CORROSION: -
What Are The Common Modes Of Deterioration Of Materials?
Materials deteriorate due to the following:
1. Corrosion by electrochemical reactions
2. Wear by mechanical rubbing
3. Erosion by high-velocity fluid, suspended particles in liquid streams or suspended droplets in gaseous streams
1. Fracture by stresses that increase with time
What Is Corrosion?
Corrosion is an electrochemical process by which a metal, such as mild steel, returns to its natural state, i.e., iron oxide or rust.
What Is A Corrosion Cell?
A corrosion cell consists of an anode, a cathode, and an electrolyte. Metal ions dissolve into the electrolyte (water) at the anode. Electrically charged particles (electrons) are left behind. These electrons flow through the metal to other points (cathodes) where electron-consuming reactions occur. The result of this activity is the loss of metal at the anode and often the formation of a deposit. Chemical reactions involved are the following:
1. At the anode
Neutral metal -y electrically charged metal + electrons Fe à Fe2+ + 2e-
2. At the cathode
Electrons + oxygen + water à charged ion
2e- +1/2O2+H2O à2O H-
What are the various forms of corrosion?
There are eight forms of corrosion that, along with their brief descriptions and examples, are listed.
Which Are The Most Common Forms Of Corrosion?
The uniform (or general) corrosion is the most common form followed closely by stress corrosion cracking and pitting corrosion.
What Factors Affect Corrosion?
These are the following:
1. Metallurgical
a. Composition of the alloy
b. Structure of the alloy (single phase, multiple phase, presence of precipitates)
c. Mechanical state of the alloy (degree of cold work, presence of defects)
2. Environmental
a. Composition and concentration of the corroding species (H+, O2, Cl-, 
HC
)
HC) b. Temperature
c. Flow velocity
d. Formation of a protective film
e. Deposits
3. Operational
a. Presence of dissimilar metals
b. Equipment design features (sharp bends, stagnant regions, etc.) c. Stresses present (tensile or compressive, static or dynamic, applied or residual) d. Thermal and mechanical history
What Are The Economic Losses Due To Corrosion?
Corrosion ultimately results in the failure of the equipment. Failure of important equipment such as heat exchanger causes direct and indirect losses besides proving dangerous to human life in certain cases. The direct losses due to corrosion are the following:
What Is Scale?
It usually refers to the deposits from water systems caused by the super-saturation of inorganic salts. However, generally no distinction is made between fouling and scale in the literature, and we shall also use the terms interchangeably.
What Are The Effects Of Fouling?
These are the following:
1. Reduced heat transfer
2. Increased pressure drop
3. Deteriorating product quality
4. Localized pitting and corrosion of the tubes
5. Reduced throughputs
6. Oversized unit at the design stage
All the above lead to economic losses. Therefore, it is important to know the salient features of the fouling of heat exchangers.
What Are The Different Kinds Of Fouling?
Fouling has been classified into eight kinds based upon the process by which it is formed (Taborek et al., 1972):
1. Crystallization: Salts dissolved in the fluid stream crystallize out when the fluid is cooled or heated (as in cooling towers and heat exchangers) or when it is evaporated (as in steam generators and reboilers). Salts that crystallize out when the liquid is heated are called the "reverse solubility salts." Their solubility decreases with increasing temperature. Some such salts are CaC03, CaS04, etc.
2. Chemical reaction: Some organic products or inorganic salts might get oxidized at high temperature and crystallize out.
3. Polymerization: Some compounds might get polymerized and then deposit on heat transfer surface. This is often found in polymer and textile industries.
4. Coking: It is found extensively in crude oil refineries and hydrocarbon processing units due to high-temperature effects on various hydrocarbons. Since it is a class in itself, it has been separated from the chemical reaction fouling.
5. Sedimentation: Dirt, clay, sand, and corrosion products suspended in the fluid stream settle on the heat transfer surface due to gravitational forces. These particles then act as nucleation sites for other kinds of deposits.
6. Solidification: When the surface temperature is lower than the solidification or freezing temperature of one or more components present in the fluid, such components solidify on the surface leading to solidification fouling. An example is the deposition of wax from crude oil in pipelines.
