Corrosion-erosion

Erosion-corrosion can be defined as the accelerated degradation of a material resulting from the joint action of erosion and corrosion when the material is exposed to a rapidly moving fluid. Metal can be removed as solid particles of corrosion product or, in the case of severe erosion-corrosion, as dissolved ions. 

Metal surfaces in a well-designed, well-operated cooling water system will establish an equilibrium with the environment by forming a coating of protective corrosion product. This covering effectively isolates the metal from the environment, thereby stifling additional corrosion. Any mechanical, chemical, or chemical and mechanical condition that affects the ability of the metal to form and maintain this protective coating can lead to metal deterioration. Erosion-corrosion is a classic example of a chemical and mechanical condition of this type. A typical sequence of events is  

A protective coating forms over the metal surface. 

The coating is mechanically removed by the abrasive effects of a high-velocity fluid. 

The protective coating reforms on the metal surface via a corrosion process. 

The term erosion-corrosion is used to describe the increased rate of attack caused by a combination of erosion and corrosion. If a fluid stream contains suspended particles, or where there is high velocity or turbulence, erosion will tend to remove the products of corrosion and any protective film, and the rate of attack will be markedly increased. If erosion is likely to occur, more resistant materials must be specified, or the material surface protected in some way. For example, plastics inserts are used to prevent erosion-corrosion at the inlet to heat-exchanger tubes. 

General Description. Erosion-corrosion is the acceleration or increase in the rate of deterioration or attack on a metal because of mechanical wear or abrasive contributions in combination with corrosion. The combination of wear or abrasion and corrosion results in mote severe attack than would be realized with either mechanical or chemical corrosive action alone. Metal is removed from the surface as dissolved ions, as particles of solid corrosion products, or as elemental metal. The spectrum of erosion-corrosion ranges from primarily erosive attack, such as sandblasting, filing, or grinding of a metal siuface, to primarily corrosion failures, where the contribution of mechanical action is quite small. 

All types of corrosive media generally can cause erosion-corrosion, including gases, aqueous solutions, organic systems, and liquid metals. For example, hot gases may oxidize a metal then at high velocity blow off an otherwise protective scale. Solids in suspension in liquids (slurries) are particularly destructive from the standpoint of erosion-corrosion. 

Erosion-corrosion is characterized in appearance by grooves, waves, rounded holes, and/or horseshoe-shaped grooves. Analysis of these marks can help determine the direction of flow. Affected areas are usually fiiee of deposits and corrosion products, although corrosion products can sometimes be found if erosion-corrosion occurs intermittently and/or the liquid flow rate is relatively low. 

Metals Affected. Most metals are susceptible to erosion-corrosion under specific conditions. Metals that depend on a relatively thick protective coating of corrosion product for corrosion resistance are frequently subject to erosion-corrosion. This is due to the poor adhesion of these coatings relative to the thin films formed by the classical passive metals, such as stainless steels and titanium. Both stainless steels and titanium are relatively immune to erosion-corrosion in many environments. Metals that 

Prevention, Erosion-corrosion can be prevented or reduced through improved design (e.g., increase pipe diameter and/or streamline bends to reduce impingement effects), by altering the environment (e.g., deaeration and the addition of inhibitors), and by applying hard, tough protective coatings. 

Sources of various mechanical forces involved in the erosion of protective films and imderl3dng metal are listed here and illustrated in Fig. 6.39 [29]  

The properties of surface films that naturally form on metals and alloys are important elements to understand the resistance of metallic materials to erosion-corrosion. Most metals and alloys used in 

However, if the flow of liquid becomes turbulent, the random liquid motion impinges on the surface to remove this protective film. Additional oxidation then occurs by reaction with the liquid. This alternate oxidation and removal of the film will accelerate the rate of corrosion. The resulting erosive attack may be uniform, but quite often produces pitted areas over the surface that can result in full perforation (Fig. 6.40). 

Obviously, the presence of solid particles or gaseous bubbles in the liquid can accentuate the attack. Also, if the fluid dynamics are such that impingement or cavitation attack is developed, even more severe corrosion can occur. 

Chromium has proven to be most beneficial toward improving the properties of the passive film of ferrous and nickel-based alloys while molybdenum, when added to these alloys, improves their pitting resistance. Oxide passive films that contain insufficient molybdenum, such as in many nickel-based alloys and stainless steels, are susceptible to pitting in stagnant and low-flowing seawater, but perform well on boldly exposed surfaces at intermediate and high flow velocities. In oilfield conditions, fluid velocity acts in 

The effect of solution velocity or the movement of a metal in a solution, on the rate and form of corrosion is extremely complex. From a fundamental viewpoint, an increase in fluid velocity can increase the corrosion rate by bringing the cathodic reactant, such as dissolved oxygen in the BWR coolant, more rapidly to the surface of the metal. 

