Chloride Stress Corrosion Cracking: Failure Analysis Guide

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: What Causes Chloride Stress Corrosion Cracking?

Chloride stress corrosion cracking is the brittle, branching failure of an austenitic stainless steel under tensile stress in a chloride-bearing environment above a critical temperature — commonly taken as about 60 °C for 304 and 316. Dry chlorides do not crack; water, oxygen and a ruptured passive film are required.

Key Takeaways

• Three conditions, not one. Chloride SCC needs a susceptible alloy, a chloride electrolyte and sustained tensile stress together; removing any one of them stops the failure.
• Temperature is the switch. Austenitic 300-series grades crack readily in chlorides above roughly 60 °C but tolerate the same chloride below it.
• The concentrating mechanism decides the outcome. Evaporation under insulation, wet–dry cycling and stagnant crevices raise chloride concentration far above the bulk fluid.
• High strength makes it worse. 17-4PH at H900 fails by hydrogen embrittlement and SCC combined; hardness control is part of the fix.
• The alloy ladder is measurable. Moving from 316L to 904L, 2205, 2507, 6Mo, Incoloy 825, Inconel 625 and Hastelloy C-276 raises PREN and raises the service limit — at a known cost.

What Is Chloride Stress Corrosion Cracking?

Chloride stress corrosion cracking (Cl-SCC) is a brittle, time-dependent failure in which a normally ductile austenitic stainless steel cracks under the combined action of tensile stress and a chloride-bearing environment. It is not a general-thickness-loss mechanism. A polished, unstressed 316L flange can sit in hot chlorides for years with almost no visible attack, while an identical stressed tube cracks in weeks. That contrast is the most useful fact in failure analysis: SCC requires a susceptible alloy, a specific environment and tensile stress at the same time, and removing any one of the three prevents it.

The mechanism is best understood as corrosion-assisted fracture rather than simple corrosion. The chromium-oxide passive film that makes stainless steel "stainless" is locally ruptured, most often in a pit, a crevice or a scratch where chloride concentration has risen. At the rupture, bare metal becomes a small anode and dissolves rapidly. The surrounding passive surface is the cathode, and because the anodic area is tiny compared with the cathode, the local dissolution rate is high. Under sustained tensile stress the crack tip stays sharp and the strain keeps re-breaking the film, so film rupture and dissolution alternate and the crack advances in steps. In austenitic 300-series stainless the result is characteristic transgranular cracking — cracks that cut across grains, branch as they propagate, and leave a fracture surface with almost no plastic deformation. In duplex and nickel alloys the path shifts: cracking in duplex grades is often mixed transgranular and intergranular, while genuinely nickel-rich alloys such as Inconel 625 and Hastelloy C-276 are effectively resistant inside the chloride envelope where 304 fails outright.

Why dry chlorides do not crack stainless steel. Solid sodium chloride, a dry salt deposit or dry chloride dust can rest on 316L indefinitely without cracking, because there is no electrolyte and no cathodic reaction to drive dissolution. Chloride SCC is an aqueous electrochemical process; it needs water — even a condensed monolayer — plus dissolved oxygen or another cathodic reactant, and a pH at which the local film is unstable. That is why failures cluster at wet–dry transitions, under wet insulation, and in stagnant pockets, and why the most durable fix is often to eliminate the water or the concentration mechanism rather than to change the alloy. It also explains why hydrotesting with chlorinated water, followed by incomplete draining, has started SCC in equipment that never saw a chloride process stream.

Which Variables Decide Whether a Chloride Will Crack?

The variables interact, so no single number decides. Temperature, chloride concentration, dissolved oxygen, pH and stress act together, and a low value in one can sometimes offset a high value in another. The professional approach is to define the service envelope and then compare it with the published limit for each candidate alloy.

Temperature is the most powerful single variable. For austenitic 300-series stainless the practical threshold for chloride SCC is often quoted as about 60 °C; below that, cracking becomes uncommon in neutral chlorides, and above it susceptibility rises steeply. The threshold is not a sharp line, and it falls as chloride concentration, oxygen or acidity increase. Duplex and super-austenitic grades push the threshold much higher, and nickel alloys push it beyond most refinery and marine service temperatures. This is why a cooling-water exchanger that runs at 40 °C for years can crack the same week it is fouled and operated at 80 °C.

Chloride concentration and the concentrating mechanism. Bulk chloride matters far less than local chloride. Evaporation under insulation (the classic corrosion-under-insulation case) can concentrate a few tens of ppm into saturated brine. Wet–dry cycling leaves a salt ring at the water line. A porous deposit or a stagnant crevice traps chloride and holds it against the metal while oxygen from the bulk solution keeps the cathode active. Any design that creates a stagnant, cyclically wetted or poorly drained region is, in effect, a chloride concentrator.

Dissolved oxygen is required for the cathodic reaction and for the local acidification inside a pit or crevice. Deaerated or oxygen-scavenged systems are markedly less aggressive. Conversely, sparging air, splash zones and open basins raise oxygen and raise risk.

pH and acid chlorides. Neutral-to-acidic chlorides are far more aggressive than alkaline ones. Acid chlorides, hydrochloric acid carry-over, and chlorides combined with an acidic process stream attack the passive film directly and can produce SCC at lower temperature and lower chloride level. Caustic (alkaline) service holds a separate cracking risk but a different mechanism and different alloys.

Stress level. Applied stress, residual stress from welding and cold work, and thermal stress all count. Cold-bent U-bends, expansion-rolled tube ends, misaligned flanges and heavy weld residual stress all supply tensile stress. The threshold stress is well below yield, so a design that is mechanically safe can still be SCC-critical. Stress relief is therefore a genuine corrective action, not a cosmetic one.

Why Do 304 and 316L U-Bend Heat-Exchanger Tubes Fail So Often?

