Stress corrosion cracking boilers

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

Boiler salts can contain chloride ions. When carried over into the steam (e.g. during priming) this can result in chloride stress corrosion cracking of austenitic stainless steel expansion bellows. In steam systems where freedom from chloride cannot be guaranteed, bellows... 

The oxygen concentration of the solution, as in many instances of corrosion, can also be critical in stress-corrosion cracking tests. Instances are available in the literature that show very markedly different test results according to the oxygen concentration in systems as widely different as austenitic steels immersed in chloride-containing phosphate-treated boiler water and aluminium alloys immersed in 3% NaCl. 

Equally, boiler surface failure may result solely from poor operational practices or other indirect problems, although more usually it is due to a combination of waterside and operational problems such as corrosion fatigue and other stress corrosion cracking (SCC) mechanisms. 

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. 

A form of boiler waterside, caustic stress-corrosion cracking corrosion affecting carbon steels and austenitic stainless steels (300 series). Particularly associated with high localized concentrations of deposited sodium hydroxide (caustic soda). 

The use of hydroxyacetic/formic acid in the chemical cleaning of utility boilers is common. It is used in boilers containing austenitic steels because its low chloride content prevents possible chloride stress corrosion cracking of the austenitic-type alloys. It has also found extensive use in the cleaning operations for once-through supercritical boilers. Hydroxyacetic/formic acid has chelation properties and a high iron pick-up capability thus it is used on high iron content systems. It is not effective on hardness scales. 

Season cracking of high zinc brasses is a severe form of embrittlement resulting in cracking or disintegration. Somewhat similar forms of stress-corrosion cracking occur in many other metals and alloys. Embrittlement of boiler plate, discussed below, may be considered a special case. 

R.G.I. Leferink and W.M.M. Huijbregts, Nitrate Stress Corrosion Cracking in Waste-Heat Recovery Boilers, Anti-Corrosion Methods and Materials, 49(2), 118-126 (2002). 

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 ... 

The empiricism that appears to follow the corrosion engineer arises from the multi-factor, multi-component situations which are present within all common articles of commerce, be they made from iron, aluminum, or more exotic material combinations. The variables are continually interacting in increasingly complex ways so we can become engrossed for even a lifetime with just one problem in performance such as airfoil stress corrosion cracking, boiler feed water stabilizations, or corrosion inhibiting paints, and many more. But does this mean the work lacks for glamour and excitement ... 

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. 

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 specimen 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.

Addition of Inhibitors. It is possible to add inhibitors for controlling two kinds of corrosion in boiler systems, namely, stress-corrosion cracking and return-line corrosion. The first can be minimized by addition of phosphates, as mentioned previously. 

Figure 19.8. Relation between chloride and oxygen content of boiler water on stress-corrosion cracking of austenitic 18-8-type stainless steels exposed to steam phase with intermittent wetting pH 10.6, 50ppm PO ", 242-260°C (467-500°F), 1-30 days exposure [W. Williams and J. Eckel, J. Am. Soc. Nav. Eng. 68, 93 (1956)].

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. 

Opinions vary concerning the observed pH value (set at room temperature) in boiler water. There is consensus that the pH value should never lie below 7 since then the magnetite scale will be attacked. In practice and research a pH value of 9.6 is seen as optimum. However, from other sources a pH value of 10.5 is considered as a prerequisite that a protective, adherent magnetite scale exists and stress corrosion cracking does not occur [55]. 

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. 

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