FORMS OF CORROSION AND PREVENTION METHODS · FORMS OF CORROSION Corrosion can be classified into following types Uniform Corrosion: Uniform corrosion involves an even rate of corrosion

FORMS OF CORROSION AND PREVENTION METHODS

FORMS OF CORROSION Corrosion can be classified into following types

Uniform Corrosion: Uniform corrosion involves an even rate of corrosion over the entire metal surface. It refers to

the corrosion that proceeds at approximately the same rate over the exposed metal surface. It

is relatively easily measured and predicted, making disastrous failures relatively rare.This

type of corrosion can be detected by acoustic emission method. . It can be controlled by

cathodic protection, use of coatings or paints.

Examples: Rusting of steels in open air, Failure of distillation column, corrosion of

underground pipes

Localized corrosion:

Localized corrosion specifically target on one place of the metal surface. Localized corrosion

is accelerated attack of a passive metal in corrosive environment. Most commonly occurring

localized corrosion include galvanic, crevice and pitting corrosions. Stress corrosion and

intergranular corrosion are environment related corrosions.

Types of localized corrosion: a. Galvanic Corrosion: Galvanic corrosion is also called bimetallic corrosion. Galvanic corrosion takes place when

one metal is in electrical contact with another in the presence of an electrolyte. Two

dissimilar metals are in the electrolyte, one metal acts as anode and another metal as cathode.

The electro potential difference between the reactions at two electrodes is the driving force

for an accelerated attack on the anode metal, which dissolves into the electrolyte. This leads

to the metal at the anode corroding more quickly than it otherwise would and corrosion at the

cathode being inhibited.

A common example of galvanic corrosion such as corrugated iron, a sheet of iron or steel

covered with a zinc coating. One of the important methods of reducing and preventing this

form of corrosion is to isolate two different metals electrically from each other so that no

galvanic coupling will occur between metals.

b. Crevice Corrosion: A crevice i.e. a narrow gap between adjacent metal surfaces. permits exposure of the

enclosed surfaces to moist air or allows seepage of liquid between the two surfacesThe major

factors influencing crevice corrosion are: (1) Crevice type.(2) Crevice geometry.(3)

Material.(4) Environment. Examples of areas where crevice corrosion can occur are gaskets,

the under surface of washers, bolt heads, gaps and contact areas between parts, under gaskets

or seals, inside cracks and seams, spaces filled with deposits and under sludge pipes.

Example: All grades of aluminium alloys and stainless steels undergo crevice corrosion.

It is prevented through welded but joints instead of riveted or bolted joints in new

equipment, eliminating crevices in existing lap joints by continuous welding or soldering, and

using solid and non-absorbent gaskets such as Teflon.

c. Pitting Corrosion: Pitting is a very localised form of attack and may be very directional. It can occur where a

small break in a protective layer, a layer of noble metal, for example, or an oxide film ,

renders the underlying metal prone to attack. Prolonged or highly localised forms of pitting

can lead to perforation of the metal. The depth of the pits is generally large compared with

their diameter. When it is occur it grows into a hole or cavity. Typically pits penetrate from

the surface downward in a vertical direction. It can cause by non-uniformities in the metal

structure itself. It is one of the most destructive types of corrosion, as it can be hard to

predict, detect and characterize.

d. Stress Corrosion Cracking

Stress Corrosion Cracking can be abbreviated to ‘SCC’ and refers to the cracking of the metal

as a result of the corrosive environment and the tensile stress placed on the metal. It often

occurs at high temperatures.

Example: Stress corrosion cracking of austenitic stainless steel in chloride solution.

e. Intergranular Corrosion

Intergranular corrosion (IGC), also known as intergranular attack (IGA), is a form

of corrosion where the boundaries of crystallites of the material are more susceptible to

corrosion than their insides. Intergranular corrosion occurs due to the presence of impurities

in the grain boundaries that separate the grain formed during the solidification of the metal

alloy. It can also occur via the depletion or enrichment of the alloy at these grain boundaries.

Example: Aluminum-base alloys are affected by IGC.

COOROSION PREVENTION METHODS

The Strategy for Corrosion Prevention

The prevention of electrochemical corrosion depends on disruption of the corrosion cell by

one of the following strategies:

(i) The anode, anode-electrolyte interface or anode reaction.

