Various Types of Corrosion in Metals | Metallurgy

Corrosion is a very complex phenomenon. However, depending on the form, appearance and the distribution of the attack, corrosion is classified industrially in following types: 1. Uniform Corrosion 2. Atmospheric Corrosion 3. Inter-Granular Corrosion 4. Pitting Corrosion 5. Crevice Corrosion 6. Stress-Corrosion Cracking 7. Hydrogen-Embrittlement 8. Corrosion-Fatigue 9. Erosion-Corrosion and Few Others.

Types # 1. Uniform Corrosion:

If corrosion is such that the whole surface of the metal is corroded to the same degree, and the metal becomes thinner uniformly to eventually fail, it is called uniform corrosion. The mechanism could be chemical, or electro-chemical in nature. In uniform corrosion, specially when a metal is placed in an electrolyte, anode and cathode regions continually shift.

The metal corrodes uniformly even without being in contact with a second material. It is possible to estimate the useful life of a part under such corrosion. Such corrosion is commonly encountered in acid solutions, or in concentrated aqueous salt solutions. Uniform corrosion is responsible for greatest loss of metals on tonnage basis. Uniform corrosion can be reduced by proper choice of material and or coating on it, or by use of cathodic protection or inhibitors.

Types # 2. Atmospheric Corrosion:

Corrosion by various atmospheres is the greatest cause of destruction of metals, ferrous as well non- ferrous, on tonnage as well as on cost basis. Rusting of iron and steel is the most common visible form of atmospheric corrosion. Rusting depends very much on humidity. If air is wet, a thin film of moisture forms to cause electro-chemical corrosion at weak spots in the oxide film.

Oxygen can easily access the surface through the thin film. Below a critical about 65% relative humidity, there is no rusting and an air-formed oxide film is present. Although iron oxide is hygroscopic, rusting severely develops above about 80% relative humidity. Iron exposed to alkaline, or neutral solutions does not have a definite corrosion product, the rust. It is a result of several chemical steps.

The overall reaction is:

Fe + 1/2 O₂ + H₂O → Fe²⁺ + 2(OH) ̅ → Fe (OH)₂ …(14.50)

As ferrous hydroxide is relatively insoluble in water, it precipitates out of solution. The dissolved O₂ oxidises ferrous hydroxide to ferric hydroxide, which is the principal constituent of rust as-

2 Fe (OH)₂ + H₂O + 1/2O₂ → 2Fe(OH)₃ …(14.51)

Strictly speaking, rust consists of hydrated iron oxides (principally hydrated Fe₂O₃). In uniform corrosion, rust forms at the anode, but in other cases, it is somewhat removed from anode regions.

Metals initially form a protective film mainly due to oxidation of metal by air. The breakdown of this film occurs due to its hygroscopic nature as well as due to pollutants in air such as sulphur compounds, sodium chloride.

The break-down causes accelerated atmospheric corrosion, which is aggravated by dust, ash, and soot particles in the atmosphere. The rates of corrosion vary widely, though the corrosion is electro-chemical in nature.

These particles absorb moisture (being hygroscopic), gases like SO₂ and produce H₂SO₄ as:

S + O₂ = SO₂

2 SO₂ + O₂ + 2H₂O = 2 H₂ SO₄

H₂ SO₄ + 2 NaCl = Na₂ SO₄ + HCl

Galvanic corrosion and differential-aeration corrosion occur. Soot particle causes local severe corrosion as it prevents spreading of moisture or acid on the metal surface. Industrial atmospheres are more corrosive than rural atmospheres. Coastal atmospheres are also more corrosive.

Weathering steels (Cu = 0.1%) resist corrosion by forming tighter and more protective rust. These fail where humidity is high. Addition of Ni and Cu resists industrial atmospheres as the insoluble sulphates are not easily washed away. For complete rust-free alloys, use stainless steels. For common atmospheres, copper, lead, aluminium, galvanised steels are commonly used. Organic, inorganic or metallic coatings are also used. If possible, the humidity may be kept below the critical value.

Types # 3. Inter-Granular Corrosion:

Because of difference of structure and composition, grain-boundaries suffer from localised anodic corrosion. In most polycrystalline metals, uniform corrosion occurs as the grain boundaries are usually only slightly more reactive than the grains. But in some particular conditions, corrosion is severe at grain boun­daries causing inter-granular corrosion.