7. Corrosion: A corrosive fluid corrodes the heat transfer surface. Part of the corrosion product shears off [erosion-corrosion (Q. 13.36)] and gets suspended in the flowing stream only to sediment out downstream as discussed in (5) above. Any corrosion products sticking to the surface reduce the heat transfer due to their low thermal conductivity and also provide nucleation sites for the fouling to deposit by other mechanisms.
8. Microbial fouling: It happens due to the presence of microbial life.
On What Factors Does The Fouling Depend?
These are the following:
1. Physical properties, chemical composition, and pH of the fluid
2. Physical state of the fluid: liquid, vapor, or two-phase
3. Types and amounts of dissolved and suspended impurities
4. Presence of dissolved air and/or carbon dioxide
5. Velocity of the fluid: average velocity as well as velocities of bypass and leakage streams and velocities in baffle windows and in cross-flow over the tube bundle (0.2.3)
6. Fluid flow regime: laminar or turbulent
7. Fluid temperature and tube wall temperature
8. Fluctuations in velocity and temperatures, either intentional or accidental
9. Mode of heat transfer: sensible heating or cooling, boiling and evaporation, condensation, or a combination of these
10. Evaporation of liquid leading to super saturation of the dissolved solids
11. Materials of construction
12. Surface roughness
13. Design of the unit as far as bypass and leakage streams and the stagnant zones are concerned
What Are The Relative Effects Of Changing Some Of The Variables On Which The Fouling Depends?
These are the following;
1. With increasing fluid velocity in turbulent flow the deposit decreases. The effect in laminar flow may not be the same.
2. With increasing tube-wall temperature, the deposit increases, specially in the case of cooling water, hydrocarbons, polymerization, and chemical reactions
3. With tube materials, such as copper or its alloys, microbial fouling is reduced.
2. Decrease in dissolved oxygen and carbon dioxide reduces corrosion fouling i. Fluctuations in velocity will momentarily increase the shear stress and hence may dislodge the deposit. Fluctuations in temperature will result in greater expansion (and contraction) of the tubes than that of the deposit layer, thus cracking and dislodging the deposit (Q. 13.62). Thus, the fluctuations in velocity and in temperature are both likely to decrease the fouling deposit. However, their adverse effects on the process and equipment should also be taken into account. i. Reduction in the amounts of leakage and bypass streams and the elimination of the stagnant zones will also reduce fouling deposit since the cross-flow velocity will be kept high.
What Are The Deposits In A Cooling Water System?
These are the following:
I. Dirt, silt, and sand
2. Corrosion products
3. Natural organics
4. Microbial slime
5. Alumina, aluminum phosphate
6. Iron phosphate
7. Calcium salts
8. Magnesium salts, etc.
How Do The Deposits Grow?
The whole field of fouling studies is fast changing and so are the concepts about the mechanisms of deposit and growth. However, it is believed that, for the inorganic salts, the three steps involved are the following:
1. Initial nucleation that produces a primary deposit occurs during an induction period where the deposit rate is low.
2. Secondary crystal growth on the primary deposit
3. An asymptotic thickness due to dynamic balance between the rate of deposit and the rate of shear removal (re-entrainment)
A cross-sectional view of water fouling deposit, and hg. A plot of the buildup of the fouling resistance (deposit) with an initial induction period. There are also models proposed that do not have the induction period. A complete discussion of the fourteen different models has been given in a most comprehensive review by Epstein (1978). His later additions to that review are also highly recommended for a complete grasp c-^'the topic (Epstein, 1981a-c).
What Are The Differences Between The Various Types Of Scale And Fouling Deposits?
The deposits differ from one another in terms of texture, strength, adhesion to the surface, color, thermal conductivity, density, rate of deposition, equilibrium thickness of the deposit, means required for the removal of the deposit, the process of formation of the deposit, or the kind of fouling phenomenon
Why Is It Important To Analyze The Fouling Deposit? How Is It Done?
Analysis of the fouling deposit tells its nature and points toward both the causes and remedial actions required. Analysis includes the following methods (Tewari and Mehandru, 1977);
1. Chemical analysis for various inorganic salts and metal ions and their concentrations
2. X-ray analysis to identify the crystal structure
3. Ignition loss and solvent extraction for organic deposits
4. Differential microbioanalysis to identify the type of microorganisms and their concentrations.
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