146_ use of phase diagrams in refractories corrosion[1]

$: 146_ Use of Phase Diagrams in Refractories Corrosion[1]$
Use of phase diagrams in studies ofrefractories corrosionS. Zhang and W. E. Lee

available in existing phase equilibrium diagrams.Although the importance of phase diagrams to studiesTypical applications of phase diagrams to specificof refractories corrosion has been highlighted,3–7 andproblems of refractories corrosion are highlighted.a few cases such as attack of Al2O3–SiO2 refractoriesSaturation solubilities of refractories components

in molten slags can be estimated using existing by alkali oxides well studied,4,6,7 their use is far fromphase diagrams. These can then be used to predict ubiquitous.the corrosion behaviour, and qualitatively compare This paper seeks to emphasise applications of phasethe corrosion resistance, of a refractory in different diagrams to specific refractories corrosion problemsslags or different refractories in the same slag.

using typical examples. Previously reported experi-If the slag is not saturated with refractoriesmental results which were not well explained havecomponents, using relevant phase diagrams, thebeen re-analysed and earlier discrepancies clarifiedconditions under which solid reaction productusing the relevant phase diagrams.phases form at refractories/slag interfaces can be

predicted, which assists understanding of directand indirect dissolution. By checking the Significance of saturation solubility ofcompatibility between impurities or additives and

refractories oxide phases in liquid slagrefractories components at high temperatures,their influence on corrosion resistance can be Dissolution of solid oxides in liquid slag is governedpredicted, aiding their selection. Phase diagrams by: (a) chemical reaction (or solution) at thealso reveal that atmosphere affects refractories

refractories/slag interface or (b) transport (orcorrosion resistance by altering the valence ofdiffusion) of reacting species.8 In the latter case,some components (in particular, iron oxide) in thedissolution rate can be expressed in terms of therefractories and/or slags. The compatibilityNernst equationbetween refractories and slags indicates the

corrosion resistance of different refractories, J=D(Cs−Cm )/d . . . . . . . . . . (1)assisting refractories selection for specificapplications. Finally, phase diagrams can be used where D is the diffusion coefficient (m2 s−1 ), Cm andto assist design of refractories composition. Cs are, respectively, concentration and saturationRepeated experimental corrosion test results show solubility of refractory in the slag (g m−3), and d isgood agreement with phase diagram predictions. the effective boundary layer thickness (m).Therefore, use of the information which is

Increasing D or decreasing d (i.e. increasingavailable in existing phase diagrams can reduceD/d) increases dissolution rate J. Besides this, it isthe need for expensive and time consumingclear from equation (1) that the value of Cs−Cmexperiments to evaluate high temperaturestrongly influences dissolution rate. If the slag iscorrosion of refractories (and other ceramics).saturated with refractory oxide, then J=0. Naturally,IMR/351to minimise the dissolution rate, it is necessary to

© 2000 IoM Communications Ltd and ASM International. minimise Cs−Cm . For example, with increasing MgOThe authors are in the Department of Engineering content in the slag, the corrosion of the periclaseMaterials, The University of Sheffield, Mappin Street,

phase in magnesite–dolomite or in magnesite–chromeSheffield S1 3JD, UK.refractories will decrease. If Cm=0, then the value ofCs−Cm reaches a maximum and thus, so does thedissolution rate.

Fortunately, the saturation solubility of the refrac-Introductiontories oxide in liquid slag can be estimated using

Refractories are non-metallic materials used to line existing phase diagrams. By then checking the differ-many industrial furnaces for high temperature pro- ence between the concentrations and saturation solu-duction of other materials such as metals, glass, bilities of the main refractories components in thecement, and petrochemicals. Decreased service life of molten slag, their relative resistance to slag attackrefractories is caused by damage arising from, for can be compared, and thus the influence of refractoriesexample, deformation at high temperature, thermal composition on corrosion resistance can be predicted.shock, mechanical abrasion, and chemical corrosion. Consider the influence of MgO concentrationAmong these factors, chemical attack by the slag is and saturation solubility in silicate slag on the dissolu-often the most critical. tion of solid MgO. Matsui et al.9 examined the

To determine chemical corrosion mechanisms and dissolution rate of sintered and fused MgO grain intoto compare corrosion resistance of different refractor- silicate slags at 1600°C by measuring the weighties, several test methods have evolved.1,2 However, changes of MgO grain before and after heating in themany of these tests are expensive and time consuming, slags given in Table 1. Using the phase diagramand in some cases, they are conducted just to (showing MgO solubility data)10 shown in Fig. 1, the

saturation solubility of MgO in each slag can bere-establish information already known and generally

ISSN 0950–6608 International Materials Reviews 2000 Vol. 45 No. 2 41

42 Zhang and Lee Use of phase diagrams in studies of refractories corrosion

2 Weight loss of fused (F-1) and sintered (S-1)MgO grain corroded for different times at 1600?Cby slags A-0, A-5, and A-10 given in Table 1

wt-

%

(after Ref. 9)1 CaO–MgO–Fe2O

3–SiO

2phase diagram at 1600?C

with MgO saturation solubilities in liquid phase:Per. periclase, Sp. spinel (after Ref. 10)

a basicity (CaO/SiO2 weight ratio (C/S)) of 1·0, thenthe saturation solubilities of MgO, Cr2O3 , and Al2O3estimated. The saturation solubilities of MgO in the in the slag at the test temperature are calculated to

