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Improving oxidation resistance and fracture strength of MgOndashC

refractory material through precursor coating

Geun-Ho Cho a Eun-Hee Kim a Yeon-Gil Jung a Yun-Ki Byeun b

a School of Advanced Materials Science and Engineering Changwon National University Changwon Gyeongman 641-773 Republic of Koreab Technical Research Laboratories Pohang Research Lab Steelmaking Research Group POSCO Pohang Gyeongbuk 790-300 Republic of Korea

a b s t r a c ta r t i c l e i n f o

Available online 21 November 2014

Keywords

Precursor

Graphite

Coating

MgOndashC refractory

Oxidation resistance

Fracture strength

An Al precursor was coatedonto the surface of graphite in a MgOndashC refractorymaterial to improvethe oxidation

resistance and the fracture strength by controlling the amount of antioxidant and the coating thickness Toenhance the coating ef 1047297ciency the surface of the graphite used as the carbon source was treated with an

acid In oxidation tests the Al-coated graphite showed a smaller weight loss than the pristine graphite

The MgOndashC refractory material with the Al-coated graphite showed a similar fracture strength to that

with commercial graphite despite the use of a smaller amount of antioxidant The highest fracture strength of

the MgOndashC refractory material was about 17 MPa it was obtained with the Al-coated graphite The increase in

fracture strength was a result of the homogeneos coating of Al precursor in the modi 1047297ed MgO-C refractory

material Based on the properties observed we discuss the relationship between fracture strength and process

parameters

copy 2014 Published by Elsevier BV

1 Introduction

MgOndashC refractory material is widely used in basic furnaces electric

arc furnaces and steel ladles because of its corrosion resistance by low

wettability with the molten metal excellent thermal shock resistance

by its low thermal expansion and high thermal conductivity [12]

These desirable properties are achieved by the use of graphite as the

carbon source in the MgOndashC However the graphite in the MgOndashC is

highly vulnerable to oxidation which causes a deterioration in the me-

chanical properties Thedamage through erosion or corrosion is serious

and structural spalling is easily generatedin real applications because of

slag penetration and pore generation by the oxidation of graphite [34]

To impede the oxidation phenomena antioxidants that have high reac-

tivity with oxygen are usually added to batches of the MgOndashC [56] The

added antioxidants include metalsalloys such as Al AlMg alloys

carbides such as B4

C and SiC and borides such as CaB6

and ZrB2

[5]

which show a stronger oxidation entropy than graphite Theoretically it

means that graphite does not react with oxygen until the oxidation of

the antioxidant has progressed considerably In practice graphite reacts

with oxygen even though the oxidation of the antioxidant has not

progressed considerably Pore generation at inner sites of the MgOndashC is

an unavoidable phenomenon because the antioxidant cannot effectively

obstruct the oxidation of graphite in real environments Further disad-

vantages include volume expansion limited content and high costIn previous works we studied methods for improving the oxidation

resistance of graphite by coating the graphite surface with a metal pre-

cursor in particular an Al precursor [78] The Al-coated graphite

showed a higher oxidation resistance due to the barrier effect of the Al

layer formed on the surface of graphite In the coating process the coat-

ing reagent should be homogeneously coated on the surface of the

graphite to obtain a suf 1047297cient coating effect [9ndash11] Four key points

should be considered in the preparation of graphite with a high coating

ef 1047297ciency (1)the processof coating thegraphite withthe metal precur-

sor is performed in an aqueous solution to increase the dispersibility of

the metal precursor on the surface of the graphite [12ndash14] (2) the

graphite surface is puri1047297ed and modi1047297ed with an acid to create hydro-

philic groups (3) to improve coating ef 1047297ciency a metal precursor that

has high water solubility is used and (4) a metal with high reactivity

with oxygen is selected

The aim of this study was to prepare a MgOndashC refractory material

not only with graphite coated with Al precursor but also with a smaller

amount of antioxidant than that used in the conventional process

Therefore in the present study Al precursor was coated onto the mod-

i1047297ed graphite with an acid reagent to enhance the coating ef 1047297ciency To

