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Corrosion of Inorganic Non-Metallic Materials part 2 Corrosion of ceramics Enamels Corrosion of concrete Ale š Helebrant Department of Glass and Ceramics I CT Prague , Czech Republic www.usk.cz Contents Main categories of ceramics Corrosion of refractories in melts

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Corrosion of Inorganic Non-Metallic Materialspart 2 Corrosion of ceramicsEnamelsCorrosion of concreteAleš HelebrantDepartment of Glass and Ceramics ICT Prague, Czech Republicwww.usk.cz

Corrosion of Materials Course

contents
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

Corrosion of Materials Course

ceramics
Ceramics
  • Porcelain, chinaware
  • Pottery
  • Bricks, roof tiles, tiles, sanitary ceramics
  • Advanced (engineering) ceramics - Al2O3, ZrO2, UO2
  • Refractories
  • Non-oxide ceramics (nitrides, carbides)

Corrosion of Materials Course

ceramics4
Ceramics
  • Polycrystalline material
  • Usually prepared by high-temperature processes
  • typical HT reaction kaolinite – mullite
  • Al2O3.2SiO2.2H2O3 Al2O3.2SiO2 + SiO2 + H2O
  • sintering process

Corrosion of Materials Course

ceramics microstructure
Ceramics - microstructure

Corrosion of Materials Course

main types of r efractories
Main types of refractories
  • silica r.
  • fireclay r. (grog, chamotte)
  • corundum r.
  • magnesite r.
  • chrome-magnesite r.
  • fusion-cast
  • refractoriness – technical property

refractoriness test

tip touching support (3-5°C.min-1)

Corrosion of Materials Course

refractories composition properties
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)
    • a-Al2O3
  • magnesite r. (>2000°C)
    • MgO periclase
  • chrome-magnesite r. (1920-2000°C)
    • MgO.Cr2O3
  • fusion-cast corundum/badelleyite r. (no open pores!)
    • a-Al2O3,ZrO2, SiO2 glass phase

acid

neutral

basic

Corrosion of Materials Course

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Corrosion of refractories in melts

refract.

refract.

melt

melt

influence of density or surface tension gradient

t=0

t>0

Corrosion of Materials Course

slide9

Testing of corrosion resistance

Corrosion of Materials Course

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Corrosion of refractories in atmosphere

  • 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
  • nepheline
  • 3(Na2O.Al2O3.2SiO2) + 2Na2O + 2SO3 5Na2O.3Al2O3.6SiO2.2SO3
  • noselite
  • mullit  nepheline  noselite = volume expansion  mechanical stress
  • similar effect - changing oxidation/reduction atmosphere
  • MgO.Fe2O3and MgO-FeO or oxidation of FeO.Cr2O3and FeO.Fe2O3

Corrosion of Materials Course

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Corrosion of non - oxide ceramics

active or pasive mechanism

pasive – high pO2, lower T

Si3N4(s) + 3O2 3SiO2(s) + 2N2

controlled by diffusion

active – low pO2, higher T

2Si3N4(s) + 3O2 6SiO(g) + 4N2

controlled by chemical reaction

Si3N4

Si3N4

SiO2

Si3N4

Si3N4

Corrosion of Materials Course

enamels
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

Corrosion of Materials Course

enamels13
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

Corrosion of Materials Course

enamels 3 main steps
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

Corrosion of Materials Course

enamels 3 main steps15
Enamels - 3 main steps

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

Corrosion of Materials Course

enamels 3 main steps16
Enamels - 3 main steps

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)

Corrosion of Materials Course

enamels17
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

Corrosion of Materials Course

corrosion of concrete
Corrosion of concrete
  • 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

Corrosion of Materials Course

corrosion of concrete19
Corrosion of concrete

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, C4AF

Reaction cement + water

  • hydrosilicates CSH phase (near to tobermorite) main connecting gel phase
  • other phases Ca(OH)2, open and closed pores

Corrosion of Materials Course

corrosion of concrete20
Corrosion of concrete

3 basic groups of concrete corrosion

  • dissolution and leaching
  • corrosion with chemical reaction – soluble products
  • corrosion with chemical reaction – insoluble products

Corrosion of Materials Course

corrosion of concrete21
Corrosion of concrete
  • 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

Corrosion of Materials Course

corrosion of concrete22
Corrosion of concrete
  • 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

Corrosion of Materials Course

corrosion of concrete23
Corrosion of concrete
  • 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

Corrosion of Materials Course