Why Vessel Corrosion Under Desiccant Beds Is the Specific Failure Mode That Matters

Three factors converge inside an adsorption dryer to make the bottom of the vessel the highest-risk corrosion location in the entire air system:

Factor 1 — chloride concentration in the desiccant phase. Molecular sieve and activated alumina both carry chloride from the manufacturing process (typically 50-200 ppm Cl^- on a dry-weight basis for fresh sieve, dropping to 20-50 ppm after the first regeneration cycle). Process-side chloride is added whenever inlet air carries HCl (compressor intake near a chlor-alkali plant, refrigeration system venting near the dryer, or combustion gas from PVC-containing waste streams). At the bottom of the vessel, the chloride concentrates in the first 100-300 mm of bed as the inlet air saturates and any entrained moisture wicks downward.

Factor 2 — wet-dry cycling at the support screen. Every regeneration cycle, the screen-to-shell interface goes from 250-350 degC wet (steam purge) to 30-50 degC dry (adsorption). This cycling pumps moisture into the crevice between the screen weld and the vessel shell, where it sits in liquid contact with the metal for hours at a time. Crevice corrosion rates in 304L under these conditions reach 0.5-1.5 mm/year — enough to perforate a 6 mm shell in 4-12 years.

Factor 3 — stress concentration at the support screen weld. The screen-to-ring weld is the most highly stressed location in the vessel. Residual welding stresses provide the driving force for chloride SCC, which can crack through a 12 mm shell in 18-36 months once the corrosion cell is established.

The combination is unique to adsorption dryers: the corrosion cell lives in a location that is (a) geometrically shielded from routine UT inspection, (b) physically covered by the desiccant bed (which must be removed for visual or coupon access), and (c) electrically isolated from any external cathodic protection. A 2018 API 571 damage mechanism review identified desiccant-bed chloride pitting as the third-most-common cause of pressure vessel failure in refinery and petrochemical air systems, behind only external atmospheric corrosion and internal HIC in wet H₂S service.

The Three Chloride Sources: Manufacturing Residuals, Process Carryover, and Regeneration Steam

Every chloride pitting incident on a desiccant-bed vessel can be traced to one of three sources, or a combination:

Source 1 — chloride residuals in fresh desiccant. Molecular sieve manufacturing uses ion-exchange and washing steps that do not remove 100 percent of the chloride from the binder. Typical fresh-sieve chloride loadings are 50-200 ppm for type 4A and 13X grades. Activated alumina typically carries 30-100 ppm chloride as NaCl residue from the precipitation step. These residuals leach into the first condensate during the first regeneration cycle and concentrate at the bottom of the vessel.

Source 2 — process-side chloride carryover. The single largest chloride source in most installations is the inlet air. Sources include:

• Compressor intake near marine or industrial atmosphere: coastal installations, chemical plants, refineries. Atmospheric chloride concentrations of 50-200 microg/m³ are common near coastlines; this chloride enters the inlet air as airborne salt aerosol.
• Compressor intake downstream of HCl / chlor-alkali / PVC operations: HCl vapor concentrations of 0.5-5 ppm in the intake air are routine in chemical plants. Each 1 ppm HCl in the inlet stream deposits approximately 1.6 mg Cl^- per Nm³ of air into the desiccant bed over the adsorption cycle.
• Lubricant carryover from oil-flooded compressors: reciprocating compressors using chlorinated paraffinic lube oils deposit trace chloride on the desiccant as the oil breaks down. Sulfur-chlorinated EP additives in compressor oils are a particularly aggressive source.
• Refrigeration system venting: if the dryer is downstream of a refrigerant-cooled aftercooler, trace refrigerant (especially R-22, R-134a, or older CFC blends) can decompose at compressor discharge temperatures to release HCl.

Source 3 — chloride in regeneration steam or purge air. Many dryers regenerate with plant steam. Steam from boilers using chloride-contaminated feedwater or from co-gen plants with deionization-system upsets can carry 5-50 ppb chloride as dissolved NaCl or HCl. Over years of operation, this chloride accumulates in the condensate pool at the bottom of the vessel and under the support screen, where it concentrates by evaporation between regeneration cycles.

