Inspectioneering Journal

99 Diseases of Pressure Equipment: Green Rot & High Hardness Fittings

By John Reynolds, Principal Consultant at Intertek. This article appears in the January/February 2003 issue of Inspectioneering Journal.
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Green Rot

Now you say, he's got to be putting me on. What is green rot? I didn't invent it. I first read about it in one of the early texts on corrosion engineering by Ughlig or Fontana, the venerable corrosion professors at MIT & Ohio State. But when I experienced it, it became very real, even though I've only seen it once in my 35-year career. It's a form of internal oxidation of chrome-nickel-iron alloys that causes embrittlement. Under some circumstances, the alloy ingredients that were originally intended to produce a protective external oxide scale, instead produce a deleterious internal oxide precipitate, which gives rise to brittle fracture. When the component fails due to "green rot", the fresh fracture surface exhibits a greenish tint caused by the internal oxide precipitates that caused the embrittlement.

The only way I know to prevent green rot is to make sure the materials selected for the environment are not susceptible to internal oxidation. So clearly this "disease" is one that can be prevented by suitable materials selection. The job of the RBI team is simply to ask if an embrittlement mechanism is possible for some high temperature - high alloy chrome-nickel steels.

High Hardness Fittings

Another affliction that is becoming more common as we start to get more and more of our components from developing countries, is that high carbon contents and high micro-alloy contents will make some components relatively unweldable under normal circumstances. Many of our ASTM and ASME carbon steel specifications allow ranges for carbon and micro-alloying elements that exceed weldability.

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