Release of Elements due to Electrochemical Corrosion of Dental Amalgam

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1994 73: 33J DENT RESS. Olsson, A. Berglund and M. Bergman

Release of Elements due to Electrochemical Corrosion of Dental Amalgam

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J Dent Res 73(l):33-43,January, 1994

Release of Elements due to ElectrochemicalCorrosion of Dental Amalgam

S. Olsson, A. Berglund, and M. Bergman

Department of Dental Materials Science, Faculty of Odontology, University of Umea, Sweden

Abstract. The corrosion pattern of dental amalgam in aqueousmedia was interpreted theoretically by means of log(a/arf)-pe diagrams. The definitions on which the diagrams werebased were given, and their features were described. Allsparingly soluble compounds which were expected to beformed in reactions with the solvents considered were listed.All the corrosion products reported in the current literaturewerefound tobeformed,and the conditions for theirformationwere established. It emerged that it was necessary to excludeother sparinglysolublecompoundswhich theoretically mightbe formed. Two compounds, CuSCN and AgSCN, which havenot been reported previously were found to be possiblecorrosion products. Corrosion products containing mercurycompound cannot be formed on amalgam restorations withno metallic contact with other materials.

Key words. Dental Amalgam, Corrosion, Mercury.

Received February 22,1993; Accepted August 8,1993This work was supported by the Swedish Medical Research Council(Project No. 07523).Correspondence and reprint requests should be addressed to: Dr. M.

Bergman, Department of Dental Materials Science, Faculty ofOdontology, University of UmeA, S-901 87 UmeA, Sweden.

IntroductionDuring the last decade, there has been an intensification inresearch activity into the mechanisms involved in the releaseof mercury from dental amalgam. For well-founded reasons,interest has focused mainly on mercury evaporation fromamalgam restorations (Svare et al., 1981; Abraham et al., 1984;Ott et al., 1984; Patterson et al., 1985; Vimy and Lorscheider,1985a,b; Berglund et aL., 1988; Olsson et al., 1989; Berglund,1990,1992; Bergman, 1992; Olsson and Bergman, 1992). How-ever, since the daily uptake of mercury from inhaled mer-cury vapor released from dental amalgam seems to make avery small contribution to the total body burden of mercury,in comparison with what can be tolerated in the work envi-ronment (Berglund et al., 1988; Berglund, 1990; Mackert, 1991;Olsson and Bergman, 1992), it is also important to considerother routes for mercury uptake from amalgam restorations-for example, the uptake in the gastro-intestinal tract. Inor-ganic mercury compounds which are swallowed will be ab-sorbed from the human gastro-intestinal tract to a level ofless than 10%, on average (WHO, 1991).

Apart from the dissolution in saliva of elemental mercuryvapor released from amalgam restorations, further contribu-tions to saliva mercury levels can occur as a result of both wearand electrochemical corrosion of the restorations. When elec-trochemical corrosion is considered, it is important to note thatdental amalgams are the most complex biomaterials from ametallurgical point of view. The set amalgam is a dynamicmaterial in which solid-state reactions occur for a long time(Marshall and Marshall, 1992). Since this takes place in thecomplex and variable oral environment, it is obvious that invitro studies can reflect the real in vivo conditions to only alimited extent. Berglund (1993) studied the release of mercuryvapor from different types of amalgam alloys in a combinedin vitro and in vivo study. He found that the electrochemical

33 by guest on July 12, 2011 For personal use only. No other uses without permission.jdr.sagepub.comDownloaded from


34 Olsson et al.

Cu(M)Cu +

-5

-0.2958

10Me(s))

A log -Me(151 (Me Z+I

Hg(l)(Hg 2+ )112

Ag(s)Ag

0

0

Hg(l)

Hg 2+

15

Eh V

10

0.2958 0.5916 0.8874

Figure 1. Log(a./are)-pe diagram of dental amalgam in synthetic saliva according to Tani and Zucci (1967) at pH = 6.7 and 25° C. The sparinglysoluble compounds which are expected to be formed in reactions with the solvent, represented by numbers, are given in Table 4 together withtheir pe-values and solubilities.

reactions occurring on the surfaces of amalgam specimens intest solutions influenced the release of mercury vapor in vari-ous ways.