The three sources are not mutually exclusive: a typical failure case shows 20 percent contribution from sieve residuals, 60 percent from process intake, and 20 percent from steam chloride. Mitigations must therefore address all three, not just one.

Why 304L Stainless Fails Where 316L Sometimes Survives: A Material Selection Comparison

Selecting the correct vessel material is the single highest-impact decision in adsorber vessel corrosion control. The trade-off is straightforward: 316L costs 25-50 percent more than 304L upfront, but the lifecycle cost difference is often negative because 316L avoids the 10-15 year emergency replacement cost that 304L incurs.

The critical numbers in this table are PREN (Pitting Resistance Equivalent Number, calculated as %Cr + 3.3 x %Mo + 16 x %N) and the chloride SCC threshold. PREN below 25 is unsuitable for any application where chloride concentration in the condensate can exceed 50 ppm — which is essentially every chloride-loaded dryer. 316L sits at PREN 25, which is the minimum acceptable threshold; for installations with documented chloride exposure above 200 ppm, alloys with PREN > 35 (904L, AL-6XN, Inconel 625) become the correct choice despite the cost premium.

The 304L chloride SCC threshold of ~50 ppm is the number that surprises most procurement engineers. 304L is widely sold as "the corrosion-resistant stainless," but in chloride-bearing service at modestly elevated temperature it stress-corrosion-cracks at chloride concentrations that are routine in any process plant with trace atmospheric or process chloride. 316L raises that threshold by 4x. That is the difference between "vessel cracks in 8 years" and "vessel survives 25 years" in the typical adsorber application.

Liner Options for Existing Carbon Steel or 304L Vessels: RB, FBE, PTFE, and Alloy Overlay

When the vessel is already installed and material upgrade is not feasible (replacement cost, downtime, or piping geometry), the correct answer is a corrosion-resistant liner. The four commercial options each address a different combination of temperature, chloride concentration, mechanical abuse, and cost:

The four liner options trade off temperature capability, chloride resistance, mechanical durability, and cost. For a typical heatless air dryer operating at 30-50 degC bed temperature with peak 80 degC during regeneration, RB lining at 4-6 mm is the dominant commercial choice — it handles the chloride exposure, tolerates the desiccant mechanical loading, and is the lowest-cost option. The critical installation detail is vacuum testing at 10-15 kPa after lining to detect any through-porosity in the rubber; a single pinhole is enough to drive localized pitting of the substrate steel within 18-24 months.

For higher-temperature service (heated-purge or externally-heated regeneration dryers running 150-200 degC), RB is unsuitable and the choice is between FBE and PTFE. FBE is the lower-cost option but requires strict surface preparation (SSPC-SP10 near-white blast, profile 75-125 microns) and 100 percent holiday detection at 1500 V DC per ASTM D5162. A single missed pinhole in FBE becomes a corrosion initiation site within 6-12 months.

For vessels too large to remove and reline, alloy 625 weld overlay on the bottom head and the lower 300-500 mm of the shell is the correct answer. The overlay is applied as 3-4 mm of Inconel 625 weld metal (ERNiCrMo-3) using GTAW with minimum 2-layer overlay (to ensure no dilution from the substrate carbon steel reaches the process surface). After overlay, the surface must be solution-annealed at 1040-1080 degC and passivated per ASTM A380. Cost is the limiting factor — overlay typically runs 4-6x the cost of RB lining and is therefore reserved for vessels larger than ~2.5 m diameter where in-place lining is impractical.

Inspection Protocol: When to Use UT, MPI, PT, and Coupon Monitoring

Inspection of the bottom head and lower-shell area requires a method that can see through the desiccant bed and reach the shell from the inside. External UT is often blocked by the support ring or insulation; internal inspection requires bed removal. The decision matrix below selects the right method by inspection trigger and access situation:

The most commonly missed inspection trigger is the outlet filter differential pressure trend. A rising DP over 3-6 months is the first externally-visible signal that either (a) sieve dust is migrating through the support screen (mechanical failure), or (b) corrosion product from the bottom head is migrating up through the bed (corrosion failure). Both require the same response: bed removal and internal inspection within the next scheduled turnaround.

