NATIONAL BUREAU OF STANDARDS REPORT

10 270

MEASUREMENT OF THICKNESS OF CHROMIUM ON CURRENCY PRINTING PLATES

Technical Report

To

Bureau of Engraving and Printing

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' was established by an act of Congress March 3, 1901. Today,

in addition to serving as the Nation’s central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

areas. These are: (1) basic measurements and standards, (2) materials measurements and

standards, (3) technological measurements and standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

Institute for Applied Technology, the Center for Radiation Research, the Center for Computer

Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and

uniform physical measurements throughout the Nation’s scientific community, industry, and commerce. The Institute consists of an Office of Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics - —Radio Engineering - —Time and Frequency - —Astrophysics - —Cryogenics.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; develops,

produces, and distributes standard reference materials; relates the physical and chemical properties of materials to their behavior and their interaction with their environments; and provides

advisory and research services to other Government agencies. The Institute consists of an Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations in the development of technological standards, and test methodologies; and provides advisory and research services for Federal, state, and local government agencies. The Institute consists of the following technical divisions and

offices: Engineering Standards—Weights and Measures — Invention and Innovation — Vehicle Systems Research—Product Evaluation —Building Research—Instrument Shops—Measurement Engineering—Electronic Technology—Technical Analysis.

THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and application of radiation to the solution of Bureau mission problems and the problems of other agencies and institutions. The Center consists of the following divisions: Reactor Radiation—Linac Radiation—Nuclear Radiation—Applied Radiation. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and

provides technical services designed to aid Government agencies in the selection, acquisition, and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques,

and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

organizational units:

Office of Standard Reference Data—Clearinghouse for Federal Scientific and Technical Information —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

- Located at Boulder. Colorado 80302. Located at 5285 Port Royal Road, Springfield, Virginia 22151.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

3120542 July 15, 1970 10 27 0

MEASUREMENT OF THICKNESS OF CHROMIUM ON CURRENCY PRINTING PLATES

Technical Report

January 1 to June 30, 1970

By

Fielding Ogburn

Electrolysis and Metal Deposition Section

Metallurgy Division

To

Bureau of Engraving and Printing

IMPORTANT NOTICE NATIONAL BUREAU OF

for use within the Governmen

and review. For this reason, whole or in part, is not aut) Bureau of Standards, Washin

the Report has been specitica

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

jress accounting documents intended

is subjected to additional evaluation

jre listing of this Report, either in the Office of the Director, National

r, by the Government agency for which

il copies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

MEASUREMENT OF THICKNESS OF CHROMIUM ON CURRENCY PRINTING PLATES

Fielding Ogburn

National Bureau of Standards, Washington, D. C. 2023^-

INTRODUCTION

There is a need for a nondestructive means of measuring the

thickness of chromium on the electrofornied nickel plates used to

print currency. The measurement should have a reliability of at

least +10% up to a thickness somewhat greater than 8 ym (0.3 mil).

Several commercially available thickness gages can be used to

measure the thickness of chromium layers on nickel, but their accuracy for this application is eitehr too poor or unknown. In the

latter category are the magnetic type gages such as the Permascope

and Accuderm and the beta backscatter type, such as the Beta Scope

and Micro-derm. In addition there is the fluorescent x-ray method.

This report covers an evaluation of these methods of measuring

the thickness of the chromium and the initial stages of developing

a fluorescent x-ray technique using a radioisotope instead of an

x-ray generator.

1

PREPARATION OF STANDARD THICKNESS SAMPLES

To evaluate the thickness gages , we prepared specimens of

chromium plated nickel with known thicknesses of chromium.

We started with ,5x8 inch sheets of low carbon steel which

had been plated with at least 50 pm ( 2 mils) of Watts nickel

(buffed). These we plated uniformly with conventional chromium

in a cell designed to obtain uniform thickness distribution and

then cut them up into 3 cm ( 1 inch) squares.