investigate the effect of the precursor coating on the oxidation resis-

tance of the MgOndashC the content of antioxidant in the MgOndashC was con-

trolled The oxidation behavior of the MgOndashC is discussed based on the

results of oxidation tests and the measured fracture strength during the

development of the new process

Surface amp Coatings Technology 260 (2014) 429ndash432

Corresponding author Tel +82 55 213 3712 fax + 82 55 262 6486

E-mail address jungygchangwonackr (Y-G Jung)

httpdxdoiorg101016jsurfcoat201411035

0257-8972copy 2014 Published by Elsevier BV

Contents lists available at ScienceDirect

Surface amp Coatings Technology

j o u r n a l h o m e p a g e w w w e l s e v i e r c o m l o c a t e s u r f c o a t

7232019 1-s20-S0257897214010561-main


2 Experimental procedure

21 Materials and preparation methods

Aluminum nitrate (Al(NO3)3) (Sigma-Aldrich Korea Yongin Korea)

was used as a metal precursor [15] Platy graphite (POSCO Pohang

Korea) usually used in commercial MgOndashC refractory materials was

suspended in a mixture of sulfuric and nitric acids (31 by vol) that

servedas a modi1047297cationreagent forthe surface of thegraphiteThe mix-

ture wassonicated for 3 h and stirred for 24 h The acid-treated graphite

was 1047297ltered and washed with distilled water until pH 7 and then dried

at80 degCfor 48h Al(NO3)3 was 1047297rst dissolved in a 3 M aqueous solution

The graphite particles modi1047297ed by the acid were mixed with the

Al(NO3)3 solution and then stirred for 1 h at room temperature The

graphite coated with Al precursor was 1047297ltered and then dried at 80 degCfor 1 h The prepared powder was held at 500 degC for 1 h under H2 atmo-

sphere to increase the adhesion of the Al precursor onto the graphite

surface Commercially fused magnesia (MgO particle size le 1 mm)

platy graphite or modi1047297ed graphite antioxidant (AlndashSi mixture

POSCO Pohang Korea) and phenolic resin were used to prepare the

MgOndashC refractory material The formulations used to prepare the

MgOndashC are shown in Table 1 The modi1047297ed or pristine graphite MgO

antioxidant and phenolic resin were mixed by a dry ball-milling pro-

cess for 24 h at room temperature and then formed under a 60 MPa

pressure with a cuboid shape of 10 mm times 10 mm times 50 mm followed

by cool isostatic pressing at 200 MPa for measuring fracture strengthand at a pressure of 110 MPa with a cylindrical shape of 50 mmdiameter

and50 mm length for oxidation tests After pressing the MgOndashC refracto-

ry samples were heat-treated at 300 degC for 3 h

22 Characterization

The combustion tests for the graphite particles with and without

modi1047297cation were conducted at 1000 degC for 1 2 and 3 h The prepared

refractory samples were also subjected to combustion tests at 1000 degC

for 3 h to investigate the oxidation resistance of the MgOndashC refractory

material depending on the formulation The Al coating on the graphite

surface was analyzed with X-ray photoelectron spectroscopy (XPS VG

Scienti1047297c ESCALAB 250 East Sussex United Kingdom) The fracture

strength of the samples before and after heat treatment was measuredusing a universal testing machine (Instron 5566 Instron Norwood

MA USA) in a four-point bending mode at a rate of 05 mm minminus1

Tests were carried out at room temperature At least1047297ve runs were per-

formed to determine the standard deviation of the strength

3 Results and discussion

The Al precursor has to be coated onto the surface of the graphite to

achieve suf 1047297cient coating ef 1047297ciency and impart oxidation resistance to

the MgOndashC The XPS results are shown in Fig 1 for the elements of

Table 1

Formulations used to prepare the MgOndashC refractory materials in this work

Run number MgO (g) Graphite (g ) Antioxidants (g) Phenol ic res in (g) Pre heating (Al-coated powder) Al precursor

