Corrosion of Inorganic Non-Metallic Materials part 2 Corrosion of ceramics Enamels Corrosion of concrete Ale

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Corrosion of Inorganic Non-Metallic Materials
part 2 Corrosion of ceramicsEnamelsCorrosion
of concrete Aleš HelebrantDepartment of Glass
and Ceramics ICT Prague, Czech
Republicwww.usk.cz
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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

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Ceramics

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

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Ceramics

• Polycrystalline material
• Usually prepared by high-temperature processes
• typical HT reaction kaolinite mullite
• Al2O3.2SiO2.2H2O ?3 Al2O3.2SiO2 SiO2 H2O
• sintering process

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Ceramics - microstructure
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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
(3-5C.min-1)
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Refractories composition, properties

• silica r. (dinas) (refractoriness 1710-1750C)
• gt93 silica quartz, tridymite, cristobalite,
  glass
• fireclay r. (grog, chamotte) (1600-1750C)
• 15-46 Al2O3 - SiO2, mullite 3Al2O3.2SiO2
• corundum (1850-2000C)
• a-Al2O3
• magnesite r. (gt2000C)
• MgO periclase
• chrome-magnesite r. (1920-2000C)
• MgO.Cr2O3
• fusion-cast corundum/badelleyite r. (no open
  pores!)
• a-Al2O3, ZrO2, SiO2 glass phase

acid neutral basic
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Corrosion of refractories in melts
influence of density or surface tension gradient
t0
tgt0
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Testing of corrosion resistance
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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.Fe2O3 and MgO-FeO or oxidation of FeO.Cr2O3
  and FeO.Fe2O3

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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
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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

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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

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Enamels - 3 main steps

• 1) the manufacture of enamel frits
• glass melting (1000-1400C) 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) Na2OK2O (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

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Enamels - 3 main steps

• 2) treatment of the metal materials
• degreasing (trichlorethylene or alkaline
  solutions
• or anneling at approx 750C burning off organic
  substances
• removing iron oxides from the metal surface
• cold bath 5-20 HCl
• or 6-17 H2SO4 at 50-70C
• inhibitors needed (e.g. phenols) Fe dissolution
  without them faster then oxides dissolution

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Enamels - 3 main steps

• 3) preparation of enamelling slips and the
  enamelling itself
• frit is ground in ball mills (lt0.1 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 (780-900C)

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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 (550C)
  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

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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

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Corrosion of concrete

• Manufacture of cement
• raw materials CaCO3, clays, bauxit...
• firing in rotary cement kilns (1400C) ? clinker
  ? milling (particles lt0.1 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

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Corrosion of concrete

• 3 basic groups of concrete corrosion
• dissolution and leaching
• corrosion with chemical reaction soluble
  products
• corrosion with chemical reaction insoluble
  products

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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

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Corrosion of concrete

• corrosion with chemical reaction soluble
  products
• dissolution in acid solutions pHlt6 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

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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