Corrosion in Metals
The destruction of metals by chemical or electro-chemical reactions with the environment is called as corrosion. Freedom of valence electrons and their conduction is responsible for corrosion in metals.
Gold and platinum, the noble metals, do not corrode as they are most inactive.
Products of corrosion are either porous or non-porous. These products interfere in the process of corrosion and therefore influence the rate of corrosion. Rate of corrosion also depends on the relative activity or the passivity of metals in different environmental conditions. The rate of corrosion of some metals in different environments is given in Table 1.
Tendency of some metals is towards passivity while other metals are made passive deliberately. Passivity is desired to prevent corrosion.
A metal-liquid and a metal-gas system develop corrosion when a galvanic cell is formed. Corrosion can precede either linearly or nonlinearly. Corrosion can occur at all temperatures. High temperature corrosion is termed as oxidation.
Corrosion causes huge losses; therefore various methods are adopted to prevent it. Special purpose chrome alloy steel, various kinds of protective coatings and cathodic protection are employed to save the metals from detrimental effects of corrosion.
Types of Corrosion in Metals
- There are different types of corrosion in metals. Main among them are:
- Oxidation that occurs in metals at high temperatures.
- Degradation that takes place in polymers on reacting with oxygen.
- Radiation damage that happens with materials in nuclear reactors.
- Rusting that occurs in iron under atmospheric condition.
Dry and wet corrosion: Corrosion can also be classified as follows.
- Dry or gaseous corrosion, and
- Wet or liquid corrosion.
Dry corrosion occurs in gaseous environments. Wet corrosion takes place in the surroundings of liquids and water.
Porous non-porous corrosion: Corrosion may also be categorized as follows.
- Porous, and
When the corrosion products (powder form) are very dense and do not allow corrosion to proceed further, then it is termed as non-porous corrosion.
If the corrosion products are not dense, and corrosion can further occur (because the electrolyte is able to penetrate through the powder) then it is known as porous corrosion.
Corrosion that takes place in the presence of a liquid (or water) is called as wet corrosion. Wet corrosion reduces the life of bridges, warships and containers. It is caused due to the formation of a galvanic cell.
Besides an ordinary galvanic cell which comprises of two metals and an electrolyte, some other kinds of galvanic cell may also form under vivid situations. They are given as follows.
- Phase cell or galvanic couple.
- Concentration cell, and
- Stress cell.
All these cells will be discussed exhaustively in the following sections.
A galvanic cell forms in the presence of two dissimilar electrodes (metals) and an electrolyte. Out of the two electrodes, one becomes anode which is the positive end, and the other cathode, which is the negative end. The criteria of being anode or cathode depends on the relative position of a metal in electromotive force (e.m.f.) series (also called Galvanic series) given in Table 2.
Lithium (Li) is shown as most anodic and gold (Au) as most cathodic. As an illustration in Cr-Co system, we can see that chromium is anodic than cobalt which is cathodic.
Environmental effect on galvanic series: The tendency of metals and alloys to corrode in various environments is different. Therefore, they acquire different positions on galvanic series. One such series under sea water environment is shown in Table 3. It shows position of some elements which is different from those shown in Table 2.
Cathodic (protected) end
Anodic (corroded) end
Gold Titanium Silver 18-8 stainless steel (passive) Nickel Bronze Copper Brass 18-8 stainless steel (active) Cast iron Steel Aluminum Zinc Magnesium
Illustration of Mg-Ni galvanic cell: A galvanic cell of magnesium-nickel combination is shown in Figure below. Based on relative position in e.m.f. series, magnesium forms the anodic end. When the circuit is closed by key, magnesium starts dissolving by the anode reaction given as,
Mg → Mg2+ + 2e–
Nickel is cathode in the galvanic cell and deposits by reduction reaction. It is expressed by,
Ni2+ + 2e– → Ni
Due to continuous thermal agitation of the ions in metals, all or some ions escape into the surrounding electrolyte and dissolve into it. Anode and cathode metals dissolve at different rates and set-up a different potential. Anodic metal loses its valence electron with relative ease. With the loss of more positive ions, the metal is left with a slight negative charge. Now the anodic ions are attracted back to the metal, and equilibrium is reached.
