MEASUREMENT OF NICKEL COATINGS ON NON MAGNETIC BASE METALS

Principle -Nickel is Ferro-magnetic. The attractive force between a bar magnet and nickel coatings is practically proportionally to the thickness of the coatings.

The Mange-Gage measures thickness by measuring the attractive force. Since the relation between thickness and attractive force is nearly linear, dial readings may be converted to thickness by multiplying by a constant. For higher accuracy the calibration curve is used.

Base Metal -The kind of base metal on which the nickel is plated does not affect the magnetic measurements, provided the base metal is non-magnetic. Nickel- brass, which contains 18% of nickel, is non-magnetic, and therefore this method can be applied to nickel coatings over this alloy.

Magnetic Properties of nickel deposits - Nickel deposited from a given bath will have fairly constant magnetic properties, provided that the operating conditions do not vary too widely. However, nickel deposited from different baths or wider widely different operating conditions has variable magnetic properties.

Annealing - Practically all nickel deposits can be brought to a common magnetic state by annealing that at 400°C (750°F.) for 30 minutes. If an oven is not available, the specimen may be heated in a Bunsen Flame until the nickel surface (if chromium is absent) becomes brown and begins to turn blue. The heating is then immediately discontinued.

NOTE: Annealing of electrodeposits is required only for nickel coatings on non-magnetic base metals and is not required for the measurement of nickel coatings on steel, which is described on the following pages.

Methods of using the Magne-GageŽ when Measuring Nickel Coatings on Non-Magnetic Base Metals - The Magne-GageŽ is calibrated with annealed nickel deposits, as these are the most reproducible. The instrument may be used in two ways for testing nickel coatings of unknown thickness:

(I) It can be used to test nickel coatings of any origin, provided the specimens are annealed before testing.

(2) It can be used for routine testing of coatings as deposited, if the coatings are all plated under similar conditions. In this case it is necessary to calibrate the instrument for the type of coating deposited, and to check the calibration occasionally. Making measurements at different points on several specimens of coatings as deposited, then annealing them and measuring again at the same points does the calibration. The latter measurements give the thickness of coating at these points from which the calibration for the unannealed coating can be calculated.

The accuracy of thickness determinations by method (1) is plus or minus 10%, and by method (2), plus or minus 15%.

Bright Nickel Coatings - Bright nickel as deposited (that does not contain appreciable cobalt of iron) is more magnetic than ordinary nickel deposits; in fact it is almost as magnetic as annealed nickel. If the calibration for annealed nickel is used for bright nickel as deposited, the thickness will not be more than 15% low. More accurate results are obtained by annealing, since bright nickel comes to about the same magnetic state as other annealed nickel deposits.

Effect of Iron or Cobalt in The Deposit - Iron and cobalt are more magnetic than nickel, and if present in a nickel deposit will cause the apparent thickness determined by the magnetic method to be too high. Each percent of iron causes about a +4% error and each per cent of cobalt caused about a +2%error. Nickel deposited from baths operated with 99% or more pure nickel anodes will not contain enough iron or cobalt to interfere with magnetic measurements. However, some bright nickel-cobalt alloys that are now being plated contain about 20% cobalt, and are more magnetic than nickel deposits. Magnetic measurements of thickness can be made on such coatings if the composition is fairly constant and if the instrument is calibrated with coatings of similar composition.

Nickel-cobalt coatings containing 15% or more cobalt distinguishes them from ordinary nickel coatings, which usually turn yellow or light brown when annealed.

  Effect of A Parallel Nickel Coating - The attraction of a magnet extends for a short distance beyond its end. Therefore, when measurements are made on a sheet of metal plated on both sides, the nickel on the opposite side may affect the readings on one side. An annealed deposit on the back of a specimen has more effect than an untreated coating. This effect of a parallel coating decreases as the thickness of the base metal in creases. It is negligible with annealed coatings if the thickness of the base metal is at least three times the diameter of the magnet, and with untreated coatings if the thickness is at least 1.5 times the magnet diameter. These thickness’ amount to 1/4 in. and /8 m. for annealed and annealed coatings, respectively.