slags at 1600°C thus obtained and the Cs−Cm values be ~24·3 wt-% (point L1 on Fig. 3a), ~7·1 wt-%are listed in Table 1, showing the Cs−Cm values (point L2 on Fig. 3b), and ~63·2 wt-% (point L3 oncorresponding to slags A-15, B-10, and C-10 are Fig. 3c), respectively. Clearly, Cr2O3 has the lowestnegative (-ve). These slags are thus oversaturated with solubility in the slag whereas Al2O3 has the highest,respect to MgO at 1600°C, so they cannot dissolve so that in a CaO–SiO2 based slag (with C/S ratioMgO at this temperature. On the other hand, the around 1·0), increasing the Cr2O3 content in MK willCs−Cm values for slags A-0, A-5, and A-10 are be expected to increase the corrosion resistance ofpositive with A-10<A-5<A-0. Since the D/d values the refractories, but increasing the Al2O3 content inof these three slags are only slightly different9 accord- MK would decrease slag corrosion resistance. Finger,ing to equation (1), the dissolution rate of solid MgO induction furnace, and rotary slag test results12–15in them will increase in the following order, agree with the above predictions based on the calcu-A-0>A-5>A-10. Consistent with these predictions, lation of saturation solubilities. Figure 4, for example,Matsui et al.9 found that slags A-15, B-10, and C-10 shows the rotary slag test results of Ichikawa et al.12did not dissolve MgO at 1600°C, while the order of and Hiragushi et al.13 In Fig. 4, the corrosionMgO dissolution rate in slags A-0, A-5, and A-10 was resistance of refractories is indicated by the relativeas predicted based on the calculation of saturation corrosion (or wear) index. Higher corrosion (or wear)solubilities (Fig. 2). index means greater corrosion. With higher Al2O3Refractories are composed of many components content in the brick, the corrosion is greater (Fig. 4a),and by checking the saturation solubility of each in and with higher Cr2O3 content in the brick, themolten slag, the influence of refractories composition corrosion is lower (Fig. 4b).on corrosion resistance can be predicted. A good Another relevant example is the influence of carbonexample is the influence of relative contents of Cr2O3 , (C) content and periclase crystal size in MgO grainAl2O3 , and MgO in magnesia–chrome refractories on the corrosion resistance of MgO–C refractories(MK) on their corrosion resistance. Figure 3 shows used in basic oxygen furnaces (BOFs). Figure 5 showsCaO–MgO–SiO2 , CaO–Cr2O3–SiO2 , and CaO– the phase relations in the CaO–MgO–Al2O3–SiO2Al2O3–SiO2 ternary phase diagrams.11 Assuming a system at liquidus temperatures in planes of constanttest temperature of 1700°C and CaO–SiO2 slag with Al2O3 spaced at 5 wt-% intervals.16 From the iso-

therms shown in Fig. 5, the saturation solubilities ofTable 1 Chemical composition and saturation MgO in CaO–Al2O3–SiO2 (CAS) slags with various

solubility Cs

of MgO in various slags basicities can be calculated. Figure 6, as an example,(wt-%) (data from Ref. 9) shows the saturation solubility of MgO in the CAS

Slag slags with 10 wt-% Al2O3 but different basicities at1700°C. With increasing slag basicity from 1·0 to 3·0,A-0 A-5 A-10 A-15 B-10 C-10the saturation solubility of MgO in the slag decreases

CaO 43·0 40·1 39·2 36·9 43·4 47·4from~29 to~12·5 wt-%. The BOF slag in the ‘early’SiO

221·5 19·3 20·8 19·9 15·2 11·7

stage is acid (C/S around 1·0), but changes to basicFe2O

335·0 32·7 32·0 29·9 30·5 31·3

MgO 0 4·8 8·3 12·4 8·8 8·8 in the ‘late’ stage (C/S above 3·0).17 In the early stage,C

s~9·0 ~8·5 ~10·0 ~10·0 <8·0 <8·0 the solubility of MgO in the slag is high (Fig. 6) and

Cs–C

m~9·0 ~3·6 ~1·7 -ve -ve -ve the dissolution of MgO from the MgO–C refractories

International Materials Reviews 2000 Vol. 45 No. 2

Zhang and Lee Use of phase diagrams in studies of refractories corrosion 43

wt-%

(a)

wt-%

(c)

wt-%

(b)

3 Phase diagrams of a CaO–MgO–SiO2, b CaO–Cr

2O

3–SiO

2, and c CaO–Al

2O

3–SiO

2systems (Ref. 11)

into the slag is a dominant factor affecting their ( lower corrosion index), but in a basic slag (slag B,C/S=3·3), increasing the graphite content from 5 tocorrosion resistance. So in this case, increasing the

content of C (which is an inert component to silicate 20 wt-% does not obviously improve corrosion resist-ance, and increasing its content to above 20 wt-%slag) or using MgO with larger crystal size (e.g. fused

MgO) will be expected to improve the corrosion lowers corrosion resistance. Figure 8 shows that, inan acid slag (C/S=1·2), increasing C content (fromresistance of the MgO–C refractories. However, in

the late stage, the saturation solubility of MgO in the 5 to 15 wt-%) and use of MgO with larger crystalsize (B), increases the corrosion resistance, but in aslag becomes low (Fig. 6), so dissolution of MgO in

the slag is not a serious problem. In this case, C basic slag (C/S=2·5) these changes have little effect.oxidation becomes the dominant factor affecting thecorrosion resistance so that increasing the C contentor using MgO with larger crystal size will not greatly Direct and indirect dissolution:improve the corrosion resistance of the MgO–C prediction of formation of reactionrefractories. In some cases using too much C decreases products at refractories/slagthe corrosion resistance because of the oxidation of interfacesC and the weak nature of the resulting bond. Theseconclusions are supported by rotary slag test results Dissolution of refractory oxide into silicate slag gener-

ally can be homogeneous (congruent or direct) or(Figs. 7 and 8). Figure 7 shows that in a more acidslag (slag A, C/S=1·5), increasing the graphite con- heterogeneous (incongruent or indirect). The former