The movement of solutions above a certain threshold velocity level can result in another form of attack that is the result of the interaction of fluid-induced mechanical wear or abrasion plus corrosion. The general term erosion corrosion (E/C) includes all forms of accelerated attack in which protective surface films and/or the metal surface itself are removed by this combination of solution velocity and corrosion such as impingement attack, cavitation damage and fretting corrosion. 

Recently, terms flow-assisted corrosion and flow-accelerated corrosion (FAC) have been used to describe the erosion (or thinning) of carbon steel in nuclear and fossil power plants where there is no threshold solution velocity. FAC is a complex phenomenon that is a function of many parameters of water chemistry, material composition and hydrodynamics. FAC involves the electrochemical aspects of general corrosion plus the effects of mass transfer and momentum transfer. 

When there is a relative motion between the corroding liquid and the metal or rubber surface the rate of attack of the damage to the surface is increased. The process is called sweating off with the corrosion product thus exposing the base surface again to corrosion. Otherwise the corrosion product (as a newly formed protective layer) would have prevented or slowed down further corrosion, just as in the case of hypochlorous acid solution on natural rubber lining, where the protective corrosion products exhibit very low cohesion and as such prone to be wiped off by the liquid unlike wet chlorine or hydrochloric acid which form a strong layer of corrosion product well adhered to the rubber surface. 

A stainless steel pump impeller with a projected life of 2 years failed in three weeks in a reducing solution. Soft metals such as copper and lead are readily damaged. Even noble metals such as silver, gold, and platinum are subject to 

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Erosion is the deterioration of a surface by the abrasive action of solid particles in a liquid or gas, gas bubbles in a liquid, liquid droplets in a gas or due to (local) high-flow velocities. This type of attack is often accompanied by corrosion (erosion-corrosion). The most significant effect of a joint action of erosion and corrosion is the constant removal of protective films from a metal s surface. This can also be caused by liquid movement at high velocities, and will be particularly prone to occur if the solution contains solid particles that have an abrasive action. 

Localized erosion-corrosion caused by turbulence or impinging flow at certain points of the surface. In the majority of cases of impingement attack, a geometrical feature of the system results in turbulence at one or more parts of the surface. 

Levy A 1995 Solid Partiole Erosion and Erosion-Corrosion of Materials (Materials Park, OH ASM International)... 

The heat-transfer quaUties of titanium are characterized by the coefficient of thermal conductivity. Even though the coefficient is low, heat transfer in service approaches that of admiralty brass (thermal conductivity seven times greater) because titanium s greater strength permits thinner-walled equipment, relative absence of corrosion scale, erosion—corrosion resistance that allows higher operating velocities, and the inherently passive film. 

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Erosion Corrosion. Erosion corrosion is the increase in the rate of metal deterioration from abrasive effects. It can be identified by grooves and rounded holes, which usually are smooth and have a directional pattern. Erosion corrosion is increased by high water velocities and suspended soHds. 

It is often localized at areas where water changes direction. Cavitation (damage due to the formation and coUapse of bubbles in high velocity turbines, propellers, etc) is a form of erosion corrosion. Its appearance is similar to closely spaced pits, although the surface is usually rough. 

High Water Velocities. The abiUty of high water velocities to minimize fouling depends on the nature of the foulant. Clay and silt deposits are more effectively removed by high water velocities than aluminum and iron deposits, which are more tacky and form interlocking networks with other precipitates. Operation at high water velocities is not always a viable solution to clay and silt deposition because of design limitations, economic considerations, and the potential for erosion corrosion. 

Impingement Corrosion This phenomenon is sometimes referred to as erosion-corrosion or velocity-accelerated corrosion. It occurs when damage is accelerated by the mechanical removal of corrosion products (such as oxides) which would otherwise tend to stifle the corrosion reac tion. 

Leaky valves are also a cause of erosion. Most turbine erosion-corrosion problems come from damage that takes place when the unit is not running. A shght steam leak into the turbine will let the steam condense inside the turbine, and salt from the boiler water will settle on the inside surfaces and cause pitting, even of the stainless blading. There must be two valves with a drain between them, i.e., a block valve on the header and an open drain in the line before it reaches the closed trip-throttle valve. 

In a turbine that is running, erosion-corrosion is pretty much confined to units that are operating on saturated steam with inadequate boiler-water treatment. This type of erosion takes place behind the nozzle ring and around the diaphragms where they fit in the casing. 

Indirect attack can also occur because of turbulence associated with flow over and around a deposit. Increased turbulence may initiate attack (see Chap. 11, Erosion-Corrosion and Chap. 12, Cavitation Damage ). 