To determine the chromium thickness we turneu to x-ray techniques which we found to be quite sensitive to the chromium thickness. We measured each of 18 squares cut from the center of a panel

to establish the relative thickness of each square . To establish

the absolute thicknesses, we determlend the weight of chromium on

each of three or four squares by conventional stripping-weight loss

methods. Then by treating these stripped specimens as calibrating

standards we calculated the weight per unit area of chromium on each

of the other squares. We thus built up a set of standard thickness

samples with thicknesses of approximately 0.02, 0.1, 0.2, 0.3, 0.4,

and 0.5 mil.

EVALUATION OF METHODS

To determine the sensitivity of each instrument we measured

one specimen of each thickness with each instrument. The accompanying figures show a plot of thickness vs instrument reading for each

instrument

2

Magne-gage

With this instrument, one dial division corresponds to 0.02 mil

or 6% at 0.3 mil. This corresponds to the uncertainty of the gage

if it is carefully calibrated and operated by the same individual.

Errors double this can be expected if the operator does not exercise

care

Permascope

The uncertainty of this instrument appears to be between +5

and 10%. We have not had much experience with this instrument

,

but it appears that the reproducibility can be improved with experience. We also understand that a new probe is available which

incorporates a spring device enabling more reproducible positioning.

Beta Scope

At 0.3 mil, our measurements with this instrument had an uncertainty of about 0.05 mil. This instrument counts backs cattered

beta particles. If the time of counting is increased, the precision

is improved. Double the time improves the precision by a factor

of 1.1. The uncertainty of 0.05 mil is based on a 1 minute count.

Fluorescent x-rays

With fluorescent x-ray techniques, one has the option of measuring either the x-rays characteristic of the coating, Cr-Ka , or of

the substrate, Ni-Ka. We found that for thicknesses less than about

0.1 mil, Cr-Ka measurements give the best results and for thicker

coatings, Ni-Ka is best. The precision of the method is better

than 1% in the 0.1 to 0.5 mil range using Ni-Ka radiations.

3

Kocour Thickness Tester

This instrument measures the current required to anodically

dissolve the chromium over a well defined area about 1/8 inch in

diameter. The readout is in thickness -units. It would be suitable

for the currency plates as far as accuracy is concerned, but it is

destructive to the coating.

Summary of Evaluations

Both the Magne-gage and the Permascope appear to have the precision required by the Bueau of Engraving and Printing, but only if

used by a reliable operator. The Accuderm is, we believe, equivalent

to the Permascope.

The Beta Scope, and its equivalent, the Microderm, do not meet

the precision requirements.

The fluorescent x-ray method offers much better precision than

required and is rapid, but requires rather elaborate equipment costabout $25,000.

All these methods must be calibrated with standard thickness

samples. Suitable standards can be prepared easily by the National

Bureau of Standards and it is possible that they are available commercially.

h

RADIOISOTOPE TECHNIQUE

The fluorescent x-ray method can be modified by replacing the

x-ray generator with a radioisotope and doing away with the spectrometer. This would allow a more compact unit and less expensive by a

factor of 10. In principal it would be equally reliable.

To evaluate this method, it was first necessary to assemble

from scratch the detector and the electronic equipment. We obtained

the components at no cost to this project. Actually the equipment

we were able to borrow was suitable for gold coatings and not for

chromium. With the intention of lauer converting the assembly to

something suitable for chromium we went ahead with the setup for

gold to obtain needed background and experience.

At this time we nave demonstrated that the method works well

for gold coatings on nickel. The critical aspects are kind and activity

of isotope; the design of a jig to accurately and reproducibly align

the radioactive source, detector, and specimen; and the design of

the jig to minimize interference of scattered and stray radiation.

These three factors determine the reliability of the thickness determinations and will have to be changed when switching from gold to

chromium measurements.

In brief, the radioisotope tecnnique is promising, but needs

to be specifically designed for chromium and tested.

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