Mercury is the most noble of the major elements in dentalamalgam, and under the conditions prevalent in the oral cav-ity, silver and mercury have the lowest electrochemical activ-ity (Gross and Harrison, 1989). However, despite their limitedtendency to dissolve electrochemically, these elements mayform some almost-insoluble sulfides. Among the corrosionproducts of various kinds which have been identified on cor-roded in vitro specimens and in retrieved amalgam restora-tions are SnO, SnO2, Sn4(OH) 6C2, Cu20, CuCl2 3Cu(OH)2, andCuCl (Mateer and Reitz, 1970; Otani et al., 1973; Holland andAsgar, 1974; Sarkar et al., 1975; Espevik, 1977; Marshall andMarshall, 1980; Marshall et al., 1980, 1982,1987; Jensen, 1982;Lin et al., 1983a,b; Sutow et al., 1991). The formation ofZnSn(OH)6andCuSn(OH)6, whichdemandshighpH values, isnot likely to occur in vivo, according toJensen (1982).

Since certain of these corrosion products may form pro-tective films, thus influencing the release of mercury vaporfromamalgam restorations, itwas considered to be of interestto relate the experimental findingsfrom the abovementionedstudieson corrosion products toknown electrochemical data.The aim of the present work was therefore to evaluate pos-

sible redox-reactions, with regard to the major elements indental amalgam and the possible corrosion products formedin reactions with the environment.

Materials and methodsThe corrosion pattern of dental amalgams in the aqueousmedia of the oral cavity is very complex, owing to manycompeting corrosion reactions and the complexity of humansaliva. Since the latter changes rapidly outside the oral cavity,it is usually very difficult or indeed impossible to use humansaliva for in vitro corrosion studies. Therefore, so-called syn-thetic saliva solutions are generally used. There are severalcompositions reported in the literature which has been re-viewed by Marek (1983). Among those, the two compositionsgiven in Tables 1 and 2 were selected for this paper. TheFusayama solution is very often used in corrosion studies andis most suitable, since it contains sulfide. The synthetic salivaaccording to Tani and Zucci (1967), which contains HCO3-ions, was selected so that the CO2/HCO3-buffering systemwhich exists in human saliva could be discussed.

In most corrosion studies where corrosion products wereanalyzed, the reactions have reached a steady state which isclose to equilibrium. Real equilibrium cannot be attained, due

Zn(s)Zn 2+

I IIIIIIIIIIIIIIIIIIIIIII

-15 7 \

-0.8874

-10

-0.59160 0 n n

j Dent Res 73(l) 1994

0211111111111111

IIIIII

5 -2'4



Electrochemical Corrosion ofDental Amalgam

pH

2 4 6 8 10-4

-6

-8pe

I I I I I I I I N

-1.183 -0.8874 -0.5916 -0.2958 0 0.2958 0.5916 0.8874

Figure 2. Log(a/arf)-pe diagram of dental amalgam in Fusayamasolution at pH = 5.35 and 25 C. The sparingly soluble compoundswhich are expected to be formed in reactions with the solvent, repre-sented by numbers, are given in Table 5 together with their pe-valuesand solubilities.

to the fact that solid-state reactions change the phase compo-sition in the amalgam fillings. However, the solid-state reac-tions are very slow compared with the corrosion reactions. Itseems therefore reasonable to assume that the corrosion prod-ucts can be treated as if chemical equilibrium exists.

One way to evaluate complex redox-reaction patterns is touse log(a,/aref)-pe diagrams (Silkn, 1952; Sillen and Martell,1964). The equations for possible redox reactions occurring inthe media used and the reaction products of the corrosion ofdental amalgams are given in Table 3. The basic data wereselected from Stability Constants (Sillen and Martell, 1964).Since there are many reactions possible in the media used, andsince only a few of the reaction products are determined at37°C but all are determined at 25C, all the reactions in Table 3are given at 25'C, for comparability. In Figs. 1 and 2, the corre-sponding log(a,/aref)-pe diagrams from the reactions given inTable 3 are shown for the solvents given in Tables 1 and 2. InTable 3, there are two kinds of reactions. The first reaction inTable 3, a redox reaction, corresponds to a line with a slope of-2, denoted Zn(s)/Zn2+ in Fig. 1. The line cuts the pe axis at -12.9.The independent coordinate, pe, of the pe-diagram is defined:

pe = - logl0fe-I = E/(RT ln 10) (1)F

-10

-12

-14

Figure 3. pe as a function of pH and ion concentration of In com-pounds in synthetic saliva (Tani and Zucci, 1967) at 25°C. In(OH)3;* ln(OH)1.75C11.25

where E is the electrode potential in the cell which containsthe redox couple and the standard Pt, H2IH+ reference elec-trode, R is the gas constant, T absolute temperature, and FFaraday constant. The value of (RT/F) ln 10 at 25°C is 0.05916V.The pe-scale is used in order for the redox data to becombin-able with the equilibrium data. The relation between the scalesgiven in Figs. 1 and 2 is obtained from:

