Avoiding Stress Corrosion Cracking of Carbon Low Alloy and Austenitic Stainless Steels in Chloride and Caustic Environments

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Maintenance Best Practice Guide:
GBHE_MBPG_1614
Avoiding Stress
Corrosion Cracking of
Carbon Low Alloy and
Austenitic Stainless
Steels in Chloride and
Caustic Environments
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Maintenance Best Practice Guide:
Avoiding Stress Corrosion Cracking of Carbon Low Alloy and Austenitic
Stainless Steels in Chloride and Caustic Environments
CONTENTS
0 PURPOSE
1 SCOPE
2 KEY DEFINITIONS
3 RELEVANT DOCUMENTATION
4 SPECIFIC LEGAL REQUIREMENTS
5 FACTORS PROMOTING STRESS CORROSION CRACKING (SCC)
IN CHLORIDE AND CAUSTIC ENVIRONMENTS
6 AVOIDING STRESS CORROSION CRACKING (SCC)
IN CHLORIDE AND CAUSTIC ENVIRONMENTS
6.1 CONTROL OF STRESS
6.2 CONTROL OF ENVIRONMENT
7 SUMMARY

APPENDICES
A GUIDANCE ON HYDROTEST WATER QUALITY FOR AUSTENITIC
STAINLESS STEEL EQUIPMENT TO AVOID CHLORIDE SCC
B GUIDANCE ON THE PREVENTION OF CHLORIDE SCC FOR
AUSTENITIC STAINLESS STEEL HEAT EXCHANGERS ON COOLING
WATER DUTIES
C SUMMARY FOR GENERAL GUIDANCE ON SCC OF CARBON STEELS
AND AUSTENITIC STAINLESS STEELS IN CHLORIDE AND CAUSTIC
ENVIRONMENTS
FIGURES
1 SCC OF AUSTENITIC STAINLESS STEELS AS A FUNCTION OF
CHLORIDE CONCENTRATION AND TEMPERATURE
2 CAUSTIC SODA SERVICE FOR AUSTENITIC STAINLESS STEELS
3 GBHE CAUSTIC SODA SERVICE CHART FOR CARBON/LOW ALLOY
STEELS
4 SCC AT TUBE/TUBEPLATE JOINTS DUE TO CONCENTRATION OF
CHLORIDE OR CAUSTIC (HYDROXIDE).

0 PURPOSE
The purpose of this Maintenance Best Practice Guide is to assist
engineers to avoid stress corrosion cracking when designing or specifying
equipment, to aid in diagnostic work and to guide inspection personnel
dealing with equipment which may hold fluids containing chloride or
caustic (hydroxides).
1 SCOPE
This Maintenance Best Practice Guide is concerned with the performance
of carbon, low alloy steels, and austenitic stainless steels, in fluids
containing chloride or caustic (hydroxides). Those factors which are
known to promote stress corrosion cracking are outlined, and service
charts defining environmental boundaries for stress corrosion cracking in
chloride and caustic containing fluids are presented. General guidance on
the avoidance of stress corrosion cracking is provided.
The avoidance of 'external' stress corrosion cracking of austenitic
stainless steels beneath thermal insulation is covered in a separate
Maintenance Best Practice Guide.
2 KEY DEFINITIONS
Stress Corrosion Cracking Cracking Cracking produced by the
(SCC) combined action of corrosion
and tensile stresses.
3 RELEVANT DOCUMENTATION
GBHE-MBPG-1814 Materials of Construction Review
GBHE-MBPG-1914 Unfired Fusion Welded Pressure Vessels
GBHE-MBPG-2014 Light Duty Items in Stainless Steel
GBHE-MBPG-0714 The Pressure Testing of In-Service Pressurized
Equipment.

4 SPECIFIC LEGAL REQUIREMENTS
Account shall be taken of relevant local legislation.
5 FACTORS PROMOTING STRESS CORROSION CRACKING (SCC) IN
CHLORIDE AND CAUSTIC ENVIRONMENTS
The common grades of carbon and low alloy steels are vulnerable to
stress corrosion cracking (SCC) in fluids containing sodium or potassium
hydroxide (caustics). They are not vulnerable to SCC in chloride
containing fluids, but can suffer from general corrosion. High strength
grades of steel (>950MPa UTS) can suffer SCC due to hydrogen
produced by corrosion reactions on their surfaces, including those induced
by chloride containing fluids. However, this topic is beyond the scope of
this Best Practice Guide,
All of the common grades of 18%Cr, 8-10%Ni austenitic stainless steel,
including types 304, 304L, 316, 316L, 321 and 347, are vulnerable to SCC
in both chloride and caustic containing fluids. Other grades of stainless
steel, including the more highly alloyed austenitic grades (e.g. those
grades containing >25Ni such as types 904L and 825) and 'duplex' grades
(i.e. those containing 5-7%Ni such as types 2205 and 2507) are also
vulnerable to SCC in chloride and caustic containing fluids, but are beyond
the scope of this Best Practice Guide.
Guides to safe operating conditions in chloride and caustic containing
fluids are shown in Figures 1 to 3. Although this data can be used for
general guidance, it is important to be alert to the effects of local
concentration and heating. Various factors can concentrate otherwise
benign levels of corrodent to concentrations which promote SCC
including:
(a) boiling and evaporative concentration due to locally high heat fluxes/skin
temperatures;
(b) intermittent wetting and drying of surfaces associated with liquid injection
into hot pipelines, level variation in storage vessels, etc.;
(c) the presence of crevices and surface deposits leading to diffusional
concentration.

