Embrittlement caustic

This results in the prevention of caustic buildup beneath deposits or within a crevice where leakage is occurring. Caustic corrosion (and caustic embrittlement, discussed later) does not occur, because high caustic concentrations do not develop. 

Virtuallv evety alloy system has its specific environment conditions which will prodiice stress-corrosion cracking, and the time of exposure required to produce failure will vary from minutes to years. Typical examples include cracking of cold-formed brass in ammonia environments, cracking of austenitic stainless steels in the presence of chlorides, cracking of Monel in hydrofluosihcic acid, and caustic embrittlement cracking of steel in caustic solutions. 

In handhng caustic, as-welded steel can be used without developing caustic-embrittlement cracking if the temperature is below 50°C (I20°F). If the temperature is nigher and particularly if the concentration is above about 30 percent, cracking at and adjacent to non-stress-relieved welds frequently occurs. 

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. 

Caustic Embrittlement—a form of stress corrosion cracking that occurs in steel exposed to alkaline solutions. 

Caustic Embrittlement stress-corrosion cracking of carbon steels caused by the presence of caustic alkali. 

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. 

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

Caustic embrittlement corrosion is intergranular, and cracks appear along the grain boundaries. The process is accelerated by ... 

In addition to stress, the development of caustic embrittlement requires a caustic corrosive in a concentration of at least 7.5 to 10% NaOH. It also requires some silica to be present (typically = 0.2%). Thus, the prevention of this form of corrosion primarily requires the elimination of either stress or the concentration of corrosive. 

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. 

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. 

Yet another problem associated with ammonia is stress corrosion cracking (SCC or caustic embrittlement) of brasses (such as brass valves and other stressed components). Stress corrosion cracking of brass may develop in systems where ammonia steadily becomes available from a suitable source (such as the breakdown of sodium nitrate when it is added to inhibit SCC of steel) because it can concentrate in the steam. 

Antifoams to reduce the risk of carryover employed emulsions of castor oil and similar substances. Caustic embrittlement was avoided by the use of quebracho tannins, chile niter (sodium nitrate), and salt cake (sodium sulfate), which was later discredited as an inhibitor. 

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 corrosion (caustic induced, stress corrosion cracking), which occurs as an intergranular form of corrosion where localized stresses and strains are present (and some silicate, which acts as a general corrosion inhibitor that protects grains at the expense of the grain boundaries). 

The presence of free caustic does not automatically lead to caustic gouging corrosion, caustic embrittlement, or other related problems. 

Caustic embrittlement (caustic stress corrosion cracking) ... 

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. 

Other factors are temperature, ionic strength, rate of flow of corrosive fluid, etc. At pH >11 corrosion rate decreases due to the formation of a protective film of ferric hydroxide/oxide. Although general corrosion rate decreases above this pH, the metal becomes susceptible to intercrystalline attack (at defects in oxide film) and thus fails due to caustic embrittlement. 

Branched cracks, transgranular except for caustic embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). 

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