The steady value forthe 0.90% Al alloy is reached aft er about}

100 hours. As has already been said the steady value was taken to in-dicate that after this period no further solution of the aluminium or any other structural changes took place. To be on the safe side a stan-dard solution annealing time of 7 days was adopted for all experiments.

Wherever reference to the solution annealing of the zinc-aluminium

alloys is made, a standard annealing time of seven days is implied. As these high temperature internal friction measurements do not contribute anything further to the general investigation described in this report, they will not be referred to again.

The results of the continuous measurement of the internal friction

as a function of time after quenching for alloys with various amounts.

FIG. IV.3 2 <4xK)-4

~

"erna! frJion 1!\ _ . _ _ f~y , 905

-

... _1;

""...,

1'-, ....

.

-._~ _. Je' to"ó

..-B !\B75 4 1 Hl 50 100 500 F\re Zinc

Internal friction and resonant frequency as a function of time af ter quenching. FIG. IV.4 4 14.290

7x10-~~fJ.,

_._._fre~ 5 14.280 .< ~

ij\

l4.270

!

!.

,---

rr1

' v

0

,

_...., .< _,., 14.'260 ~ 3· 9 3·7 0'1

--

...

_...

_,

0·5

...

,

'.

t---... ~. 1·0

-'

~-

...

-5·0 10 hours after qu<nehng

50 100 500 14.250

1 4,240

1000

Internal friction and resonant frequency as a function of time after

quenching. Note the beginning of a peak in the internal friction at

approximately 4 hours.

FIG. IV.S 15 xl0-4 o 0 0 tt t I ï ; '$ 'i I: 13 \ . . \ ~ Intema fnction _ . _ e _ freqJency 1 4,560 14.550 u

.

~ 11 1 4.540 ~ 0 - _

i'

I " ~

..

~ S

>-/

\

._e_

I ~-. 1

V

~

~',

.

I 1' ....

o

~

.

... 4,530

1

5 01 1{l ...

,

"'

I I \

_...

....

\

i'...

50 10

heus otter q.Jenchng

1 4.520

~

1

1()() 500 ~.51O

Intemal friction and resonant frequency as a function of time aft er quenching. Note the enormous increase of the intemal friction at approximately 4 hours after quenching.

15 x1o-13 11 9 5 01 4

fî

.

-l

~_

...

.-...

00

!

\ I

v

G

_{0·5 1'0} FIG. IV.6

~

intemal

fricL

14.660 _._._t~ 14.650 1 .... ,-, /

'

...

__

.

_-

.

...

r-

--1

\

I'--. 4.620 5·0 10 50 100 500 ~,6lO hoursofter ~

Intemal friction and resonant frequency as a function of time after quenching. Note that the height of the peak in the intemal friction at approximately 4 hours after quenching is less than for 0.12% Al

(fig. IV.5).

38 14 .10-4 2 10 8 4 01

'\.

,FIG. IV.7

f

\

~

_\

~

\

_r-.... ...

'"

0·5 '·0 50 100 500 1000

hours aft~ quenching 0·22% Aluminium

Intemal friction as a function of time aft er quenching. Note that

the height of the peak in the intemal friction is less than for the

0.12 andO.14%AI alloys (fig. IV.S and IV.6) and that it now occurs

at approximately 1 . 2 hours after quenching. FIG. IV.8 xU-~ "tema

(riL

4 1tj010 _._._f~ 5 _1t\ ()()() ---.~

.

u ~

Î"'-.

-

....

-4 990~ ·0_.

--e __

1--

-_e __ .

~

Q ..0. _5,

-

_~

3 """-. 5, gJO 1 01 50 10 ~O 100 500 10005 960

Intemal friction and resonant frequency as a function of time after quenching. Note that the peak in the intemal friction has practically disappeared.

.~ t ;f

]

:Ë FIG. IV.9 6 .Kr 4

~nta'YlCllf

ric

lton

15,910

_._._f~ ~ 900 4

~

_~ '\ 3

----

...

_....

~

...

"-

5,8

...

-

...

~

..

.

.

.

-~870 5860 05 50 10 50 100 500 1000

hOlrS aftel" quenching

Intemal friction and re sonant frequency as a function of time after quenching. FIG. IV.IO

.

~

intern<i frieL., 1 5,040 _e_e_freq..aenc.y !\030 ~ ""'-~ """--. ~

r-.

4 3

r-.----.. - ... --

"....-

... /,;--

...

.

; ' .... r-- ... "0

_-.

,,0

-4,990 1<) 5{) 10 50 100 500 1000

hours otter q.JenChi'1g

Intemal friction and resonant frequency as a function. of time after quenching.