When the corrosion is severe at and adjacent to grain boundaries with little corrosion of the grains, so that the alloy even disintegrates there, it is called inter-granular corro­sion. The classical example is stainless steel, which in certain metallurgical conditions becomes so sensitive that complete corrosion occurs of grain boundaries for the part to fail.

Chromium in such steel is added to be present in solid solution state, and protects the steel by forming a passivating film over its surface. Such a steel if heated to 500-600°C, causes the chromium near the grain boundaries to be removed from the solid solution to form chromium carbide precipitates as illustrated in Fig. 14.14 (b).

The thin layers alongside the grain boundaries, being depleted of chromium are then no longer protected by the passivating film and become anodic relative to rest of the surface. Being very narrow, they are severely attacked by the corrosion current produced by the cathodic reactions over the rest of the metal surface.

Such a specific corrosion is commonly called weld decay, because during welding such a temperature range is attained at some distance away from the weld. One of the methods to overcome such an effect is to use stabilised stainless steels, which have small amount (0.5%) of a strong carbide-forming element such as titanium, columbium, or niobium, which have much greater affinity for carbon than does chromium to completely capture the carbon, and leave the chromium to remain in solution.

Keeping the carbon below 0.03% in stainless steels also does not cause inter-granular corrosion, or using high temperature (1050-1100°C) solution-heat-treatment followed by quick-cooling through the sensitizing range called quench-annealing of the weld-decay part too avoids the inter-granular corrosion.

Knife-line attack (KLA) is also inter-granular corrosion in the stabilised stainless steels similar to weld decay due to chromium carbide precipitation under certain conditions. KLA occurs in a narrow band adjacent to the weld where the metal attained 1400°C. At such a temperature, columbium/titanium carbides dissolve in that narrow zone, and fast cooling does not allow any carbide formation.

If heated to 500-800°C, chromium carbide-precipitates form to deplete the thin area of chromium and causes corrosion. To avoid KLA after welding, the part be reheated to 1060-1100°C, so that chromium carbide dissolves and columbium or titanium carbide precipitates to capture completely the carbon.

Age-hardenable type of aluminium alloys, duralumin type, are also susceptible to inter-granular corrosion though to a lesser extent. Precipitates of CuAl₂, Mg₂Si, Mg₅Al₈ etc. in these alloys along the grain boundaries result in depleted zones which are corroded in suitable environments. Some magnesium and copper base alloys also suffer from inter-granular corrosion.

Types # 4. Pitting Corrosion:

Pitting is one of the most destructive and dangerous form of corrosion. It is a type of corrosion where small localised areas of the metal are attacked to produce pits of sizes varying from shallow hemi-spherical form to pin-holes, and where the depth of penetration is normally far greater than the diameter.

Often, the pipe or the vessel gets perforations even though the amount of metal lost due to corrosion is very small. Pitting is particularly vicious because it is highly localised and intense form of corrosion. The failure often occurs very suddenly.

Initiation of Pit:

The occurrence and distribution of pits is a complicated process. Pitting probably nucleates due to the formation of a corrosion cell between a small anodic area and a large cathodic area, or the rate of dissolution is momentarily high at some particular point.

This small anodic area, or this point, with which corrosion manifests as pits, may be caused by heterogeneity existing in the metal, or in the film at the metal surface, or and in the solution in the immediate vicinity of the metal. An emergent screw dislocation may be a probable anodic site for the pit to be formed on the metal surface. Selective attack of a less noble metal of the widely separated galvanic couple, or presence of small areas of more plastic strain, which become anodic to the rest of metal to cause pits.

Once, metal has momentarily dissolved at high rate at a point, as in Fig. 14.32, the chloride-ions migrate to this point, but chloride quickens the dissolution further. The rate of dissolution increases in the pit, which produces an excess of positive charge (M⁺ ions) in that area to cause more migration of chloride-ions to maintain electroneutrality. This increases the concentration of MCl in the pit.

The reaction:

M⁺Cl ̅+ H₂O = MOH + H⁺ Cl ̅ …(14.52)

produces high concentration of hydrogen ions. H⁺-ions as well as Cl ̅ -ions help to dissolve most metals, and the entire process accelerates with time. Thus, pitting is self-stimulating and self-propagating. The pit has more concentrated solution. No oxygen reduction occurs within a pit as no oxygen is present inside.

