Types Of Corrosion

General Corrosion General corrosion is corrosion that proceeds without an appreciable localization of attack. This type of corrosion occurs on metals or alloys that do not develop an effective passive film on the surface. Usually, the corrosion mechanism is oxidation with the formation of metal oxide corrosion products. General corrosion is most often encountered in pumps with carbon steels and copper base alloys. Cast irons also experience a specialized form of general corrosion, known as graphitic corrosion, which will be considered separately.

Carbon steel does not develop a protective oxide film and will corrode at a rate dependent upon several characteristics of the water or other fluid, including temperature, oxygen content, pH, and fluid chemistry. Several empirical indices based on water chemistry exist and can be used to calculate the relative corrosivity of natural waters to carbon steel and similar ferrous alloys. The Langelier Index is best known. The rate of corrosion is also very dependent on velocity and increases with an increasing velocity. In most pump applications, with the notable exception of hydrocarbons, the corrosion rate of carbon steel is too high for this material to provide a useful life. However, carbon steel is frequently used, particularly in vertical pumps, with some form of protective coating to prevent corrosion. Coal tar epoxy is a preferred coating for many water services.

Copper alloys, including both brasses and bronzes, are also subject to general corrosion in the water applications where they are most commonly used in the pump industry. The corrosion rate will be increased by the presence of small amounts of sulfides in the water. Copper alloys gradually develop a protective copper oxide corrosion film in most applications. The corrosion rate gradually decreases over time as this film develops. The rate of general corrosion varies with the specific type or grade of copper alloy. Among the alloys commonly used in pumps, nickel aluminum bronzes have the lowest corrosion rate and best tolerance for higher velocities.

The general corrosion of a Ductile Ni-Resist casing from a vertical pump is shown in Figure 1. A metallographic cross section was removed to show the depth of the corrosion attack.

FIGURE 1 A small fragment of Ductile Ni-Resist from the lower casing of a vertical pump. The microstructure is also shown on the right side of this figure, illustrating the depth of the corrosion's penetration. This is a classic example of general corrosion (right photo at 100X).

Dealloying Dealloying is the preferential removal of one phase from a multi-phase alloy, or one element from a material. Several types of dealloying occur in the pump industry. One of the most common is the graphitic corrosion of gray cast iron. This material is low cost, easy to machine, and well suited for a variety of applications, especially in the waterworks industry. It is probably the most widely used material in the pump industry.

Gray cast iron corrodes by a fundamentally different mechanism than carbon steel or ductile cast iron. The structure of gray cast iron consists of interconnected graphite flakes in a matrix that is predominantly iron. In the presence of an electrolyte, which is usually water, a galvanic cell is established between the iron and graphite. The iron corrodes, and the corrosion products are largely flushed away with the fluid passing through the pump. The original casting is gradually reduced to a porous graphite structure that may contain some iron oxide corrosion product. This is frequently referred to as graphitization. The surface of a gray iron casting that has suffered graphitic corrosion will retain its original shape and dimensions, but the surface will be largely graphite, which can be cut with a knife. The casting will lose some fraction of its mechanical properties and become increasingly susceptible to brittle failure, resulting from modest shock or impact loads. This is also the corrosion mechanism for Ni-Resist in seawater. Figure 2 shows the interface between the sound base metal and the graphitzed front.

It is important to recognize that the rate of graphitic corrosion varies with the water chemistry, and that this type of corrosion can occur in both fresh and salt waters. The high conductivity of salt water corresponds to a higher corrosion rate. Graphitic corrosion will proceed at a slower pace in waters that have a high mineral content. Minerals tend to plug the graphitic layer on the surface, sealing off the base metal from exposure to the fluid, thereby reducing the corrosion rate.

As the surface of a cast-iron component, such as a pump casing, gradually graphitizes, the galvanic relationships with other components within the pump will be altered. It has been observed that the bronze impeller originally supplied in a cast-iron pump handling seawater will provide a significantly longer life than bronze impellers that are installed after the pump has been in service for several years. The reduced life of the replacement impellers is caused by an altered galvanic relationship with the pump casing. Initially, the casing was cast iron, which is anodic to a bronze impeller. With time, as the casing graphi-tizes, it gradually becomes cathodic, due to the influence of the graphite. The bronze impeller is now the anode and corrodes at a much higher rate. This example highlights the influence that graphitic corrosion can have on other components within the pump and the importance of carefully selecting materials for use in conductive fluids, such as salt water.