When the anode and cathode are connected through a conductor, this equilibrium is disturbed at both electrodes. This causes removal of excess electrons from anode and addition of electrons at cathode. Consequently more and more anodic atoms dissolve into electrolyte, and therefore anodic metal corrodes.
Main examples of corrosion caused due to galvanic cell formation are as under:
- Corrosion of steel propeller shafts of ships running on bronze bearings is an example of corrosion under salty (sea) environment due to galvanic cell formation.
- The galvanized iron pipe (G.I. pipe) used in drinking water supply employs zinc coating on iron pipe. Zinc provides mechanical protection and prevents electrolyte from reaching iron. A galvanic cell forms when the zinc coating gets scratches, cuts or other failure exposing iron to the electrolyte in the process. Then zinc becomes anodic to iron, and corrodes.
- In an iron-tin (Fe-Sn) system, iron corrodes.
- In aluminum-copper (Al-Cu) system, aluminum corrodes being on the anodic side than copper.
Types of Galvanic Cell Corrosion
A galvanic cell may also set-up due to some other reasons such as
- Two different phases in the same metal.
- Fine and coarse grains in the same metal.
- Difference in metal ion-concentration.
- Difference in oxygen concentration.
- Different residual stresses in the same metal.
These reasons have their effects on corrosion and will be discussed in detail now.
Single Metal Corrosion: Corrosion in a single metal can occur due to presence of different phases, or different grain sizes. Pearlite, a micro constituent, has a mixture of two phases viz. ferrite and cementite of which ferrite is more anodic than cementite. A phase cell or galvanic couple is formed by them.
Under corrosive conditions ferrite corrodes, and thus causes single metal corrosion. The two phases in a metal may possess grains of different sizes and orientations. Presence of finer grains results in the formation of more galvanic cells; hence the rate of corrosion is fast. Tempering of steel during heat treatment produces finer ferrite and cementite below 400°C and coarser cementite above 500°C. Therefore number of galvanic cells formed below 400°C is more. Hence rate of corrosion is faster below 400°C and slower above 500°C.
Concentration Corrosion: Such corrosion may occur due to difference in concentrations of metal-ions. Let the concentrations of metal-ions in an electrolyte at two locations A and B be CA and CB (CA> CB). Then the metal near location B will be anodic to the metal near location A. Difference of CA and GB induces the formation of an ion-concentration cell which is responsible for concentration corrosion.
Crevice Corrosion: Galvanic cell is also formed due to concentration of oxygen in the path of electrolyte flow. Oxygen plays an important role and corrosion depends on the rate at which oxygen diffuses to cathode. The oxygen rich region becomes cathodic with respect to oxygen scare region, thus resulting in crevice corrosion.
Corrosion at the corners, holes and discontinuities such as in coupled pipe joints and threaded connections are the examples. Figure given below explains such corrosion.
Here a baffle plate is submerged under water in a container. Water from side A flows to side B through the holes in baffle plate. In due course, oxygen concentrates tin and around holes. Thus the concentration cell forms and corrosion occurs.
Water Line Corrosion: Corrosion occurs in a water storage tank just below the water line. The same is true for an object which is partly dipped in water and partly is above the water. It also corrodes below the water line. This is known as water line corrosion, and is shown in Figure below.
The concentration of oxygen is higher just above the water line as compared to the submerged portion near the water line. The latter zone acts as anode and therefore corrodes. Water line corrosion is a type of crevice corrosion.
An analogues situation is encountered in an underground pipeline passing through the surrounding of impervious clay and porous sand. Pipeline corrodes in the more wet clay region.
Stress Corrosion: Difference in stresses at various regions of a metal is responsible for creating a stress galvanic cell in the metal. This cell is also known as stress cell. The stress variation may be due to the presence of residual stresses produced during cold working or hardening of metals. Hence
- The stressed region form anodic end of the galvanic cell and corrodes.
- The grain boundaries in a polycrystalline metal also corrodes being anodic to the crystals as shown in Figure (a).
