⛽ Corrosiveness of LPG to copper — the copper strip test
In short
A polished copper strip immersed for 1 h in 100 ml of water-saturated LPG at 40 °C in a test bomb; visual assessment against the corrosion standards (classes 1–4). The test is performed according to PN-EN ISO 6251:2001; Test conditions: 1 h, 40 °C, a water bath. The procedure comprises 6 steps (about 1 h in total); it is used for: Quality control of LPG autogas according to PN-EN 589 (class 1 acceptable), Control of LPG in the distribution chain (no corrosion at the producer, corrosiveness often found in distribution), Diagnostics of the causes of failures of vehicle gas installations (injectors, reducers, filters).
At a glance
- Standard: PN-EN ISO 6251:2001
- Category: Petrochemistry
- Procedure steps: 6
- Total time of stages: 1 h
- Test conditions: 1 h, 40 °C, a water bath
- Sample: 100 ml of water-saturated LPG in the test bomb
- Strip: Copper, freshly polished
Overview
The test of the corrosiveness of LPG to copper according to PN-EN ISO 6251:2001 is the oldest and still the basic method of assessing whether a gaseous fuel does not attack the non-ferrous metal parts of an installation; it is referred to in the autogas specification PN-EN 589. Pure LPG is not corrosive — the problem arises when sulfur and its compounds (hydrogen sulfide, mercaptans, free sulfur), water and solid contaminants appear. Studies have shown that hydrogen sulfide at a concentration of as little as 5 ppm produces multicoloured green-pink-violet corrosion products (copper sulfides) on the strip, and elemental sulfur S8 at 5 ppm a black film of copper(II) sulfide; carbonyl sulfide (COS) does not corrode the strip even at 100 ppm, but in the presence of water it hydrolyses to H2S and CO2; a mixture of sulfur compounds is more corrosive than any of them separately. The present description is based solely on the literature (the Oil and Gas Institute, Archiwum Motoryzacji, SGS) — the text of the standard was not available.
The test is not quantitative but qualitative (a test of the effect): after 1 h the strip is compared with a reference scale, the discolouration, film and pitting being described; on the four-class scale only class 1 is acceptable for autogas. The effects of corrosive LPG in a vehicle are rough running of the engine, blocking of the injectors by particles of sulfides, unstable pressure at the reducer and clogging of the liquid-phase filters with black dust.
Polish interlaboratory studies (the Oil and Gas Institute, 2010–2011) showed a lack of repeatability and reproducibility of the results of this determination for part of the LPG on the market: the same samples gave classes from 1 to 3 in different accredited laboratories, and the degree of corrosion of the same sample changed with time. As possible causes were indicated an incorrect visual interpretation of the class, incorrect execution (a badly filled bomb, a wrong temperature, badly cleaned strips, failure to wet the bomb with water) and the presence in the LPG of foreign substances (amines, alcohols, elemental sulfur); corrosion inhibitors (fatty amines, imidazolines) in ppm amounts mask the corrosion without changing the composition.
Method principle
Polished copper strips are immersed for 1 h in 100 ml of a water-saturated sample of LPG at a temperature of 40 °C, in a "test bomb" intended for corrosion tests, placed in a water bath. After that time the strip is taken out of the bath and the corrosiveness of the LPG is assessed by comparing the strip with the copper strip corrosion standards and determining the class of corrosion: class 1 (slight tarnish: a light orange colour, almost as freshly polished, dark orange), class 2 (moderate: claret, light violet, multicoloured with a violet-blue or silver film on a claret ground, silver, brassy or gold), class 3 (dark: a violet film on a brassy ground, multicoloured with red and green patches — peacock, not grey), class 4 (corrosion: transparent black, dark grey or brown with green patches, graphite or matt black, glossy black).