Several other types of dealloying can also occur in pumps. Brass and bronze alloys containing more than about 14 percent zinc are subject to a form of dealloying known as dez-incification. The zinc is preferentially corroded from the matrix of the material, leaving a spongy, copper-rich residue. Dezincification can occur either uniformly in a shallow layer

FIGURE 2 The interface between the advancing graphitized front and the sound base metal. Graphitic corrosion propagates along the path of the graphite flakes (50X).
FIGURE 3 The dealloying of a vertical turbine pump impeller. Note the change in color across the cross section. The unaffected bronze (light color) material is surrounded by a dezincified layer (1.3X).

over the surface of the casting or as a distinct plug confined to a small area. Plug-type dez-incification is a more serious problem because the plug is weak and will cause leakage if it penetrates a pressure boundary, but it should be emphasized that copper alloys containing less than 14 percent zinc are not susceptible to this form of corrosion. Consequently, the requirement often imposed upon pump manufacturers for zinc-free bronzes to avoid dezincification is without technical justification. Figure 3 shows the dealloying of an impeller.

The final type of dealloying that occasionally occurs in pumps is dealuminification in aluminum bronzes. These are metallurgically complex materials. Some compositions can form an aluminum-rich phase that can be preferentially corroded in aggressive fluids, especially seawater. The detrimental phase can be mitigated by a special heat treatment known as temper annealing. This heat treatment must be specified by the designer for susceptible compositions, because it is not a mandatory requirement of national material specifications. The chemistry of some aluminum bronze alloys from Europe has been adjusted to preclude the formation of the detrimental aluminum-rich phase without the need for the temper annealing heat treatment. The temper anneal can serve as a stress relief operation for fabricated aluminum bronze structures, which is a secondary benefit for products in this category.

Galvanic Corrosion Galvanic corrosion refers to the corrosion that occurs when one alloy is electrically coupled to another and exposed in a conductive liquid. Usually, the corrosion rate of the more noble alloy will be less than if it were exposed uncoupled. The corrosion rate of the less noble material will be greater than if it were exposed uncoupled.

Several factors influence the rate of galvanic corrosion of both metals. This corrosion is greatly influenced by the conductivity of the fluid. In a fluid such as fresh water, which has a low conductivity, galvanic corrosion will be less severe and generally confined to the immediate location where the metals contact one another. However, in a highly conductive fluid, such as seawater, galvanic corrosion will be more severe and will occur over a wider area. The pump designer needs to consider the possibility of such corrosion when using dissimilar metals in a conductive fluid.

Galvanic corrosion problems in seawater and other conductive fluids can be avoided by the careful use of materials. Galvanic corrosion is related to the area ratios of the coupled metals. It is always desirable to have the area of the anode, or less noble metal, equal to or greater than that of the more noble metal. In this way, the additional corrosion experienced by the less noble metal will be spread over a relatively large area and will not be excessive because of being coupled. An example of the effective use of this galvanic relationship involves centrifugal pumps having a Ni-Resist casing and austenitic stainless steel internals. This combination is often specified for seawater services. The Ni-Resist is anodic to the stainless steel and will protect it from localized corrosion when the pump is shut down and contains stagnant water. The area of Ni-Resist is considerably larger than that of stainless steel. The increased galvanic corrosion of the Ni-Resist is spread over a large area and is negligible.

The amount of corrosion that will occur in a galvanic couple also depends on the freely corroding potentials of the coupled metals. Less corrosion-resistant metals, such as zinc, cast iron, and steel will usually have more negative potentials when measured against a standard reference electrode. More corrosion-resistant metals, such as stainless steels, will have less negative potentials.

The corrosion potentials for many commonly used engineering alloys in slowly moving seawater are shown in Table 1. The alloys are listed in the order of the potential that they exhibit in flowing seawater. Certain alloys (indicated by solid colored boxes preceding the name of the alloy) in low-velocity or poorly aerated water and at shielded areas may become active and exhibit a potential near —0.5 volts. The extent of galvanic corrosion that will occur when two metals are electrically coupled will depend on the potential difference between the metals. The corrosion rate of zinc coupled to stainless steel will increase dramatically because of the large potential difference between these two metals. A nickel aluminum bronze coupled to austenitic stainless steel will experience little galvanic corrosion because the potentials of these two metals are close to one another. The pump designer needs to be aware of the corrosion potentials of dissimilar metals used in conductive fluids in order to avoid unanticipated galvanic corrosion problems.

