How Does the Erosion-Corrosion Resistance of 70-30 Cupronickel Compare to PTFE in High-Velocity Seawater?

Moving saltwater, particularly when it carries suspended sand or shell fragments, functions as a liquid sandblaster inside a heat exchanger tube. Two materials typically used in seawater service--70-30 copper-nickel and PTFE--respond quite differently to this mechanical abuse. The challenge is more than just corrosion resistance. The actual fight is against degradation due to high-velocity particle impact.

The result may appear contradictory in the 70-30 cupronickel versus PTFE erosion corrosion seawater comparison. Since PTFE is chemically inert to the seawater, while copper alloys are still electrochemically active metals. But in abrasive high velocity flow circumstances the tougher metallic alloy can sometimes outlast the softer polymer when erosion becomes the dominant failure mode.

Seawater System Erosion-Corrosion
Erosion-corrosion is a combined mechanical and electrochemical phenomenon.

Effect of Fluid Velocity
As the velocity of the seawater increases,

Turbulence is worsening

Thin boundary layers

Increase in particle impact energy

surface wear speeds up

With suspended solids the material is physically removed from the tube wall by the fluid stream.

Sand as abrasive agent
Particles of fine sand can cause:

Surface gouging

Strip film

Microcutting

Impact fatigue repeated

Severity varies on:

Concentration of particles

Particle sizes

Particle hardness

Shape of the particle

Stream velocity

Quartz particles are highly hostile due of their sharpness, hardness and angular geometry.

How 70-30 Cupronickel Protects Itself
70-30 copper-nickel alloy is approximately:

70% copper

30 % nickel

For a long time this alloy has been utilized for maritime heat exchangers because of the formation of a protective oxide covering.

The Self-Healing Oxide Layer
The alloy develops on exposure to sea water:

A cuprous oxide coating adhering firmly

This oxide coating forms a protective layer against further damage.

The metal is covered with a self-healing ceramic skin. The only defence of the polymer is its slipperiness.

Resistance at High Velocity of Flow
In clean sea water the oxide film can be stable at velocities of the order of:

3-4 m/sec

In this range:

Film removal still restricted

Surface Attack Still Under Control

The protective layer is always being reformed

If the oxide film is slightly disturbed by abrasion, the exposed metal quickly reoxidizes and reinstates protection.

Alloy Toughness (Mechanical)
The copper-nickel base material underneath provides mechanical resilience, too.

The alloy is

Flexibility

Difficult

Crack resistant

Impact Energy Absorption Capacity

This mechanical strength helps to prevent catastrophic surface damage even under turbulent flow conditions.

PTFE Reaction to Abrasive Seawater
The difference with PTFE is that it is not a metal and does not rely on electrochemical protection.

Complete Chemical Inertness PTFE is:

Corrosion resistance to seawater

Not susceptible to assault by chloride

Resistant to chemical biofouling

Stable in salt water.

There is no corrosion process and so no oxide film to break down.

Pure mechanical rubbing
For PTFE, the failure process is virtually exclusively mechanical.

The polymer surface can be subjected to:

scrapes

Erosion of the surface

Erosion of particles

Progressive loss of material

PTFE is about as hard as a human fingernail, therefore at high flow velocity it can be damaged by repeated impacts with sand.

PTFE Velocity Limits
PTFE does not have the hardness of metallic alloys, and its safe operating velocity in abrasive service is substantially less.

Clean Water Flow vs Sand Laden Flow
PTFE performs well in clean seawater with moderate flow conditions.

However, for sandy slurry service:

Practical velocity limits may reduce to about 1-2 m/s

Outside this range:

Wear rates can increase fast

The surface may be thinned.

Long-term structural integrity deterioration

The permissible limit is very dependent on the concentration and aggressiveness of suspended solids.

Significance of Particle Characteristics
The erosion of PTFE is far greater if particles are:

Round not angular

Big, not fine

Tough not soft

Although smooth silt at low concentrations may cause very little damage, sharp silica particles can dissolve the polymer far more violently.

Why 70-30 Cupronickel is sometimes more durable than PTFE
When considering pure chemical corrosion media, PTFE generally is preferable.

However, the comparison is dissimilar in the case of abrasive seawater conditions.

Inertness and hardness
70-30 Cu-Ni advantages of

Increased surface hardness

Oxide protection self-healing

Better resilience to particle cutting

PTFE provides:

Total chemical immunity

Behavior of low friction surfaces

No electro-chemical degradation

But its besetting flaw in erosive service is softness.

PTFE's Sole Protection is Friction
The benefit is that PTFE has a low coefficient of friction.

The slippery surface could:

Decrease particle adhesion

Reduced sliding friction

Dissipate some impact energy transfer

However, this advantage may not be sufficient to compensate for the reduced hardness of the material under strong abrasive flow.

Typical Applications of Each Material
The optimal choice of material depends on whether corrosion or erosion is the main factor in the application.

Where 70-30 Cupronickel Shines
Cupronickel continues to perform very well in:

condensers for marine use

Refrigeration equipment, naval

High flow Saltwater Piping

Sand tolerant exchanger service

especially when the flow velocity is large but the chemistry is somewhat typical.

Where PTFE Shines
PTFE is outstanding in the following areas:

Seawater Blends of Highly Corrosive Chemical

Chlorination systems

Acidification of marine waters

Biofouling Control Systems

provided that suspended abrasive solids are limited.

The Impact of System Design
Material performance is also strongly dependent on exchanger shape and hydraulic design.

Control of Flow
Reducing erosion can include:

Decreasing the speed of flow

With smoother elbows

Eliminating abrupt shifts of direction

Sand separator installation

Hybrid Design Strategies
Some systems mix materials in a deliberate fashion:

PTFE in Chemically Aggressive Environments

Cu-Ni in high flow areas of abrasives

This way one can selectively take advantage of each substance.

Summary
The evaluation of 70-30 cupronickel over PTFE in high velocity seawater is a classic engineering trade off of chemical inertness versus mechanical durability. PTFE is totally resistant to corrosion by seawater, although its relatively soft surface can be highly eroded when subjected to fast-moving, sand-laden flow. In practice, under such conditions, velocities are limited to perhaps 1-2 m s-1.

Conversely, 70-30 copper-nickel shields itself with a robust, self-healing cuprous oxide coating which can tolerate saltwater velocities of 3-4 m/s in clean service and continue to recover from moderate abrasive damage. This protective coating, together with the toughness and mechanical resilience of the alloy, can enable a longer service life in extremely erosive maritime settings.

Thus, under severe sandy seawater use, the more difficult metal alloy may be more durable overall than the chemically superior polymer. In the end, material selection for maritime thermal systems is a battle between two different strengths: the metal's ability to heal itself and to take a physical beating, and the polymer's ability to simply disregard chemistry.