304 and 316L U-bend heat-exchanger tubes are the classic chloride SCC failure for one reason: they combine every contributing factor in a single component. The U-bend has very high residual tensile stress from cold forming on its outer crown. The tube wall is thin, so the stress is high relative to the section. The shell side often runs a chloride-bearing cooling water at a surface temperature above 60 °C. And the outside of the bend is frequently the wettest, most deposit-prone area of the bundle.

In practice the crack appears on the outside of the U-bend, on the tension side, running transgranularly through the wall, usually with little or no general corrosion nearby. Failures are commonly found during a routine hydrotest or on shutdown inspection, when a fine surface-breaking crack is opened by the test pressure. Because the crack is tight and often hidden by deposit, dye penetrant on a cleaned bend, or eddy-current testing of the full tube length, finds failures that a visual walk-down misses.

316L is more resistant than 304 because of its molybdenum and its lower carbon, but the difference in chloride SCC resistance between the two is smaller than most engineers assume. Molybdenum improves pitting and crevice resistance and raises the temperature threshold modestly, but it does not make 316L a hot-chloride alloy. The frequent error is to specify 316L "because it is the better stainless" and then to operate the tube outside at 80 °C with a concentrating deposit. The material choice was defensible; the operating envelope was not.

The correct response is not to blame the grade. It is to verify the tube-side and shell-side chemistry, the metal temperature, the velocity and the deposit pattern, and then either change the alloy (see the ladder below), reduce the metal temperature, or remove the concentrating mechanism. Our duplex & PH stainless page shows the grades that sit immediately above 316L on that ladder, and our chloride service field guides cover the inspection techniques that catch U-bend cracking before it becomes a leak.

2205 Duplex in Hot Chloride: A Different Failure Mode

Duplex 2205 (UNS S32205) is far more resistant to chloride SCC than 316L, and it is a common upgrade for chloride-bearing water. It is not immune. In hot, concentrated chlorides — typically above about 120–150 °C, or in acidic chloride with high oxygen — 2205 can crack, and the crack path is often mixed with a strong intergranular component rather than the clean transgranular path of 304. That mixed path is easy to misread if the analyst assumes SCC in duplex must look like SCC in austenitic stainless.

Two further duplex failure modes are frequently confused with chloride SCC. First, 2205 is susceptible to hydrogen embrittlement at high hardness, particularly in the weld metal or the heat-affected zone of a poorly controlled weld, and to sulphide stress cracking in sour service. Second, the 50/50 ferrite–austenite balance can be upset by wrong heat treatment or wrong filler, leaving an unbalanced microstructure that is more susceptible to selective attack. When a 2205 failure appears, the analysis must therefore confirm the phase balance and the hardness of the affected zone, not only the crack path — the metallographic examination of the microstructure is as important as the chemistry check. Grades such as 2507 (UNS S32750) and the 6Mo super-austenitics raise the limit further, and are discussed under the upgrade ladder.

High-Strength 17-4PH: Where SCC and Hydrogen Embrittlement Combine

Precipitation-hardening martensitic stainless steel such as 17-4PH (UNS S17400) fails differently from the austenitic grades, and the difference matters when a component is misapplied. In the H900 condition, 17-4PH reaches roughly 1310 MPa minimum tensile strength per ASTM A564, and at that strength it has very little tolerance for absorbed hydrogen. In wet chloride service, and especially under cathodic protection, atomic hydrogen is generated at the surface and absorbed; the part then fails by hydrogen embrittlement, often with no visible corrosion and often shortly after start-up or commissioning rather than after years. Sustained tensile stress in hot chloride adds classic SCC on top of the hydrogen mechanism, so the two act together.

The engineering consequence is a hardness and condition rule rather than an alloy rule alone. Move the part to a lower-strength overaging condition such as H1150 to raise its tolerance, or leave the martensitic family altogether and use a nickel alloy such as Monel K-500 or Inconel 718 where strength and chloride resistance are both required. The Inconel alloy supplier range is where most of those substitutions land, and the relevant hardness controls are set out by the applicable AMS and ASTM bar and forging specifications. The single most common field error is to specify "17-4PH" with no condition; the drawing must state the condition and the governing standard, for example "17-4PH, H1150, per ASTM A564", because the condition, not the name, controls the failure risk.

Sensitisation, Welded 316 and the Intergranular Corrosion Question

Intergranular corrosion is a second, separate failure mode that is often found during the same investigation as chloride SCC, and it must be distinguished from it. In unstabilised austenitic stainless such as 304 and 316, welding or high-temperature exposure in the range of roughly 425–815 °C can precipitate chromium-rich carbides at the grain boundaries. The adjacent zone is depleted in chromium and becomes an anodic path, so attack follows the grain boundaries — typically a short distance from the weld fusion line. This is sensitisation, and it produces intergranular corrosion (IGC) that can look superficially like cracking when the grains fall out and leave a shallow, stepped surface.

The two failure modes are diagnosed with different tests. IGC susceptibility is assessed with ASTM A262, which provides several practices for austenitic stainless, and, for the nickel-rich Cr-bearing alloys, with ASTM G28. Chloride SCC is assessed with stress-applied tests such as ASTM G36 (boiling magnesium chloride) for austenitic stainless and ASTM G39 bent-beam specimens under applied or residual stress. A failure investigation that finds intergranular attack in a welded 316 component should test the material to the appropriate A262 practice and check the carbon content and the stabilisation of the weld, because the corrective action — low-carbon or stabilised grade, or correct solution treatment — is entirely different from the SCC fix.

The practical rule for welded 316 is straightforward. Use the low-carbon L grade or a stabilised grade for welded assemblies that see elevated temperature or aggressive chemicals; specify the correct filler; and, where the service justifies it, verify the IGC resistance with a coupon test rather than assuming it from the certificate. Our alloy technical knowledge center holds the test-selection notes that we use when a customer requests corrosion coupons alongside a material order.

