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1. Nondestructive testing (NDT) - the process of a flux leakage from a discontinuity, the small
inspecting, testing, or evaluating materials, ferromagnetic particles are pulled in,
components or assemblies for discontinuities, or attracted by the flux leakage. In short, ung
differences in characteristics without destroying the naattract na fm sa crack.
serviceability of the part or system. *noninvasive
techniques, di nasisira ang specimen 2. Advantages:
 It is quick and relatively uncomplicated
2. History:  It gives immediate indications of defects
 It shows surface and near surface defects, and
Legend has it that the ancient Greek King Hiero II these are the most serious ones as they
(also known as the Greek Sicilian Tyrant of concentrate stresses
Syracuse) tasked Archimedes with discovering  The method can be adapted for site or
whether his crown was pure gold or if the workshop use
blacksmith cheated him and mixing the gold with  It is inexpensive compared to radiography
another substance. This led to Archimedes’ “Eureka”  Large or small objects can be examined
moment by using the principle of displacement to  Elaborate pre-cleaning is not necessary
prove that the crown wasn’t pure gold.
However, Formal NDE dates back to early railroad Disadvantages:
days when a mixture of oil and talc were used to
 It is restricted to ferromagnetic materials -
detect cracking in axles and wheels.
usually iron and steel, and cannot be used on
austenitic stainless steel
3. Advantages of NDT:  It is messy
(1) Material is preserved;  Most methods need a supply of electricity
 It is sometimes unclear whether the magnetic
(2) Less costly in Money and Time
field is sufficiently strong to give good
Disadvantages of NDT:  The method cannot be used if a thick paint
(1) Requires a trained personnel coating is present
 Spurious, or non-relevant indications, are
probable, and thus interpretation is a skilled
Factors to consider if there is inaccuracy of task
results:  Some of the paints and particle suspension
(1) Fault in the equipment used fluids can give a fume or fire problem,
particularly in a confined space
(2) Misjudgment of results
(3) Misapplication of Methods

MAGNETIC PARTICLE TESTING Penetrant Testing - there are three types but the
1. MAGNETIC PARTICLE TESTING - A common ones are those using either visible or
relatively simple non-destructive testing (NDT) fluorescent dyes.
technique that can be used in the detection of surface - there are also solvent cleanable and water
and slight sub-surface discontinuities in magnetic washable tests.
 Cracks (which intersect the magnetic field
lines at 90o and can be as low as 45o can still Procedures for Penetrant Testing:
give a response) can be highlighted with the
application of detection media. 1. Clean (5 to 10 minutes)
 Detection Media – is applied to highlight the 2. Spray penetrant (5 to 25 minutes)
crack. Can be liquid based (ink), or dry 3. Dwell
powder. Has ferromagnetic (fm) particles 4. Carefully clean off excess penetrant (5 to 10
 Cracks act as barrier, flux jumps over the minutes)
crack. 5. Spray developer (5 to 15 minutes)
6. Wait and watch
 Flux Leakage – Site where flux leaves the
metal surface
 Indication – Form when the detection
medium is applied to a surface that contains
Applications: Phased Array Ultrasonic Testing or Phased
Array UT - This is essentially the same as the
1. Parts with cracks open to the atmosphere that can
ultrasound tests used on humans and animals.
be cleaned well
- There are multiple crystals in the transducer and
2. For mechanical components - structural
the computer software analyzes them.
components or aluminum, stainless, or steel, etc.
Common Applications:
3. Relatively inexpensive initial cost for materials
1. Complex and Internal Welds
2. Shafts
Penetrant Testing Problem Areas:
3. Bolts
1. Requires careful and thorough precleaning before
penetrant application. 4. Internal areas that are hard to inspect with other
methods but have a known crack geometry.
2. Grinding and other mechanical power cleaning
can hide indications.
3. Time consuming, manpower intensive and Ultrasonic Testing Problem Areas:
greatest opportunity for human error.
1. Very operator intensive
4. It is probably the most frequently misused NDT
2. Requires good knowledge of the part geometry
technique. Frequently done by inexperienced
personnel leading to gross procedure errors, 3. Insensitive to flaws that are basically parallel to
misinterpretation, and wasted money. the sound path

Ultrasonic Testing
Common Applications:
1. Thickness testing of tanks, vessels, pipings, etc.
2. Quality inspection of components such as
forgings, castings, welds, etc., looking for internal
3. Flaw testing of materials looking for process
induced cracking (bolts, shafts,etc.)