Eh = pe x 0.05916 (2)

The Eh-scale is given only for orientation to the electrodepotential data. The value 12.90 in the first reaction of Table 3 isdenoted pe', which is defined:

pe = E°/(RT ln 10) = 1 logl0KF Z

(3)

where E is the standard electrode potential, z is the number ofelectrons of the redox reaction, and log10K is given in StabilityConstants (Sillen and Martell, 1964). The reactions 2 to 9 ofTable 3 represent insoluble or sparingly soluble compoundsof the Zn case. The corresponding equations are straight linesparallel with the pe axis. Their intersections with the lineZn(s)/Zn2+ are denoted by means of a horizontal line and afigure given in Tables 4 and 5 for the media used.

In amalgam, the activity of the metallic components is less

35j Dent Res 73(l) 1994



J Dent Res 73(1)1994

Table 1. Composition of the modified Fusayama solution

Compound g/L mol/L log(conc)a

NaCl 0.4KCI 0.4CaCl2 0.795 0.0176b log[CI-I = -1.75Urea 1.0 0.067NaH2PO4 H20 0.78 0.005 log[H2PO4 I = -2.3Na2S x H20 (X = 7 - 9) 0.005 0.000021 log[H2ScI = -4.7pH = 5.2 - 5.5 for freshly made solution

a Real concentration at the pH used.b Total chloride concentration.c H2S concentration not stabilized.

Table 2. Composition of synthetic saliva (Tani and Zucci, 1967)

Compound g/L mol/L log(conc)a

KSCN 0.517 0.00532 log[SCNi = -2.27NaHCO3 1.253 0.0149 log[HCO3 b1=-2.02KCI 1.471 0.0197 log[Cl-I = -1.70NaH2PO4 H20 0.1878 0.00136 log[H2P04-I = -2.98Lactic acid (HL) 0.90 0.010 log[HLI = -4.84pH = 6.7

a Real concentration at the pH used.b HCO3- concentration kept constant by means of 10% CO2 in the gas mixture (Wald and Cocks, 1971).

than unity. The activity of the pure metal in the redox reactioncan be replaced with:

{Me(s)) = [amalgam]f., (4)

where [amalgam] denotes theamalgam used in the specimensand still has the activity equal to unity, andfMe is the activityfactor of the least noble metallic component of the corrodingphase of the amalgam used. When Eq. (4) is introduced intothe equation of the redox couple, Eq. (5) is obtained:

log [amalgam] = - z [pe - pe + 1 logfe] (5)tMez+l Z

When fMe decreases, the line representing the redox coupleMe(s)/Mez+ is shifted to the right in the pe-diagram.

Since the activity of the solid metal Me(s) or the amalgamand that of the insoluble solid compounds are unity, the sameconcentration of Mez+ is obtained when in Table 3, for instance,the first equation and one of the equations of the solid com-pounds of the same redox couple are put equal. Then, pe corre-sponding to the solubility of, for instance, Zn3(PO4)2(s) is ob-tained from:

pe = -11.73 - 2/6 logfH2PO4-I - 4/6 pH

In order for correct pe-values to be attained, the anion concen-trations corresponding to the pH-value used must be calcu-lated from the pKa of the corresponding acids and the totalconcentrations given in Tables 1 and 2. The pe-values aremeasures of the reactivity of the redox couple at the solubilitylevel of the solid compound formed. The pe-values and thesolubility, given by the concentration of Mez+ in the mediumused, are given in Tables 4 and 5 in order of increasing pe-values. The more negative the pe-value, the more corrosion-prone the metal. Therefore, the corrosion will start with theleast noble component of the amalgam and with formation ofthe corrosion product which has the most negative pe-value.The corrosion reaction will proceed until the activity factor ofthe corroding component of the amalgam has decreased somuch that the next pe-value has been reached [cf Eq. (5)] oruntil the corrosion is strongly reduceddue topassivation. If thepassivation is not efficient enough to stop the corrosion, thelatter will continue with the second noblest metallic compo-nent and will form the compound which has the second mostnegative pe-value.

