Internal stress exists as an inherent force within electroplated and chemically applied metallic deposits. This induced stress can be tensile or compressive in nature, causing the deposit to contract or expand in relation to the base material. High levels of stress in deposits produce micro-cracking and macro-cracking, and in severe cases produce a lack of deposit adhesion in the form of blistering, peeling, and flaking. Two ways to evaluate the internal deposit stress in metallic coatings are the bent strip Deposit Stress Analyzer and the Spiral Contractometer methods. These represent two stress test procedures that have approval status by the American Society for Testing Metals Standards.
Spiral Contractometer Instructions for Applied Metalic Coatings
1. Specialty Testing and Development Company Spirals (Helices)
are constructed from 0.010 inch thick stainless steel and each has a precise surface area of 13.57 square inches. The spiral (helix) mounts on the contractometer in a manner that permits plating on the entire outside surface of the spiral (helix) and discourages deposition on the inside of the spiral (helix). Thus, there is no need to estimate the surface area that has been plated.
2. Properties and Test conditions for Stainless Steel Spirals (Helices)
Spiral / Helix for the New Spiral Contractometer Design Plating Test Conditions
Surface Area, in² 13.57
Square Feet 0.0942
Stock Thickness, inches 0.010
3. Test Procedure Spiral (Helix) Contractometer Method – Reference ASTM Standard B636 Equipment
Spiral contractometer (PN: KSC114)with calibration weights
Adjustable support stand (PN: SAS141)
Spiral (helix) test piece(PN: CTS214 Fluoropolymer coated or PN: NCS214 non-coated).
Titanium anode basket PN: ABL14 to hold pure nickel buttons.
4,000 ml Pyrex beaker – for plating.
Power supply (PN: HY3005 0-5 Amp) sufficient for the amperage desired.
Magnetic stir hot plate (PN: MSH4000)
Digital Temperature Controller (PN: TC590) with probe to place within the plating solution – recommended since the deposit stress varies with the bath temperature.
Spiral (Helix) preparation and Use
Spiral (Helix) preparation and use video
Step 1: Place the anode basket containing anodes or buttons in a 4,000 ml Pyrex beaker, then place the beaker over a magnetic stirrer hot plate, add the desired plating solution to the beaker, then place the spiral contractometer adjustable support stand over the beaker and anode basket and warm the plating solution to the desired temperature.
Step 2: If a nickel strike is necessary for adhesion of the applied coating, pour the nickel strike solution into a 4,000 ml beaker use the circular titanium anode basket containing nickel anodes or buttons.
Step 3: Clean a spiral (helix) as the cathode (positive charge) in an alkaline steel electrocleaner at 3 amps for 30 seconds and water rinse.
Step 4: Nickel strike the spiral (helix) at 3 amps for 1 minute. Remove the spiral (helix) from the spiral contractometer and water rinse, dip in an isopropyl alcohol rinse and dry.
Step 5: Weigh the spiral (helix) to the nearest milligram and record the weight in grams. Note: use gloves to prevent contamination of the spiral (helix). This is your Start Weight SW=___________.
Step 7: Place the spiral (helix) contractometer assembly and the attached spiral (helix) into the adjustable support stand holes located on the top surface of the stand. Then allow two minutes for the spiral (helix) to reach the plating bath temperature.
Step 6: Mount the spiral (helix) onto the spiral contractometer and tighten the Polycarbonate screws through the holes provided sufficiently to secure the spiral (helix) so as to prevent slippage during the plating process. Note: the wire contact end must be positioned over the top of the spiral (helix) with the screw provided.
Calibration of the Spiral (Helix) How to Find Your (K)
Step 1: Loosen the pulley wheel screw that holds the pulley wheel tight against the top of the center rod and position the dial by rotating it to match the zero with the arrow, then tighten the screw to secure the rod so slippage cannot occur during the plating cycle.
Note: It is important that when the spiral (helix) is attached to the contractometer, a space of about 3/16 inch should be allowed between the bottom of the contractometer spiral shaft and the top of the spiral (helix) holder knob at the bottom where the spiral (helix) is attached.
Step 2: Place the eye loop thread of one of the calibration weight string over the pulley calibration pin at the zero and wrap the thread clockwise part way around the pulley calibration wheel, then suspend the thread and weight over the grooved guide wheel near the Kc marker.
Step 3: Place the eye loop thread of second calibration weight string over the remaining pulley calibration pin and wrap the thread clockwise part way around the pulley calibration wheel, then suspend the weight over the grooved guide wheel near the Kc marker.
