Corrosion Problems and Cotrol

8/2/2019 Corrosion Problems and Cotrol

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CORROSION PROBLEMS AND COTROL

Corrosion is a chemical or electrochemical process between a material,

usually a metal and its environment by which the subjective metal is pittedand so thinning of metal occurs in a random manner. The general definition ofcorrode is to eat into or wear away gradually, as if by gnawing.

When corrosion is discussed, it is important to think of a combination of a material

and an environment. The corrosion behavior of a material cannot be described

unless the environment in which the material is to be exposed is identified.

Similarly, the corrosivity or aggressiveness of an environment cannot be described

unless the material that is to be exposed to that environment is identified. In

summary, the corrosion behavior of the material depends on the environment to

which it is subjected, and the corrosivity of an environment depends on the

material exposed to that environment. It is useful to identify both naturalcombinations and unnatural combinations in corrosion. Examples of natural or

desirable combinations of material and environment include nickel in caustic

environments, lead in water, and aluminum in atmospheric exposures. In these

environments, the interaction between the metal and the environment does not

usually result in detrimental or costly corrosion problems. The combination is a

natural combination to provide good corrosion service. Unnatural combinations, on

the other hand, are those that result in severe corrosion damage to the metal

because of exposure to an undesirable environment. Examples of unnatural

combinations include copper in ammonia solutions, stainless steel in chloride-containing environments (e.g., seawater), and lead with wine (acetic acid in wine

attacks lead). It has been postulated that the downfall of the Roman Empire can be

attributed in part to a corrosion problem, specifically the storage of

Wine in lead-lined vessels.

CASE STUDY: Mild steel is corroded quickly by sulfuric acid in the

concentration range of 5 to 60%; but becomespassive at higher concentrations, and

it is used to store 90 to 98%.

A water hose used to clean down the conical roof of a storage tank containing 95%

sulfuric acid, without it being realized that the conical roof had corroded internallyand had at least some holes. Next morning the tank was in two pieces with a clean

cut running round the tank, just above the acid level. This incident drew the

attention to the corrosion problem by dilute acid formed by the condensation of

acid fumes underneath the roof and absorption of atmospheric water vapor by

condensed acid.


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Four Requirements of Corrosion Cell: There are four requirements for an

electrochemical corrosion cell. These are-

Two dissimilar metals to form a galvanic cell;

The aqueous solution to act as electrolyte;

An ionic current path from anode to cathode; and An electronic path from cathode to electrode.

The anode and cathode are connected through the solution by an ionic current

path, and they are connected through the metal by an electronic path. An

electrochemical reaction involves the transfer of electrons from one species to

another, causing direct current flow through the corrosion cell.

EIGHT FORMS OF CORROSION:

Uniform or general corrosion;

Pitting corrosion;

Crevice corrosion;

Galvanic corrosion;

Erosion corrosion;

Intergranular corrosion;

Dealloying; and

Stress corrosion cracking.


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Planning for the graceful aging of a structure or system during materials selection

requires a firm understanding of the environment it will be operating in. The word

environment describes the conditions a system may be exposed to while in service.

For example, a ship floating in the ocean is thought to be in a marine environment.

Another example is the high temperature environment turbine blades experience

inside a jet engine during operation. In reality though, things are not quite that

simple because systems experience a variety of simultaneous environmental

conditions. Systems often contain many fluids and chemicals that are necessary for

its components to operate, but some of these can be very corrosive and cause a

material to degrade. For instance, designers must consider cleaning chemicals and

hydraulic fluids as sources of contamination that can cause a material to corrode.

There are many other materials and contaminants that exist within the operational

environment that can influence the rate at which a structure or component

corrodes. Designers must take a step back and gain a firm understanding of allenvironmental factors that can influence corrosion before selecting the construction

materials. Again, it's important to note that the environment as is described here

isnt a single condition but rather a combination of factors, which work in concert,

such as operating temperature and humidity, salinity, and mechanical loading.

Of far more serious consequence is how corrosion affects our lives during travel

from home to work or college. The corrosion of steel reinforcing bar (rebar) in

concrete can proceed out of sight and suddenly (or seemingly so) result in failure

of a section of highway, the collapse of electrical towers, and damage to buildings,

parking structures, and bridges, etc., resulting in significant repair costs and

endangering public safety.

Perhaps most dangerous of all is corrosion that occurs in major industrial plants,

such as electrical power plants or chemical processing plants. Plant shutdowns can

and do occur as a result of corrosion. This is just one of its many direct and indirect

consequences. Some consequences are economic, and cause the following:

Replacement of corroded equipments;

Preventive maintenance, for example, painting;

Shutdown of equipment due to corrosion failure; Sudden release of flammable chemical cause fire, explosion or other safety

problems;

Sudden releases of toxic chemicals cause health and environmentalproblems.


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Material Factors Affecting Corrosion in Chemical & Petrochemical Factories:Corrosion is the primary means by which metals deteriorate. Most metals corrode oncontact with water (and moisture in the air), acids, bases, salts, oils, aggressive metalpolishes, and other solid and liquid chemicals. Metals will also corrode when exposed to

gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfurcontaining gases. The production of oil and gas, its transportation and refining, and itssubsequent use as fuel and raw materials for chemicals constitute a complex anddemanding process. Various problems are encountered in this process, and corrosion isthe major one. Since metals are the principal material suffering corrosive deterioration, itis important to develop a background in the principles of metallurgy to fully understandcorrosion. The control of corrosion through the use of coatings, metallurgy, nonmetallicmaterials for constructions cathodic protection and other methods has evolved into ascience in its own right and has created industries devoted solely to corrosion control.Metallurgical factors that affect corrosion are chemical composition, material structure,grain boundaries, alloying elements, mechanical properties, heat treatment, surface

coating, welding and manufacturing conditions. Understanding these factors are of greatimportance to decrease and control corrosion problem in many industrial applications.

