Caustic Environments


Caustic versus Carbon- & Low-alloy Steels

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General
"Caustic embrittlement" of steel boilers or the SCC of steels in NaOH was one of the first known forms of stress-corrosion cracking. It is still one of the most common.

Carbon steel is the material of choice for handling caustic solutions at low temperatures. However, there is a pronounced temperature threshold above which caustic can cause cracking.

The HSLA steels such as ASTM A 242 show approximately the same SCC resistance as the common carbon steels
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NACE caustic soda service chart

This information is from the NACE Corrosion Data Survey (5th edition)

The well known NACE caustic soda service chart (above), showing the effect of temperature and concentration on SCC susceptibility, was derived from a NACE survey of field experience in a wide variety of applications.

The temperature threshold curve is most clearly established in the middle concentration ranges, from about 10 to 50%, since the majority of applications surveyed were in these concentration ranges. At lower concentrations, particularly below 5%, the chart as published by NACE is probably conservative since there were relatively few data in the very low concentration ranges. Laboratory investigators have difficulty producing SCC on steels in pure caustic solutions with less than 4% NaOH, even at high temperatures. Field failures in weak caustic solutions can usually be traced either to concentration to higher caustic percentages (from evaporation or at liquid-vapor interfaces) or to the presence of impurities such as lead oxide or copper oxide which tend to accelerate caustic SCC.
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Electrochemical Potential
The SCC of steels in several solutions of economic importance (caustic, nitrates, carbonates and ammonia) apparently takes place by accelerated corrosion along active paths. These active paths operate in specific ranges of electrode potential, which vary with the solution (figure 2 below).
Some of the solutions have very wide ranges of cracking potentials, such as the nitrates and NH3. By contrast, caustic cracking occurs in two rather narrow (approximately 100 mV wide) potential bands, which would correspond approximately to the active nose and transpassive regions on the potentiostatic polarization curve.

However, significant uncertainty exists. Practically speaking, this means that relatively small changes in composition, temperature or concentration sometimes shift the free-corrosion potential of the system into or out of the cracking range.

Consequently, both cathodic protection and inhibitor additions (certain tannins, NaH2PO4, Na2SiO3 or Oxygen) can inhibit cracking.

Figure 2:
Schematic diagram showing potential ranges over which SCC of carbon steels occurs in various solutions.

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Heat Transfer
On heat transfer surfaces, such as boiler tubes, crevices or leaks are not necessary for the concentration of entrained caustic if the temperature difference between the boiling point of the water and the tubes exceeds certain levels. Note that the most damaging concentration (33% NaOH) may be produced with a DT of only 25°C.
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Steam Systems
In steam transfer lines, most problems occur at leaks and liquid-vapor interfaces, where low levels of caustic can concentrate by evaporation to form deposits very high in caustic. Valve-bonnet leaks, for example, often lead to SCC of the bonnet bolts.

Low-alloy steels such as AISI 4140 (ASTM A 193 Grade B7) show a parallel susceptibility to the low-strength carbon steels. However, low-alloy steels are susceptible over a much wider range of potential.

Consequently, a wider spectrum of impurities and operating conditions can result in cracking. On valve-bonnet bolts, this can be dangerous since a highly stressed bolt can become a projectile when its elastic strain energy is suddenly released by SCC.
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Prevention
On bolting which might be exposed to steam leaks, electroless nickel plating is widely used to prevent SCC. Nickel is practically immune to caustic SCC, and the electroless plating process, properly applied, can provide a high-quality barrier coating over the steel.


Plating is not often economical on carbon steel piping or vessels. In power plants, the standard method of preventing caustic SCC is by water chemistry control. Nitrates are an effective inhibitor for caustic SCC in the concentration ranges encountered in steam plants.