- A plastically deformed metal piece corrodes more readily than the elastically deformed metal piece.
- A folded metal sheet is likely to corrode more at the folds due to higher plastic deformation as shown in Figure (b).
- A bent wire corrodes more at its bends as regions of bends are over stressed than rest of the wire as shown in Figure (c).
- A nail corrodes more at its head and tip than the shank as head and tip regions are heavily stressed than rest of the body as shown in Figure (d).
Corrosion of a metal by a gas is referred to as dry corrosion. The molecules of gas are adsorbed on the surface of metal during the reaction. This reaction yields corrosion products (or powder) in the form of oxides or salts. For example, in following reactions
4Fe + 3O2 → 2Fe2O3
2Na + Cl2 → 2NaCl
Iron oxide is a red-brownish colored oxide and sodium chloride is a salt. Some combinations of metals and gases are more active at room temperature while others are active at elevated temperatures only. A clean surface of aluminum door handle reacts quickly with air but iron in dry air requires high temperature to produce a significant reaction.
Types of Corrosion Film: During the process the corrosion products form a film on the metal surface that can be either non-porous type i.e. adherent or protecting, or porous type i.e. non-protecting.
The actual type of film may be determined by comparing the molar or specific volume of corrosion product. Molar volume is expressed as M/ ρ mm3/mol where M is the molecular weight and ρ is the density. Thus an adherent film will form when
Mc/ρc > Mm/ρm
Where the subscripts c and m stand for corrosion product and metal respectively. Most metals with the exception of alkali metals satisfy this equation. In adherent film, corrosion products compress upon the surface.
Contrary to this, in porous corrosion the film expands to cover the surface and therefore
Mc/ρc < Mm/ρm
In this case the gas remains in direct contact with metal and the chemical attack continues at a constant rate.
Mechanism of Dry Corrosion: The mechanism of corrosion by diffusion in a non-porous medium formed between copper and oxygen is shown in Figure. The metal-film interface acts as anode and the gas-film interface behaves as cathode. The film acts as external circuit in conducting the electrons from anode to cathode and as internal circuit in conducting the ions.
Laws of Corrosion
Based on nature of corrosion films produced in metals, the following law of corrosion can be interpreted.
- Linear law (in porous film).
- Parabolic law (in non-porous film).
- Logarithmic law (in non-porous film).
The rate of corrosion is measured in terms of film thickness x as function of time t may be expressed as:
- for linear law by x = CLt
- for parabolic law x = CP√t
- for logarithmic law by x = Co log (1 + t/t1)
Here CL is linear rate constant, CP is parabolic rate constant, Co is logarithmic rate constant.
The corrosion in plain carbon steel, Mg, St, Ba, Ca, Ta and Nb follows linear law. It follows parabolic law in copper, stainless steel, Co and Si. Whereas in nickel, aluminum, zinc, Be and Cr corrosion follow logarithmic law.
Oxidation is a high temperature environmental effect on materials. Problem of oxidation becomes more severe at the increasing functional temperatures. An adherent oxide layer offers good oxidation resistance. Addition of nickel, chromium and aluminum in steel improves its oxidation resistance.
Rate of Oxidation: While designing with oxidation-prone materials, it is desirable to know how fast or slow the oxidation process is going to be. The rate of oxidation may-be very fast as in tungsten and molybdenum or very slow as in tin, silver and gold. This rate varies between a few minutes to many million hours. Table 4 shows oxidation time for some materials.
|Oxidation time (hour)
|Melting point (K)
|1.8 x 105
Mo and W lose weight linearly on oxidation. It is due to volatile nature of their oxides MoO3 and W03. At high temperatures, these oxides evaporate just after their formation and therefore do not offer any barrier to oxidation. Due to loss of oxide, the material loses its weight.
Methods to Reduce Oxidation: Oxidation of materials follows linear and parabolic laws. Their rates can be expressed by an Arrhenius type equation. Oxidation rates can be decreased by addition of some oxidation-resistant elements. Rate of oxidation in steel decreases by
- about (0 – 30 times on adding 5% Al,
- 20 times on adding 5% Si, and
- 100 times on adding 18% Cr at 900°C.