The structure of metals and alloys is of decisive importance in determining theircorrosion characteristics which cannot, therefore, be discussed without the use ofmetallurgical terms. For example of these factors, sulfide inclusions in pure iron have a

marked tendency to react in even mild corrosive environments. Also heavily deformedmaterial by plastic deformation at room temperature (cold worked, cold rolled, colddrown, etc.) have a remarkable effect on the corrosion resistance of the materials.

Metals listed as pure or commercially pure, actually contain a variety of

impurities and imperfections. These impurities and imperfections areinherent cause of corrosion in aggressive environment. It has been shownthat, as purity increases, the tendency for a metal to react inelectrochemical environments reduces proportionality. However, high puritymetals generally have low mechanical strength; hence they are rarely usedin engineering applications. It is necessary, therefore to work with metallicmaterials which are stronger and which are usually formed from acombination of several elemental metals. The common alloys are thosewhich have a good combination of mechanical, physical and fabrication

qualities which tend to make them structurally, as well as economically,useful. Although an alloys resistance to corrosion is very important, thisfactor is sadly neglected in the aim to improve the mechanical properties.

Many of the final mechanical properties and the corrosion resistance of amaterial can be related to its heat treatment. That is its metallurgicalprocessing using heat treatment to transform or anneal the structure of the


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metal or alloy. Annealing; is very important heat treatment process usuallydone to produce one of two effects affecting corrosion resistance of thematerial:

First, homogenization of cast alloys which may exhibit chemicalinhomogeneity (coring); and second, remove residual stress and cold workeffects in deformed metals. This annealing treatment to a great extendreduces the corrosion tendency of the materials and alloys. Chemicalinhomogeneity of an alloy cause reduction in its mechanical properties, inaddition, reducing in corrosion resistance often occurs. Homogenizinganneal is important or chemical homogeneity of an alloy by diffusion(movement of atoms in the solid state).

Residual stresses and cold work in deformed metals reduces its resistance

to general corrosion. Deformation decreases corrosion resistance basicallybecause it increases dislocations in the cold deformed metals; these areasof high dislocation density are usually subjected to pitting corrosion. Oftenimpurities or atoms of alloying metals migrate to these imperfections tocause an even greater change in the electrochemical characteristics ofthese defects. Thus annealing a cold worked metal may result in adecrease in dislocations and important improvements in the corrosionresistance.

Aluminum, magnesium, nickel, copper and some forms of stainless steel,when alloyed with certain elements can be strengthened by age hardeningor precipitation hardening treatments. It is well known that pure aluminumforms a thin film of aluminum oxide which provides a very effective barrierto environmental attack. This explains the extremely good corrosionresistance of aluminum. Aluminum alloys do not readily form this protectiveoxide. Therefore, the corrosion resistance of the important aluminum agehardening alloys is not as good as pure aluminum with its atmosphericallyoxidized surface. To develop this protection on alloys, a thin layer of purealuminum is often clad to the outer surface. This layer develops a

protective surface oxide.

The corrosion engineer is often required to consider one or more propertiesin addition to corrosion resistance and strength when selecting a material.The selection criteria used by materials engineers in choosing from a groupof materials includes a list of qualities that are either desirable ornecessary. Unfortunately, the optimum properties associated with each


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selection criteria can seldom all be found in a single material, especiallywhen the operating conditions become aggressive. Thus, compromisesmust frequently be made to realize the best performance of the selectedmaterial. A wide variety of iron- and nickel-based materials are used forpressure vessels, piping, fittings, valves, and other equipment in chemicalindustries. The most common of these is plain carbon steel.

Thus it is possible to reduce or prevent corrosion by application of thefollowing metallurgical principals:

1. Use of high purity material,2. Use of alloying additions,3. Effective heat treatments,4. Use surface coatings, and

5. Knowledge of the metallurgical history of the used material.

CATHODIC PROTECTION:

Cathodic protection is a widely used electrochemical method for protecting a

structure or important components of a system from corrosion. A Cathodic

Protection system is essentially an electrochemical cell and must have a cathode,

an anode, an electrical connection between them and an electrolyte. The principle

behind Cathodic Protection is that dissolution of a metal (cathode) can be

suppressed by supplying it with electrons, and in effect, controlling the corrosion.

Corrosion is then targeted on the anode instead of the metal. Since an electrolyte is

required for this method of protection, CP is not effective for systems in air or

other environments that resist current flow between the anode and cathode.

There are two main classes of CP: active and passive. Active cathodic protection,

also called impressed-current, requires the use of an external power supply. In this

type of protection, the negative terminal of the power supply is connected to the

metal to be protected, and the positive terminal is connected to an inert anode. The

anode, however, is often not more anodic to the metal, and can be even more

cathodic than the metal. The impressed-current ensures that current flows such that

the metal acts as the cathode and is therefore protected from corrosion.Passive CPsystems are simpler than impressed-current systems and involve the galvanic

coupling of the metal to be protected to a sacrificial anode, which corrodes

preferentially. The anode in this type of system must be more anodic than the metal

and must also readily corrode without passivation in order for the system to be

successful. In some instances the sacrificial anode must be replaced after it has

been consumed to ensure protection of the structure.

Corrosion Problems and Cotrol

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