Corrosion of Materials Course Corrosion of Inorganic Non-Metallic Materials part 2 Corrosion of ceramics Enamels Corrosion of concrete Aleš Helebrant Department of Glass and Ceramics ICT Prague, Czech Republic

Corrosion of Materials Course 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 Porcelain, chinaware Pottery Bricks, roof tiles, tiles, sanitary ceramics Advanced (engineering) ceramics - Al 2 O 3, ZrO 2, UO 2 Refractories Non-oxide ceramics (nitrides, carbides)

Corrosion of Materials Course Ceramics Polycrystalline material Usually prepared by high-temperature processes typical HT reaction kaolinite – mullite Al 2 O 3.2SiO 2.2H 2 O  3 Al 2 O 3.2SiO 2 + SiO 2 + H 2 O sintering process

Corrosion of Materials Course Ceramics - microstructure

Corrosion of Materials Course 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 silica r. (dinas) (refractoriness °C) –>93% silica quartz, tridymite, cristobalite, glass fireclay r. (grog, chamotte) ( °C) –15-46% Al 2 O 3 - SiO 2, mullite 3Al 2 O 3.2SiO 2 corundum ( °C) –  -Al 2 O 3 magnesite r. (>2000°C) –MgO periclase chrome-magnesite r. ( °C) –MgO.Cr 2 O 3 fusion-cast corundum/badelleyite r. (no open pores!) –  -Al 2 O 3, ZrO 2, SiO 2 glass phase acid neutral basic

Corrosion of Materials Course refract. melt refract. melt t=0 t>0 Corrosion of refractories in melts influence of density or surface tension gradient

Corrosion of Materials Course Testing of corrosion resistance

Corrosion of Materials Course corrosion leading to mechanical degradation regenerators in gas heated glass melting furnaces – preheating of gas by products of combustion 3Al 2 O 3.2SiO 2 + Na 2 O  Na 2 O.Al 2 O 3.2SiO 2 + 2Al 2 O 3 nepheline 3(Na 2 O.Al 2 O 3.2SiO 2 ) + 2Na 2 O + 2SO 3  5Na 2 O.3Al 2 O 3.6SiO 2.2SO 3 noselite mullit  nepheline  noselite = volume expansion  mechanical stress similar effect - changing oxidation/reduction atmosphere MgO.Fe 2 O 3 and MgO-FeO or oxidation of FeO.Cr 2 O 3 and FeO.Fe 2 O 3 Corrosion of refractories in atmosphere

Corrosion of Materials Course active or pasive mechanism pasive – high pO 2, lower T Si 3 N 4 (s) + 3O 2  3SiO 2 (s) + 2N 2 controlled by diffusion active – low pO 2, higher T 2Si 3 N 4 (s) + 3O 2  6SiO(g) + 4N 2 controlled by chemical reaction SiO 2 Si 3 N 4 Corrosion of non - oxide ceramics

Corrosion of Materials Course 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 Fe 3 C) other applications –Al – building industry –Cu, Ag, Au, Pt - jewellery –Ti - biomaterials

Corrosion of Materials Course 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 1) the manufacture of enamel frits –glass melting ( °C) in gas fired furnaces –quenching in water  granulation –(rotating cylinders  glass ribbon crushed into scales) –low silica glasses (about 50-47%), B 2 O 3 (16-11), Al 2 O 3 (7) Na 2 O+K 2 O (20-15) fluorides (6-20) different composition for ground and cover layers –other components 0.5% CoO (ground), SnO 2 in mill (cover) –TiO 2 enamels – nucleation (anatase)  opacity, thinner layer

Corrosion of Materials Course 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% H 2 SO 4 at 50-70°C inhibitors needed (e.g. phenols) – Fe dissolution without them faster then oxides dissolution

Corrosion of Materials Course 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 NaNO 2 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 ( °C)

Corrosion of Materials Course Enamels Special enamels –acid-resitant for food or pharmaceutical industry (higher content of SiO 2 and ZrO 2, 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 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 concrete Manufacture of cement raw materials CaCO 3, clays, bauxit... firing in rotary cement kilns (1400°C)  clinker  milling (particles <0.1 mm) theoretical clinker composition: C 3 S, C 2 S, C 3 A, C 4 AF 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 concrete 3 basic groups of concrete corrosion 1.dissolution and leaching 2.corrosion with chemical reaction – soluble products 3.corrosion with chemical reaction – insoluble products

Corrosion of Materials Course Corrosion of concrete 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(HCO 3 ) 2 = 2CaCO 3 + H 2 O

Corrosion of Materials Course Corrosion of concrete 2.corrosion with chemical reaction – soluble products dissolution in acid solutions: pH<6 dissolution of all cement phases – CSH, Ca(OH) 2, CaCO 3 –treatment – decrease porosity, protective layers (water glass, pure cement mortar) corrosion by ammonium salts Ca(OH) 2 + 2NH 4 = 2NH 3 + CaCl 2 + 2H 2 O

Corrosion of Materials Course Corrosion of concrete 3.corrosion with chemical reaction – insoluble products sulphate corrosion – most dangerous product ettringite 3CaO.Al 2 O 3.3CaSO 4.32H 2 O 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 Ca 2+ for ettringite