Audible Sound - 20 to 20,000 cps or Hz

Normal Ultrasonic Range - 100,000 to 25,000,000
Hz or 0.1 to 25 MHz
Wavelength - Velocity/Frequency
Infrasound - below 20 Hz
Ultrasound - over 20,000 Hz

Black box - generates a signal and sends it to the

transducer, then reads the signals returning from
changes in “Acoustic Impedance”.

Newer Ultrasonic Testing Method


Corrosion - the deterioration of a metal as a result of chemical reactions between it and the surrounding

All metals can corrode. Some, like pure iron, corrode quickly. Stainless steel, however, which
combines iron and other alloys, is slower to corrode and is therefore used more frequently. All small group
of metals, called the Noble Metals, are much less reactive than others. As a result, they corrode rarely.
They are, in fact, the only metals that can be found in nature in their pure form. The Noble Metals, not
surprisingly, are often very valuable. They include copper, palladium, silver, platinum, and gold.

 Factors of corrosion
o nature of the metal
o environment
o concentration of electrolyte
o temperature
o electrode potential
o hydrogen over voltage

 Types of Corrosion
o Uniform corrosion - this type of corrosion develops as pits of very small diameter, in the order of a
micrometer, and results in a uniform and continuous decrease in thickness over the entire surface area
of the metal. The rate of uniform corrosion can be easily determined by measuring the mass loss, or
the quantity of released hydrogen

o Pitting corrosion - this localized form of corrosion is characterized by the formation of irregularly
shaped cavities on the surface of the metal. Their diameter and depth depend on several parameters
related to the metal, the medium and service conditions. — Unlike uniform corrosion, the intensity
and rate of pitting corrosion can be assessed neither by determining the mass loss nor by measuring
released hydrogen

o Transgranular - Within the metal, at the level of the grain, corrosion may propagate in two different
ways: (i) It spreads in all directions, corrosion indifferently affects all the metallurgical constituents;
there is no selective corrosion. This is called transgranular or transcrystalline corrosion because it
propagates within the grains. (ii) It follows preferential paths: corrosion propagates at grain
boundaries. Unlike transgranular corrosion, these forms of intercrystalline corrosion consumes only a
very small amount of metal, which is why mass loss is not a significant parameter for assessment of
this type of corrosion.

o Exfoliation corrosion - a type of selective corrosion that propagates along a large number of planes
running parallel to the direction of rolling or extrusion. Between these planes are very thin sheets of
sound metal that are not attacked, but gradually pushed away by the swelling of corrosion products,
peeling off like pages in a book; hence the term exfoliation corrosion. The metal will swell, which
results in the spectacular aspect of this form of corrosion

o Crevice corrosion - a localized corrosion in recesses: overlapping zones for riveting, bolting or
welding, zones under joints and under various deposits. These zones also called crevices, are very tiny
and difficult to access for the aqueous liquid that is covering the rest of the readily accessible surfaces.
This type of corrosion is also known as deposit attack.

o Galvanic - when two dissimilar metals are in direct contact in a conducting liquid, experience shows
that one of the two may corrode. This is called galvanic corrosion. The other metal will not corrode;
it may even be protected in this way. — This corrosion is different in its kind and intensity from the
one that would occur if they were placed separately in the same liquid.
o Corrosion by erosion - this type of corrosion is related to the flow speed of the fluid. It leads to local
thinning of the metal, which results in scratches, gullies, and undulations, which are always oriented
in the same direction, namely the flow direction.