Coupon monitoring is the underused third tool. A pre-weighed carbon steel or 304L coupon installed in the condensate pocket under the support screen, retrieved annually and re-weighed, provides a direct corrosion rate measurement in mm/year. If the measured rate exceeds 0.1 mm/year on 304L or 0.05 mm/year on 316L, you have a quantitative case for either material upgrade or liner installation before the next planned turnaround.

The 9-Step Economic Decision: Replace, Reline, or Accept the Risk

The economic decision between full vessel replacement, in-place relining, and accepting continued corrosion risk is driven by vessel age, current wall thickness, exposure severity, and remaining service life of the connected plant. The following nine-step decision sequence is the framework used by most fixed-equipment groups for this call:

1. Step 1 — Confirm the corrosion mechanism: pitting vs SCC vs general corrosion. Different mechanisms drive different replacement economics. Pitting allows relining; SCC mandates replacement.
2. Step 2 — Measure remaining wall thickness on a 20-point grid across the bottom head and lower shell. Compare to the original + corrosion allowance. If remaining wall is below ASME Section VIII minimum, immediate replacement.
3. Step 3 — Characterize the chloride exposure via inlet air sampling (3-day average), steam chloride analysis, and sieve chloride analysis on a fresh sample. Quantify the driving force.
4. Step 4 — Estimate remaining service life using measured corrosion rate (from coupons or previous UT) and exposure trend. If <5 years remain, plan replacement in next turnaround.
5. Step 5 — Evaluate the relining option: vessel diameter, internal geometry, access for surface prep. Vessels >2.5 m diameter typically require in-place work; <2.5 m diameter can be removed and sent to a rubber-lining shop.
6. Step 6 — Get budgetary pricing for both options: relining (4-8 week shop time + 2-3 week install) vs replacement (16-24 week fab + 4-6 week install). Compare to cost of unplanned outage (typically $50k-200k per day for refinery-scale air systems).
7. Step 7 — Decide on material upgrade scope: if replacing, is 316L sufficient or does the exposure justify 904L or Inconel 625 clad? The cost delta is significant but the service-life extension is 10+ years.
8. Step 8 — Commit to a 10-year inspection plan that includes annual filter DP trending, coupon retrieval at every turnaround, and a planned internal inspection at year 10 for 304L or year 15 for 316L.
9. Step 9 — Document the decision basis: a 2-page memo in the equipment file capturing exposure severity, measured wall loss, options evaluated, and the chosen path. This memo is the basis for any future warranty or insurance claim.

The most common mistake in this decision is to skip Step 1 and assume the corrosion is general wall loss when it is actually chloride SCC. SCC requires full vessel replacement because the cracking can extend through the wall in areas where UT thickness readings still show acceptable wall — the cracking is too tight for UT to detect reliably, and it propagates rapidly once the corrosion cell is established. MPI or PT inspection of suspect welds is mandatory before committing to a relining-only path.

Five Procurement Pitfalls That Drive 30 Percent Vessel Cost Overruns

The five most common procurement mistakes in replacement or relining projects all stem from inadequate specification at the RFQ stage:

Pitfall 1 — Specifying 304 "stainless" without the L (low-carbon) designation. Standard 304 has 0.08 percent max carbon, which precipitates chromium carbide at 450-850 degC during welding. Sensitized 304 is dramatically more susceptible to intergranular corrosion in chloride service. Specify 304L or 316L (both 0.03 percent max C) for chloride-loaded service.

Pitfall 2 — Forgetting the pickling and passivation step in the RFQ. New vessels ship with embedded iron, weld discoloration, and heat-tint scale — corrosion initiation sites in chloride service. ASTM A380 pickling (HNO₃ + HF bath) and passivation (HNO₃ bath) must be in the RFQ and verified by water-break test before vessel acceptance.