The presence of concentrated solution within a pit is essential for further pitting, and thus pits are most stable when growing in the direction of gravity. li has also been seen that just outside the pit-periphery, the pit corrosion product and (OH) ̅ ions interact to produce ‘rust’ consisting of Fe(OH)₃, Fe₃O₄, Fe₂O₃ (in different oxidised states) in the form of tube. Pitting corrosion does not require a crevice as it creates its own.

It has been seen that pitting is mostly caused by chloride and chlorine-containing ions or rather halide ions as cupric, ferric and mercuric halides. Pitting increases with the increase of pH. Pitting is usually associated with stagnant areas having liquids.

Active metals like chromium, aluminium, and particularly stainless steels, are more susceptible to pitting. It is better to avoid stagnant conditions, avoid halide ions, reduce O₂ content and use polished clean surface.

Types # 5. Crevice Corrosion:

When intense localised corrosion occurs within crevices and other shielded-regions on metal surfaces exposed to corrosive environments, it is called crevice corrosion. There is a small volume of stagnant solution in crevices under bolt and rivet heads, in holes, gasket-surfaces, tubular sleeves; or create stagnant conditions under deposits such as sand, dirt, corrosion products. It is also called deposit or gasket corrosion.

Metals and alloys such as aluminium having oxide or passive film for corrosion resistance are also susceptible to crevice corrosion when such a film is destroyed by high concentration of chloride or hydrogen-ions.

The basic mechanism of crevice corrosion is metal-environment interaction by the reactions:

M → M⁺ + e̅

O₂ + 2H₂O + 4e̅ → 4(OH) ̅ …(14.52)

The depletion of O₂ to produce oxygen concentration-cell in the crevice does take place, and may be contributing to such a corrosion, but its depletion stops the reaction (14.52). Thus, crevice corrosion requires increased concentration of both metal, and chloride-ions in the crevice. Hydrolysis increases the hydrogen ion concentration too (pH is seen as 2-3). Thus, dissolution rate of metal is increased.

Crevice corrosion can be reduced by using proper alloying elements in metals, using welded joints, using non-absorbent gaskets, maintaining clean surface free of deposits, etc.

Types # 6. Stress-Corrosion Cracking (SCC):

When cracking occurs due to simultaneous presence of tensile stress and a specific corrosive environment, it is called stress-corrosion cracking. In SCC, most of the metal-surface is virtually unattacked while the fine cracks progress through it. The stress level whether residual or applied is within the range of design stress.

Two classical examples of stress-corrosion cracking are the ‘season cracking’ of brass and the ‘caustic embrittlement’ of steel. Cartridge-brass cracks when in residual-stressed-state in ammonia atmosphere. Explosions of the steel-boilers due to cracks at rivet holes too was due to the stresses produced during riveting alongwith caustic or sodium hydroxide atmosphere.

The cracks, in some cases are fine and numerous having ‘river delta’ pattern, but in other cases cracks are randomly oriented without branching. Normally, the crack proceeds perpendicular to the applied stress. Both intergranular and transgranular cracks have been observed. Stress-corrosion-cracking of a metal occurs in a specific environment.

For example, stainless steels crack in chloride environment, and not in ammonia environment, but reverse is true for brasses. In the same way, for each alloy-environment, there probably is an effective minimum, or threshold stress for stress corrosion-cracking to occur. But this minimum depends on the temperature, alloy composition and environmental composition.

The stress should be tensile in nature, whether applied, residual, thermal or due to welding. The width of the crack is narrow during the early stages of cracking, and little change in extension is observed. During later stages, the crack widens. Immediately preceding rupture, cross-section is reduced to the point where the applied stress is equal to or greater than the UTS of the metal. Large plastic deformation occurs and a large change in extension is observed. Finally, the failure occurs by mechanical rupture.

Stress-corrosion-cracking is a very important corrosion problem, but is not understood well, because of the complex interplay of metal, interface and environment properties. Moreover a specific mechanism may not apply to all metal-environment systems. The tensile stress ruptures the protective films during both initiation and propagation of the cracks. The breaks in the passive film (in stainless steels) allow more rapid corrosion at various points on the surface, and thus, initiate the cracks.

