Understanding Corrosion in Materials Selection

Recently, Flow Control magazine published an article by Gerry Berry, a metallurgical engineer working with Emerson’s Micro Motion Coriolis flowmeters. The article, Strategies for Proper Material Selection–Lessons Learned from 30 Years of Application Experience, shares considerations in selecting materials suitable for reliable fluid handling systems.

The article gleans a few key nuggets from the comprehensive Micro Motion Corrosion Guide and describes this guide as:

a repository of test data that has been accumulating over decades of testing and field experience with customers on hundreds of thousands of applications.

Over the years, Gerry and the team have used tools such as x-ray equipment, positive material identification (PMI), scanning electron microscopes, ultrasonic thickness measuring devices, Hall-Effect gauges, potententiostats, and hardness and microhardness testers to accumulate this valuable test data. The team also takes advantage of the National Association of Corrosion Engineers’ (NACE) body of knowledge.

For those like me who may not be versed on the subject of corrosion, the article provides an excellent overview on corrosion and its causes and begins with a good definition:

Corrosion is the degradation of a metal or alloy caused by its reaction with an environment. Metals and alloys rely on the formation of an oxide layer for protection. The integrity of the oxide layer is dependent upon both the metal and the environment. For reliable protection, the oxide layer must be uniform.

Gerry provides several fundamental questions you need to ask to assess material compatibility:

  • What corrosive agents are in the process and in what concentration range?
  • What is the process temperature range?
  • What material is being used for the piping?
  • What cleaning cycles exist, and what fluids are used in these cycles?
  • What is the velocity (particularly important when handling sulfuric acid)?

After addressing these questions, there are process-specific considerations like erosion caused by solids, liquid slurry, or even gaseous steam moving through a pipe at high velocity. Also, as I can attest from my earlier years on the oil and gas platforms in the Gulf of Mexico, humidity, salt water and other ambient environmental conditions must be considered. For processes requiring sterilization between batches, the clean-in-place/sterilize-in-place operations, the draining capabilities of the piping, and dwell time between batches should be considered.

Gerry provides some other scenarios like processes with chlorine, fluorine, changing chemical mixtures, and large temperature swings and the challenges they bring from a corrosion standpoint.

If your responsibilities include the selection of materials for your instrumentation, I highly recommend the article and the wealth of great information in the Corrosion Guide.

Posted Tuesday, June 12th, 2007 under Asset Optimization, Plant Equipment.

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