In this article we will discuss about:- 1. Meaning of Corrosion 2. Causes of Corrosion 3. Factors 4. Theories 5. Forms 6. Effect 7. Corrosion of Ferrous Metals 8. Corrosion of Non-Ferrous Metals 9. Standard Electrode Potential 10. Prevention of Corrosion of Steel in Concrete.
Meaning of Corrosion:
The term corrosion is defined as an act or process of gradual wearing away of a metal due to chemical or electro-chemical reaction by its surroundings such that the metal is converted into an oxide, salt or some other compound. A substance known as the rust results from such a process. In other words, the corrosion indicates the deterioration and loss of material due to chemical attack.
Following are the two types of attack on the metals for causing corrosion:
(1) Dry Corrosion:
The chemical reaction between metal and gas or liquid in the absence of electrolytes is known as the dry corrosion. An electrolyte is a substance that dissociates into ions in solution or when fused, thereby becoming electrically conducting.
(2) Wet Corrosion:
If the electrolyte causing corrosion is an aqueous solution of acid, salt or alkali, it is known as the wet corrosion.
If the corrosion is localized, severe cavities or depressions may lead to the formation of holes. Such a condition may prove disastrous in case of liquid vessels because extensive corrosion reduces mechanical strength of the section.
The corrosion has been a serious problem and it becomes necessary for the engineer to understand the mechanisms of corrosion if its effects are to be minimized. He will then be in a position to avoid severely the corrosive environments and to provide necessary protective measures against corrosion.
Causes of Corrosion:
Following are the factors which are responsible for causing corrosion especially to the reinforcing bars in concrete:
(i) Congested reinforcement in small concrete sections,
(ii) Excessive water-cement ratio,
(iii) Improper construction methods,
(iv) Inadequate design procedure,
(v) Incompetent supervising staff or contractor,
(vi) Initially rusted reinforcement before placing concrete,
(vii) Insufficient cover to steel from the exposed concrete surfaces,
(viii) Permeability of concrete which depends on various factors such as water- cement ratio, size of aggregate, curing, grading of aggregates, etc.,
(ix) Poor workmanship,
(x) Presence of moisture in concrete,
(xi) Presence of salts,
(xii) Type of atmospheric conditions surrounding the region of concrete,
(xiii) Unequal distribution of oxygen over the steel surface,
(xiv) Weak and porous cover blocks which are tied directly to the reinforcement or stirrups, etc.
Factors Influencing Corrosion:
The corrosion is influenced by the following factors:
(i) Blow holes, inclusions and trapped gases,
(ii) Chemical nature of the metal,
(iii) Distribution of secondary phases,
(iv) Eddy electric currents,
(v) Internal structure of metal,
(vi) Nature of engineering application,
(vii) Presence of dust, dirt or other foreign matter,
(viii) Surface film,
(ix) Working conditions or environment like temperature, concentration of stresses, etc.
Theories of Corrosion:
The various theories of corrosion have been developed.
But the four important theories of corrosion are as follows:
(1) Chemical action theory or direct corrosion
(2) Electrolytic theory or electro-chemical corrosion
(3) Galvanic action theory
(4) High-temperature oxidation.
Each of the above theory of corrosion will now be briefly described.
(1) Chemical Action Theory or Direct Corrosion:
The direct corrosion is the simplest corrosion produced by means of a chemical attack and it includes oxidation in which the oxygen of the atmosphere combines with all or a part of the surface of material.
The chemical reactions involved are as follows:
The combined action of oxygen, carbon dioxide and moisture on iron results into soluble ferrous bicarbonate Fe(HCO3)2 as shown by reaction (i).
This ferrous bicarbonate is then oxidized to basic ferric carbonate 2Fe(OH)CO3 as shown by reaction (ii).
This basic ferric carbonate is converted into hydrated ferric oxide and carbon dioxide is liberated as shown by reaction (iii).
The above theory of corrosion is supported by the following two observations:
(i) The analysis of rust shows small amounts of ferrous bicarbonate, ferric carbonate and hydrated ferric oxide.
(ii) If carbon dioxide is excluded by immersing iron into a solution of sodium hydroxide or lime water, the intensity of rusting is considerably decreased.
The chemical reaction can also take place at the interface between a liquid and a metal.
