{"id":82635,"date":"2021-01-11T00:00:00","date_gmt":"2021-01-11T00:00:00","guid":{"rendered":"https:\/\/www.corrosionpedia.com\/2021\/01\/11\/caustic-cracking-of-austenitic-stainless-steel"},"modified":"2021-01-05T19:59:52","modified_gmt":"2023-12-09T19:06:05","slug":"caustic-cracking-of-austenitic-stainless-steel","status":"publish","type":"post","link":"https:\/\/www.corrosionpedia.com\/caustic-cracking-of-austenitic-stainless-steel\/2\/7231","title":{"rendered":"Caustic Cracking of Austenitic Stainless Steel"},"content":{"rendered":"

Caustic cracking<\/a> of austenitic stainless steels<\/a> 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<\/a> is one of the oldest forms of stress corrosion cracking<\/a> 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.<\/p>\n

The good corrosion resistance of austenitic stainless steels to caustic<\/a> 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<\/a>.)<\/p>\n

Caustic Conditions are Prevalent<\/h2>\n

Caustic or strongly alkaline<\/a> process streams are common in many industries. These conditions include concentrations of sodium hydroxide (NaOH) or caustic soda<\/a>, potassium hydroxide (KOH) or caustic potash<\/a> and calcium hydroxide (Ca(OH)2<\/sub>) or caustic lime<\/a>.<\/p>\n

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<\/a> and in bleaching the brown pulp from the pulping process. Typically, the pH is greater than about 10.5 for many of these processes.<\/p>\n

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<\/a> 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<\/a> 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.<\/p>\n

Requirements for Cracking<\/h2>\n

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?<\/a>) Above this temperature austenitic stainless steels tend to exhibit unstable passivity<\/a> that can cause severe general corrosion. Above about 93°C traditional stainless steels are also susceptible to caustic cracking<\/a>. 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<\/a> (chromium carbides formed in the grain boundaries<\/a> from extended elevated temperature exposure) tends to be most detrimental at all NaOH concentrations.<\/p>\n

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.<\/p>\n

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<\/a>.<\/p>\n

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<\/a> current. Cl–<\/sup> does not appear to aggravate caustic cracking and may play an inhibitive role.<\/p>\n