Corrosion of Ceramic and Composite Materials Part 10 potx

A list of about 30 glass compositions with theirresistance to weathering, water, and acid has been publishedto the effects of glass composition upon dissolution, the actualcorrelation is

be devoted mostly to aqueous attack.

In general, very high silica (>96% SiO2), aluminosilicate,and borosilicate compositions have excellent corrosionresistance to a variety of environments Silicate glasses, in

general, are less resistant to alkali solution than they are toacid solution A list of about 30 glass compositions with theirresistance to weathering, water, and acid has been published

to the effects of glass composition upon dissolution, the actualcorrelation is with glass structure not composition This is sobecause composition determines structure An example of thiswas indicated by Brady and House [6.3] They determined thatglasses that were silica-rich and highly polymerized dissolvedmore slowly than those containing large amounts of othercations The key structural factor is that highly polymerizedglasses dissolved more slowly

The deterioration of a glass surface by atmospheric

conditions, commonly called weathering, is very similar to that

described above If droplets of water remain on the glasssurface, ion exchange can take place with a subsequent increase

in the pH As the volume of the droplets is normally small

types is depicted in Figure 6.1

A weight loss of 1 mg/cm 2 is equivalent to a depth loss of 0.01 mm/(specific gravity of glass) for those cases where the attack is not selective (From Ref 6.1, Copyright © 1966 by John Wiley & Sons, Inc This material is used by permission of John Wiley & Sons, Inc.)

glass being annealed, allows the sodium in the surface layers to

is then washed off prior to inspection and packing The firststep in weathering is then diminished because of the low alkalicontent of the surface

According to Charles [6.6], the corrosion of an alkali silicateglass by water proceeds through three steps These are:

bonds, forming nonbridging oxygens, and

another OH- ion This OH- repeats step 2 The silicic acidthus formed is soluble in water under the correct conditions

of pH, temperature, ion concentration, and time

It is questionable as to whether the first step described aboveinvolves the penetration of a proton or a hydronium, H3O+ ion.There is evidence that supports the exchange of hydronium foralkalies [6.7] In addition, the dissolution of silicate minerals,which is very similar to silicate glasses, has been reported to

The development of films on the glass surface has beendescribed by Sanders and Hench [6.8] They showed that a 33

glass by 2 orders of magnitude This difference was caused by

content Scratching the glass surface produced an unusually high

surface films or go into solution The thickness of this film andits adherence greatly affected the corrosion rate In Na2O SiO2

glasses, Schmidt [6.9] found that films formed only on glasses

2.36 in Chapter 2]

-–

Several workers have investigated the concentration profiles

of glass surfaces after leaching by water and attempted toexplain the variations observed Boksay et al [6.10] postulated

a theory that fit the profiles observed in K2O–SiO2 glass, but

due to a concentration-dependent diffusion coefficient.Doremus [6.11] developed a theory that included aconcentration-dependent diffusion coefficient to explain theprofiles in Li2O–SiO2 glass; however, his theory still did not fitthe observations for sodium determined by Boksay et al [6.12].Das [6.13] attributed the differences in the profiles betweenthe sodium and potassium glasses as being a result of adifference in the structure of the leached layer caused by therelative difference in size between the H3O+ and the Na+ ionsand the similarity in size between H3O+ and K+ ions In general,the dissolution rate (i.e., dealkalization) decreased as the ionradius of the alkali decreased

Douglas and coworkers [6.14–6.17] found that alkaliremoval was a linear function of the square root of time inalkali-silicate glass attacked by water At longer times, the alkaliremoval was linear with time Silica leached from

glasses decreased as the amount of silica in the glass increased,unlike that of the alkalies Wood and Blachere [6.18]

not find a square root of time dependence for removal of K or

Pb but found a dependence that was linear with time Thisbehavior was also reported by Eppler and Schweikert [6.19]and by Douglas and coworkers Wood and Blachere proposedthat an initial square root of time dependence occurred butthat the corrosion rate was so great that it was missedexperimentally

The pH of the extracting solution is also very important asfound by Douglas and El-Shamy [6.17] They found that abovepH=9, the leaching rate of alkalies decreased with increasing

pH, whereas below pH=9 the leaching rate was independent

of pH A somewhat different relationship was found for theleaching rate of silica—above pH=9 the rate increased with

alkali-silicate

increasing pH, whereas below pH=9 the amount of silicaextracted was close to the detection limits of the apparatus.Two reactions were identified: one where alkalies passed intosolution as a result of ion exchange with protons from thesolution and one where silica passed into solution as aconsequence of the breaking of siloxane bonds by attack fromhydroxyl groups from the solution Thus removal of silica wasfavored by an increase in hydroxyl ion activity (i.e., increasedpH), which was accompanied by a reduction in proton activityand thus a reduction in alkali extraction.