Sodium nitrate and sodium sulfate, which is sometimes used for the same purpose, inhibit caustic SCC by shifting the potential of the solution away from the cracking range. If operating conditions or other impurities result in a shift back into the cracking potential range, the mere presence of nitrates or sulfates in caustic will not inhibit SCC. As a result, NaNO3 and Na2SO4, although widely used, are referred to by some authorities as "unsafe inhibitors". True inhibitors of caustic SCC include valonea and quebracho tannins, NaH2PO4 and (to a lesser extent) Na2SiO3.
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Other Hydroxides
Limited data suggest that as SCC agents, calcium hydroxide and potassium hydroxide are not as potent as sodium hydroxide. However, since the dangerous temperature and concentration limits have not been thoroughly established for any solutions but sodium hydroxide, current industry practice is to use the NACE caustic soda service chart guidelines for all caustic solutions.
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(Bi-)Carbonates
Slightly alkaline sodium, potassium and ammonium carbonate-bicarbonate solutions have produced intergranular SCC of steels in buried pipelines and treating plants removing CO2 from natural gas.

Steels with less than 0.2% carbon are most affected by carbonate SCC. Therefore, most common carbon steels used in CPI plants would show only mild susceptibility to this sort of attack were it not for the decarburized layer often present on hot-rolled plate and pipe. Once cracks initiate in this decarburized layer, they can propagate more easily into the less sensitive base material. An exception to the trend noted above is that the Nb-bearing, low-carbon HSLA steels are comparable in SCC resistance to medium-carbon steels such as API 5LX-52.

Carbonate SCC is observed in the pH range 8-10.5. Cracking has occurred in concentrations as low as 0.25N. Several silicate and phosphate compounds are effective inhibitors for carbonate SCC.

Cracking can occur from the freezing point up to 90°C. However, most field failures take place above 60°C. Increasing temperature exponentially increases the crack growth rate and the probability of failure; the crack growth rate follows an Arrhenius law with increasing temperature such that every 10°C increase in temperature will approximately double the crack velocity.

Threshold stresses for carbonate SCC are fairly high, and field failures are extremely rare at design stresses below 60% of the specified minimum yield strength. Consequently stress-relief is an effective preventive measure.

The range of electrode potentials over which cracking occurs averages about 100 mV, but the width of the range varies with pH.
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Amines
Some corrosion engineers consider monoethanolamine (MEA) and diglycolamine (DGA) as stress-cracking agents on steels and refineries routinely stress-relieve equipment which handles them.

However, there is some confusion over the circumstances of cracking. No stress-cracking normally takes place in steel equipment used to manufacture and store a wide variety of ethylene and ethanol-based amines. It could be that the stress-cracking in MEA and DGA service in refineries is related not to the amine itself but rather to CO2, CO or CN- compounds in the gas, or to NaOH from regeneration systems. More research is needed on this point.
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Caustic versus Austenitic Stainless Steels

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Introduction
The 18-8 type stainless steels, exemplified by Types 304 and 316, have a useful low corrosion rate in caustic at up to 50% concentration to about 70°C. In 40-50% NaOH from 80 to 100°C, the 18-8 austenitic stainless steels have unstable passivity and may exhibit severe general corrosion if activated.

Note that Diagram 1 below (based on Copson's work; LaQue & Copson, Corrosion resistance of metals and alloys, Reinhold Publ. Corp., NY, 1963) is somewhat more optimistic, showing less than 1 mpy up to about 95°C. This is probably due to dissolved oxygen or traces of oxidizing species (cf. infra) because it is known that Type 304 can go active in 40% caustic at about 80°C and in 50% caustic at 70°C. In the active state, the 18-8 alloys corrode faster than C-steel!

Wrought austenitic stainless steels also suffer rapid SCC in caustic solutions above certain threshold temperatures related to solution composition. Below these temperature limits, however, the austenitics are serviceable in caustic and offer an alternative with better corrosion resistance than carbon steel and lower cost than nickel.

