When process equipment is manufactured, it is invariably designed to offer the maximum life for its particular construction. (For more on the importance of design, see Corrosion Control Considerations in the Equipment Design Process.) A common starting material used for many types of process equipment is cast or carbon steel, which has a predictable corrosion life for a given medium and temperature.
Where heavy abrasive media is anticipated, alloys that will offer the best compromise of service life to cost and operation are normally selected.
However, it can take only a small geometry change for cavitation to manifest itself in relatively benign fluids, such as cooling water and seawater. Furthermore, operator errors—such as lower-than-designed operating pressures, or liquids with lower boiling temperatures than the equipment was designed for—can compound the problem.
One of the biggest challenges is that externally and while in operation, cavitation is not easily identifiable other than an increase in vibration and noise. It is usually during shutdowns, unplanned outages or breakdowns that the operator is able to see any symptoms.
Where an active alloy such as steel is used, the problems of erosion corrosion accelerate the deterioration of the asset. Thus the design of any protective coating system needs to address all of the potential forces likely to be encountered in operation.
Understanding Cavitation
Cavitation is a complex science, but one that is often overcomplicated in its explanation and diagnosis. Within certain equipment, the creation of localized low pressure areas is often required as part of their function. For example, a propeller on a ship works in accordance with Bernoulli’s principle and is best understood by the manner in which an airplane stays in the air. As air (in the vapor phase) passes over the wing, it generates a high-pressure side under the wing and a low-pressure side above the wing, forcing it upwards.
A ship’s propeller works identically—its rotation in water creates a high-pressure side on the back of the blade, and a low-pressure side on the front of the blade, driving the ship forward.2
If the low-pressure area is permitted to reach vapor temperature, then steam will be generated on the front side of the blade, and these vapor bubbles will be carried in the flow away from their inception area.
As the vapor bubbles move from a low-pressure area to a high-pressure one, the bubbles will collapse in on themselves and return to their liquid state. Where this happens within the body of the liquid, i.e. evenly surrounded, there are still significant shock waves but the collapse is even.
However, where bubbles enter the high-pressure region close to a surface, the collapse is uneven because there is high pressure on all sides of the bubble except where the bubble is in contact with the surface.
Figure 1: The cavitation process.
Cavitation damage occurs as a result of an extreme pressure difference in the fluid near the substrate surface. To identify cavitation, we need to look for a “peppering” of the surface and geometry, which might cause pressure variation through fluid flow.
Hence, cavitation in fluid flow equipment will not be found in conditions of stagnant water such as those that might contribute to pitting, deposit or crevice corrosion.
A further clue to cavitation damage is found by looking for geometry that might create low-pressure areas and erosion slightly downstream of this.
Approaches to Resisting Cavitation
As we have seen, the forces involved in cavitation damage are extreme and can result in damage, vibration, noise and loss of performance. The first effort by any operator or designer should be to attempt to operate out or design out the cavitation effect. However, this is not always possible to achieve for various reasons, such as function and cost, and so the operator is left with the consideration of material selection.
There are two common material methods employed to combat the effects of cavitation:
- Hard alloys
- Ductile, elastic systems
Both methods have benefits and limitations. Hard alloys offer good entrained solids erosion resistance, so they are commonly found in Pelton turbines, for example. Weld overlay is a consideration and it is not unusual to see hard metals being used to weld up cavitation damage. However, introducing different metal alloys into the system brings galvanic forces into play, thereby causing additional problems. (See 5 Ways to Avoid Galvanic Corrosion for more information.)
Flexible systems, however, aim to work with the problem, and protective coatings are commonly used in this respect. Coating selection is important because all of the above forces (corrosive and erosive) are still present, as is the potential for impact damage during installation or service.
