DEAERATION AND DEACTIVATION

18.1 DEAERATION AND DEACTIVATION

In accord with principles described in Sections 7.1 and 7.2 , corrosion of iron is negligible at ordinary temperatures in water that is free of dissolved oxygen. An effective practical means, consequently, for reducing corrosion of iron or steel in contact with fresh water or seawater is to reduce the dissolved oxygen content. In this way, corrosion of copper, brass, zinc, and lead is also minimized.

Removal of dissolved oxygen from water is accomplished either by chemi- cally reacting oxygen beforehand, called deactivation , or by distilling it off in suitable equipment, called deaeration . Deactivation can be carried out in practice by slowly fl owing hot water over a large surface of steel laths or sheet contained in a closed tank called a deactivator. The water remains in contact long enough to corrode the steel, and, by this process, most of the dissolved oxygen is con- sumed. Subsequent fi ltration removes suspended rust. Water so treated is much less corrosive to a metal pipe distribution system. Deactivators of this kind have been employed in some buildings, but, since use of deactivation requires regular attention in addition to periodic renewal of the steel sheet, this approach to

Corrosion and Corrosion Control , by R. Winston Revie and Herbert H. Uhlig Copyright © 2008 John Wiley & Sons, Inc.

318 TREATMENT OF WATER AND STEAM SYSTEMS

oxygen removal is usually too cumbersome compared with the use of chemical inhibitors or deaeration.

Deactivation of industrial waters (not potable waters, because the chemicals used are toxic) is possible by using oxygen scavengers (strong reducing agents), such as sodium sulfi te (Na 2 SO 3 ) and hydrazine (N 2 H 2 ), which function as corro- sion inhibitors by removing the oxygen. Since hydrazine has been identifi ed as a possible carcinogen, there have been considerable efforts to replace it.

Sodium sulfi te scavenges oxygen in accord with the reaction

Na SO 2 2 + O 2 → Na SO 2 4 (18.1)

for which Na 2 SO 3 reacts with oxygen in the weight ratio of 8 : 1 (8 kg Na 2 SO 3 to

1 kg O 2 ). The reaction is relatively fast at elevated temperatures, but slow at ordinary temperatures. It can be accelerated by adding catalysts, such as Cu 2+ or Co 2+ , salts [1, 2] .

The rapid decrease with time of dissolved oxygen in the San Joaquim, Cali- fornia, river water after treatment with 80 ppm Na 2 SO 3 (0.67 lb/1000 gal) plus copper or cobalt salts is shown in Fig. 18.1 . Water so treated using CoCl 2 as cata- lyst was found by Pye [1] to be noncorrosive to a steel heat - exchanger system that, without treatment, had previously suffered serious corrosion and loss of heat transfer. Tests showed a reduction in corrosion rate from 0.2 mm/y (0.008 ipy, pitting factor = 7.4) before treatment to 0.004 mm/y (0.00016 ipy) afterward.

Hydrazine (N 2 H 4 ), supplied as a concentrated aqueous solution, also reacts with dissolved oxygen, according to

Figure 18.1. Effect of cobalt and copper salts on reaction rate of Na 2 SO 3 with dissolved oxygen at room temperature [1] . [ Reprinted from Journal AWWA 39 (11) (November 1947), by permission. Copyright 1947, American Water Works Association .]

DEAER ATION AND DEAC TIVATION

NH 2 4 + O 2 → N 2 + 2 HO 2 (18.2) in the weight ratio of 1 : 1. This reaction is slow at ordinary temperatures, but can

be accelerated by using catalysts (e.g., activated charcoal, metal oxides, alkaline solutions [3] of Cu 2+ and Mn 2+ ) and by raising the temperature. At elevated temperatures, decomposition occurs slowly at 175 ° C (345 ° F) and more rapidly at 300 ° C (570 ° F), producing ammonia [4] :

3 NH 2 4 → N 2 + 4 NH 3 (18.3) The normal reaction products — nitrogen, water, and a small amount of NH 3 — are

all volatile, and, unlike sulfi te addition, no dissolved solids accumulate in the treated water.

Two additional oxygen scavengers are carbohydrazide, (NH 2 NH) 2 CO, and diethylhydroxylamine (DEHA), (C 2 H 5 ) 2 NOH; the reactions with oxygen are

( NH NH CO 2 ) 2 + 2 O 2 → 2 N 2 + 3 2 H O CO + 2 (18.4) and

4 ( CH 2 52 ) NOH + 9 O 2 → 8 CH COOH 3 + 2 N 2 + 6 HO 2 (18.5) Both these oxygen scavengers have the additional advantage that they form

protective fi lms on both iron and copper [5] . Deaeration is accomplished by spraying water or fl owing it over a large surface, countercurrent to steam. Oxygen distills off and also some dissolved carbon dioxide (Fig. 18.2 ). Water is heated in the process and is suitable, there- fore, as feedwater for boilers. Steam deaerators of this kind are standard equip- ment for all high - pressure stationary boilers. On the other hand, if the water is to be used cold, dissolved gases are distilled off by lowering the pressure, employ- ing a mechanical pump or steam ejector instead of a countercurrent fl ow of steam. This is called vacuum deaeration. Equipment of this kind has been designed to deaerate several million gallons of water per day.

In principle, it is more diffi cult and more expensive to remove the last traces of dissolved oxygen by distillation compared to the fi rst 90 – 95%, and it is more diffi cult at low temperatures than at high temperatures. To achieve a low enough oxygen level in cold water, it is often necessary to use multiple - stage vacuum treatment. Fortunately, acceptable levels of dissolved oxygen for corrosion control in cold water are higher than in hot water or in steam. Allowable levels established through experience are given in Table 18.1 [6] .

320 TREATMENT OF WATER AND STEAM SYSTEMS

Figure 18.2. Sketch of one type of steam deaerator.

T A B L E 18.1. Approximate Allowable Oxygen Concentration in Deaerated Water for Corrosion Control in Steel Systems

Maximum Oxygen Concentration

mL/liter Cold water

ppm

0.3 0.2 Hot water

0.1 0.07 Low - pressure boilers ( < 1.7 MPa, < 250 psi)

0.03 0.02 High - pressure boilers

HOT- AND COLD-WATER TREATMENT