It may take one of the following three forms:
(i) The atoms from the liquid may diffuse into the surface of the solid metal and cause serious changes in the mechanical properties of the metal.
(ii) The liquid atmosphere may dissolve the surface of the parent metal by a simple solution action.
(iii) The liquid may react with the parent metal surface to form a compound similar to the oxide films.
The surface with direct corrosion has an etched or worn away appearance and its common examples are as follows:
(i) Acid pickling or chemical bath used to clean the metal surfaces;
(ii) Corrosion of copper flashing or sheet metal in the atmosphere;
(iii) Reactions of dry chlorine, hydrogen, etc.;
(iv) Rusting of iron and steel;
(v) Tarnishing of silverware; etc.
(2) Electrolytic Theory or Electro-Chemical Corrosion:
This is the commonly accepted theory of corrosion. According to this theory, the corrosion takes place due to chemical reaction in combination with electrolysis. It takes place at or near room temperature when the metal comes into contact with moisture or with aqueous solutions of salts, acids or bases.
For electro-chemical corrosion to occur, the following conditions should be satisfied simultaneously:
(i) There should be an electrolyte.
(ii) The current should be passing through the circuit.
(iii) There should be a difference of potential between a metal and its surrounding or between different parts of the same metal.
(iv) The circuit must be closed.
In electro-chemical corrosion, the cathodic and anodic regions of the metal surface are involved. The metal surface from which current leaves the electrolyte and returns to the metal is called the cathode. The cathodic area does not corrode and it remains unchanged by the corrosion attack.
The cathodes and anodes may be separate and independent units. The different areas on the same piece of metal may also represent the anode and cathode. The rate of corrosion will depend on the intensity of current between the anodic and cathodic sites and the nature of electrolyte. The multi-phase metals and alloys corrode at higher rates than pure metals.
(3) Galvanic Action Theory:
The galvanic corrosion occurs when two dissimilar metals are in electrical contact with each other and are exposed to an electrolyte. For instance, a less noble metal like zinc will dissolve and form the anode whereas the more noble metal such as copper will act as the cathode.
Thus the anode metal is made to corrode or dissolve continuously by the galvanic action. It is therefore necessary to observe that the direct contact between dissimilar metals is avoided in the fabrication work to prevent the corrosion of the anodic metal.
(4) High-Temperature Oxidation:
The rusting of ferrous alloys at high temperatures forms scales and oxides. It indicates the high-temperature dry corrosion. The other form of the high-temperature corrosion occurs when the liquid metals flow through other metals. The corrosion is due to the tendency of the solid to dissolve in the liquid metal upto the solubility limit at the given temperature.
The liquid-metal attack may take any of the following three forms:
(i) Simple solution of the solid metal, or
(ii) Formation of chemical compound, or
(iii) Selective extraction of one of the component metals in a solid alloy.
Forms of Corrosion:
In actual practice, the specific terms are used to denote a particular type of corrosion.
The meanings of such terms are as follows:
(1) Atmospheric Corrosion:
The rain water and humid air act as electrolytes and they are mainly responsible for this type of corrosion. It is very frequent on ferrous materials.
(2) Corrosion Fatigue:
The combined action of corrosion and repeated stresses result in the corrosion fatigue. It is most common to the environments which cause pitting on the surface of the material.
(3) Erosion Corrosion:
The combined effect of the basic corrosion mechanism on a metallic surface and mechanical abrasion produce erosion corrosion and it is associated with the formation of cavities in the metal by fast moving liquids.
(4) Fretting Corrosion:
A fret indicates a hole, worn spot or path made by abrasion or erosion. This type of corrosion occurs when contact areas between the surfaces of any two materials are subjected to vibrational stresses. It is common in bolted and riveted joints, clamped surfaces, machine slides, etc.
(5) Inter-Granular Corrosion:
This type of corrosion occurs when a pronounced difference in reactivity exists between grain boundaries and the remainder of the alloy. It is observed in defective welding and heat treatment of stainless steels, copper and aluminium alloys.
(6) Pitting Corrosion:
It is a localized type of corrosion and it is recognized by the presence of holes or pits. The shapes of pits vary widely. But generally, they are hemispherical with electro-polished inner surfaces. The pitting is observed in aluminium, steel, copper and nickel alloys.