Caustic and Ammonia Cracking: Adjacent Failure Modes to Rule Out

Two adjacent cracking mechanisms are routinely mislabelled as chloride SCC, and mislabelling them leads to the wrong alloy change. Caustic cracking occurs in alkaline (caustic soda) service and is concentrated at welds and heat-affected zones under high residual stress; it is managed by post-weld heat treatment to relieve stress and by correct alloy selection, and it is temperature- and concentration-dependent. Ammonia cracking affects copper alloys and some carbon steels in ammonia-bearing environments, particularly where oxygen and moisture are present, and is a reason to avoid copper-based tubing in ammonia service.

The diagnostic value is this: a crack in a caustic circuit is not fixed by removing chlorides, and a crack in an ammonia circuit is not fixed by changing to a more corrosion-resistant stainless. The analyst must confirm the process chemistry first — sodium hydroxide, ammonium compounds, hydrogen sulphide, chlorides — and only then compare the crack path and the test evidence with the known mechanism. This is why the correct order of a failure investigation is chemistry and service condition first, microscope second.

How Do You Analyse a Chloride SCC Failure?

A chloride SCC investigation follows a fixed sequence, and the sequence matters because the fastest way to a wrong conclusion is to look at the crack before you understand the chemistry. Establish the service condition first, then document the crack, then verify the material, and only then decide the corrective action.

Start with the process and the paperwork. Recover the operating temperature, the chloride analysis of both sides of the exchanger, the pH, the oxygen level, the flow velocity and the cleaning history. Compare the actual chemistry with the design basis; more than half of the SCC cases we review involve a chloride source that was not in the original design — a change of cooling-water source, a chlorination regime, a cleaning chemical, or a hydrotest that was not properly drained.

Next, examine the crack visually and under a stereo microscope, in place and then on a section. Note the orientation relative to stress, the presence of branching, whether the crack is single or multiple, and whether it originates at a pit, a crevice or a weld. Transgranular branching with minimal deformation points to chloride SCC in austenitic stainless; a mixed path with an intergranular component points to duplex or to a different mechanism.

Then cut and mount a metallographic section across the crack and along the grain structure. The metallography confirms the crack path, reveals secondary cracking, shows sensitisation at the grain boundaries if present, and shows the microstructure of duplex grades. This is also where an intergranular corrosion pattern is separated from a genuine SCC pattern.

Deposit analysis by SEM/EDS is the next step when a deposit or corrosion product is present. The chemistry of the deposit is often the only direct evidence of the chloride source, and it frequently identifies an unexpected contaminant. Alongside this, verify the base material chemistry by optical emission spectrometry or portable PMI to ASTM E1476, and confirm the hardness, because a substitution or a wrong heat-treatment condition changes the risk profile entirely.

For in-service screening rather than a single failure, the tools are eddy-current testing of non-ferromagnetic tubes for cracking and wall loss, dye penetrant on cleaned welds and bends, surface replica metallography for creep or cracking, and ultrasonic examination of heavy sections. The choice of method follows the material: eddy current works well on austenitic and nickel-alloy tubes but needs the correct probe and reference for duplex, and magnetic-particle inspection cannot be used on austenitic material.

Code	Scope	What it detects	Applies to
ASTM G36	Boiling magnesium chloride SCC test	Austenitic SCC susceptibility ranking	austenitic stainless
ASTM G48	Pitting and crevice corrosion in ferric chloride	Localized corrosion, PREN ranking	all families
ASTM G28	Intergranular corrosion of Ni-rich Cr-bearing alloys	Sensitisation / IGC	nickel alloys, 6Mo
ASTM A262	Intergranular attack in austenitic stainless	Sensitisation	304, 316, 321, 347
ASTM G39	Bent-beam SCC specimens	SCC under applied stress	all families
ASTM G30	U-bend SCC specimens	SCC under residual stress	all families
NACE TM0177	Sulfide stress cracking in H2S service	SSC / SCC	carbon, low-alloy, stainless
ASTM G61	Cyclic potentiodynamic polarization	Localized corrosion tendency	all families

Table note: Test methods are listed with their stated scope from the sponsoring body. Screening test results rank candidate alloys; they are not a substitute for testing the specific heat in the specific chemistry.

The Alloy Upgrade Ladder: PREN, Temperature and Chloride Limits

The way out of a chloride SCC problem is almost always to move up an alloy ladder whose steps are measurable. PREN — the pitting resistance equivalent number, calculated from chromium, molybdenum and nitrogen — is the standard way to express that step, and although PREN is a calculated index and not a standard acceptance value, it tracks pitting and crevice resistance well and correlates with the chloride SCC limit for the austenitic and duplex families. The ladder below runs from the weakest practical grade to the strongest, with the reason for each step.

Alloy (UNS)	Cr	Ni	Mo	N	Cu / other	PREN (typical)	Per standard
304 (S30400)	18.0-20.0	8.0-10.5	—	—	—	18-19	ASTM A240
316L (S31603)	16.0-18.0	10.0-14.0	2.00-3.00	0.10 max	—	24-26	ASTM A240
904L (N08904)	19.0-23.0	23.0-28.0	4.00-5.00	0.10 max	Cu 1.00-2.00	34-36	ASTM B625 / A240
2205 (S32205)	22.0-23.0	4.5-6.5	3.00-3.50	0.14-0.20	—	34-36	ASTM A240
2507 (S32750)	24.0-26.0	6.0-8.0	3.00-5.00	0.24-0.32	—	40-43	ASTM A240
254SMO (S31254)	19.5-20.5	17.5-18.5	6.00-6.50	0.18-0.22	Cu 0.50-1.00	42-44	ASTM A240
AL-6XN (N08367)	20.0-22.0	23.5-25.5	6.00-7.00	0.18-0.25	Cu 0.75 max	44-47	ASTM B688
Incoloy 825 (N08825)	19.5-23.5	38.0-46.0	2.50-3.50	—	Cu 1.50-3.00, Ti 0.6-1.2	29-31	ASTM B424
Inconel 625 (N06625)	20.0-23.0	58.0 min	8.0-10.0	—	Nb+Ta 3.15-4.15	50-52	ASTM B446 / B444
Hastelloy C-276 (N10276)	14.5-16.5	balance	15.0-17.0	—	W 3.0-4.5, Fe 4.0-7.0	65-70	ASTM B575 / B622

Table note: Composition limits are from the listed ASTM specifications. PREN is a calculated index, not a standard requirement, and is intended as guidance for ranking, not as an acceptance criterion.

