Fig. 1. Gold-silicon phase diagram[1]

I want to subcritical anneal a part in hydrogen that is made from 304 stainless steel. The annealing temperature (approximately 1700°F) is a concern to me in that the parts are gold plated. The plated gold is causing a eutectic or eutectic reaction (lowering the materials’ melting point). Is there a method to gold plate 304 stainless steel and then anneal it at 1700°F?

To begin, it is necessary to understand that a eutectic (or eutectic mixture) involves an alloy (i.e. a mixture of two or more solids) of two or more elements whose combined melting point depends on the relative proportions of the alloying elements present. The eutectic point for a given concentration is a local temperature minimum, which means that all the constituents crystallize simultaneously at this temperature from molten liquid solution. Such a simultaneous crystallization of a eutectic mixture is known as a eutectic reaction; the temperature at which it takes place is the eutectic temperature; and the composition and temperature at which it takes place is called the eutectic point.

There are five recognized classes of gold-plating chemistries:
  • Alkaline gold cyanide (for gold and gold-alloy plating)
  • Neutral gold cyanide (for high-purity plating)
  • Acid gold plating (for bright, hard gold and gold-alloy plating)
  • Non-cyanide – generally sulfite-based (for gold and gold-alloy plating)
  • Miscellaneous procedures (for general-purpose applications)
As you already know, gold coatings are often used for their corrosion-resistant properties and electrical conductivity. The composition of 304 stainless steel and a review of the phase diagrams tell me that you will NOT be able to gold plate and then anneal at 1700°F.

In particular, the gold-silicon phase diagram (Fig. 1), using a nominal 1% silicon (usually present in 304), tells me that a eutectic melting point occurs at approximately 1470°F (800°C). I do not know if you can buy 304 with a limited silicon content, but you would need to have less than 0.30% silicon in order to prevent melting at 1700°F.

In addition, the gold plate may not involve pure gold, which can change the eutectic point depending on what elements are present. For example, if a gold-compatible solder is involved (often 80% gold/20% tin alloys), the eutectic melting point drops to around 530°F.