Cathodic and Anodic protection

Cathodic protection This is method of reducing or preventing corrosion of a metal by making it a cathode in the electrolytic cell. This can be achieved by means of an externally impressed current or sacrificial anode. An electrolyte is needed to ensure the passage of current through the part to be protected. This is effective only in soils or aqueous media where part to be protected is immersed. It is not effective in the atmosphere.

(1)   Impressed –current method an external DC power supply is connected to the metal be protected. The negative terminal of power supply is connected to the part to be protected and the positive to an Auxiliary or inert anode eg: graphite. Steel scrap, Al, Si-Fe are also can be used. Si-Fe and graphite are suitable for ground-beds-buried or sea-bed for marine projects.

Applications: pipe-lines, underground cables of Al, Pb; storage tanks, heat-exchangers, steel-gates exposed to sea water, hulls of ships, highways and bridges.

(2)   Sacrificial anode (or galvanic coupling) in this metal which has more negative electrode potential than the structure to be protected is connected electrically to the part or structure to be protected. The structure is protected at the sacrifice of another metal. Mg alloys, Zn, Al-5%Zn are widely used. These anodes are replaced as soon as consumed.

Applications: under-water parts of ships, ship hull, underground pipes, steel water tanks, water heaters, condenser tubes, oil-cargo-ballest tanks. Galvanized sheet is sacrificial protection of steel (Zn on steel).

Anodic protection This is based on the formation of a protective film on metals by externally applied anodic currents. An external current i_crit is initially applied impressed on the metal so as to passivate it. Then the current density is reduced to i_passive and maintained at that value to ensure the passive film does not dissolve. Material must exhibit passivity in corrodent eg: Ni, Fe, Cr, Ti and their alloys. A potentiostat is used to maintain the metal at a constant potential w.r.t a reference electrode. If the control is lost temporarily and the potential strays into the anodic region, the corrosion can be disastrously high. The primary advantage is its applicability in extreme corrosive environments with low current requirements.

Comparison of Anodic and Cathodic protection

	Anodic	Cathodic
Applicability	Active-passive matals/alloys	All metals/alloys
Nature of corrosive medium	Weak to aggressive	Weak to medium
Cost: Installation           Maintenance	High Very low	Low Medium to high
Operating conditions	Can be accurately determine	Determined by empirical testing
Significance of applied current	Direct measure of protected corrosion rate	Complex to indicate corrosion rate

Strenghtening from fine particles

Small second phase particles (Hard) distributed in a ductile matrix are a common source of alloy strengthening. 

Precipitation hardening is produced by solution treating (heating to a single phase region) and quenching an alloy in which a second phase is in solid solution at the elevated temperature but precipitates upon quenching and aging at room temperature (natural aging) or at slightly higher temperature (artificial aging, 100-200degrees). for this to occur the second phase must be soluble at an elevated temperature but must exhibit decreasing solubility with decreasing temperature. coherency between the precipitates and matrix is essential. The hardness increases with the formation of GP zones (Guinier-preston) and the intermediate transition precipitates.. A peak in hardness results due to critical dispersion of GP zones , further aging leads to decrease in hardness due to coarsening of precipitates (incoherent). This is called over-aging. eg:Al-Cu (aerospace industry) and Cu-Be (sparking tools in coal-mines).

The peak hardness depends on

1. Average particle size (fine particles)

2. Number of Particles (more finer particles)
3. Inter particle distance (less)

Methods of studying precipitation
1.Mechanical properties During aging as the amount of precipitate increases with time which increases the strength or hardness of the alloy. tension test or hardness measurement can be used to know the changes in mechanical properties.
2. Electrical resistivity During aging the excess solute comes out gradually hence strains in the crystal lattice decreases hence resistivity decreases.
3. X-ray diffraction Its application is to measure strain in the crystal. strains in the lattice will decrease with time.
4. Electron Microscopy as precipitates are very small in size (few nano meters) we have to use electron microscopy to observe the precipitation.

The fraction of second phase is limited by solubility limit. Higher supersaturation causes faster precipitation. The degree of super saturation decreases with the increase of aging temperature resulting in lower peak hardness at high temperatures. since the amount of second phase is less and inter particle distance is high.