Run-1 84 10 30 3 500 degC1 h Without coating

Run-2 30 With coating



Fig 1 XPS results for (a) pristine graphite and (b) Al-coated graphite

430 G-H Cho et al Surface amp Coatings Technology 260 (2014) 429ndash432

7232019 1-s20-S0257897214010561-main


carbon(C) oxygen(O) and aluminum (Al) TheAl peak was detected inthe Al-coated graphite from the short wavelength as shown in

Fig 1(b-3) while only C and O peaks were detected in pristine graphite

(Fig 1(a-1) and (a-2)) The O was detected by the carboxylic group

which was reformed along with C on the surface of the graphite with

the acid indicating that graphite was modi1047297ed by the acid It was also

veri1047297ed that the Al precursor wascoated and then the Al layer wasgen-

erated on the surface of the graphite The generated coating layer

restricts the access of oxygen to the graphite and hence adequately

controls the oxidation of the graphite

Thepristine graphite wasdrastically affected by oxidationafter com-

bustion tests showing a weight loss of 822 after heat treatment for

3 h The oxidation rate was calculated from the weight loss of graphite

it was found to increase with time The oxidation rate decreased with

theprecursor coatingdue to theAl layer formed on thegraphite surfaceIn addition the weight loss also increased signi1047297cantly from 89 to

761 with an increase in test time from 1 to 3 h showing the same

oxidation trend as that for pristine graphite When graphite was modi-

1047297ed with the Al precursor its color changed to white after heat treat-

ment because of the formation of the alumina phase converted from

the Al precursor on the graphite surface as presented in Fig 2(b) This

means that the graphite was well coated with the Al precursor It was

therefore evident that the Al coating enhanced the oxidation resistance

of graphite even though the weight loss caused by the oxidation of

graphite should be taken into consideration

The fracture strength values of MgOndashC samples prepared using

pristine and Al-coated graphite as a function of composition are

shown in Fig 3 The fracture strength of the MgOndashC with Al-coated

graphite in the Run-2 composition was slightly increased independent

of heat treatment even though the increase was within the error rangeThehighest fracture strength valuesof about 17 and7 MPawere obtain-

ed in Run-2 before and after heat treatment respectively The as-

prepared samples showed values similar to each other meaning that

the antioxidants added to the MgOndashC did not affect the green strength

However the effect of the antioxidant on the strength after heat treat-

ment was undesirable The sample prepared with the conventional

composition Run-1 showeda similar fracture strength to that prepared

Fig 2 Photographs of graphite particles with and without precursor coating (a) pristine graphite and (b) Al-coated graphite Each number indicates graphite particles after combustion

tests conducted for 1 2 and 3 h

Fig 3 Fracture strengthvalues of MgOndashC samples depending on formulations Each number

