Stress corrosion cracking caustic

Caustic Cracking/Caustic Stress Corrosion Cracking/ Caustic Embrittlement... 

Usually, carbon steels are also resistant in alkaline environments such as NaOH and KOH solutions without contaminations, but may be subject to stress corrosion cracking (caustic embrittlement) at certain concentrations and temperatures (Section 7.12). 

Virtuallv evety alloy system has its specific environment conditions which will prodiice stress-corrosion cracking, and the time of exposure required to produce failure will vary from minutes to years. Typical examples include cracking of cold-formed brass in ammonia environments, cracking of austenitic stainless steels in the presence of chlorides, cracking of Monel in hydrofluosihcic acid, and caustic embrittlement cracking of steel in caustic solutions. 

Caustic Embrittlement—a form of stress corrosion cracking that occurs in steel exposed to alkaline solutions. 

Alloy 400 has good mechanical properties and is easy to fabricate in all wrought forms and castings. K-500 is a modified version of this alloy and can be thermally treated and is suitable for items requiring strength, as well as corrosion resistance. Alloy 400 has immunity to stress corrosion cracking and pitting in chlorides and caustic alkali solutions. 

Certain environments containing nitrate, cyanide, carbonate, amines, ammonia or strong caustic, due to the risk of stress corrosion cracking. Temperature is an important factor in assessment of each cracking environment ... 

In tests lasting for 14 days, Copson found that the susceptibility of steel to stress-corrosion cracking in hot caustic soda solutions increased with increase in nickel content up to at least 8-5%. Alloys containing 28% and more of nickel did not fail in this period. In boiling 42% magnesium chloride the 9% nickel-iron alloy was the most susceptible of those tested to cracking (Table 3.38). Alloys containing 28 and 42% nickel did not fail within 7 days. 

The corrosion rate of nickel in sodium hydroxide is adversely affected by heat transfer by small amounts of oxidisable alkaline sulphur-containing salts, e.g. Na2SOj, NajS Oj, Na S and, at high temperatures, by alkaline oxidising agents, viz. NaClOj and NajOj. In the former circumstance Alloy 600 is more resistant than nickel, but not in the latter. When Alloy 600 is used for service in caustic alkalis, it should be stress relieved after fabrication to minimise the possibility of stress-corrosion cracking. 

Austenitic stainless steels will exhibit stress-corrosion cracking in hot aqueous chloride solutions, in acid chloride containing solutions at room temperature, in hot caustic solutions and in high-temperature high-pressure oxygenated water. 

For carbon steels, however, a full stress-relief heat treatment (580-620°C) has proved effective against stress-corrosion cracking by nitrates, caustic solutions, anhydrous ammonia, cyanides and carbonate solutions containing arsenite. For nitrates, even a low-temperature anneal at 350°C is effective, while for carbonate solution containing arsenite the stress-relief conditions have to be closely controlled for it to be effective . 

Caustic Embrittlement stress-corrosion cracking of carbon steels caused by the presence of caustic alkali. 

For boiler plants over 900 psig (6.21 mpa, 63.07 bar absolute), the water chemistry is particularly carefully controlled, with no free caustic alkalinity permitted. This is partly to reduce the risks of localized caustic deposits forming, which may cause caustic gouging or caustic-induced, stress corrosion cracking to develop. 

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

Where a deposit contains an adequate concentration of sodium hydroxide and the affected area is stressed to a sufficiently high level, stress-corrosion cracking or caustic embrittlement (SCC) may occur. This type of caustic corrosion is different from caustic gouging, which does not require the presence of stress. 

Yet another problem associated with ammonia is stress corrosion cracking (SCC or caustic embrittlement) of brasses (such as brass valves and other stressed components). Stress corrosion cracking of brass may develop in systems where ammonia steadily becomes available from a suitable source (such as the breakdown of sodium nitrate when it is added to inhibit SCC of steel) because it can concentrate in the steam. 

Caustic embrittlement corrosion (caustic induced, stress corrosion cracking), which occurs as an intergranular form of corrosion where localized stresses and strains are present (and some silicate, which acts as a general corrosion inhibitor that protects grains at the expense of the grain boundaries). 

A form of boiler waterside, caustic stress-corrosion cracking corrosion affecting carbon steels and austenitic stainless steels (300 series). Particularly associated with high localized concentrations of deposited sodium hydroxide (caustic soda). 

Caustic embrittlement (caustic stress corrosion cracking) ... 

Stress-Corrosion Cracking. The conditions for SCC to occur are (i) a crack-promoting environment (ii) the susceptibility of material to SCC (iii) tensile stresses must exceed the threshold value. SCC is distinguished by the fact that the stress corrosion faces suffer very low corrosion, even in solutions that cause some damage to the free surfaces. As an example is the SCC of stainless steel at 200°C in a caustic solution or in aerated chloride solution where almost no traces of dissolution are visible on the crack faces (Figure 6.53).84,95 For example, SCC of metals has been by far the most prevalent... 

UNS S30409) stainless steel (SS) for metal temperatures above 1,200°F (650° C). Caustic stress corrosion cracking (SCC) from solids can occur in the steam preheat coils if solid carry-over is excessive (see Chapter One, Steam and Condensate section). The inlet connections to the steam methane reformer furnace tubes are either IViCr-V Mo (1,100°F [595°C] maximum) or 21/4Cr-1Mo (1,200°F [650°C] maximum). 

---