(ii) The cathode, cathode-electrolyte interface or cathode reaction.

(iii) The electrolyte.

(iv) The cathodic reactant

(v) The electrical connection between anodic and cathodic sites.

It is important to recognise that, in a real corrosion situation, the corrosion cell and the

electrochemical reaction will not always be readily identified or isolated.

Prevention methods

Protective methods are broadly subdivided into four categories:

(1) Surface coating

(2) Materials and design.

(3) Modification of the electrolyte

(4) Change in the electrode potential

1. Surface Coatings A diverse range of surface coatings may be applied to protect a metal surface. Due

consideration must be paid to the avoidance of localised damage that could render areas of

the surface vulnerable to corrosion. Examples of surface coating protection layers include:

1. Paint or a polymer cladding.

2. Metal oxides, as in the anodising of aluminium.

3. Metal coating, eg., steel sheet electroplated with zinc or hot-dipped galvanised iron.

Electroplating is the process by which the coating metal is deposited on the base metal by

passing a direct current through an electrolytic solution containing the soluble salt of the

coating metal.

Theory of electroplating:- suppose the anode is made up of coating metal (M) itself, during

electrolysis the concentration of the electrolytic bath remains unaltered since the metal ions

deposited from the bath are replenished continuously by the reaction of the free anions of the

electrolyte with the anode metal. Thus for e.g. if CuSO4 solution is used as an electrolyte it

ionizes as

CuSO4 ---------> Cu+2 + SO4-2

On passing current Cu+2 ions go to the cathode (the article to be plated) and get deposited

there.

Cu+2 + 2e- –-----> Cu (at cathode)

The free sulphate ions migrate to the Cu anode and dissolve an equivalent amount of the Cu

to form CuSO4

Cu + SO4-2 –--------> CuSO4 + 2e- (at anode)

The CuSO4 thus formed dissolves in the electrolyte thus there is continuous replenishment of

electrolytic salt during electrolysis.

Procedure:- The article to be electroplated is first treated with organic solvent like

tetrachloroethylene to remove oil, grease etc. then it is treated with dil HCl or H2SO4 to

remove surface scales, oxides etc. For Ni and Cu HCl is used while for Cr plating H2SO4 is

used. The cleared article is then made as cathode of an electrolytic cell. The anode is either

the coating metal itself or an inert material of good electrical conductivity like graphite. The

electrolyte is a solution of a soluble salt of the coating metal. When direct current is passed

coating metal ions migrate to the cathode and get deposited there. Thus a thin layer of coating

metal is obtained on the article made as cathode. For brighter and smooth deposits, favorable

conditions such as low temp, medium current density and low metal ion concentrations are

used.

In all cases of protective coatings, the consequences of defects in the coating or damage

of the coating for corrosion must be taken into account. For example galvanic corrosion can

result at pore sites or damaged sites in metal coatings and atmospheric corrosion may result at

a damaged zone in a paint coating on a metal.

2.Materials Selection and Design

Due consideration to the suitability of a material coupled with care in design can prevent

many corrosion problems. Amongst the most important factors to be considered are:

(i) Materials (metals, alloys, non-metallic materials) – Material should be chosen taking into

account its cost, availability and its suitability to the environment in which the item is to be

used.

(ii) Contact between metals of different standard electrode potentials - designs should avoid

contact between dissimilar metals where the kinetics of attack on one metal surface are

enhanced by the presence of the second.

(iii) Geometry - localised attack can be minimised by the avoidance of areas particularly

susceptible to erosion or cavitation.

(iv) Mechanical factors – excessive stress, internal or externally applied, should be avoided

especially in metals known to be vulnerable to stress-corrosion cracking. This will reduce the

instances of corrosion fatigue or fretting corrosion.

(v) Surface conditions - surface conditions that enhance susceptibility to localised attack

should be avoided, e.g. roughened surfaces, broken films of metal or oxide and weld spatter.

(vi) Electrochemical protection - where possible, designs should include provision for

cathodic or anodic protection or for the application of protective coatings.

(vii) Alloying speciality alloys provide an excellent means of corrosion prevention for certain

applications. In tidal zones, for example, the use of Ni-Cu alloys in the construction of jetties

and offshore oil platforms has proved effective against corrosive attack by seawater.

