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What is Corrosion? • Corrosion may be defined as the gradual destruction of metals by the chemical or electrochemical reaction with the environment. During corrosion, the metals are converted to their metallic compounds at the surface. The loss of materials due to corrosion has become a great problem. The most common example for corrosion is the rusting of iron when it is exposed to atmospheric conditions. The rusting is due to the formation of hydrated ferric oxide on the surface. Another example is the formation of green film of basic copper carbonate on the surface of Cu, when exposed to moist air containing CO2.
Case Studies of Corrosion Failure 198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft
Contd. • In the early morning of December 3, 1984, water inadvertently entered the methylisocyanate storage tank, where >40 metric tons of methylisocyanate were being stored. The addition of water to the tank caused a runaway chemical reaction, resulting in a rapid rise in pressure and temperature. The heat generated by the reaction, the presence of higher than normal concentrations of chloroform, and the presence of an iron catalyst, produced by the corrosion of the stainless steel tank wall, resulted in a reaction of such momentum that gases formed could not be contained by safety systems.
Consequently, methylisocyanate and other reaction products, in liquid and vapor form, escaped from the plant into the surrounding areas. There was no warning for people surrounding the plant since the emergency sirens had been switched off. The effect on the people living in the shanty settlements just over the fence was immediate and devastating. Many died in their beds, others staggered from their homes, blinded and choking to die in the street. It has been estimated that at least 3000 people died as a result of this accident, while figures for the number of people injured currently range from 200,000 to 600,000, with an estimated 500,000 typically quoted.
So why study Corrosion? • Materials are Precious resources • Engineering design is incomplete without knowledge of corrosion • Applying knowledge of corrosion protection can minimize disasters • Corrosion- may contaminate stored food, dairy products , etc • Corrosion products cause pollution • Artificial implants for the human body ?
Types of Corrosion • Direct Chemical Corrosion or Dry Corrosion • 2. Electrochemical Corrosion Or Wet Corrosion
1. Direct chemical corrosion or dry corrosion • This type of corrosion occurs mainly through the direct chemical action of atmospheric gases such as O2, halogens, H2S, CO2, SO2, N2, H2 or liquid metals on metal surface in the absence of moisture. • There are 2 types of chemical corrosion.
i. Oxidation corrosion • It is brought about by the direct action of O2 present in the atmosphere on metals at low or medium temperature in the absence of moisture to form metallic oxides. Alkali and alkaline earth metals are rapidly oxidisesd even at low temperature. At high temperature almost all metals except (Ag, Au, Pt, Pd) are attacked. ii. Liquid metal corrosion • It is due to the action of following liquid metal at high temperature on solid metal or alloy. This type of corrosion is found in nuclear power plants. Corrosion takes place either due to the dissolution of solid metal by the liquid metal or due to the penetration of liquid metal in to the solid metal.
II.ELECTROCHEMICAL CORROSION OR WET CORROSION • This type of corrosion occurs when : • A metal is in contact with a conducting liquid • Two dissimilar metals or alloys are immersed partially in a conducting solution. • This corrosion is due to the existence of separate anodic and cathodic areas between which current flows through the conducting solution.
Mechanism • Electrochemical corrosion involves the flow of electron current between the anodic and cathodic areas. One part of the metal becomes anode and the flow of electron current between the anodic and cathodic areas takes place through an electrolyte in contact with it. The anodic reaction involves the dissolution of metal to metallic ions with the liberation of electrons. Hence corrosion always takes place at the anodic area.
At anodic area, • M Mn+ + ne- (oxidation) • The cathodic reaction consumes ne- with either by the evolution of H2 or absorption of O2 depending on the nature of environment.
A. Evolution of H2 • In acidic medium, the cathodic reaction takes place by the evolution of H2 . Consider the corrosion of iron. The anodic reaction is the dissolution of Fe as Fe2+ ions with the liberation of electrons. • Fe Fe2+ + 2e- (Oxidation) • Now these electrons flow through the metal from anode to cathode where these electrons are taken by H+ ions present in acidic solution forming • 2H+ + 2e- H2 (Reduction)
B. Absorption of O2 • In neutral or slightly alkaline medium, the cathode reaction takes place with the absorption of atmospheric O2. In this type, the electrons liberated from Fe flow from anodic to cathodic areas where these ne- are received by O2 dissolved forming OH- ions • ½ O2 + H2O + 2e- 2OH- (reduction)
The liberated Fe2+ ions at the anode and OH- ions at the cathode diffuse and combine to form the first corrosion product namely Fe(OH)2. • Fe2+ + 2OH- Fe(OH) 2 • In presence of excess of O2, Fe(OH)2 is changed in to Fe(OH)3 with the composition Fe3+2O3. 2H2O which is known as rust. • 2Fe(OH)2 + ½ O2 Fe2O3. 2H2O rust
TYPES OF ELECTROCHEMICAL CORROSION • When two dissimilar metals or alloys are electrically connected and exposed to an electrolyte, the metal higher in electrochemical series under goes corrosion. Thus when Zn and Cu are connected, Zn being higher in the series act as anode and undergoes corrosion and Cu which is lower in the series act as cathode and gets protected.
