AES Research Project 3, Adhesion of Electrodeposits, Part 4, Commentary on Measurement
This is Part 4 of a four-part article consisting of the full report of AES Research Project #3, Adhesion of Electrodeposits, done at the University of Michigan in the mid-1940s, following the end of World War II. It contains a commentary by industry people on the published methods for measuring adhesion.
Project Director: A.L. Ferguson
Associate: Elmer F. Stephan
Chemistry Department, University of Michigan, Ann Arbor, Michigan
Part 4. Correlated Abstract of Comments on Methods for Measuring the Degree of Adhesion
This is Part 4 of a four-part article consisting of the full report of AES Research Project #3, Adhesion of Electrodeposits, done at the University of Michigan in the mid-1940s, following the end of World War II. It contains a commentary by industry people on the published methods for measuring adhesion. A printable version of this section can be downloaded by clicking HERE.
The previous report was a summary and critical analysis of information contained in the literature. The present report is a summary of information contained in communications resulting from a circular letter sent to the total membership of the American Electroplaters' Society, and from many personal letters sent to outstanding men in electroplating and related fields, requesting that they submit their personal experiences in the use of methods for measuring the adhesion of electrodeposits. The information presented in the previous report was organized on the basis of the various methods used. The same plan is followed in this report.
The general tone running through most of the replies was a genuine regret that no satisfactory quantitative method is available for measuring the degree of adhesion. There is a strong feeling that even the fairly quantitative methods for thick deposits are not practical because of the time and care needed in preparation of samples and the nature of the testing equipment required. It is the general belief that the use of present quantitative methods is limited largely to research laboratories. There is a strong desire for a simple, standardized and quick method suitable for regular production.
Much interest is shown in the problem. Some of the typical statements are quoted below. A.W. Hothersall of England has probably done more work on the subject of adhesion than any other person. A letter contains these interesting comments:
"There is much confusion of thought on the subject of adhesion, partly due to failure to define terms properly. It is most important that descriptions of the results of qualitative adhesion tests are intelligible to those not associated with the workers, and also that different people dealing with this question do mean the same thing when they use the same words. For a long time, I have stressed the importance of using defined terms to describe the degree of adhesion found in a qualitative test. I am extremely glad that the AES, which has long been prominent for its work on improvement in the quality of electroplated coatings, is now taking up the study of adhesion. There is much to be done in this direction, particularly in developing methods for assessing the adhesion of relatively thin deposits. I am not aware of any other laboratory in this country or in Europe, where the question of adhesion is being specifically studied. I shall look forward with great interest to the results of your work and if there is any way in which I can be of assistance to your Committee, please do not hesitate to write to me."
M.B. Diggin, Chief Chemist, Hanson-Van Winkle-Munning Company, expresses his feelings as follows:
"I do hope that your Committee makes some progress in developing quantitative methods for determining adhesion and that the methods are simple enough to be applied to formed articles. We wish you success in this undertaking and shall be glad to assist in any way possible."
As stated in the previous report it is the policy of the author to submit the comments and ideas of the various contributors in their own words. In this way, it is hoped to avoid misinterpretations and coloring which might result from personal prejudice. The reader is also freer to form his own opinions.
Bending and twisting tests
Tests of this nature are in more widespread use than any others. In some cases, the manner of application is extremely crude; in others, a distinct effort has been made to carry out the tests so as to permit some degree of standardization and reproducibility.
Judging from the replies received, the Meaker Company ranks very high among those giving consideration to adhesion. E.H. Lyons, Jr., Chief Chemist, furnished considerable detailed information. The following are his statements on the bending and twisting methods used by his company as well as a number of steel companies engaged in electrogalvanizing:
"Our work deals principally with electroplated zinc on steel strip and wire, for which the adhesion requirements are extremely severe. We also test some zinc coatings on fabricated parts and castings, and occasionally cadmium coatings. Because cast and cold-worked zinc or cadmium have much lower tensile strength and hardness than the steel base and, we believe, than the bond between the base metal and the electroplate, the quantitative tests applied to copper and nickel coatings are believed to be inapplicable, and we are not aware of any attempts to apply them. Therefore, as far as we know, no quantitative tests for adhesion have been made on zinc and cadmium plates.
