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IS 12554-1 (1988): Non-destructive coating thickness

testing instruments, Part 1: Eddy current instruments [ETD
18: Industrial Process Measurement and Control]

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IS : 12554 ( Part 1 ) - 1988

( Reaffirmed 2004 )

Indian Standard

SPECIFICATIONFOR
NON-DESTRUCTIVECOATINGTHICKNBSS
TESTINGINSTRUMENTS
PART

1 EDDY CURRENT

UDC

INSTRUMENTS

62017914208 : 669058

0 CopyGght 1989

BUREAU
MANAK

Gr 3

OF
BHAVAN,

INDIAN

STANDARDS

9 BAHADUR
SHAH
NEW DELHI 110002

ZAFAR

MARC

July

1989

IS : 12554 ( Part 1 ) - 1988

Indian Standard
SPECIFICATION FOR
NON-DESTR-UCTIVE COATING THICKNESS
TESTING INSTRUMENTS
1

PART

EDDY

CURRENT

INSTRUMENTS

0. FOREWORD
0.1 This Indian Standard
( Part 1 ) was adopted
by the Bureau of Indian
Standards
on 31 May
1988, after the draft finalized
by the Industrial
Process
Measurement
and Control
Sectional
Committee
had been approved
by the Elcctrotechnical Division Council.
0.2 Thickness
as destructive

measuring
techniques
and non-destructive.

particle reflected by the sample

related to the thickness.
Eddy
changes
inducing

The most direct

method
is to take a cross
section of the sample and measurement
of the
plating
under
a microscope.
In practice,
the
procedure
can be quite time-consuming
and
tedious.

2. SCOPE
1.1 This standard
specifies the
eddy current
instruments
for
nleasurement
of thickness
of a
or less conductive
coating
on
base metal.
NOW -The
cc)nductivity ratio
base metal shall be greater than 4.

requirements
of
non-destructive
non-conductive
a non-magnetic
of

coating

and

2. TYPE
2.1 This tyx
non-destructive
ncss.
Two

of portable instrument
is used for
measurement
of coating
thickmodels
are
available,
each
set

current
instruments
measures
the
in apparent
impedance
of the
coil
the eddy currents into the base metal.

0.5 This standard ( Part 1 ) specifies the requirements of instrument

which can measure metallic
or non-conductive
coatings
on metallic
bases
and metallic coatings
on conductive
bases eddy
current
provided.
The conductivity
ratio
of
coating material and base metal ( and vice-vcrsu )
is greater than 4.

Another a destructive
method
is the couloThe coating is dissolved
metric thickness gauge.
anodically
( that is, using a process,
the reverse
of plating ) and
the total electrical
charge
required to dissolve a known area is determined.
Thickness of the deposit is related to the charge.

0.3 Magnetic gauges can be used when the base

is magnetic
and the coating
is non-magnetic.
The gauge measures the reduction
in magnetic
attraction
as the coating
thickness
increase.
Magnetic gauges using permanent
magnets
and
Nuclear
also
electromagnets
are
available.

and

0.4 All the above methods except cross-scctioning for microscopic

measurement
require
an
empirical calibration
of the instrument.
In most
cases, the signal obtained is not a linear function
of the deposited thickness and this can add to
the measurement
inaccuracies.
The inaccuracies
are of the order of f5 percent.

are classified

methods
of
thickness
Non-destructive
measurement
are based on the
principles
of
eddy currents
or nuclear
particle
magnetism,
scattering,
The methods
are quick and nondestructive
to facilitate routine
testing
in mass
production.

is collected

0.6 Part 2 of this standard

specifies
the
requirements
non-destructive
coating
instruments.

( under preparutiotz )
of magnetic
type
thickness
testing

0.7 For the purpose

of deciding
whether
a
particular
requirement
of this standard
is complied wilh, the final value, observed
or calculated, expressing
the results
of a test, shall bc
rounded
off in accordance
with IS : 2- 1960*.
The number of signilicant
places retained in the
rounded off value should be the same as that of
the specified value in this standard.
*Rulei for rounding

off numerical values ( revised ),

specifically for combination

of coating/coating
and base material; direct type for paint, varnish,
lacquer, teflon,
lead on aluminiurn
or copper
and indirect type for nickel, zinc, tin, c;tdnlitlu1,
chromium
on steel.
2.2 This method
is particularly
i~sclul 101
measuring
thu thickness of an anodic Coating on
aluminium
alloys. Chemical conversion
c~~~tings
arc too thin to be mcasurcd by this method.
3. OPERATING
PROCEDURE
3.1 The conttng
clcctromagnctic
1

