Some common terms are used throughout this report. Although they are discussed in detail later, the reader will find it useful to be familiar with the following definitions from the start:
Erosion: The progressive loss of original material from a solid surface due to mechanical interaction between that surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles (ASTM G 73 1993). As used in this report, the term refers specifically to slurry erosion, which is caused when a solid surface is impinged upon by solid particles suspended in a liquid stream.
Cavitation: In the literature and the field (and in this report), erosion caused by cavitation is generally referred to simply as cavitation. Cavitation is the progressive loss of original material from a solid surface due to the formation and collapse, within a liquid, of cavities or bubbles (ASTM G 32 1992).
Corrosion: The deterioration of a material because of reaction with its environment (Fontana and Green 1979).
Thermal spraying: A process by which finely divided metallic or nonmetallic materials are deposited in a molten or semimolten condition on a prepared substrate to form a sprayed deposit (AWS 1991).
Welding: A metal working process in which metals are joined by heating them to the melting point and allowing the molten portions to fuse or flow together (Althouse, Turnquist, and Bowditch 1967).
The formation and collapse of vapor bubbles or cavities in a fluid can produce extremely high pressures, frequently damaging adjacent surfaces and causing material loss (March and Hubble 1996). An example of cavitation damage observed on a Francis hydroelectric turbine located at the TVA Raccoon Mountain pumped-storage plant, Chattanooga, TN, is shown in Figure 1.
Pressures greater than 100,000 psi have been measured in materials by the shock wave from cavitation bubbles (Vyas and Preece 1976). A consensus has developed that material removal by cavitation is caused by a cyclic fatigue process (Richman and McNaughton 1995). The pressures can be transmitted from the collapsing bubbles to the surface either in the form of a shock wave or by microjets, depending on the distance from the surface. The cycle of formation and collapse of the bubbles occurs at a high frequency and the dynamic stress generated can cause the damage of the material by fatigue (Schwetzke and Kreye 1996).
Figure 1. Cavitation damage on a TVA hydroelectric turbine blade.
The basics of cavitation have been reviewed for the Electric Power Research Institute (EPRI) (Rodrigue 1986). Various factors that influence cavitation pitting include:
· velocity effects
· material
· size effects
· corrosion
· roughness effects
· temperature effects
· thermodynamic effects
· fluid properties
· gas content.
Therefore, due to the large number of factors that influence cavitation, qualitative approaches have been developed to assist the plant manager to make cavitation repair decisions. EPRI gives plant owners several options for when to make cavitation repairs (Rodrigue 1986):
· Make all repairs during each inspection period.
· Repair only areas where cavitation damage exceeds 1/8 inch.
· Repair areas on stainless steel overlays where pitting is 1/8 inch or deeper. On carbon steel, repair areas even with light damage using stainless steel weld materials.
· Allow cavitation to progress to the maximum depth that can be repaired with two weld passes-about 3/8 inch.
Low, medium, and high cavitation have also been defined in terms of the wear rate for a normal operational year of 8000 hours. Low cavitation is defined as 1/16 to 1/8 inch-deep damage in carbon steel occurring in two year; medium cavitation is defined as more than 1/16 inch damage in austenitic stainless steel in 1 year; and high cavitation is defined as more than 1/8 inch damage in stainless steel in 6 months or less (Spicher 1994). It should be noted that repair or replacement shall be made whenever cavitation damage threatens the structural integrity of a mechanical component.
The cavitation material-loss process usually involves erosion, but erosion may have various causes. As noted at the beginning of this chapter, for the purposes of this report the term erosion will refer specifically to slurry erosion, which occurs at a surface impinged upon by solid particles suspended in a liquid stream.
Corrosion occurs by an electrochemical process. Two dissimilar metals (forming an anode and a cathode), an electrolyte, and an electrical circuit connecting them are required for corrosion. Dissolution of the metal into the electrolyte occurs at the anode. Cavitation may combine with corrosion to create much greater damage rates than the sum of the two if each acted alone. Metals usually develop passive films or layers on the surface that inhibit further corrosion and metal removal. Cavitation removes this passive film exposing a fresh metal surface that can readily corrode. The increased surface roughness caused by corrosion may also promote cavitation (Rodrigue 1986).
Techniques available for cavitation damage repair including: (1) weld overlays and inlays, (2) reinforced epoxy coatings, and (3) thermal spray coatings. Of these methods, the one most commonly used is the weld overlay because it produces the most durable coating. Two weld repair processes generally used for cavitation repair are: (1) gas metal arc welding (GMAW) or metal-inert gas (MIG) welding, and (2) shielded metal arc welding (SMAW) or stick electrode welding (Rodrigue 1986).
Due to the condition of most cavitated surfaces, damage generally cannot be repaired by directly filling the pitted areas. The pitted surface is usually undercut to remove the damaged area and to provide a surface that can be adequately cleaned before filling repair. The resulting space is normally filled by welding with a common stainless steel alloy such as 308L or 309L. The top 0.25 in. layer is usually 308L stainless steel. 309L stainless steel is used when the first pass is on mild steel. 309L has higher Cr and Ni content, and can withstand dilution with the mild steel without a loss of properties for cavitation resistance. However, if the substrate to be repaired is stainless steel, 308L can be used.
