Corrosion of Materials Course

Corrosion of Inorganic Non-Metallic Materials

part 2 Corrosion of ceramicsEnamels

Corrosion of concrete

Aleš Helebrant

Department of Glass and Ceramics ICT Prague, Czech Republic

www.usk.cz


Contents

• Main categories of ceramics• Corrosion of refractories in melts• Corrosion of refractories in furnace atmosphere• Corrosion of non/oxide ceramics

• Protection of metals by enamels• Corrosion of concrete


Ceramics

• Porcelain, chinaware• Pottery• Bricks, roof tiles, tiles, sanitary ceramics

• Advanced (engineering) ceramics - Al2O3, ZrO2, UO2

• Refractories• Non-oxide ceramics (nitrides, carbides)


Ceramics

• Polycrystalline material• Usually prepared by high-temperature processes• typical HT reaction kaolinite – mullite

• Al2O3.2SiO2.2H2O 3 Al2O3.2SiO2 + SiO2 + H2O

• sintering process


Ceramics - microstructure


Main types of refractories

• silica r.• fireclay r. (grog, chamotte)• corundum r.• magnesite r.• chrome-magnesite r. • fusion-cast

• refractoriness – technical property

refractoriness testtip touching support (3-5°C.min-1)


Refractories – composition, properties

• silica r. (dinas) (refractoriness 1710-1750°C)– >93% silica quartz, tridymite, cristobalite, glass

• fireclay r. (grog, chamotte) (1600-1750°C)– 15-46% Al2O3 - SiO2, mullite 3Al2O3.2SiO2

• corundum (1850-2000°C)– -Al2O3

• magnesite r. (>2000°C)– MgO periclase

• chrome-magnesite r. (1920-2000°C)– MgO.Cr2O3

• fusion-cast corundum/badelleyite r. (no open pores!)– -Al2O3, ZrO2, SiO2 glass phase

acid

neutral

basic


refract.

melt

refract.

melt

t=0 t>0

Corrosion of refractories in melts

influence of density or surface tension gradient


Testing of corrosion resistance


corrosion leading to mechanical degradation• regenerators in gas heated glass melting furnaces – preheating of gas by products of combustion

3Al2O3.2SiO2 + Na2O Na2O.Al2O3.2SiO2 + 2Al2O3

nepheline3(Na2O.Al2O3.2SiO2) + 2Na2O + 2SO3 5Na2O.3Al2O3.6SiO2.2SO3

noselitemullit nepheline noselite = volume expansion mechanical stress

similar effect - changing oxidation/reduction atmosphereMgO.Fe2O3 and MgO-FeO or oxidation of FeO.Cr2O3 and FeO.Fe2O3

Corrosion of refractories in atmosphere


active or pasive mechanism

pasive – high pO2, lower TSi3N4(s) + 3O2 3SiO2(s) + 2N2

controlled by diffusion

active – low pO2, higher T2Si3N4(s) + 3O2 6SiO(g) + 4N2

controlled by chemical reaction

SiO2

Si3N4

Si3N4

Si3N4

Si3N4

Corrosion of non - oxide ceramics


Enamels

• glassy layers on metals (glazes on ceramics)– protecting metals– improving appearance

• mostly on Fe - steel sheets, cast iron (low content of C in the form of free cementite Fe3C)

• other applications– Al – building industry– Cu, Ag, Au, Pt - jewellery – Ti - biomaterials


Enamels

• Main criteria for enamelling– similar thermal expansion coefficient– adhesion (physical, chemical)– wettability of metal by enamelling suspension

Three steps of enamelling• the manufacture of enamel frits• treatment of the metal materials• preparation of enamelling slips (suspensions)

and the enamelling itself


Enamels - 3 main steps

1) the manufacture of enamel frits– glass melting (1000-1400°C) in gas fired furnaces– quenching in water granulation– (rotating cylinders glass ribbon crushed into scales)

– low silica glasses (about 50-47%), B2O3 (16-11), Al2O3 (7) Na2O+K2O (20-15) fluorides (6-20) different composition for ground and cover layers

– other components 0.5% CoO (ground), SnO2 in mill (cover)

– TiO2 enamels – nucleation (anatase) opacity, thinner layer



2) treatment of the metal materials– degreasing (trichlorethylene or alkaline solutions– or anneling at approx 750°C – burning off organic

substances– removing iron oxides from the metal surface

• cold bath 5-20% HCl

• or 6-17% H2SO4 at 50-70°C

• inhibitors needed (e.g. phenols) – Fe dissolution without them faster then oxides dissolution



3) preparation of enamelling slips and the enamelling itself– frit is ground in ball mills (<0.1 mm) (in wet or dry process)– adding other ingredients during wet process

• quartz, MgO etc. – adjusting the firing interval

• colorants or opacifiers

• NaNO2 for preventing the metal from rusting

• clays and electrolytes to adjust rheological properties of the slip

– dipping or pouring, electrostatic wet spraying– dry process for large cast-iron objects – application of dry

powdered frit onto a preheated product– drying, firing (780-900°C)


Enamels

Special enamels– acid-resitant for food or pharmaceutical industry (higher content

of SiO2 and ZrO2, lower of alkaline oxides– non-alkaline enamels for engines – resistant to T changing– enamels for Al – low firing temperatures (550°C) – higher content

of PbO

Testing– in organic acids – enamels for household or food industry– in mineral acids – enamels for technical reasons

General chemical durability of enamels and corrosion mechanisms – same as for glass



• Concrete = material prepared by chemical reaction between inorganic binder (cement) and water

• other components – gravel aggregate (stones), quartz...

• corrosion of concrete – corrosion of binder• cement – fine grain hydraulic binder• Portland cement – mixture of Ca silicates and

alumosilicates



Manufacture of cement• raw materials CaCO3, clays, bauxit...• firing in rotary cement kilns (1400°C) clinker

milling (particles <0.1 mm)• theoretical clinker composition: C3S, C2S, C3A,

C4AFReaction cement + water• hydrosilicates CSH phase (near to tobermorite)

main connecting gel phase• other phases Ca(OH)2, open and closed pores



3 basic groups of concrete corrosion

1. dissolution and leaching

2. corrosion with chemical reaction – soluble products

3. corrosion with chemical reaction – insoluble products



1. dissolution and leaching• Ca(OH)2 dissolution• increase of porosity, increase of contact surface with

corrosive environment• decrease of pH in concrete – dissolution of CSH phase

+ influence of Fe corrosion in concrete• Ca, (Na, K) sulphates and carbonates on the concrete

surface• in “soft“ waters• “hard“ (earthy) waters - densifying of concrete

Ca(OH)2 + Ca(HCO3)2 = 2CaCO3 + H2O



2. corrosion with chemical reaction – soluble products

• dissolution in acid solutions: pH<6 dissolution of all cement phases – CSH, Ca(OH)2, CaCO3

– treatment – decrease porosity, protective layers (water glass, pure cement mortar)

• corrosion by ammonium salts

• Ca(OH)2 + 2NH4 = 2NH3 + CaCl2 + 2H2O



3. corrosion with chemical reaction – insoluble products• sulphate corrosion – most dangerous

• product ettringite 3CaO.Al2O3.3CaSO4.32H2O

• volume expansion (400%) increase mechanical degradation

• seawater, waste waters

protection:– low content of aluminates

– active silica in cements – reaction with Ca(OH)2 to silicates – no Ca2+ for ettringite

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