It will be noticed that for pure zinc and alloys containing 0.45, 0.60 and 0.90% Al the intemal friction does not change very much during aging. However, for the 0.08% Al aHoy a smaH peak begins to appear at about 4.0 hours after quenching. At 0.12% Al this peak becomes enormous. At 0.14% Al it is still large but not quite as large as for the 0.12% Al alloy, and at 0.22% Al it has shrunk still further. This peak

is so pronounced that it must bear a significant relation to the changes

taking place in the aHoys during aging. Possible explanations for the occurrence of this peak will be discussed in chapter 6.

It has already been stated that in the case of some of the alloys meas-urements weremadeafterprolongedperiods of aging. Table IV.1 gives the results for alloys for which this was the case. For the sake of com-parison, figures for shorter aging times are included in the tabie.

TABLE IV.1

Percentage Time after Intemal friction

aluminium quenching _{in hours} x 104

0.08 24 4.08 986 4.07 0.12 72 5.39 314 5.59 1563 5.69 0.14 96 5.25 450 5.51 2150 5.60 0.22 30 4.80 1290 5.12 0.45 24 1.98 247 1.91 652 1.98 2573 2.97 0.90 21 4.05 143 4.19 980 4.75 2623 5.43

The values for the pure zinc are not given, as the specimens

re-crystallized completely at room temperature after a relatively short time.

It can be seen from this tabie that the values for the alloys with 0.12,

0.14 and 0.22% Al showed only small changes after prolonged aging, but the 0.45 and 0.90% Al alloys showed somewhat larger changes, al-though these were still small as compared with the peaks a few hours af ter quenching for the 0.12, 0.14 and 0.22% Al alloys.

From the curves presented above, the internal friction va lues for 48 hours after quenching were taken and plotted against the aluminium

concentration. In thecaseofpurezinc the value taken was 15 hours and in the case of the 0.60% Al aHoy 21 hours after quenching. This latter value would presumably have been somewhat lower after 48 hours. The curve obtained in this way is given in figure IV.U.

5 x1() 4

1\

5

t

~

3 1 00

I

Q-2 ,

\

\

0-4 FIG. IV.U

/

I

V

-05 0-8 10 12 14

48 Hours Aft ... Q.Jenemg

lntemalfriction approximately 48 hours after quenching as a function of the aluminium concentration.

From this curve it can be seen that the internal friction reaches a maximum at about 0.15% Al and a minimum at about 0.50% Al.

The results of the hardness measurements are presented in figure IV.12. 0 Si Sl! ;n lil I 1 ~ ~

2

0 !i E 80 70 60 50

}

40

vol

30 00

f

~_x-·

."

Ij FIG. IV.12

v--

_~

_...

_{. - -}~---1

-/ '

-

...

-

~--

.-.-.-.-

·-x· .X_·-·

- - 0 - 0 -24 hors alter quencmg

_ e _{___ 48 .. ..} ·-x·-x34.000 .. .. ..

I

I

10 12 lA

percentage cUrWium

Brinell .Dardness 24, 48 and 34,000 hours after quenching asa function of the aluminium concentration.

They show that, above approximately 0.30% Al the hardness only drops slightly from 24 to 48 hours aging. However, after prolonged aging these values are lowered considerably. It can also be seen th at below 0.30% Al

the hardness does not change significantly with time. The 24 and 48 hour values show a small arrest at about 0.05% Al. The inflection of

the 34,000 hour curve between zero and 0.20% Al mayor may not be

significant. This inflection depends on the value of the 0.14% Al aHoy only, and it would be unwise to artach undue significa~ce to it.

The flow stress results, 48 hours after quenching, are presented in figure IV.13.

10 8 4

~

2 o 00

V

v

0'2 FIG. IV.13

~

V

r/

V.

04 06 08 10 12 14 percmtoge durTrim

Flow stress 48 hours after quenching as a function of the alumi-nium concentration.

The only sifnificant feature of these results is the deep minimum at approximately 0.15% Al. The initiallowering of the flow stress. as the aluminium content is increased from zero to 0.15%. is perhaps some-what unexpected. It will be discussed in detail in chapter 6.

CHAPTERS

Electron Microscopy

5.1. TRANSMISSION ELECTRON MICROSCOPY

With the methods usually employed for the preparation of thin metal foils for transmission electron microscopy, these foils are subject toa certain amount of unavoidable mechanical deformation before they are mounted ready for examination in the microscope. In the case of the alloys th at were examined in the course of this investigation it was thought wise to employ a method of sample preparation that would avoid deformation of the thin foils as far as possible. The method used was suggested by Wilsdorf (private communication).