Rapid local dissolution is needed for rapid propagation. Corrosion plays an important role in the imitation of cracks. A pit, or discontinuity on the surface of the metal acts as stress-raiser As is known from theory of elasticity that stress concentration increases rapidly as the radius of the tip of pit decreases. Once a crack has started, the propagating-tip has small radius. Because of high stresses, plastic deformation can occur in the region immediately preceding the crack tip.

The cold-worked region is less corrosion resistant because of continuous projection of slip steps. Crack propagates by localised anodic dissolution, accelerated by plastic deformation. Chloride-ions could migrate to break down the film and to dissolve the metal. Surface-active species-absorb and interact with strained bonds at the crack tip to reduce the bond-strength and to cause crack propagation.

Crack propagates by formation of deep pits or tunnels due to dissolution followed by linking of these pits or tunnels by ductile rupture. The corrosion products, build up in the existing cracks, exert a wedging action.

Stress-corrosion-cracking can be prevented by using one or more of the following methods:

i. Lowering the Stress below the threshold value by annealing, or by using thicker sections, or by reducing the load.

ii. Modification or Elimination of Critical Environment:

This is accomplished by using degasification, or demineralization, or by adding sufficient inhibitors such as phosphates, etc.

iii. Selection of Proper Alloy:

Carbon steels have more resistance to stress-corrosion cracking than stain­less steels. Such a substitution may be done such as in heat exchangers used in contact with sea water.

iv. By cathodic Protection.

v. Shot-peening induces compressive residual stresses to reduce the stress-corrosion-cracking.

Types # 7. Hydrogen-Embrittlement:

The loss of ductility of a material in the presence of hydrogen is called hydrogen-embrittlement. Hydrogen-embrittlement is well known, if not well understood, phenomenon in BCC metals, specially in steels as well as in FCC metals and alloys. Generally, hydrogen-embrittlement occurs when metals are put under tensile load in an atmosphere of hydrogen.

Hydrogen penetrates into the structure of metals in atomic form. In titanium and other strong-hydride forming metals, the dissolved hydrogen forms brittle hydride compounds. In other materials like iron and steel, this dissolved hydrogen interacts to cause brittleness.

In some cases, atomic-hydrogen is absorbed during processes such as pickling, electroplating, corrosion in aqueous solutions, cathodic protection, welding etc., when the metal is not under stress. Subsequent stressing of such materials in normal atmospheres causes failures at loads much below the tensile strength. Such failures are also called delayed failures.

In steels, hydrogen-embrittlement occurs when stresses are tensile in nature. It does not effect the impact values, but a decrease in temperature decreases the susceptibility of steels to hydrogen-embrittlement. Fracture stress becomes less and fractured surface has brittle fracture.

In pressure-theory to explain hydrogen-embrittlement, the dissolved atomic hydrogen in steel lattice precipitates in molecular form at defects such as inclusions, microcracks, dislocation pile-ups, to build up high internal pressures (1000-1400 MNm^-2). In modified theory, the hydrogen precipitates in micro-cracks, voids, etc. which form in the region of triaxiality in front of a crack.

The stress-concentration increases the density of voids. As more hydrogen diffuses, voids grow in size, and coalesce with each other to form a micro-crack, which coalesces with an advancing crack to form a step in it. This process continues to produce a crack of critical length to cause fracture.

The adsorption theory profound the decrease of surface energy as the reason as hydrogen is adsorbed at the micro-crack. This decreases the fracture stress. None of these theories is able to explain all the phenomena of hydrogen-embrittlement.

However, one or more of the following methods prevents hydrogen-embrittlement:

a. Baking:

Hydrogen can be removed from steels by baking at 100°-150°C to restore almost the mechanical properties.

b. Decrease Hydrogen Pick-Up:

Pickling is very common cause of hydrogen pickup. Careful addition of inhibitor during pickling eliminates reaction of acid with base metal to drastically reduce the hydrogen pick-up to below damage level. Hydrogen pick-up during plating can be reduced by changing the baths and electroplating currents. Use of low-hydrogen welding rods and by maintaining dry conditions, hydrogen pick-up can be reduced during welding.

c. Proper Alloys:

Very high strength steels commonly suffer from hydrogen-embrittlement. Addition of nickel or molybdenum in steel reduces the susceptibility to hydrogen-embrittlement.

