Caustic cracking of austenitic stainless steels is an environmentally assisted failure mode that is often forgotten and not always considered when specifying a material of construction. This type of failure mode is one of the oldest forms of stress corrosion cracking for steels and dates back to the days of early steam locomotives when it was responsible for a large number of explosions of riveted boilers.
The good corrosion resistance of austenitic stainless steels to caustic resulted in many applications of these alloys in caustic service. However, in the 1950s it was found that these alloys also were susceptible to caustic cracking. (Get an introduction to austenitic stainless steels in the article 12 Things You Need to Know About Austenitic Stainless Steel.)
Caustic Conditions are Prevalent
Caustic or strongly alkaline process streams are common in many industries. These conditions include concentrations of sodium hydroxide (NaOH) or caustic soda, potassium hydroxide (KOH) or caustic potash and calcium hydroxide (Ca(OH)2) or caustic lime.
High temperature caustic conditions of greater than 50% alkalis are found in industries such as the production of alkalis and alumina, oil refineries, nuclear power plants, pulp and paper, manufacture of textiles and as a drain cleaner. NaOH is widely used in the pulping of wood for making paper and is a key component to separate lignin from cellulose fibers in the kraft process and in bleaching the brown pulp from the pulping process. Typically, the pH is greater than about 10.5 for many of these processes.
Selection of metallurgy for these applications is based on process factors such as alkali concentration, temperature, impurities in the caustic and necessity of product purity. For the specified process conditions, the corrosion rate and susceptibility to caustic cracking must be considered. Of course, all of these considerations must be weighed against the economics of the potential candidates. For example, caustic soda can be contained in cast iron or steel vessels as long as iron contamination is not detrimental to end use. When greater corrosion resistance is required, stainless steel or nickel alloys are often specified.
Requirements for Cracking
Austenitic stainless steels have fairly good corrosion resistance to NaOH up to about 50% concentration and temperatures of about 93°C (199°F). (Related reading: Why is Stainless Steel Corrosion Resistant?) Above this temperature austenitic stainless steels tend to exhibit unstable passivity that can cause severe general corrosion. Above about 93°C traditional stainless steels are also susceptible to caustic cracking. At NaOH concentrations below about 15% for unsensitized stainless steel alloys, the temperature to crack the metal is substantially higher. For example, at low NaOH concentrations of about 1% the tendency to crack will occur above 200°C (392°F). At temperatures above 300°C (572°F) the cracking can be quite rapid. Sensitization (chromium carbides formed in the grain boundaries from extended elevated temperature exposure) tends to be most detrimental at all NaOH concentrations.
The impact of the oxygen level in the caustic solution on cracking behavior is not well understood; but for concentrated deaerated solutions the Ni content of the alloy appears to be a critical factor. However, for Mo containing alloys there does not seem to be a relationship with Ni content at higher NaOH concentrations. Improved cracking resistance for aerated solutions of NaOH is related to higher Cr and Ni contents in the alloy.
Type of caustic on the temperature of cracking is not clear. For aqueous KOH higher temperature may be required for cracking even when boiling point elevation is used for the comparison; while another study found cracking at 100°C for various alloys. Experimental variations might be the source of the observed differences. On the other hand, aqueous LiOH solutions at 95°C were found to crack Type 316L.
The presence of sulfide in a NaOH environment will significantly decrease the temperature for cracking to as low as 50°C (122°F). It has been suggested that the greater susceptibility is from the increased anodic dissolution current. Cl- does not appear to aggravate caustic cracking and may play an inhibitive role.
Figure 1. Video about caustic stress corrosion cracking.
Identification of Caustic Cracking
Determination of the root cause of a failure is always a complex task but can be particularly the case with caustic cracking of austenitic stainless steel. The failure mode is by transgranular (through the grains) cleavage but can transition depending on conditions to an intergranular (along grain boundaries) crack propagation, particularly at higher temperatures. A metallographic cross-section usually reveals a highly branched transgranular crack(s).
Unfortunately, the crack morphology is identical to that of chloride stress corrosion cracking (Cl-SCC). (Chloride stress corrosion cracking is examined in Chloride Stress Corrosion Cracking of Austenitic Stainless Steel.) Thus, differentiating between the two failure modes is non-trivial. Both Cl and Na have high solubility in water; and therefore are often not detected by scanning electron microscopy / energy dispersive x-ray spectroscopy. The pH of the process stream can be useful for differentiating the two mechanisms since Cl-SCC does not occur at high pH. As with Cl-SCC, caustic cracking of a sensitized microstructure will also propagate in an intergranular mode.
The mechanism for caustic cracking is not fully understood but appears to be related to selective dissolution. In pits where cracking initiates, there is often found a nanoporous layer enriched in Fe and Cr. Mo may also be observed if present in the alloy.
Prevention of Caustic Cracking
When conditions for caustic cracking are suspected to be present or is identified as the root cause, a metallurgical solution is often selected. Duplex stainless steels, which have a two-phase microstructure consisting of an approximately 50/50 mix of ferritic and austenitic grains, are often specified. Although the general corrosion resistance may be somewhat lower than for the austenitic alloys, caustic cracking seems to be less of a problem for Alloys 2205, 2304 and 2906. Alloy 2205 has been successfully used up to 60% concentration of NaOH at 90°C (194°F); cracking was noted for a U-bend study for 50% concentration of NaOH at 140°C (284°F).
Higher temperature resistance to cracking appears to be a function of the ferrite content and stress loading. Based on U-bend tests of Alloy 2304 in white liquor at 170°C (338°F), no cracking was observed. Welding these alloys may increase the susceptibility of these alloys to caustic cracking because of the deviation of the balanced microstructure caused by heat inputs during welding and cooling rates of welds.
Alloys with higher Ni, such as Alloys 20Cb, AL-6XN, 904L, 800 and 825, are more resistant to caustic corrosion and cracking than the 300 series stainless steels.
In more severe environments, Ni and Ni-based alloys are often specified. Commercially pure Ni-alloys 200 and 201 are most resistant to caustic but Alloy 200 is susceptible to intergranular corrosion in concentrated caustic above 300°C (572°F). Alloy 600 can suffer caustic cracking in hot caustic at 150-200°C (302-392°F) in air. Under deaerated conditions cracking may occur at higher temperatures. Results tend to be quite varied depending on the test conditions.
Ni-Cu alloys such Alloys 400 and K500 are fairly resistant to caustic environments and approach the resistance of Ni-Alloy 200. However, under very high stress and elevated temperatures of about 215°C (419°F) these materials can suffer from caustic cracking.
Finally, the addition of phosphates to reduce the amount of caustic has been used in boiler water to reduce the potential of caustic cracking.