Corrosion

Corrosion is defined as any unwanted destruction of a metal or non metal substance by its surroundings due to chemical,  electrical,  electrochemical attack is known as corrosion.

Types: #

Dry corrosion #

Wet corrosion/electrochemical corrosion #

It can take place via two different mechanisms 

1. Evolution of hydrogen gas: this mechanism generally takes place in acidic solutions where fair amount of h+ ions are present. 

   1. In this mechanism the metal first gets oxidized to an ionic state with release of electrons. M-> M^n+ + ne- 

   2. In the second step the h+ ions present in the solution or atmosphere accept the e- and get reduced to h2 gas. The amount of gas is very low and it is also a slow process. nH+ + ne- -> n/2H2 

   3. Hydrogen gas evolved is very little and the rate is very slow. 

2. Absorption of oxygen gas/dissolved oxygen 

   1. This mechanism takes place where metals are in constant contact with stagnant water. In stagnant water, the amount of dissolved oxygen increases due to microbial activity and prescence of aquatic life forms. This dissolved oxygen accepts electrons and gets reduced to hydroxide ions. These hydroxide ions may further react with the oxidized metal resulting in formation of metal hydroxides.  1/2O2 + H2O + 2e- -> 2OH- 

Galvanic corrosion/bimetallic corrosion   #

This type of corrosion takes place when two dissimilar metals are electrically connected or exposed to an electrolyte solution. The metal which lies at a higher position in the electrochemical series gets oxidized. The electrons which are released are consumed for either hydrogen evolution or oxygen adsorption. 

Concentration cell corrosion #

• It is defined as the electrochemical attack on the metal surface which is exposed to electrolyte solutions of varying concentrations or varying aeration. 
• The concentration cell corrosion due to varying aeration is also known as differential aeration corrosion.  
• This kind of corrosion takes place at a faster rate in inaccessible parts of the metal
• It is also accelerated due to accumulation of dirt, sand, mud, scale, sludge etc. 

Pitting corrosion #

• It is a highly localized and accelerated attack on the metal surface resulting in formation of cavities around the metal, most of which is unattached. It leads to development of pinholes,  pits, cavities. 
• It is characterized by very small anodic part and large cathodic part. 
• It is accelerated near rough surfaces or non uniform protective layers, sharp edges, scratches, local straining of metal, turbulent flow of chemical over a metal, deposition of dust or moisture. 

Intergranular corrosion #

• This kind of corrosion does not take place on the metal surface but takes place on the grain boundary when corrosive liquid or moisture seeps through the metal surface. 
• It is most dangerous type of corrosion because it is not apparently visible on the surface and it takes place inside alloys at a much faster rate compared to pure metals. 
• It can be prevented by decreasing the amount of doping atoms within the alloy. For example, corrosion of chromium carbide can be decreased by decreasing the carbon content. 

Waterline corrosion #

It takes place in metallic storage tanks where water is stored for a long period of time. It can be also called a special type of concentration cell corrosion due to long storage of water the amount of dissolved oxygen and salts get accumulated in the tank. Again, growth of algae and aquatic plants also increases the oxygen content. Happens due to difference in concentration of dissolved oxygen and ions. 

Stress corrosion #

• Takes place due to a combined effect of static tensile stresses and corrosive environment on the metal.
• It is a highly localized attack around the areas under stress. However, the overall corrosion is relatively low. This type of corrosion is observed in chemical industries and laboratories where the presence of chemical fumes and gasses makes the environment highly corrosive.
• For example, fumes of caustic alkalis and nitrate based compounds can even induce corrosion in mild steel. Brass is readily attacked by fumes of ammonia whereas stainless steel gets corroded by vapors of acid chloride. 

Factors influencing corrosion   #

1. Nature of the metal 

   1. Position of the metal in the galvanic series: based on how readily substances undergo corrosion. The substance or the metal which is at a higher position in the galvanic series gets easily corroded. 

   2. Overvoltage: less the overvoltage,  higher will be the corrosion rate. Overvoltage is the difference between actual and observed voltages of reactions. 

   3. Relative areas or cathodic and anodic parts 

   4. Purity of the metal 

   5. Physical state which includes grain size, orientation of Crystal’s 

   6. Nature of the surface film.  

      1. Pascivity is defined as reluctance of a metal to undergo corrosion at the regular rate due to formation of a very thin stable oxide layer on the metal surface. Generally the thickness of such an oxide film is around 0.0004mm. Metals showing passivity generally occupy …. position in the position of galvanic series. 

      2. Order of passivity: Tl, Al, Cr, Be, Mo, Mg~Ni, Co, Fe (least, highest) 

      3. Sometimes, metal may form an insoluble salt on the surface thus protecting it. Eg. Lead gets covered with lead sulphate. 

   7. Solubily or volatility of the corrosion product

2. Nature of environment 

   1. Temperature influences the rate of corrosion highly. In summer, when the temperature is high corrosion takes place at a faster rate compared to winter 

   2. As the humidity increases in the environment,  the moisture content also increases which leads to faster corrosion. The value of relative humidity from where the rate of corrosion increases sharply is known as critical humidity  

   3. Presence of impurities like carbon dioxide, sulphur dioxide, NO_x increases the ionic character of the environment and which increases the rate of corrosion. 