Alloy (UNS)	Condition	UTS min	0.2% YS min	Elongation min	Hardness	Per standard
316L (S31603)	annealed	485 MPa	170 MPa	40%	95 HRB max (typical)	ASTM A240
904L (N08904)	annealed	490 MPa	220 MPa	35%	—	ASTM B625 / A240
2205 (S32205)	annealed	620 MPa	450 MPa	25%	~30 HRC max (typical)	ASTM A240
2507 (S32750)	annealed	795 MPa	550 MPa	15%	~34 HRC max (typical)	ASTM A240
254SMO (S31254)	annealed	650 MPa	300 MPa	35%	—	ASTM A240
AL-6XN (N08367)	annealed	690 MPa	310 MPa	30%	—	ASTM B688
Incoloy 825 (N08825)	annealed	586 MPa	241 MPa	30%	—	ASTM B424
Inconel 625 (N06625)	annealed	827 MPa	414 MPa	30%	—	ASTM B446
Hastelloy C-276 (N10276)	annealed	690 MPa	283 MPa	40%	—	ASTM B575
17-4PH (S17400)	H900 aged	1310 MPa	1170 MPa	10%	40 HRC min (typical 40-47)	ASTM A564

Table note: Room-temperature minimum values from the cited specifications; confirm against the current edition before design use. Hardness figures marked typical are orientation values, not guaranteed minima.

Alloy family	Typical chloride SCC threshold (typical)	Acid or concentrated chloride	Basis / test
304 / 316L	about 60 °C in neutral chlorides	Cracks below 60 °C with acid chloride or high oxygen	ASTM G36 screening
904L	typically above ~100 °C	Limited	ASTM G36 screening
2205 duplex	typically above ~120-150 °C	Risk in acid chloride	ASTM G36 / G48 screening
2507 super duplex	Above the 2205 limit	Better tolerance	published test data (typical)
6Mo: 254SMO / AL-6XN	High; used in hot seawater and brine	Limited	ASTM G48 screening
Incoloy 825	Very high; resistant in most chloride waters	Good	ASTM G48 / G28
Inconel 625	Effectively resistant in chloride waters	Good	ASTM G48
Hastelloy C-276	Resistant, including acid chloride	Best of the ladder	ASTM G48 / G28

Table note: Threshold temperatures are typical engineering values from published industry guidance and screening tests based on ASTM G36 boiling magnesium chloride; they are not standard acceptance limits and must be confirmed for the specific chloride chemistry, oxygen level and stress state.

Item	ASTM	AMS	ASME	GB (separate system)
Austenitic plate and sheet	ASTM A240	—	BPVC II-A	GB/T 4237
Austenitic bar	ASTM A276 / A479	—	BPVC II-A	GB/T 1220
Stainless heat-exchanger tube	ASTM A213 / A249	—	BPVC II-A	GB/T 13296
Duplex tube	ASTM A789	—	BPVC II-A	GB/T 21833
Titanium tube	ASTM B338	—	BPVC II-B	GB/T 3625
Ni-Cr-Mo tube (C-276)	ASTM B622	—	BPVC II-B	GB/T 15007
Incoloy 825 tube	ASTM B423	—	BPVC II-B	GB/T 15007
17-4PH bar	ASTM A564	AMS 5643	BPVC II-A	GB/T 1220

Table note: The GB column is from the Chinese national standard system and is shown for cross-reference only; GB values are not identical to ASTM values and must not be substituted directly.

Service condition	Recommended family	Avoid	Why
Seawater, ambient, low velocity	316L with velocity control, or 2205	304	Pitting and crevice risk
Seawater, ambient, high velocity	2507, 6Mo, titanium Gr 2	316L	Erosion-corrosion
Hot chloride water above 100 °C	Incoloy 825, Inconel 625	304, 316L	Chloride SCC
Acid chloride or HCl carry-over	Hastelloy C-276	All 300-series	Passive film breakdown
Caustic service	Nickel 200 or low-carbon steel with PWHT	High-strength martensitic	Caustic cracking
Ammonia-bearing	Carbon steel or stainless	Copper alloys	Ammonia cracking
Sour H2S service	Per NACE MR0175 limits	High-strength martensitic	Sulfide stress cracking
High-strength fastener in chloride	Monel K-500, Inconel 718	17-4PH H900	Hydrogen embrittlement plus SCC

Table note: Selection guidance is qualitative engineering judgment supported by the test methods listed in this article; every change must be verified against the actual chemistry, temperature and stress for the specific item.

Field Case Histories

Case 1 — A cooling-water exchanger that cracked after a water-source change. A petrochemical plant replaced its cooling-water source and, without re-checking the tube-side chemistry, kept a 316L U-bend bundle in service. Within a season the bundle leaked on hydrotest. Sectioning found transgranular branched cracking on the outside of the U-bends on the tension side, with no general corrosion and no wall loss; EDS on the deposit found chloride consistent with the new water source and a concentration pattern at the bend. The chloride level in the bulk water was unremarkable, but the surface temperature at the bend and the deposit had concentrated it well above the threshold. The fix was to move the bundle to duplex 2205 and to add a strainer and a cleaning cycle to remove the deposit. The direct cost was the replacement bundle and the lost production; the avoidable part was the missing chemistry review.