(7) Selective Corrosion:
When electro-chemical corrosion encourages preferential corrosion of one of the component metals, it is known as the selective corrosion. It is common in brass pipes and for brass articles containing more than 15% of zinc.
(8) Stress Corrosion:
The combined effect of mechanical stress and a corrosive environment produce this type of corrosion on the metal. The stress may either be applied or residual. The stress corrosion is usually confined to a local area which ultimately gives rise to small cracks and finally results in the failure of the material in service.
(9) Uniform Corrosion:
When the whole surface of the metal is corroded uniformly, it is known as the uniform corrosion and it is observed in metals like aluminium, lead and zinc.
Effect of Corrosion:
The action of corrosion especially on the steel used as reinforcement in concrete is very slow and except under exceptional circumstances, such corrosion does not decrease the life of concrete member. It should however be remembered that action of corrosion becomes more intensive when it is combined with adverse effects of internal and external stresses.
One important effect of corrosion is the formation of cracks and these cracks usually progress or advance most rapidly where shearing stresses are the greatest and where slipping occurs due to loss of bond.
Corrosion of Ferrous Metals:
The main constituent of the ferrous metals is iron. The term rusting is sometimes used to refer the corrosion of ferrous metals. The ferrous metals corrode most easily.
The three important ferrous metals are cast-iron, wrought-iron and steel. It is observed that rusting of cast-iron is less, that of steel is much more and that of wrought-iron is medium.
Corrosion of Non-Ferrous Metals:
Following are the two reasons for the corrosion to develop in the non-ferrous metals i.e., metals which do not contain iron as their main constituent:
(i) Contact between unprotected dissimilar metals where moisture is present; and
(ii) Contact with masonry, lime, cement and some varieties of timber which may release acids, alkalies or salts when damp.
The two important non-ferrous metals liable to corrode are lead and zinc. The copper is unaffected by cement or lime.
A coating of bitumen or one coat of hot bitumen or two thick coats of bituminous paints may be adopted for non-ferrous metals to giant them protection against corrosion.
Standard Electrode Potential:
All the common commercial metals and alloys are classified in an order which is known as the standard, electromotive force series, as mentioned in table 13-1.
The study of table 13-1 reveals the following facts:
(i) The hydrogen is considered as a neutral element.
(ii) The metal magnesium is the most reactive in the whole series and the reactivity decreases in the descending order until it is the minimum for lead.
(iii) The metals below hydrogen become inert in the increasing order and the gold is the most inert being last in the series.
(iv) The lead is highly resistant towards corrosion and it is for this reason that it is widely used with great advantage in storage batteries and as lining for acid tanks.
(v) Any metal is capable of displacing the metal below it in the series. For instance, the iron can displace copper from solution by the reaction –
Fe + CuSO4 → FeSO4 + Cu.
(vi) When two metals are in contact with each other, the metal which is nearer the top in the series will function as the anode and undergoes accelerated corrosion whereas the metal nearer the bottom of series will receive galvanic protection.
For instance, for the steel sheet which has been galvanized, the zinc will get corroded in preference to iron. Hence, the iron is protected until the zinc is completely destroyed by corrosion.
Prevention of Corrosion of Steel in Concrete:
The steel is the most liable to the corrosion and hence the study of steel corrosion is of paramount importance.
To minimize the chances of development of corrosion of steel in concrete, the following preventive measures may be taken:
(i) Avoiding heavily congested reinforcement especially at the intersection of beams and columns;
(ii) Avoiding the steel to come into contact with bricks, soil, wood and other porous non-alkaline materials;
(iii) Avoiding the use of materials which accelerate the process of corrosion i.e., aggregates with high salt contents, water containing salts, etc.;
(iv) Cleaning the reinforcement with wire-brush to remove the rust scales before placing of concrete;
(v) Maintaining a high degree of workmanship;
(vi) Proper structural design with due provision of cover;
(vii) Providing cathodic protection to the reinforcement by some suitable method;
(viii) Providing surface coatings with paints, tars, asphalts, etc.;
(ix) Use of high quality and impermeable concrete;
(x) Using stone pebbles in place of badly made cover blocks;
(xi) Using the correct water-cement ratio; etc.