Reasons for hardening in precipitation-hardening

1. Internal strain-hardening by elastic coherency strains around GP zones.

2. Chemical-hardening due to precipitates being sheared (cut) by moving dislocations.

3. Dispersion-hardening due to formation of loops of dislocations around precipitates.

The requirement of a decreasing solubility with temperature places a limitation on the number of useful precipitation-hardening alloy systems. Precipitation hardened alloys can't be used at higher temperatures  precipitates dissolve in the matrix at higher temperature. To overcome these difficulties a dispersion strengthening is developed.

Dispersion hardening  The hard and strong foreign particles are dispersed in a metal/alloy (matrix). Powder metallurgy is the best route to consolidate these dispersion alloys. These particles are oxides, carbides, nitrides etc. Such alloys are called dispersion strengthened alloys. The second phase alloys has very little solubility in the matrix even at elevated temperatures. No coherency between the second-phase particles and the matrix. These alloys are much more resistant to recrystallization and grain growth than single-phase alloys. Second phase particles donot dissolve even at high temperature.

Advantages

1. can be used for High temperature applications

2. we can use this for any alloy system

3. No limitation on the fraction or amount of dispersoid

Exam Pattern

There will be a total of 65 questions carrying 100 marks.

Q.1 to Q.25 (25 questions) carry one mark each (sub-total 25 marks). Q.26 to Q.55 (30 questions) carry two marks each (sub-total 60 marks). Questions Q.56 - Q.65 belong to General Aptitude (GA). Questions Q.56 - Q.60 (5 questions) carry 1 mark each (sub-total 5 marks) and questions Q.61 - Q.65 (5 questions) carry 2-marks each (sub-total 10 marks).

Questions Q.48 - Q.51 (2 pairs) are common data questions. and Qquestion pairs (Q.52, Q.53) and (Q.54, Q.55) are linked answer questions. The answer to the second question of the linked answer questions depends on the answer to the first question of the pair. If the first question in the linked pair is wrongly answered or is unattempted, then the answer to the second question in the pair will not be evaluated.

NEGATIVE MARKING:For Q.1 - Q.25 and Q.56 - Q.60, ⅓ mark will be deducted for each wrong answer. For Q.26 - Q.51 and Q.61 - Q.65, ⅔ mark will be deducted for each wrong answer. The question pairs (Q.52, Q.53), and (Q.54, Q.55) are questions with linked answers. There will be negative marks only for wrong answer to the first question of the linked answer question pair i.e. for Q.52 and Q.54, ⅔ mark will be deducted for each wrong answer. There is no negative marking for Q.53 and Q.55.

Questions on Engineering Mathematics will carry about 15% of the total marks (excluding General Aptitude section)

Cold working and Annealing

Cold working is deformation carried out under conditions where recovery processes are not effective. Hot working is deformation under conditions of temperature and strain rate such that recovery processes take place simultaneously with the deformation.

Structural changes during cold working of polycrystalline metals and alloys

(1)   Changes in shape and size of grains: The equiaxed grains on deformation are elongated in the direction of acting force i.e. stretched in the direction of main tensile deformation stress–say, in the direction of rolling or wire drawing.

(2)   Changes in orientation of grains: Preferred orientation or texture of is the state of severely cold worked metal in which certain crystallographic planes of the grains orient themselves in a preferred manner with respect to the direction of the stress (or maximum strain).

(3)   Changes in internal structure of grains: during cold working around 15% of the work of the deformation gets absorbed in the material (rest is lost as heat). This stored energy is the form of energy of crystal defects. Plastic deformation increases the concentration of point defects. With increase of cold working, the number of stacking-faults increases, thus density of extended dislocations increases. The number of kinks, jogs, dipoles, prismatic loops increase. The most important internal change of structure is increase in density of dislocation from 10⁶ – 10⁸ cm^-2 in annealed state to 10¹⁰ – 10¹² by moderate cold working.

Effect of cold work on properties

Cold working or strain hardening is the increase in the stress required to cause further slip because of previous plastic deformation. This is an important industrial process that is used to harden metals or alloys that do not respond to heat treatment. It changes various mechanical, physical and chemical properties of metals and alloys.