Types # 8. Corrosion-Fatigue:

The reduction of the fatigue strength due to the presence of a corrosive medium is called corrosion-fatigue as illustrated in Fig. 14.34. Steels have a fatigue limit in air, i.e., below a certain stress, it does not fail by fatigue, but steel having suffered from corrosion-fatigue does not have such a critical value of stress, i.e., Corrosion-fatigue-failure occurs even at lower stress values.

Fig. 14.33 illustrates a corrosion-fatigue failure. There is a large area covered with corrosion products and a smaller roughened area resulting from the final brittle sudden fracture. The presence of corrosion products need not necessary indicate corrosion- fatigue as the corrosion might have taken place later after fatigue failure had occurred.

But there are present a number of cracks rather than one crack, and these cracks are usually perpendicular to principal tensile stress. These originate at the surface where the stresses were at a maximum in corrosion-fatigue fractured surface. Corrosion-fatigue failure is usually transgranular.

Corrosion-fatigue is commonly seen in marine platforms, submarines, marine-propeller shafts, boilers, turbine rotors, blades, casing, rock drills, and in aerospace and nuclear plants.

Corrosion-fatigue is most pronounced at low stress frequencies as then there is greater contact time between the metal and corrosive medium. Corrosion-fatigue is effected by such factors of corrosive medium as oxygen content, pH, its composition, temperature. For example, stainless steel has only 70-80% of fatigue resistance in sea water as compared to normal water.

Corrosion-fatigue curves of iron and steel resemble the non-ferrous metals as indicated in Fig. 14.34. It has been seen that the corrosion-fatigue is more prevalent in environments which produce pitting corrosion. These pits act as stress-raisers and initiate fatigue cracks. Corrosion is most intense at the crack-tip.

Fatigue cracks are propagated from the bottom of these pits. Once, the crack has been initiated, the removal of corrosive medium does not effect the further propagation. That is why final stages of corrosion-fatigue are identical to ordinary fatigue, i.e., final fracture is purely mechanical.

Corrosion fatigue can be prevented by one or more of the following methods:

a. Lowering the tensile stress on metal or alloy.

b. Reducing the stress on the component by changing the design, or suitable heat treatment.

c. Inducing compressive stress in surface layers by shot-peening, nitriding etc.

d. Removing critical ions from the environment.

e. Addition of inhibitors to eliminate effects of environmental species.

f. Coatings to exclude the metal from direct contact of environment.

g. Avoid structural vibrations in service.

Types # 9. Erosion-Corrosion:

It is defined as the increase in the rate of corrosion of a metal due to relative motion between the metal surface and the corrosive environment which may be liquid or gas. This is a common type of corrosion where high velocities of corrosive fluid are encountered. It is also called impingement-corrosion.

The metal is removed from the surface as dissolved-ions, or if it forms solid corrosion-products, these are mechanically swept from the metal surface.

The erosion-corroded surface has a bright appearance with characteristic pits, grooves, waves, rounded holes and valleys, and exhibits a directional pattern. Most metals and alloys suffer from erosion-corrosion attack, but metals that are soft to wear off mechanically, such as copper and lead are more prone to erosion-corrosion. All the machines exposed to moving fluid suffer from erosion-corrosion such as bends, elbows, tees, nozzles, ducts, baffles, valleys, pumps, blowers, propellers, impellers, agitators and agitated vessels, turbine-blades, etc.

Factors Effecting Erosion-Corrosion:

i. Nature of Surface Film:

Some metals form a protective film. A hard, dense, adherent and continuous film having resistance to wear in the specific environment to which the metal is exposed in service, can provide better protection. Metal should be able to form the film quickly and easily when exposed fresh, or reform quickly when damaged or destroyed.

A film which is brittle or spalls may not be able to protect. The passive film on stainless steels is easily erosion-corroded in sulphuric acid-ferrous sulphate slurry moving with velocity. Carbon steels show good erosion-corrosion resistance when the pH of the water is 6 and 10 as it has film of Fe(OH)₂ and Fe(OH)₃ respectively.

