CORROSION INSPECTION
TECHNIQUES
CORROSION INSPECTION
TECHNIQUES
• The field of corrosion measurement, control, and prevention
covers a very broad spectrum of technical activities. Within the
sphere of corrosion control and prevention, there are technical
options such as cathodic and anodic protection, materials
selection, chemical dosing and the application of internal and
external coatings.
• Corrosion measurement employs a variety of techniques to
determine how corrosive the environment is and at what rate
metal loss is being experienced. Corrosion measurement is the
quantitative method by which the effectiveness of corrosion
control and prevention techniques can be evaluated and
provides the feedback to enable corrosion control and
prevention methods to be optimized.
Variety of corrosion inspection techniques
The Need for Corrosion
Inspection and Monitoring
• The rate of corrosion dictates how long any process
equipment can be usefully and safely operated. The
measurement of corrosion and the action to remedy
high corrosion rates permits the most cost effective
plant operation to be achieved while reducing the life-
cycle costs associated with the operation.
Corrosion monitoring techniques
can help in several ways:
1. By providing an early warning that damaging process conditions exist which may
result in a corrosion- induced failure.
2. By studying the correlation of changes in process parameters and their effect on
system corrosivity.
3. By diagnosing a particular corrosion problem, identifying its cause and the rate
controlling parameters, such as pressure, temperature, pH, flow rate, etc.
4. By evaluating the effectiveness of a corrosion control/prevention technique such as
chemical inhibition and the determination of optimal applications.
5. By providing management information relating to the maintenance requirements
and ongoing condition of plant.
Consideration in choosing corrosion
monitoring techniques :
• The techniques are easy to understand and implement.
• Equipment reliability has been demonstrated in the field
environment over many years of operational application.
• Results are easy to interpret.
• Measuring equipment can be made intrinsically safe for
hazardous area operation.
• Users have experienced significant economic benefit through
reduced plant down time and plant life extension.
Corrosion monitoring is typically used in
the following situations:
• Where risks are high - high pressure, high temperature, flammable,
explosive, toxic processes.
• Where process upsets can cause high corrosivity.
• Where changes in operating conditions can cause significant changes in
corrosion rate.
• Where corrosion inhibitors are in use.
• In batch processes, where corrosive constituents are concentrated due to
repeated cycling.
• Where process feedstock is changed.
• Where plant output or operating parameters are changed from design
specifications.
• In the evaluation of corrosion behavior of various alloys.
• Where induced potential shifts are used to protect systems and/or
structures.
• Where product contamination due to corrosion is a vital concern.
Some examples of industries and specific areas of
interest include, but are not limited to:
Introduction to Nondestructive
Testing
Definition of NDT
The use of noninvasive
techniques to determine
the integrity of a material,
component or structure
or
quantitatively measure
some characteristic of
an object.
i.e. Inspect or measure without doing harm.
Methods of NDT
Visual
What are Some Uses
of NDE Methods?
• Flaw Detection and Evaluation
• Leak Detection
• Location Determination
• Dimensional Measurements Fluorescent penetrant indication
• Structure and Microstructure Characterization
• Estimation of Mechanical and Physical Properties
• Stress (Strain) and Dynamic Response
Measurements
• Material Sorting and Chemical Composition
Determination
When are NDE Methods Used?
There are NDE application at almost any stage
in the production or life cycle of a component.
– To assist in product development
– To screen or sort incoming materials
– To monitor, improve or control
manufacturing processes
– To verify proper processing such as heat
treating
– To verify proper assembly
– To inspect for in-service damage
Six Most Common NDT
Methods
• Visual
• Liquid Penetrant
• Magnetic
• Ultrasonic
• Eddy Current
• X-ray
Visual Inspection
Most basic and common
inspection method.
Tools include
fiberscopes,
borescopes, magnifying
glasses and mirrors.
Portable video inspection
unit with zoom allows
inspection of large tanks
and vessels, railroad tank
cars, sewer lines.
Robotic crawlers permit
observation in hazardous or
tight areas, such as air
ducts, reactors, pipelines.
(0)The method is not used, or it is not applicable.
