Chloride Stress Corrosion Cracking in Stainless Equipment

Short answer: Chloride stress corrosion cracking (Cl-SCC) is branched, surface-initiated cracking of 300-series austenitic stainless steels. It needs three things together: chlorides, tensile stress and enough temperature. In fully immersed service it is rare below about 60 °C (140-150 °F). Under evaporating or wet-insulation conditions, chlorides concentrate, and cracking can then start at lower bulk temperatures and lower chloride levels. Penetrant testing and eddy current find surface cracks. PAUT finds and sizes cracks that start on the inside surface.

This guide is written for refinery, chemical-plant and pharmaceutical mechanical-integrity engineers and inspectors in the US and Canada. It covers where Cl-SCC occurs, how API RP 571 describes it, how it differs from look-alike mechanisms, and which NDE methods find it reliably. It also covers what usually goes wrong in inspection plans for stainless equipment. Materials selection, acceptance and repair decisions belong to the owner and the owner's API 510 or API 570 inspector. Where cracking has to be assessed rather than simply removed, the owner may commission a fitness-for-service engineering assessment. Atlantis does not offer that assessment.

What chloride SCC is and how API RP 571 frames it

API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, lists chloride stress corrosion cracking as an environmental cracking mechanism. In its description, the cracking starts at the surface of 300-series stainless steels and some nickel-based alloys exposed to tensile stress, temperature and an aqueous chloride environment. The current edition is the third (2020). It reorganised the document, so section numbers in older study guides and many web pages no longer match. Look the mechanism up by name in your licensed copy rather than by clause number.

The cracks are usually transgranular and highly branched, often described as a "crazed" or lightning-bolt pattern under magnification. On the surface they can be very fine, and they are often hidden under deposits, scale or coatings. Cl-SCC can go through the wall without much general corrosion around it. A leak, or a weep under insulation, is often the first sign. That is why thickness surveys alone, which are the backbone of most API 510/570 programmes, do not detect it.

The Specialty Steel Industry of North America (SSINA) lists the same drivers from the materials side. Susceptibility goes up with temperature, chloride concentration and tensile stress, and goes down at higher pH. API RP 571 also treats oxygen and alloy composition as critical factors. Remove any one leg (no chloride, no tensile stress, or temperature kept low) and the risk falls sharply. That is the logic behind every mitigation discussed below.

Temperature, chloride level and alloy: the numbers that are actually published

Web pages often quote very specific thresholds. Here is what the public authoritative sources actually say.

• Temperature. SSINA states that for stainless steels fully immersed in chloride solutions, Cl-SCC is rare below 60 °C (150 °F). API RP 571 study material commonly cites a metal temperature of about 60 °C (140 °F) as the usual lower bound in process service. Both sources warn that this is not a hard floor. Evaporation, crevices and heat transfer surfaces can produce cracking well below it.
• Chloride concentration. SSINA reports failures with as little as 10 ppm chlorides in the bulk water. Evaporation at wet/dry interfaces can concentrate a few ppm in the bulk liquid into hundreds of ppm at the surface. Water that "meets spec" can therefore still crack equipment if it is allowed to dry out on hot stainless.
• Alloy. SSINA ranks the standard austenitic grades (304/304L, 316/316L, with roughly 8-10% nickel) as the most susceptible. Higher-nickel austenitics such as Alloy 20, 904L and the 6% molybdenum super-austenitics are much more resistant. Duplex stainless steels have intermediate to high resistance. Ferritic grades such as 430, 439 and 444 show excellent resistance. The low-carbon "L" grades do not make 304L or 316L resistant to Cl-SCC. The "L" addresses sensitisation, which is a different problem.

Treat any single number as a screening value and not as a design limit. The owner's damage-mechanism review should record the actual metal temperature range, including excursions, start-ups and steam-outs, and the actual chloride sources for each circuit.

Where Cl-SCC shows up in refineries and chemical plants

Most real Cl-SCC findings fall into a handful of situations.