At a pH of 8, film is granular of Fe₃O₄ whereas below 5 pH, the film cracks due to internal stresses, thus causing high rates of erosion-corrosion. TiO₂ film on titanium has good erosion-corrosion resistance in sea water, chloride solutions and fuming nitric acid.

ii. Velocity of Medium:

The increased velocity of medium may effect the mechanisms of corrosion reactions, as well as increase the mechanical wear specially when it has solids in suspension. Normally, the rate of attack increases rapidly when a critical velocity is attained, specially when it accelerates the corrosion mechanism. For example in steels by increased supply of O₂, CO₂ or H₂S.

It may decrease the corrosion by increasing effectiveness of inhibitors by supplying chemicals to the metal-surface at a higher rate, or by preventing the deposition of silt, dirt, which would cause crevice-corrosion, or by removing the corrosive agent present or newly formed. Turbulent-flow of fluid results in better intimate contact between the environment and the metal to enhance the erosion-corrosion.

Turbulent-flow-conditions exist very often when flowing-fluid changes from a large pipe into a smaller diameter-pipe, or due to presence of ledges or other obstructions. Impingement attack is very severe when fluid is forced to change its direction of flow. Solids, bubbles of gases, air bubbles in the liquid accelerate the impingement attack. For example, in steam-turbine blades, in exhausts, bends, tees, external components of aircraft, parts in-front of inlet pipes, etc.

iii. Galvanic Effect:

The contact of another metal in a flowing system can increase corrosion such as of stainless steel (316) with lead as it destroys the passive film on stainless steel by combined forces of erosion-corrosion and galvanic corrosion. In high velocities, steel is less attacked when coupled with stainless steel and titanium than when coupled with copper or nickel as in former there is more effective cathodic polarisation.

iv. Nature of Material:

The composition of the alloy largely determines its corrosion-resistance. An active-metal, or an alloy having active-metal resists corrosion due to its protective film. A noble metal has its own inherent corrosion-resistance such as nickel, so that 80 Ni-20 Cr alloy is superior to 80%Fe-20%Cr alloy.

Addition of third element such as iron to cupronickel, aluminium in brass increases resistance to erosion-corrosion by sea-water because of more stable-protective film due to third element. Soft metals are more subject to mechanical wear. Solid-solution hardening results in good erosion-corrosion resistance.

High silicon (14.5% Si) iron having non-precious metals, perhaps is the most universal corrosion resistant, and the only alloy that can be used in many severe erosion-corrosion conditions. Cast irons generally show better results than steels, particularly in hot strong H₂SO₄ acid.

Prevention:

i. Choice of material can be- done as explained above.

ii. Better Designs:

It includes change in shape or geometry and not in the selection of material. Some simple cases are illustrated in Fig. 14.36, to reduce impingement effects.

iii. Changes in Environment:

Deaeration as well as settling and Alteration of the fluid can be done. Inhibitors can be added. The temperature may be reduced, if possible without affecting the process. Temperature is the worst enemy in all types of corrosion including erosion-corrosion.

iv. Electro-chemical protection:

a. Cathodic Protection:

It reduces attack. Steel plates on condenser-heads protect inlet-ends of tubes in heat-exchanger using sea water. Zinc plugs are used in water pumps.

b. Other Methods:

Hard-facings or welded-overlays can be used provided facing has good corrosion resistance. Inhibitors can be used. Bichromates as inhibitor protects 70/30 brass against sea-water.

Types # 10. Cavitation-Corrosion:

It is a special form of erosion-corrosion caused by the formation and collapse of vapour-bubbles in a corrosive liquid near a metal surface. It occurs on surfaces which are in contact with high-velocity-flowing liquids with frequent pressure changes, such as in hydraulic turbines, ship pro­pellers, pump impellers, etc.

If pressure inside a water-filled container drops, water vapourises to form bubbles. The increase of pressure now, makes the bubbles to collapse. These two-steps if repeated at high speed, then shock waves with pressures as high as 400 MN/m^-2 are generated to cause plastic deformation of some metals. Slip lines have been seen in pump- parts. This can produce dents and can easily break any protective film. This is the process of cavitation.

Cavitation-corrosion is a combined action of mechanical effect and corrosion. Due to the former, i.e., the collapsing-vapour-bubbles destroy the protective surface films as illustrated in step 2 and 5 in Fig. 14.37, and thus increases the corrosion as the new exposed area corrodes and the film reforms.