The use of coatings can decisively alter the galvanic relationships in a pump. If the more anodic component, such as a steel casing, is coated, one can expect a high rate of corrosion at those locations where the coating eventually begins to fail. This will be caused by a very unfavorable area ratio, with a small area of exposed carbon steel coupled to a large area of some more noble metal, such as stainless steel or bronze. For this reason, coatings should be employed with caution in pumps handling conductive fluids that are constructed of dissimilar metals. It is generally advisable in these applications not to coat the anodic component. Figure 4 documents the galvanic corrosion on the interior diameter of a carbon steel flange connected to a stainless steel shroud. The accelerated corrosion is due to the unfavorable ratio of stainless steel to carbon steel in this component.

Stress Corrosion Cracking Stress corrosion cracking (SCC) is a particularly dangerous form of corrosion because it is not easily detected before it has progressed to such an extent that it can cause sudden catastrophic damage. Although relatively uncommon in pumps, it can occur in several classes of materials. The pump designer should be aware of the potential combinations of material and environment that can cause SCC.

Stress corrosion requires that several factors be present. These include tensile stress, which can be either residual or applied, a susceptible material, an environment capable of causing stress corrosion, and time.

The materials used in the pump industry that may experience SCC include austenitic and martensitic stainless steels, some copper base alloys, and, occasionally, Ni-Resist. The austenitic stainless steels are susceptible to stress corrosion in aqueous chlorides at temperatures above about 140°F (60°C). Cast alloys, which contain some fraction of ferrite in the microstructure, are significantly more resistant to stress corrosion than their wrought counterparts. The possibility of cracking is increased in situations where chlorides are concentrated, as by evaporation. High residual stress, often present in as-welded structures, also enhances the possibility of cracking. Increasing nickel content in austenitic stainless alloys enhances the resistance to SCC. The high nickel grade, commonly known as Alloy 20, is often used in chemical applications where the optimum resistance to stress corrosion is necessary. The SCC of austenitic stainless steels in pumps is relatively uncommon.

Martensitic stainless steels are susceptible to cracking in the presence of hydrogen sulfide and is often referred to as sulfide stress corrosion cracking (SSC). These steels, particularly CA-15 and CA-6NM, are commonly used in pumping applications in oil production and refining where hydrogen sulfide can be present. SCC can be avoided by giving these materials a special heat treatment intended to reduce hardness below a certain threshold level, below which cracking will not occur. This has also been correlated to the yield strength of a material. It is often seen in literature that ferrous materials used

TABLE 1 Corrosion potentials in flowing seawater (8-13 ft/s, 50-80°F/2.4-4.0 m/s, 10-26°C)

Volts: Saturated Calomel Half-Cell Reference Electrode

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in these services should have a hardness no greater than 22 Rc or a yield strength no higher than 90,000 lb/in2 (620MPa). Technical standards, including API 610 and NACE MR-01-75, can be used to specify appropriate requirements for martensitic steels, which will be used in environments containing hydrogen sulfide.

FIGURE 4 Galvanic corrosion is evident on this pump section. Note the high corrosion rate on the interior diameter of the carbon steel flange that is attached to the stainless steel shroud.

Copper alloys are susceptible to SCC in the presence of ammonia, although considerable variations take place in the susceptibility of the various types of bronzes, with aluminum bronzes being the most resistant. Polluted natural waters can contain ammonia, and for this reason, bronze pumps are usually not a good choice for these applications.

High-strength manganese bronzes are susceptible to cracking in natural waters. Cast impellers in these alloys have been known to suffer severe cracking. Residual stress in the casting may also be sufficient to induce cracking. These alloys should not be used in pumps because of their susceptibility to such problems.

Ni-Resist is an austenitic cast iron that contains 15 to 20% nickel. This material is commonly used in large, seawater vertical pumps. Experience has shown that it is subject to SCC, especially in the diffuser section of these pumps, unless the castings are furnace stress-relieved. This must be specified by the purchaser, as it is not a requirement of national material specifications.

Hydrogen Embrittlement Hydrogen damage is a form of environmentally assisted failure that results from the combined action of hydrogen and residual or applied tensile stress. Hydrogen damage to specific alloys or groups of alloys manifests itself in many ways, such as cracking, blistering, hydriding, or as a loss of tensile ductility. Collectively, these various forms of damage are often referred to as hydrogen embrittlement.

