Caustic cracking boilers

Sulphate in general appears to behave very similarly Hatch and Rice have shown that small concentrations in distilled water increase corrosion more than similar concentrations of chloride". In practice, high-sulphate waters may attack concrete, and the performance of some inhibitors appears to be adversely affected by the presence of sulphate. Sulphates have also a special role in bacterial corrosion under anaerobic conditions. Both sulphates and nitrates are acceptable in low-pressure boiler feed water as they are believed to be of value in controlling caustic cracking. 

Symposium on Caustic Cracking in Steam Boilers, Society of Chemical Industry London, (1953)... 

Where MU is intended for power boiler FW, particularly strict limits are imposed on sodium leakage (breakthrough) and silica leakage to prevent FW contamination and subsequent downstream caustic gouging corrosion, caustic cracking corrosion, or silica deposition. 

The most outstanding mechanism which has found a wide acceptance is the film rupture mechanism. This mechanism has been extensively studied in stress corrosion cracking of alpha-brass in ammoniacal environment, although it was originally proposed for caustic cracking of boiler steel. Here are some salient features of the mechanism. The items are illustrated in Fig. 4.43. 

Austenitic nickel-chromium stainless steels and mild steel are subject to stress corrosion cracking in caustic soda (caustic cracking) at elevated temperatures. The phenomenon, caustic cracking is mostly encountered in boilers. Caustic is added as an additive to boiler water in order to preserve the thin film of magnetic iron oxide by raising the pH. Caustic addition creates problems only when it becomes concentrated. It can become concentrated by one of the mechanisms summarized below. Four instants in the life of a steam bubble are shown in Fig. 4.58. 

It is possible also for the caustic to concentrate at the waterline. Generally, the waterline area is always most sensitive to corrosion. Two types of failures are common in boilers and both are related to the effect of concentration of caustic. One which forms discontinuous microcracks and results in the bursting of tubes is called hydrogen damage, and the other which results in the formation of continuous microcracks leading to intergranular corrosion is called caustic cracking. Both are briefly discussed below ... 

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. 

State three major factors which cause the concentration of caustic. Explain briefly the mechanism of caustic cracking in boilers. 

CO2 is not removed by water treatment. Scales and deposits may also be formed by dissolved and suspended solids. Excessive alkalinity in boilers can lead to caustic cracking. High alkalinity is caused by high TDS (total dissolved solids) and alkaUnity. External treatment includes demineralization and reduction of alkalinity, corrosion inhibition and biological control. Morpholine inhibitor is added as inhibitor for treatment of condensate corrosion. 

In this method, no free caustic is maintained in the boiler water. Figure 11.14 shows resulting pH versus phosphate concentration, the resulting pH on addition of trisodium phosphate. Trisodium phosphate hydrolyzes to produce hydroxide ions. Phosphate acts as buffer to minimize caustic cracking. 

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 ... 

F ure 11.16 Caustic cracking of a boiler steel tube, showing the microstructure (500X)... 

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. 

For boiler plants over 900 psig (6.21 mpa, 63.07 bar absolute), the water chemistry is particularly carefully controlled, with no free caustic alkalinity permitted. This is partly to reduce the risks of localized caustic deposits forming, which may cause caustic gouging or caustic-induced, stress corrosion cracking to develop. 

This form of SCC affects both carbon steels and austenitic stainless steels (300 series) that are under stress. It is particularly associated with the inducement of boiler waterside metal-surface fractures (cracking) under localized deposits containing high concentrations of sodium hydroxide (caustic soda). 

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. 

The high nitrogen content of Bessemer steel makes it more sensitive than open-hearth steels to stress-corrosion cracking in hot caustic or nitrate solution. For this reason, open-hearth steel is usually specified for boilers. 

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. 

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