Why Some Stainless Steel Pipes Must Be Degreased: Oxygen Line Cleaning

Pickling and Passivation Are the Normal Mill Finish

Stainless steel pipe leaving a mill is usually treated by pickling and passivation. Pickling dissolves the hot-worked oxide scale and the chromium-depleted layer beneath it, and passivation then rebuilds the thin chromium-rich oxide film that gives the alloy its corrosion resistance. For austenitic grades such as 304 and 316 supplied to ASTM A312 or GB/T 14976, this treatment is sufficient for the great majority of water, steam and general process duties.

Cleanliness, however, is a separate requirement from corrosion resistance. A pipe can pass every corrosion test and still carry drawing lubricant, cutting fluid or temporary rust-preventive oil on its inner wall. That residue only becomes critical in a small group of services where the transported medium reacts with hydrocarbons.

Why Oxygen and High-Purity Gas Lines Are Degreased

Oxygen pipelines are the classic case. Much of the oxygen handled in coal-chemical, metallurgical and medical gas plants is more than 99.99% pure, and it is moved at significant pressure and high flow velocity. In that environment a trace of oil or grease on the pipe wall can ignite on contact with the gas stream. Loose particles that break free can also strike the wall at high velocity, and the impact energy is enough to start a fire in an oxygen-enriched atmosphere. Compressor suction lines, instrument air circuits and breathing-air systems raise comparable concerns.

The practical consequence is that degreasing is written into the project specification rather than the mill order. Contract practice is to clean the assembled piping system after installation, because welding, cutting and site handling reintroduce contamination that shop cleaning cannot control. Factory degreasing is therefore the exception, and it is requested mainly when the pipe will be installed in a controlled clean environment or when the fabricator has no suitable cleaning facility on site.

The Standard Four-Step Cleaning Sequence

The sequence below is the one generally specified for stainless steel oxygen piping. Each stage has a defined purpose, and skipping the intermediate rinse simply carries contamination further down the system.

Step	Operation	Main parameters	Purpose
1	High-pressure water flushing	Pressure approximately 0.6 MPa	Removes dust, sediment and loosely adhering metal oxide
2	Manual wiping with degreasing agent	Aqueous alkaline cleaner, mixed to the supplier ratio and monitored for colour change	Removes mineral oil, graphite, rust-preventive oil and other organic films
3	Second water flushing	Continued until the discharge water runs clean	Flushes out residual cleaning agent and loosened residue
4	Oil-free air or nitrogen purge	Oil-free compressed air or nitrogen, then sealed with clean plastic wrap	Dries the bore and prevents recontamination before erection

During step 2 the cleanliness of the cleaning bath must be watched. Once the bath turns visibly dirty it should be drained and re-mixed, because a saturated bath will redeposit the very oil it is meant to remove. Wiping cloths should be lint-free and used once.

Correcting an Outdated Solvent Recommendation

Older degreasing procedures often name carbon tetrachloride as an acceptance solvent. That recommendation should not be followed today: carbon tetrachloride is toxic and its production is controlled under the Montreal Protocol because of its ozone-depleting potential. Modern practice uses aqueous alkaline cleaners, hydrocarbon-free solvents or neutral pH degreasers that are compatible with the pipe material, followed by a solvent-free wipe check. The same principle applies to chlorinated solvents in general, which can leave chloride residues that attack austenitic stainless steel.

Acceptance Criteria After Degreasing

Once cleaning is complete the bore is verified before the line is closed. Typical acceptance requirements are:

Inspection of the inner wall under ultraviolet light at a wavelength of 320 to 380 nm, with no grease fluorescence visible.

Wiping of the bore with clean, dry white filter paper, leaving no oil trace on the paper.

A solvent-based solvent residue check showing hydrocarbon content below the limit set by the purchaser.

Any additional technical index agreed in the project specification, such as particle count or final dew point for the drying step.

Records matter as much as the test itself. Cleaning agent batch numbers, bath changes, flushing water quality and the result of each visual and ultraviolet check should be logged, because the piping will normally be closed and sealed immediately afterwards. ASTM G93 describes cleaning methods and cleanliness levels for equipment used in oxygen-enriched service and is a useful framework when a project has no internal standard of its own.

Frequently Asked Questions

Q: Does every stainless steel pipe need degreasing?
No. Pickling and passivation are sufficient for most water, steam and general process lines. Degreasing is specified for oxygen service, high-purity gas and other duties where hydrocarbon residue creates a genuine ignition or purity risk.

Q: Should degreasing be done at the mill or on site?
For an installed piping system, cleaning is normally carried out on site after erection and welding are complete, because site work reintroduces oil, scale and particles that shop cleaning cannot prevent.

Q: What pressure is used for the initial water flush?
Small high-pressure cleaning units are commonly run at approximately 0.6 MPa, which is enough to lift loose dust, sediment and detached oxide without damaging the pipe bore.

Q: Why is the line purged with oil-free air or nitrogen?
The purge dries the bore after wet cleaning. If oil is present in the purge gas it becomes a new contaminant, so oil-free compressed air or dry nitrogen is specified and the pipe is then wrapped and sealed.

Q: How is a degreased pipe checked before it is sealed?
Fluorescence under 320 to 380 nm ultraviolet light and a white filter-paper wipe are the two most common field checks, supported by a solvent residue test against the purchaser limit.

Q: Can chlorinated solvents be used for degreasing stainless steel?
They should be avoided. Chlorinated solvents such as carbon tetrachloride leave chloride-bearing residues that can break down the passive film on austenitic stainless steel and cause pitting.

Q: Does degreasing replace passivation?
No. Degreasing removes organic contamination, while passivation restores the chromium oxide film. A pipe that has been degreased may still require pickling or passivation after welding.