Equations equivalent to Eq. (6) also give the possibility forpe to be calculated due to variations in the concentration ofanions and pH in the aqueous solution used. When pe is plot-ted against pH, it is possible for pe-values of several possiblereaction products of the same redox couple to be compared,

36 Olsson et al.




6

4

2

pe0 NmA

4 6 8 10

-2

-4

pHFigure 4. pe as a function of pH and ion concentrations of Cucompounds in synthetic saliva (Tani and Zucci, 1967) at 25 C. -O-represents CuSCN; 0, Cu 0 A Cu2(OH)2CO3; A, Cu3(OH)2(CO3)2; 0Cu (PO4)2; 0Cu(OH) 5C1 * CuO; X, CuCI; X AgSCN.

and a decision made as to which of the products might beformed. Since the comparisons are made between compoundsbelonging to the same corroding metallic component, it is notnecessary to study the decreasing activity of the corrodingcomponent simultaneously. The extension of the curves wasfound by means of calculations of AG vs. the saturated calomelelectrode (SCE, 0.2446 V) for the processes where the solidmetal was directly electrochemically converted to the corro-sion products in synthetic saliva (Tani and Zucci, 1967).

ResultsIn Fig. 1 it can be seen that the most probable corrosionproducts in Zn-containing amalgams are those of Zn and Snin conventional amalgam and those of Zn, Sn, andCu in high-Cu amalgam. It can also be seen that corrosion of Ag and Hgis less probable. Furthermore, it was found that indium in In-containing amalgams is the most reactive metal componentafter Zn. If the medium contains sulfide, the most probablecorrosion products will be metal sulfides (Fig. 2). In Figs. 3and 4, the pe-values of the expected corrosion products wereplotted againstpH within the region where AG vs. SCE for thecorresponding corrosion process is negative. The conditionsfor the formation of the compounds are shown in the Figs.,

and the corrosion products found in experimental studieswere identified.

Discussion

An alloy surface which containsdifferent phasesconsisting ofmetal components with different electrode potentials willform a corrosion cell if the surface comes in contact with anaqueous medium. General as well as local corrosion-for ex-ample, pitting-can appear. At the anodic site, the metal ionsreleased as a result of corrosion react with the medium. Incertain cases, the ions are to a large extent hydrolyzed and amore acid solution is generated, causing a local rise in thepassivation potential with increased pe-values. In addition, anincreased chloride ion concentration corresponding to de-creased pe-values may occur in a pit or crevice as a conse-quence of the chloride ions migrating with the corrosion cur-rent, i.e., due to the charge transport in the solution. The de-crease in pH increases the dissolution of the passive layer, andthe increase in chloride concentration increases the corrosion,i.e., both factors further corrosion and prevent or render itdifficult for repassivation to be obtained.

The corrosion of amalgam is very complex, but by means oflog(a/aref)-pe diagrams, it is possible to obtain a survey of thecorrosion pattern. From Fig. 1, it is seen that in synthetic saliva,according to Tani and Zucci (1967), the reactivity of the metalcomponents is ranked Zn, In, Sn, Cu, Ag, and Hg.

In the case of tin, the composition of corrosion productssuggested has been discussed. The composition of basic Sn(II)chloride was originally assumed to be Sn(OH)ClH2O (Sarkaret al., 1975), but was later found to be Sn4(OH)6C12 (Marshalland Marshall, 1980). Old solubility data for Sn(OH)ClH2O aregiven in Stability Constants (Sillen and Martell, 1964), butthere are no solubility data for Sn4(OH)6C12. Furthermore, thereare no reliable data for the Sn(s)/Sn4+ system. It is therefore notpossible to give the pe-values for Sn C2 and Sn02. Fortu-nately, Donaldson et al. (1963) studied the composition ofbasic Sn(II) chlorides and found that all precipitates fromoxygen-free solutions of pH 1.4-4.5 had the compositionSn4(OH)6C12 and that above pH 4.5, Sn4(OH)6C12 appeared to-gether with increasing content of hydrous Sn(II) oxide, withincreasing pH. The system was sensitive to oxygen, and ifoxygen was present, hydrous Sn(IV) oxide was also formed.These results are in agreement with clinical experience. In pitsand deep crevices, where pH may be low and the chlorideconcentration may be increased due to ion migration,Sn4(OH)6C12 can be formed from the y2-phase (Marshall andMarshall, 1980; Moberg and Oden, 1985). The formation ofhydrous SnO2 may be reduced due to lack of oxygen in thecrevices. Above pH 4.5, SnO and SnO2 can also be formed.

