ASTM B636/B636M-15 Standard Test Method for Measurement of Internal Stress of Metallic Coatings Coated with Spiral Contractome

The standard "ASTM B636/B636M-15 Standard test method for measurement of internal stress of metallic coatings coated with spiral contractome" developed by the American Society for Testing and Materials (ASTM) describes the use of a spiral contractome to measure the internal stress of metallic coatings produced from coating solutions on a spiral cathode. This test method can be used with electrolytic and autocatalytic deposits.

For the purposes of this standard, the following terms and definitions apply:

• Compressive stress refers to the stress that causes expansion of a sediment.
• Internal stress refers to the net stress that remains in a sediment when it is free from external forces. Internal stress tends to compress or stretch sediments.
• Tensile stress refers to the stress that causes a sediment to shrink.

The test method for measuring stress with a spiral counter-meter is based on coating the outside of a helix. The helix is ​​formed by winding a strip around a cylinder and then annealing it. During operation, one end of the helix is ​​fixed and the other is allowed to move as the stresses develop. The free end is attached to a gauge needle by means of gears that magnify the movement of the helix. As the helix is ​​coated, the stress in the deposit causes the helix to be wound tighter or looser, depending on whether the stress is compressive (−) or tensile (+). From the amount of needle deflection and other data, the internal stress is calculated. By changing the instrument, the movement of the helix can be measured electronically rather than mechanically as described above.

The spiral contractometer, when used properly, gives repeatable results over a wide range of stress values. With this method, internal stress limits can be specified for use by both the purchaser and the manufacturer of coated or electroformed parts.

Coating with large tensile stresses reduces the fatigue strength of a product made of high strength steel. To minimize this, maximum stress limits can be specified. Other properties affected by stress include corrosion resistance, dimensional stability, cracking and peeling.

In the control of electroforming solutions, the effects of stress are more widely recognized and control of stress is often necessary to obtain a usable electroform. Internal stress limits can be determined and specified for production control.

The internal stress values ​​obtained with a spiral countermeter do not necessarily reflect the internal stress values ​​found in a part coated in the same solution. The internal stress varies depending on many factors, including coating thickness, substrate preparation, current density and temperature, and solution composition. A closer correlation is obtained when the test conditions match the conditions used in coating the part.

Factors affecting accuracy include:

• Variations in helix preparation can cause significant changes in results.
  • The stop material should be applied evenly to the inside of the helix. The stop material should be thin and flexible to allow the helix to move freely during testing. A coating weight of less than 3 mg/in is sufficient. The inside should be stopped with some inert, flexible coating. An acceptable stop material is microstop. One part of Microstop is diluted with two parts of acetone before use. Any nickel deposited on the inside of the helix will have an adverse effect.
  • Helices permanently coated on the inside with TFH-fluorocarbon may give variable results when testing near-zero voltages.
  • Changes in the cleaning and surface preparation of the helix prior to testing can produce variable results. For example, the electro cleaning of the helix should always be cathodic and controlled in terms of current, time and temperature. Anodic cleaning at this stage can provide wide variations. Abrasive cleaning of the helix and the use of abrasives should be avoided.
  • Very fine sediments, less than about 3 pm, are more affected by surface conditions and helix preparation than coarser sediments.
• The internal voltage varies with the current density used in electroplating. The change is not predictable and depends on the metal being deposited, the impurities or additives, and the current density range being considered. It is important that the current be measured and closely controlled throughout the voltage test. Changes in currents should be kept to less than 2 percent.
• Since the temperature of the coating solution may affect the internal stress, it should be kept within 2 degrees during the test. The initial rest point and final rest point of the indicator are taken at the operating temperature of the coating solution to eliminate thermal stresses.
• The solution composition should not change during testing. Generally, if repeatability tests are met, it can be assumed that the solution has not changed during the test run. Conversely, if repeatability tests are not met, the coating solution should be analyzed to determine if there has been any change in solution composition during testing.
  • Tests on electroplating solutions using insoluble or inert anodes may result in significant solution changes during testing.
  • When testing autocatalytic coating solutions, the components of the coating solution may be significantly depleted during the test if not regenerated.
• There is a relationship between the surface area to be coated and the volume of autocatalytic coating solutions which can affect the character of the deposit. When testing autocatalytic coating solutions, the ratio of the surface area coated to the volume of solution normally used in the coating tank should be maintained. When using proprietary solutions, the supplier's recommendation should be followed.

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