Contents

1. 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

2. 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

3. 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

4. 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

5. Ceramics - microstructure Corrosion of Materials Course

6. 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

7. 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

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

9. Testing of corrosion resistance Corrosion of Materials Course

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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