Caustic Stress Corrosion Cracking
Definition - What does Caustic Cracking mean?
Caustic cracking is 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 temperatures of 482°F (250°C).
Caustic cracking is 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 temperatures of 482°F (250°C). This phenomenon mostly occurs in boilers where caustic soda (NaOH). Stress corrosion cracking (SCC) of nickel-based alloys. Environments that may cause stress corrosion cracking in nickel alloys include hot caustic solutions, high temperature water and hot and wet hydrofluoric acid. This chapter addresses the cracking behavior of nickel alloys in industries such as chemical process, nuclear power generation and in oil and gas exploration and production. Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material.
This phenomenon mostly occurs in boilers where caustic soda (NaOH) has been added in small amounts to boiler water to prevent scaling. However, caustics (alkalis) may become concentrated amounts in crevices around rivet heads and at hot spots. When combined with the considerable fabrication stresses around rivet holes, this can cracking of the steel boiler shells and tube plates.
Caustic cracking is also known as caustic embrittlement.
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Improved resistance to caustic stress corrosion cracking can be given to Inconel by heat treating it at 620 o C to 705 o C, depending upon prior solution treating temperature. Other problems that have been observed with Inconel include wastage, tube denting, pitting, and intergranular attack. The useful safe limit of carbon steel is approximately 150 o F/65 o C, both with regard to caustic stress corrosion cracking (CSCC) and corrosion. Stainless steels are more resistant to general corrosion compared with carbon steel; however, they can suffer CSCC at approximately 250 o F/121 o C.
Corrosionpedia explains Caustic Cracking
The following conditions are necessary for this type of cracking to occur:
- The metal must be stressed
- The boiler water must contain caustics
- At least a trace of silica must be present
- Some mechanisms, such as a slight leak, must be present to allow the boiler water to concentrate on the stressed metal
Caustic Embrittlement In Carbon Steel
Caustic cracking can be prevented through:
- Control of stress level (residual or load) and hardness
- Avoiding alkalis
- Use of materials known not to crack in the specified environment
- Controlling temperature and/or potential