Mechanism of Oxidation: Generally metals are stable in oxide form at room temperature. Gold is an exception that occurs in metallic form at room temperature. Change in the free energy of oxide formation at zero and room temperatures are shown in Table 5. The values are negative except for gold and alkali halides indicating that these oxides are in stable form at room temperature.
|Free energy (kJ/mol)
|Free energy (kJ/mol)
|Pilling-Bedworth Ratio (PBR)
|(at 273 K)
|(at 300 K)
|Alkali halides (LiCl, NaCl, KCl ect.)
|+400 to 1400
|+600 to 1500
Protection against Oxidation: Protection against oxidation may be endangered when the protective film breaks due to some reasons, such as in following cases.
- Alternate heating and cooling.
- Thermal shock caused by sudden cooling or sudden heating.
- Volatile layer in tungsten and molybdenum.
An adherent layer is not a guarantee for protection against oxidation as further oxidation can occur by diffusion through the oxide layer itself. This may proceed through either oxygen anions or metallic cations. Hence thickness of the oxide layer x increases as a function of time t at constant temperature T, and is expressed as
x2 α Dxt
where Dx is the diffusivity. The diffusivity increases exponentially with temperature. Therefore oxidation seizes when the oxide layer reaches a critical thickness. At this juncture the PER is much more than unity.
Condition of a metal or an alloy that exhibits higher corrosion resistance than its position on the galvanic series is termed as passivity or passivation.
Chromium, aluminum, stainless steels show passivity as a thin, invisible, nonporous and insoluble film forms on their surfaces. The protecting film on the metal surfaces is formed when a layer of oxygen is adsorbed; or some chemical compound reacts.
Iron when exposed to concentrated nitric acid gets oxidized and becomes passive in the process, and is further not attacked by its strong solution. Contrary to this, iron is not oxidized by dilute solution of nitric acid and hence is attacked by its weak solution.
Improving the Passivity of Metals: To improve passivity the potential of metal electrodes can be increased electrically. During passivation, the current density attains a peak value and then falls abruptly. This occurrence is associated with the formation of thin oxide layer on the metal.
The peak current density just before and just after passivation of chromium is approximately 200 A/m2 and 0.1 A/m2 respectively. Zinc chromates and nitrates are used as passivators in paints.
Special Types of Corrosion
Corrosion is known to occur in metals under different conditions in addition to the environmental effects. Mechanical stress, fatigue behavior, cavitation and erosion phenomena, dissolution, and residual stresses etc. favor the situation of corrosion. We shall now study the special types of corrosion.
Stress Corrosion Cracking
A mechanically stressed metal component develops local disorder and tends to be anodic. This induces cracks in the metal. The cracks produced in stress corrosion cracking are of two types.
- Transcrystlline, and
Transcrystlline Corrosion: Cold-worked areas, sharp bends and heavily distorted slip band regions under the combined action of mechanical stress and corrosive environment developed transcrystalline crack or corrosion. A crack develops across the metal through the grain boundaries perpendicular to the direction of load applied as shown in Figure. Corrosion of austenitic stainless steel in chloride environment is an example.
Intergranular corrosion: Intergranular cracking or corrosion occurs in 18-8 stainless steel which contains 18% chromium. Due to its prolonged high temperature exposure, chromium carbide precipitates at grain boundaries, as shown in Figure below. The grain boundaries become anodic to the grains. Notches are formed between the grains, and local stress increases. Fracture takes place almost without plastic strain.
Corrosion Environments that are Responsible for Stress Corrosion Cracking: Various corrosive environments responsible for stress corrosion cracking in different alloys are given in Table below.
|Distilled water, sea atmosphere
|Sea water, air, solution of NaCl
|Concentrated NaOH, KOH
|Water, NH3 vapors and solutions
|Monel metal (Ni-Cu)
|Solution of HF
|Lead acetate solution
|Sea water, solutions of NaOH and nitrates
|Solution of caustic, solution of HNO3 and H2SO4, solution of H202 and NaCl
|Chlorinated hydrocarbons, elevated temperature chlorides
Corrosion in materials due to corrosion fatigue is a result of combined effect of surface irregularities and fatigue. The progress of corrosion depends on time while that of fatigue depends on number of cycles of repeated stress. Pits and surface irregularities caused by corrosion act as stress raisers and are responsible for enhanced fatigue. This enhanced fatigue, in turn, increases the rate of corrosion.