With increase in amount of cold work, Ultimate Tensile Strength, Yield Strength, Hardness increases but ductily (elongation and reduction in area) decreases. Cold worked texture and mechanical fibering leads to Anisotropy in in properties of materials. The ductility and impact toughness is much lower in transverse section rather than in longitudinal section. As the internal energy of cold worked state is high, the chemical reactivity of the material increases i.e. the corrosion resistance decreases, and may cause stress corrosion cracking in certain alloys. The rate of strain hardening (slope of flow curve) is generally lower in HCP metals than cubic metals. High temperatures of deformation also lower the rate of strain-hardening.

Annealing of Cold worked materials

In certain applications materials are used in the cold-worked state to derive benefits of increased hardness and strength. The cold worked dislocation cell structure is mechanically stable, but not thermodynamically stable. It is necessary to restore the ductility to allow further cold deformation or to restore the optimum physical properties such as electrical conductivity essential for applications. The treatment to restore the ductility or electrical conductivity with a simultaneous decrease in hardness and strength is Annealing (or Recrystallization annealing). It is heating cold worked metal to a temperature above recrystallization temperature, holding there for some time and then slow cooling.

The process of Annealing can be divided into three fairly distinct stages (1) Recovery (2) Recrystallization (3) Grain growth. There is no change in composition or crystal structure during annealing. The driving force for recovery and recrystallization is the stored cold-worked energy, whereas for grain growth is the energy stored in grain boundaries.

Recovery It is restoration of the physical properties of the cold worked metal without of any observable change in microstructure. It is the Annihilation and rearrangement of point imperfections and dislocations without the migration of high angle grain boundaries. Recovery is initially very rapid, and more when the annealing temperature is high. Electrical conductivity increases rapidly toward the annealed value and lattice strain measured using XRD is appreciably reduced. Properties those are sensitive to point defects are affected, and strength properties are not affected. With increasing time at constant temperature the recovery becomes slower. The greater the initial cold work, the more rapid is the initial rate of recovery. The rate of recovery of fine grains is higher than that of coarse grains.

Polygonization one of the recovery processes which leads to rearrangement of the dislocations, with a resultant lowering of the lattice strain energy. It is a process of arranging excess edge dislocations in the form of tilt boundaries, and the excess screw dislocations in the form of twist boundaries, with the resultant lowering of the elastic strain energy. Climb and slip of dislocations are essential for polygonization. The presence of solute atoms in a metal reduces the rate of polygonization.

Recrystallization It is nucleation and growth of new strain-free crystals from the cold worked metal. Kinetics of recrystallization resembles a phase transformation. Two distinct nucleation mechanisms have been identified. (1) Strain-induced boundary migration, where a strain-free nucleus is formed when one of the existing grain boundaries into its neighbour, leaving a strain-free recrystallized region. (2) new grains are formed in the regions of sharp lattice curvature through subgrain growth. This seems to predominate at high strains, with nuclei appearing at grain boundaries or at inclusions or second phase particles. Mechanical properties change drastically over a very small temperature range to become typical of the annealed material. Electrical resistivity decrease sharply.

Factors influence recrystallization behavior are (1) Amount of deformation (2) temperature (3) time (4) initial grain size (5) composition (6) amount of recovery or polygonisation (7) Method of deformation. Hence recrystallization temperature is not a fixed temperature in the sense of a melting temperature. It can be defined as the temperature at which a given alloy in a highly cold-worked state completely recrystallizes in 1h. The laws of recrystallization are: (1) a minimum amount of deformation is needed to cause recrystallization. (2) Smaller the degree of deformation, higher the temperature required to cause recrystallization. (3) Recrystallization rate increases exponentially with temperature. Doubling the annealing time is approximately equivalent to increasing the annealing temperature 10°C. (4) Greater degree of deformation and lower annealing temperature, the smaller the recrystallized grains. (5) Larger the original grain size, the greater the amount of cold-work required to produce equivalent recrystallization temperature. (6) The recrystallization temperature decreases with increasing impurity of motel. Alloying always raise recrystallization temperature. (7) The amount of deformation required to produce equivalent recrystallization behavior increases with increased temperature of working.