refers to the values before and after heat treatment at 1000 degC for 3 h

431G-H Cho et al Surface amp Coatings Technology 260 (2014) 429ndash432

7232019 1-s20-S0257897214010561-main


with less antioxidant Run-3 When the antioxidant was not added to

the batch the fracture strength value was signi1047297cantly lower than that

of the as-prepared sample indicating that the Al-coated graphite had

a limited effect on improving the strength value in terms of the antioxi-

dant content Oxidation by heat treatment leads to a deterioration in

the thermal and mechanical properties of the MgOndashC which shortens

the lifetime The sectional images measured after combustion tests at

1000 degC for 1 h carried out for an application are shown in Fig 4 The

size of MgOndashC samples prepared using standard amount of the antiox-

idant added in the conventional process remained unchanged after

combustion tests at 1600 degC independent of the precursor coating

However when the content of antioxidant was reduced to 50 in

Run-3 size reduction caused by the oxidation of graphite was detected

and this was even further advanced in the case of no antioxidant in

Run-4

4 Conclusions

MgOndashC samples were prepared withthe Al-coated graphite and the

effect of the antioxidant content on the oxidation behavior of the MgOndash

C with the pristine and Al-coated graphite particles was investigated

The coating of the Al precursor onto the surface of thegraphite was con-

1047297rmed by XPSresults TheAl-coated graphite showeda relatively higher

oxidation resistance than the MgOndashC with pristine graphite and less

antioxidant The fracture strength of the MgOndashC was successfully

increased by using the Al-coated graphite even when the antioxidant

content was reduced to 50 of the standard composition The highest

fracture strength values of about 17 and 7 MPa (before and after heat

treatment respectively) were obtained in the MgOndashC with the Al-

coated graphite in the standard antioxidant content The thermal stabil-

ity of the MgOndashC was con1047297rmed in combustion tests The Al precursor

coating had limited effects on the fracture strength and oxidation in

samples containing up to 50 of the standard antioxidant content

Acknowledgment

This research was 1047297nancially supported by Changwon National

University in 2013ndash2014

References

[1] S Zhang WE Lee In1047298uence of additives on corrosion resistance and corrodedmicrostructures of MgOndashC refractories J Eur Ceram Soc 21 (2001) 2393

[2] B Hashemi ZA Nemati MA Faghihi-Sani Effects of resin and graphite content on

density and oxidation behavior of MgOndashC refractory bricks Ceram Int 32 (3) (2006)313

[3] L Xiaowei R Jean-Charles Y Suyuan Effect of temperature on graphite oxidationbehavior Nucl Eng Des (2004) 273

[4] AP Luz DO Vivaldini F Lopez PO Brant VC Pandolfelli Recycling MgOndashC refrac-toriesand dolomite1047297nes as slag foaming conditioners experimental and thermody-namic evaluations Ceram Int 39 (2013) 8079

[5] S Zhang NJ Marriott WE Lee Thermochemistry and microstructures of MgOndashCrefractories containing various antioxidants J Eur Ceram Soc 21 (2001) 1037

[6] SK Sadrnezhaad S Mahshid BH Ashemi ZA Nemati Oxidation mechanism of Cin MgOndashC refractory bricks J Am Ceram Soc 89 (2006) 1308

[7] EH Kim GH Cho YS Yoo SM Seo YG JungDevelopmentof a newprocessin highfunctioning ceramic core without shape deformation Ceram Int 39 (2013) 9014

[8] GH Cho EH Kim J Li JHLee YG Jung YKByeunCY Jo Improvement of oxida-tion resistance in graphite for MgOminusC refractory through surface modi1047297cationTrans Nonferrous Met Soc China 24 (2014) s119

[9] M Barsoum Fundamentals of Ceramics McGraw-Hill Seoul 1997[10] EH Kim JH Lee YG Jung CS Lee U Paik A new in situ process in precision cast-

ing for mold fabrication J Eur Ceram Soc 31 (2011) 1581[11] DA Buttry FC Anson New strategies for electrocatalysis at polymer-coated elec-trodes Reduction of dioxygen by cobalt porphyrins immobilized in na1047297on coatingson graphite electrodes J Am Chem Soc 106 (1) (1984) 59

[12] EH Kim D LeeU Paik YG Jung Sizeeffectin Ni-coated TiC particles formetal ma-trix composites J Nanosci Nanotechnol 11 (2011) 1746

[13] EH Kim JH Lee YG Jung CG Lee MK Lee JJ Park Preparation of Ni-coated TiCparticles using potential hydrogen (pH) for dispersion into a molten metal ProgOrg Coat 70 (2011) 310

[14] PCPandey SUUpadhyay NKShukla S Sharma Studies on theelectrochemical per-formance of glucose biosensor based on ferrocene encapsulated ORMOSILand glucoseoxidase modi1047297ed graphite paste electrode Biosens Bioelectron 18 (2003) 1257