3.Modification of the Electrolyte

Electrolyte modification may involve following tactics:

(i) Removal of the aggressive species through suitable strategies such as -

1. Elimination of dissolved oxygen usually by evacuation, nitrogen saturation or by means of

oxygen scavengers such as hydrazine.

2. Elimination of acidity by neutralisation or by addition of lime

3. Elimination of dissolved salts by means of reverse osmosis or ion exchange

4. Reduction of humidity by means of desiccants such as silica gel.

5. Reduction of local humidity by means of a localised temperature increase of around 5°C.

6. Elimination of solid particles in order to prevent deposit corrosion.

(ii) Addition of corrosion inhibitors

A corrosion inhibitor is an inorganic or organic species that retards corrosion when

introduced, in low concentration, into an aqueous solution in contact with the metal surface.

Several different mechanisms exist for the inhibition. Inhibitors may act by adsorption on the

surface of the metal. Adsorption occurs around the corrosion potential and reduces the rate of

either the anodic or cathodic reaction. Various sulphur, arsenic or phosphorus compounds act

in this way.

An alternative mechanism for inhibition involves the formation of the precipitate on the

metal surface or catalysis of a passivating reaction. Phosphonate, polyphosphate or hydrogen

carbonate salts belong to this category of inhibitor.

A third group of inhibitors comprises redox reagents which act by shifting the surface

potential to a region where cathodic or anodic protection occurs. The dichromate ion acts by

this type of mechanism.

4.Change of Electrode Potential

It is, theoretically, possible to raise the electrode potential away from the corrosion potential

to a value where a stable passive state is attained, i.e., the surface is anodically protected.

Conversely, the potential may be lowered, removing it away from the corrosion potential and

into a stable zone, i.e., the surface is cathodically protected.

(i) Cathodic protection : The principle involved in this method is to force the metal to be

protected to behave like a cathode thereby corrosion doesn’t occur. There are two types of

cathodic protection. Cathodic protection may be achieved by means of a sacrificial anode or

by the use of impressed current.

Sacrificial anodic protection method: - In this method the metallic structure to be protected

is connected by a wire to a more anodic metal, so that all the corrosion is concentrated at this

more active metal implies the more active metal itself gets corroded slowly. While the parent

structure which is cathodic is protected .The more active metal so employed is called

“sacrificial anodic” .Whenever the sacrificial anode is consumed completely. It is replaced by

a fresh one.

To operate successfully a sacrificial anode must dissolve at a uniform rate at a potential

negative with respect to the corrosion potential of the metal it protects. It will then provide a

consistent protective current of sufficient magnitude. Dissolution of the second metal results

in a negative shift of the electrode potential of the protected metal. As long as the two metals

have approximately equal potentials the second will act as the anode and the protected

surface as the cathode. Thus, while the overall rate of loss of metal is increased, it is the

auxiliary metal that dissolves. This anode is, therefore. 'sacrificed' in order to protect the

cathode.

Zinc, magnesium and certain aluminium alloys are commonly used as sacrificial

anodes in the protection of steel. Sacrificial anodes are used for the protection of buried pipe

lines underground cables, marine structures, ship hulls, water tanks etc. A steel ship's hull is

often fitted with zinc blocks, for example, which are simply removed and replaced when

necessary.

Impressed current – cathodic protection - In this method an impressed current is applied in

opposite direction to nullify the corrosion current and current the corroding metal from anode

to cathode. Usually the impressed current is derived from a direct current source (like battery

or rectifier on ac line) with an insoluble anode, like graphite, high silica iron, scrap iron,

stainless steel or platinum. Usually a sufficient d.c is applied to an insoluble anode, buried in

the soil ( or immersed in the corroding medium) and connected to the metallic structure to be

protected. The anode is buried is a backfill such as coke breeze or gypsum to increase the

electrical contact between itself and the surrounding soil.

Thus, a power supply is employed to drive current to the protected metal which serves as a

cathode with respect to an auxiliary anode. The cathode potential is maintained within the

required limits by current or cell voltage regulation in a case such as a pipeline or by a large

scale potentiostat (controlled potential power supply) for an offshore construction. The cost

of maintaining an electrical current must. of course, be taken into account.

Applications:- This type of protections is used in buried structures such as tanks and

pipelines, transmission line towers, marine piers, laid up ships etc.,

Advantages: - They can be automatically controlled which reduce maintenance and

operational costs.