   The thickness of coatings on one side of thin sheet metal, which has been nickel-plated on both sides, can be measured if the coating on the opposite side is first removed by grinding or by solution in acid. The thickness of coatings on each side can be determined without destroying either coating if the effect of separation upon the attraction of the magnet has been determined.

The following formula may be used to correct a dial reading for the coating on the back side of a flat sheet:

Corrected dial reading =  D1 -(FxD2)

(On front side) 1 - F2

D1 and D2 are the dial readings on the front and back sides, respectively.

   F is the fraction of the attractive force of a coating that is obtained on the opposite side of a sheet. F is readily determined by taking a dial reading on a nickel-plated surface, then placing a piece of the unplated sheet metal on the nickel surface and taking another reading. This latter reading will be smaller, since the magnet is not in contact with the nickel. F is equal to the dial reading on the sheet metal divided by the dial reading on the nickel surface.

MEASUREMENT OF NON-MAGNETIC COATINGS ON IRON AND STEEL

Magnetized steel - If the steel base metal has retained magnetism through exposure to a strong magnetic field, for example, on a magnetic chuck or on magnafluxing; it should be demagnetized before testing with the Magne-GageŽ. This may be done by holding the specimen in a strong alternating magnetic field and slowly withdrawing it beyond the range of the field. Steel that is weakly magnetized may not occasion any difficulty, especially if the surfaces to be measured are of fiat curvature, as the magnetic field is weak on such surtaxes as compared to areas of sharp curvature.

Soft Coatings -The magnet may stick to soft metal surfaces, such as lead, or more rarely to a freshly deposited cadmium coating, and cause low readings. This difficulty maybe obviated be placing a film of very light mineral oil on the specimen and then wiping it off until no excess remains or by using a plastic film of known thickness which must be subtracted to obtain the true thickness.

Testing Coatings Not Produced by Electroeposition - Measurements of hot-dipped zinc coatings are usually 10% low (compared with the antimony trichloride stripping method as standard), probably because of the alloying between the zinc and the steel. Sheradized coatings are rough and always should be burnished before testing. Magnetic measurements on them are usually about 10% high (compared with the antimony trichloride stripping method as standard).

Many commercial hot-dipped tin coatings are about 0.0001 in. thick. To obtain satisfactory measurements on these coatings, the apparent thickness given by the Magne-GageŽ must be multiplied by the factor of 1.3, whence the results will be corrected within about 15%.

More accurate determinations of the thickness of hot-dipped zinc and tin can be obtained by calibrating against similar coatings.

The thickness of paint and lacquer coatings on steel is measured in the same manner as metallic coatings. If the coatings are tacky, they may cause the magnet to stick. This difficulty can be obviated by applying a film of light oil to the paint surface, or to uses a plastic ilm as suggested under "soft Coatings."

Thickness of Sheet Material -Occasionally it may be desired to measure the thickness of a non-magnetic sheet material, such as paper, plastic or metal which does not have a backing of steel. To perform a thickness measurement, the material should be placed in intimate contact with steel backing as any space between the specimen and the backing will be included in the measured thickness. Thin sheet material may be cemented to a steel surface with an adhesive (for example, Duco cement) and the excess pressed out with a roller, or the material maybe tightly stretched over a steel rod about 1 inch in diameter, which serves as the backing.

Testing Sticky Coating -When sticky coatings, such as tar or gum are to be tested, place a piece of Cellophane (approximately 0.001 inch thick) over the first test sample of the "A" calibration and check the thickness of the cellophane as outlined above. Then place the Cellophane on the specimen to be tested, making certain that there is no air gap between the two surfaces, and measure the total thickness in the usual manner. Subtraction of the Cellophane thickness from this total thickness gives the thickness of the coating being measured.

Measurement of Polished Nickel Coatings on Iron or Steel- Nickel is magnetic but less so than iron. When a nickel coating is present on a steel base, it decreases the attractive force of the magnet on a manner similar to non-magnetic coatings (as previously explained) but to a smaller extent. A separate calibration is required for nickel coatings.