   4. Suspended particles like particulate matter, soot when present in air trap moisture for a long period of time which increases the corrosion rate. (Pm2.5, PM 100) 

   5. Rate of corrosion is minimum when the pH=7, as the ph increases or decreases from this value (7) the rate bnb of corrosion increases rapidly. 

   6. Nature of ions present in solution: the presence of certain ions such as silicate ions which form insoluble salts reduces the rate of corrosion whereas the ions like h+, NH4, and copper increases the rate of corrosion.  

   7. Flow velocity of processed steam: in boilers and steam processing industry, the rate of formation of stem contributes to corrosion. At higher flow velocities, steam carries small water droplets which increases the corrosion of metal bodies. 

   8. Polarization of electrodes: electrodes made up of less polarizable material are prone/susceptible to corrosion at a faster rate. 

Control of corrosion, preventing corrosion   #

1. Proper choice of material and proper designing : corrosion even if it occurs, proper designing can slow down the corrosion process.  

   1. In highly corroding atmosphere avoid contact of dissimilar metals. Even if two dissimilar metals are used, the area of contact between them should be minimum.  

   2. The dissimilar metals should be very close to one another in the galvanic series. If possible, insulating fittings should be used to avoid metal-metal contact. 

   3. Reduce inhomogenities of sharp corners, stress on metal parts. 

   4. If possible, use pure metal. Alloys should be used in parts which undergo regular wear and tear. 

2. Environment 

   1. Modify the environment by making it less corrosive. Regular deaeration should be carried out for removal of dissolved oxygen or carbon dioxide from water containing … Oxygen deactivation by the use of chemicals such as sodium sulphide and ammonia. Silica gels and alumina can be used to absorb moisture from the humid atmosphere. 

   2. For neutralization of acidic environments, fumes of calculated amount of ammonia can be used

3. Cathodic protection involves converting the more reactive anode into a less reactive material by making it function as a cathode. The anode involved in the corrosion can be converted into cathode by the following two methods 

   1. Sacrificial anodic protection: where a more reactive or alloy is connected or coated in the top of the anode. The metal which is used for covering or coating gets itself corroded and protects the actual anode. The metal used for covering or coating is known as the sacrificial anode. 
   2. Impressed current cathodic protection: in this method, the anodic and cathodic materials are connected to an external battery in such a way that it nullifies the flow of electrons taking place due to corrosion.  

4. Use of inhibitors 

   1. There are certain metals which get corroded easily but their insoluble salts do not undergo any oscillate. Anides like chromate, sulphate, arsenate, phosphates form insoluble salts with the anodic material and covers it up which prevents further corrosion. 

   2. Organic inhibitors such as glycerol, mercaptans, amines, petrocyclic nitrogen compounds are added to the solution which either slow down the movement of h+ ion, or consume it up. 

5. Use of protective coatings such as paints or metal coatings.  

   1. The coating must have the following two properties. 

      1. It should be inert or it should have a inert layer on top of it. 

      2. It should completely block the penetration of the corrosive environment.  

   2. Protective coatings can be of two types: 

      1. Anodic (sacrificial anode): ex: coating of zinc, aluminium or cadmiumon steel or fe 

      2. In cathodic coating, the anodic metal is coated with a noble/inert metal. Eg: coating of chromium, tin, on iron

Refractory metals : high bp/mp 

Methods of application of protective coatings  #

1. Hot dipping: in this method, metals or alloys which are to be protected (iron, stainless steel) are dipped into a hot solution of another relatively inert metal having lower melting point such as zinc, copper, tin, lead. Based on the type of metal used for protective coating it can be classified into many types, but the major two types are: 

   1. Galvanizing: coating with zinc 

      1. Metal is dipped in dil H2SO4 at 60-90C  

      2. Acid is removed by washing 

      3. It is then heated for drying 

      4. Dipped into a solution of molten zinc which is covered with a layer of molten NH4Cl to prevent the oxidation of it. 

      5. Passed through rollers to remove excess molten zinc. 

   2. Tinining: coating with tin 

      1. Same as galvanizing 

      2. Molten tin is covered with two layers: palm oil and molten ZnCl2 (binding agent, increases the stickiness of the molten on the metal sheet) 

      3. Doesn’t include washing and drying processes. 

2. Metal cladding: protecting of mild steel by sandwiching it between stainless steel. Will also increase the overall strength. 

3. Electroplating: doing electrolysis by keeping one end with the metal we want to coat. It is the best method for durability. 

4. Potentiometric anodic  protection: this method is similar to impressed cathodic current method. 

   1. In this method the metal which need to be protected is connected to a potentiometric setup and the potential is slowly increased in the negative direction and at the same time the current is monitored.
   2. Critical potential is defined as the minimum potential required to render the surface of the metal passive.