PRINCIPLE

AND

thickness is dctcrmincd
with :in
instrument
that measures
tlm

IS : 12554 ( Part 1 ) - 1988

not supplied by the manufacturer of the mesuring instrument

since it depends on both the
measuring frequency of the probe system and the
electrical conductivity of the base metal.
4.3.1 General Rule - For a given measuring
frequency,
the higher the conductivity of the
base metal, the smaller is its critical thickness.
For a given base metal, the higher the measuring
frequency, the smaller is the critical thickness of
the base metal.
4.4 Edge Effect - The method is sensitive to
abrupt changes in the surface contour of the
test specimen.
Therefore, measurements
made
too near an edge or inside corner will not be
valid unless the instrument
is specifically calibrated for such a measurement.

changes in apparent
impedance of the coil
inducing the eddy currents into the base metal.
The design of the instrument shall be such that
variations in apparent impedance, produced by
variations in coil to base metal spacing, can be
calibrated to indicate the thickness of coatings.
3.2 Procedure
3.2.1 The instrument
shall be operated
in
accordance with the instructions
of the manufacturer, and consideration to the factors given
m 4.
3.2.2 Check the calibration of the instrument
at the test site each time the instrument
is put
into service and at frequent intervals during use
to assure proper performance.
3.2.3 Observe the following precautions:
thickness Check
3.2.3.1 Basis
metal
whether the basis metal thickness exceeds the
critical thickness. If not, either use the back-up
method in 5.4 or make sure that the calibration
has been made on a standard having the same
thickness and electrical properties as the test
specimen.
3.2.3.2 Edge effects - Do not make readings
close to an edge, hole, inside corner, etc, of a
specimen unless the validity of the calibration
for such a measurement has been demonstrated.
3.2.3.3 Curvature - Do not make readings
on curved surface of a specimen unless the
validity of the calibration for such a measurement has been demonstrated.
3.2.3.4 Number of readings - Because of
normal instrument variability, it is necessary to
make several readings at each position. Local
variations in coating thickness may also require
that a number of measurements be made in any
given area ; this applies particularly to a rough
surface ( see 4.9 >.
3.2.3.5 Surface cleanliness - Before making
measurements, clean any foreign matter such as
dirt, grease, and corrosion products from the
surface without removing any coating material.
4. FACTORS

AFFECTING THE
MEASURING
ACCURACY

4.1 Inherent

in the method is a measuring

uncertainty that for thin coatings, is constant
and independent of the coating thickness, and
for a single measurement, not less than 05 pm;
for
thicknesses
greater
than about 25pm,
this uncertainty is proportional
to the coating
thickness.
4.2 Electrical Properties of the Basis Metal Eddy current measurement are allected by the
electrical conductivity of the basis metal, which
itself is often affected by heat treatment.
- For each measurcmcnt, there is a critical thickness of the base
metal above which the measurements will not be
affected by an increase in that thickness. Its
value should be determined experimentally,
if
4.3 Base Metal Thickness

4.5 Curvature - Measurements are affected by

the curvature of the test specimen. The influence
of curvature varies considerably with the make
and the type of the instrument
but always
becomes more pronounced
as the radius of
curvature decreases.
4.6 Surface Roughness
4.6.1 Measurements
are influenced
by the
surface topography
of the substrate and the
coating, and a rough surface will give individual
instrument readings that will vary from point to
point. In this case, it is necessary to make
many
readings at different
positions
to
obtain a mean value that is representative of the
average coating thickness.
4.6.2 If the base metal is rough, it may also
be necessary to check the zero of the instrument
at several positions on a sample of the uncoated
rough substrate. If the roughness of the substrate surface is small relative to the coating
,thickness, its effect will probably be negligible.
4.7 Foreign Particles - The probes of eddy
current instruments must make physical contact
and shall be free of foreign material that
prevents intimate contact
between the probe
and the coating surface.
4.8 Pressure - The pressure with which the
probe is applied to the test specimen affects the
instrument readings and should, therefore, be
kept constant.
4.9 Number of Readings - The precision of the
measurements can be improved by increasing the
number of readings in accordance with statistical
principles.
5. CALIBRATION

OF INSTRUMENTS

5.1 Before

USC, each instrument

shall be calibrated in accordance with the instructions
of the
manufacturer,
employing
suitable
thickness
standards.
During
USC, the calibration shall
be checked
at frcqucnt intervals, at least
one an hour.
Attention shall be given to the
procedure described in 3 and to the factors
listed in 4.