When the activity of Sn in the amalgam surface decreases,the line in Fig. 1 denoted Sn(s)/Sn2+ will be shifted to the right[Eq. (5)]. The pe-value for SnO (No. 10, Table 4 and Fig. 1) mustthen be shifted to that for CuSCN (No. 12, Table 4 and Fig. 1).Then, the line according to Eq. (5) is to be shifted from -8.20

J Dent Res 73(l) 1994 37



38 Olsson et al. J Dent Res 73(1)1994

Table 3. Reactions on which (Log(ai/aref)-pe)-diagrams are based [data from stability constants (Sillen and Martell, 1964)1Redox couple Reaction at 25 C

Zn log{( 1 = -2 [pe + 12.901Zn2+ fZn2+1

log Zn(OH)2(s)1 = -11.5 + 2 pH{Zn241

log Zn(°OH')5C(l.(s = -7.60 + 0.5 logiCI-1 + 1.5 pH

logfZn(OH)½tClo4(s)1 = -8.20 + 0.4 logiCl- + 1.6 pHZn +1

log{ZnCO3(s)1 = 0.37 + log{HC03 la + pH

logIZn(OH)1j8(CO3)0.3s)1 = -7.22 + 0.36 logIHCO3ila + 1.64 pH{Zn ~1

loglZn(Lb)2(s)1 = -3.97 + 2 log{HLtI + 2 pHfZn2-rI

lOgiZn3(P4)2(S) "3 = -2.34 +2/3 log {H2PO4-) + 4/3 pHIZn2+I

log 2S) = 4.91 + log IH2SIC + 2 pHIZnI

In logIIn(s)i = -3 [pe + 5.681In3+ {In3+1

log in(OH)3 s)1 = -8.65 + 3 pH{ln3+)

loglIn(OH)u C.2sl( = 0.70 + 1.25 ICI-l + 1.75 pH[In +I

log1In2SPS) 1/2 = 3.37 + 1.5 logfH2SIC + 3 pHfin +I

Sn log[ = -2 [pe + 2.381Sn2+ (Sn2+1

log 21 = -1.76 + 2 pH(Sn2~

log{Sn(OH)5CJ0.5U(s) = xxx + 0.5 logIClI) + 1.5 pH[Sn +1

log[S2) = 4.98 + log [H2SI + 2 pH[Sn2~

Cu logfCu(s)1 --1 [pe - 8.801Cu+ [GuI+

log[CuCl(s)) = 6.50 + log(ClI}{Cu+}

lOg{Cu20(s)/2 = 0.84 + pH

log[CuSCN(s)1 = 14.32 + log(SCN-[Cu+1

1ogKCu2S(s)l /2 = 13.31 + 0.5 1ogfH2S)C + pHICU+) (Table continues next page)



Electrochemical Corrosion ofDental AmalgamJ Dent Res 73(1)19

CuCu2+

AgAg+

H g-(Hg22+)1/2

HpHg2+

log CU(S)1 = -2 [pe - 5.701tCu2+i

logCUO(s)1= -8.37 + 2 pH{Cu2+1

logICu(OH)1.5ClO.5(s)) = -3.65 + 0.5 log{ClI} + 1.5 pH(Cu2+1

log{Cu3(OH)2(C0322(s)jll/3 = -0.90 + 2/3 log{HCO3- a+ 4/3 pH{Cu2+1

log1CU2(OH)2,C3(s)j1'/2 = -2.28 + 0.5 logfHCO3 la + 1.5 pH(Cu +}

logICU3(po )2(s))L/3 = -0.72 + 2/3 logIH2PO4 I + 4/3 pHICu }

log Cu(SCN),(s)I = 3.65 + 2 logISCN-IICu2+,

log(Cu(Lb)2(s)) =-2.88 + 2 logtHLb +2 pHICu2+I

logC2uS(s)i = 15.53 + logIH2SIc + 2 pHfcu2+

log Ag(s)I = -1 [pe -13.511{Ag+I)