Solute and Pinning effects The impurity in metal segregate at grain boundary and retard the migrating boundaries during recrystallization. This is known as the solution drag effect. When fine second phase particle (carbides) lies on the migrating boundary, the grain boundary area is reduced by an amount equal to cross sectional area of particle. When the boundary moves further, it has to pull away from the particle and thereby create new boundary are equal to cross sectional area of particle. This increases energy and manifests itself as a pinning acting on the boundary. Consequently the rate of recrystallization decreases.
Grain growth It is uniform increase in the average grain size following recrystallization. The grain size distribution does not change during normal grain growth.  During abnormal grain growth called secondary recrystallization because the phenomenon shows kinetics similar to recrystallization, the grain size distribution may radically change i.e. some very large grains present along with the fine grains. The driving force for abnormal growth is decrease in surface energy. Solute drag and pinning action of second phase particles retard movement of a migrating boundary during grain growth as well.

comparison of mechanical properties during Recovery, Recrystallization and Grain growth.

Yield point and strain aging

Yield point phenomenon

localized heterogenous type of transition from elastic to plastic deformation produces a yield point in stress-strain curve. The load increases steadily with elastic strain, drops suddenly, fluctuates approximately at constant value of load, and then rises with further strain. The load at which the sudden drop ours is called the upper yield point. The constant load is called the lower yield point.

The deformation occurring throughout the yield-point elongation is heterogenous. At upper yield point a discrete band of deformed metal at a stress concentration such as a fillet, and coincident with the formation of the band the load drops drops to the lower yield point. The band then propagates along the length of the specimen, causing the yield-point elongation.

These bands are generally 45 degrees to the tensile axis. They are usually called Luders bands, hartmann lines or stretcher strains, and this type of deformation is sometimes referred to as the Piobert effect.Yield point was found originally in Low-carbon steel. It is also been observed in Iron, Polycrystalline Mo, Ti, Zn and Al alloys and in single crystal of Fe, Cd, Zn, alpha and beta brass  and Al.

This can be associated with small amounts of Interstitial or Substitutional  impurities. carbon and nitrogen are more effective.

Strain-aging

This is reappearance of yield point  in which strength of metal is increases and ductility decreases and a low value of strain rate sensitivity (m) on heating at a relatively low temperature after cold-working.

Plain carbon steel strained plastically through the yield-point elongation to a particular strain X (region A). If it is unloaded and reloaded again without any appreciable delay or any heat treatment yield point doesn't occur since the dislocations have been torn away from the atmosphere of carbon and nitrogen atoms (region B). if it is unloaded and reloaded after aging for several days at room temperature or several hours at an aging temperature like 400 K then yield point reappears and moreover, the yield-point will be increased (region C). This is due to the diffusion of C and N atoms to the dislocations during the aging to form new solute atmospheres anchoring the dislocations.

Nitrogen plays a more important role in the strain-aging of iron that carbon because it has a higher solubility and diffusion coefficient and produces less complete precipitation during slow cooling.

It is important to eliminate strain aging in deep drawing steels because the reappearance of yield point can lead to difficulties with surface markings or stretcher strains due to localized heterogenous deformation.

Remedies

(1) lower the free amount of carbon and nitrogen by adding strong carbide and nitride formers (Al, V, Ti, Cb, B)

(2) deform the metal by roller leveling or a skin-pass rolling and use it immediately before it can age.

Strain-aging is also associated with the occurrence of serrations in the stress-strain curve (discontinuous or repeated yielding). This dynamic strain-aging behavior is called the portevin-Lechatelier effect. Dislocation arrest and release by solute atoms are reasons for these serrations.Mechanical twinning during deformation or stress-assisted martensitic transformation also produce the same effect.

Plain carbon steels heated in 500 to 650 K shows a decreased tensile ductility and notched impact-resistance and a minimum strain rate sensitivity and maximum strain aging rate due to discontinuous yielding. This region is known as blue brittleness. This is just an accelerated strain aging. Hence plain carbon steels are not worm worked in this temperature range.

Solid solution strengthening

If the solute and solvent are roughly similar in size, the solute atoms will occupy lattice points in the crystal of solvent. This is called substitutional solid solution.

If the solute atoms are much smaller than the solvent atom they occupy interstitial positions in the solvent lattice. eg: C, N, O, H, B etc.

Smaller atoms produce a local tensile stress field in the crystal where as larger atoms produce a local compressive field. In both cases, the stress field of solute atom results in increase in stress required to move dislocation. Hence strength increases and hardness also increases.