(ii) Anodic protection

The principle of anodic protection depends on maintaining a stable passivating layer on the

metal surface. Addition of elements such as palladium or copper as low concentration

components in alloy steel produces galvanic anodic protection of the steel. Impressed current

anodic protection is used to a much lesser extent than its cathodic counterpart

CORROSION MONITORING

1. Non-Destructive Method:

Non-Destructive Testing (NDT) is a wide group of analysis techniques used in science and

technology industry to monitor or evaluate the properties of a material, component or system

without causing damage. Types of non-destructive method:

a. Ultrasonic and Acoustic Method:

The main principle of ultrasonic testing method is two probes are connected with the testing

material and on the basis of transmitting and receiving pulse it is measures the thickness of

the material and calculate the corrosion rate. Acoustic wave devices design in such way that

they can measure the atmospheric corrosion of thin plat. Very short ultrasonic pulse –waves

are transmitted through the material to detect the internal flaws or to characterize the

material. Acoustic Emission sensors are generally piezoceramic transducers; the basic

principle is convert physical displacement into voltage. According to change in voltage we

can calculate stress-corrosion cracks and ductile fracture of the materials. The main

advantage is high penetrating power means we can detect the deep internal flaws. It is

capable to estimating the size and orientation of defects.

b. Electromagnetic Method:

The main principle of method is inducing electric currents or magnetic fields or both into the

test object and observe the electromagnetic response of the object there are many types of

electromagnetic method like:

i. Magnetometer and di-electrometers:

Hidden corrosion under paint and to measure the depth of moisture within barrier paint

coatings are detect by Meandering Winding Magnetometers (MWM) and interdigital

Electrometers (IDED) because of the reducing the conductivity near a metal surface there

may be chance of oxygen diffusion layer on the metal surface, it is early stage of corrosion.

MWM uses magnetic fields and inductive coupling to measure property profiles of the

material. The IDED uses electric fields and capacitive coupling to measure the properties of

multi-layered insulating media, such as paint on metal oxides formed during corrosion.

ii. Magnetic flux leakage (MFL):

Basic principle of MFL technology is saturating magnetic field is applied to the test material

through large magnet and sensing local change in the applied field. Basically MFL is use in

larger diameter pipes over a long distance. MFL tools consist of total two parts (1)

magnetizer with magnet and sensor,(2) electronics and batteries. The magnets are mounted

between the brushes and tool body to create the magnetic circuit along with the pipe wall. As

the tool travels along the pipe, the sensors detect interruptions in the magnetic circuit.

Interruptions are typically caused by metal loss and which in most cases are corrosion and the

dimensions of the potential metal loss is denoted previously as "feature." Other features may

be manufacturing defects and not actual corrosion.

iii. Eddy current testing (ECT):

Given an Alternating Current (AC) to a wire coil, this wire coil produces alternating magnetic

field around itself. Frequency of current and magnetic field is same. When the coil

approaches a conductive material, currents opposed to the ones in the coil are induced in the

material this called eddy current. It is basic principle of eddy current testing. There two major

applications of ECT (1) Surface inspection: used in aerospace industries and petrochemical

industries. Surface inspection can be applied to both ferromagnetic and non-ferromagnetic

material, (2) Tubing inspection: It is only applicable to non-ferromagnetic material. An

inversion algorithm for the reconstruction of cracks from eddy current signals is developed in

this study and applied to the profile evaluation of natural stress corrosion cracks that were

found in steam generator tubes of a nuclear power plant. Using ECT testing cracks, laminar

defects and assess wall thickness types of flaws can be detected. Merits of ECT are Non-

contact test, No residual effects, MFL induced currents can be detected by ECT sensor. There

are some more techniques used in electromagnetic testing like (1) Remote field testing, (2)

Wire rope testing,(3) Magnetic particle inspection (MT OR MPI), (4) Alternating current

field measurement (ACFM) and (5) Pulsed eddy current.

c. Radiographic Method:

A radiographic method is use of high frequency gamma radiation and x-ray, are employed to

produce images of physically inaccessible metallic components obstructing the radiation

path. This radiation method of corrosion monitoring has low sensitivity and requires radiation

safety. In the chemical and petroleum industries radiography is extensively used in measure

pipe thickness. Applications of radiographic method are (1) Internal deposits: radiography

has been proven to be very useful in detecting different kinds of internal deposit.