ioglAgCl(s)} = 9.75 + logICli(Ag+i

logAg2(s1/2 = -6.31 + pHJAg+I

logg2CO3(s)_2 = -0.54 + 0.5 logIHCO3 la + 0.5 pH(Ag+I

1OglAg3PO4(s)'/3 =-0.56 + 1/3 log(H2PO4 1+ 2/3 pH{Ag+}

log(AgSCN(s)I = 11.94 + log{SCN-I{Ag+)

logLAg2S(S)/2 = 14.64 + 0.5 logIH2S)c + pH{Ag+I

log H = -1 [pe -13.401IHg22+I1/2

logtg2Cl2(S)1I/2 = 8.87 + logICl

logf g2CO3S)I = 2.86 + 0.5 log(HCO3 }a + 0.5 pH

logtg2(SCN) (s)1'/2 = 9.76 + log(SCN-I{Hg22+ii/2

log{Hg(l), = -2 [pe - 14.381{Hg2+}

log{HgO(s)I = -2.54 + 2 pHIHg2+I

g(HgS(s)} = 30.77 + log{H2S1C.+ 2 pH{Hg2+)

94

a HCO3- concentration kept constant by

means of 10%CO2in the gasmixture(Waldand Cocks, 1971).

b Lactic acid (HL), lactate (L).c H2S concentration not stabilized.d Azurite.e Malachite.

39



J Dent Res 73(1)1994

Table 4. pe-values and solubilities of compounds in synthetic saliva solution (Tani and Zucci, 1967)

No. Compound Concentration of Mez+ (mol/L) pe-value

1 ZnCO3 10-5.05 -15.42 Zn3(PO4)2 10-4.60 -15.23 Zn(OH)1.28(CO3)0.36 10-3 04 -14.44 Zn(OH)2 10-1.9 -13.95 Zn(OH)1.6Cl0.4 10-1.84 -13.86 Zn(OH)1.5C10.5 lo-1.60 -13.77 Zn lactate 1o+0.25 -12.88 In(OH)3 lo-11.5 -9.499 In(OH)1.75C11.25 lo-10.3 -9.1110 SnO 10-11.6 -8.2011 Sn(OH)1.5C10.512 CuSCN lo-12.1 -3.2513 Cu2O 10-7.54 +1.2614 CU2(OH)2CO3 10-6.76 +23215 CU3(OH)2(CO3)2 10-6.68 +2.3616 Cu3(PO4)2 10-6.23 +2.5917 Cu(OH)1.5C10.5 10-5 55 +2.9318 CuO 10-5.03 +3.1919 AgSCN 10-9.67 +3.8420 CuCl 10-480 +4.0021 Cu(II)lactate 10-0.84 +5.2822 AgCl 10-8.05 +5.4623 Hg2(SCN)2 10-7.49 +5.9124 Cu(SCN)2 10+0 90 +6.1525 Hg2C12 10-717 +6.2326 Hg2CO3 10-5.20 +8.2027 HgO 10-10.9 +8.9528 Ag2CO3 10-2.87 +10.629 Ag3PO4 10-2.91 +10.630 Ag2O 10-0.39 +13.131 02 (Activity = 0.2) +13.9

to -3.25, i.e., -1/2 logfs. = 4.95 andfs. = 10-99,and thus theoreti-cally the concentration of Sn in the amalgam surface must bevery low in order for the corrosion of Sn and Cu to be ex-

changed. Since the amalgam surface of a _y2-amalgam prob-ably never loses all the -y2-phase in the surface or in the pores,it seems to be less probable that copper will corrode in thisparticular case.

In dental amalgam containing indium, twocorrosion prod-ucts are possible (Nos. 8 and 9, Fig. 1and Table 4). In Fig. 3, theirpe-functions were plotted against pH. It is seen that ln(OH)3 isformed above pH = 5.8 and that In(OH)175Cl125 is the corrosionproduct at pH below 5.8. Since these compounds are sensitiveto variations in pH (Table 3), their ability to cause passivationisprobablyreduced. Thisis supported byexperimental resultsobtained by Berglund (1993), who found, in isotonic salinesolution at pH = 6, that breakthroughs in the layer of corrosionproducts had occurred, resulting in mercury vapor release

with peaks in mercury vapor measurements recorded duringa ten-day interval.

