"Accelerated Corrosion Testing"

Question:

"I am looking for an accelerated corrosion test to evaluate the resistance to interfacial corrosion of brazing alloys. I would like to be able to correlate the accelerated test to an equivalent long term exposure. The problem I have with ASTM G31 is that it does not offer any method for dealing with the localized type of corrosion we are experiencing. The interfacial corrosion phenomenon is a crevice corrosion-type problem. Are there any generally accepted methods for figuring corrosion rates for localized corrosion phenomena? I´m looking for some information about corrosion maintenance of petroleum and gas equipment, like vessel, heat exchanger, tanks,colums, pipelines, flowlines, etc. I would like to now some monitoring methods."

Answer:

If you are interested in resistance to aqueous immersion environments, the standard laboratory corrosion coupon weight loss test method (ASTM G31) normalizes corrosion rates to units of time, typically mm/yr or mils per year (mpy). As long as you test for a sufficient time to account for time dependent polarization effects, a short term test (40-200 hours) can accurately predict long term behaviour. NACE test method TM0169-95 (281-228-6200) provides recommendations for determining optimum test times for immersion service testing. I personally know of no way to eliminate the influence of corrosion of the brazed material unless you can mask it or in some way isolate it from the test environment. Careful application of an epoxy resin over the material you wish to isolate may work. There are three ASTM methods that may help you in the determination of localized corrosion (e.g. pitting and crevice). 1) ASTM F746: Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials. This is an electrochemical method. 2) ASTM G78: Guide for Crevice Corrosion Testing of Iron Base and Nickel Base Stainless alloys in seawater and other chloride containing aqueous environments. This method consist of exposing the alloy with special washers to form crevices. It can be used as a base document for your specific alloy. 3) ASTM G46: Practice for examination and evaluation of pitting corrosion. This practice shows how to determine corrosion rate for localized corrosion. Predicting the effect of time on corrosion is never easy. Temperature has been used as an accelerator of corrosion. However, in the presence of dissolved gases, higher temperatures may lower corrosion rates. Another way is to change the solution to the solution inside the pit or crevice which is usually acidic and/or concentrated in ferric chloride (see ASTM G48). I think that the best way is to perform several tests with different exposure time. It will give you first the localized corrosion initiation time, then the corrosion rate increase versus time. Rohrback Cosasco Systems (310)949-0123 sells all kinds of corrosion monitoring equipment. They have a web site at www.corrpro.com.

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