Stress–Corrosion Cracking

For just about every alloy, there is some chemical environment that, combined with stress, will cause cracking. For brass that environment is ammonia or other nitrogen compounds.

The source of stress is usually residual forming and welding stresses, which may reach the yield point of the material. Operating stress is rarely the issue.

For austenitic stainless steels chlorides are the major cause of stress–corrosion cracking (SCC). An example is hot potable water under heat transfer conditions which permit chlorides to concentrate locally. Susceptible alloys include 304L, 316L, 321, and 347. Some 95% of 316L chemical plant equipment failures may be attributed to chloride SCC. The chlorides concentrate from trace amounts present in steam for heating or the cooling water in heat exchangers as well as from the product Chloride SCC occurs most quickly in austenitic stainless steels with about 8–10% nickel, alloys with much lower or much higher nickel content being less susceptible. As thermal stress relief is rarely practical with stainless fabrications, the metallurgical solution is a change in alloy. Nickel-free ferritic steels, such as E-BRITE, are highly resistant to chloride SCC but impractical to fabricate into a vessel.

The traditional solution in North America has been to go to a higher nickel alloy. Alloys with about 30% or more nickel are generally considered to be good engineering solutions to most chloride SCC problems, although they will crack under very severe conditions. 20Cb- 3 at about 34% and 825 at 40% nickel have long been chosen for this service. Likewise the fine-grained Incoloy 800, UNS N08800, 31% Ni, had been used in years past. However, this low-carbon, fine-grained version of 800 is now rarely available. Regardless of what it is called, ‘‘800’’ today usually is 800HT , UNS N08811, a higher carbon, coarse-grained version. This grade is designed to maximize creep rupture strength for high-temperature applications.

Since the mid-1980s the 6% molybdenum superaustenitics have become available.

Grades such as 254 SMO with only 18% nickel, or AL-6XN at 24% nickel, have been

used effectively to resist chloride SCC. The material cost of the ‘‘6-moly’’ grades is between that of 316L stainless and nickel alloy C-276. Although lower in nickel, molybdenum contents above 2% tend to decrease susceptibility of austenitic stainless steel to chloride SCC.3

For greater resistance to both corrosion and chloride SCC the most used grade has been
C-276, at 57% Ni 15.5% Mo. There are now a number of alloys in this class, including C-
22, Inconel 686, C-2000, 59, and a new Japanese grade, MAT 21. These very high nickel
alloys can easily reach five times or more the cost of 316L stainless steel. They are metallurgically
excellent solutions to chloride SCC, particularly in severe environments. However,
they are expensive choices for service conditions under which 316L lasted a few years before
cracking. There is a less expensive choice, one that has long been used in Europe—a duplex
stainless steel which is about half austenite and half ferrite. Duplex stainless steel is a practical
solution to most 304L or 316L SCC failures. The most commonly available duplex in
North America is 2205, at a cost roughly 20% above that of 316L.
Other forms of SCC in stainless steels include caustic cracking and polythionic acid
SCC. Caustic may crack carbon as well as stainless steel. High-nickel alloys, such as alloy
600 or, better, commercially pure nickel (UNS N02201), are used for hot concentrated
caustic.
Polythionic acid stress–corrosion cracking (PASCC) is caused by sulfur compounds in
the environment and most often encountered in refineries. Any stainless or nickel alloy that
has been sensitized can be subject to PASCC. High-nickel alloys do not help; even 600 alloy
will crack when sensitized.4 To resist this form of SCC the alloy must contain a strong
carbide-forming element, or ‘‘stabilizing’’ element, such as columbium or titanium. Examples
2 Some Forms of Corrosion 45
include 321, 347, 20Cb-3, 825, and 625. In addition, the alloy must be given a stabilizing
anneal so that the carbon is effectively combined with the Cb or Ti. RA333, because of its
tungsten and molybdenum content, resists PASCC when stabilize annealed about an hour at
1700 F.
Normally, 304H or 316H would be quite sensitive to polythionic acid stress cracking,
as these higher carbon, solution-annealed alloys readily sensitize. The matter has been addressed
at one refinery by fabricating the equipment from one of these H-grade stainless
steels, then stabilize annealing the completed fabrication. A temperature of about 1650 F
(900 C) for a minimum of 1 h is used. This does precipitate carbides at the grain boundaries,
but the temperature is high enough to permit chromium to diffuse back into the Cr-depleted
grain boundary zone. In addition, this treatment relieves over half of the residual fabricating
stress, thus reducing susceptibility to chloride SCC as well.

READ MORE.......

STAINLESS STEELS

James Kelly

Rochester, Michigan




Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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