The next pe-value after those of Sn is that of CuSCN (No.12, Table 4 and Fig. 1). In Fig. 4, the pe-values of CuSCN areplotted against pH. It is seen that the pe-function of CuSCNis below that of the other Cu-compounds. Since CuSCN isinsoluble in hydrochloric acid (Partington, 1947) and has avery low solubility in the solution used (Table 4), it should bestable in the oral cavity and also in crevices. Therefore, ifthiocyanate is present in the solution, CuSCN can be ex-pected to be present among the corrosion products of dentalamalgams. The reason for the fact that CuSCN has neverbeen identified as a corrosion product is, perhaps, that no onehas ever looked for it.

The corrosion of Cu has been observed in dental amalgam.The determining pe-value in solutions which do not containthiocyanate is seen to be that of Cu20(No. 13, Table 4 and Fig.

40 Olsson et al.




Table 5. pe-values and solubilities of compounds in Fusayama solution

No. Compound Concentration of MeZ+ (mol/L) pe-value

1 ZnS 10-10.9 -18.42 Zn3(PO4)2 10-3.26 -14.53 Zn(OH)1.6C10.4 i0+° 34 -12.74 Zn(OH)1.5C10.5 10+0.45 -12.75 Zn(OH)2 10+0.80 -12.56 In2S3 lo-124 -9.817 In(OH)1.75CI125 10-788 -8.308 Inl.OH)3 10-7.40 -8.149 SnS 10110 -7.8810 Cu2S lo-16.3 -7.5211 SnO 10-8.94 -6.8512 Sn(OH)1.5C10.513 uO5 lo 21.5 -5.0814 A2S -17.6 -4.14

15 HgS 10 368 -4.02

16 Cu2O 10-619 +2.6117 CU3jPO4)2 10-4.8 +3.2618 Cu(OH)1.5C105 10-3580 +3.9519 CuCn10337 +4.0520 CuO 10 233 +4.5321 AgCl 10-8.00 +5.5122 Hg2C12 10-712 +6.2823 HgO 10 8.16 +10.324 Ag3PO4 10-224 +11.325 Ag2O 10+0.96 +14.526 02 (Activity = 0.2) +15.3

1). This compound is stable in contact with Cu and has beenreported as a corrosion product on copper-rich dental amal-gams (Marshall et aL, 1982). In Fig. 4, the pe-values [cf Eq. (6)] ofall the copper compounds except that of Cu(II) lactate given inTable 4 are plotted against pH. It is seen that CU20, AG < 0above pH = 4, has the lowest pe-values from pH 10 to 4, andthat below pH = 4, CuCl determines the corrosion of copper(AG < 0 for formation of CuCl when the chloride concentra-tion is higher than 0.015 mol/L, and the Cu activity is equal tounity). The formation of CuCl below pH = 4 is in agreementwith the results of Sutow et al (1991), who found CuCl at pH =1 and a chloride concentration not less than 0.068 in crevices.This means that the pe-values of Cu2O and CuCl, i.e., the redoxcouple Cu(s)/Cu+, determines the corrosion of copper.

From Fig. lit is seen that there are five Cu(II) compounds(Nos.14 to 18, Table 4 and Fig. 1) within a small range of pe-values. (The curves are shown in the region where AG < 0 andthe Cu activity is equal to unity.) In Fig. 4 it is seen that they areso close together that the composition of the solvent willprobably be the factor determining which one of them will beformed. In synthetic saliva (Tani and Zucci, 1967), basic Cu(II)carbonate should be formed after oxidation of Cu2O. Of the

Cu(II)compoundsshownin Fig. 4,onlyCuCl2 3Cu(OH)2,whichcanalsobewrittenCu(OH)15 5 (No. 17, Table 4and Fig. 1), hasbeen found in Ringer's solution at 37TC by Marshall et al.(1982), who found that green CuCI23Cu(OH)2 covered thelayer of red Cu20 on the surfaces of copper-rich dental amal-gam specimens. Sutow etaL. (1991) alsofound that CuCl, formedin crevice corrosion at pH < 4 at high chloride concentration,could be oxidized to CuCl23Cu(OH)2 in moist air. But when thespecimens were properly stored and analyzed immediatelyafter the experiment was ended, only CuCl was formed. Theseresults are in agreement with the present calculations of AGdemonstrating that it is always possible to obtain all the pro-posed Cu(II) compounds (Fig. 4) from both CuCl and CU20 bymeans of oxidation by moist air.

