Process analyzers and process sampling systems often times are exposed to challenging corrosion and wear environments both internally and externally.  Many sample streams are corrosive or contain active compounds that reduce equipment lifetime or require extended preventative maintenance due to wear.  Some systems are exposed to environments such as sea water, which cause rapid deterioration of equipment, requiring extra cost to keep them operating.  For systems that are required to give accurate, reliable and repeatable data in such conditions, the cost of upkeep and maintenance is much larger than systems in more benign environments. 

Chloride environments and chloride containing streams can greatly reduce the lifetime of process systems.  Coatings, paints and costly super alloys have been used to increase the lifetime of components in salt water and/or chloride containing environments.  Table I provides the results obtained from ASTM G31 testing.  This method is an immersion test for 24 hours in a 6M HCl (18%) solution at room temperature and pressure.  After immersion, differential weighing allows the amount of material loss to be determined.  The sample size for each configuration was 3 samples.  The amorphous silicon coated stainless steel shows greater than 20 times the resistance of non-treated stainless steel in these environments and the Dursan coating creates greater than 200 times the resistance.  Any loss in the coated samples occurred as a result of pitting corrosion.  The pitting is an indication that there are still pin-holes present in the surface which allowed corrosive attack to initiate.

Table I:  Weight loss after 24 hour exposure to 6M (18%) HCl

Another factor for consideration is the wear resistance of coatings applied to analytical sampling equipment.  This factor is critical, especially in applications where there mechanical contact such as valve actuation or physical abrasion like particulates moving through the sampling equipment at high velocity.  Valve seat movements or particulate in these applications can quickly erode a soft coating such as silicon creating sites for adsorption to occur.  Table II summarizes the data obtained from wear studies conducted on both non-treated and treated surfaces.  Data was generated using a pin-on-disk tribometer (Nanovea, Irvin, CA).  The experiment uses a flat plate loaded onto the test rig and the indenter applies a precise force to the surface.  The plate is then rotated and forces are measured between the pin and the disc.  Results from this experimental method can produce wear behavior and friction coefficients of the plate surface¹.  Results from this study demonstrate that the carboxysilane coatings wear less than untreated steel and silicon coated surface.  The improved wear resistance as a result of the coating will lead to longer lifetimes of system components in extreme environments.

Table II:  Physical PROPERTIES of coatings