Cracking caustic

Alkaline solutions above about pH 11 due to general dissolution and caustic cracking ... 

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. 

In addition to carbonates and/or phosphates, a variety of substances have also been used to act as sludge conditioners —e.g. starches, lignins, polyacrylates etc. Other substances, such as nitrites and sulphates are claimed to have some value in the suppression of caustic cracking (see Section 5.1). 

Apart from caustic cracking, the main forms of corrosion in low-pressure plant are oxygen pitting and scab formation. The residence of oxygen at sensitive locations on-load, possibly following off-load initiation, means that... 

For the particular problem of caustic cracking remedial measures include ... 

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. 

Caustic Cracking/Caustic Stress Corrosion Cracking/ Caustic Embrittlement... 

Caustic cracking (caustic embrittlement) Intergranular corrosion affects both carbon steels and austenitic steels and accelerated by high stress, higher temperatures, and impurities in grain boundaries. 

Carbonate/nitrite passivator Based on 1.0% Na Oj and 0.5% NaN02. The carbonate may be replaced by caustic, except where there is a risk of caustic cracking. 

The user of cathodic protection must also consider the material being protected with regard to caustic cracking a cathodic potential driven to the negative extreme of—0.95 V for microbiological protection purposes can cause caustic cracking of a steel structure. The benefits and risks of cathodic protection must be weighed for each material and each application. 

Nickel is resistant to chloride-induced SCC, but subject to caustic cracking in aerated solutions under high stress. Nickel is highly resistant to corrosion in natural fresh water and flowing seawater. Pitting occurs under stagnant or crevice conditions. 

The alloy Haynes 6B is resistant to corrosion in organic acids, but subject to pitting and crevice corrosion and SCC in chloride media. The corrosion rate of 0.3 mm/yr or 12mpy has been observed in 30 wt % of NaOH it is likely that caustic cracking will occur at high concentrations of NaOH and temperatures in the case of all the cobalt alloys. The nominal composition of high-temperature cobalt alloys is given in Table 4.54. 

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. 

CAUSTIC EMBRITTLEMENT - An obsolete term replaced by caustic cracking. 

The reported data on failures was collected from five companies and a total of more than 1272 failures. The failure mode of cracking includes SCC, fatigue cracking and caustic cracking. 

Ferritic stainless steels in general are less resistant than the austenitics and are subject to caustic cracking. Cupronickel tubes are occasionally used for low- to medium-concentration caustic at mild temperatures and with restrictions on fluid velocity. These alloys are also resistant to hot 70% NaOH, but only in the complete absence of oxidants. Monel has a number of applications, but its use can be restricted by the slow development of color due to copper contamination. It is a frequent choice for processing equipment in the salt-recovery section of the diaphragm-cell process (Sections 9.3.2.S and 9.3.2.6). 

FIGURE 14.29. Temperature and NaOH concentration limits fw caustic cracking of carbon steel. 

FIGURE 14.30. Regions of caustic cracking of metals and alloys. (Courtesy of M. Okubo.)... 

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