(1)The method is possible, but not suitable.
(2)The method is suitable, however, there are other methods that are preferable.
(3)The method is suitable.
*Special probes for inspection of small tubes, for example, in heat exchangers
Liquid Penetrant Inspection
• A liquid with high surface wetting characteristics
is applied to the surface of the part and allowed
time to seep into surface breaking defects.
• The excess liquid is removed from the surface
of the part.
• A developer (powder) is applied to pull the
trapped penetrant out the defect and spread it
on the surface where it can be seen.
• Visual inspection is the final step in the
process. The penetrant used is often loaded
with a fluorescent dye and the inspection is
done under UV light to increase test
sensitivity.
Magnetic Particle Inspection
The part is magnetized. Finely milled iron particles coated with a
dye pigment are then applied to the specimen. These particles
are attracted to magnetic flux leakage fields and will cluster to
form an indication directly over the discontinuity. This indication
can be visually detected under proper lighting conditions.
Magnetic Particle Crack Indications
Radiography
The radiation used in radiography
testing is a higher energy (shorter
High Electrical Potential
wavelength) version of the
electromagnetic waves that we
see as visible light. The radiation can Electrons
come from an X-ray generator or a + -
radioactive source.
X-ray Generator
or Radioactive
Source Creates
Radiation
Radiation
Penetrate
the Sample
Exposure Recording Device
Principle of radiography
Film Radiography
The part is placed between the
radiation source and a piece of film.
The part will stop some of the
radiation. Thicker and more dense
area will stop more of the radiation.
The film darkness
(density) will vary with
the amount of radiation
X-ray film reaching the film
through the test object.
= less exposure
= more exposure
Top view of developed film
Radiographic Images
Eddy Current Testing
Coil's
Coil magnetic field
Eddy current's
magnetic field
Eddy
currents
Conductive
material
Eddy Current Testing
Eddy current testing is particularly well suited for detecting surface
cracks but can also be used to make electrical conductivity and
coating thickness measurements. Here a small surface probe is
scanned over the part surface in an attempt to detect a crack.
Ultrasonic Inspection (Pulse-Echo)
High frequency sound waves are introduced into a
material and they are reflected back from surfaces or
flaws.
Reflected sound energy is displayed versus time, and
inspector can visualize a cross section of the specimen
f
showing the depth of features that reflect sound.
initial
pulse
back surface
echo
crack
echo
crack
0 2 4 6 8 10 plate
Oscilloscope, or flaw
detector screen
Ultrasonic Imaging
High resolution images can be produced by plotting
signal strength or time-of-flight using a computer-
controlled scanning system.
Gray scale image produced using Gray scale image produced using the
the sound reflected from the front sound reflected from the back surface
surface of the coin of the coin (inspected from “heads” side)
Common Application of NDT
• Inspection of Raw Products
• Inspection Following Secondary
Processing
• In-Services Damage Inspection
Inspection of Raw Products
• Forgings,
• Castings,
• Extrusions,
• etc.
Inspection Following
Secondary Processing
• Machining
• Welding
• Grinding
• Heat treating
• Plating
• etc.
Inspection For
In-Service Damage
• Cracking
• Corrosion
• Erosion/Wear
• Heat Damage
• etc.
Power Plant Inspection
Periodically, power plants are
shutdown for inspection.
Inspectors feed eddy current
probes into heat exchanger
tubes to check for corrosion
damage.
Pipe with damage Probe
Signals produced
by various
amounts of
corrosion
thinning.
Wire Rope Inspection
Electromagnetic devices
and visual inspections are
used to find broken wires
and other damage to the
wire rope that is used in
chairlifts, cranes and other
lifting devices.
Storage Tank Inspection
Robotic crawlers
use ultrasound to
inspect the walls of
large above ground
tanks for signs of
thinning due to
corrosion.
Cameras on
long
articulating
arms are used
to inspect
underground
storage tanks
for damage.
Aircraft Inspection
• Nondestructive testing is used
extensively during the
manufacturing of aircraft.
• NDT is also used to find cracks
and corrosion damage during
operation of the aircraft.
• A fatigue crack that started at
the site of a lightning strike is
shown below.