Step 4: Tap the pulley calibration wheel at the top center of the contractometer lightly and read the degrees compressive stress. Record this degree reading as Kc. Note: The compressive stress values are identified with a minus sign. If when the test is finished you find the plating bath is compressive this is your (K)______________.
Step 5: Remove the eye loop threads from the pins and repeat the above procedure except for wrapping the strings counterclockwise and suspending the threads over the grooved guide wheels near the Kt markers. Again, tap the pulley calibration wheel at the top center of the contractometer lightly and read the degrees tensile stress. Record this reading as Kt. Finally, remove the eye loop calibration string and weights. If when the test is finished you find the plating bath is tensile this is your (K)______________.
Plating the Spiral (Helix) How to Find Your (d)
Step 1: Make sure the solution temperature is the desired temperature within ± 3°F.
Step 2: Set the timer for the desired plating time.
Step 3: Connect the positive lead (usually red) of the power supply to the anode basket and the remaining lead (usually black) to the metal contact at the top of the spiral contractometer.
Step 4: With the rectifier plugged into the timer, turn on the timer and begin the plating process. For critical work, maintain the bath temperature within ± 3°F. It is helpful to tap the top of the pulley calibration wheel every 3 minutes or so with a blunt instrument to assist in stabilizing the degree reading.
Step 5: When the plating time expires, tap lightly on the top of the pulley calibration wheel to stabilize the degree value, then read the final degrees value and note if the stress is compressive (negative) or tensile (positive). Record the degree value as Kc- or Kt+ accordingly. This is your (d) value in Deposit Stress Formula______________.
Step 6: Remove the spiral contractometer from the adjustable support stand, rinse the spiral (helix) in water, and rinse it in 70 % isopropyl alcohol. Remove the spiral (helix) from the contractometer using gloves. Dry thoroughly. Note: It is helpful to pull part of a rolled sheet of paper towel gently through the interior of the spiral (helix) to assist in drying.
Step 7: When the spiral (helix) is completely dry, weigh the spiral (helix) and record the weight in grams. This is your finished weight FW= _______________.
Calculations for the Deposit Thickness
1. Subtract from this weight the before plating weight to obtain the weight of metal deposited. Post Weight – pre-weight = W
2. Calculate the average deposit thickness in inches.
W
T = ────────────────────────── = Inch
D x 13.57in² x 6.45cm²/in² x 2.54 cm/in
T = ────────────────────────── = Inch
D x 13.57in² x 6.45cm²/in² x 2.54 cm/in
W= Grams of nickel deposit
D= Density of plated material g/cm³, see chart below<
T= Deposit thickness in inches
Note: The surface area for Specialty Testing spirals (helices) is 13.57 in² or 13.57 square inches.
W
T = ──────────── = _________Inch
D(222.32cm³/inch
T = ──────────── = _________Inch
D(222.32cm³/inch
Note: If calculation of the internal deposit stress is desired based on the pitch of the spiral (helix), since all of the Specialty Testing and Development stainless steel spirals (helices) are manufactured in the same manner from 0.0100 inch thick stainless steel, t = 0.0002540 meters, and are formed in the same manner, the pitch, p, of each spiral (helix) is 0.000254 meters. The pitch is determined by slipping a spiral (helix) over a 0.019 m, 0.75 inch, diameter rod, compressing the spiral (helix) so the edges of the loops tightly touch each other and dividing the length in meters by the number of loops. This length is 14.3 cm = 0.143 m, and 0.143m ÷ 7.73 loops equals 0.0196 m.
Note: For the spirals (helices) plated on the old design spiral contractometers, the surface area plated must be determined by wrapping the spiral (helix) tightly around a 3/4 inch diameter rod. Then the diameter and estimated plated length in inch values are used to calculate the plated surface area as follows: Surface Area = πdh = cm² b. Solve the equation for PSI internal deposit stress as follows
Density and Modulus of Elasticity
| Deposited Metal | (D)Density | (Edeposit)Modulus of Elasticity |
|---|---|---|
| Cadmium | 8.80 | 30,600,000 |
| Copper | 8.93 | 16,000,000 |
| Gold(Soft) | 19.30 | 11,200,000 |
| Gold(Hard) | 19.32 | |
| Nickel | 8.88 | 30,000,500 |
| Palladium | 12.02 | 17,000,000 |
| Platinum | 21.45 | 24,800,000 |
| Rhodium | 12.45 | 52,100,000 |
| Silver | 12.02 | 11,000,000 |
| Tin | 7.26 | 5,900,000 |
| Zinc | 7.10 | 14,000,000 |