Causes of failure of metals: Failure of metal may occur even at (i) low cycle of repeated stress and high corrosion rate, and (ii) high cycle of repeated stress and low corrosion rate. Any corrosion film formed on the surface is broken by repeated strain (caused due to repeated stress) in each cycle. Due to this reason, fatigue cracks are formed at corrosion pits. These cracks further intensify the stress at the tip which makes corrosion rate high.
Effects of corrosion fatigue: These are given below.
- Corrosion fatigue lowers down the strength of metals considerably.
- Corrosion fatigue strength of stainless steel may be as low as that of plain carbon steels.
The surfaces of two materials in contact get damaged owing to rubbing between them due to repeated sliding motions. Repeated sliding motion is produced due to repeated stress cycles. This results in mechanical wear of surfaces.
Due to mechanical wear of the surface, protective corrosion film on it is broken and the surface gets exposed to corrosive surroundings. In addition to this, the rubbing induced tensile stress on the surface adds to increased corrosion rate.
Method to Avoid Fretting Corrosion: Fretting corrosion generally occurs in heavily loaded and tightly clamped parts. Figure given below depicts such a case. To avoid fretting corrosion, slip between mating parts A and B in this figure should be stopped. It can be done either by increasing the pressure between two surfaces in contact or by introducing compressive residual stress on the surfaces.
Fretting corrosion is noticed in following cases.
- Ball bearing race pressed on a shaft,
- shrunk-fit collars,
- bolted and riveted joints under vibrations.
Cavitation is a fluid flow phenomenon in which collapse of water vapor bubbles exerts extremely high pressure on the solid in which fluid flows. This pressure causes cracks in brittle metals and plastic deformation in ductile metals. Cavitation accelerates corrosion.
Erosion produces wearing and roughening of the surface. It removes the protective film from the surface which further leads to an increase in corrosion.
The cavitation-erosion corrosion is noticed in pumps, valves, marine propellers, and penstocks carrying water to turbines.
Methods to minimize the cavitation-erosion corrosion: These can be prevented or minimized by proper designing and fabrication of components. A proper design should not have sharp corners, projections and sudden changes in dimensions of the components.
In boilers, water is heated to generate steam. Steam at high pressure and high temperature introduces tensile stress in the boiler material. If the water used is alkaline, it evaporates leaving behind sodium hydroxide (NaOH) that causes brittleness in the boiler steel by capillary action. The occurring chemical reaction is
Na2CO3 + H2O = 2NaOH +CO2
Preventing measures: This kind of corrosion can be prevented by softening water and minimizing its alkalinity. Sodium sulphate, phosphates and tannins etc. are added in water to prevent caustic embrittlement.
Hydrogen is invariably present in various chemicals. Hydrogen present in these chemicals reacts with iron at room temperature. For example, hydrogen present in hydrogen-sulphide reacts with iron according to
Fe + H2S = FeS + 2H
in which nascent hydrogen is released.
The nascent hydrogen released in the reaction diffuses into the voids of iron and builds-up a high pressure. This pressure exceeds the yield strength of iron and lowers its ductility. High pressure also causes blustering and fissure in iron.
In due course, the metal embritles. At high temperatures, nascent hydrogen combines with carbon, sculpture, nitrogen etc. present in small amounts in iron. It produces methane at a very high pressure which reduces the strength of steel and makes it brittle.
In alloys of two or more elements, one constituent sometimes leaves the other constituent under favorable corrosive conditions. This induces corrosion in the alloy. Such corrosion is called selective corrosion. Dezincification is one such kind of selective corrosion.