Egs. are:- • Steel pipe connected to copper plumbing • Lead – antimony solder around copper wire.
Pitting Corrosion • Pitting corrosion is a localized accelerated attack resulting in the formation of pin holes, pits and cavities on the metal surface. It is due to the breakdown or cracking of the protective film on the metal at specified points. This gives rise to the formation of small anodic and large cathodic areas. Once a small pit is formed the rate of corrosion will be increased
Presence of external impurities like sand, dust, water drops etc on the surface of the metal can also be a cause for this type of corrosion. In this case, the small part below the impurity acts as the anodic area while the rest of the metal acts as the cathode area. Due to corrosion a small pit is formed at the anodic area which grows gradually.
Concentration cell corrosion • This type of corrosion is due to electrochemical attack on the metal surface exposed to an electrolyte of varying concentrations. Metal in contact with lower concentration will act as anode and undergoes corrosion. It is observed in chemical plants and storage tanks and also in marine structures like ships. Differential aeration corrosion is the most important.
a. Differential aeration corrosion • This type of corrosion occurs when one part of the metal is exposed to a different air concentration from the other part. This causes a difference in potential between differently aerated areas. It is found that less oxygenated part acts as anode more oxygenated part acts as cathode. • Corrosion of metals partially immersed in a solution is due to differential aeration corrosion. Consider a Mg rod partially immersed in NaCl solution. The areas of Mg rod above and just below the water line are more oxygenated and will act as cathode. The remaining part of the rod which is well under the solution is less oxygenated and act as anode and undergone corrosion.
Water line Corrosion It is another type of differential aeration corrosion. Corrosion in storage tanks, H2O tanks, marine structures etc is called water line corrosion. When water is stored in an iron tank, it is found that maximum corrosion occurs along a line just below the water level. The area above the H2O line is highly oxygenated and acts as cathode while the area just below the H2O line is less oxygenated and acts as anode and undergoes corrosion.
CORROSION CONTROL OR PROTECTION FROM CORROSION Cathodic Protection In this method, the corroding metal is forced to behave like a cathode. There are two types of cathodic protection a. Sacrificial Anodic protection on Galvanic protection In this method, the metallic structure which is to be protected from corrosion is connected to a more anodic metal by a wire so that the entire corrosion is concentrated on this more active metal. The more active metal loses e ns− and get corroded and this metal is called sacrificial anode. Metals commonly employed as sacrificial anode are Mg, Zn, Al and their alloys.
Impressed Current Cathodic Protection In this method, an impressed current is applied in the opposite direction to nullify corrosion current so as to convert the corroding metal from anode to cathode. Impressed current can be derived from a direct current source like battery. An inert or insoluble electrode like graphite or silica act as anode to complete the circuit. The surroundings of anode should be filled with salts and carbon to increased the conductivity.
Protective coatings or Surface coatings An important method for protecting a metal from corrosion is to apply a protective coating. The protective coatings may be of inorganic or organic. The coated surface isolates the metal from the corroding medium. The coating applied must be chemically inert towards the environment. Inorganic coatings are further classified in to metallic coatings, chemical conversion coatings and vitreous coatings. a. Metallic Coatings Metallic coatings are mostly applied on Iron and steel because these are cheap and commonly used construction materials. There are two types of metallic coatings. 1. Anodic coatings The base metal which is to be protected is coated with a more anodic metal for eg. Coatings of Zn, Al and Cd steel are anodic because their electrode potentials are lower than that of the base metal ie. Fe. 2. Cathodic Coatings It is obtained by coating a more inert metal having higher electrode potential. Than the base metal. Eg. Coating of Sn, Cr, Ni on Fe surface. The coating should be continuous and free from pores and cracks. These coating metals usually have higher corrosion resistance than the base metal.
