The Crevice Corrosion Challenge in Compact Heat Exchangers: Material Selection for Plate & Frame Units

The Crevice Corrosion Challenge in Compact Heat Exchangers: Material Selection for Plate & Frame Units

Plate and frame heat exchangers (PHEs) are efficiency marvels, offering exceptional heat transfer in a small footprint. However, their very design—with countless contact points between plates and elastomeric gaskets—creates an ideal environment for a stealthy and destructive phenomenon: crevice corrosion.

This form of localized attack occurs in stagnant micro-environments where oxygen diffusion is restricted. Inside a crevice (at gasket/plate interfaces, under deposits, or between contact points), the metal's passive layer breaks down, leading to aggressive, rapid pitting that can perforate thin plates unexpectedly. For engineers, the material selection for PHE plates is fundamentally a battle against this specific failure mode.

Why PHEs are Inherently Vulnerable

1. Ubiquitous Crevices: Every gasket groove and plate contact point is a potential site. Unlike tubular exchangers, you have hundreds or thousands of these inherent crevices.

2. Stagnant Zones: Low-flow areas near gasket channels or on the cold side of a temperature gradient allow chemistry inside the crevice to become aggressive (low pH, high chloride concentration).

3. Thin Sections: Plates are typically 0.5-1.0 mm thick. Even minor localized corrosion can lead to rapid through-wall penetration and cross-contamination of media.

The Material Selection Hierarchy: Balancing Cost & Performance

Choosing the right plate material is a function of chloride concentration, temperature, and pH. Here’s a practical guide, moving from standard to premium.

1. AISI 304 / 304L Stainless Steel

• Application: Low-risk, benign environments. Clean, chlorinated city water below 30°C, certain non-halide process streams.

• Crevice Corrosion Limitation: Very poor resistance. Susceptible at chloride levels as low as 100 ppm at ambient temperatures. Often a false economy in industrial settings.

• Best Practice: Use only when water chemistry is tightly controlled, known, and unchanging. Avoid for seawater, brackish water, or cooling tower water.

2. AISI 316 / 316L Stainless Steel (The "Default" with Caveats)

• Application: The most common industrial choice for cooling water, low-chloride process streams, and many HVAC applications.

• Crevice Corrosion Limitation: Moderate resistance. The 2-3% Molybdenum content improves performance, but failure is common in aggressive waters. A critical rule of thumb: Risk becomes high above 50°C with chlorides >200 ppm.

• Best Practice: Mandatory for operators to monitor and log chloride concentration and inlet temperature continuously. Always include a margin of safety. Not suitable for seawater.

3. High-Molybdenum Stainless Steels (The Reliable Upgrade)

• Grades: 254 SMO (6% Mo), AL-6XN (6-7% Mo), 904L (4.5% Mo).

• Application: The standard solution for severe cooling tower water, brackish water, and many chemical process streams where chlorides are present but not extreme.

• Advantage: Significantly higher Critical Crevice Corrosion Temperature (CCT). For example, while 316L may fail at 30°C in seawater, 254 SMO can resist up to 70°C+.

• Decision Point: Often the most cost-effective long-term choice when 316L is marginal. Prevents unscheduled failures and provides operational flexibility.

4. Titanium (The Benchmark for Chlorides)

• Grades: Gr.1 (commercially pure) or Gr.2.

• Application: The definitive choice for seawater, high-chloride brines, and oxidizing media. Virtually immune to chloride crevice corrosion at temperatures up to 120°C+.

• Consideration: Higher upfront cost, but offers complete reliability in the harshest chloride environments. Be aware of compatibility with reducing acids (e.g., uninhibited sulfuric) and the risk of hydriding if improperly handled.

5. Nickel Alloys (For Extreme Conditions)

• Grades: Alloy C-276 (Hastelloy), Alloy 625 (Inconel).

• Application: For processes combining very high chlorides, low pH, oxidizing agents, and high temperatures—conditions beyond the reach of titanium (e.g., hot hydrochloric acid fumes, severe sour gas coolers).

• Note: A highly specialized, premium solution. Justify based on a clear and present combination of aggressive factors.

Practical Selection & Operational Mitigation Strategy

Selecting the material is only half the battle. Implementation and operation are critical.

Essential Mitigation Practices for Any Material:

1. Water Chemistry Management: The single most important factor. Control chlorides, sulfates, pH, and oxidizing agents (like hypochlorite for biofouling). Avoid over-chlorination.

2. Design & Flow Optimization: Specify "no-contact" or "wide-gap" plate patterns where possible to minimize crevice sites. Ensure adequate flow velocity across all plates to reduce stagnation.

3. Cleaning & Maintenance: Follow regular, gentle cleaning protocols to remove deposits (which create under-deposit crevices). Avoid hydrochloric acid for cleaning stainless steels; use sulfamic, citric, or nitric-based products.

4. Inspection: During maintenance, inspect plate interiors, especially near gasket grooves, for tell-tale signs of pitting or "pepper" marks—the early stage of crevice attack.

Conclusion

Defeating crevice corrosion in plate and frame heat exchangers requires a two-pronged approach: selecting a material with a proven CCT above your operating reality and implementing operational discipline to control the environment.

The cost of a single failure—downtime, product loss, plate replacement—almost always outweighs the premium for a more resistant material. When in doubt between 316L and a 6-Mo alloy, upgrading is rarely regretted. For chloride-bearing waters, titanium is often the most reliable and ultimately economical choice.

The goal is not just to buy a heat exchanger, but to specify a system with inherent resistance to its most likely failure mode, ensuring long-term, reliable, and efficient operation.