Case 2 — 17-4PH hardware that failed without corroding. A coastal site used high-strength 17-4PH fasteners on a seawater-exposed assembly fitted with sacrificial anodes. Several bolts failed weeks after commissioning, with no visible corrosion on the fracture faces and no wall loss on the shanks. Metallography and the service condition confirmed hydrogen embrittlement, with SCC contributing under the sustained preload. The fix was to change the alloy to Monel K-500 for the exposed fasteners and to record the condition on the drawing. The lesson is that high-strength martensitic hardware plus cathodic protection plus moisture is a known combination, and that the absence of corrosion products is a clue, not evidence of innocence.

Case 3 — Welded 316 that failed intergranularly, not transgranularly. A chemical plant reported a leak near a weld in a 316 spool. The initial assumption was chloride SCC, but the metallography showed intergranular attack a short distance from the weld rather than branching transgranular cracks, and a coupon test to ASTM A262 confirmed sensitisation. The correct fix was a material and welding change — a low-carbon or stabilised grade and correct filler — not an upgrade to a nickel alloy. The two failures would have looked similar to a plant engineer in a hurry and would have led to two completely different, and differently priced, corrective actions.

Table note: Case histories are anonymised summaries of investigations consistent with published failure-analysis practice using the test methods cited in this article; they are illustrative and are not a warranty of outcome for any specific installation.

Corrective Action and a Prevention Checklist

Corrective actions form a hierarchy, and the cheapest reliable fix is usually at the bottom of it. Change the design or the operating envelope first if you can, because changing the material is expensive and changing nothing is not an option after a failure.

1. Eliminate the concentrating mechanism. Remove the wet insulation, drain the stagnant pocket, eliminate the wet–dry cycle, strain the cooling water, add a cleaning cycle. This is the most durable fix when it can be achieved.
2. Reduce stress. Relieve weld residual stress where the service justifies it, correct misalignment, avoid unnecessary cold bending, and reconsider the expansion joint or support detail that loads the joint.
3. Control the environment. Reduce chloride ingress, deaerate, control pH, avoid chlorinated hydrotest water, and drain and dry after every test.
4. Apply the right heat treatment. For carbon and low-alloy steel in caustic or sour service, correct post-weld heat treatment is specific; for austenitic stainless, solution anneal or use a low-carbon/stabilised grade to avoid sensitisation. Note that stress relief of austenitic 300-series is not a routine operation and must not be assumed to fix SCC.
5. Change the alloy. Only after the cheaper actions are evaluated. Move up the PREN ladder to the lowest-cost grade that meets the verified envelope.

Thermal practice is a family-specific question, and its value in a failure analysis is that it separates the two things heat treatment can actually do: it can restore a microstructure the alloy was designed around, and it can remove residual stress. It cannot make an austenitic 300-series stainless resistant to chloride SCC, because the mechanism initiates at stresses well below yield; the idea that post-weld stress relief of 304 or 316 is the cure for chloride SCC is wrong, and a stress relief applied in the wrong range can sensitise the material instead. What thermal practice does control, decisively, is sensitisation and intergranular corrosion in the austenitic grades, the ferrite–austenite balance in the duplex grades, and the strength-versus-hydrogen-tolerance trade in the precipitation-hardening grades. The table below lists the thermal route that matters for each family together with the standard that governs how the result is verified.

Material family	Relevant thermal treatment	Purpose	Risk if the wrong thermal route is used	Basis / standard	Note
304 / 316L austenitic (S30400 / S31603)	Solution anneal at roughly 1040-1120 °C followed by rapid quench (typical mill practice)	Dissolve carbides and restore a uniform austenitic structure with a fully passive surface	Slow cooling through roughly 425-815 °C precipitates chromium carbides at the grain boundaries and sensitises the steel to intergranular corrosion	ASTM A240 (product); IGC verified to ASTM A262	Annealing temperatures are typical practice, not a single standard value; confirm the range in the current edition
Welded 304 / 316 fabrication	Low heat input, correct filler, L-grade or stabilised grade; avoid a stress-relief cycle inside the sensitisation range	Keep the heat-affected zone out of the carbide-precipitation window	A stress relief applied in the wrong range sensitises the HAZ and creates a new failure mode instead of removing one	ASME BPVC Section VIII (fabrication and PWHT rules); ASTM A262	Post-weld stress relief does not prevent chloride SCC and must not be presented as a cure for it
Carbon and low-alloy steel in caustic or sour service	Post-weld heat treatment to the temperature and time required by the applicable code and sour-service requirement	Relieve weld residual stress to prevent caustic and sulphide stress cracking	Under-treatment leaves cracking-level residual stress; over-treatment can degrade toughness	ASME BPVC Section VIII; NACE MR0175 / ISO 15156	Applies to caustic and H2S service; it is not a chloride SCC measure
2205 / 2507 duplex (S32205 / S32750)	Anneal in the duplex solution-annealing range and quench rapidly, with no slow cooling	Restore the balanced ferrite-austenite structure and dissolve intermetallic (sigma) phase	The wrong route leaves sigma phase or an unbalanced phase ratio, raising selective-attack and hydrogen-embrittlement risk	ASTM A789 (tube) / ASTM A240 (plate); ASTM A923 for detection of detrimental intermetallic phase	Annealing ranges are typical mill practice for each grade; confirm the range and the required testing in the current editions
17-4PH (S17400)	Solution treat then age: H900 for about 1310 MPa minimum UTS, or overage to H1150 for lower strength and greater hydrogen tolerance	Trade strength against stress-corrosion and hydrogen susceptibility	H900 in wet chloride, or under cathodic protection, is a known hydrogen-embrittlement combination; a drawing that states no condition is the commonest error	ASTM A564; AMS 5643	The condition, not the grade name, governs the risk; hardness limits for sour service per NACE MR0175 / ISO 15156
Inconel 625 (N06625)	Supplied in the annealed condition, with no cold-worked temper retained for chloride service	Keep the designed annealed structure and the full corrosion resistance of the alloy	Retained cold work and unrelieved fabrication strain raise SCC and hydrogen susceptibility	ASTM B446 (bar) / ASTM B444 (tube)	Shop cold forming in aggressive chloride duty should be followed by an appropriate anneal
Incoloy 825 (N08825)	Annealed (stabilised) condition; verify intergranular resistance after welding or hot forming	The titanium addition ties up carbon; confirm that the annealed condition is met after fabrication	Loss of intergranular corrosion resistance if the annealed condition is not restored	ASTM B424 (plate) / ASTM B423 (tube); ASTM G28 (intergranular corrosion test)	A coupon test on the fabricated item is the reliable check, not the certificate alone
Hastelloy C-276 (N10276)	Supplied mill-annealed; avoid sustained exposure in the range where intermetallic phases precipitate	Retain the annealed structure that gives the alloy its acid-chloride resistance	Intermetallic precipitation at the grain boundaries reduces corrosion resistance in the most aggressive service	ASTM B575 (plate) / ASTM B622 (tube); ASTM G28	Any precipitation range quoted for fabrication guidance is typical guidance, not a standard acceptance limit

