Nitric acid passivation is commonly used in an effort to remove exogenous iron from surfaces and to prevent staining in the hope that the stainless steel flow path will become more corrosion resistant or inert.
Corrosion can not only compromise sampling system integrity and increase maintenance cost; it can also generate particulates that can result in obstruction and fouling of the system and cause adsorption of reactive compounds and ultimately compromise test results. Pitting can also create excellent hiding places for "sticky" molecules; resulting in carryover and false positive results.
Traditional passivation techniques involve the following steps:
- Thorough cleaning of surfaces
- Immersion in nitric or citric acid bath for 30 minutes. (acid concentration and additives are dependent on the grade of stainless steel)
- Rinse parts thoroughly in water
- Test part for passivation by placing in a humidity cabinet.
Here's an excellent source that explains the passivation process in detail.
Unfortunately when applications require the use of aggressive agents like bleach or hydrochloric acid, no amount of passivation or electropolishing will prevent corrosive attack.
But there is an alternative passivation technique. Bonding enhanced silicon compounds, like Dursan®, into the steel surface will both improve corrosion resistance and bio-inertness. The chemical vapor deposition process used to passivate stainless steel flowpaths will prevent disruption of grain boundaries and act as an inert barrier to aggressive and sticky compounds. Dursan coated HPLC flow paths will prevent HCl, bleach, and other corrosives from pitting and attacking stainless steel surfaces.
Learn more about improving the performance and durability of HPLC and other analytical systems.
Watch our process video and see how we bond silicon compounds to stainless steel surfaces.
Improving Metal Passivation in CEMS
The Dursan CVD passivation process is more sophisticated than typical methods:
- Surface preparation (usually aqueous sonication)
- Heating part to reaction temperature
- Deposition of silicon and other compounds onto and into the surface
- Cooling and post deposition cleaning
- Test part by surface contact angle analysis, coupon immersion into test solution, or refractometry.
Protecting stainless steel from corrosive attack in stainless steel flow paths
The Dursan coating prevents corrosion by a factor of 8x or more by preventing interaction of analyte or cleaning corrosives with stainless steel. The surface exceeds typical metal passivation capability and inertness by orders of magnitude. Immersion testing in 6M HCl shows that Dursan coated surfaces prevent surface attack by orders of magnitude compared to passivated stainless steel.
Stop Carryover and Contamination
Sticky compounds can be difficult to remove from test flow paths. Existing techniques for enhancing analyte release such as polyethylene glycol (PEG) systems lack stability and will oxidize in the presence of oxygen, rendering them useless. Others like thermoplastics fail at 250° C and risk incompatibility.
SilcoTek provides an alternative coating solution in Dursan that offers better stability in air and other oxidative environments, even under elevated (450° C+) temperatures. Dursan ensures near zero protein carryover compared to common PEG coatings or common passivation techniques.
Want to improve the performance of your CEMS, stack & flare sampling and procsss sampling systems? Read our e-book.