In this process, the zinc constituent of brass (Cu + Zn) dissolves during corrosion leaving behind a spongy copper of almost nil strength. This occurrence is known as dezincification. Addition of less than 0.1% arsenic in brass prevents dezincified corrosion (or dezincification).
Season Cracking | Caustic Embrittlement
Season cracking is a special type of stress corrosion cracking. In stress corrosion cracking the metals under constant stresses fails in tension after some time. The first requirement for this to occur is the presence of an environment where the grain boundaries are sufficiently anodic to the grains.
The second requirement is the presence of a steady tensile stress of magnitude somewhere around the yield stress. Under these conditions an attack begins at grain boundaries of the metal. Stress corrosion cracking requires varying amount of time in different situations. Time ranges from a few minutes to many years.
This phenomenon has been observed in almost all metals. One example is the season cracking noticed in brass exposed to moist atmosphere having traces of ammonia. Various examples of season cracking/caustic embrittlement are given below.
- Caustic embrittlement of mild steel in steam boilers where steam leakage permits high ion concentrations to build up,
- intergranular cracking in aluminum alloys containing more than 12% zinc or 6% magnesium,
- cracking in brass made kerosene stove, and
- transcrystal-line cracking in stainless steel in acid chlorides is some other examples.
Formation of pits or cavities at one or more locations on the material’s surface is called pitting corrosion. It occurs when oxide film on a metal breaks due to abrasion or other such reasons. Pitting corrosion also happens at dispersed particles in dispersion-hardened metals.
It is shown in above Figure (b). Pitting corrosion is of localized nature. Pits formed during this process may be small or large. Pitting will be severe if the cathodic area is large.
Prevention of Corrosion in Metals
Loss due to corrosion has been estimated to be about 3% of annual income of an industrially advanced country. To prevent this huge loss, various methods are adopted to protect metals from corrosion. Important amongst these are
- Use of noble metals.
- Use of corrosion and oxidation resistant materials.
- Use of protective coatings.
- Use of inhibitors.
- Design of components to avoid formation of galvanic cell.
- Deaeration of water, and
- Cathodic protection. All these methods are based on the principle of passivation.
Use of Noble Metals
Noble metals such as gold and platinum lie on cathodic end of e.m.f. series hence become natural choice metals in corrosion prevention. But these metals cannot be used for most engineering applications due to cost considerations. They are generally used in making ornaments, delicate components of instruments and objects of international standards.
Use of Corrosion and Oxidation Resistant Materials
- Many alloys offer good resistance to corrosion in different environments. For example
- Copper alloys, brass (Cu + Zn) and bronze (Cu + Sn) have good resistance against the environments of water and salty air.
- Titanium and zirconium resist chlorine environment.
- 18-8 stainless steel (18% Cr and 8% Ni) is excellent against all types of environments.
- Addition of niobium in steel offers very good corrosion resistance. Chromium, nickel and aluminum as alloying elements in steel provide excellent resistance at elevated temperatures. Various combinations of steel alloys showing their important applications are depicted in Table given below.
|Name of alloy
|Composition of chromium and others in steel
|Cr up to 10%
|Cr 10% and Ni
|Cr 12 to 17%
|Gas and steam turbine blades, ovens and furnaces, exhaust valves of I.C. engines
|Cr 16%, Ni 76%
|18-8 Stainless steel
|Cr 18%, Ni 8%
|Utensils, pressure vessels, heat exchangers
|Cr 20%, Ni 80%
|Cr 24%, Al 6%, Co 2%
|Windings of ovens and furnaces
|Ni 95% Al 2%, Mn 2%, Si 1%
|Heat resistant wires
Above Figure shows the effect of chromium content on high temperature corrosion rate of iron. It shows that rate of corrosion drops down on increasing the chromium content.
Use of Protective Coatings
Various kinds of protective coatings are provided on materials as they offer
- mechanical protection.
- galvanic protection by being anodic to the base metal.
- passivating action to shift the base metal towards cathodic end.
The mechanical protection is provided by
- Organic coatings such as paints, grease etc.
- Inorganic coatings such as enamels etc.
- Electroplating by tin and noble metals.
