Metal

Oxide

PR Ratio

Protectiveness

Type of Oxide

Aluminum

Al₂O₃

1.28

P

n

Calcium

CaO

0.64

NP

n

Cadmium

CdO

1.42

NP

n

Cobalt

Co₂O₃

2.40

P

p

Copper

Cu₂O

1.67

P

p

Chromium

Cr₂O₃

2.02

P

p

Iron

FeO

1.78

P

p

Magnesium

MgO

0.81

P

n/p

Manganese

MnO₂

2.37

P?

n

Molybdenum

MoO₃

3.27

NP

n

Nickel

NiO

1.70

P

p

Lead

PbO

1.28

NP

p

Silicon

SiO₂

2.15

P

n

Tantalum

Ta₂O₅

2.47

NP

n

Titanium

Ti₂O₃

1.76

NP

n/p

Uranium

UO₂

1.97

NP

p

Tungsten

WO₃

1.87?

NP

n

Zinc

ZnO

1.58

NP

n

Zirconium

ZrO₂

1.57

P

n