The repetition of these steps results in deep-holes. In absence of protective film, the imploding-cavitation-bubble forces to tear the metal particles away from surface leaving behind a rough surface, where new cavitation bubbles can nucleate to continue the damage. Cavitation-damage increases as entrained air, dust in air, and corrosiveness of liquid increases with temperature having maximum damage at 50°C.

Similar preventive methods may be used as prescribed for preventing erosion-corrosion. The specific-measures are to use better material such as stainless steels instead of brasses. Extra-smooth-surface makes nucleation of bubbles difficult. Cathodic protection helps to reduce this damage. Rubber and plastic coats on metals reflect the shock-waves without intense damage. Designs could be improved of parts to minimise the difference of pressures in process-flow streams.

Types # 11. Fretting-Corrosion:

It is a corrosion occurring at contact areas between two surfaces under load subjected to vibrations and slip. Pits or grooves in metal surrounded by corrosion products (usually finely divided particles) are observed. Fretting-corrosion occurs in the atmosphere (and not in aqueous conditions), and has also been called friction-oxidation, wear-oxidation, false-brinelling (pits resemble brinell indents).

The oxide debris causes seizing and galling. Dimensional accuracy of closely fitting parts is lost. Frequent tightening of parts is needed. Press-fitted ball-bearings get loosened to ultimately fail. The rough surface decreases the fatigue resistance of the parts. The classic case of fretting-corrosion is at bolted-tie-plates on rail road rails.

Basic requirements for fretting-corrosion to occur are:

i. The surfaces in contact must be under load.

ii. Vibration, or repeated relative motion as small as 10^-8 cm must occur between two surfaces.

iii. The load as well as relative motion should be able to cause plastic-deformation on the surfaces. Two mechanisms have been proposed to explain the fretting-corrosion. Both the mechanisms operate during fretting-corrosion, i.e., both combine to cause fretting-corrosion.

a. Wear-Oxidation Mechanism:

Under the applied load, cold-welding or fusion occurs at the high points of-the two surfaces, Fig. 14.38 (a). During subsequent relative motion, the contact-points are ruptured, and as a result fragments of metal are removed. The frictional-heat immediately oxidises these small-sized fragments of metal. This process is continuously repeated to result in loss of metal, and production of oxide residue.

b. Oxidation-Wear Mechanism:

Most metals have a thin, adherent protective oxide film on the surfaces due to atmospheric oxidation. When the metals are put in contact under load, and subjected to repeated relative-motion, the oxide films are ruptured at high points to produce oxide debris. The freshly exposed surfaces get oxidised. The repeated steps cause films loss of metal and the oxidi-sed-debris.

Parkarised (phosphate-coating) parts with lubrication with low-viscosity oils and greases reduce fretting- corrosion. Using hard-metals, or even cold-worked or shot-peened metals reduce fretting-corrosion. Use gaskets to absorb vibrations and exclude oxygen at bearing-surfaces to prevent this corrosion. Increase the load on surfaces to prevent slip of surfaces.

Types # 12. Selective-Leaching or De-Alloying:

These two-terms in general mean preferential removal of an element from the solid-alloy leaving behind the altered structure. The most common examples are dezincification (removal of Zn from brasses) and graphitisation (removal of ferrite from gray cast-iron), though others like de-aluminification (Al from Al- bronzes), de-cobaltification, etc. are also seen. Fine metal powders could be produced by-such an attacked alloy, such as of nickel.

Dezincification is selective corrosion of Zn from brasses having more than 15% Zn. The rate increases with the increase of zinc content of brasses. The colour of yellow-brass becomes red or copper-colour, but it becomes weak, porous having little strength. Dezincification is uniform in acid medium of high zinc brasses, but plug-type in low zinc brasses.

The process intensifies in stagnant conditions, at high temperatures, in high acid and chloride content solutions. Probably, zinc atoms leave the brass lattice with vacant sites behind, or dissolution of brass occurs to reprecipitate copper back. Dezincification is reduced by oxygen removal, cathodic protection, or using alloys having 1% Sn too, or small amounts of As, Sb or P.

Graphitisation occurs of gray cast iron. Graphite acts as cathode to preferentially corrode anodic ferrite. The end result is weak porous mass of graphite network with voids and rust, which can be easily cut by knife. Graphitisation is a very slow process and takes place in mild mediums such as soil or water. Graphitisation does not occur in S.G. iron or malleable iron (absence of graphite network) and white cast irons.