Since the interposition of nickel coatings does not decrease the magnetic attraction as much as do non-magnetic coatings, the measurements of nickel coatings might be expected to be less accurate. How ever, most nickel coatings are deposited over a smooth steel surface and are them polished, so that more reproducible readings are obtained.

The calibration curve for nickel should be checked more frequently than the one for non-magnetic coatings, because an error on the dial reading produces a larger error in determining the thickness of nickel than of non-magnetic coatings.

FACTORS AFFECTING PRECISION AND ACCURACY

The effect of roughness of the base metal or coatings on the accuracy is most pronounced with coatings less than 0.001 m. thick. The smoothness of the coating is even more important than that of the base metal; in fact the reproducibility of readings is fairly good on smooth coatings, even though they are produced on a rough base metal. For example, fairly reliable readings are obtained on galvanized zinc surfaces (which are wiped smooth in the manufacturing process), even though the base metal may have been severely etched prior to galvanizing.

Measurements on polished surfaces are reproducible to about two divisions. The precision is lower on rough surfaces and in some cases it may be necessary to take the average of three or more readings. Measurements of coatings on sheet steel products, on machined parts, or of enamel coatings are satisfactory, but coatings on castings and on sand-blasted surfaces are too rough to be measured unless they are at least 0.00l in. thick hot dipped zinc coatings, for example).

The precision of measurements on rough surfaces is improved by burnishing a small area with a 1/2-in. to 1-in. ball bearing and a drop of oil. Coarse or crystalline deposits are burnished to advantage, but it is usually unnecessary to burnish galvanized sheet, terne plate, tin plate, or most deposits thicker thanO.00l in.

Occasionally it is necessary to test uncoated steel for the purpose of comparing it with the steel used in the calibration. For securing reproducible readings on bare steel, the surface must be smoother than is required for measurements on coatings. Some cold-rolled steel has a sufficiently smooth surface, but in general, the surface of steel must be polished before testing, either by buffing, or if the surface is too rough, by use of successively finer polishing papers.

Thickness of The Base Metal - Measurements are not appreciably affected by the thickness of mild steel, if it is over 10 mil. thick. Base metals of high carbon steel must be several times thicker if the magnetic measurements are to be independent of the thickness.

If a base metal is too thin for satisfactory measurements a flat specimen can be placed on a heavier flat piece of steel, which will permit the magnet to more nearly develop its maximum attractive force.

Magnetic Properties of The Base Metal - The nature of the ferrous base metals affects magnetic measurements, and separate calibrations should be made for metals which vary much in magnetic properties (as measured by their magnetic attraction). The attractive forces between a small magnet and many different specimens of mild steel such as are used in stamping and forming operations, were determined and found not to vary more than a few percent. Hence, with one calibration, measurements can be made on the majority of plated iron or steel products.

Whether or not coatings on a given ferrous metal will fit a certain calibration curve can be determined by taking a reading on the uncoated metal (suitably polished). If the attractive force of a particular metal does not vary by more than five scale divisions from that of the steel used for calibration, no appreciable error will be made in the measurements of coatings thicker than 0.0002 in. If the difference is larger (for example, on a high-carbon steel), the standard calibration curve can still be used by applying a simple correction to the apparent thickness obtained on the coated specimen.

Additional Factors Affecting Precision and Accuracy- The previous discussion of the factors affecting precision and accuracy applies without essential change to the measurement of nickel coatings. However, a variation in a given factor will produce a lager error on the measurement of the thickness of nickel coatings than in the measurement of non-magnetic coatings. Measurements should be made only on smooth nickel surfaces, for example, on "bright" nickel deposits or on buffed or burnished surfaces. For the thinner nickel coatings (Calibration-B) the correction for the effect of variations in base metal is made in the came manner as for non-magnetic coatings on steel (see preceding page)

Measurements on composite coatings of copper and nickel will give too high a result if the calibration curve for nickel is used and too low a result if the curve for non-magnetic coatings is used. However, if the copper layer is a flash coating only several hundred-thousandths of an inch thick, the use of the curve for nickel will not cause an error larger than l0%in the total thickness.

Correction curves and Instructions for measuring composite coatings containing copper in higher percentages are available and should be requested when ordering an instrument for this service.