"Corrosion Resistance in Carbon Steels"

Question:

"I am employed as a product engineer at a corporation which developes and produces automotive airbag inflators. The current product I am working on is fabricated from 304L SS. For cost savings purposes, we intend to go to an HSLA carbon steel for the inflator housing. However, we are trying to maintain a hermetic seal at one of the machined components by using an o-ring. My concern in switching to a carbon steel is that corrosion (both short-term and long-term) will destroy the capability to achieve and maintain that seal. These components will be shipped to our plant with a rust inhibiter, however, they will be cleaned in an ultrasonic aqueous solution wash prior to assembly, as all dust must be removed to obtain the seal, and all oil must be removed so that the part can be welded. What I am looking for is a low-cost, readily available, carbon steel alloy which is compatible with HSLA carbon steels for welding, and is most importantly corrosion resistant. Resistant to the extent that a reasonably corrosion free surface could be maintained after the wash for a period of up to two days prior to assembly, and could maintain a hermetic seal with a lubricated o-ring for a period of 15 years. Bear in mind that this is only a static seal to provide resistance against moisture ingress into the inflator assembly, and will not be exposed to any fluids other than air, at a max pressure differential of 1 atm. One of our materials specialists had recommended a "copper steel" which supposedly has a copper content of ~0.2%. I have been unable to find any information regarding this type of material, and am concerned that it would not be cost effective. Could you recommend a material, or even an aqueous inhibiter that would suit these purposes?"

Answer:

You are asking for quite a lot out of a "low cost, readily available, corrosion resistance carbon steel that can be welded to HSLA". There are many HSLA alloys and you don't say where the inflator is to be located in the automobile. I assume that it will be located with the interior of the car and that it will not be exposed to the elements. The copper containing carbon steel (commonly known as CorTen, a US Steel trademark, there are two types) is probably not the answer. It will rust, although with more protective properties than non Corten steels and with a more visually pleasing patina. Therefore, it will not necessarily remain free of rust in storage any more than ordinary carbon steel. So, I share your concern that it may not be cost effective vs. non Corten steel. If you are wanting temporary resistance to flash rusting after cleaning and welding, one solution (short of staying with 304LSS) is to control the storage/holding area environment so that the components are not exposed to excessive humidity or the elements. The storage environment can be reinforced, if required, with a vapor phase inhibitor, for added protection. Once in service in the interior of an automobile, the sealing surface under the O-ring should not corrode to the point where the seal is compromised, provided that the seal pressure is adequate. All this, of course, is dependent on the range of expected environments the inflator body may be exposed to.

 

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