Passive corrosion caused by chemically inert substances is the same whether the substance is living or dead. The substance acts as an occluding medium, changes heat conduction, and/or influences flow. Concentration cell corrosion, increased corrosion reaction kinetics, and erosion-corrosion can he caused by biological masses whose metabolic processes do not materially influence corrosion processes. Among these masses are slime layers. 

Shells, clams, wood fragments, and other biological materials can also produce concentration cell corrosion. Additionally, fragments can lodge in heat exchanger inlets, locally increasing turbulence and erosion-corrosion. If deposits are massive, turbulence, air separation, and associated erosion-corrosion can occur downstream (see Case History 11.5). 

After only 4 months of service, the main condenser at a large fossil utility began to perforate. Initial perforations were due to erosion-corrosion (see Case History 11.5). Small clumps of seed hairs entering the condenser after being blown into the cooling tower were caught on surfaces. The entrapped seed hairs acted as sieves, filtering out small silt and sand particles to form lumps of deposit (Fig. 6.24A and B). Immediately downstream from each deposit mound, an erosion-corrosion pit was found. 

Flow effects may be pronounced. High-turbulence areas can become preferred attack sites (Fig. 7.17). Erosion-corrosion phenomena are important (Fig. 7.18) (see Chap. 11, Erosion-Corrosion ). 

Visual examination of external surfaces revealed grooves and general metal loss (Figs. 7.24 and 7.25). Metal loss was caused by erosion-corrosion. However, the corrosive loss was more important than erosive loss, since metal loss was also substantial in low-flow regions. 

Most metals are subject to erosion-corrosion in some specific environment. Soft metals, such as copper and some copper-base alloys, are especially susceptible. Erosion-corrosion is accelerated by, and frequently involves, a dilute dispersion of hard particles or gas bubbles entrained in the fluid. 

When very high velocities are encountered, metal loss from erosion-corrosion can be general. T ically, however, erosion-corrosion produces localized metal loss in immediate proximity to the disrupted flow. Smooth, rolling, wavelike surface contours are often produced, or distinct, horseshoe-shaped depressions (Fig. 11.1) or comet tails... 

Figure 11.3 Sand dunelike erosion-corrosion patterns on the inlet end of a steel heat exchanger tube. (Magnification 7x.)...

Figure 11.4 Deposits covering intact metal surfaces at sites of horseshoe-shaped erosion-corrosion depressions. (Magnification 15x.)...

Affected areas are essentially free of deposits and corrosion products, although these may be found nearby (Fig. 11.4). Affected areas may be covered with deposits and corrosion products if erosion-corrosion occurs intermittently, and the component is removed following a period in which erosion-corrosion was inactive. 

Favored locations for erosion-corrosion are areas exposed to high-flow velocities or turbulence. Tees, bends, elbows (Fig. 11.5), pumps, valves (Fig. 11.6), and inlet and outlet tube ends of heat exchangers (Fig. 11.7) can be affected. Turbulence may be created downstream of crevices, ledges (Fig. 11.8), abrupt cross-section changes, deposits, corrosion products, and other obstructions that change laminar flow to turbulent flow. 

Erosion-corrosion problems on the outside of tubes are frequently associated with impingement of wet, high-velocity gases such as steam. This typically involves peripheral tubes at the shell inlet nozzle (Fig. 11.9). Baffle and tube interfaces may also be affected. 

Figure 11.5 Erosion-corrosion at elbow of a brass tube. Note also the borseshoe-shaped depressions and comet tails aligned with flow direction in the straight section.

Erosion-corrosion is a fairly complex failure mode influenced by both environmental factors and metal characteristics. Perhaps the most important environmental factor is velocity. A threshold velocity is often observed below which metal loss is negligible and above which metal loss increases as velocity increases. The threshold velocity varies with metal and environment combinations and other factors. 

Figure 11.8 Turbulence created by the ledge at the tube end caused erosion-corrosion immediately downstream.

Figure 11.9 Erosion-corrosion damage on the surface of a brass tube facing the steam inlet nozzle.

Although it is entirely possible for erosion-corrosion to occur in the absence of entrained particulate, it is common to find erosion-corrosion accelerated by a dilute dispersion of fine particulate matter (sand, silt, gas bubbles) entrained in the fluid. The character of the particulate, and even the fluid itself, substantially influences the effect. Eight major characteristics are influential particle shape, particle size, particle density, particle hardness, particle size distribution, angle of impact, impact velocity, and fluid viscosity. 

In addition to fluid velocity, other characteristics of the eroding fluid can exert a marked influence on the erosion-corrosion process. Among the important factors are the following ... 

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