5.2 Calibration
of thickness
standards
( of
known thickness available either as foils or as
2

IS : 12554 ( Part 1 ) - 1988

coated
specimens ) used
instrument,
shall be tested
in Appendix
A.
5.2.1

for calibrating
by the method

the
given

6.2 Reading -- Reading

direct reading scales.

shall

be

in

microns

6.3 Power Requirements - Power requirements

shall be 230 V ac, 50 Hz. single phase.
Power
needed shall not exceed 10 W.

Calibration Foils

5.2.1.1 The calibration

foils used for
the
calibration
of eddy current
instruments
are
They are advantagegenerally made of plastic.
ous for calibration
on curved
surfaces
and are
more readily available than coated standards.

6.4 Operating
Temperature - Operating
temperature shall be 15 to 3OC.
Calibration
once
set ~111 hold for f5C
variations
in ambient
temperature.

5.2.1.2 To prevent measurement

errors, it is
necessary
to ensure
that intimate
contact
is
Resiestablished
between
foil and substrate.
lient foils should be avoided if possible. Calibration foils are subject to indentation
and should,
therefore,
be replaced frequently.

6.5 Dimensions
than 250 mm.

5.2.2 Coated Standards - These

calibration
standards
consist of non-conductive
coatings
of
known, uniform
thickness
permanently
bonded
to the substrate material.

7.1 The instrument

is normally
used for the
preset combination
and rang! specified by the
manufacturer.
However, it IS possible
to use
the instrument
for more precise
mcastlrtnlcnts
by suitably scaling and ranging the instrument.

The length

shall not be

6.6 Weight - Unless otherwise

shall be less than 4 kg.
7. SPECIAL

5.3 The base metal of the calibration

standards
shall have electrical properties
similar
to those
of the base metal of the coated
test specimen.
To confirm their suitability,
a comparison
of the
readings obtained
with the base metal of the
bare standard and that of the test specimen
is
recommended.

specified,

more
weight

APPLICATION

7.2 Range Adjustment

7.2.1 The instrument
is made such that the
range can be adjusted
over a ratio of I : 4 b}
using the range pot. For example, if the instrument of maximum
sensitivity
has a full scale
range of, say, O-100 microns,
then at its least
sensitive
position
it would be
approximately
O-400 microns.

5.4 The base metal thickness

for the test and
the calibration
shall be the same if the critical
thickness, defined in 4.3, is not exceeded.
When
possible, back up the base metal of the standard
or of the test specimen with a sufficient thickness
of similar
material
to make
the
readings
independent
of the base metal thickness.

7.2.2 Non-standard
range is possible by using
suitable thickness
standard
and lower scale, say,
O-100 with appropriate
multiplying
factor.
7.3 A note for nickel
Appendix
B.

A way to determine
if the base metal thickness
to
make
exceeds
the critical
thickness
is
measurements
before
and after
backing
up
the base metal with copper
or aluminium
at
least 3 mm thick.
If there is no difference,
between the readings,
the critical
thickness
is
exceeded.

measurement

is given

in

8. MARKINGS
8.1 The following
marked
on
the
position.

particular
instrument

shall
be clearly
in a measured

8.1.1 Reodiing Range - For

cxamplc,
micrometre
for conductive
coatings
on
conductive
bate and 0250 micrometre
for
conductive
coatings on conductive
base.

5.5 If the test specimen is soft and thin, it is

susceptible to indentation
by the probe.
Because
of this, and despite the use of special probes or
fixtures,
measurements
on such specimens
are
sometimes impossible
to make.
5.6 If the curvature
of the test specimen
to be
measured is such as to preclude calibration
on a
flat surface, the curvature
of the coated standard
or of the substrate on which the calibration
foil
is placed shall be the same as that of the test
specimen.
6. GENERAL

8.1.2

Accuracy

8.1.3

Operating

8.1.4

Power Requirements

8.1.5

Operating

Voltage NII~ Frequency

Temperature

8.2 The instruction

manual
manufacturer
shall have the
lion:

REQUIREMENTS

a) Installation
calibration

6.1 Accuracy - The instrument,

its calibration,
and its operation
shall be such that the coating
thickness
can bc determined
within
10 percent
of the true thickness
or 1 pm, whichcvcr
is
greater.

supplied
following

proccdurc
and
standard
supplictl;

bv the
informndet:liIs

and precnuLions
b) Safety rcyuirenient
taken during mcasurctncnt;
c> Critical

thickness

on a magnetic base metal, its use for coating

below
25 pm is not recommended,
and the magnetic method
shall be used.

