Basic Cracking of Austenitic Stainless Steels
Basic Cracking Introduction
Basic cracking of austenitic stainless steels is an environmentally assisted failure mode that is often overlooked when specifying materials of construction. This failure mode is one of the oldest forms of stress corrosion cracking in steel and dates back to the early days of steam locomotives when it resulted in the explosion of heavily riveted boilers.
Alkaline Conditions Prevalent
Alkaline or highly alkaline processes common in many industries involve concentrations of sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2). High temperature alkaline conditions (greater than 50% alkali) are common in industries producing alkalis, alumina, refineries, nuclear power plants, pulp and paper, textile manufacturing, and as drain cleaners.
Crack Formation Conditions
Austenitic stainless steels are very resistant to the corrosive properties of NaOH until concentrations of approximately 50% and temperatures of 93°C (199°F). Above this temperature, austenitic stainless steels exhibit unstable passivation that can result in severe general corrosion. Above approximately 93°C, conventional stainless steels are also susceptible to basic cracking.
Identification of Basic Cracking
Determining the root cause of failure is a complex task, especially for basic cracking in austenitic stainless steels. This failure mode is through-the-grain (through-the-grain) fracture, but depending on conditions can transition to intergranular (along-the-grain-boundary) crack propagation, especially at higher temperatures.
Prevention of Basic Cracking
When conditions for basic cracking are suspected or have been identified as the root cause, a metallurgical solution is often chosen. Duplex stainless steels are considered first, as they have a two-phase microstructure consisting of approximately a 50/50 mixture of ferrite and austenite grains. Although the general corrosion resistance of these alloys may be slightly lower than that of austenitic alloys, basic cracking appears to be less common in 2205, 2304, and 2906 alloys.
Alloys with higher nickel contents, such as 20Cb, AL-6XN, 904L, 800, and 825 alloys, are more resistant to alkaline corrosion and cracking than the 300 series stainless steels.
Nickel and nickel-based alloys are generally considered for use in more severe environments. Commercially pure nickel alloys 200 and 201 are the most resistant to alkalinity, but alloy 200 is susceptible to intergranular corrosion in concentrated alkalinity above 300°C (572°F). Alloy 600 may suffer from alkaline cracking in hot alkalinity at 150-200°C (302-392°F), and cracking may occur at higher temperatures under degassing conditions.
Nickel-copper alloys, such as alloys 400 and K500, are fairly resistant to corrosive environments, approaching that of nickel alloy 200. However, at very high stresses and temperatures of about 215°C (419°F), these materials may experience corrosion cracking.
Finally, reducing the amount of alkaline material by adding phosphates has been used in boiler water to reduce the potential for alkaline cracking.
