Cleaning & Repair
November/December 2019
B ULK D ISTRIBUTOR
9
conventional methods such as dye penetrant testing (PT) have been
the only way to assess pits visually and these are completely
dependent on the operator’s trained eye with no recordability of
results.
This technique provides no measurement of the depth of pits
which often have a below surface cavity. This means that the part
has to be inspected every time it undergoes grinding repair, over
and over until the pit is removed, which is very time consuming.
A solution
Rapid motion scanning technology image to examine welds and general corrosion Test scan data overlaid to give a feel of the real-time data acquisition
be tested on completion by the oxilyser 3 passivity tester.
In the case of a clean polished tank surface it is even better to
carry out only chemical passivation. Furthermore, it is also very
important to degrease the surface prior to pickling and/or
passivation, even when grease or other dirt is not observed on the
surface.
Other aspects also need to be checked. What is the right chemical
mixture for pickling and passivation? What concentration of nitric
acid and HF will be used by the repair depot? What kind of materials
will be used for grinding and polishing? What is an acceptable
passivity value for the stainless steel, especially tank containers? For
example, will a reading of 65 on the oxilyser passivity test be high
enough or should it over 80 to have a much stronger resistance
again corrosion attacks? radiographic (x-ray), ultrasonic, magnetic particle, liquid penetrant
and visual testing.
They are split into two categories - conventional techniques and
advanced techniques.
In recent years advanced techniques have taken NDT to new
heights thereby increasing the probability of detection, accuracy,
recordability and traceability of testing, Brookes Bell says.
The research partners, and their peers, have devised many ways of
assessing the integrity of tanks, finding means for detecting general
corrosion, pitting corrosion, stress corrosion and fatigue cracking
(see the rapid motion scanning technology image).
Pitting and stainless steel
The environment within the pit is different from the environment
outside. Aggressive ions accumulate within the pit, combining with
hydrogen forming acidic species. This accumulation of acidity within
the pit accelerates the corrosion reaction.
Pitting is further compounded by several factors. The area of the
pit is active whereas the surround steel is passive creating an anode–
cathode galvanic cell. This drives the corrosion in the same way as
an electrochemical cell.
Also, the geometry of the pit restricts the flow of oxygen within
the body of the pit preventing self-passivation.
Visual inspection of the pit will not reveal what is happening
below surface and it is very difficult to know how the pit has
penetrated and can result in a dangerous situation.
Owing to the localised nature of pitting corrosion rates in terms of
their depth of penetration are typically several orders of magnitude
greater than those experienced by general corrosion.
Following the onset of pitting, if the environment/cargo is
sufficiently aggressive then penetration of the full thickness can
happen quickly resulting in a breach.
Pits cannot self-repair in the same way as the surface can and,
once initiated, pitting will become progressively worse.
Pitting also presents a problem with cleanliness and, owing to all
these factors, pitting must be removed. This is achieved by
mechanical means.
The problem
Although good for general corrosion all current NDT scanning
methods cannot accurately detect and size micro-pits in stainless
steel. Also, magnetic wheel tracks render the scanners useless on
non-magnetic materials such as stainless.
For many years, the most difficult defects to detect have been
small pits that accelerate corrosion and wastage of assets, these are
some of the most incipient and dangerous defects. Until now,
Quala expands in Gulf
E
arlier this year, US bulk container cleaning specialist Quala
acquired Birmingham Tank Wash in Birmingham, Alabama,
expanding its presence in the US Gulf region.
“We look forward to bringing the Birmingham Tank Wash into the
Quala family,” said Jeff Noble, senior vice-president of operations at
Quala. “The acquisition allows us to expand our network in the Gulf
region with Quala’s best-in-class service”.
The new facility is located at 2420 18th Ave N, Bessemer, Alabama
35020, a few minutes away from Interstate 20 West / Interstate 59
South.
Quala also appointed Angela Samuel to the role of vice-president of
sales for IBC services.