- Plastic coatings and adhesives.
The galvanic protection is given to
- iron by zinc coating as in galvanized iron (G.I.) pipes.
- iron by aluminum and cadmium etc.
Zinc chromate is applied as pigment in the paints.
Use of Inhibitors
Inhibitors are the chemicals added to the electrolyte to form an insoluble layer on the metal. By doing so, they make the system passive. Different kinds of inhibitors are
- Anodic inhibitors such as chromates and nitrites.
- Cathodic inhibitors.
- Vapor phase inhibitors such as bicarbonate.
Anodic inhibitors form a thin passivating oxide film while cathodic inhibitors promote a thick film on the metallic surface. The vapor phase inhibitor compounds are placed in the vicinity of metals to be protected.
Design To Avoid Formation of Galvanic Cell
To prevent corrosion, formation of galvanic cell can be avoided by logical design considerations. Some important considerations are
- avoiding physical contact between two metals,
- preferring larger surface area for anodic metal,
- streamlining the components carrying fluids,
- selection of single phase alloys as they, have better corrosion resistance than the two or more phase alloys,
- use of two dissimilar metals which are closer to each other in electromotive force series,
- preference to welding over riveted or bolted joints.
Salient design features to avoid formation of galvanic cell are given below.
- Design of a system should be such that the physical contact between two metals is avoided. In this situation a galvanic cell will not form. But this situation cannot he eliminated in practical applications such as steel screw in brass components or steel shaft in a bronze bearing.
- In case, the mechanical contact between dissimilar metals is unavoidable, the anodic metal should be made of larger surface area in comparison to the cathodic metal. This design will cause low current density at anode and therefore low rate of corrosion. A steel nut and bolt on large aluminum sheet will be more admissible than an aluminum nut and bolt on a steel component of large surface area.
- Design of components should be such that the sharp corners are avoided. The component profile should be streamlined so as to minimize stagnation areas and accumulation of flowing substances. Figure 20.18 shows both recommendable and improperly profiled components.
Deaeration of water: Corrosion of iron and steel by water may be controlled by removing dissolved oxygen, a cathodic reactant, from water. Dissolved oxygen is removed by either deaeration or by chemical reagents such as sulphites.
Deaeration is done by spraying water in a low pressure or vacuum chamber. This method is employed in boilers where feed water is recirculated. A level of 0.03 to 0.3 ppm of oxygen is acceptable, yet this level should be reduced to about 0.005 ppm.
In this method a galvanic cell is deliberately created. The items to be protected such as precision instruments, underground pipelines, offshore structures etc. are made cathode, and metals like magnesium, aluminum or zinc are made anode. The cathode is protected at the cost of dissolution of anode. The anodic material is replaced at pre-known intervals. The components of process industries, petroleum pipelines, ships and jetties etc. are also protected by this method.
The scheme of corrosion protection of mild steel tank in a process industry is shown in Figure below. The tank acts as cathode and magnesium as anode. Magnesium sacrifices itself by dissolving and thus protects the mild steel tank.
Working Arrangement of Cathodic Protection: An external direct current (d.c.) source may also be used to get cathodic protection. The metal to be protected is connected to the negative end (cathode) of d.c. source and the positive end (anode) to an inert metal. The supply voltage should be such that the metal to be protected remains cathodic. The anode is kept at a higher voltage than the cathode.
A battery is generally employed as an external d.c. source. The protecting current Ip flowing from anode through the electrolyte may be determined from
Ip = (Ec – Ea)/(Rc + Rp)
Where Ec and Ea are electrode potentials at the cathode and anode respectively, Rc is the resistance of local cathode area, and Ra denotes resistance on cathode due to paint coating if any.
Cathodic protection of petroleum pipelines is an example of this method. The schematic arrangement is shown in Figure below.
Protection of underground oil and natural gas pipelines in contact with damp ground is of great importance. If the depth of soil is such that oxygen is not excluded effectively, then the following reactions take place:
- Oxygen reduction reaction: O2 + 2H2O + 4e → 4 OH and,
- Metal-corrosion reaction: Fe → Fe++ + 2e
These reactions cause corrosion to the pipeline.