• External SCC under insulation. This is the most common case in North American plants. Rain, deluge or cooling-tower drift soaks insulation that leaches chlorides, or chlorides come in from the atmosphere near the coast. Hot stainless pipe or vessel walls then evaporate the water and concentrate the chloride. It is the stainless steel branch of corrosion under insulation. API RP 583 covers it alongside carbon steel CUI. ASTM C795 specifies thermal insulation for use in contact with austenitic stainless steel, which limits leachable chlorides relative to inhibiting silicate and sodium ions.
• Cooling-water heat exchangers. Stainless tubes or shells exposed to cooling water, especially where deposits form, flow is low or stagnant, or tube-to-tubesheet crevices exist. The hot side of the metal surface is where the risk is highest.
• Hydrotest water left in equipment. Stainless vessels or piping hydrotested with water that is not controlled for chlorides, then left wet or allowed to dry slowly, can crack during the shutdown or soon after start-up. Many owners specify low-chloride test water and prompt draining and drying. Check your owner specification for the limit.
• Process streams with chloride carryover. Examples are overhead systems, water-wash points, and units where chloride slips through desalting or catalyst regeneration and condenses on stainless surfaces.
• Bellows and expansion joints. They combine thin stainless walls, high residual stress from forming, and crevices.
• Welds and cold-worked areas. Residual stresses from welding, bending and forming supply the tensile stress, so cracking clusters at heat-affected zones, attachment welds and bends.

The external case deserves particular attention because it is usually invisible until the insulation comes off. For the insulation side of the problem, see our corrosion under insulation inspection page and the CUI monitoring guide.

Look-alike mechanisms: do not mix them up

Several other cracking mechanisms attack stainless steel. They have different causes, different mitigations and sometimes different inspection timing. Getting the mechanism right matters for the inspection plan.

If a crack report says only "SCC", ask which one. Metallographic confirmation (a field replica or a boat sample examined by a laboratory the owner engages) is how branched transgranular cracking is told apart from intergranular cracking. That answer drives the fix. For a broader map of mechanisms, see our SCC types overview and the API 571 reference page.

NDE methods for chloride SCC: what each finds and where it struggles

Because Cl-SCC starts on the surface exposed to chlorides, the first question is which surface. For external SCC under insulation, the cracks are on the outside surface, which you can reach once insulation is removed. For process-side or cooling-water SCC, the cracks start on the inside surface. They are reached either from inside during an entry or by volumetric methods from outside.

Magnetic particle testing does not work on austenitic stainless steel because the material is not ferromagnetic. If an inspection plan for 304/316 equipment lists MT for crack detection, it was copied from a carbon steel template. That is a common audit finding. Penetrant materials used on austenitic stainless also need controlled contaminant (halogen and sulfur) content. ASME Section V requires certification of penetrant contaminant content when it is used on these alloys, so check the batch certificates.

For method detail, see penetrant testing in refining, eddy current tube inspection and phased array inspection services. For the general logic of matching methods to damage, see the NDE method selection by damage mechanism guide.

How the inspection is applied in practice

A workable Cl-SCC inspection plan under API 510 or API 570 usually follows this sequence.

• Screen the population. List the austenitic stainless equipment and piping, its operating and excursion temperatures, whether it is insulated, its exposure to deluge, rain, coastal air or cooling-tower drift, and known process chloride sources. The mechanism review assigns Cl-SCC susceptibility per circuit or item.
• Pick the right surfaces. On insulated lines, focus on places where water gets in and stays: low points, supports, penetrations, damaged jacketing, insulation rings on vertical runs, and areas below leaking flanges. On exchangers, focus on hot-end tubes and deposit zones.
• Prepare the surface properly. For PT, remove deposits with methods that do not smear metal over crack openings, and use stainless-only brushes and tools so the surface is not contaminated with carbon steel. Fine SCC is easy to miss on poorly cleaned surfaces.
• Size what you find. PT and ET locate the cracks. PAUT or other qualified UT techniques, or progressive grinding with repeat PT where the owner's procedure allows it, establish depth. The inspector of record needs both length and depth.
• Feed results back. Findings go back to the mechanism review: whether to extend the inspection to similar circuits, change insulation and coating practice, or change material. Crack findings are not given a corrosion rate. They change the inspection strategy and may lead the owner to order an engineering assessment or a replacement.

Worked example (qualitative). During a turnaround, a 316L line running at around 90 °C under calcium silicate insulation shows rust-coloured staining at a pipe support. Insulation removal reveals wet insulation and a fine branched crack pattern next to a support weld. PT confirms a cluster of indications. PAUT from the outside shows the cracks are shallow. The inspector records the findings and expands the inspection to other supports on the same circuit and on similar insulated stainless lines. The owner then decides on repair or replacement, re-insulation with insulation suited to stainless steel, and adding a protective coating or foil barrier under the new insulation. Industry CUI guidance describes these barriers as a recognised mitigation for stainless steel.

