Stainless Steel: A metal for all seasons

A metal for all seasons By George Genevro The exhaust manifold, waste gate, hot side of the turbo, and the exhaust elbow are made of various grades of stainless steel. Exhaust system and turbosupercharger for the Orenda engine. The...


Galvanic Series Chart
Galvanic Series Chart

Applications
What does all this mean for those who design and also maintain and repair aircraft? Are stainless steels the answer to all of our fabrication and corrosion problems? Not really. While stainless steels have many admirable characteristics, we must remember that in a structure as complex as an airplane many different metals are used, almost always in close contact with each other. A quick look at the listing of common metals in the galvanic series chart tells us that only three of the noble metals - silver, gold, and platinum - are more corrosion resistant than stainless steels. The metals at the top of the chart corrode more readily than those at the lower end and we should note that aluminum, in third position on the chart, corrodes readily when not coated with pure aluminum by the Alclad process or protected by other means. Two important factors to keep in mind: when two metals come in contact, the one above it on the chart serves as a sacrificial anode to the one below; and, the farther apart the metals are on the chart the more severe the corrosion will be on the sacrificial metal when they are in direct contact.

We all enjoy the clean, shiny appearance of stainless-steel fasteners for cowlings, inspection hole covers, and other parts on our aircraft, but we should be aware that aluminum and stainless steel are quite far apart on the galvanic series chart and that when these two materials are in unprotected contact corrosion of the aluminum can be quite severe. On unpainted aircraft one solution is the use of non-metallic washers while on painted aircraft a well-applied and properly maintained paint film will act as an effective barrier to corrosion. Another factor to keep in mind is that the typical stainless-steel machine screw that has a completely threaded shank is not a structural fastener and should never be substituted for regular aircraft hardware that is designed to carry shear as well as tension loads.

In the past, several attempts have been made to build entire airplanes out of stainless steel but none have been produced in large numbers. The Budd Oonestoga, a high-wing freighter type airplane, of which 11 were built in the 1940s, has faded into oblivion and only two examples of the Fleetwings Seabird amphibian were still flying in recent years. On these aircraft spot welding was used extensively in fabricating the airframe. Will someone ever tackle the demanding task of designing and building another all-stainless steel airplane for the civilian market? Probably not.

Conclusion
In the time that stainless steels have been available, their contributions to the aircraft industry have been both valuable and widespread. Aircraft parts made from these materials are used whenever their unique properties can make a product more reliable, safer, more efficient, longer lasting, as well as more attractive. Since safety is a matter of paramount importance in aircraft, it is comforting to know that the work of many metallurgists has provided us with many critical items, such as exhaust systems, turbosuperchargers, and gas turbine parts that are durable and cost-effective.

Stainless steels are used effectively in a wide variety of aircraft. And it is comforting to know that various types of stainless steels are quietly doing their diverse tasks well and safely.

George Genevro, a retired college professor at Cal State Univ. at Long Beach, CA, is an A&P mechanic, pilot, and aircraft owner.

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