The dependence of dissolution upon pH can be seen by an

minerals Similarly, glasses in contact with aqueous solutionscan be represented by the following ion exchange reaction:

(6.1)which has as the equilibrium constant:

(6.2)Expressing this in logarithm form then gives:

(6.3)Thus it should be obvious that the exchange reaction of aproton for the leachable ionic species in the glass is dependentupon the pH of the solution and also the leached ion activity

in the solution

increased dissolution occurred to higher values, creating glassesthat were more durable and less sensitive to pH changes Paul[6.21] has also reported the beneficial effects of alumina andzirconia upon durability

Manufacturers of soda-lime-silicate glasses have known for

a long time that the addition of lime to sodium silicate glassincreased its durability Paul [6.21] reported that substitutions

Na2O remained constant With the larger amounts of CaOdevitri-fication problems during manufacture occurred,requiring the substitution of MgO for some of the CaO.According to Paul [6.21] calcium-containing glasses shouldexhibit good durability up to about pH=10.9 He also indicatedthat replacement of CaO by ZnO extended this durability limit

to about pH=13, although these compositions were attacked

in acid solutions at pH<5.5

The effects of MgO, CaO, SrO, and BaO upon leaching of

[6.21] At higher temperature, the durability decreased withincreasing ionic size, whereas at the lower temperature, thedurability was relatively the same for all four alkaline earths.This was attributed to the restricted movement at the lowertemperature for the larger ions

Expanding upon the ideas originally proposed by Paul andcoworkers [6.22–6.24], Jantzen and coworkers [6.25–6.27]have shown that network or matrix dissolution wasproportional to the summation of the free energy of hydration

of all the glass components as given by the equation:

(6.4)where A is the proportionality constant and L is a normalizedloss by leaching in mass per unit area Jantzen [6.28] has shownthat high-silica glasses exhibited weak corrosion in acidic-to-neutral solutions and that low-silica glasses exhibited activecorrosion at pH from <2 to 3 Between pH 2 and 10 in anoxidizing solution, hydrolysis occurred through nucleophilicattack with the formation of surface layers by reprecipitation

or chemisorption of metal hydroxides from solution Inreducing solutions, surface layers tended to be silicates thatexhibited weak corrosion or were even immune In alkalinesolutions at pH greater than about 10, both low- and high-silica glasses exhibited active corrosion with low-silica glasseshaving a potential for surface layer formation

Ernsberger [6.29] has described the attack of silica or silicateglasses by aqueous hydrofluoric acid in detail and related it to

the structure of silica glasses The silicon-oxygen tetrahedraare exposed at the surface in a random arrangement of fourpossible orientations Protons from the water solution will bondwith the exposed oxygens, forming a surface layer of hydroxylgroups The hydroxyl groups can be replaced by fluoride ions

in aqueous hydrofluoric solutions Thus the silicon atoms may

in the silicon atom coordination, which is six with respect tofluorine This causes the additional bonding of fluoride ions,with a particular preference for bifluoride Thus the fourfluoride ions near the surface provide an additional four-coordinated site for the silicon A shift of the silicon to form

ready availability of additional fluoride ions will then cause

that show a maximum in corrosion rate with bifluoride ionconcentration Although giving a slightly different description

of the possible reactions, Liang and Readey [6.30] reportedthat the dissolution of fused silica varied with HF concentrationand was controlled by a surface reaction rather than diffusionthrough the liquid

The solubility in nitric acid has been reported by Elmer andNordberg [6.31] to be a function of acid concentration;however, the rate decreased with increasing concentration (from0.8 to 7.0 N), just the opposite as that found in HF Inconcentrations greater than 3 N, saturation was reached inabout 24 hr At 0.1 N, the rate was considerably lower thanthe other concentrations, not reaching saturation even after

