Corrosion Monitoring

1. Maintenance engineering
Module III
PREPARED BY
DHANESH S
ASSISTANT PROFESSOR
AHALIA SCHOOL OF ENGINEERING AND TECHNOLOGY

2. CORROSION MONITORING
Principle of corrosion monitoring equipment is based on corrosion or chemical
wear of the material
Use of such techniques for condition monitoring of machines/components is
very limited and selective
Some of these may not give accurate machine deterioration rate
Corrosion is usually a greater problem for structural components than for
rotating machines and a number of techniques are commonly used to determine
its extent
Common corrosion monitoring techniques are-Weight loss method, electrical
resistance method, linear polarization method, corrosion potential measurement,
ultrasonic testing, sentinel hole method

3. CORROSION MONITORING
Weight Loss Method:
Commonly used for general and localized corrosion
Weight loss of a material of known surface area is determined in specific environment
for a known time period from which corrosion rate can be calculated
Electrical Resistance Method:
Probe which consists of thin wire inserted into process fluid
As wire corrodes, its cross-sectional area reduces and electrical resistance increases
Can be measured with a suitable bridge circuit to give a continuous signal related to
rate of corrosion

4. CORROSION MONITORING
Linear Polarization Method:
Electrochemical reaction and slope of the potential/applied current curve is inversely
proportional to reaction rate on electrode surface
Related to rate of corrosion
Probes are widely used to facilitate these measurements
Used for finding out rate of corrosion in electrically conductive corrosion fluid
Corrosion Potential Measurement:
Electrical potential between plant being maintained and a reference electrode can be
related to rate of corrosion
Useful where the material shows both active and passive corrosion behaviour in
process and can be used to indicate the development of active corrosion

5. CORROSION MONITORING
Ultrasonic Testing:
Ultrasonic pulse is sent from one side of component and time is measured to reflect
from other side and return
Indicates the thickness of material and it is a valuable method for monitoring pipe
wall thickness and pressure vessel wall thickness
Sentinel Hole Method:
Small holes are drilled to specific depths in a pipe wall or similar components
When corrosion reaches the bottom of the hole, wall will leak
Single or multiple holes of varying depths can be used to access corrosion
Holes are often plugged, once the leakage has been detected so that plant may run
until a suitable outage can be arranged

6. CRACK MONITORING
Mostly used for quality assurance and metallographic analysis to assess the
quality of metals and quality of procedures during making, shaping and treating
of metals in industries
Also used in condition monitoring of many metallic components
Crack monitoring program does not measure total crack depth and width but
change in crack width
This change in crack width is called crack displacement

7. CRACK MONITORING
Crack displacement measured by sensors may be driven by any combination of
factors as follows
Differential thermal expansion
Structural and machine overloading
Chemical changes in various components of machine
Shrinkage and twisting of different components temperature and humidity changes etc
Fatigue and aging of components etc
Non-destructive testing plays an important role in crack monitoring
NDT is defined as a method of inspecting an object without impairing its future
usefulness

8. CRACK MONITORING
A simple technique such as a visual inspection is a form of non-destructive testing
Major methods include:
Penetrant testing
Magnetic particle testing
Ultrasonic testing
Radiography testing
Some new techniques are:
Eddy current testing
Acoustic emission methods
Thermography
Holography
Leak testing