Pitfall 3 — Specifying rubber lining thickness without the test method. "6 mm RB lining" is meaningless without specifying ASTM testing: D5162 for holiday detection at 1500 V DC, D1415 for hardness, D412 for tensile. The rubber shop quotes to the cheapest interpretation unless the test methods are in the spec.

Pitfall 4 — Skipping the support screen design review. The support screen is the most highly stressed and corroded component in the vessel, but is often specified by the vendor as a "standard" detail. Screen-to-shell weld geometry, screen-to-ring weld procedure, and screen open area (30-40 percent) all need explicit engineering review. Poor screen design concentrates corrosion at the weld interface and shortens vessel life by 30-50 percent.

Pitfall 5 — Underestimating bed removal cost. Removal takes 8-16 hours with a vacuum truck. Chloride-contaminated spent sieve disposal runs $500-1500 per ton. Include in the economics.

Six Signals That Mean You Need to Act This Turnaround, Not the Next One

Six inspection findings should trigger immediate vessel replacement or relining, regardless of the planned turnaround schedule:

1. Filter DP has risen 50 percent over baseline in the last 6 months with no change in operating conditions. This indicates either sieve attrition or corrosion product migration; either way, the bottom of the vessel needs visual inspection.
2. Corrosion product (red-brown or black oxide) is visible in the bottom 100 mm of the desiccant bed during scheduled sieve top-up. This is direct evidence that active corrosion is occurring on the shell or bottom head.
3. Coupon weight loss exceeds 0.1 mm/year for 304L or 0.05 mm/year for 316L. Quantitative evidence that the corrosion rate is in the "action required" range.
4. Condensate chloride analysis exceeds 50 ppm in any single sample. Indicates the chloride driving force is higher than 304L can tolerate indefinitely.
5. Visible cracks on the screen-to-shell weld during a scheduled screen inspection. SCC is now active; replacement is mandatory.
6. UT thickness readings below ASME Section VIII minimum at any point in the lower shell or bottom head. The vessel is no longer fit for service per the original design code.

Any one of these signals should trigger a 90-day plan to either replace or reline the vessel. The cost of acting on a single signal is dramatically less than the cost of an unplanned outage caused by a vessel failure, which typically runs $200k-500k per day for a refinery-scale air dryer plus the cost of emergency repair at premium fabrication rates.

Specification for a New Chlorides-Resistant Adsorber Vessel: The 12-Line RFQ Checklist

If you are specifying a new vessel for chloride-loaded service, the following 12 specification points must be in the RFQ to avoid ending up with a vessel that fails in the same mode as the one you are replacing:

1. Material: SA-240 316L (UNS S31603) plate + SA-182 316L forgings + SA-403 316L fittings. No substitutions to 304L permitted without explicit engineering review.
2. Internal weld procedure: GTAW root + SMAW fill, no autogenous welds on the internal surface. Solution anneal at 1040-1080 degC after all major welds.
3. Pickling and passivation: ASTM A380 nitric-hydrofluoric acid pickle followed by nitric acid passivation. Verify with water-break test before vessel is closed.
4. Surface finish on all wetted surfaces: ≤ 0.8 micron Ra (32 microinch), electropolished optional for high-purity service.
5. Support screen: 316L wedge-wire screen, 30-40 percent open area, welded to a 316L support ring with full-penetration welds. Screen-to-shell weld geometry reviewed and approved by vessel engineer before fabrication.
6. Bottom head: 2:1 elliptical, minimum 8 mm thickness with 3 mm corrosion allowance. UT thickness map baseline at commissioning.
7. Inspection ports: minimum two DN150 flanged ports in the lower 1 m of the shell for coupon access and internal UT without full bed removal.
8. Drain port: DN50 at the lowest point of the bottom head, with a 316L block valve and a sample port for condensate retrieval.
9. External connections: all piping 316L, butt-welded where possible. Threaded connections limited to instrument ports.
10. Insulation: calcium silicate or mineral wool with aluminum cladding. Avoid foam-glass in chloride-loaded service (chloride leaches from foam glass over decades).
11. Documentation: material test reports (MTRs) for all pressure-retaining materials, weld procedure specifications (WPS), procedure qualification records (PQR), and welder qualification records (WPQ) per ASME Section IX.
12. Vendor qualification: vendor must demonstrate 5+ years experience fabricating 316L pressure vessels for chloride service. Reference list with at least 3 operating vessels >10 years old in similar service.