[15] CH Hou C Liang S Yiacoumi S Dai C Tsouris Electrosorption capacitance of nanostructured carbon-based materials J Colloid Interface Sci 302 (2006) 54

Fig 4 Photographs of MgOndashC samples with different formulations after combustion tests at 1000 degC (a) Run-1 (b) Run-2 (c) Run-3 and (d) Run-4


7232019 1-s20-S0257897214010561-main


2 Experimental procedure

21 Materials and preparation methods

Aluminum nitrate (Al(NO3)3) (Sigma-Aldrich Korea Yongin Korea)

was used as a metal precursor [15] Platy graphite (POSCO Pohang

Korea) usually used in commercial MgOndashC refractory materials was

suspended in a mixture of sulfuric and nitric acids (31 by vol) that

servedas a modi1047297cationreagent forthe surface of thegraphiteThe mix-

ture wassonicated for 3 h and stirred for 24 h The acid-treated graphite

was 1047297ltered and washed with distilled water until pH 7 and then dried

at80 degCfor 48h Al(NO3)3 was 1047297rst dissolved in a 3 M aqueous solution

The graphite particles modi1047297ed by the acid were mixed with the

Al(NO3)3 solution and then stirred for 1 h at room temperature The

graphite coated with Al precursor was 1047297ltered and then dried at 80 degCfor 1 h The prepared powder was held at 500 degC for 1 h under H2 atmo-

sphere to increase the adhesion of the Al precursor onto the graphite

surface Commercially fused magnesia (MgO particle size le 1 mm)

platy graphite or modi1047297ed graphite antioxidant (AlndashSi mixture

POSCO Pohang Korea) and phenolic resin were used to prepare the

MgOndashC refractory material The formulations used to prepare the

MgOndashC are shown in Table 1 The modi1047297ed or pristine graphite MgO

antioxidant and phenolic resin were mixed by a dry ball-milling pro-

cess for 24 h at room temperature and then formed under a 60 MPa

pressure with a cuboid shape of 10 mm times 10 mm times 50 mm followed

by cool isostatic pressing at 200 MPa for measuring fracture strengthand at a pressure of 110 MPa with a cylindrical shape of 50 mmdiameter

and50 mm length for oxidation tests After pressing the MgOndashC refracto-

ry samples were heat-treated at 300 degC for 3 h

22 Characterization

The combustion tests for the graphite particles with and without

modi1047297cation were conducted at 1000 degC for 1 2 and 3 h The prepared

refractory samples were also subjected to combustion tests at 1000 degC

for 3 h to investigate the oxidation resistance of the MgOndashC refractory

material depending on the formulation The Al coating on the graphite

surface was analyzed with X-ray photoelectron spectroscopy (XPS VG

Scienti1047297c ESCALAB 250 East Sussex United Kingdom) The fracture

strength of the samples before and after heat treatment was measuredusing a universal testing machine (Instron 5566 Instron Norwood

MA USA) in a four-point bending mode at a rate of 05 mm minminus1

Tests were carried out at room temperature At least1047297ve runs were per-

formed to determine the standard deviation of the strength

3 Results and discussion

The Al precursor has to be coated onto the surface of the graphite to

achieve suf 1047297cient coating ef 1047297ciency and impart oxidation resistance to

the MgOndashC The XPS results are shown in Fig 1 for the elements of

Table 1

Formulations used to prepare the MgOndashC refractory materials in this work

Run number MgO (g) Graphite (g ) Antioxidants (g) Phenol ic res in (g) Pre heating (Al-coated powder) Al precursor

Run-1 84 10 30 3 500 degC1 h Without coating




Fig 1 XPS results for (a) pristine graphite and (b) Al-coated graphite


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Run-4

4 Conclusions
















Acknowledgment



References


















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Run-4

4 Conclusions
















Acknowledgment



References


















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Run-4

4 Conclusions
















Acknowledgment



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