Residual fabrication stresses commonly trigger SCC at welds and at cold worked
areas, including cold-formed bends, pressed components, expansions, etc.
Highly stressed equipment such as bellows are also particularly vulnerable.
Heat exchangers of all types are potentially vulnerable to SCC. In the case of
tubular exchangers, problems are more likely if the corrodent is on the shellside,
with heat transfer into the tube/tubeplate crevice. Favored cracking sites are in,
and adjacent to tube/tubesheet welds, and at the overlap zones of the expansion
stages. Vertical or inclined bundles can be particularly vulnerable to such
problems if the top tubesheet is inadequately vented, and a vapor space
develops, as shown schematically in Figure 4. In such cases, even high quality
demineralized waters with trace levels of chloride or caustic can concentrate and
promote cracking in time periods as short as a few days.
In the case of chloride induced SCC of austenitic stainless steels, the qualities of
waters used for pressure testing and cooling are significant issues. Some general
guidance is provided in Appendices A and B.
6 AVOIDING STRESS CORROSION CRACKING IN CHLORIDE AND
CAUSTIC ENVIRONMENTS
6.1 Control of Stress
In the case of carbon and low alloy steels, tensile stresses around yield are
required to promote SCC, which as a result is commonly associated with residual
rather than operating loads. Cracking can then be prevented by appropriate
stress relief procedures, of which thermal stress relief is the more widely
practiced and effective. Equipment design codes contain procedures for thermal
stress relief, but do not give information on when it is required to prevent SCC.

In the case of austenitic stainless steels, stress corrosion cracking can be
initiated by relatively low stresses. Thermal stress relief in austenitic stainless
steels requires much high temperatures than carbon steels, it is therefore
considered to be a less practicable procedure than for carbon steels. As a result,
thermal stress relief is rarely used to control SCC in austenitic stainless steels.
Dissimilar welds between carbon or low alloy and austenitic materials cannot be
thermally stress relieved effectively, and their use should be avoided in
circumstances where they are vulnerable to SCC.
The use of mechanical stress relief procedures such as shot peening can be
beneficial in specific circumstances. However, such techniques are specialized
and require careful control; a materials engineer should be consulted about their
use whenever possible.
6.2 Control of Environment
Figures 1 to 3 provide guidance on safe operating windows to avoid SCC of
equipment, but care is needed to ensure that local heating/concentration effects
are anticipated successfully at the design stage and avoided. In certain
circumstances, corrosion inhibitors can be used to reduce the propensity of an
environment to promote SCC, e.g. the use of phosphates in waters. However,
there are many traps for the unwary, and some factors which have resulted in
SCC in practice which were not anticipated at the design stage are:
(a) Injection of water with traces of free chloride or caustic to desuperheating
steam, resulting in concentration due to wetting/drying of downstream
surfaces;
(b) Failure to appreciate that some sources of caustic have high chloride
levels;
(c) Hydro testing equipment with poor quality water;
(d) Allowing excessive fouling of tubular heat exchanger shellside surfaces,
resulting in local heating/concentration;
(e) Gagging back on water supply to heat exchangers to control process
temperatures resulting in local heating/concentration;
(f) Using high temperature tubeside process cleaning procedures resulting in
dry out and therefore concentration, of water in the shellside.

7 SUMMARY
The main factors outlined in this Good Practice Guide are summarized in
Appendix C.
Stress corrosion cracking is a complex topic, and a materials engineer should be
Consulted whenever possible.
Effect of Cu-content on crack growth rate. The effect of initial stress
on time to failure of maraging
steel in 3.5% NaCI solution.