Prior to joining Quala, she worked at Union Pacific Railroad (UP) for 16
years, having started her career with UP as a sales and marketing intern.
Commenting on the appointment, the company said: “Since joining
Quala in August 2018, Angela’s work performance and attitude have
been exemplary. She excels in leading sales organisations, navigating
through highly complex accounts, and generating profitable revenue.
She’s a perfect fit for this role. We are all very excited to see Angela
taking on this new position in what is now one of our largest lines of
business.
“At Quala, our employees are our greatest assets, by supplying great
support to our employees we are able to provide an excellent
experience to our customers.”
cleaning
your
wörld
Repair
Conventional methods of inspection do not reveal how deep pitting
extends below the surface. Visual methods will not reveal the true
extent of pitting as pits in the early stages often fall below the level
of visual acuity – to locate all the pits techniques such as dye
penetrant inspection can be used, but again no information is
revealed regarding the depth.
To date, finding information regarding the true extent of pitting to
stainless steel structures (including ISO tank containers) has not
been possible. This results in many unknowns when it comes to
repairs and operators can find themselves in the position of
embarking on a repair which can turn out to be very labour
intensive or even a waste of time if the tank is unsalvageable. It is
this dilemma which has been the focus of a collaborative research
project between Istech, Brookes Bell’s materials team, and Eddyfi (a
leading manufacturer of NDT eddy current technologies).
Brookes Bell and Eddyfi engaged in long research and development
and discovered a technological breakthrough in detecting and sizing
pits and stress corrosion cracking (SCC).
This is electromagnetic testing with eddy currents (ECT). When a
wire is shaped into a coil, the interaction of each turn produces a
magnetic field around the coil. This magnetic field oscillates at the
same frequency as the electric current injected into the coil.
When the coil is placed over a conductive material, opposed
alternating currents are generated - eddy currents (ECs). ECs also
oscillate at the same frequency as the current in the coil, and this
principle can be used to detect defects.
If we supercharge this coil principle, we can create eddy current
array sensor probes which contain a large number of elements
(coils) arranged and activated in specific patterns, they explained.
This is called ‘multiplexing’ and it is with this process that the
partners were able to create the new scanning technology which
enables users to: detect very small pits and cracks on large surfaces
at high speed; perform highly accurate depth analysis; improve flaw
detection and sizing with C-scan (birdseye view) imaging; integrated
encoder for accurate defect positioning; be adaptable to any surface
contour; and vastly reduce inspection time.
Once the sensor is calibrated on the material to be tested the
signal is normalised. The angle of signal rotates clockwise with
depth (phase–shift). Signals are then obtained from various
anomalies: lift-off (any air gap between the sensor and test
material); shallow-surface pit; deep-surface crack; subsurface defect;
and deeper subsurface defect.
The novel eddy current array (ECA) sensor can contain up to 128
coils that when energised in various sequences, allow the inspector
to detect the smallest of defects and, in many different orientations,
pits and cracks are all detected and sized accurately.
Tankcleaning manufacturer:
Silo / bulk cleaning
Chemicals cleaning
Food cleaning
Railway carriage cleaning
Container cleaning
Container heating
IBC cleaning
CIP / latex cleaning
Tank Cleaning
Systems
Food / Non-Food
Cleaning Systems
NDT what is it?
NDT is a wide group of analysis techniques used in science and
technology industries to evaluate the properties of a material,
component or system without causing damage.
The main purpose of this process is to look for anomalies or
defects that may compromise the integrity and reliability of the
product being tested.
NDT guarantees a uniform quality level across the transport
industry and ensures that any remediation and repairs are carried
out cost effectively and efficiently.
The six most frequently used methods are eddy current,
Parts & Service
The reference in new tankcleaning projects and renovations!
CTW
Everdongenlaan 9/19
2300 Turnhout
Belgium
T. +32 (0) 14 27 00 80
[email protected]
www.ctwcleaning.com
www.groninger.eu
Overschieseweg 111
3044 EH Rotterdam
The Netherlands
Telephone +31 10 437 10 22