8.3 The instrument

Standard
Mal-k.
3

of

to bc

of base mct~tl;

and calibration
d) Operation
proccdurc.
e) Maintenance

this test method

NOTE - Although, theoretically,
can be used for rneasul-ing non-conductive
coatings

O-30
nonnon-

PI-occd~re;

may also bc markcti

and

\vith the

IS : 12554 ( Part 1 ) - 1988

NOTE- The use of the Standard Mark is governed
by the provisions of the Bureau of Indian Standards
Act, 1986 and the Rules and Regulations
made therexnder.
The Standard Mark on producrs covered by
an Indian Standard conveys the assurance
that they
have been produced to comply with the requirements
of that standard under a well defined
system
of
and quality
control
which
is
inspection,
testing
devised and supervised by BIS and operated by the
producer.
Standard
marked
products
are also
continuously
rheckcd by BIS for conformity
to that
standard as a further safeguard. Details of conditions
under which ;: licence for the use of the Standard
Mark may be granted to manufacturers
or producers
may be obtained from the Bureau of Indian Standards.

9. TESTS
9.1 Insulation
Resistance
Test Insulation
rcsistnncc bct\veen tern-tinal and enclosure
shall
bc not less thnn 50 megohm at 500 volts d.c.

9.2 High
Voltage
Test No
break-down,
arcing or sparking shall occur when 2 kV rms is
applied between terminals
and enclosure
for a
period of one minute.
9.3 Accuracy

DIS

APPEN

Test

9.3.1 The melcr shall be calibrated

accorriing
to 5 or manufacturers
instruction
with the help
of thickness
standard.
The different
plating
thickness
shall be measured
and compare each
plating thickness within the testing
by destructive tester.
The dilferencc in reading
shall not
be more -1710 percent
of the reading
measured
by generalized
destructing
thickness
tester or
1 pm, whichever
is grcatcr.
The method
of
checking
thickness
standards
is given
in
Appendix
A.

( Clauses
5.2and 9.3.1 )
A TYPICAL

A-l. Thickness
indicating:

METHOD
FOR PREPARATION
AND
THICKNESS
STANDARDS

standards

will

carry

b) Current
c) Plating

density
time -

and

A-3. Four point measurement

for following
reasons:

A/dm2!

minutes;

d) Method of checking
thickness
- destructive
- coulometric:

of
and

of Thickness

2) non-parallelism
plates, and

Standard

4) Destructive
circular.

chemicals
from standard
manuC) Plating
facturers arc used. The procedure
outlined
by them is adopted
to carry
out
the
required plating.

1) Microscopic

thickness

is

measured

cross sectional

method,

;ind time
is considered
101plating thickness

anode-cathode

test holes arc 10 mm2

with
5) If rending difrerence
is more than one micron,
rejected.

by

near

four points
the plutc is

of approxi6) The area with diameter

mately IO mm is sufficient for standard
probe
or microprobe
to carry
out
calibration.
7) Destructive
tester mcasurcs
the thic~kness with chemical etching.
Cunductivily of the plated
material
doe> not

or

2,) Coulomctric
destructive
test.
NOTE-~ M~croxct)pic examination
tctiiolts
method

between

for
\t:lndar:lizing
purpose to avoid
reasons
given above.
NOTE 2 - On one plate only half area is plated
in order to ensure that the same plate gives
conductivity
of bare steel.
Otherwise,
other
MS bare plate may have different conductivity.
This is for adjusting zero on deltameter.

Electroplaling
is carried
out in perspex
both of length
150 mm, width 100 mm
and height 75 mm.
LOW voltage,
high
current
dc power supply
is used
for
electroplating.

d) The plating
either:

essentially

3) concentration
of current
density
at
corners of cathode plate.
NOTE1 - A central portion of plating is used

a) Small steel plates of approximately

70 mm
x 40 mm x 1 mm are taken.
Near mirror
finish buffing is carried out. Cleaning agent
is used to rernovc any oil contamination.

b)

is done

a) To ensure good average reading;

b) To take care of non-uniformity
of plating
which results due to:
I ) anode/cathode
area difference,

plating

c) Special note - if any, for example, electric

conductivity
of electroplated
metal when
different chemical/process
is used.
A-2. Preparation

OF

e) Four corner measurements

are carried out
by destructive
test ( coloumetric
type )
for a circle of approximately
10 mm
diameter.

label

a) Plating chemical used : manufacturer

brand name of the chemical;

CHECKING

being very
coulomctric
conxun~ing,
to be reference method
measurement.

