Failure analysis · Semi-welded plate heat exchanger
Pitting Corrosion in a Semi-Welded Plate Exchanger
Failure analysis of chloride-induced pitting during steam heating of vegetable oil.
Project summary
Challenge
Premature leakage developed in an exchanger whose stainless-steel surface experienced a more aggressive chloride environment than expected.
Engineering response
The investigation separated thermal duty from corrosion risk and reviewed water chemistry, deposits, temperature and material suitability.
Outcome
The case demonstrates why complete fluid analysis and worst-case temperature are required before final material selection.
- Chloride-induced pitting
- Steam heating
- Vegetable-oil duty
- Material review
Original project record
Pitting Corrosion in a Semi-Welded Plate Heat Exchanger
Steam Heating of Vegetable Oil – Material Selection Risk Analysis. This case explains why a semi-welded plate heat exchanger can improve sealing reliability, but does not automatically increase corrosion resistance when chloride contamination exists.
Project Overview
A semi-welded plate heat exchanger supplied as a technical replacement equivalent to ALFA LAVAL MK15BW was installed for steam heating of vegetable oil .
Operating Conditions
Why semi-welded? Semi-welded construction was selected to improve sealing reliability in oil service and reduce cross-contamination risk.
Observed Failure – Localized Pitting Corrosion
After a period of operation, the unit developed internal leakage. Inspection revealed:
- Multiple deep pitting cavities
- Localized corrosion spots
- No weld seam cracking
- No mechanical damage
- No manufacturing defect
Failure morphology: consistent with chloride-induced pitting corrosion of stainless steel 316 .
Why This Is Not a Structural Issue
Semi-welded plate heat exchangers provide:
- Improved safety
- Reduced cross-contamination risk
- Higher pressure stability
However: semi-welded construction does not increase corrosion resistance. Corrosion resistance depends on alloy composition and the chemical environment.
Medium Evaluation – Is Vegetable Oil Corrosive?
Under standard conditions, refined vegetable oil typically:
- Contains negligible chloride
- Has low water content
- Does not typically cause pitting corrosion
For pitting corrosion in AISI 316, the following conditions are commonly required:
- Chloride presence
- Elevated temperature
- Moisture or a water phase
- Deposit formation
Conclusion: the corrosion indicates the actual process medium contained aggressive components beyond normal vegetable oil characteristics.
Temperature Effect on Pitting Risk
Cold side outlet temperature: 80°C
Chloride pitting susceptibility increases significantly above ~60°C. At elevated temperatures:
- Passive film stability decreases
- Pitting initiation potential drops
- Corrosion propagation accelerates
Engineering note: temperature acts as a corrosion multiplier in chloride-contaminated systems.
Root Cause
Failure mechanism: chloride-induced localized pitting corrosion of AISI 316 plates under elevated temperature conditions.
The aggressiveness of the actual operating medium exceeded the corrosion resistance limit of 316 stainless steel.
Material Selection Lessons for Plate Heat Exchangers
When selecting materials for semi-welded plate heat exchangers in oil heating applications:
- Verify chloride concentration (including contamination and upstream cleaning chemicals)
- Confirm water contamination risk (condensation, wash water, steam traps, ingress)
- Consider worst-case chemical scenarios (CIP residues, additive packages, impurities)
- Evaluate operating temperature impact (pitting accelerates above ~60°C)
If chloride uncertainty exists, consider material upgrades: Duplex 2205, 254SMO, or Titanium. Upgrading during design is significantly less costly than premature failure and unplanned shutdown.
Related article:
- Material selection for heat exchangers