In Fig. 4, the comparison of Cu(I) and Cu(II) compoundswas carried out with the Cu activity equal to unity. This wasdone due to the lack of knowledge of the values of theactivity factor at the start of the corrosion. When the amal-gam surface is depleted of Cu, 'cu [Eq. (5)] decreases, and,since the shift of the line Cu(s)/Cu+ is not equal to that ofCu(s)/Cu2+, pe of Cu20 will be shifted closer to pe of, e.g.,Cu(OH)1 5C105 (No. 17, Table 4 and Fig. 1). But before equal pe-

j Dent Res 73(l) 1994 41



42 Olsson et al.

values of these two compounds were obtained (pe = 4.6 atfcu= 0.0005), a pe-value of 3.84 for AgSCN (No. 19, Table 4 andFig. 1) will be reached. (The activity shift of the line Ag(s)/Ag+ is negligible.) Therefore, the redox couple Cu(s)/Cu+determines the corrosion of Cu and Cu(OH)15C105 can beformed only through air oxidation of Cu2O or CuCl in solu-tions such as saliva.

Fig. 1 shows that the pe-value ofAgSCN (No. 19, Table 4andFig. 1) is very close to that of CuCl (No. 20, Table 4 and Fig. 1).From Table 4 it can be seen that the difference in their pe-values is only 0.16. Fig. 4 shows the pe-function of AgSCN, andit is obvious that it is very close to that of CuCl. Therefore,belowpH = 4, it is expected that a mixture of CuCl and AgSCNwill be formed. AgSCN is a white precipitate insoluble in nitricacid (Partington, 1947). It is therefore stable even in crevicesand should be formed in solutions containing thiocyanate.However, AgSCN has not been identified among the corrosionproducts of dental amalgam.

In amalgam containing Zn, the Zn content is low, and thecorrosion of Zn is of minor importance. As can be seen in Fig.l and Table 4, ZnCO3 (No. 1) and Zn3(PO4)2 (No. 2) and, in Fig.2 and Table 5, ZnS (No. 1) can be expected to be formed.

The corrosion of dental amalgam in the Fusayama solutionis shown in Fig. 2. It can be seen that the sulfide content of thesolution changes the corrosion pattern considerably comparedwith that in Fig. 1.Of special interest is the region around the pe-values of the Sn compounds, where all the sulfides except thatof Zn are present. All pe-functions of the sulfides vary withpHin the same way, and their curves are therefore parallel to eachother. This means that the sulfide with the most negative pe-value determines the corrosion of amalgam at all values of pH.In dental amalgams containing indium, the corrosion leads totheformation of InS2 (No. 6, Table 5 and Fig. 2). In conventionaldental amalgam, the corrosion is determined by SnS or Cu2S(Nos. 9 and 10, Table Sand Fig. 2). Since the activity shift of thepe-values [Eq. (5)] is different for Sn(II) and Cu(I), and since thedifference in their pe-values is only 0.36, the activity of Sn andCu in the amalgam surface at the moment of corrosion is thedetermining factor for the formation of the corrosion product,i.e., either SnSorCu2Scan be formed. In high-Cuamalgam, Cu2S(No. 10, Table Sand Fig. 2) is most likely to be formed. It is alsoseen that theformation of CuS,Ag2S,andHgS(Nos. 13, 14, and 15,Table 5 and Fig. 2) is not possible when amalgam without anyelectron-conducting contact with other metallic materials isconcerned.

In conclusion, it was found that, by means of log(a,/aedf)-pediagrams, it is possible to interpret the corrosion pattern ofdental amalgam. All thecompounds reported in experimentalstudies were confirmed, and the conditionsfor their formationwere established. Other sparingly soluble compounds whichmight be expected to form can now be excluded. It was alsofound that CuSCN and, at pH < 4, AgSCN could not be ex-cluded, and therefore these compounds may exist, althoughthey have not been identified. Most important, however, cor-rosion products containing mercury cannot be formed on

amalgam restorations with no metallic contact with othermaterials. This is in agreement with the opinions expressed byGross and Harrison (1989).

AcknowledgmentWe are grateful to Professor PhD Staffan Sjoberg, Departmentof Inorganic Chemistry, University of Umea, for valuable ad-vice and fruitful discussions.

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Release of Elements due to Electrochemical Corrosion of Dental Amalgam