Coating Selection for Cavitation Risk
For immersion conditions, coatings fall into different categories, and we will review four of the most popular coating choices, which currently include:
- Glass flake coating systems
- Modified solvent-free epoxy systems
- Modified solvented epoxy coatings
- Thermosetting polyurethane coatings
1. Glass Flake Coating Systems
Glass flake coatings use a binder, often polyester or vinylester, which is bulked out using glass flakes. High-quality binders can provide good chemical resistance, but this is not normally an issue in benign conditions and therefore cheaper resins are often used. These resins have good dielectric strength for insulation and can be sprayed, keeping labor costs down for large areas.
Vinyl and polyesters are, by nature, quite flexible resins; however, the use of glass as a bulking agent results in a thick (0.04–0.12'' / 1–3 mm) system, which is brittle with relatively poor adhesion, impact, cavitation and impingement erosion resistance. A common misconception is that glass flake systems offer good impact (and thus cavitation) protection, but in fact they offer limited protection. Furthermore, the presence of styrene monomers and volatile organic compounds (VOCs) can also cause problems for applicators if they are not suitably protected.
2. Modified Solvent-free Epoxy Systems
Solvent-free epoxy resin systems are among the most modifiable group of resins. They show very good adhesion to all metals—up to 230 kg/cm2 to grit-blasted surfaces. Using correct binders and fillers results in systems capable of high compressive strengths and good erosion corrosion resistance, while maintaining high dielectric strength and low moisture diffusion. These attributes make them popular for galvanic corrosion protection.
They do not contain solvents and so do not carry the associated health and safety risks of conventional paints and glass flakes, but not all systems can be sprayed. This applies especially to epoxy resins designed with high entrainment erosion resistance in mind, as the fillers tend to cause severe wear to spray units.
Epoxies can also be modified to offer different levels of elasticity and thus cavitation resistance, but they are still second to polyurethane technology for resilience. These hybrid epoxy systems, using rubber or urethane as a flexibilizer, have been shown to compromise the diffusion resistance and thus adhesion in long-term immersion.
3. Modified Solvented Epoxy Coatings
These epoxies have similar properties to the solvent-free version; however they are compromised by adding solvents to aid application. The use of solvents in the coating application process results in shrinkage and coating stress (not ideal in cavitation environments), as well as other hazards such as solvent entrapment and capillary formation during evaporation, which again weakens the system.
4. Thermosetting Polyurethane Coatings
The design of thermosetting polyurethane coatings allows these coatings to be as stiff or flexible as required, offering good curing at low temperatures, and cavitation and impact-erosion resistance. Their disadvantage tends to be in long-term immersion, as some can be moisture-sensitive (in general, absorbing water more readily than other coatings). They are applied at a greater thickness to help to avoid this, and there have recently been developments in diffusion resistance to provide systems that overcome this shortfall. (For the results of laboratory and field testing of a cavitation-resistant coating, see 30-Year Development of a Cavitation-Resistant Elastomer.)
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References:
- National Institute for Science and Technology / The International Association for the Properties of Water and Steam http://www.iapws.org/, April 26, 2000
- S A KINNAS, “A Note on the Bernoulli Equation for Propeller Flows: The Effective Pressure”, Journal of Ship Research, Volume 50, Number 4, December 2006 , pp. 355-359(5), SNAME 2006
- Y TOMITA and A SHIMA, “High-speed photographic observations of laser-induced cavitation bubbles in water”. Acustica, 71, No. 3, 161—171, 1990.
- Y TOMITA and A SHIMA “On the behaviour of a spherical bubble and the impulse pressure in a viscous compressible liquid”. Bull. JSME, 20, 1453-1460, 1977
- L. SCHREIR, “Corrosion Vol. 1”, Newnes Butterworth, 1963
- G BOWERS, “Theoretical and Practical Aspects of Erosion-Corrosion Control and Repair in Waterjet Installations”, Waterjet Propulsion III, RINA, 2001
- K Alexander, “Corrosion resistance of aluminium and stainless steel in waterjets”, 13th Fast Ferry International Conference, 1997
- H Tietgen, “Experience gained with plastic materials in hull repairs”, Germanische Lloyd, 1982