occurs between refractory oxides and molten slagtent in the brick improves the corrosion resistance



invariant point E corresponding to DCAS2–A–CA6 is~1500°C, with a test temperature above 1500°C, CA6layer formation is not expected in this slag. In otherwords, if CA6 is observed in the microstructure, it hasformed on cooling not at the test temperature. Thelines bA, cA, and dA intersect with the boundarycurve EF, but the temperatures corresponding to theintersection point are different (~1720, 1780, and1810°C, respectively). Assuming a test temperatureof 1600°C and a composition of 90 wt-% solidAl2O3+10 wt-% slag, then the amounts of precipi-tated CA6 corresponding to the slags b, c, and d arecalculated to be ~15·6, 27·8, and 39·7 wt-%, respect-ively. These results indicate that increasing thebasicity of the CAS slag will favour the formation ofsolid CA6 at high temperature, i.e. favour the indirectdissolution of Al2O3 in the slag.

For the case of CMAS slag, whether solid CA6forms or solid MA spinel forms at high temperaturesdepends on the relative CaO and MgO contents inthe slag, which can be estimated using the CaO–MgO–Al2O3 (Fig. 9)27 and CaO–MgO–Al2O3–SiO2phase diagrams (with 5–15 wt-% MgO) (Fig. 10).28Although the CaO–MgO–Al2O3 phase diagram is toosimple to predict the detailed corrosion process ofAl2O3 in the CMAS slag, it provides some usefulinformation regarding the formation of solid CA6 orMA at high temperature. Draw a straight line betweenthe Al2O3 apex (A) and invariant point (X) corres-ponding to DA–CA6–MA in Fig. 9. The CaO/MgO

(a)

(b)

ratio corresponding to this line is ~4·4. Therefore, if4 Wear index (rotary slag test) of MgO–Cr2O

3brick

as function of a Al2O

3content12 or b Cr

2O

3the CaO/MgO ratio in the CMAS slag is above~4·4,

content,13 showing that increasing brick Al2O

3then, the joint line between the point A and slag

content increases corrosion, but increasing composition point will hit the boundary curve (XY)Cr

2O

3content decreases corrosion: slag between the Al2O3 primary field and the CA6 primary

composition (A) C/S=1·0, Al2O

3=10 wt-% and

field. Consequently, a CA6 layer is expected to form(B) C/S=1·0, Al2O

3=30 wt-%

adjacent the solid Al2O3 . If the CaO/MgO ratio inthe CMAS slag is below ~4·4, then the joint line

when ions from the refractory oxides dissolve directly will hit the boundary curve (XZ) between the Al2O3into the melt, while the latter occurs when one or primary field and the MA primary field, so that inmore solid reaction products form at the refractory this case an MA layer is expected adjacent the solidoxide/slag interface separating the oxide from the Al2O3 .slag.2,20–26 Because of the formation of a solid If more components are included in the system, theinterface product layer (or layers) in the latter case, critical CaO/MgO value will be a little different fromthe dissolution rate will be decreased. Obviously, to the value (4·4) estimated above. For example, if SiO2understand the corrosion mechanism clearly and is included in the system, based on the CaO/SiO2–evaluate the corrosion resistance, it is necessary to Al2O3–MgO phase diagrams given by Kitai,29 theknow the conditions under which solid reaction prod- critical CaO/MgO value is calculated to be ~4·5ucts form at the refractories/slag interface at high when the C/S ratio of the slag is between 1·0 and 3·0.temperature. The following two examples, dissolution The above CaO/MgO ratio (~4·5) can be used toof Al2O3 and MgO in CAS (or CMAS) slag, illustrate predict experimental results approximately. Sandhagehow to use phase diagrams to predict formation of and Yurek21 found a CA6 layer formed adjacent toreaction product layers at high temperatures. solid Al2O3 in a slag with CaO/MgO=7·0, but an

MA layer formed adjacent solid Al2O3 in slags withCorrosion of Al

2O

3by CAS or CMAS slag CaO/MgO<~4·5. The present authors examined

the dissolution of Al2O3 in a FexO and MnO contain-Consider the dissolution of Al2O3 in four CAS slagsing CMAS slag with CaO/MgO=6·0, so a CA6 layerwith the same Al2O3 content (25 wt-%) but differentformed adjacent the solid Al2O3 .30C/S ratios (0·5, 1·0, 1·5, and 2·0). These four slags are

The fact that increasing MgO concentration inindicated on the CaO–Al2O3–SiO2 phase diagramCMAS slag favours the formation of solid MA spinel(Fig. 3c) by points a, b, c, and d, respectively. Drawat high temperature is also reflected in the phasea straight line between each of these points and thediagrams shown in Fig. 10 which shows that the areaAl2O3 apex (A). Because the line aA does not hitof MA liquidus surface increases with increasing MgOthe boundary curve (EF) between the CA6 primary

field and Al2O3 primary field, and the temperature of content in the system.