Table note: Thermal routes are stated as typical mill and fabrication practice, and the governing requirement in every row is the standard cited in the Basis column (ASTM A240, ASTM A262, ASTM A564, ASTM A789, ASTM A923, ASTM B423, ASTM B424, ASTM B444, ASTM B446, ASTM B575, ASTM B622, AMS 5643, ASME BPVC Section VIII, NACE MR0175 / ISO 15156). Annealing and ageing ranges must be confirmed against the current edition of the applicable specification before they are written into a purchase order or a welding procedure.

A prevention checklist for owners and inspectors should include: a documented chloride and oxygen analysis for both sides of every exchanger; a metal-temperature check against the threshold for the installed grade; a review of any change to cooling-water source, chlorination or cleaning chemicals; a design review for stagnant pockets, dead legs and wet insulation; a check that tube ends and U-bends are not over-expanded or over-stressed; a schedule of eddy-current or penetrant inspection for susceptible services; verification of the material and condition against the drawing at every shutdown; and a standing rule that stressed high-strength fasteners are never used in wet chloride service. Our Hastelloy C-276 plate & bar and Incoloy 825 / 800H supplier pages describe the grades at the top of the ladder, and our related failure-analysis notes set out the inspection intervals we recommend for chloride-critical services.

Price Reference (2026, EXW Shanghai)

Grade	Tube (USD/kg)	Bar (USD/kg)	Plate (USD/kg)	Comment
304	6-10	4-7	4-7	Baseline austenitic
316L	9-14	6-10	6-10	Molybdenum addition
904L	20-30	16-25	16-25	High nickel and molybdenum
2205 duplex	14-20	10-16	10-15	Duplex step
2507 super duplex	22-32	18-28	18-28	Higher alloy duplex
254SMO / AL-6XN	28-40	24-36	22-34	6Mo super-austenitic
Incoloy 825	25-38	22-32	20-30	Ni-Fe-Cr, chloride SCC resistant
Inconel 625	45-65	40-60	38-58	Ni-Cr-Mo-Nb
Hastelloy C-276	55-80	50-75	48-70	Molybdenum-rich, acid chloride

Table note: Reference range only — 2026, EXW Shanghai, USD/kg — floats with nickel price. Actual quotation depends on form, size, quantity and delivery.

Standard Index

Standard	Title / scope	Covers	Form
ASTM G36	Stress-corrosion cracking in boiling magnesium chloride	SCC susceptibility, austenitic	coupon
ASTM G48	Pitting and crevice corrosion in ferric chloride	Localized corrosion resistance	coupon
ASTM G28	Intergranular corrosion of Ni-rich Cr-bearing alloys	Sensitisation, IGC	coupon
ASTM A262	Intergranular attack in austenitic stainless steels	Sensitisation, IGC	coupon
ASTM A923	Detecting detrimental intermetallic phase in duplex stainless steels	Microstructure, phase balance	test method
ASTM G39	Preparation and use of bent-beam SCC specimens	SCC under applied stress	specimen
ASTM G30	Making and using U-bend SCC specimens	SCC under residual stress	specimen
NACE TM0177	Metals resistant to sulfide stress cracking in H2S	SSC / SCC testing	specimen
ASTM G61	Cyclic potentiodynamic polarization measurements	Localized corrosion	specimen
ASTM E1476	Guide for metals identification and sorting (PMI)	Alloy verification	test method
ASTM E8/E8M	Tension testing of metallic materials	Mechanical properties	test method
ASTM E18	Rockwell hardness of metallic materials	Hardness	test method
ASTM E112	Determining average grain size	Microstructure	test method
ASTM A240	Cr and Cr-Ni stainless plate, sheet, strip	Composition and mechanical	plate, sheet, strip
ASTM A213 / A249	Seamless / welded alloy steel heat-exchanger tubes	Composition and mechanical	tube
ASTM A789	Duplex stainless tubing for general service	Composition and mechanical	tube
ASTM B163 / B622	Ni and Ni-alloy seamless condenser and HX tubes	Composition and mechanical	tube
ASTM B424 / B423	Ni-Fe-Cr-Mo-Cu (Incoloy 825) plate / tube	Composition and mechanical	plate, tube
ASTM B446 / B444	Ni-Cr-Mo-Nb (Inconel 625) bar / tube	Composition and mechanical	bar, tube
ASTM B575 / B622	Low-C Ni-Mo-Cr (Hastelloy C-276) plate / tube	Composition and mechanical	plate, tube
ASTM A564	Age-hardening stainless bar and shapes	Composition and mechanical	bar
AMS 5643	17-4PH bar, forging and tubing	Composition, heat treatment, mechanical	bar, forging, tube
NACE MR0175 / ISO 15156	Materials for use in H2S-containing environments	Sour service limits	all forms
ASME BPVC Section II / VIII / IX	Materials, pressure vessels, welding	Design and fabrication basis	all forms
GB/T 1220 / 4237 / 13296 / 3625	Chinese stainless and titanium product standards	Composition and mechanical	bar, plate, tube

Table note: Standards are listed with their stated scope. Where a Chinese (GB) standard is shown, it belongs to a separate system and its values are not interchangeable with ASTM values.