96 hr

that caused surface corrosion also caused enhanced crackgrowth The environments studied were distilled water,hydrazine, formamide, acetonitrite, and methyl alcohol White

et al found that acetonitrite was noncorrosive and that waterwas the most effective in leaching alkali, while hydrazine was

the most effective in leaching silica Formamide was only mildlyeffective in leaching alkali The mechanism of corrosion forwater, formamide, and hydrazine was reported to be alkali ionexchange with H+ or H3O+.

glass subjected to various temperatures in deionized water wasstudied by Hench et al [6.33] They concluded that both lower

previous study [6.34] and higher densities improved thedurability

The effect of dissolved water in soda-lime glass upon therate of dissolution in water was related to the influence ofabsolute humidity at the time of forming and annealing byBacon and Calcamuggio [6.35] Very high resistance wasobtained by use of very dry air Similar results were obtained

and ZnO with dissolved water contents between 4 and 8 wt.%

Wu, however, reported leach rates independent of watercontents at concentrations less than 4 wt.% Tomozawa et al.[6.37] concluded that many Si–O bonds in the glass are possiblyhydrolyzed by the dissolved water content, thus eliminatingsome steps during the dissolution of the glass in water andincreasing the rate of attack

Little information seems to have been published in the area ofmolten salt attack on glasses The dissolution of several glasscompositions was reported by Bartholomew and Kozlowski [6.38]

to be extensive and nonuniform in molten hydroxides Samplesattacked by sodium hydroxide exhibited an opaque and frostedsurface, whereas those attacked by potassium hydroxide weretransparent Bartholomew and Kozlowski used the mechanismproposed by Budd [6.39] to interpret the attack shown in theirstudies Considering the hydroxide ion as basic, a vigorous reactionshould take place with an acidic glass This was confirmedexperimentally by testing glasses of different chemistries.Loehman [6.40] reported no trends in leaching with nitrogencontent for several Y–Al–Si–O–N glasses, although two of hiscompositions exhibited lower weight losses by at least a factor

of 2 than fused silica when tested in distilled water at 95°C for

350 hr In their study of soda-lime-silicate glasses, Frischat andSebastian [6.41] reported that a 1.1 wt.% addition of nitrogenconsiderably increased the leach resistance to 60°C water for

49 hr The release of sodium was 55% less and calcium 46%less for the nitrogen-containing glass An additional indication

of the greater resistance of the nitrogen-containing glass wasthe change in pH of the leaching solution with time Startingwith a solution pH of 6, the solution pH drifted to 9 for thenitrogen-free glass after 7 hr, but reached 9 for the nitrogen-containing glass after only 25 hr The improved leach resistance

of this glass was attributed to a greater packing density for thenitrogen-containing glass

White and Day [6.42] reported no detectable weight loss of

a 1×1×0.2 cm rare-earth aluminosilicate (REAS) glass samplebefore 6 weeks in 100 mL of distilled water (pH=7) or saline

g/

of fused silica, a Corning glass (CGW-1723™*) and yttriaaluminosilicate (YAS), Oda and Yoshio [6.43] showed thatYAS was significantly more durable than fused silica insaturated steam at 300°C and 8.6 MPa The dissolutionmechanism is very important for applications in the humanbody; however, it is very difficult to determine whether theseglasses exhibit congruent or incongruent dissolution Surfaceanalyses of microspheres and bulk glasses indicated that themechanism was congruent [6.42] Using inductively coupledplasma and atomic adsorption spectroscopy it has beendetermined that the yttrium release from YAS microspheres indistilled water or saline at 37 or 50°C was below detectablelimits [6.44]

cooperative diffusion process takes place where tin diffuses

* CGW-1723™ is a clear aluminosilicate glass.

The float process for the manufacture of flat glass involves floating molten glass onto molten tin in a chamber, called the float bath, containing a reducing atmosphere.