(a)

(b) (c)

(d) (e)

(f) (g)

a 5 wt-%Al2O

3; b 10 wt-%Al

2O

3; c 15 wt-%Al

2O

3; d 20 wt-%Al

2O

3; e 25 wt-%Al

2O

3; f 30 wt-%Al

2O

3; g 35 wt-%Al

2O

3

5 Phase relations at liquidus temperature in different Al2O

3content planes (5–35 wt-%) of CaO–MgO–

Al2O

3–SiO

2system (Ref. 16)



7 Influence of graphite content on corrosion ofMgO–C bricks in acid (A, C/S=1·5) and basic(B, C/S=3·3) slags at 1650?C (rotary slag test),18

6 Saturation solubility of MgO in CAS slag as showing that in acid slag (A) increasing graphitefunction of slag basicity at 1700?C (calculated content in brick decreases corrosion, but infrom Fig. 5), showing saturation solubility of basic slag (B) increasing graphite content fromMgO in acid slag is much higher than in basic 5 to ~20 wt-% has little effect on corrosionslag resistance and further increasing graphite

content from 20 to 30 wt-% lowers corrosionresistance

Now consider the influence of slag basicity andMgO content in the slag on the dissolution of Al2O3 . that increasing the slag MgO content will greatlyFigure 11 shows the saturation solubilities of Al2O3 decrease the dissolution of Al2O3 in the slag. Fingerat 1600°C in CM(A)S slags with 10 wt-% MgO but

test results given by Sandhage and Yurek22 (Fig. 14)different basicities (calculated by using the 1600°C

verify this conclusion.isotherms shown in Fig. 10b). As shown in Fig. 11,with increasing slag basicity, the saturation solubility

Corrosion of MgO by CAS (or CMAS) slagof Al2O3 in the slag increases. Based on this considera-tion, increasing the basicity of the CMAS slag will Dissolution of MgO into CAS (or CMAS) slag can

also be direct or indirect, dependent on whether anbe expected to increase the dissolution of Al2O3 .However, as stated above, increasing the slag basicity MA spinel layer forms at the MgO/slag interface at

high temperatures. This can be predicted using thefavours CA6 layer formation at high temperature andmakes Al2O3 dissolution become indirect, which will phase diagrams shown in Fig. 5.

As shown in Fig. 5a and b, there is no liquidusdecrease the dissolution rate. Therefore, the overalldissolution rate of Al2O3 is determined by the balance surface of MA in the phase diagrams with 5 and

10 wt-% Al2O3 , indicating that to form an MA layerbetween these two contradictory factors. Millerand Shott31 examined the influence of CMAS slag at high temperatures, the Al2O3 content in the CMAS

slag should be above 10 wt-%. In other words, if the(with 10 wt-% MgO) basicity on corrosion resistanceof Al2O3-based refractories. Their results (Fig. 12) Al2O3 content in CMAS slag is below 10 wt-%, no

MA reaction layer forms at high temperatures. Thereshowed that CMAS slag corrosiveness increases withincreasing basicity, and for fused cast alumina, the is a small area of MA liquidus surface shown in the

diagram with 15 wt-% Al2O3 (Fig. 5c). The area onlyslag with a basicity (C/S) of 2·14 is ×10 morecorrosive than that with a ratio of 0·76. These results covers small temperature (~1300 to ~1580°C) and

slag basicity (C/S from ~0·55 to ~1·2) ranges, indi-suggest that for Al2O3 dissolution, the influence ofincreasing saturation solubility is greater than that of cating that solid MA spinel may form only when

temperature and slag basicity are within these ranges.CA6 layer formation, and explains why Al2O3 basedcastables are not successful in the slag lines of steel- When the Al2O3 content in the slag increases to above

15 wt-%, the area of MA surface becomes moremaking ladles, where because of the high basicityslag (in some cases, the lime content in the slag is extensive and covers a wider temperature range. In

this case, as shown in Fig. 5d–g, the formation of MA>70 wt-%),32,33 basic refractories such as MgO–C(Ref. 34), and MgO–Al2O3 (Ref. 35) bricks are gener- solid phase is mainly controlled by the slag basicity.

For example, at 1600°C, as shown in Fig. 5d and g,ally used.Figure 13 shows the variation of saturation solu- the slag basicity (C/S) at 20 wt-% Al2O3 should be

less than 1·1, and at 35 wt-% Al2O3 less than 2·1.bility of Al2O3 at 1600°C in CMAS slag (C/S=1·0)with MgO content in the slag (calculated from Based on the above analyses, increasing Al2O3

content in the slag and decreasing the slag basicityFig. 10). It can be seen that with increasing slag MgOcontent, the saturation solubility of Al2O3 in the slag generally will favour the formation of solid MA at

high temperature. If the slag Al2O3 content is lowerdecreases. Since increasing the MgO content in theslag also favours MA spinel layer formation at high than a critical value and/or the slag basicity is higher

than a critical value, then the solid MA layer attemperature (indirect dissolution), it can be deduced



8 Influence of graphite content and MgO grain quality on corrosion of MgO–C brick in acid (C/S=1·2)and basic (C/S=2·5) slags,19 showing that in acid slag, increasing graphite content or use of MgOwith larger crystal size improves corrosion resistance, but in basic slag it does not; A MgO withsmaller crystal size, B MgO with larger crystal size

wt-%

9 CaO–MgO–Al2O

3phase diagram (Ref. 27)



(a)

(b) (c)

a 5 wt-%MgO; b 10 wt-%MgO; c 15 wt-%MgO

10 Phase relations at liquidus temperature in different MgO content planes (5–15 wt-%) of CaO–MgO–Al

2O

3–SiO

2system (Ref. 28)

high temperature will not be expected to form, i.e. to the indirect dissolution and the lower saturationsolubility of MgO in the slag. On the other hand, indirect dissolution of MgO in the slag will occur.

