Coatings and Surface Treatments

Overview

The internal surface condition of sample transport tubing, probes, fittings, and valves has a direct and measurable impact on analytical accuracy. Untreated metal surfaces are chemically active -- they adsorb reactive compounds, catalyze molecular transformations, and release corrosion products into the sample stream. For trace-level analysis of sulfur compounds, mercury, moisture, ammonia, and other reactive species, surface treatment is not optional. It is a fundamental requirement for obtaining reliable data.

Surface treatments fall into two categories: inert barrier coatings that chemically isolate the sample from the metal substrate, and electrochemical processes that refine and passivate the native metal surface. The appropriate treatment depends on the analytes of interest, process conditions, and regulatory requirements.

SilcoNert 2000

Description

SilcoNert 2000 is a chemical vapor deposition (CVD) applied silicon coating that creates an amorphous silicon layer bonded to the metal substrate. This coating renders the surface chemically inert to virtually all reactive analytes encountered in process gas sampling.

Performance Characteristics

• Coating Thickness: Approximately 100-500 nanometers
• Temperature Tolerance: Continuous service up to 750 degrees F (400 degrees C)
• Surface Energy: Extremely low, preventing molecular adsorption
• Inertness: Exceeds electropolish by 10x to 50x for reactive compounds

When to Specify SilcoNert 2000

• Trace sulfur analysis (H2S, COS, mercaptans) at sub-ppm levels
• Mercury monitoring in natural gas, refinery gas, and flue gas streams
• Moisture analysis at ppm and sub-ppm levels
• Ammonia and amine measurement in process gas
• Any application where ASTM D5504, EPA Method 30B, or similar trace-level test methods are in use
• Regulatory compliance sampling where data defensibility requires demonstrated sample path inertness

Applicable Components

SilcoNert 2000 can be applied to tubing, probes, fittings, valves, regulators, filters, and sample cylinders. All wetted surfaces in the sample path should be treated for maximum effectiveness.

SilcoNert 1000

Description

SilcoNert 1000 is a hydrogenated amorphous silicon coating applied via CVD. It provides a corrosion-resistant barrier with moderate inertness, positioned between electropolish and SilcoNert 2000 in terms of both performance and cost.

Performance Characteristics

• Coating Thickness: Approximately 100-500 nanometers
• Temperature Tolerance: Continuous service up to 750 degrees F (400 degrees C)
• Corrosion Resistance: Significantly exceeds uncoated 316SS in acidic and chloride environments
• Inertness: 5x to 20x improvement over electropolish for moderately reactive compounds

When to Specify SilcoNert 1000

• Total sulfur analysis at low-ppm levels where SilcoNert 2000 performance is not required
• Corrosion protection for sample systems in mildly aggressive environments
• Applications where budget constraints preclude SilcoNert 2000 but electropolish alone is insufficient
• Stack gas moisture monitoring and general combustion gas analysis

Electropolish

Description

Electropolishing is an electrochemical metal removal process that preferentially dissolves surface peaks, micro-burrs, and inclusions from the internal bore of tubing and components. The result is a smooth, chromium-enriched passive layer with reduced surface area and improved corrosion resistance.

Performance Characteristics

• Surface Finish: Typically achieves 10-15 Ra (microinches), down from 20-32 Ra for mechanically polished tubing
• Chromium Enrichment: Surface chromium-to-iron ratio increases significantly, improving passive layer stability
• Surface Area Reduction: Smoother surface presents fewer adsorption sites compared to as-drawn tubing

When to Specify Electropolish

• General analytical sampling where trace-level reactive species are not the primary concern
• Pharmaceutical and biotech sampling systems requiring cGMP-compliant surface finishes
• Food and beverage process sampling where cleanability and corrosion resistance are priorities
• As a pre-treatment step before SilcoNert coating application (improves coating adhesion and performance)
• ASME BPE compliant systems requiring documented surface finish specifications

Limitations

Electropolish improves but does not eliminate surface activity. For trace sulfur, mercury, and moisture analysis, electropolished surfaces still exhibit measurable adsorption and response time delays compared to SilcoNert-treated surfaces.

Passivation

Description

Passivation is a chemical treatment process, typically using nitric acid or citric acid, that removes free iron and surface contaminants from stainless steel and enhances the native chromium oxide passive layer. It is the baseline surface treatment for all stainless steel sample system components.

Performance Characteristics

• Process: Immersion in acid bath per ASTM A967 or ASTM A380
• Result: Clean, uniform passive layer free of embedded iron, scale, and shop contamination
• Surface Finish: Does not change the mechanical surface finish (Ra value)

When to Specify Passivation

• All new stainless steel sample system components as a minimum treatment
• After any welding, brazing, or mechanical fabrication that disrupts the native passive layer
• When components have been exposed to contamination, carbon steel contact, or improper storage
• Routine maintenance and reconditioning of existing sample system tubing and fittings

Treatment Selection Guide

Compliance and Standards

Surface treatments for analytical sampling systems may be driven by regulatory requirements including EPA test methods, ASTM analytical standards, and industry specifications such as ASME BPE. When specifying treatments for compliance-driven applications, ensure the selected coating or process meets the documentation and traceability requirements of the applicable standard.

Ordering Information

Surface treatments can be applied to new components at the time of fabrication or to existing components as a retrofit service. Specify the treatment type, the components to be treated, and any applicable compliance standards. Contact our engineering team for guidance on treatment selection and turnaround times.