Three Diagrams provided below (Copson 1963; INCO Databooks 1973; and Swandby, Chemical Engineer, Vol. 69, Nov.1972) differ slightly in the boundaries at both very low or very high concentrations. Moreover, the INCO limits are significantly more conservative in the 50-75% concentration range.
Diagram 1
Isocorrosion chart for Type 304 and Type 316 stainless steels in NaOH, with SCC boundary superimposed (Copson 1963 / Schillmoller for NiDI, 1988).


Diagram 2
Isocorrosion chart for austenitic Cr-Ni stainless steels in NaOH (INCO Databook, 1973)


Diagram 3
Caustic soda service chart for austenitic stainless steels in NaOH (Swandby 1972)

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Temperature
After passing through the temperature range where activation can take place, passivity is apparently once more assured and SCC may occur. The bottom temperature range for SCC is still uncertain, but rapid cracking will occur in the 300-350°C range. Cracking of as-welded Type 304 (but not Type 304L) has been reported in 3-5% caustic at 50°C - presumably intergranular in nature. It is also reported that at 150°C, SCC occurred on U-bend specimens.

With austenitic stainless steels, the morphology of cracking is often transcrystalline, indistinguishable from that of chloride cracking. However, intergranular cracking is also observed. At high temperatures (e.g., 300°C) a characteristic "blueing" (similar to the color of gun metal) is observed on austenitic stainless steel cracked by caustic.
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Oxygen & oxidizers
Oxygen is reportedly not required for environmental cracking of stainless steels by caustic. Data from laboratory tests in 10-30% sodium hydroxide at 330°C showed that the presence of chromate ions prevented SCC of stainless steels.
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Anhydrous salt
Although some authors] state that anhydrous caustic does not cause SCC of stainless steel, very severe and rapid (i.e., in less than 24 hours) SCC has been observed in molten sodium at 330°C under a high-pressure hydrogen atmosphere. This was occasioned by the presence of parts per million of water in an organic reactant. Thus, the resulting environment was 100% anhydrous caustic dissolved in molten sodium under anaerobic conditions.
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Other alkali metal salts
Although sodium salts are more commonly encountered, other alkali metal compounds cause similar problems. Potassium and lithium hydroxides, carbonates and bicarbonates are capable of inducing environmental cracking in a manner directly analogous to that of their sodium counterparts.
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Caustic versus Ferritic Stainless Steels

Description
Many of the high-chromium ferritic stainless steels are outstanding in their resistance to hot caustic soda. 26-1 is widely used in caustic evaporators as heat exchanger tubing, in 177°-204° C (350°-400° F), 45% NaOH. Although general corrosion, erosion-corrosion and intergranular attack are sometimes observed, no SCC has been encountered below 290° C (550° F). 29-4 and 29-4-2 alloys look good in laboratory tests. As with any stainless material, their general corrosion resistance is improved by the presence of small amounts of oxidizing species such as chlorates or oxygen.
Type 430, Type 439 and 18-2 stainless steels are also resistant to caustic SCC. However, these 16-19% Cr grades should be used with caution. Their general corrosion resistance is poor in 10% NaOH in the range of temperatures encountered in boiling water reactors 315° C (600° F).
The "lean alloy" ferritics (12-14% Cr) also show high rates of general corrosion in hot concentrated caustic solutions and should be approached with caution as a result.
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Caustic versus Duplex Stainless Steels

Description
The duplex stainless steels are a relatively new class of alloys; their caustic corrosion and SCC performance have not yet been clearly defined. Some tests on AISI 329 suggest that it can be used in up to 50% NaOH to at least 135° C without SCC.
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Caustic versus Hardenable Stainless Steels

Description
SCC of hardenable stainless steel turbine components is a serious source of forced outages in power plants. The alloys normally used are 12 Cr martensitic grades such as 403 and 410. NaOH is used in power plants to control feedwater pH and can concentrate due to carryover into drum-type boilers or misoperation of condensate polishers. In addition, pH control with trisodium phosphate can give rise to locally high hydroxyl ion concentrations. If the boiler water treatment system allows the presence of free hydroxide ion (OH-), carryover from the steam drum can cause SCC of downstream components.

