Fasteners and Finishes

Often a customer requests a recommendation for the best finish available for areas of high corrosion. If the area involves high heat or heat over a long operating life, care should be taken that coatings high in zinc, cadmium, tin, and some rare earth metals not be chosen for steel parts. Other base metals have other groups to be avoided. The problem that may occur is called liquid metal embrittlement. The lower melting point metals diffuse into the base material forming intermetallic compounds of alloy and molten metal similar to solid corrosion compounds. In the presence of stresses, as in installed fasteners, the molten alloy penetrates the base material and produces cracks very similar in appearance to stress corrosion cracks. The speed of this process varies with temperature and the metal involved. The closer to the metal's melting point, the faster the action. The failures are catastrophic and are often mistaken for hydrogen embrittlement failures. As the photo shows, the cracks are intergranular in nature and are often numerous. Cadmium should not be used in exhaust areas since temperatures often exceed 650F. Zinc is also susceptible to liquid metal cracking, but only when the temperatures are much higher (750F range). The most recently reported case of this type of failure (August 1998) was on a fleet of school buses that had manifold stud nuts replaced with cadmium dichromate plated parts, resulting in a 100% failure starting within 15 minutes.

Tin has become a hot new automotive coating choice these days. Replacing lead in electrical areas and used in conjunction with zinc in tin-zinc multiplex alloy type coatings, little work has been done with its potential for failure under moderate heat conditions. Melting points are as low as 449F for pure metal and about 512F for tin-zinc alloys. Tests show that screws plated with the material do not stand much of a chance in hot areas. One test run several years ago by J. Bicknell, Bendix Missile Systems, on tin-plated screws showed 100% failure on stressed (installed) screws after 48 seconds at 450F (50% failed within four seconds). While there are not many fasteners, beyond battery parts, currently plated with pure tin, the use of tin-zinc alloy coated parts is rapidly increasing. Little research has been run by the chemical manufacturers to ascertain the susceptibility of tin-zinc plated parts to liquid metal embrittlement at high (area of more than 400F) temperatures and the effect of prolonged exposure to 350F temperatures. Temperatures in the 350F range are common in today's underhood automotive uses. Little literature is available and no information has been published about the effects of diffusion over time at temperatures below the melting point of the metal. Another phenomenon of tin-zinc alloy plating that seems to have little press is that there appears to be a stage where prolonged heat exposure causes the tin dispersion in the alloy to slowly coalesce into nodules. This leaves the coating with tin nodules scattered throughout a zinc-rich matrix. This condition is an ideal medium for differential corrosion, as the zinc will corrode between the tin "balls" (stovepiping).

In addition to the combinations mentioned above, aluminum, brass, magnesium, and titanium parts also show tendencies towards liquid metal embrittle- ment failures. A few instances that the plater may encounter are: On aluminum parts—mercury, sodium, tin, zinc and rare earth metals Ga and In; On magnesium products—zinc should be avoided if the parts will see temperatures in the 700F range; On titanium—avoid cadmium. Finally, brass parts that see any adverse temperatures should not be plated with tin, zinc, or low-melting-point metals. Some reports show a tendency of alloyed metals to cause metal embrittlement even when the individual components do not, as one reported case of a stress cracked high-strength alloy steel fastener that failed while in contact with a tin-lead solder even though there is no susceptibility of steel to either molten lead or molten tin.

It is a wise policy to always ask the customer how he intends to use his parts. The recommendation should take into account conditions of stress, environment and, as this article discussed, heat.