Pilling-Bedworth ratio
In chemistry and materials science, the Pilling-Bedworth ratio (PBV, or PBR from Pilling-Bedworth ratio ) is the ratio of the molar volumes of a metallic element and its oxide. The PBV can be used to estimate whether a metal is permanently resistant to dry air through the formation of a protective oxide layer ( passivation ). Since the oxide formation sometimes takes place in several stages and mixed oxides can be formed in alloys, the PBV does not allow any reliable statements. This applies in particular to atmospheric corrosion, which almost always occurs via hydroxides due to the ubiquitous water.
definition
M denotes the molar mass , ρ the density , V the (molar) volume and n the number of metal atoms in the empirical formula of the oxide.
effect
The following relationship can be shown on the basis of measurements:
- PBV <1: tearing of the oxide layer, no protective effect (example magnesium oxide on magnesium )
- PBV> 2: flaking of the oxide layer, no protective effect (example rust on iron )
- PBV = 1 ... 2: Formation of a permanent, passivating oxide layer (examples aluminum , titanium , chrome layer on steel)
Numerical values
| metal | Metal oxide | PBV |
|---|---|---|
| magnesium | Magnesium oxide | 0.81 |
| aluminum | Alumina | 1.28 |
| Zirconium | Zirconium (IV) oxide | 1.56 |
| nickel | Nickel (II) oxide | 1.65 |
| iron | Iron (II) oxide | 1.7 |
| titanium | Titanium (IV) oxide | 1.73 |
| chrome | Chromium (III) oxide | 2.07 |
| iron | Iron (II, III) oxide | 2.10 |
| iron | Iron (III) oxide | 2.14 |
| iron | Iron (III) oxide hydroxide ( rust ) | 3.6 |
| Silicon | Silicon dioxide | 2.15 |
| Tantalum | Tantalum (V) oxide | 2.47 |
| Vanadium | Vanadium (V) oxide | 3.25 |
Web links
- Script of the TU Dortmund declaration on the PBV on page 33 (PDF file; 5.44 MB)
- Dissertation. Contains a definition of the PBV. (PDF file)
- Dissertation with numerical values of the PBV , u. a. for Cr, Ni, Al (page 27). (PDF file; 4.53 MB)
literature
- Ralf Bürgel: Handbook high temperature material technology basics. Page 272ff. ISBN 3528231076
- NB Pilling, RE Bedworth: The Oxidation of Metals at high Temperatures. J. Inst. Met 29 (1923), pp. 529-591.
- ND Tomashov: Theory of Corrosion and Protection of Metals (The Science of Corrosion). Translated and edited by BH Tyrtell, I. Geld, HS Preiser, New York (1965).