IS : 12554 ( Part 1 ) - 1988

play any
role in this
measurement
whereas eddy current
base instrument
thickness
dependent
measures
the
entirely on conductivity
of the plated
material.

different
thickness

9) In fact it is advisable
that chemicals
and brighteners
are used
from
the
same manufacturer.
In case the user
has standard
set process,
the sample
plated can be destructively
tested and
used
as standard
for eddy currcnt
instruments.

8) Brightness
added
to solutions
while
plating, particularly
in case of nickel
Eddy
current
affect
conductivity.
for
instrument
readings
will
vary

APPENDIX

( Clause 7.3
NOTE
B-l. Nickel
plating
dependent
on method
be as follows:

FOR NICKEL

measurement
which
is
of plating employed,
shall

B-l.1 With Copper Flash

Undercoat Few
microns of copper is sometimes deposited before
coating nickel. If the nickel is measured directly,
it would give excessive thickness
readings.
This
is due to copper having much
higher conductivity than nickel. This error can bc corrected as
follows.
Calibrate
the instrument
for nickel
as usual.
Measure
the nickel
equivalent
of
copper,
say, 8 microns.
Subtract
this reading
from the total nickel plating reading.
For getting approximate
copper
thickness
reading,
divide the nickel equivalsnt
of copper
reading
by four.
plating
B-l.2 Chrome
Flash - Bright chrome
has one or two micron
chromium
flash over
the nickel.
This thin chrome coating
does not
make significant
difference in total nickel thickness
reading.
If chrome
is to be measured
accurately,
first measure nickel at the same spot.
Measure again after chrome
plating.
Multiply
the difference by a factor of two.
B-1.3 Bright Nickel - Variety
of brighteners
are added to the nickel plating
bath to get the
These brighteners
contain sulphur
bright finish.
which changes
the conductivity
of the nickel
Therefore,
if substantial
deposit substantially.
bright chrome
is
deviation
from the expected
observed, it may be necessary to make a special
thickness
standard
for the particular
plating
process.

brightener
content
the same
shown on destructive
tester.

MEASUREMENT
B-l.4 Semi-Bright
Nickel - Semi-bright
nickel
platings are usually
free from sulphur
content
and, therefore,
have much higher conductivity
than the bright nickel.
Due to this property,
it
is unlikelv
that thickness
measurement
errors
would a&e from the use of chemicals
from
different manufacturers.
B-l.5 Duplex or Semi-Bright
Nickel - These
plating have a semi-bright
nickel coat followed
by bright finish.
Presently,
a standard
containing 50 percent semi-bright
and 50 percent
bright
nickel is available.
For other percentages,
special
standards
would be required.
B-l.6 Plating
More
Than
30 Microns - ln
certain
articles,
heavy nickel
plating
that
is
higher than 30 microns,
may be specified.
For
few such cases, erratic
measurement
have been
observed on the deltameter.
Thus a reading less
than
10 microns
may be obtained
even when
actual thickness is in excess of 30 microns.
Two possible causes of these errors have been
suggested.
Permanent
magnetization
of nickel
is one possibility.
Higher sulphur
concentration
in the upper
layers of the nickel plating
is
another possibility.
So far, no solution
is in sight
to solve this problem.
A suggested
approach
to overcome
this
problem would be to measure
nickel
thickness
at other points on the article, say, the undcrsidc
where the thickness is known
to be much less.
If these positions
show acceptable
thickness,
then the whole article may bc accepted.

BUREAU OF INDIAN

STANDARDS

Headquartersr
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Telephones : 3310131, 3311375

NEW DELHI

110002

Telegrams : Manaksanstha
(Common to all Offices)

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gouthern

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53627

Pushpanjali,
1st Floor, 205A West High Court Road, Shankar
Square, NAGPUR 440010
Institution

560058

751002

53/5 Ward No. 29, R.G. Barua Road, 5th By-Lane,

5-8-56C

Road, BANGALORE

Road, T.T. Nagar, BHOPAL

Plot No, 82183, Lewis Road, BHUBANESHWAR

26348, 26349

380001

1332 Shivaji

Approach,

Nagar
25171

Nagar, PUNE 411005

P.0, Princep Street, Calcutte

700072

Chambers, Grant Road, Bombay 400007

Prrntefl at Swetmntrs Bharot Prey.

52435

276800
896629
Delhi (India)