Boilers caustic embrittlement

A boiler s water may have caustic embrittling characteristics. Only a test using a U.S. Bureau of Mines Embrittlement Detector will show whether this is the case. If the water is found to be embrittling, it is advisable to add sodium nitrate inhibitor lest a weak area of the boiler be attacked. 

Today, boilers are welded and stress-relieved, thus ostensibly eliminating a primary component of the SCC process however, modem boilers operate at higher heat fluxes, which imposes a strict requirement for cleaner metal surfaces. The presence of deposits on any waterside surface may provide an opportunity for the concentration of free alkali under the deposit, and so caustic embrittlement still occurs today, depending on inherent stress levels and the particular water chemistry involved. 

Boiler compound formulations containing aluminate, silicate, and carbonate ingredients have commonly utilized various ratios of some or all of these constituents, together with caustic soda (for alkalinity control), sodium nitrate or sulfate (to prevent caustic embrittlement), sodium lig-nosulfonate (sludge conditioner), and other compounds. 

Caustic embrittlement is the development of brittleness iit metals such as steel or ferrous alloys, upon prolonged exposure to alkaline substances, like caustic soda, in solution. Failures and explosions in boilers and evaporators have been caused by this action. Effective water treatment essentially has eliminated this condition in boilers. See also Corrosion Embrittlement. 

Caustic embrittlement is a particular version of stress corrosion cracking that sometimes occurs in boilers. Three contributory factors are considered to be involved in the problem [Kemmer 1988] and include ... 

Two of the earliest and classic examples of stress corrosion are the seasonal cracking of brass cartridge cases and the caustic embrittlement of riveted steel locomotive steam boilers. Ammunition becomes worthless during the wet seasons, and boilers explode because of cracks that started near the rivets or stressed areas. 

Stress-corrosion cracking of steel was first encountered in a practical way in riveted steam boilers. Stresses at rivets always exceed the elastic Unfit, and boiler waters are normally treated with alkalies to minimize corrosion. Crevices between rivets and boiler plate allow boiler water to concentrate, until the concentration of alkali suffices to induce S.C.C., sometimes accompanied by explosion of the boiler. Because alkalies were recognized as one of the causes, failures of this kind were first called caustic embrittlement. With the advent of welded boilers and with improved boiler-water treatment, S.C.C. of boilers has become less common. Its occurrence has not been eliminated entirely, however, because significant stresses, for example, may be established at welded sections of boilers or in tanks used for storing concentrated alkalies. 

Carbon steels are susceptible to stress corrosion cracking in alkaline environments at elevated temperature. At the start of the industrial era, many steel-riveted boilers burst due to SCC because the water treatment used permitted the establishment of alkaline conditions in crevices underneath the rivets. The phenomenon was referred to as caustic embrittlement. 

The previously feared caustic embrittlement of steel boilers has practically vanished after the introduction of high purity, i.e. deionised water [60]. Low carbon steels are sufficient for those sections of the steam boiler which come into contact with liquid water less than 330 °C. For the hot side of turbines a steel with 12% Cr is recommended. The remaining sections of the steam boiler system consist of carbon steel or low alloyed steel [22]. Steam boiler operation in the super heater section (500-600 °C) affects the diffusion of hydrogen in the steel, and will thereby be dec-arburised [14]. The methane that is formed caimot escape from the iron, stresses are formed and the formation of cracks and blisters occurs. A remedy was found by adding 0.5% Mo and 1% Cr, whose carbides at boiler temperature do not transform into methane [14, 61]. The limit of the use of these steels is portrayed in Figure 7. 

Caustic corrosion of unalloyed and low-alloy steel is encountered in some unusual situations. For example, in boilers traces of sodium hydroxide can become concentrated and cause local corrosion and caustic embrittlement. This occurs usually in boiler tubes that alternate between wet and dry conditions or in which deposits form. Boiler feed water permeates the deposits and evaporates. This causes concentration of the caustic material, to up to several percent, which is enough to destroy the protective magnetite and/or to initiate caustic embrittlement (Effertz et al., 1982 Hersleb, 1982). 

A final step in boiler feedwater treatment consists of pH adjustment as a further aid to corrosion control. Usually the pH is adjusted to a range of 10 to 11 with trisodium phosphate (or combinations of caustic with sufficient mono- or disodium phosphate to form trisodium phosphate upon inadvertent evaporation of the water). This "coordinated phosphate" treatment is intended to preclude the environmental cracking of steel by free sodium hydroxide (caustic embrittlement), a catastrophic form of corrosion described in Chap. 6. Caustic carryover with the steam can present severe corrosion problems (Fig. 8.16). 

In the presence of sufficient tensfle stress and traces of silicon, hot caustic solutions can induce see of boiler steels. This phenomenon is not called caustic embrittlement, as no loss of ductility occurs in caustic cracking. Tensile stress and caustic concentration cause the formation of continuous intergranular cracks in the metal. As the cracks progress, the strength of the metal is exceeded and fracture occurs. 

This denotes a type of stress corrosion cracking encountered in steel exposed to high concentration of hydroxide at a temperature of200-250°C. Caustic cracking of steel is sometimes called as caustic embrittlement (Fig. 11.16). It is a special type of stress corrosion cracking that occurs in boilers. The tube cracks under the influence of stress and high concentration of hydroxide which causes it to corrode. There are two conditions which must be satisfied ... 

The boiler metal must be under high stress such as the areas where the boiler tubes are rolled into the drum. The welded joints become highly susceptible to caustic embrittlement. The attack proceeds on the grain boundaries, hence, it is an inter-crystalline phenomena. Post-weld heat treatment or stress relieving heat treatment can preclude cracking problems. 

Traces of caustic can become concentrated in boiler feedwater and cause SCC (caustic embrittlement). This occurs in boiler tubes that alternate between wet and dry conditions (steam blanketing) because of overfiring. Locations such as cracked welds or leaky tube rolls can form steam pockets with cyclic overheating and quenching conditions. These freqnently lead to caustic embrittlement. 

Caustic embrittlement is actually only one type of stress corrosion cracking. It is the one most frequently found in boilers and for that reason merits special consideration. 

Another approach to the prevention of caustic cracking involves maintenance above a certain value of the ratio of sodium sulfate to alkalinity in the boiler water. If chemically treated water is used along with condensate as the feed water, then the ratio of d plus 804 " to NaOH should be no less than five. Excessive alkalinity may be reduced by neutralizing with H2SO4 and then using an ion-exchange resin to free the water of excess alkalinity by replacing Na" " ions with H ions. The most widely accepted chemicals for the prevention of caustic embrittlement are the nitrate ion and quebracho extract. The amount of nitrate used is critical and must be 35 to 40% of the total alkalinity, calculated as NaOH. 

Highly alkaline waters being usually unpalatable, upper limits with respect to phenolphthalein alkalinity and total alkalinity have been specified for municipal water supplies. Alkaline waters when u.sed in boilers for steam generation may lead to precipitation of sludges, deposition of scales and cau.se caustic embrittlement. A knowledge of the kinds of alkalinity present in water and their magnitudes is important... 

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