Boiler Corrosion

18.3.1 Boiler Corrosion

Steam boilers are constructed according to various designs, but they consist essentially of a low - carbon steel or low - alloy steel container for water that is heated by hot gases. The steam may afterward pass to a superheater of higher - alloy steel at higher temperature than the boiler itself. For maximum heat

324 TREATMENT OF WATER AND STEAM SYSTEMS

transfer, a series of boiler tubes is usually used, with the hot gases passing either around the outer surface or, less frequently, through the inner surface of the tubes. The steam, after doing work or completing some other kind of service, eventually reaches a condenser that is usually constructed of copper - base alloy tubes. Steam is cooled on one side of the tubes by water passing along the oppo- site side of a quality ranging from fresh to polluted, brackish, or seawater. The condensed steam then returns to the boiler and the cycle repeats.

The history and causes of corrosion in power - station boilers have been reviewed by Mann [11] ; case histories of corrosion and its prevention in industrial boilers have been presented by Frey [12] .

Some boilers are equipped with an embrittlement detector by means of which the chemical treatment of a water can be evaluated continuously in terms of its potential ability to induce stress - corrosion cracking (Fig. 18.3 ) [13] . A speci- men of plastically deformed boiler steel is stressed by setting a screw; adjustment of this screw regulates a slight leak of hot boiler water in the region where the specimen is subject to maximum tensile stress and where boiler water evaporates.

A boiler water is considered to have no embrittling tendency if specimens do not crack within successive 30 - , 60 - , and 90 - day tests. Observation of the detector is

Figure 18.3. Embrittlement detector which, when attached to an operating boiler, detects tendency of a boiler water to induce stress - corrosion cracking.

BOILER-WATER TREATMENT

a worthwhile safety measure because the tendency toward cracking is more pro- nounced in the plastically deformed test piece than in any portion of the welded boiler; hence, water treatment can be corrected, if necessary, before the boiler is damaged.

Corrosion of boiler and superheater tubes is sometimes a problem on the hot combustion gas side, especially if vanadium - containing oils are used as fuel. This matter is discussed in Sections 11.8 and 11.9 . On the steam side, since modern boiler practice ensures removal of dissolved oxygen from the feedwater,

a reaction occurs between H 2 O and Fe, resulting in a protective fi lm of magnetite (Fe 3 O 4 ) as follows:

3 Fe + 4 HO 2 → Fe O 3 4 + 4 H 2 (18.6) The mechanism of this reaction, so far as it is understood, indicates that Fe 3 O 4

is formed only below about 570 ° C (1060 ° F) and that above this temperature, FeO forms instead. The latter then decomposes on cooling to a mixture of mag- netite and iron in accord with

4 FeO → Fe O 3 4 + Fe (18.7) Measurements of hydrogen accumulation in boilers as a function of time, as well

as laboratory corrosion rate determinations, indicate that growth of the oxide obeys the parabolic equation [14] . Hence, the rate is diffusion - controlled, in line with the mechanism depending on ion and electron migration through solid reac- tion products as described in Chapter 11 .

At lower temperatures (e.g., room temperature to about 100 ° C) and proba- bly at higher temperatures before a relatively thick surface fi lm develops, experi- ments show that Fe(OH) 2 is the initial reaction product and not Fe 3 O 4 [15] . The mechanism of corrosion in this temperature range follows that described for anode and cathode interaction on the plane of the metal surface in contact with an electrolyte. Ferrous hydroxide eventually decomposes, at a rate depending on temperature, into magnetite and hydrogen in accord with a reaction fi rst described by Schikorr (Schikorr reaction) [16]

3 Fe OH ( ) 2 → Fe O 3 4 + H 2 + 2 HO 2 (18.8) Any factors that disturb the protective magnetite layer on steel, either chem-

ically or mechanically, induce a higher rate of reaction, usually in a localized region, causing pitting or sometimes grooving of the boiler tubes. In this regard,

the specifi c damaging chemical factor of excess OH − concentration is discussed later; mechanical damage, on the other hand, may take place each time the boiler is cooled down. Differential contraction of the oxide and metal causes some degree of spalling of the oxide, thereby exposing fresh metal. Accordingly, it is observed that rate of hydrogen evolution is momentarily high after a boiler is started up again, with hydrogen production then falling to normal values

326 TREATMENT OF WATER AND STEAM SYSTEMS

presumably after a reasonable thickness of oxide has again built up at damaged areas.

Conditions of boiler operation that lead to metal oxide or inorganic deposits (from the boiler itself or from condenser leakage) on the water side of boiler tubes cause local overheating accompanied by additional precipitation of solutes from the water. Pitting usually results, or so - called plug - type oxidation occurs, which accentuates local temperature rise, leading eventually to stress rupture of

the tube. Furthermore, hydrogen resulting from the H 2 O – Fe corrosion reaction may enter the steel causing decarburization, followed by microfi ssuring along grain boundaries and eventual blowout of the affected tube. The latter type of failure may take place without any major loss of tube wall thickness. In the absence of deposits within boiler tubes, these types of damage are not observed