FAQ

Q1: What is chloride stress corrosion cracking?

Chloride stress corrosion cracking is a brittle, time-dependent failure of a susceptible alloy under tensile stress in a chloride-bearing aqueous environment. It produces branching cracks that can run through the wall with almost no general corrosion and no visible warning, so a component can look sound and leak without warning. Three conditions must coexist: a susceptible alloy, a chloride electrolyte with the right chemistry, and sustained tensile stress. Removing any one of them stops the mechanism. In austenitic 300-series stainless such as 304 and 316L the crack path is characteristically transgranular, with cracks cutting across grains and branching as they grow. In duplex grades it is often mixed, and in genuinely nickel-rich alloys such as Inconel 625 and Hastelloy C-276 the resistance is high enough that the mechanism is not usually the governing failure mode in chloride water service.

Q2: At what temperature does chloride SCC start in 316L?

The commonly quoted threshold for 316L and similar austenitic grades is about 60 °C, above which susceptibility to chloride SCC rises steeply in neutral chlorides. This is a practical engineering threshold rather than a sharp physical line, and it falls as chloride concentration, dissolved oxygen or acidity increase. In a very concentrated or acidic chloride, cracking can occur below 60 °C; in a dilute, deaerated, near-neutral water it may not occur until well above it. Because the threshold is not a standard acceptance value, the correct approach is to establish the metal temperature and the actual chemistry, then compare them against published guidance and, where the consequence of failure is high, against a screening test such as ASTM G36. Never assume that "316L is fine" simply because the bulk water temperature is below 60 °C — local concentration can defeat that assumption.

Q3: Why does 316L crack if it is the better stainless steel?

Molybdenum and lower carbon make 316L better than 304 at resisting pitting and crevice corrosion, and slightly better against chloride SCC, but they do not make it a hot-chloride alloy. The step from 304 to 316L is small compared with the step from 316L to duplex or to a nickel alloy. The frequent failure is not a material defect but a service-envelope error: a 316L tube operating at a metal temperature well above the threshold, under a deposit that concentrates chloride, with residual stress from a cold-formed bend. The material was appropriate for a moderate chloride duty and was then exposed to an aggressive one. The correct analysis separates the material question from the operating question, and the fix may be an operating change, a design change or an alloy change rather than simply blaming the grade.

Q4: Can 2205 duplex suffer chloride stress corrosion cracking?

Yes, but at a much higher temperature and chloride level than 316L. Duplex 2205 is far more resistant than the austenitic grades and is a common upgrade for chloride-bearing water, but it is not immune. Failures are reported in hot, concentrated chlorides, typically above about 120-150 °C, and in acidic chloride with high oxygen. The crack path in duplex is often mixed or partly intergranular rather than cleanly transgranular, which makes the metallographic interpretation different from an austenitic failure. Duplex is also sensitive to hydrogen embrittlement at high hardness — particularly in a poorly controlled weld or heat-affected zone — and to sulfide stress cracking in sour service. When a 2205 failure is found, the analysis must confirm the ferrite-austenite balance and the hardness of the affected zone, as well as the crack path.

Q5: Why is 17-4PH dangerous in seawater?

In the H900 condition, 17-4PH reaches about 1310 MPa minimum tensile strength per ASTM A564, and at that strength it has very little tolerance for absorbed hydrogen. In seawater, and especially under cathodic protection where atomic hydrogen is generated at the surface, it fails by hydrogen embrittlement, often with no visible corrosion and often soon after commissioning. Sustained tensile stress in a hot chloride adds classic SCC on top of the hydrogen mechanism. The practical rules are: never specify H900 hardware for immersed chloride or cathodically protected service; move to a lower-strength overaging condition such as H1150 if the martensitic family must be retained; or change the alloy to Monel K-500 or Inconel 718 where strength and chloride resistance are both required; and always record the condition on the drawing.

Q6: Do dry chlorides cause stress corrosion cracking?

No. Dry chloride salts, dry dust or dry deposits do not crack stainless steel because there is no electrolyte to carry the electrochemical reaction and no cathodic reactant to drive dissolution. Chloride SCC is an aqueous process and needs water — even a condensed film — plus dissolved oxygen or another cathodic reactant. This is why failures cluster at wet-dry transitions, under wet insulation and in stagnant pockets where moisture condenses and chloride concentrates. It also explains a frequent field surprise: equipment that never saw a chloride process stream can still crack after a hydrotest with chlorinated water that was not fully drained and dried. The practical implication is that controlling moisture and drainage is a genuine engineering countermeasure, not housekeeping.

Q7: How is chloride SCC different from pitting and crevice corrosion?

Pitting and crevice corrosion are localized loss-of-metal mechanisms: the passive film breaks down in a small area and a pit or a shielded gap deepens, removing wall thickness. Chloride SCC is a cracking mechanism: the metal separates by a brittle crack under stress, often with negligible thickness loss. The two are related, because a pit or crevice is usually where SCC initiates, and both respond to the same alloying elements — chromium, molybdenum and nitrogen, summarized by PREN. But the diagnostic signatures differ, the inspection methods differ, and the corrective actions differ. A pit is found by wall-thickness measurement and by the deposit that forms over it; an SCC crack is found by penetrant, eddy current or sectioning, and it may be invisible to a thickness survey because the wall is intact until the crack breaks through.

