Use of handheld X-ray fluorescence (XRF) in the FAC protocol

Topic :

• Non-Destructive Examination methods

• Inspection methods

• Chromium/copper measurements

ABSTRACT :

Research conducted through the Electric Power Research Institute (EPRI) has demonstrated that FLow Accelerated Corrosion (FAC) is a complex process influenced by a number of variables, including :

• The composition of the steel - principally the alloying elements of chromium (CR), copper (Cu), and molybden (Mo).
• The water chemistry in use - pH at temperature in the water, dissolved oxygen, and temperature.
• The flow variables - fluid velocity, diameter, fitting geometry, and upstream influences.

Of the variables presented above, both laboratory testing and plant experience have shown that material composition exerts the most influence on FAC.

It has been widely demonstrated that small quantities of alloying elements, chromium in particular, greatly reduce the rate of FAC. Research conducted by Michel Bouchacourt of Electricité de France has shown that at higher levels of trace chromium (above ~ 0.1%), substitution of chromium atoms for iron atoms occurs in the oxide layer, creating an oxide structure of FeCr₂O₄, which is much less soluble than the normal magnetite (Fe₃O₄) oxide layer present in carbon steel piping. As a result, it has become industry convention when inspecting for FAC to closely monitor trace alloy content, because it is the lack of these elements that allows FAC to occur. Further, monitoring of trace alloy content helps you facilitate improved planning of inspection protocol. For example, if you find inspected piping contains sufficient chromium content, then you can omit it from future FAC inspection. The composition data is also assists in interpreting the overall FAC data model.

Traditionally, chemical analysis of carbon steel piping in FAC inspection has been performed by laboratory analysis of filings, or more recently, by use of spark-based optical emission spectroscopy (OES) instrumentation, due to the need for detection of very low levels of chromium (~0.02%). 

With its dramatically improved detection limits, handheld x-ray fluorescence (XRF) technology has become a valid alternative method for the FAC analysis application. The Thermo Scientific Niton XL3t900 Series with geometrically optimized large area drift detector (GOLDD^TM) technology is the latest in the Niton® XL3t Series analyzers, offering vast improvements in sensitivity and speed. 

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