For anyone new in the finishing business, there appears to be an array of finishes and coatings required by OEMs and major industrial customers. Why are so many different, yet similar coatings needed? The answer is that the finish on a fastener has more than one purpose these days, including: providing corrosion protection in different areas in different conditions; controlling the torque/tension of the fastener into the joint; assisting in product identification; enhancing the appearance of the joint; and meeting special handling conditions.
The corrosion properties of fastener finishes are usually rated in years to red or white rust, since salt-spray testing is generally not accepted as an accurate method for rating a part's life in the real world. Cyclic corrosion testing has been found to be more representative of the conditions that actually occur and is rapidly becoming the method of choice. However, there are few coaters that have access to corrosion cabinets for cyclic testing or a proving ground like most major OEMs, which makes the callout of an ASTM B117 test a necessary default. Roughly one year is considered 96 hours neutral salt spray exposure, two years is 168 hours and five years is about 400 hours.
With a few variations Table I lists the requirements of most OEMs for the various areas of their vehicles.
The main reason for the large number of finishes needed to meet these requirements is the increased diversity of the conditions in which these finishes are asked to serve. Acid rain and salt use have increased the amount of corrosion resistance that a fastener needs outside.
Various types of fasteners cannot accommodate the same finish. Small parts with recess drives easily clog with organics and render the assembly process difficult, if not impossible. These same organics fill up the thread recesses on small diameter parts and make washer assembly parts stick together or freeze at an angle.
|TABLE I - OEM Requirements
|Area||Corrosion Resistance (yr)||Salt Spray Hours (Approx.)|
|All areas shall pass 10 years corrosion without loss of function. This is defined as the ability to remove the part without failure of the fastener. Grounding applications shall be capable of conducting a current without loss of function for 10 years. Special areas subjected to unusual conditions or chemicals shall not fail at less than the hours specified for cosmetic areas and shall maintain functional operation for 10 years.|
The trend has been to color coordinate fasteners. Black seems to be the preferred color on underhood parts with silver the other select color. While import vehicles show only yellow dichromate or a Dacromet® in response to the wishes and preferences of the customer, many domestic vehicles increase cost and proliferation by offering a rainbow of identification, or harmony, colors to fulfill the requests of internal management directives. Of course all of these colors and variations will be asked of the finisher.
Handling and interface concerns cause variation in finishes as well. The old oily types do not feed well through assembly tubes, pick up dirt easily and dirty the hands of workers. When these finishes are used in areas involving interior materials (seat fabrics and soon), the oil transfers, which results in a rejected seat cover. Environmental concerns have lead to the replacement of some finishes because of possible effects on the workers (cadmium and some "dusting" of metallic-based organics).
Many differences appear in the market because of the old "Not Invented Here" (NIH) syndrome. NIH leads many companies into developing their own coatings instead of using those available on the market.
Finally, cost drives an increasing number of coatings onto the market. There are finishes available that give more protection or better appearance but are more expensive as well. Most OEMs have ongoing programs to develop less expensive coatings that do the same job as the present ones, the sage about getting what you pay for not withstanding.
As the domestic OEMs continue their recently started commonization work and begin to look at finishes, some reduction in the number and types of finishes will certainly occur. More resistance to corrosion at lower cost is the top requirement. While there is always some concern about the effects of hydrogen embrittlement, no great drop in electroplated coatings will probably occur. The reason for this is that although most users would not like to use the electrodeposited finishes because of the tales of hydrogen embrittlement, the organics are just too thick to use effectively in many sizes and types of fasteners. The organics will continue to develop more of the qualities that the customers demand and will continue to work on thickness reductions.