†

into the glass and the constituents of the glass diffuse into thetin The reaction zone in the glass is about 25 µm thick Manyinvestigators have studied the tin oxide gradient of float glassand have reported a rather complex behavior [6.45–6.54].Stannous tin is dominant at the near surface A typical humpoccurs in the tin profile at between 5 and 10 µm where stannic(or oxidized tin) is predominant This hump has been attributed

to the additional tin from the ion exchange with calcium byFranz [6.55] Investigation of extremely thin layers of glasshas indicated tin oxide contents as high as 36% at the surface[6.52] (see Fig 6.2) The amount of the tin contained withinthe glass surface and the depth to which it penetrates isdependent upon the exposure time and temperature (whichrelates to glass production tonnage and thickness), and theamount and type of impurities (especially sulfur) contained inthe tin Thin glass travels through the bath faster than thickglass and therefore has less time for the various reactions totake place

At the hot end of the bath, iron oxide in the glass will migratetoward the bottom surface where it is reduced (by reactionwith either stannous oxide or hydrogen) to iron metal anddissolves into the tin At the cooler end of the bath, this tin/iron alloy will oxidize (oxygen coming from air ingress) formingboth iron oxide and tin oxide Iron has a greater potential tooxidize than tin and therefore acts as a scavenger for oxygen

FIGURE 6.2 Tin oxide penetration into bottom surface of float

glass (From Ref 6.52.)

Being essentially insoluble in tin these two oxides will enterinto the glass either by diffusion or by exchange for calciumoxide [6.55] Calcium oxide is also insoluble in molten tin andwill therefore form a deposit on the bottom surface of the glass.The deposit can be washed from the bottom surface of theglass by a vinegar solution.* Thus iron that enters the tin atthe hot end of the bath will reenter the glass at the cold end,setting up an equilibrium concentration of iron in the tin Thisequilibrium can be altered if the glass composition is changedfrom one of high iron content to one of lower iron content (orvice versa).

Although the interaction layer thickness is quite small, thepresence of tin in the surface of the glass ribbon causes somesecondary fabrication problems Many fabrication methodsrequire that the flat glass piece be bent This is carried out byreheating the glass on a metal frame and allowing the glass tosag to the desired shape This reheating process can provideadditional oxidation of the tin (from stannous to stannic oxide)

in the bottom surface This oxidation is accompanied by anexpansion of the tin-rich layer causing a microwrinkled surface.This wrinkled surface becomes visible as a faint iridescent

haze—known as the defect bloom [6.56] This phenomenon

can also occur when glass is reheated for tempering

6.3 BOROSILICATE GLASSES

The durability of borosilicate glasses has been extensivelyinvestigated by the nuclear waste glass community Noattempt will be made here to review all the literature related tonuclear waste glasses; however, the article by Jantzen [6.28]described quite well the use of Pourbaix diagrams inpredicting the dissolution of nuclear waste glasses Jantzen

* The deposit of calcium oxide reacts with atmospheric carbon dioxide forming calcium carbonate on the glass surface that is insoluble in water and must be washed off with a vinegar solution.

has performed a very thorough job in explaining theint errelationship of pH, Eh, activity, free energy of hydration,and glass dissolution It was shown that solution Eh had aneffect upon network dissolution that was 20 times less thanthat of pH But when redox-sensitive elements were leachedfrom the glass, the solution Eh could have a much largereffect Jantzen also concluded that less durable glasses had amore negative free energy of hydration and thus releasedmore silicon and boron into solution Higher boron releaseover that of silicon was attributed to the greater solutionactivity of vitreous boria compared to that of vitreous silica at

any given pH Refs 5.28–5.32 listed at the end of the previous

chapter are a good source of information for the readerinterested in the aqueous attack upon borosilicate glasses andnuclear waste materials in general

In borosilicate glasses requiring a heat treatment step afterinitial melting and cooling to produce phase separation, a

workers This silica-rich surface layer can influence thesubsequent leaching process that would be needed to produceVycor™*-type glass [6.57] If the hydrated surface layer wereremoved before heat treatment, the silica-rich layer would bealmost entirely eliminated

The leaching rate in 3 N HCl solution for borosilicatesglasses with an interconnected microstructure was shown byTakamori and Tomozawa [6.58] to be dependent upon thecomposition of the soluble phase The composition and size ofthis interconnected microstructure was also dependent uponthe temperature and time of the phase separation heattreatment process Taylor et al [6.59] have shown that phaseseparated low soda borosilicate glasses form a less durable

* Vycor™ is manufactured by Corning, Inc.