Applications
- Quality control of LPG autogas according to PN-EN 589 (class 1 acceptable)
- Control of LPG in the distribution chain (no corrosion at the producer, corrosiveness often found in distribution)
- Diagnostics of the causes of failures of vehicle gas installations (injectors, reducers, filters)
- Interlaboratory and proficiency testing of LPG laboratories
Key parameters
| Parameter | Value |
|---|---|
| Test conditions | 1 h, 40 °C, a water bath |
| Sample | 100 ml of water-saturated LPG in the test bomb |
| Strip | Copper, freshly polished |
| Assessment | Visual against the corrosion standards: classes 1–4 (1 = no corrosion) |
| The requirement of PN-EN 589 | Class 1 |
| Nature of the result | Qualitative — no statistical assessment (z-score) in interlaboratory studies |
Standard
- Standard number
- PN-EN ISO 6251:2001
- Title (PL)
- Skroplone gazy węglowodorowe — Korodujące działanie na miedź — Badanie na płytce miedzianej
- Title (EN)
- Liquefied petroleum gases — Corrosiveness to copper — Copper strip test
Step-by-step procedure
-
Preparation of the strip
Polish the copper strip immediately before the test; do not touch the surface.
-
Filling of the bomb
Wet the bomb with water and fill it with 100 ml of the water-saturated LPG under test; place the strip in the bomb and close it tightly.
-
Exposure
Place the bomb in the water bath at a temperature of 40 °C for 1 h.
-
Removal of the strip
After 1 h release the gas safely and take the strip out of the bomb.
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Assessment
Compare the strip with the corrosion standards and determine the class 1–4 (discolouration, film, pitting); for autogas only class 1 is acceptable.
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Test report
Give the class of corrosion with a reference to PN-EN ISO 6251, the conditions (1 h, 40 °C) and the identification of the sample; in doubtful cases let more than one person make the assessment.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Test bomb for the corrosion of LPG | A pressure vessel for copper strip corrosion tests, filled with 100 ml of LPG | — |
| Water bath | Maintaining 40 °C | — |
| Copper strips and polishing materials | Freshly polished before the test | — |
| Copper strip corrosion standards | A colour scale with classes 1–4 for comparison | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Water | — | For saturating the LPG and wetting the bomb (failure to wet the bomb with water was indicated as a cause of discrepancies) |
Health and safety (OHS)
- LPG is extremely flammable and under pressure — fill and empty the bomb in a ventilated place free of sources of ignition, with earthing
- The test bomb under the vapour pressure of LPG at 40 °C — use only sound pressure vessels, open them after the pressure has been released
- Hydrogen sulfide in LPG is toxic — release the gas into a fume hood
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 6251:2001 — “Liquefied petroleum gases — Corrosiveness to copper — Copper strip test”.
How does this method work?
Polished copper strips are immersed for 1 h in 100 ml of a water-saturated sample of LPG at a temperature of 40 °C, in a "test bomb" intended for corrosion tests, placed in a water bath. After that time the strip is taken out of the bath and the corrosiveness of the LPG is assessed by comparing the strip with the copper strip corrosion standards and determining the class of corrosion: class 1 (slight tarnish: a light orange colour, almost as freshly polished, dark orange), class 2 (moderate: claret, light violet, multicoloured with a violet-blue or silver film on a claret ground, silver, brassy or gold), class 3 (dark: a violet film on a brassy ground, multicoloured with red and green patches — peacock, not grey), class 4 (corrosion: transparent black, dark grey or brown with green patches, graphite or matt black, glossy black).
What is the measuring range and accuracy?
Test conditions: 1 h, 40 °C, a water bath; Sample: 100 ml of water-saturated LPG in the test bomb; Strip: Copper, freshly polished; Assessment: Visual against the corrosion standards: classes 1–4 (1 = no corrosion).
How long does the test take?
The times given for the individual stages add up to approximately 1 h. The procedure comprises 6 steps.
What equipment is required?
Test bomb for the corrosion of LPG, Water bath, Copper strips and polishing materials, Copper strip corrosion standards.
Where is this test used?
Quality control of LPG autogas according to PN-EN 589 (class 1 acceptable); Control of LPG in the distribution chain (no corrosion at the producer, corrosiveness often found in distribution); Diagnostics of the causes of failures of vehicle gas installations (injectors, reducers, filters); Interlaboratory and proficiency testing of LPG laboratories.
What safety precautions apply?
LPG is extremely flammable and under pressure — fill and empty the bomb in a ventilated place free of sources of ignition, with earthing; The test bomb under the vapour pressure of LPG at 40 °C — use only sound pressure vessels, open them after the pressure has been released; Hydrogen sulfide in LPG is toxic — release the gas into a fume hood.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 6251.