It is used in metal degreasing and cleaning processes in a wide range of industry applications. It is one of the most prevalent afflictions in the industry and one of many environmental cracking mechanisms that typically occur in carbon steel equipment, but it can also afflict low alloy and austenitic stainless steel. Stress corrosion cracking of stainless steels in high temperature. Caustic cracking is also known as caustic embrittlement. Caustic cracking can occur in carbon steel piping and equipment handling. It is one of the most prevalent afflictions in the industry and one of many environmental cracking mechanisms that typically occur in carbon steel equipment, but it can also afflict low alloy and austenitic stainless steel equipment. For example, caustic crack propagation rate of a steel was reported 5,6 to be 25 times greater in a 3.
However, caustics alkalis may become concentrated amounts in crevices around rivet heads and at hot spots. It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. The useful safe limit of carbon steel is approximately 150. It becomes increasingly more corrosive with increasing temperature and concentration. Stress corrosion cracking international institute of. Alkali caustic cracking accidents also occurred in pressure vessels of. Caustic cracking is a form of stress corrosion cracking most frequently encountered in carbon steels or ironchromiumnickel alloys that are exposed to concentrated hydroxide solutions at temperatures of 482f 250c.
Corrosion by caustic sodium or potassium hydroxide at all concentrations is easily handled at room temperature with a variety of metals and alloys, including carbon steels. At temperatures above ambient, corrosion rates of carbon steel become greater and is accompanied by a risk of cscc. Low concentrations of caustic can be safely handled by carbon steel up to 180 o f82 o c, where cscc starts to become a risk factor, while the safe upper limit for a 50% solution is approximately 150 o f65 o c, although cracking has occurred at temperatures as low as 120 o f48 o c. Both 304 and 316 stainless steel types are resistant to a wide range of concentration and temperature. Industrial experience on the caustic cracking of stainless steels. When combined with the considerable fabrication stresses around rivet holes, this can cracking of the steel boiler shells and tube plates.
Stainless steels are more resistant to general corrosion compared with carbon steel. Caustic cracking can occur in carbon steel piping and equipment handling caustic and also in downstream equipment, if caustic carryover occurs. Corrosion behaviour of type 316l stainless steel in hot caustic. Below 80 degc they can be considered resistant to any concentration of sodium hydroxide, up to the limit of solubility. Stainless steels types 304 and 316 can be considered resistant below 80 degc, up to. Caustic stress corrosion cracking occurs at higher temperatures than. Stress corrosion cracking of an austenitic stainless steel in. Caustic cracking of pressure vessels mainly occurs in steam and other places prone to repeated evaporation and concentration or in hightemperature vessels in contact with caustic soda. Three cases of caustic cracking are illustrated in. The scc susceptibility is not reduced by increasing. Selection of stainless steels for handling sodium hydroxide.
Austenitic stainless steels such as type 304 and 316 seem less resistant to caustic conditions than even plain carbon steel. Sodium and potassium hydroxides both hereafter referred to as caustic are commonly handled and stored at ambient temperatures in all concentrations with carbon steel equipment. The useful safe limit of carbon steel is approximately 150 o f65 o c. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. Three basic mechanisms of stress corrosion cracking have been identified as described below. Caustic stress corrosion cracking, also referred to as caustic embrittlement, is a form of degradation that is caused when a component operates in a caustic environment. Caustic cracking is a form of stress corrosion cracking most frequently encountered in carbon steels or ironchromiumnickel alloys that are.
The intersection of the naohho boiling point curve with regions of caustic scc. How to avoid caustic cracking of carbon steel and low. The corrosion of stainless steel in hot caustic environment has been the. There can be a risk of stress corrosion cracking scc attack at higher temperatures, which is common to both the 304 and 316 types. Susceptibility to caustic scc is dependent on several. Caustic cracking of carbon steel occurs over a wide range of caustic. It may occur in equipment of carbon steel, low alloy steel, ferritic steel and austenitic stainless steel in contact with caustic soda.
The role of hydrogen embrittlement in caustic scc of. Industrial experience on the caustic cracking of stainless. Stainless steels types 304 and 316 can be considered resistant below 80 degc, up to the limit of solubility. As a general rule, the resistance to caustic solutions increases with increasing nickel content. Caustic stress corrosion cracking inspectioneering.
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