Jet Engine Inspection
• Aircraft engines are overhauled
after being in service for a period
of time.
• They are completely disassembled,
cleaned, inspected and then
reassembled.
• Fluorescent penetrant inspection
is used to check many of the parts
for cracking.
Crash of United Flight 232
Sioux City, Iowa, July 19, 1989
A defect that went
undetected in an
engine disk was
responsible for
the crash of
United Flight 232.
Pressure Vessel Inspection
The failure of a pressure vessel
can result in the rapid release of
a large amount of energy. To
protect against this dangerous
event, the tanks are inspected
using radiography and
ultrasonic testing.
Rail Inspection
Special cars are used to
inspect thousands of miles
of rail to find cracks that
could lead to a derailment.
Bridge Inspection
• The US has 578,000
highway bridges.
• Corrosion, cracking and
other damage can all
affect a bridge’s
performance.
• The collapse of the Silver
Bridge in 1967 resulted in
loss of 47 lives.
• Bridges get a visual
inspection about every 2
years.
• Some bridges are fitted
with acoustic emission
sensors that “listen” for
sounds of cracks growing.
Pipeline Inspection
NDT is used to inspect pipelines
to prevent leaks that could
damage the environment. Visual
inspection, radiography and
electromagnetic testing are some
of the NDT methods used.
Remote visual inspection using
a robotic crawler.
Magnetic flux leakage inspection.
This device, known as a pig, is
placed in the pipeline and collects
data on the condition of the pipe as it
is pushed along by whatever is being
transported.
Radiography of weld joints.
Special Measurements
Boeing employees in Philadelphia were given the privilege of
evaluating the Liberty Bell for damage using NDT techniques.
Eddy current methods were used to measure the electrical
conductivity of the Bell's bronze casing at various points to
evaluate its uniformity.
Treatment and Analysis of Inspection Results
1. Description and Evaluation of Observations and Conditions
• Description of the construction/equipment as regards location, shape,
dimensions, relationships between components, age and delivering company.
• Description of the appearance of the attack both before and after surface
cleaning, by means of photographs, sketches, text and quantitative
expressions. Both distribution and intensity (depths) of attacks must be
documented.
• Evaluation of data from physical and chemical detection
• Reports on service conditions such as continuity/intermittence and duration of
service, temperature, pressure, flow conditions and rates, chemical composition
of the corrosive environment, possible mixture of phases, mechanical loads and
effects, as well as the variation of the mentioned physical, chemical and
mechanical conditions. Operation by personnel and possible operational
irregularities should also be included.
• Data of corroded material and possibly other materials in the plant with some
connection to the former. Such data must include composition and possible
welding, cold work, heat treatment, surface treatment, etc.
2. Diagnosis Based on Description of Attack and Service
Conditions – Definition of Corrosion Forms
• Corrosion is often classified by corrosion forms characterized
by the appearance of the attack, possibly combined with
information about important service conditions. The benefit of
such classifications is that a real corrosion attack can usually
be identified as a certain form of corrosion, by means of a
simple visual examination, by the naked eye, by a magnifying
glass or a microscope.
• Because each form of corrosion has its characteristic
conditions and causes that are well known to the corrosion
engineer, it is often possible to make the diagnosis after such
an examination (supported by information on material and
service conditions).
• Using this approach, one can define the forms of
electrochemical corrosion
Corrosion Monitoring
• Corrosion Coupons (Weight Loss)
The Weight Loss technique is the best known and
simplest of all corrosion monitoring techniques. The
method involves exposing a specimen of material
(the coupon) to a process environment for a given
duration, then removing the specimen for analysis.
The basic measurement which is determined from
corrosion coupons is weight loss; the weight loss
taking place over the period of exposure being
expressed as corrosion rate.
• The technique is extremely versatile, since
weight loss coupons can be fabricated from any
commercially available alloy. Also, using
appropriate geometric designs, a wide variety of
corrosion phenomena may be studied which
includes, but is not limited to:
• • Stress-assisted corrosion
• • Bimetallic (galvanic) attack
• • Differential aeration
• • Heat-affected zones
