“Pilling–Bedworth ratio”的意思、由来-开放百科全书

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Pilling–Bedworth ratio

释义

Definition
History
Application
Values
References

The Pilling–Bedworth ratio (P–B ratio), in corrosion of metals, is the ratio of the volume of the elementary cell of a metal oxide to the volume of the elementary cell of the corresponding metal (from which the oxide is created).

On the basis of the P–B ratio, it can be judged if the metal is likely to passivate in dry air by creation of a protective oxide layer.

Definition

The P–B ratio is defined as:

where:

– Pilling–Bedworth ratio
– atomic or molecular mass
– number of atoms of metal per molecule of the oxide
– density
– molar volume

History

N.B. Pilling and R.E. Bedworth^[1] suggested in 1923 that metals can be classed into two categories: those that form protective oxides, and those that cannot. They ascribed the protectiveness of the oxide to the volume the oxide takes in comparison to the volume of the metal used to produce this oxide in a corrosion process in dry air. The oxide layer would be unprotective if the ratio is less than unity because the film that forms on the metal surface is porous and/or cracked. Conversely, the metals with the ratio higher than 1 tend to be protective because they form an effective barrier that prevents the gas from further oxidizing the metal.^[2]

Application

On the basis of measurements, the following connection can be shown:

R_PB < 1: the oxide coating layer is too thin, likely broken and provides no protective effect (for example magnesium)
R_PB > 2: the oxide coating chips off and provides no protective effect (example iron)
1 < R_PB < 2: the oxide coating is passivating and provides a protecting effect against further surface oxidation (examples aluminium, titanium, chromium-containing steels).

However, the exceptions to the above P-B ratio rules are numerous. Many of the exceptions can be attributed to the mechanism of the oxide growth: the underlying assumption in the P-B ratio is that oxygen needs to diffuse through the oxide layer to the metal surface; in reality, it is often the metal ion that diffuses to the air-oxide interface. {{citation needed|date=February 2015}}

Values

Metal	Metal oxide	Formula	R_PB
Potassium	Potassium oxide	2}}{{Oxygen}}	0.474^[4]
Sodium	Sodium oxide	2}}{{Oxygen}}	0.541^[4]
Lithium	Lithium oxide	2}}{{Oxygen}}	0.567^[4]
Strontium	Strontium oxide	{{Strontium}}{{Oxygen}}	0.611^[4]
Calcium	Calcium oxide	{{Calcium}}{{Oxygen}}	0.64 ^[2]
Barium	Barium oxide	{{Barium}}{{Oxygen}}	0.67^[4]
Magnesium	Magnesium oxide	{{Magnesium}}{{Oxygen}}	0.81
Aluminium	Aluminium oxide	2}}{{Oxygen\|3}}	1.28
Lead	Lead(II) oxide	{{Plumbum}}{{Oxygen}}	1.28 ^[2]
Platinum	Platinum(II) oxide	{{Platinum}}{{Oxygen}}	1.56 ^[2]
Zirconium	Zirconium(IV) oxide	2}}	1.56
Zinc	Zinc oxide	{{Zinc}}{{Oxygen}}	1.58
Hafnium	Hafnium(IV) oxide	2}}	1.62 ^[2]
Nickel	Nickel(II) oxide	{{Nickel}}{{Oxygen}}	1.65
Iron	Iron(II) oxide	{{Iron}}{{Oxygen}}	1.7
Titanium	Titanium(IV) oxide	2}}	1.73
Iron	Iron(II,III) oxide	3}}{{Oxygen\|4}}	1.90
Chromium	Chromium(III) oxide	2}}{{Oxygen\|3}}	2.07
Iron	Iron(III) oxide	2}}{{Oxygen\|3}}	2.14
Silicon	Silicon dioxide	2}}	2.15
Tantalum	Tantalum(V) oxide	2}}{{Oxygen\|5}}	2.47 ^[2]
Niobium	Niobium pentoxide	2}}{{Oxygen\|5}}	2.69^[4]
Vanadium	Vanadium(V) oxide	2}}{{Oxygen\|5}}	3.25 ^[2]

References

1. ^N.B. Pilling, R. E. Bedworth, "The Oxidation of Metals at High Temperatures". J. Inst. Met 29 (1923), p. 529-591.
2. ^¹²³⁴⁵⁶"ASM Handbook Vol.13 Corrosion", ASM International, 1987
3. ^¹²³⁴⁵⁶{{Cite web |title=Pilling–Bedworth ratios (PBRs) for metals and their oxides |url=http://pubs.acs.org/subscribe/archive/ci/31/i11/html/11sheppard_tab1.ci.html}}

^[3]
}}{{DEFAULTSORT:Pilling-Bedworth ratio}}

1 : Corrosion prevention

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