Furthermore, because the saturation solubility of a high basicity slag (e.g. C/S=3·0, Fig. 15), on increas-ing the slag Al2O3 content from 10 to 35 wt-%,MgO in the slag increases with increasing slag Al2O3

content (Fig. 15),36 so will dissolution of MgO in the because no MA layers are expected to form(Fig. 5b–g), direct dissolution will occur. Nevertheless,slag. However, if slag Al2O3 content is higher than

the critical value and the basicity of the slag is lower because the solubility of MgO only changes slightlywhen the slag Al2O3 content increases from 10 tothan the critical value, then a solid MA layer at high

temperature will be expected to form, so again indirect ~25 wt-% (Fig. 15), the Al2O3 content in the slagshould have little influence on the dissolution ofdissolution of MgO occurs. In this case, the dissolu-

tion rate of MgO in the slag will be determined by MgO. However, when the slag Al2O3 contentincreases above ~25 wt-%, the MgO solubility gre-the balance between the effect of formation of an MA

layer and that of changing saturation solubility caused atly increases with the slag Al2O3 content. Since theresulting dissolution of MgO is direct, it will increaseby increased Al2O3 content in the slag. As shown in

Fig. 15, in a low basicity slag, when the content of with the slag Al2O3 content.The above predictions are verified by the rotaryAl2O3 increases above a critical value (for example

for the slag with C/S=1·0, it is 30 wt-%), further slag test results given by Herzog37 (Fig. 16). In theslag with basicity (C/S) of 2·3, direct dissolution ofincreasing Al2O3 content will greatly decrease the

saturation solubility of MgO in the slag. In this case, MgO should occur, because the slag basicity (2·3)is higher than the upper limit (2·1) for formationthe dissolution rate of MgO will greatly decrease due



11 Effect of slag basicity on saturation solubility13 Saturation solubility of Al

2O

3in CM(A)S slagof Al

2O

3in CM(A)S slag (MgO content=

10 wt-%) at 1600?C (calculated from Fig. 10b) at 1600?C as function of slag MgO content(calculated from Fig. 10)

14 Corrosion rate of sapphire in CMAS slag at1550?C as function of slag MgO content (fingertest) (after Ref. 22)

12 Effect of slag basicity on corrosion of Al2O

3based bricks, showing that increasing slagbasicity markedly increases corrosion (in24 h¬1·06 mm h−1) (after Ref. 31)

of solid MA at the test temperature (1600°C).Consequently, increasing the slag Al2O3 content from20 to 35 wt-% increases the MgO refractories cor-rosion rate (Herzog did not discuss this in his paper).On the other hand, in the low basicity slags (e.g.C/S=1·0), when the content of Al2O3 in the slag isabove, for example, 35 wt-%, indirect dissolution ofAl2O3 should occur and the saturation solubility ofMgO becomes low (Fig. 15), so the MgO dissolution 15 Saturation solubility of MgO in CMAS slag atrate decreases with increasing slag Al2O3 content 1700?C as function of slag Al

2O

3content (after

Ref. 36)from 35 to 40 wt-% (Fig. 16).



TE

MP

ER

AT

UR

E, °

C

wt-%

17 MgO–B2O

3phase diagram (Ref. 11)

16 Corrosion rate of MgO brick in slags withdifferent basicities as function of slag Al

2O

3content (rotary slag test).37 In basic slag (C/S=2·3), corrosion increases with increasing slagAl

2O

3content due to direct dissolution and

increased saturation solubility of MgO, but inacid slag (C/S=1·0), it decreases withincreasing slag Al

2O

3content because of

indirect dissolution

Influence of refractories purity oncorrosion resistanceExisting phase diagrams provide useful informationon the compatibility between the main refractory MgO C2S

B2O3

components and impurities at high temperature and 18 Composition triangle of MgO–C2S–B

2O

3thus can be used to predict the influence of impurities section of CaO–MgO–B2O

3–SiO

2system (Ref. 3)

on corrosion resistance. Consider the influence ofB2O3 impurity in MgO refractories. Figure 17 shows Influence of refractories additives onthe MgO–B2O3 phase diagram.11 Even a small their corrosion resistanceamount of B2O3 markedly decreases the temperatureof liquid formation in MgO from the melting point To improve specific properties, minor levels of addi-

tives may be made to refractories such as the use ofof pure MgO (~2800°C) to 1358°C. Dicalcium sili-cate, C2S, is commonly formed as a secondary phase antioxidants (e.g. Al (Refs. 39–42)) in carbon contain-

ing refractories to improve oxidation resistance andat MgO grain boundaries. Examination of the MgO–C2S–B2O3 phase diagram3 at 1550°C (Fig. 18) reveals high temperature mechanical strength. Aluminium

metal is also known to influence corrosion resistance.that the liquid phase coexisting with periclase+C2Scontains some 80 wt-% C2S and 10 wt-% B2O3 , i.e. Whether addition of Al to MgO–C refractories is

beneficial or harmful is the subject of some contro-in the presence of periclase, B2O3 will flux some 8times its weight of C2S at this temperature. Therefore, versy.43,44 Although disagreement has been attributed

to factors such as graphite purity45 or MgO grainthe presence of even tiny amounts of B2O3 in MgOmay disintegrate the MgO grain by fluxing the C2S quality,46 the influence of slag composition needs

consideration.grain boundary phase. Figure 19 shows the results ofWatanabe et al.38 in which wear increases markedly Two main factors in corrosion of MgO–C refractor-

ies are dissolution of MgO in the slag and oxidationby increasing B2O3 to only ~0·07 wt-%.



rosion resistance. On the other hand, in a basic slag,because the saturation solubility of MgO in the slagis low and is not sensitive to the change of Al2O3content in the slag (Fig. 15), the problem of C oxi-dation becomes more serious than the dissolution ofMgO in the slag. Consequently, because Al additionencourages MgO dense layer formation (which canremain at the refractories/slag interface for a longtime) and effectively inhibits C oxidation, it has apositive effect on corrosion resistance. Ikesue et al.49tested the effectiveness of Al addition to MgO–Cbricks in a basic slag (C/S=3·0). They found thatthe Al addition improves the corrosion resistance. Onthe other hand, Baker and Brezny50 found that Aladdition to MgO–C bricks resulted in severe cor-rosion in an acid slag (C/S=1·0). Asano51 used arotary slag test to examine the effect of Al in twoslags with basicity of 1·0 (slag A) and 2·0 (slag B). Hisresults (Fig. 20) show that Al addition to MgO–C19 Influence of B