Q8: Which tests compare alloys for chloride SCC resistance?

The classic screening test is ASTM G36, which uses boiling magnesium chloride solution to rank austenitic stainless steels by their resistance to stress-corrosion cracking under stress. Pitting and crevice resistance are compared with ASTM G48 in ferric chloride, which is also the basis for many PREN correlations. Intergranular corrosion and sensitisation are assessed with ASTM A262 for austenitic stainless and ASTM G28 for nickel-rich chromium-bearing alloys. Stress-applied specimen designs are standardized in ASTM G39 for bent beams and ASTM G30 for U-bends. Sour-service cracking is tested to NACE TM0177, and the acceptance limits for H2S service are set by NACE MR0175 / ISO 15156. These tests rank candidate materials; they do not replace testing the specific heat in the actual chemistry.

Q9: How do I verify the material before ordering a replacement?

Verify chemistry, condition and heat identity, not just the grade name. Optical emission spectrometry or portable PMI to ASTM E1476 confirms the actual composition, which is the only reliable way to detect a substitution — for example a 304 tube supplied as 316L, or a 15-5PH bar supplied as 17-4PH. Hardness testing to ASTM E18 confirms the heat-treatment condition, which matters most for martensitic and precipitation-hardening grades. Reconcile the heat number against the mill certificate and, where required, an EN 10204 3.1 or 3.2 certificate, and photograph the marking into the batch record. For duplex materials, confirm the phase balance and the ferrite content as well. A certificate alone is not verification; the material must be checked against it.

Q10: What is PREN and can I rely on it?

PREN, the pitting resistance equivalent number, is calculated from the chromium, molybdenum and nitrogen content of the alloy and is used to rank resistance to pitting and crevice corrosion. It correlates reasonably well with the chloride SCC limit across the austenitic and duplex families, which is why it is a convenient ladder index. It is a calculated value, not a standard acceptance criterion, and different formulas weight nitrogen differently, so the same alloy can have slightly different quoted PREN values. Use PREN to compare grades and to place a candidate on the ladder, but confirm the actual behaviour in the specific chemistry and temperature with testing where the consequence of failure is high. Never use PREN as a substitute for a service-envelope check against published limits.

Q11: What is the cheapest reliable fix for a chloride SCC problem?

The cheapest reliable fix is usually to remove the concentrating mechanism or to reduce the metal temperature, not to change the alloy. Draining a stagnant pocket, removing wet insulation, eliminating a wet-dry cycle, straining the cooling water, adding a cleaning cycle, or lowering the operating temperature can all move a component out of the cracking envelope at low cost. Reducing stress — correcting misalignment, avoiding unnecessary cold bending, relieving weld residual stress where it is applicable — is the next step. Changing the alloy is effective but usually the most expensive option, and it should come after the cheaper actions have been evaluated. Only when the envelope cannot be changed should the design move up the PREN ladder to the lowest-cost grade that meets the verified service condition.

Q12: Can stress relief prevent chloride stress corrosion cracking?

Not in the way it prevents caustic or sour-service cracking in carbon and low-alloy steels. Chloride SCC initiates at stresses well below yield, so simply reducing stress does not eliminate the mechanism unless the stress is removed almost entirely, which is rarely practical. Post-weld heat treatment is a specific and effective measure for carbon and low-alloy steel in caustic and sour service, where the applicable code and NACE requirements define the treatment. For austenitic 300-series stainless, stress relief is not a routine fix — it can cause sensitisation if done wrongly, and solution annealing is the correct heat treatment where the aim is to restore the microstructure. Reducing stress helps, but it is a contribution, not a cure; removing the chloride or the moisture is far more decisive.

Q13: Are caustic and ammonia cracking the same as chloride SCC?

No, and confusing them leads to the wrong corrective action. Caustic cracking occurs in alkaline (caustic soda) service, concentrates at welds and heat-affected zones under high residual stress, and is managed by correct post-weld heat treatment and alloy selection for the temperature and concentration involved. Ammonia cracking affects copper alloys and some steels in ammonia-bearing environments, particularly with oxygen and moisture present, and is a reason to keep copper-based tubing out of ammonia service. Chloride SCC is an aqueous chloride mechanism with its own temperature and concentration thresholds. The correct order of investigation is to confirm the process chemistry first, then interpret the crack path, because a caustic crack is not fixed by removing chlorides, and an ammonia crack is not fixed by upgrading to a more corrosion-resistant stainless.

Conclusion: The Selection Rule

The rule that prevents almost every chloride stress corrosion cracking failure is short: define the service envelope first, then choose the material from it. Establish the metal temperature, the chloride concentration and its concentrating mechanism, the dissolved oxygen and pH, and the actual stress state at the joint — then compare that envelope against the published limit for each candidate alloy and select the lowest-cost grade that sits safely inside it. Do not choose the alloy from the grade name alone, do not assume 316L is safe because the bulk water is cool, and do not specify a precipitation-hardening stainless without the heat-treatment condition. Where the envelope is uncertain, test the candidate alloys with the standard methods listed above rather than relying on judgment. Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier supplies the full upgrade ladder, from 316L and duplex through 904L, 6Mo super-austenitic, Incoloy 825, Inconel 625 and Hastelloy C-276, with full certification.

To discuss a specific failure or to select a replacement grade, send the process condition and the failed part to sales@hangboalloy.com, contact us on WhatsApp (Lisa) at +86 13611656360, or use our contact page to request a material recommendation and a quotation.

Contact & Complete Product Range

Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier

Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com

All Grades

Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA

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