Protective coatings on metals are used to prevent corrosion and to enhance the outer appearance. The coatings may be broadly classified as follows.
- Metallic coating.
- Non-metallic coating, and
- Chemical protective coating.
Electroplating: Electroplating is an electro-deposition process of metals. The coating material is deposited on the base metal by passing d.c. through an electrolytic solution. Quality of the coating depends on the composition of electroplating solution, current density, agitation, temperature of solution etc. Electroplating is done for corrosion protection, and decoration purposes.
Nickel, tin and zinc are coated on iron to protect against corrosion whereas silver is used for plating fancy articles to enhance their beauty. Machinery parts are electroplated with chromium to protect them from wear and corrosion. The electro-deposited metals possess crystalline structure. Finer the crystals; brighter, smoother and stronger is the deposit.
Dipping: In this method, the article to be coated is cleaned and dipped in a bath of molten metal. The coating metal has melting point lower than the base metal. Galvanizing and tinning are the applications of this method.
In the galvanizing process, coating of zinc is done on iron. Galvanized iron is used for making buckets, roofing articles and G.I. pipes etc.
In the tinning process, a coating of tin is obtained on iron. Tinning process is carried out in the same way as galvanizing. The iron sheets are cleaned and passed through a layer of molten zinc chloride and ammonium chloride. The tin layer so formed on the finished plate is very thin. Tin is highly resistant to corrosion and is used in production of tin cans used for packing food.
Spraying: A metallic coating is obtained on the base metal surface by spraying heated metal on it. The molten metal particles interlock the irregularities of the base metal surface. The sprayed coating is applied by spray gun and other specialized guns.
Cladding: It is a method of putting a thick lining of one metal on the surface of another metal. It is done by hot rolling process. Nickel claded steel, copper and aluminum alloys are the examples of cladding.
Cementation: In this process the base metal is heated with another powdered metal. Diffusion of powdered metal takes place on the hot base metal surface. Examples of cementation are sheavadizing, calorizing, and chromizing.
In the sheavadizing process, thoroughly cleaned iron or steel is packed in zinc dust. When it is heated for an hour or so up to a temperature of about 350° to 450°C, it becomes coated with a continuous film of zinc.
In the calorizing process, aluminum is used to coat the steel surface. Steel thus obtained is called calorized steel. This is highly resistant to oxidation.
In the chromizing process, steel is coated with a thin layer of chromium. Chromised steel exhibits a greater resistance to oxidation.
Vitreous or porcelain enamels: Enamels are non-metallic materials. Porcelain enamel is essentially a vitreous coating containing an oxide colored pigment. Vitreous enamel coat consists of a thin adherent layer composed of borosilicate. The chemical resistance of vitreous enamels increases with silica content.
High silica compositions give very hard enamels of high fusion, low tensile strength, low thermal coefficient of expansion and poor adhesion to the base metal. Low silica compositions give enamel a low chemical resistance.
Anodized Coatings: Anodized coatings are produced by electrolyte processes on zinc, aluminum, magnesium and their alloys. Aluminum coatings are formed in an acid electrolyte at moderate temperature and current densities. The base metal is made anodic. The nature and thickness of the coating depends on the type of electrolyte, temperature, current density and duration of application.
Surface Conversion or Chemical Dip Coating: These coatings are obtained by immersion or spraying. The base metal is covered with a chemical solution. This solution reacts with the metal surface to produce an adherent coating. These coatings are insoluble in the environment. They increase resistance to corrosion but do not afford permanent protection.
Example of such coating is ‘chromate conversion coating’ used for the protection of zinc and cadmium plated parts. These coating are applied by immersion method. Phosphate coatings are applied to iron, steel, zinc, aluminum, cadmium and tin by chemical reaction of aqueous solution of phosphate and phosphoric acid.
The chemical reaction between the phosphatic solution and the base metal results in the formation of a film which consists of crystalline zinc-iron or manganese-iron phosphates firmly embedded into the crystal lattice of the metal surface. For more information, you may visit Wikipedia.