Cracking is worst in deoxygenated solutions; the presence of oxygen tends to retard the attack. The higher the metal hardness, the shorter the time to failure. In up to 10% NaOH solutions, fully-annealed material and material with hardness less than about Rc30 show good resistance to cracking up to 315°C. However, the material can be made to crack in U-bends. At higher concentrations, the 12 Cr alloys show high rates of general corrosion and are not suitable for use.
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Caustic versus Nickel-based Alloys

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Overview
In caustic of 70% concentration or more, even the high-nickel alloys will crack over prolonged periods of time at temperatures above 300°C and at high stress levels. In ascending order of merit are Alloy 400, Alloy 600, Alloy 200 and Alloy 201.

- Alloy 400 can suffer SCC at concentrations in excess of 80% if temperatures exceed 175°C.
- Alloy 600 is more readily cracked by hot concentrated caustic when oxygen is present.
- Nickel Alloy 200 fails only under very high stress at temperatures above 300°C after prolonged periods.
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Chlorides
Chlorides are a common contaminant in caustic and many SCC failures in alkaline environments have been ascribed to them.

It is important, although admittedly difficult, to differentiate their influence, as this can profoundly affect the selection of alternative materials. For example, when both chlorides and caustic are present, Alloy 600 is to be preferred to Alloys 800 and 825, which can be cracked by caustic more easily than Alloy 600.
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Oxidizers
Oxidizing contaminants, such as chlorates or hypochlorites, can affect the corrosivity of caustic streams in an unpredictable fashion, either aggravating or inhibiting specific phenomena.
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Sodium salts
Although some sources indicate that sodium salts other than halides do not cause SCC, this allegation must be tempered with the knowledge that some sodium salts can and do suffer either hydrolysis or thermal decomposition with attendant liberation of caustic, which does cause SCC. The thermally unstable bicarbonates and carbonates are frequent offenders.
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Steam
Steam is a common environment for the SCC of stainless steel, due to carryover from the boiler drum of either chlorides or caustic or both. Parts per million of free caustic may accumulate or develop in crevices (e.g., from sodium carbonate) to concentrations that will cause SCC in a matter of hours. Gasket areas, welds lacking complete penetration, and bellows-type expansion joints are frequent victims. Steam at 300 psia (215°C) and 400 psia (230°C) has caused SCC of both 18-8 stainless steel and Alloy 400.
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Caustic versus Copper Alloys

Description
Copper (ETP, DLP or DHP) shows useful general corrosion resistance even in 50% NaOH at temperatures up to 130°C. Although copper equipment is not widely used for handling concentrated caustic, it is used in process streams where caustic is used as a neutralizing agent, inhibitor, etc., with good success. Stress-corrosion cracking has not been reported.

In the laboratory, 70-30 brasses have been cracked in NaOH solutions with pH greater than 12. Cracking apparently does not take place unless the potential is displaced at least 100 mV less negative than the free corrosion potential. At a pH of 12 or less, cracking of brass in NaOH has not been observed. These laboratory data would seem to confirm a long-held CPI tradition against the use of high-Zn brasses such as CA 270 and 280 in caustic solutions.

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Caustic versus Aluminum Alloys

Description
General corrosion rates of aluminum alloys in caustic solutions are so high that stress-corrosion cracks never get a chance to initiate. Even mildly alkaline solutions (pH>10.5) should not be handled in aluminum equipment.

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Caustic versus Titanium and its Alloys

Description
Strong, hot caustic can strip the oxide film off titanium, leading to rapid general corrosion. However, stress-corrosion has not been reported.

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Caustic versus Zirconium

Description
Zirconium shows good corrosion and stress-corrosion cracking resistance in tests in 50-90% NaOH up to 260°C. Since the installed cost of zirconium equipment has historically ranged from 50 to 100% higher than that of comparable nickel equipment, zirconium has not been widely used for caustic service. However, changing prices and availability problems with nickel may result in more widespread use of zirconium in caustic.

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