Damage caused by hydrogen is occasionally encountered in pumps. Some plating processes, such as chrome plating, which is often used to rebuild pump shafts, generate hydrogen. This hydrogen can enter the surface of the metal. Microscopic cracks can occur in higher strength steels (greater than a 90,000-lb/in2 or 620-MPa yield strength). Abusive grinding can work-harden the surface of lower strength steels and increase the probability that hydrogen will cause cracking. Microscopic cracks resulting from hydrogen damage act as stress risers and can propagate failure catastrophically by mechanical fatigue. This problem can be avoided by utilizing proper grinding practices before plating. Higher strength steels should be baked, to drive off hydrogen, immediately after plating.

Hydrogen can also be introduced into metals during welding. In order to avoid the hydrogen damage associated with welding, ferritic and martensitic steels should be welded with low hydrogen electrodes. Coated electrodes should be baked, in accordance with manufacturer's instructions, prior to usage in order to drive off moisture, which is the major source of hydrogen contamination of welds.

Microbiologically Induced Corrosion Living organisms can promote corrosion in many different environments. A variety of biological organisms thrive in both aerobic and anaerobic environments. Corrosion attributable to microbiological activity occurs most frequently in stagnant water, which remains in a pump when it is shut down for an extended length of time.

Sulfate-reducing bacteria are found in many waters. They will form slimy, reddish hemispherical shaped mounds or colonies on cast iron or carbon steel. These are known as tubercles. If scraped off, there will invariably be a saucer-shaped pit beneath the tubercle. The inside of the pit will contain a wet, black deposit. The pitting is caused by traces of sul-furic acid excreted by the bacteria. This type of corrosion will usually not result in premature failure.

Several more serious types of microbiologically induced corrosion afflict stainless steels. A certain class of metal ion concentrating/oxidizing microbes appears to concentrate ferric and manganic chlorides, both of which are potent pitting agents. These bacteria form colonies preferentially at welds in austenitic stainless steels and are capable of causing severe pitting corrosion in a relatively short time. This problem has been encountered in a variety of equipment in both salt and fresh water. It is often discovered only when the welds begin leaking. Pumps employing welded stainless steel fabrications can be afflicted by this problem if permitted to sit idle with stagnant water, either fresh or salt, for an extended period. Biocides can be used to mitigate this problem in some instances.

Finally, the decay of biological organisms can generate hydrogen sulfide, which adversely affects the protective oxide film on copper base alloys. The enhanced biological activity in warmer tropical waters, especially under stagnant conditions, can impair the corrosion resistance of bronzes and reduce the threshold velocity at which accelerated corrosion will occur. Bronzes should be used with caution in applications where macrobiolog-ical activity is anticipated and the possibility of extended shutdowns is possible.

Intergranular Corrosion This infrequent type of corrosion preferentially attacks a material at the grain boundaries. This is caused by local chemical differences such as the chrome-depleted regions of an austenitic stainless steel. Bronze alloys susceptible to this type of corrosion include aluminum brasses, silicon bronzes, Muntz metal, and admiralty metal. Two things are necessary: a sensitized material and a corrosive media, such as sea-water. Sensitization can occur during heat treatment or more commonly during weld repair. This type of corrosion often leads to corrosion-assisted fatigue cracks when cyclic loading is present.

The improper heat treatment of 300 series austenitic stainless steels can result in sen-sitization to intergranular corrosion. Sensitization occurs when stainless steels that contain more than .03% carbon are held at temperatures between 800 and 1550°F (between 425 and 850°C). At these temperatures, chrome carbides precipitate along the grain boundaries, resulting in chrome depletion in the adjacent areas. These adjacent areas have reduced corrosion resistance. Austenitic stainless steels contain approximately 16 to 18% chrome. The chromium content in the areas surrounding a chrome carbide particle can drop below the 12% necessary to maintain a passive state. A galvanic cell is set up with a large cathode (grains) and a small anode (grain boundaries). In this undesirable scenario, corrosion occurs along the anodic grain boundaries. The extent of the corrosion damage depends on the length of time held within the sensitization temperature range. The degree of sensitization is a function of the carbon content; the higher the carbon content, the shorter the period of time the material can be held within this range without sensitization occurring. A graph of the temperature versus time for various carbon contents illustrates this point in Figure 5. Intergranular corrosion of an improperly heat-treated stuffing box cover is shown in Figure 6.

Austenitic stainless steels can also be sensitized during normal welding procedures. Care must be taken to avoid the sensitization range during welding followed by proper post-weld heat treatment when necessary.

Sensitization can be avoided or corrected by several methods:

• Heat the material to a temperature high enough to dissolve the chrome carbides, typically 1900 to 2100°F (1040 to 1150°C), followed by rapid cooling through the sensitization range. Localized heat treatment of welded areas will not desensitize a material.

