Hydroxides stress corrosion cracking

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. 

Leidheiser, H. and Kissinger, R., Chemical Analysis of the Liquid Within Propagatory Stress-corrosion Cracks in 70 30 Brass Immersed in Concentrated Ammonium Hydroxide , Corrosion, 28, 218 (1972)... 

The corrosion rate of nickel in sodium hydroxide is adversely affected by heat transfer by small amounts of oxidisable alkaline sulphur-containing salts, e.g. Na2SOj, NajS Oj, Na S and, at high temperatures, by alkaline oxidising agents, viz. NaClOj and NajOj. In the former circumstance Alloy 600 is more resistant than nickel, but not in the latter. When Alloy 600 is used for service in caustic alkalis, it should be stress relieved after fabrication to minimise the possibility of stress-corrosion cracking. 

Fig. 8.21 Current density dilTerences between fast and slow sweep rate polarisation curves and stress corrosion cracking suspectiblity as a function of potential for a C-Mn steel in nitrate, hydroxide and carbonate-bicarbonate solutions...

Low-carbon and chromium-nickel steels, certain copper, nickel and aluminium alloys (which are all widely used in marine and offshore engineering) are liable to exhibit stress-corrosion cracking whilst in service in specific environments, where combinations of perhaps relatively modest stress levels in material exposed to environments which are wet, damp or humid, and in the presence of certain gases or ions such as oxygen, chlorides, nitrates, hydroxides, chromates, nitrates, sulphides, sulphates, etc. 

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

Where a deposit contains an adequate concentration of sodium hydroxide and the affected area is stressed to a sufficiently high level, stress-corrosion cracking or caustic embrittlement (SCC) may occur. This type of caustic corrosion is different from caustic gouging, which does not require the presence of stress. 

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). 

Copper and its alloys are resistant to alkalies with the exception of ammonium hydroxide and cyanides. Ammonium ions promote stress-corrosion cracking of copper and its alloys. Ferric and stannic salts are aggressive towards copper alloys. Ammonia and cyanide ions form tetramine copper and tetracyano copper complexes in ammonia and cyanide solutions, respectively. 

Sodium hydroxide, even at low concentrations, will cause stress corrosion cracking in stainless steel when subjected to temperatures above 100°C. Caustic solutions are much better handled in alloys with high concentrations of nickel, such as Alloy 400 or Alloy 600. Alloy 400 has also found success in applications involving fluoride, hydrogen fluoride, and hydrofluoric c dP ... 

M.J. Humphries and R.N. Parkins, Stress-Corrosion Cracking of Mild Steels in Sodium Hydroxide Solutions Containing Various Additional Substances, Corros. Sci., Vol 7, 1967, p 747-761... 

R.N. Parkins and N.J.H. Holroyd, Stress Corrosion Cracking of 70/30 Brass in Acetate, Formate, Tartrate, and Hydroxide Solutions, Corrosion, Vol 38, 1988, p 245-255... 

CAUSTIC CRACKING - A form of stress-corrosion cracking most frequently encountered in carbon steels or iron-chromium-nickel alloys that are exposed to concentrated hydroxide solutions at temperature of 200 to 250°C. 

J.H. Zheng, W.F. Bogaerts, Effects of cold work on stress corrosion cracking of type 316L stainless steel in hot lithium hydroxide solution, Corrosion 49 (1993) 585—593. 

Figure 20.34 shows intergranular stress corrosion cracking of Thomas steel normalized in a nitrogen-purged 20 wt% sodium hydroxide solution at 110°C. 

Stress corrosion cracking in alkali-metal hydroxide solutions shows a number of variations. Whereas in nitrate solntions the grain bonndary breakthrough potential only means a restriction of... 

In addition to the known corrosive effects of alkali-metal hydroxide and nitrate solutions, intergranular stress corrosion cracking in unalloyed and low-alloy steels in contact with ammonium carbonate and crude methanol (methanol with low concentration of impurities) has also been observed (Matsukura and Sato 1977 Wendler-Kalsch 1983). When this group of materials comes into contact with various othCT aggressive substances, stress corrosion cracking occurs, primarily with transgranular characteristics. 

Alkali-metal hydroxide solutions can also cause stress corrosion cracking in austenitic stainless steels (Figure 20.38), albeit only at elevated temperature, often above the boiling point of the... 

FIGURE 20.39 Stress corrosion cracking regions of various metallic materials in sodium hydroxide. 

FIGURE 20.40 Stress corrosion cracking of austenitic stainless steels in sodium hydroxide solution, temperature, and concentration limits. SCC = stress corrosion cracking. (Speidel, M. O., Sedriks, A. J. Corrosion of Stainless Steels, p. 173. 1979. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.)... 

Anodic inhibition of stress corrosion cracking was first provided on an industrial scale in a large plant for the production of hydrogen by electrolysis of potassium hydroxide solution. After preliminary trials on a laboratory scale, the chemical industry s first anodically protected large-scale plant, a sodium hydroxide solution evaporator with a capacity of 142 t, was put into operation in 1968. Since then, plants have been equipped in the same way (Grafen 1971). 

Production of a corrosive product (aqueous sodium hydroxide). Caustic stress corrosion cracking risks arise at around 120°C on stainless steel (Phenix superheater and resuperheater material is an austenitic stainless steel). 

In a similar manner carbon steel is sensitive to pH. In the pH 4.5-9 range the corrosion rate is governed by dissolved oxygen. (See the following discussion on velocity effects.) Below pH 4.5 the corrosion rate is controlled by hydrogen evolution and above about pH 9, the rate is suppressed by an insoluble film of ferric hydroxide (Fig. 5) [IS]. At very high pH levels, especially at elevated temperatures, steel is susceptible to stress corrosion cracking [2i]. 

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