"The qualitative tests can be classed as bend tests and blister tests. The bend tests comprise bending or flexing of the plated steel in such a manner as the nature of the plated object will permit. The bending is usually done by hand and, because no mandrel is employed, the radius of bending is not standardized. It is usually small, however, less than 1/8 in., unless prohibited by the nature of the work. The bending is frequently reversed and repeated until the steel base is fractured. Any sign whatsoever, even microscopic, of peeling, chipping or flaking of the zinc or cadmium plate, is taken as cause for rejection.
"There is a machine put out by Amsler for testing the bending properties of steel strip. A sample of specified size is clamped between a pair of jaws machined to provide mandrels of specified radius. The opposite end of the specimen is seized by a clamp which applies a specific tension. The sample is first bent 90° to one side and straightened, then 90° to the other. This is repeated until the specimen breaks, and the number of bends withstood is recorded. Its chief use, so far as we have observed, is in detecting hydrogen embrittlement of the steel. This machine has been proposed for a standardized bend test, but inasmuch as almost any electroplated zinc or cadmium coating will withstand the test, it has never come into use. Our own laboratory routine for testing adhesion on steel strip includes:
- 90° bend test: Performed by seizing the specimen in a pair of pliers, and bending it as sharply as possible, first to one side and then to the other, until the steel breaks.
- Crimp test: The steel is bent 180° back on itself and the bend is flattened by pliers or in a vise. On opening, the steel usually breaks before the bend has been straightened, the fracture having been initiated during the flattening.
- Twist or rocking test: Outlined in Trans. Electrochem. Soc., 84, 288 (1943).
"The last test is the most severe because specimens which withstand the first two may fail in the last. Attempts to apply the Erichsen cup test to electrogalvanized strip steel as a test for adhesion have been discouraging, doubtlessly because the zinc is ductile enough to flow in this test.
"The requirements on strip steel are unusually severe because the material is frequently severely formed, bent, twisted, folded, stamped or drawn after plating. The same is even more true of steel wire.
"The standard test for adhesion on steel wire consists in wrapping the wire around a mandrel having the same diameter as the wire. It is important to twist the wire around its own axis as well while making this wrap, so that all elements of the surface will be subjected both to stretch on the outside and to compression on the inside. The resultant product is a coil of wire having twenty or thirty turns around the circumference of a straight portion of the wire. Close straight turns must be made to provide the greatest stringency of test. A crude lathe is often helpful.
"Although it is very difficult for most hot galvanized wire to meet this test without flaking of the coating, it is passed so readily by electrogalvanized coatings as to be meaningless for all intents and purposes. Nevertheless, it is commonly carried over from hot dip practice.
"Failures are sometimes noted on wrap testing hard steel wire and copper wire, but on stripping the electrogalvanized coating it will be found that the base metal itself has cracked. This is not regarded as the fault of the coating or its adhesion.
"Sometimes failures are noted on the inner smaller part of the wrap where the coating is under compression, although the stretched portion is perfectly adherent.
"Our laboratory routine for testing adhesion on round wire is as follows:
- Wrap or 'button' test: Includes the steps described above: (a) wrap, (b) unwrap, (c) reverse wrap and (d) unwrap. In (c) the identical wire specimen which has been unwrapped from the coil formed in (a), is rewrapped in the opposite direction to form a second coil, inside-out with respect to the first coil. Some steels will break in (b), (c) or (d).
- Crimp test: The wire is bent 180° sharply back on itself, and squeezed as far as possible in pliers or vise. The highly stretched outer bend is examined.
- Hinge test: The crimp formed in (2) is opened out carefully. The steel which has started to fracture in forming the crimp now parts along the fracture line. While the break is being observed with a low power magnifying glass, the fracture is opened until the steel has entirely parted and a portion of the zinc coating remains intact (though stretched) as a hinge joining the two pieces of steel. The adhesion of the hinge, and of the broken coating around the remainder of the circumference, is examined under the glass.
"As previously stated, the slightest sign of detachment of the zinc coating from the steel is cause for rejection, in any of the above tests."
The twist or rocking test referred to was briefly described by Lyons during the discussion of his paper on "Electrogalvanizing of Strip Steel". In answer to the question by C.G. Fink, "Do you not make any ductility tests?" Lyons replied:
"Yes, they are usually bend tests. We have recently been introduced to a test which is more severe. The strip is bent into a V-shape with handles [Fig. 1] and then it is given repeated bending in a direction perpendicular to the plane of the V, which results in a rolling twist at the point. A satisfactory electrogalvanized coating will stand such twists until the steel breaks, which in some instances may be after sixty or eighty twists."
It may be of interest to mention that the war pennies were blanked out of zinc-plated sheet steel, tested in this manner.