The 12 points cover material, fabrication, inspection, and vendor qualification. Following them adds approximately 15-25 percent to the new vessel cost compared to a baseline carbon-steel vessel, but typically saves 100 percent of the replacement cost over a 25-year service life.

Quick-Reference Decision Card

For engineers who need to make the call in a single meeting, the following 5-question decision card works:

• Q1 — What is the measured chloride exposure in the condensate? Below 50 ppm → 304L is acceptable. 50-200 ppm → 316L required. Above 200 ppm → 904L or Inconel 625 required.
• Q2 — What is the measured remaining wall thickness vs the original + corrosion allowance? If below ASME Section VIII minimum → immediate replacement. If above but with >0.1 mm/yr corrosion rate → plan replacement in next turnaround.
• Q3 — Is chloride SCC present (PT or MPI evidence of tight surface cracks)? Yes → full replacement mandatory, relining will not stop crack propagation.
• Q4 — Is the vessel <2.5 m diameter and removable? Yes → relining is cost-effective. No → in-place alloy overlay or replacement required.
• Q5 — What is the cost of unplanned outage for this dryer? Daily outage cost x 7-14 days repair time. If exceeds planned replacement delta, replace in the next turnaround.

Working through these five questions with the operations team, the inspection data, and the procurement budget will produce a defensible replacement-or-reline decision in under an hour.

Next Steps and How to Engage Aluminaworld

Adsorber vessel corrosion under chloride-loaded desiccant beds is a solvable problem. The right combination of material selection, liner specification, and inspection discipline will deliver a 20+ year service life.

If you are evaluating replacement vessel options, lining a 304L dryer that is showing early pitting, or specifying a new vessel for known chloride service, Aluminaworld can supply:

• Activated alumina and molecular sieve with documented low-chloride specifications (typical 30-80 ppm Cl on fresh material, lot-certified analysis).
• Lined or 316L adsorber vessels for chloride-loaded service in standard sizes from DN600 to DN3000.
• Engineering support for vessel specification review, RFQ development, and lining-vs-replacement economics.
• Sample COAs and chloride specifications for incoming-material QC programs.

For a vessel specification review or to request a budgetary quote on a chloride-resistant 316L dryer, contact our engineering team:

WhatsApp: +86 133 2522 2240 (English / Chinese, replies within 12 hours CST)
Email: barry@aluminaworld.com (full vessel specification packages returned within 48 hours)

Free sample shipments of low-chloride activated alumina and molecular sieve are available for qualification testing (typically 1-2 kg, shipped via DHL within 5 business days).

Related Reading from the Aluminaworld Engineering Library

For readers working on adjacent failure modes in adsorption dryers, the following five guides cover the most common companion topics. Each is engineered for the same procurement / fixed-equipment audience:

• Why Your Regeneration Cycle Keeps Getting Longer: 7 Root Causes Ranked by Frequency — the upstream diagnostic that often precedes vessel-corrosion investigation, since longer regeneration cycles concentrate more chloride per cycle at the bottom of the bed.
• Molecular Sieve Dusting Downstream of a Dryer: Filter DP Trending and Screen Mesh Selection — the downstream consequence when corrosion product from the support screen migrates up through the bed and loads the outlet filter.
• Desiccant Bed Channelling: How to Diagnose Maldistribution from Outlet Dew-Point Sawtooth Traces — channelling bypasses that allow wet air to reach the bottom head before full adsorption, accelerating the chloride corrosion cell.
• Activated Alumina Ball Attrition in Pneumatic Conveying Lines: How to Specify 5 N/mm Crush Strength Minimum — mechanical load on the support screen that drives the residual stress component of chloride SCC at the screen-to-shell weld.
• Molecular Sieve Purification of Electronic-Grade Specialty Gases: NF3, SiH4 and Moisture Below 100 ppb — high-purity service context where the chloride tolerance is <1 ppb and the vessel material must be AL-6XN or Inconel 625 from the outset.

