Albert Emery, Contributing Writer
Why did corrosion form so quickly during thermal cycling in a precise optical assembly?
My company designed a very precise optical assembly that could maintain focus as the ambient temperature fluctuated. The assembly consisted of two stainless-steel barrels machined from bar stock, both of which contained lenses that were adhesively bonded in place. One barrel fit inside the other, and a spring-loaded mechanism adjusted force - that is, the position of one barrel with respect to the other - as the temperature varied.
The fit between the barrels was very tight, and in some areas there was actual contact, so a thin deposit of nickel plating was applied to one of the barrels to provide lubricity and prevent seizing. Also of significance was the fact that the space between the barrels was enclosed so that the air inside was entrapped and stagnant.
Every lens assembly was tested by subjecting it to temperature cycling. A few years ago, there was an unusually high failure rate. Failure was evident as binding of the barrels and inability to maintain focus. Close inspection of failed units after disassembly revealed very small corrosion deposits in the gap between the barrels, causing them to bind. It was a mystery as to how corrosion had formed so quickly during thermal cycling.
The failures initiated a detailed investigation that included additional testing, laboratory failure analysis, and visits to the hardware manufacturers and the plating shop. A review of production records determined that the high failure rate was limited to one specific lot of the nickel-plated barrel. For this particular lot only, the thickness of the plating had exceeded the engineering drawing tolerances, so the plating was chemically stripped from the barrels and reapplied to the correct thickness.
Meanwhile, the failure analysis laboratory cross-sectioned a nickel-plated barrel from a binding optical assembly and photographed it under high magnification. Upon inspecting the photographs, I discovered missing sulfide stringers. The final piece was in place to solve the mystery!
The stainless-steel alloy used to fabricate the barrels was 416. It was not only relatively inexpensive, but also a free-machining grade. This characteristic was achieved by adding sulfur to the alloy at the steel mill. The sulfur combined with manganese, and the resulting compound formed numerous, thread-like “stringers” that ran parallel to the axis of the stainless-steel bar as it was formed. As the cutting tool contacted a stringer during machining, the brittle stringer broke and facilitated the formation of small chips.
When the nickel-plated barrels had been chemically stripped to remove the over-tolerance plating, the aggressive chemical agent also dissolved any exposed sulfide stringers, and then remained entrapped inside the resulting hollow “tunnels.” Subsequent rinsing did not completely remove the stripping agent, and reapplication of nickel plating did not plug the “tunnels.”
It was this entrapped chemical agent which caused the corrosion deposits. It existed in the nickel-plated barrels at the time the lens assemblies were built. Once built, the area of stagnant air was created, and corrosive vapors from the chemical agent quickly caused corrosion deposits to form. More than likely, the high temperature extremes during thermal cycling aided the release of corrosive vapors. Because the gap between the barrels was so narrow, a very small corrosion deposit was sufficient to cause the binding.
About the Author:
Albert Emery is a materials engineer at Lockheed Martin Corp. During his 28 years at the company, he has supported several military defense programs, including the PATRIOT surface-to-air missile. He has a BS degree in metallurgical engineering from the University of Notre Dame. He lives in Orlando, FL with his wife and three children, and enjoys reading, traveling and bicycling.
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