Mitigation and design measures the inspection plan should check

Mitigation belongs to the owner and the engineering function, but inspectors should verify that it is in place:

• Insulation specified for austenitic stainless (ASTM C795-compliant) and jacketing that actually sheds water.
• Coatings or foil wrapping under insulation on susceptible stainless surfaces.
• Control of hydrotest water chloride and prompt draining and drying.
• Avoiding chloride-containing cleaning agents, markers and tapes on stainless.
• Stress relief or design changes at high-stress details where appropriate.
• Upgrading to duplex or higher-alloy materials where the environment cannot be controlled. SSINA's ranking shows these alloys are far more resistant.

Each of these leaves something you can see, and the external visual inspection is where gaps are found.

Jurisdiction and programme overlay (US and Canada)

Cl-SCC itself is not regulated separately. It is handled through the mechanical-integrity programme:

• OSHA PSM (29 CFR 1910.119(j)). Covered processes need inspection and testing that follow recognised and generally accepted good engineering practice. API 510, API 570 and API RP 571 are the usual references.
• State boiler and pressure-vessel laws, and the NBIC. These may apply to vessels in some jurisdictions.
• Canada. Pressure equipment falls under provincial safety authorities such as ABSA in Alberta and TSSA in Ontario, each with its own integrity-management requirements. Confirm with your jurisdiction.
• California refineries. They also face process safety rules that require damage-mechanism reviews, a requirement introduced after the 2012 Chevron Richmond fire. Cl-SCC is one of the mechanisms those reviews have to consider.

Related guides: SCC detection methods and CUI detection guide.

How Atlantis supports chloride SCC inspection

Atlantis NDT performs the NDE that Cl-SCC inspection plans call for, on equipment and circuits the owner has identified. ASNT-certified technicians under ASNT Level III oversight carry out PT (colour-contrast and fluorescent, with contaminant-controlled materials for stainless), eddy current and eddy current array screening, exchanger tube eddy current, PAUT for ID-initiated cracking and depth sizing, and visual examination during CUI campaigns. The Level III selects techniques suited to austenitic welds. Results are reported by location, with length and depth, so the owner's API 510/570 inspector can evaluate them. Atlantis is not the inspector of record and does not make materials, repair or fitness-for-service decisions. See pressure vessel inspection services. Quotes are returned within 24 hours. Request Cl-SCC inspection support.

Frequently asked questions

What temperature does chloride stress corrosion cracking occur at?

In fully immersed service it is rare below about 60 °C (140-150 °F), according to SSINA and API RP 571 descriptions. Under evaporating conditions or wet insulation, chlorides concentrate and cracking can occur at lower bulk temperatures.

Which stainless steels are most susceptible to chloride SCC?

The standard austenitic grades 304/304L and 316/316L are the most susceptible. Duplex grades, higher-nickel austenitics such as Alloy 20 and 904L, the 6% Mo super-austenitics, and ferritic grades are much more resistant.

How much chloride causes stress corrosion cracking?

SSINA reports failures with as little as 10 ppm chlorides where evaporation concentrated them. No bulk chloride level is safe if water can dry out on hot stainless.

What is the best NDT method for chloride stress corrosion cracking?

On accessible surfaces, penetrant testing after thorough cleaning, or eddy current, for detection. PAUT for inside-initiated cracks and for depth sizing. Radiography often misses tight branched SCC, and magnetic particle testing does not work on austenitic stainless.

Can magnetic particle testing find SCC in stainless steel?

Not in austenitic (300-series) stainless, which is non-magnetic. Use PT or ET instead. MT is used for cracking in ferromagnetic steels.

What is external chloride SCC under insulation?

Cracking on the outside of insulated stainless equipment. It happens when water carrying chlorides from the insulation, the atmosphere or deluge systems evaporates on the hot surface. API RP 583 covers it with carbon steel CUI.

What does chloride SCC look like?

Fine, highly branched, mostly transgranular cracks, often under deposits or staining. Metallographic examination confirms the branched transgranular pattern.

Is 316L immune to chloride stress corrosion cracking?

No. The "L" grade reduces sensitisation but does not give Cl-SCC resistance. 316L is among the susceptible grades.

How is chloride SCC different from polythionic acid SCC?

Cl-SCC is transgranular and needs aqueous chlorides. Polythionic acid SCC is intergranular, attacks sensitised stainless, and forms during shutdowns when sulfide scale meets air and moisture.

Does API 570 require inspection for chloride SCC?

API 570 requires the inspection plan to address the damage mechanisms that apply to each circuit, using API RP 571 as the reference. If Cl-SCC is credible, the plan must include suitable crack-detection methods. Thickness readings alone are not enough.

Ask a Level III to review your stainless CUI scope or get a quote for PT, ET and PAUT on stainless equipment.