Frequently Asked Questions on Adsorber Vessel Corrosion

The following 11 questions cover the most common engineering and procurement decisions on chloride pitting and stress corrosion cracking in molecular sieve and activated alumina dryer vessels. Each answer is written for fixed-equipment engineers, materials specialists, and procurement engineers working on the 10-25 year service-life decision.

What chloride concentration in the condensate is the threshold between 304L being acceptable and 316L being required?

Below 50 ppm chloride in the condensate, 304L is generally acceptable with normal inspection intervals. 50-200 ppm requires 316L with annual coupon monitoring. Above 200 ppm requires 904L, AL-6XN, or Inconel 625, or a full rubber lining over carbon steel. The threshold is driven by the chloride SCC susceptibility: 304L cracks at ~50 ppm chloride at 60 degC, while 316L raises the threshold to ~200 ppm. These numbers come from ASTM G48 critical pitting temperature testing in 6 percent FeCl3 and from field experience compiled in NACE / AMPP technical reports.

How do I know whether the vessel has general pitting or active chloride stress corrosion cracking?

General pitting shows as scattered shallow pits with no preferred orientation, detectable by UT thickness mapping or visual + pit-depth gauge. Active SCC shows as branched or single tight surface cracks, often with no visible wall loss, detected only by PT (liquid penetrant) or MPI (magnetic particle) inspection. UT alone will miss SCC because the cracks are tight and oriented perpendicular to the wall surface. If you see SCC, the vessel must be replaced; relining will not stop crack propagation because the lining does not address the residual welding stresses that drive the cracking mechanism.

What is the difference between RB (rubber) lining and FBE (fusion-bonded epoxy) lining for a desiccant dryer?

RB lining is 4-6 mm of natural or chlorobutyl rubber vulcanized to a shot-blasted steel substrate, suitable for service up to 80-100 degC, with excellent chloride resistance and full vacuum capability. FBE is 0.3-0.5 mm of epoxy powder sprayed and oven-cured, suitable for 110-120 degC service, good chloride resistance but pinhole-sensitive (requires 100 percent holiday detection per ASTM D5162). RB is the lower-cost and more forgiving option for heatless dryers operating under 80 degC; FBE is the only practical lining for heated-purge dryers in the 100-200 degC range. PTFE is reserved for high-purity or food/pharma service where extractables must be minimized.

How often should I inspect the bottom head of a 316L desiccant dryer in chloride-loaded service?

For a 316L vessel in documented chloride service, the recommended inspection cadence is: annual filter DP trending (quarterly preferred), annual coupon retrieval and weight-loss measurement, internal UT thickness mapping at 15-year intervals (assuming coupon rate below 0.05 mm/year), and full visual + MPI of suspect areas at any time the filter DP rises 50 percent above baseline. If the measured coupon rate is above 0.05 mm/year, shorten the UT inspection interval to 10 years. If PT or MPI evidence of cracking appears at any inspection, plan immediate replacement.

Why is pickling and passivation per ASTM A380 mandatory for new stainless vessels in chloride service?

Pickling (nitric-hydrofluoric acid bath) removes the chromium-depleted layer beneath heat tint and weld discoloration, restoring the passive chromium oxide film that gives stainless steel its corrosion resistance. Passivation (nitric acid bath) thickens the chromium oxide layer for additional protection. Without these steps, the heat-affected zones around welds are sensitized and anodic relative to the rest of the vessel, and these sensitized zones become pitting initiation sites in chloride service within 12-24 months. The water-break test (water sheets uniformly on a clean, passivated surface) is the standard acceptance criterion.

What is the cost difference between RB lining an existing vessel and replacing the vessel with a 316L unit?