• Use a stainless steel that is stabilized by the addition of niobium or titanium. These two elements will tie up the carbon, thus preventing chrome carbides.

• Reduce the carbon content to a low level (less than .03 percent). The lower the carbon content, the longer it takes chrome carbide precipitation to occur.

When austenitic stainless steels are necessary in the pump industry, materials commonly used in services where intergranular attack is anticipated include 316L, 304L, CF-3, and CF-3M. Intergranular corrosion is not a concern in alloys containing 25% or more chromium.

Cavitation Erosion Cavitation erosion is primarily a mechanical process, although it acts synergistically with corrosion and is often considered with other forms of corrosion. Cavitation erosion can be defined as metal removal from the surface caused by high

FIGURE 5 Time-temperature sensitization curves as determined by the Strauss Test for 18-8 stainless steel. Note that a low carbon grade of stainless (0.03% C) requires five to 10 hours exposure, while a standard grade (0.08%) need only minutes of exposure time.
FIGURE 7 Cavitation erosion of an impeller, indicated by the porous appearance of cavitated regions on the surface

stresses associated with the collapse of vapor bubbles in the fluid. Cavitation occurs in a pump when the local pressure of the fluid is reduced to the vapor pressure. In a multistage pump, vapor bubbles form in the low-pressure areas at the impeller inlet and are swept by the flow into regions of higher pressure where they collapse. A great many bubbles may form and collapse in a small area, producing many microjets of high kinetic energy. The energy released by the bubble collapse is expended as impact loading on the metal surface. This situation is aggravated if protective oxide films are present because these are damaged, exposing fresh metal to the corrosive action of the fluid. This cyclic loading eventually causes the formation of microscopic fatigue cracks. These cracks propagate and intersect, resulting in the removal of metal from the surface and the characteristic spongy or porous appearance of cavitation damage. An example of a cavitated impeller is shown in Figure 7.

Although every effort should be made in the design and application of centrifugal pumps to prevent cavitation, it is not always possible to do so at capacities less than the rated maximum efficiency capacity of the pump. It must be recognized that at a low flow operation, the stated NPSH required curve is not usually sufficient to suppress all cavitation damage. The stated NPSH required is that needed to produce the head, capacity, and efficiency shown on the rating curve. At low flows, some cavitation damage should be expected. It may be impractical to supply an NPSH that would suppress all cavitation at these low flows, as it could be many times that it is required at the best efficiency point. Therefore, the possibility of cavitation damage frequently becomes a consideration when selecting material for impellers.

Open-type mixed flow impellers that produce heads in excess of 35 ft (10.7 m) are particularly susceptible to cavitation erosion in the clearance space between the rotating vanes and the stationary housing. This is usually referred to as vane tip erosion and is caused by a cavitating vortex in the clearance space between the vane and the housing. It is also impractical in this instance to provide sufficient NPSH to eliminate the cavitation. Any evaluation of the impeller and housing for a pump of this type should include the possibility of vane tip erosion.

It was conventional wisdom in the pump industry until recent years that the cavitation resistance of a material was directly related to its hardness. A more sophisticated under standing has been developed in recent years that has led to the development of a new class of nonstandard stainless steels with exceptional cavitation resistance.

The relationship between cavitation resistance and hardness was first critically investigated in the 1970s when it was observed that cobalt base alloys of a modest hardness developed a very high resistance to cavitation damage. Cavitation resistance was related to the capability of the material to transform at the surface when subject to cavitation loading into a harder, more resistant metallurgical phase. This work was extended to austenitic stainless steels, whose chemical composition was adjusted to promote the formation of a stress-induced martensite under cavitation loading. New alloys were developed initially as weld filler metals to repair cavitation damage and later as impeller castings for pumps. These alloys have relatively low hardness in the solution-annealed condition, comparable to standard austenitic grades, but transform to a much harder martensite at the surface upon exposure to cavitation loading. The hard surface layer resists the initiation of fatigue cracks. If these cracks eventually develop after extended exposure to cavitation bubbles, propagation into the soft ductile base metal is difficult. Cavitation-resistant austenitic stainless steel castings, alloyed with chrome and manganese, develop cavitation resistance similar to that of cobalt base alloys.

Extensive laboratory tests of the resistance of a wide range of materials to cavitation erosion have produced data for all the materials commonly used in centrifugal pump construction. It is possible to make a good correlation between the laboratory data and field experience to develop the following tabulation of the cavitation-resistance properties of pump materials, listed in order of decreasing cavitation resistance:

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