Limitation: each alloying element has got its own solubility limit.

Factors favour extensive substitutional solid solubility (Hume rothery rules) 
(1) Crystal structure and valency of the both metals should be same.
(2) The size difference between the parent atom and solute atom must be    <15%.
(3) The electro negativity difference between two metals must be small.

Mechanism of solute atom-dislocation interaction

Elastic, modulus, stacking-fault, electrical, short-range order, long-range order.

Transformation (nucleation and growth)

The driving force for phase transformation arises if the free energy of parent phase becomes higher than that of product phase through a change in temperature or pressure. Below melting point, liquid can spantaneously transform to the solid. but the transformation can’t take place all at once.

The transformation can be divided into two steps that occur sequentially. 

Nuclaetion: The formation of tiny particles (nuclei) that are stable to further fluctuations and will not dissolve. 
Growth: The increase in the size of thse stable nuclei particles.

The difference in the volume free energy helps to create the interface between tiny solid and the liquid. But a very small particle has a large surface area to volume ratio. i.e. the volume free energy available is lesser than required to create the surface area and thus is unstable. Hence during nucleation stage surface energy is the dominant factor or inhibiting factor for phase change to occur.

Homogenous nucleation

The probability of nucleation remains constant throughout the volume of the parent phase. i.e. solid begins to nucleate throughout the bulk of the liquid without preference of any point.

A grater amount of supercooling is needed for homogenous nucleation to occur. Homogeneous nucleation is a difficult process and almost never occurs in industry. In practice, nucleation occurs at preferred sites. Heterogeneous nucleation is a much easier process.

Heterogenous nucleation

The probability of nucleation occuring at certain preferred sites in the liquid is much more than that at other sites i.e. solid nucleates preferentially at certain sites in the liquid phase. The preferred sites are the walls of container (in the case of liquid), inclusions, grain boundaries, stacking faults and dislocations (incase of solids). 

The key to reduction of the nucleation berrier is a small value of θ.

Note: critical radius remains same in both homogeneous and heterogeneous (independent of contact angle).

The rate of heterogenous nucleation is much higher than the homogenous nucleation (because Δf^*_het < Δf^*_hom).

Nucleation rate (I) and growth rate (U): both are highly dependent on thermal fluctuations. I and U are zero at melting temperature as well as at 0 K. they passes through a maximum at some intermediate temperature. growth maximum turns out to be higher than that for the nucleation maximum. 

Overall transformation rate: This is a function of both nucleation rate and growth rate. It has the same temperature dependence as nucleation and growth i.e. zero at T_m, increases with decreasing temperature (or increasing super cooling), reaches a maximum and then decreases to zero at 0 K.

T-T-T diagram: The data on transformation rate are usually plotted in the form of a T-T-T (temperature – time – transformation) diagram. The time for a fixed fraction of transformation is plotted as a function temperature. This has a C-shape where the nose of the C-curve corresponds to the minimum time (or maximum in rate). The transformation is delayed at high temperatures due to low driving force, and delay at lower temperatures due to low diffusion.

Importance of I and U: The grain size of the product phase depends on the relative rates of nucleation and growth. The combination of high nucleation and a low growth rate yields a fine grain structure. A low nucleation rate combined with a high growth rate yields a coarse grain size. If the cooling rate is very high then results in combination of low nucleation rate and slow or no growth yields a meta stable structures.

why substitutional diffusion is much slower than interstitial diffusion?

Diffusion is the mass flow process in which atoms change their positions relative to their neighbours in a phase under the influence of thermal energy and a concentration gradient.

Interstitial diffusion

The diffusing atoms jumps from one interstitial to neighboring interstitial site in executing a unit step. The neighbouring site is usually vacant in dilute solutions. The activation energy is simply the energy barrier along the path from one interstitial to next.

Substitutional diffusion

This occurs with the aid of vacancies. A substitutional atom jumps from a regular site into a neighbouring vacant site in executing the unit step. In addition to the probability of an atom crossing the activation barrier along the path, the probability of the neighbouring site being vacant has to be considered. 

Hence, substitutional diffusion is generally 4 – 6 orders of magnitude slower than interstitial diffusion.