For a 1.5 m diameter, 4 m tall dryer with 6 mm wall, RB lining in a qualified shop typically runs $40,80,000 depending on geometry, plus $15-25k for bed removal, sieve storage, and disposal. Total relining cost ~$60-110k with 12-16 week turnaround. Replacement with a new 316L vessel of equivalent size runs $120-180k for the vessel alone, plus $30-60k for installation and commissioning, total $150-240k with 20-24 week delivery. The economic crossover point depends on the cost of unplanned outage: at $50k/day outage cost, relining is preferred if the existing vessel is otherwise sound; replacement is preferred if the existing vessel has SCC or wall thickness below code minimum.

Can I use cathodic protection on a desiccant dryer vessel to control internal chloride corrosion?

Cathodic protection is generally not applicable to internal surfaces of adsorption dryers because the desiccant bed electrically shields the bottom head and lower shell from any external anode. Sacrificial anodes placed inside the vessel quickly become buried in sieve and lose electrical contact. The two practical corrosion control options for existing vessels are (a) material upgrade to 316L or higher-PREN alloy, or (b) installation of a corrosion-resistant liner (RB, FBE, PTFE, or alloy overlay). For new vessels, the correct answer is to specify 316L from fabrication with ASTM A380 pickling and passivation, and to control the chloride driving force at the inlet (filtration, intake location, compressor type).

What is the maximum allowable chloride concentration in steam used for thermal regeneration of molecular sieve?

Industry practice for molecular sieve regeneration steam is below 50 ppb total dissolved solids with chloride below 5 ppb, per the Honeywell UOP and Engelhard technical bulletins on molecular sieve care. Steam chloride above 5 ppb accumulates in the condensate pool at the bottom of the regeneration gas stream and concentrates by evaporation. Boiler feedwater chloride limits of 0.05 ppm in the drum and 0.5 ppb in the condensate are typical targets. If the steam source cannot meet these limits, an in-line condensate dump valve on the regeneration gas outlet is the standard mitigation — dump the first 5-10 minutes of condensate every regeneration cycle to prevent chloride accumulation in the vessel.

How do I detect chloride SCC in a vessel that cannot be removed from service for internal inspection?

Externally accessible NDT for chloride SCC is limited but not zero. ACFM (alternating current field measurement) can detect surface cracks from the outside through insulation if the insulation is removed first. Eddy current array (ECA) is sensitive to surface cracks on non-ferrous vessels from the outside. For ferrous vessels (carbon steel, 400-series SS), low-frequency UT with shear wave probes can detect cracks in the bottom head from external access if the geometry permits. None of these methods are as reliable as internal PT or MPI, but they provide risk-prioritization data when internal inspection is not feasible. If any of these methods indicates suspect cracking, plan an internal inspection at the next opportunity.

What alloy is the correct upgrade path for a vessel in 200-500 ppm chloride service where 316L is not adequate?

Above 200 ppm chloride, the correct alloy upgrades are 904L (UNS N08904, PREN 36), AL-6XN (UNS N08387, PREN 47), or Inconel 625 (UNS N06625, PREN 50). 904L is the lowest-cost upgrade at roughly 4x the cost of 316L and is suitable for chloride service up to about 1000 ppm at 60 degC. AL-6XN is a super-austenitic stainless with PREN 47, suitable for chloride service up to 5000 ppm. Inconel 625 is a nickel-base alloy with PREN 50, suitable for the most aggressive chloride service including hot concentrated chloride brines. For vessels in the 200-500 ppm range, 904L is typically the economic optimum; above 500 ppm, AL-6XN becomes justified.

Why does chloride pitting tend to concentrate at the support screen weld rather than the bottom head in general?

The support screen weld is the highest-stress, highest-chloride, and most-crevice location in the vessel. Stress concentrates at the weld toe because of differential thermal expansion between the screen material and the shell, residual welding stresses, and mechanical loading from the desiccant bed weight (which can be 5-15 tonnes on a large dryer). Chloride concentrates at the weld because condensate wicks into the screen-to-shell crevice by capillary action and evaporates slowly between regeneration cycles, leaving chloride residues. Crevice geometry prevents oxygen from reaching the metal surface, which would otherwise support the passive chromium oxide film. The combination of stress, chloride, and crevice geometry makes the support screen weld the statistically dominant initiation site for both pitting and SCC in chloride-loaded dryers.