ASTM G93 & CGA G-4.1 Oxygen Cleaning Standards Explained

Reviewed by the Harrison Electropolishing oxygen cleaning team · Updated October 2026

The short answer: ASTM G93 and CGA G-4.1 are the two standards that define what "cleaned for oxygen service" means. Both exist because ordinary shop contaminants — cutting oil, grease, thread sealant, lint, metal particles — can ignite in an oxygen-enriched environment, and the fire burns the metal itself. ASTM G93 sets the cleanliness levels (how much residue and how many particles are allowed) and describes the cleaning and verification methods. CGA G-4.1 is the Compressed Gas Association's practical guide to cleaning equipment for oxygen service and is the reference most industrial gas, aerospace, and medical oxygen specifications cite. If your component will contact oxygen at elevated pressure or concentration, it needs to be cleaned to one of these and verified.

Why oxygen service is different

In air, a fleck of grease on a valve seat is a housekeeping issue. In a pipeline carrying oxygen at 300 psi, it is fuel. Oxygen doesn't burn, but it makes everything around it burn more easily, hotter, and faster. The most common ignition mechanism in oxygen systems is adiabatic compression: when a valve opens quickly, the gas downstream is compressed and heats, and a hydrocarbon residue on the surface reaches its ignition temperature. Once it ignites, it can kindle the metal. Stainless steel, aluminum, and carbon steel all burn in high-pressure oxygen; only a few copper- and nickel-based alloys resist it.

Particles matter for a second reason. A metal particle carried at velocity through a system can strike a surface, generate heat by impact, and ignite. So oxygen cleanliness has two dimensions: non-volatile residue (NVR, the hydrocarbon film) and particulate.

ASTM G93: cleanliness levels and methods

ASTM G93, Standard Guide for Cleanliness Levels and Cleaning Methods for Materials and Equipment Used in Oxygen-Enriched Environments, gives the owner a vocabulary for specifying how clean is clean enough. It defines cleanliness levels for NVR and for particles, and it describes the accepted cleaning and inspection methods.

Non-volatile residue levels (per 0.1 m², approximately one square foot) (confirm against the ASTM G93 edition your specification references):

Particle levels are specified by the largest permitted particle size and a count distribution (for example, Level 100: no particles over 100 µm, with limits on smaller sizes). The specifying engineer selects an NVR level and a particle level based on the system's pressure, oxygen concentration, flow velocity, and consequence of failure. Most industrial oxygen piping and valves are specified at Level B or C for NVR; medical, aerospace, and cryogenic systems commonly require Level A.

G93 is a guide, not a specification; it doesn't tell you which level to use. It gives you the definitions so your specification can.

CGA G-4.1: the cleaning practice

CGA G-4.1, Cleaning Equipment for Oxygen Service, is the Compressed Gas Association's document describing how equipment should be cleaned, inspected, packaged, and maintained for oxygen service. It is written for the people doing the work and is the standard most purchase orders reference. It covers:

• Contaminants to remove: hydrocarbons (oils, greases, cutting fluids, thread compounds), particulates (metal chips, weld spatter, grinding dust, lint), and other organics (fingerprints, solvent residues, adhesives).
• Cleaning methods: mechanical pre-cleaning; aqueous alkaline detergent cleaning; solvent cleaning; ultrasonic cleaning; acid cleaning or pickling where scale is present; and rinsing with deionized water.
• Inspection methods: direct visual inspection under white light; ultraviolet (black light) inspection for fluorescing hydrocarbons; wipe tests with clean white cloth or filter paper; water-break test; and, for quantitative verification, solvent extraction with gravimetric NVR analysis and particle counting.
• Packaging and preservation: drying with clean, oil-free nitrogen or air; capping all openings; bagging in clean polyethylene, usually double-bagged; labeling "Cleaned for Oxygen Service"; and handling only with clean gloves in a controlled area.

What Harrison's oxygen cleaning process includes

Every oxygen-cleaning job at Harrison follows a written procedure aligned to CGA G-4.1, with the cleanliness level set by your specification per ASTM G93.

1. Receipt and inspection — parts are logged, photographed, and inspected for scale, heavy oil, or damage that needs pre-treatment.
2. Pre-cleaning — disassembly where required, mechanical removal of gross contamination, and pickling or passivation if scale or free iron is present.
3. Precision cleaning — aqueous detergent cleaning matched to the component and contaminant, followed by deionized-water rinsing.
4. Verification — inspection by the CGA G-4.1 methods appropriate to the specified cleanliness level, from white-light and ultraviolet inspection and wipe testing through quantitative NVR and particle verification where the level requires it.
5. Drying and packaging — dried with filtered, oil-free gas, openings capped, double-bagged, and labeled per CGA G-4.1.
6. Documentation — certificate of conformance stating the standard, the cleanliness level, and the verification method, plus test results on request.

Oxygen cleaning is often combined with electropolishing or passivation when the component is stainless steel: electropolishing produces a smooth, low-particle surface that is easier to clean and verify, and passivation ensures the surface is free of embedded iron.

What needs oxygen cleaning

Anything that will contact oxygen at a concentration above about 23.5% (the threshold CGA uses to define oxygen-enriched) or at elevated pressure, including:

• Valves, regulators, fittings, and gauges for oxygen service
• Piping, tubing, and manifolds for industrial, medical, and aviator's oxygen systems
• Cryogenic liquid oxygen (LOX) equipment: vaporizers, transfer lines, storage vessels
• Components for oxygen-enriched combustion in steel, glass, and petrochemical plants
• Hyperbaric and life-support system components
• Aerospace propellant and ground-support equipment

If a component has been in service, repaired, or handled outside a controlled environment, it needs to be re-cleaned before it goes back into oxygen service. Cleanliness doesn't survive a trip through a machine shop.

Industries we serve

Harrison provides oxygen cleaning from our Houston facility for industrial gas suppliers, petrochemical and refining plants along the Gulf Coast, medical gas equipment manufacturers, and aerospace suppliers. Because the Houston region is the largest industrial gas and petrochemical market in North America, oxygen-service equipment is a large share of what comes through our shop.

Request an oxygen cleaning quote

Tell us the component, material, quantity, the standard and cleanliness level your spec calls for, and whether the parts also need electropolishing or passivation. We'll quote it within one business day.

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