The cast zinc alloys with various aluminium concentrations were rolled into strips about 1 mm thick; Round discs 2.4 mm in diameter werepunched from these strips. In an accurately aligned press a hemi-spherical hollow was pressed into the cent re of these discs. The wall thickness at the bottom of the depression was approximately 0.1 to 0.2 mmo These discs were then annealed and quenched. After quenching the specimens were rapidly electro polished in a 10% chromic acid solution until the wall thickness was about 0.03 to 0.05 mmo After this, they were slowly thinned further in a phosphoric acid-alcohol mixture while being continuously viewed through an ordiriary optical microscope. At the appearance of a sm all hole at or near the centre of the depression, the polishing was interrupted and the specimen carefully rinsed in water and acetüne and dried. In the majority of the specimens the area around the hole was thin enough over a sufficiently large area to permit a con-venient examination of the specimen. Due to the support of the rigid ring of thick material around the thin section, the specimens could be handled conveniently without disturbance of the thin foil in the centre.

The specimens were mounted without any support in the micro scope. The examination was carried out in a Siemens Elmiskop at 100 kv.

Most of the specimens were examined at 20,000 times magnification on the screen, but in a few cases this was increased to 40,000 times.

Some of the specimens were polished immediately af ter quenching and others 24 or 48 hours after quenching. The examination of the foUs was carried out either 48 or 72 hours af ter quenching. No detect-able differences were observed between foils polished or examined at different times after quenching.

Apart from the alloys that were heat treated and quenched in the normal way, a 0.60% Al alloy was aged for 48 hours at room

tempera-ture and then re-annealed for 24 hours at 1200 _{C. Another specimen,}

also with 0.60% aluminium was not quenched but slowly cooled in the fumace after solution annealing. These specimens were then polished,

thinned and examined in the same manner as described above.

5.2. SURFACE ELECTRON MICROSCOPY 1)

Specimens ofthe alloys prepared in the usual way were ground down to the finest grade ofwaterpaper and then annealed and quenched. Forty eight hours after quenching they were electro polished in a 20% chromic acid solution and replicas of the surface prepared the same day.

5.3. RESULTS

Typical electro~ micrographs of solution annealedand quenched thin

foils of pure zinc and zinc alloys with various amounts of aluminium are given in figures V.l, V.3 to V.8, V.lO and V.ll. The diffraction pattems associated with figures V.l and V.8 are given in figures V.2 and V.9 respectively. Micrographs representative of the structures observed in the re-annealed and fumace cooled 0.60% Al alloy are given in figures V.ll and V.12 respectively.

1) The author is indebted to the Council for Scientific and lndustrial Research, Pretoria, for the assistance rendered with this part of tlÎe investigation.

FIG. V.I

Pure zinc quenched from 3500_{C showing stacking faults and}

dis-locations. (x 20,(00)

FIG. V.2

Diffraction pattern of fig. V.l. Basal plane orientation. Bright left spot is centre spot.

48

FIG. V.3

0.03% Al alloy quenched from 3S00C showing extended

disloca-tions. (x 20,(00)

FIG. V.4

Same as fig. V.3 except that this represents a 0.08% Al aHoy. (x 20,(00)

FIG. V.5

0.12% Al alloy quenched from 3500C. Note absence of stacking faults and extended dislocations. (x 20,000)

FIG. V.6

0.35% Al alloy quenched trom 3500C.

FIG. V.7

0.45% Al aHoy quenched from 3500C.

(x 20,000)

FIG. V.8

0.60% Al aHoy quenched from 3S00C. Note loops aligned in rows with dislocations attached or partiaHy attached to rows.

FIG. V.9

Diffraction pattem of fig. V.S showing basa!. plane orientation.

FIG. V.lQ

Same as fig. V.S.Note absence of contrast between inside of loops

52

FIG. V.ll

0.9% Al aHoy quenched from 3S00C. Loops are too numerous to

distinguish rows. (x 20,000)

FIG. V.12

0.60% Al aHoy quenched from 3S00C and re-annealed for 24 hour ü

FIG. V.13

0.60% Al aHoy furnace cooled from 3500C. Note large hexagonal precipitate partic1es. (x 20,000)

When interpreting electron micrographs of thin metal foils, it is of the utmost importance to keep in mind that the images produced on the screen (and on the photographic plate) are contrast effects mainly re-sulting from diffraction phenomena. Contrast effects due to differences in absorption in the foil are only of minor importance.

A brief description of the manner in which the images of crystalline metal foils are formed in the electron miscroscope will assist the in-terpretation of the micrographs that have been presented. A compre-hensive discus sion on the subject of electron microscopy of thin foils and a survey of the available literature has been published by Thomas (1962 (a», to which is referred for detailed information.

When an electron be am pàsses through a thin foil of a single crystal a Laue diffraction pattem is formed. Due to the extreme thinness of the foil only two of the Laue diffraction conditions need to be satisfied for a spot to result in this case. In the electron micro scope an image of th is Laue pattern is formed in the back focal plane of the objective lens. lf

a wide enough objective aperture is used and the magnification of the subsequent lens system (e.g. an intermediate and projection lens) is