Remarks:
The establishment of protective metal oxide lay-ers through the use of reducing agents (such as hydrazine, hydroquinone, and other oxygen scavengers) is known as metal passivation or metal conditioning. Although "metal passivation" refers to the direct reaction of the compound with the metal oxide and "metal conditioning" more broadly refers to the promotion of a protective surface, the two terms are frequently used interchangeably.
The reaction of hydrazine and hydroquinone, which leads to the passivation of iron-based metals, proceeds according to the following reactions:
N2H4 |
+ |
6Fe2O3 |
® |
4Fe3O4 |
+ |
2H2O |
+ |
N2 |
hydrazine |
|
hematite |
|
magnetite |
|
water |
|
nitrogen |
C6H4(OH)2 |
+ |
3Fe2O3 |
® |
2Fe3O4 |
+ |
C6H4O2 |
+ |
H2O |
hydroquinone |
|
hematite |
|
magnetite |
|
benzoquinone |
|
water |
Similar reactions occur with copper-based metals:
N2H4 |
+ |
4CuO |
® |
2Cu2O |
+ |
2H2O |
+ |
N2 |
hydrazine |
|
cupric oxide |
|
cuprous oxide |
|
water |
|
nitrogen |
C6H6O2 |
+ |
2CuO |
® |
Cu2O |
+ |
C6H4O2 |
+ |
H2O |
hydroquinone |
|
cupric oxide |
|
cuprous oxide |
|
benzoquinone |
|
water |
Magnetite and cuprous oxide form protective films on the metal surface. Because these oxides are formed under reducing conditions, removal of the dissolved oxygen from boiler feedwater and condensate promotes their formation. The effective application of oxygen scavengers indirectly leads to passivated metal surfaces and less metal oxide transport to the boiler whether or not the scavenger reacts directly with the metal surface.
A significant reduction in feedwater oxygen and metal oxides can occur with proper application of oxygen scavengers.