2O

3content in MgO grain on

refractories improves their corrosion resistancecorrosion rate of MgO–C bricks, showingslightly in the more basic, but not in the acid, slag.that slight increase of B

2O

3content in MgO

Silicon (or SiC) and B4C are also used as additivesgrain markedly decreases corrosionto carbon containing refractories.52,53 The CaO–SiO2resistance (Ref. 38)and CaO–B2O3 phase diagrams (Fig. 21)11 show aproblem when using such additives in ZrO2–C refrac-of C by oxidising atmosphere or FexO and MnOtories. In these refractories, oxides (SiO2 and B2O3 )from the slag.47,48 Addition of Al metal powder toformed from oxidation of Si (or SiC) and B4C areMgO–C refractories has a positive effect on corrosionnot in equilibrium with the stabilising agents (e.g.resistance by encouraging formation of a dense MgOCaO) in the ZrO2 grain at high temperature, so theylayer and inhibiting C oxidation, but it also has awill react with each other to form CaO–SiO2 andnegative effect by increasing the saturation solubilityCaO–B2O3 compounds. The formation of these com-of MgO in the slag (Fig. 15). As shown in Fig. 15, inpounds will cause ZrO2 destabilisation so the ZrO2an acid slag, the saturation solubility of MgO in thegrain disintegrates, adversely affecting the corrosionslag is high, so MgO dissolution in the slag is theresistance.54dominant factor affecting the corrosion resistance. In

this case, even if a dense MgO layer forms, it willquickly dissolve in the slag. Furthermore, Al2O3 Influence of atmosphere onformed from oxidation of Al will increase the satu- refractories corrosion resistanceration solubility of MgO in the slag (Fig. 15), counter-acting the positive effect of Al addition, so that it The oxygen partial pressure in a refractory lined

enclosure may vary in service, leading to a change ofmay not have an overall positive effect on the cor-

20 Corrosion of MgO–C bricks with and without Al addition in acid slag A (C/S=1·0) and more basicslag B (C/S=2·0):51 Al addition slightly improves corrosion resistance in more basic slag, but doesnot in acid slag



TE

MP

ER

AT

UR

E, °

C

(a)

(b)

21 Phase diagrams for a CaO–SiO2

and b CaO–B2O

3systems, compositions in wt-% (Ref. 11)

valence of some components (in particular, iron oxide) (Fig. 22a), a liquid will develop at as low as 1210°Cwhen a fireclay brick consisting initially of mullitein the refractories and/or slag. This can significantly

affect the refractories corrosion resistance. and tridymite (or cristobalite) absorbs a small amountof iron oxide. If, however, the brick is higher in Al2O3Consider the influence of atmosphere on corrosion

resistance of Al2O3–SiO2 refractories containing, or and originally consists of mullite and corundum, aliquid will not form until a temperature of 1380°C isin contact with, iron oxide. Figure 22 shows solidus

surfaces in the Al2O3–SiO2–FeO and Al2O3–SiO2– reached, even after a considerable amount of ironoxide has been absorbed by the brick. In air, Fig. 22b,Fe2O3 systems.27 Under strongly reducing conditions



(a) (b)

22 Projections of solidus surface in iron oxide–Al2O

3–SiO

2system in a reducing and b oxidising

atmospheres (Ref. 27)

(a) (b)

a in reducing atmosphere; b in oxidising atmosphere

23 1500?C isothermal sections through CaO–MgO–iron oxide system; MW magnesiowustite, MFmagnesioferrite, L liquid (Ref. 27)

the lowest temperatures of liquid formation for bricks cause a liquid to develop at temperatures above 1210and 1380°C, depending on the Al2O3/SiO2 ratio ofof the same compositions as above are considerably

higher. On addition of iron oxide a fireclay brick the original brick (Fig. 22a). In air, on the other hand,a substantial proportion of the iron is present asdevelops a liquid phase only after a temperature of

1380°C or higher is reached, while a high alumina Fe3+. Hence moderate amounts of iron oxides can beabsorbed by alumina–silica refractories in air withoutbrick with absorbed iron oxide withstands temper-

atures at least as high as 1460°C before liquid starts formation of a liquid phase even at temperaturesabove those of the three-phase triangles labelled 1380forming.27

An important difference between the two diagrams and 1460°C in Fig. 22b. Composition triangles inwhich liquid is one of the phases present therefore(Fig. 22a and b) is related to the manner in which

iron can be accommodated in the crystal structures extend all the way over to the Al2O3–SiO2 side ofFig. 22a, whereas the analogous areas in Fig. 22bof the phases present. Two of the crystalline phases

present in alumina–silica refractories, mullite and terminate before reaching the Al2O3–SiO2 side.27The above analyses indicate that Al2O3–SiO2corundum, can accommodate iron ions in their

lattices, but only in the ferric state (Fe3+). Under refractories have better corrosion resistance in oxidis-ing than in reducing atmosphere. Based on this con-reducing conditions, where the iron in the oxide

phases is present almost exclusively as Fe2+, incorpor- clusion, it can be deduced that C in Al2O3 (–SiO2 )–Crefractories may have different effects on the corrosionation of Fe in the crystalline phases cannot take place.

