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||Application|
|Chromium steel||Cr up to 10%||Oil refinery|
|Chromel||Cr 10% and Ni||Thermocouples|
|Stainless steel||Cr 12 to 17%||Gas and steam turbine blades, ovens and furnaces, exhaust valves of I.C. engines|
|Inconel||Cr 16%, Ni 76%||–|
|18-8 Stainless steel||Cr 18%, Ni 8%||Utensils, pressure vessels, heat exchangers|
|Nichrome||Cr 20%, Ni 80%||Strain gauges|
|Kanthal||Cr 24%, Al 6%, Co 2%||Windings of ovens and furnaces|
|Alumel||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.
Also Read: Types of Corrosion