Prevention of Corrosion
o Environmental modification. Since, corrosion is caused by chemical interactions between metal and
gasses in the surrounding environment. By removing the metal from, or changing, the type of
environment, metal deterioration can be immediately reduced. This may be as simple as limiting
contact with rain or seawater by storing metal materials indoors or could be in the form of direct
manipulation of the environmental affecting the metal.

o Metal selection and surface conditions, no metal is immune to corrosion in all environments, but
through monitoring and understanding the environmental conditions that are the cause of corrosion,
changes to the type of metal being used can also lead to significant reductions in corrosion.Metal
corrosion resistance data can be used in combination with information on the environmental conditions
to make decisions regarding the suitability of each metal. The development of new alloys, designed
to protect against corrosion in specific environments, is constantly under production. Hastelloy®
nickel alloys, Nirosta® steels, and Timetal® titanium alloys are all examples of alloys designed for
corrosion prevention. Proper monitoring and the elimination of unnecessarily vulnerable surface
conditions, along with taking steps to ensure that systems are designed to avoid reactive metal
combinations and that corrosive agents are not used in the cleaning or maintenance of metal parts are
all also part of effective corrosion reduction program.

o Cathodic protection works by converting unwanted anodic (active) sites on a metal's surface to
cathodic (passive) sites through the application of an opposing current. This opposing current supplies
free electrons and force local anodes to be polarized to the potential of the local cathodes. Cathodic
protection can take two forms. The first is the introduction of galvanic anodes. This method, known
as a sacrificial system, uses metal anodes, introduced to the electrolytic environment, to sacrifice
themselves (corrode) in order to protect the cathode. The second method of cathodic protection is
referred to as impressed current protection. This method, which is often used to protect buried
pipelines and ship hulls, requires an alternative source of direct electrical current to be supplied to the

o Corrosion inhibitors are chemicals that react with the metal's surface or the environmental gasses

o Paints and other organic coatings are used to protect metals from the degradative effect of
environmental gasses. Coatings are grouped by the type of polymer employed. Common organic
coatings include:
o Alkyd and epoxy ester coatings that, when air dried, promote cross-link oxidation
o Two-part urethane coatings
o Both acrylic and epoxy polymer radiation curable coatings
o Vinyl, acrylic or styrene polymer combination latex coatings
o Water-soluble coatings
o High-solid coatings
o Powder coatings

o Plating - Metallic coatings, or plating, can be applied to inhibit corrosion as well as provide aesthetic,
decorative finishes. There are four common types of metallic coatings, the electroplating, mechanical
plating, electroless and hot dipping.
Aqueous corrosion is electrochemical in nature. This means that the corrosion reaction involves electrons. All
aqueous corrosion can be divided into ANODIC and CATHODIC reactions.
An Anodic Reaction is an oxidation process through which the valence of a metal increases from zero to a more
positive value. Aqueous corrosion involves the loss of metal to the aqueous phase. When this occurs, the metal
releases a number of electrons into the metal/alloy. While the Cathodic reaction is a reduction process during
which the valence of the species is reduced. The electrons produced during the anodic corrosion reaction must be
consumed at the same rate by a corresponding cathodic reaction.

 Corrosion rate determination

A weighed sample coupon of metal introduced into the corrosion process and after desired exposure
period removed, cleaned of all corrosion products and reweighed. Weight loss can be converted to average
corrosion rate (mpy) using Faraday’s law. There are ASTM standards G1, G4 and G31 for preparing, cleaning
and evaluating corrosion test specimens, conducting corrosion coupon tests in plant equipment and laboratory
immersion corrosion testing. Using corrosion coupons for weight loss (corrosion rate) measurements has
advantages such as cheap and simple, permits analysis of corrosion products and can easily be done in a
laboratory or on a service equipment. However, it requires long term exposures to be more accurate as short-
term tests can yield misleading information. Different shapes of corrosion coupons, such as flat, ring type or
cylindrical can be used. Coupons can be placed in industrial equipment using holders (electrically isolated).
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