Extensive weld repair can introduce stresses in the area being repaired and can damage the component. Entire throat rings have required stainless steel weld repair. Complete welding of the throat ring produces thermal stresses on cooling that cause the weld overlay and liner to pull away from the concrete support. The detached steel liner is subject to buckling and damage. In order to prevent this disbonding and overstressing of the liner, anchors and grout are used.
Thermal spraying is a process by which finely divided metallic or nonmetallic materials are deposited in a molten or semimolten condition on a prepared substrate to form a sprayed deposit. Thermal spray processes include combustion powder flame spray, combustion wire flame spray, wire arc spray, plasma spray, and high velocity oxyfuel (HVOF) spray (Figures 2 and 3).
Thermal spraying that uses the heat from a chemical reaction is known as combustion gas spraying, or flame spraying. Any material that does not sublime (i.e., does not transform directly from a solid to gas) and has a melting temperature of less than 5000 _F may be flame sprayed. Materials that are applied by flame spray include metals or alloys in the form of wire, cord, or powder; ceramics as powder, cord, or rod; and polymers as powder.
Combustion wire flame spray feedstock material is mechanically drawn by drive rollers into the rear of the gun. The feedstock proceeds through a nozzle where it is melted in a coaxial flame of burning gas. One of the following gases may be combined with oxygen for use in flame spraying: acetylene, methylacetylene-propadiene stabilized (MPS), propane, hydrogen, or natural gas. Acetylene is the gas most widely used because of higher flame temperature. The fuel gas flame is used for melting only-not for propelling or conveying the material. To accomplish spraying, the flame is surrounded with a stream of compressed gas-usually air-to atomize the molten material and to propel it onto the substrate.
The combustion powder flame spray process is similar to the wire process but the powder feedstock is stored in a hopper that can either be integral to the gun or externally connected to the gun. A carrier gas is used to convey the powder into the oxygen fuel gas stream where the powder is melted and carried by the flame onto the substrate.
In the wire arc process, two consumable wire electrodes, which are at first isolated from each other, automatically advance to meet at a point in the atomizing gas stream. An electrical potential difference of 18 to 40 volts, applied across the wires, initiates an arc that melts the tip of the wire electrodes. An atomizing gas, usually compressed air, is directed across the arc zone, shearing off the molten droplets that form the atomized spray.
Plasma spray technology uses a plasma-forming gas (usually either argon or nitrogen) as both the heat source and the propelling agent for the coating. A high-voltage arc (up to 80 kW) is struck between the anode and cathode within a specially designed spray gun. This energy excites the plasma gas into a state of ionization. The excited gas is forced through a convergent/divergent nozzle. Upon exiting the nozzle, the gas returns to its natural state, liberating extreme heat. Powder spray material is injected in the hot plasma stream, in which it is melted and projected at high velocity onto a prepared substrate. The resulting coatings are generally dense and strongly bonded with high integrity (AWS 1985).
The HVOF process efficiently uses high kinetic energy and controlled heat output to produce dense, very-low-porosity coatings that exhibit high bond strength. The HVOF gun consists of a nozzle to mix the combustion gases, an air-cooled combustion chamber, and an external nozzle (air cap). The process gases enter through several coaxial annular openings. A central flow of powder and carrier gas is surrounded by air, fuel, oxygen, and the remaining process air. This focuses the spray stream and prevents the powder from contacting the gun walls. The oxygen and fuel burn as they enter the rear portion of the combustion chamber. Most of the process air is used to cool the combustion chamber and, in the process, is preheated before entering the air cap. As it enters, the process gas forms a thin boundary layer that minimizes the contact of the flame with the walls of the air cap and helps to reduce the quantity of heat transferred to the air cap. Hot gases with a combustion temperature of up to 6000 _F exit through a converging nozzle with a gas velocity that can approach 4500 ft/sec (Metco 1996).
For the application of polymeric or thermal spray coatings the surface must be cleaned and have a suitable profile that will enhance the coating adhesion. Cleaning procedures are designed to remove specific types of contaminants without changing the physical or chemical properties of the substrate surface. Cleaning can be done with solvents that dissolve the contaminants. A rough profile has a greater surface area, which increases bonding capability. Surfaces can be roughened by machining or grit blasting (Ruzga, Willis, and Kumar 1993).
Figure 2. Schematic of various thermal spray processes (Irons 1992).
Figure 3. Schematic of High Velocity Oxyfuel (HVOF) thermal spray process (Metco 1996).
Thermal spray coatings are generally limited in the thickness of material that can be deposited. This limit can be as low as 0.030 in. for plasma spray and HVOF coating processes (Irons 1992). However, in some cases 1 in. thick coatings have been applied (Musil, Dolhof, and Dvoracek 1996). Due to thickness limitations, deep cavitation damage would have to be repaired by welding, but thermal spray coatings could be applied to the welded surface to provide additional protection to the component. Thermal spray coatings could also be applied directly to properly cleaned and roughened surfaces that do not require weld repair.
It is anticipated that once a sprayed coating is applied, this coating will prevent damage to the underlying base metal. Because the sprayed coating becomes the active surface, future repairs of the affected area can be made using thermal spray coatings deposited by the HVOF process rather than by weld repair of the substrate, which costs approximately three times as much as flame spraying.