Corrosion monitoring and preventive methods.

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CORROSION MONITORING AND
PREVENTION METHODS
INTRODUCTION
Corrosion monitoring and prevention methods involve the use of
techniques which measure corrosivity of assets in a particular
environment, so that the harm which could occur to the
machinery/plant could be prevented or delayed.
CORROSION MONITORING METHODS
Corrosion Monitoring is the regular measurement of corrosivity
or rate of corrosion. It helps to diagnose a particular corrosion
problem and provide information related to maintenance
required by plant/system.
The various methods include:
1. Corrosion Coupons (Weight Loss)
It involves exposing of specimen alloy (the coupon) to a
process environment for a given duration (usually 90 days)
and then analysing the weight loss which is indicative of the
rate of corrosion.
2. Electrical Resistance (ER) Monitoring
ER technique measures the change in Ohmic resistance of
the corroding metal element using ER probes, in the
corroding environment. Due to the action of corrosion, the
resistance of metal increases (because surface area

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decreases) as a function of time, giving the rate of
corrosion.
3. Linear Polarisation Resistance (LPR) Monitoring
This technique is based on electrochemical theory. Here a
small voltage (polarisation potential) is applied to an
electrode in a clean aqueous electrolytic environment. The
measurement of current (the current needed to maintain a
specific voltage change) gives instantaneous rate of
corrosion.
4. Galvanic Monitoring
In this method, two electrodes of dissimilar metals are
exposed to the process fluid. The current generated due to
the potential difference relates to the rate of corrosion.
5. Specialised Monitoring
Examples are:
a) Biological Monitoring: By identifying the presence of
Sulphate Reducing Bacteria (SRBs).
b) Hydrogen Penetration Monitoring: H2 is produced as
a by-product during corrosion reaction which is
absorbed by steel. This can be analysed and used as an
indicator of corrosion rate.

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CORROSION PREVENTION METHODS
The main objective of corrosion prevention measures is to
control the process to acceptable limits. The common methods
employed are:
1. Proper Material Selection, Design and Construction
 The surface of metal or alloy used for construction
should be smooth and homogenous.
 If two metals are to be in contact, they should be so
selected that their oxidation potentials are very near.
 If contact of dissimilar metals with larger difference in
oxidation potential is unavoidable, then they should be
insulated.
 Shapes like sharp corners which favour the formation
of stagnant areas and accumulation of solids should be
avoided.
2. Use of Corrosion Resistant Alloys
Alloys like stainless steel, monel metal, duriron should be
used instead of pure metals.
3. Use of Protective Coatings
The type of protective coating depends on the corrosivity of
atmosphere, nature of structure (thickness), its life
expectancy and utility.
The various types of protective coatings are:
a) Metallic Coatings: Such coatings are usually applied
on iron and steel. The metals like Zn, Sn, Ni, Cu, Cr, Al

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and Pb are used to form coating by methods such as
electroplating, hot dipping, vapourising, etc.
b) Inorganic Non-Metallic Coatings: These are of
following types-
i. Chemical Dip Coating or Surface Conversion:
The metal is immersed in a solution of suitable
chemical which reacts with the surface to produce
adherent coating. Examples are chromate,
phosphate or oxide coatings on surface of metal or
alloy.
ii. Anodized Oxide Coating: Protective oxide films
are produced on Aluminium by making Aluminium
as anode in electrolytic bath containing oxidizing
agents. This is then immersed in boiling water to
decrease porosity and increase resistance to
corrosion.
iii. Vitreous Enamel Coating: These are usually
applied to cleaned steel and cast iron equipment in
the form of a powder mix or frit, which is later
fused onto the metal surface by heating.
c) Organic Chemical Coatings: These include paints,
varnishes, enamels, etc which are effective inert
barriers.
4. Cathodic or Sacrificial Anodic Protection
The metal to be protected is made cathode and another more
corrodible metal is made the anode which is replaced again
and again to protect the metal.

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Example: Use of an alloy of Zn and Mg as a sacrificial
anode for the protection of ship hulls.
5. Elimination of Corrosive Agents from Environment
This can be done in the following ways:
a) Deaeration: Removal of oxygen from aqueous
environment or deactivation by adding chemical which
combine with oxygen like sodium sulphite.
b) Dehumification: Reducing the amount of moisture in
the immediate environment using alumina or silica gel.
c) Alkaline Neutralisation: Neutralising the acidic
character of corrosive environment using alkaline
neutralizers like NH3, NaOH, lime, etc.
References:-
1. Advanced Physical Chemistry; Gurtu, J.N. and Gurtu, A.
2. Applied Chemistry for Home Science and Allied Sciences;
Jacob, Thankamma.
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