Hence, even small amounts of iron oxide absorbed resistance of Al2O3 (–SiO2 ) grain phases, depending



TE

MP

ER

AT

UR

E, °

C

TE

MP

ER

AT

UR

E, °

C

(a)

(b)

(c) (d)

24 Phase diagrams of a MgO–FeO (Ref. 11), b MgO–Fe2O

3(Ref. 11), c CaO–FeO (Ref. 28), and d CaO–Fe

2O

3(Ref. 28) systems

on the local reducing conditions. If C only reduces reducing conditions (Fig. 23a), a CaO–MgO mixtureof composition A can take up about 22% iron oxideFe2O3 in the refractories or slag to FeO, then

Al2O3 (–SiO2 ) grains resistance to slag attack will without a liquid phase developing at 1500°C. In air(Fig. 23b), however, only 3% iron oxide can be takenbe decreased. However, if C further reduces FeO to

metallic Fe, the liquid formation temperature and up before a liquid phase develops at the same temper-ature. Hence, maintaining strongly reducing con-the slag viscosity are expected to increase so improv-

ing the resistance of the Al2O3 (–SiO2 ) grains to slag ditions within a dolomite refractory body exposed toiron oxide attack is desirable.27 The carbon inattack.

Now consider the influence of atmosphere on the MgO–CaO–C or tar bonded MgO–CaO bricks foroxygen steelmaking vessels is believed to serve thiscorrosion resistance of MgO–CaO refractories con-

taining, or in contact with, iron oxide. The MgO– purpose. In addition, however, carbon probably playsother important roles in the chemistry involved duringCaO–FeO and MgO–CaO–Fe2O3 phase diagrams at

1500°C (Fig. 23)27 reveal a large difference in the use of such bricks. For example, the carbon reducessome iron oxide in the refractories and/or in the slagextent of the area where no liquid phase is present at

the two different oxygen pressures. Under strongly to metallic iron, which enhances the liquid formation



(a) (b) (c)

a 0 wt-%C; b 10 wt-%C; c 20 wt-%C

25 Influence of brick CaO content, slag basicity (C/S), and slag iron oxide content (T. Fe) on corrosionrate of MgO(–C) and MgO–CaO(–C) refractories (Ref. 57). In low basicity and low iron oxide slag(C/S=1·2 and T. Fe=9·1 wt-%), MgO–CaO(–C) refractories have better slag resistance than MgO(–C)refractories, but in high basicity and high iron oxide content slag (C/S=3·1 and T. Fe=19 wt-%) theyexhibit poorer corrosion resistance

temperature in the refractories and/or slag increasing saturation solubility, its dissolution in the slag is aserious problem. Increasing the CaO (decreasingviscosity and reducing corrosion.MgO) content in the refractories usefully decreasestheir dissolution. Furthermore, increasing the CaOComparing corrosion resistance ofcontent may lead to the formation of a C2S layer

refractories (Fig. 21a) at the refractory/slag interface,55,56 whichcauses indirect dissolution and retards corrosion.In many cases, phase diagrams can be used to com-Therefore, in low basicity slag, the MgO–CaO(–C)pare qualitatively the corrosion resistance of differentshow better corrosion resistance than MgO(–C)refractories by checking the compatibility betweenrefractories.the refractories and slag. A good example of this is

Considering both the influence of Fe oxide and ofthe comparison of corrosion resistance of MgO(–C)slag basicity on the corrosion resistance of MgO(–C)and MgO–CaO(–C) refractories.and MgO–CaO(–C) refractories indicates that inThe MgO–FeO (Ref. 11), MgO–Fe2O3 (Ref. 11),low basicity and low Fe oxide content slag,CaO–FeO (Ref. 28), and CaO–Fe2O3 (Ref. 28)MgO–CaO(–C) refractories will show better cor-phase diagrams are shown in Fig. 24. After MgOrosion resistance than MgO(–C) refractories, but inabsorbs 70 wt-% FeO or 70 wt-% Fe2O3 , the liquidthe high basicity and/or high Fe oxide content slag,formation temperature is still higher than 1600 orthe latter will show better corrosion resistance.1700°C (Fig. 24a and b), but after CaO absorbsInduction furnace and rotary slag test results57–63only <15 wt-% FeO or a small amount of Fe2O3 , support these predictions. Figure 25, as an example,the liquid formation temperature falls to ~1160 orshows the rotary slag test results of Toritani et al.571438°C (Fig. 24c and d), and after CaO absorbs aboutExcept for the two bricks with CaO of ~8 and60 wt-% Fe2O3 , all of it becomes liquid at ~1450°C~17 wt-% in the slag with C/S=3·1 and total(Fig. 24d).Fe oxide (T. Fe)=19 wt-% (Fig. 25a), MgO–CaOComparing Fig. 24 with Fig. 23, reveals that after(Fig. 25a) and MgO–CaO–C refractories (Fig. 25bthe MgO–CaO refractory (A) takes up ~22 wt-%and c) show better corrosion resistance than MgOFeO or 3 wt-% Fe2O3 , liquid develops at 1500°Cand MgO–C refractories in the low basicity (C/S=(Fig. 23), however, after MgO refractory absorbs the1·2) and low Fe oxide content (T. Fe=9·1 wt-%)same amount of FeO or Fe2O3 , the liquid formationslag, but exhibit poorer corrosion resistance in thetemperature remains higher than 2300 or 2700°Chigh basicity (C/S=3·1) and high Fe oxide content(Fig. 24a and b).(T. Fe=19 wt-%) slag.The above differences indicate that with higher Fe

oxide content slags, the MgO(–C) corrosion resistanceis better than that of MgO–CaO(–C) refractories. Refractories selection for specific

The corrosion behaviour of MgO(–C) is also applicationsdifferent from that of MgO–CaO(–C) refractories inslags with different basicities. As shown in Fig. 6, in By checking the compatibility between refractories

and surrounding slags, the corrosion behaviour ofa slag with low basicity, because MgO has high



(a)

(e) (f)

(b)

Mol.-%

TE

MP

ER

AT

UR

E

(c) (d)

Mol.-%

26 Phase diagrams of a Al2O

3–CuO/Cu

2O (Ref. 64), b CaO–CuO/Cu

2O (Ref. 65), c ZrO

2–CuO/Cu

2O (Ref. 66),

d SiO2–CuO/Cu

2O (Ref. 67), e MgO–CuO/Cu

2O (Ref. 68), and f Cr

2O

3–CuO/Cu

2O (Ref. 69) systems

different refractories can be predicted and compared, amounts of Cu2O/CuO fluxes these oxides (i.e. lowersthe melting temperature). For example, just a tinyassisting refractories selection for specific applications.