APPENDIX A GUIDANCE ON HYDROTEST WATER QUALITY
FOR AUSTENITIC STAINLESS STEEL EQUIPMENT TO
AVOID CHLORIDE SCC
Control of the quality of hydro test water is necessary for the prevention of stress
corrosion cracking by chlorides present in the test water, which remain in the
equipment after the hydraulic test. The greatest susceptibility is at high chloride
concentrations and service temperatures more than 60o
C.
(a) Do not use salt or contaminated water for testing, e.g. borehole, river,
canal, lake, estuarine, or sea water.
(b) When the process operating temperature is less than 60o
C then use of
potable quality test water is acceptable.
(c) When the operating temperature is above 600
C but the metal is flushed by
process fluids or condensing steam at start up, then the use of potable
quality test water is acceptable, provided the equipment is completely self-
draining.
(d) When the operating conditions are above 60o
C and the metal is not
flushed by process fluids or condensing steam on start up, then, provided
the equipment is completely self-draining, test water of potable quality can
be used, followed by flushing with water containing less than 1ppm
chloride.
(e) When the operating temperature is above 60o
C, and the vessel is not
flushed by process fluids or condensing steam and is NOT completely
self-draining, test water with less than 1ppm chloride should be used. Dry
out carefully by swabbing, and/or blowing with warm air (less than 60oC).
(f) Reference (c) and (d) above, enclosed spaces such as shell sides of heat
exchangers or vessel or pipe jackets which operate above 600C should
always be tested with water containing less than 1ppm chloride.
Experience dictates that residual chloride cannot be removed successfully
by flushing.
Note:
Less than 1ppm chloride = demineralized water, or pure condensate.

APPENDIX B GUIDANCE ON THE PREVENTION OF CHLORIDE
SCC FOR AUSTENITIC STAINLESS STEEL HEAT
EXCHANGERS ON COOLING WATER DUTIES
The '300' series of 18%Cr, 8-10%Ni austenitic stainless steels are used
commonly for heat exchanger construction. Their weakness is their susceptibility
to stress corrosion cracking in chloride-containing media, and, materials
engineers are often asked, 'What is the critical level of chloride ions which
stainless steel will tolerate?’ This is not a question to which there is a simple
answer.
A much easier question to answer is, ‘what is the critical temperature below
which chloride stress corrosion cracking will not occur?' There is no unique
answer to this question either, but most materials engineers would agree that
below 700
C stress corrosion cracking is unlikely, and below 600
C it is extremely
rare. For this reason, prevention of stress corrosion cracking in heat exchangers
in the Company cooling systems has been based upon control of skin
temperature rather than chloride level per se.
In broad terms, the chloride contents of the Company cooling systems in the US
are generally < 200 ppm, and rarely, if ever, exceed 600 ppm. In Europe,
somewhat higher levels have to be tolerated, but even so, chloride contents
rarely exceed 800 ppm. Where flow rates are high and crevices/deposits are
absent, i.e. when cooling water flow is through the tubes, US experience
indicates that skin temperatures up to 100O
C can be tolerated in such fluids.
However, stress corrosion cracking commonly initiates from pitting or
crevice/deposit corrosion, and is thus favored by low flow conditions and
occluded areas associated with joints, scales, foulants, etc. Thus for water-in-
shell bundles or plate exchangers, the only reliable defense against stress
corrosion cracking is to restrict skin temperatures to a maximum of 600
C.
In practice most cases of stress corrosion cracking in austenitic stainless steel
heat exchangers are associated with operational practices resulting in higher skin
temperatures and/or higher chloride contents than anticipated at the design
stage. Thus, excessive fouling of shellside tube surfaces, restricting water flow to
adjust process temperatures, high temperature tubeside cleaning resulting
in shellside dry out etc, etc, can lead to failure by stress corrosion cracking in
systems which would have otherwise proved benign. If you have any doubts
about the integrity of your own system, contact a materials engineer.

APPENDIX C
SUMMARY FOR GENERAL GUIDANCE ON SCC OF CARBON STEELS AND
AUSTENITIC STAINLESS STEELS IN CHLORIDE AND CAUSTIC
ENVIRONMENTS
CHLORIDE ENVIRONMENTS (CL¯ )

CAUSTIC ENVIRONMENTS (OH¯ )
Notes:
1 The term austenitic stainless steels as used in this note includes grades
304, 304L, 316, 316L 321 and 347.
2 The term carbon steels includes carbon-manganese and low alloy steels.

FIGURE 1 SCC OF AUSTENITIC STAINLESS STEELS AS A FUNCTION
OF CHLORIDE CONCENTRATION AND TEMPERATURE

FIGURE 2 CAUSTIC SODA (SODIUM HYDROXIDE) SERVICE CHART FOR
AUSTENITIC STAINLESS STEELS

FIGURE 3 GBHE CAUSTIC SODA (SODIUM HYDROXIDE) SERVICE
CHART FOR CARBON/LOW ALLOY STEELS

FIGURE 4 SCC AT TUBE/TUBEPLATE JOINTS DUE TO
CONCENTRATION OF CHLORIDE OR CAUSTIC (HYDROXIDE)

Avoiding Stress Corrosion Cracking of Carbon Low Alloy and Austenitic Stainless Steels in Chloride and Caustic Environments

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