A good example of this is selection of refractories to amount of Cu2O/CuO will decrease the liquid forma-tion temperature from 2051°C (the melting point ofbe exposed to copper melting slags.

Figure 26 shows the Al2O3–Cu2O/CuO (Ref. Al2O3 ) to 1238°C with Al2O3 (Fig. 26a), and from2730°C (the melting point of ZrO2 ) to 1130°C with64), CaO–Cu2O/CuO (Ref. 65), ZrO2–Cu2O/CuO

(Ref. 66), SiO2–CuO (Ref. 67), MgO–Cu2O/CuO ZrO2 (Fig. 26c).However, the cases of MgO and Cr2O3 are quite(Ref. 68), and Cr2O3–Cu2O/CuO (Ref. 69) phase dia-

grams. As can be seen from Fig. 26a–d, CuO and different. As shown in Fig. 26e, MgO can take up20 wt-% Cu2O/CuO without liquid formation, andCu2O are corrosive to Al2O3 , CaO, SiO2 , and ZrO2

refractories, because the presence of even small even after Cr2O3 absorbs ~65 wt-% Cu2O/CuO,



27 Composition triangle of MgO–CuO–Cr2O

3showing saturation solubility of MgO of 28 Solidus surface of CaO–MgO–SiO2

systemcomposition E and MgCr

2O

4of composition F shown in Fig. 3a (Ref. 27)

in liquid at this temperature is low (Ref. 3)system in Al2O3-based castables. However, if CAcement is added to an MgO–MA castable, the liquidno liquid forms (Fig. 26 f ). Consequently, MgO andformation temperature is lower than 1400°C (point QCr2O3 are much more resistant to Cu2O/CuO thanon Fig. 9). A similar situation arises in MgO–CaOAl2O3 , CaO, SiO2 , and ZrO2 . The MgO–Cr2O3– castables. CA cement thus does not provide a suitableCu2O diagram at 1400°C (Ref. 3) (Fig. 27) furtherbond system for basic castables.reveals that the solubilities of MgO and MgCr2O4 in

To improve flowability and other properties ofliquid (L) at 1400°C are low. The above analysescastables, superfine (submicrometre) powders suchexplain why direct bonded magnesia–chrome refrac-as Al2O3 and SiO2 are usually added.79–81 A smalltories are so successful in copper smelting furnaces.70amount of Al2O3 added to a MgO–CaO castable willdecrease the liquid formation temperature to lower

Designing refractories composition than 1500°C (Fig. 9). However, examining the CaO–MgO–SiO2 phase diagram (Fig. 3a) shows that whenProper raw materials selection and optimisation ofa small amount of SiO2 is added to a MgO–CaOrefractories compositions are important factors incastable, C3S will form and the liquid formationdeveloping refractories with high refractoriness andtemperature remains as high as ~1850°C. This indi-good corrosion resistance. A good example of this iscates that using superfine SiO2 powder in MgO–CaOcontrol of the C/S ratio in MgO grain materials orcastables is better than using superfine Al2O3 powderrefractories.71,72 The solidus surfaces of the CaO–if corrosion resistance is paramount.

MgO–SiO2 system (Fig. 28) reveal that the liquidformation temperature generally increases with

Summaryincreasing C/S ratio in the MgO grain. For example,the liquid formation temperature corresponding to Understanding corrosion mechanisms and comparingC/S(mol ratio)=1·0–1·5 is 1490°C, to C/S=1·5–2·0 corrosion resistance of refractories in various slags isis 1575°C, to C/S=2·0–3·0 is 1790°C, and to C/S>3·0 important for evaluating their service life. Phase dia-

grams provide useful information about the compati-is 1850°C. Therefore, it is necessary to maintain thebility between refractories components and slags,C/S ratio of MgO grain or MgO based refractorieswhich can be used to interpret corrosion mechanismsabove 2·0 to retain high refractoriness and corrosionand predict corrosion behaviour. In this review, typi-resistance.71–73cal applications of phase diagrams to specific prob-A further example is selection of a suitable bondlems of refractories corrosion have been highlighted.system for castable refractories. Calcium aluminateExperimental corrosion test results show good agree-cement (CA cement) is widely used in Al2O3-basedment with the phase diagram predictions (thoughcastable refractories, though in recent years, cement-phase diagrams do not contain any information re-free castables have been developed (e.g. Refs. 74–76).lating to the kinetics of the reactions involved).According to the CaO–MgO–Al2O3 phase diagramTherefore, use of the information which is available(Fig. 9), small additions of CA cement whose mainin existing phase diagrams can reduce the need forcomponents are CA, CA2 , and C12A7 (Refs. 77, 78)expensive and time consuming experimentation toto Al2O3 castable will lead on firing to formation ofevaluate the high temperature corrosion of refrac-CA6 , and in this case the liquid formation temperaturetories.is ~1870°C (point Y on Fig. 9). With addition of a

small amount of CA cement to Al2O3–MA castable,Referencesthe liquid formation temperature remains higher than

1800°C (point X on Fig. 9). Therefore, if SiO2 is not 1. . , . , and . : T aikabutsu Overseas,1982, 2, (2), 5–13.added, CA cement gives a highly refractory bond



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