Liquefied Petroleum Gas (LPG) Copper Strip Corrosion Test: Principle, Standards, and Procedure

The liquefied petroleum gas (LPG) copper strip corrosion test is a essential quality control procedure used to detect corrosive sulfur compounds, such as hydrogen sulfide and mercaptans, that can damage copper and brass components in storage, distribution, and end-use equipment. Performed in accordance with recognized standards such as ASTM D1838, ISO 6251, and SH/T 0232, the test determines whether an LPG sample meets the corrosion limits specified in product specifications like GB 11174, ASTM D1835, and EN 589. This article explains the technical principle, sample requirements, test methods, classification system, and practical applications of the LPG copper strip corrosion test.

Technical Overview

The copper strip corrosion test is the most widely used screening method for evaluating the corrosiveness of liquefied petroleum gas toward copper and copper alloys. During refining, storage, or transportation, LPG can become contaminated with so-called active sulfur compounds, primarily hydrogen sulfide (H2S), elemental sulfur, and reactive mercaptans. Even at low concentrations, these species react with metallic copper surfaces to form copper sulfide films that discolor, tarnish, or blacken a polished copper strip.

In the test, a freshly polished copper strip is sealed inside a corrosion test bomb (pressure cylinder) together with a liquid sample of LPG. The cylinder is immersed in a water bath maintained at a controlled temperature, typically 37.8 °C (100 °F) per ASTM D1838 or 40 °C per ISO 6251 and SH/T 0232, for a contact period of one hour. After exposure, the strip is removed, gently cleaned, and immediately compared against standardized copper strip corrosion color standards. The degree of tarnish or corrosion is reported as a classification ranging from 1a (slight tarnish, essentially the color of a freshly polished strip) to 4c (glossy black, heavy corrosion).

The significance of the test is substantial. Copper, brass, and bronze are used extensively in LPG pressure regulators, valves, meters, burner nozzles, and appliance fittings. Corrosive sulfur species can pit these components, generate deposits, cause seal leakage, and ultimately lead to equipment failure or safety incidents. The test therefore serves as a rapid, sensitive pass/fail indicator of whether the gas has been adequately treated, for example by amine sweetening or mercaptan removal units, and whether it complies with commercial and regulatory specifications.

Testing Samples

Commercial LPG (Propane, Butane, and Mixtures)

Typical samples include commercial-grade propane (such as HD-5 propane), commercial butane, and propane-butane blends supplied for residential, commercial, and industrial use. These products are routinely tested at refineries, import/export terminals, and cylinder-filling plants to verify conformance with specifications such as ASTM D1835 and GB 11174.

Automotive LPG (Autogas)

Automotive LPG is subject to stricter quality requirements because it feeds engine fuel systems containing precision copper-alloy parts. Specifications such as EN 589 impose a copper corrosion requirement, and accredited third-party laboratories frequently perform the copper strip test as part of autogas certification and dispute resolution.

Refinery, Terminal, and Process Stream LPG

Intermediate streams from gas fractionation units, sweetening unit outlets, and storage spheres are tested as process quality checks. Monitoring the copper strip result upstream allows operators to detect breakthrough of hydrogen sulfide or mercaptans before the product is transferred or shipped.

Field Samples in Pressure Cylinders

LPG is volatile and must be sampled into purpose-designed stainless steel sample cylinders that maintain the product in its liquid phase. The cylinder should be purged with the sample, filled with an adequate volume of single-phase liquid, kept cool, and delivered for testing as quickly as possible to prevent compositional changes and loss of volatile corrosive species. Because LPG is highly flammable and is handled under pressure, sampling must follow strict safety procedures by trained personnel.

Testing Items

The primary testing item is the copper strip corrosion classification of the LPG sample under the specified conditions of temperature and contact time, for example one hour at 37.8 °C or 40 °C. The result is reported as the matching classification designation, such as 1a, 1b, 2a, 3b, or 4c, together with a statement of conformance or non-conformance against the applicable specification limit.

In practice, the copper strip corrosion test is often combined with related sulfur and quality tests to fully characterize the sample, including:

• Hydrogen sulfide content, for example by the ASTM D2420 lead acetate method
• Total sulfur content
• Mercaptan sulfur content
• Residue on evaporation and visible contamination of the residue
• Vapor pressure and compositional analysis of the gas

Testing Methods

ASTM D1838

ASTM D1838, Standard Test Method for Copper Strip Corrosion by Liquefied Petroleum (LP) Gases, is the dominant method in North America and in many international contracts. A polished copper strip is placed in a stainless steel corrosion test cylinder, which is then charged with liquid sample and immersed in a water bath at 37.8 °C (100 °F) for one hour. Two strips are normally exposed simultaneously as a built-in check. The exposed strips are compared with the ASTM copper strip corrosion standards, which are identical to those used in the ASTM D130 method, and the classification is reported.

ISO 6251

ISO 6251, Liquefied petroleum gases and dimethyl ether and dimethyl ether blends and LPG blends to dimethyl ether or to LPG is superseded in scope by the current edition, which specifies the copper strip test for assessing the corrosiveness of LPG to copper. The exposure is performed at 40 °C for one hour in a pressure vessel, and the resulting tarnish level is classified against the standard reference strips. The method is referenced by European and international LPG specifications, including EN 589 for automotive LPG.

SH/T 0232 and Chinese National Specification GB 11174

SH/T 0232 is the Chinese petrochemical industry standard for the copper strip corrosion test of LPG, and its conditions (40 °C, one hour) align closely with ISO 6251. The Chinese national product standard GB 11174 for liquefied petroleum gas specifies that copper strip corrosion shall be no worse than Class 1. These standards are routinely applied for domestic trade, regulatory supervision, and import/export inspection in the Chinese market.

General Test Procedure

Although details vary slightly between standards, the essential workflow is consistent:

• Polish the copper strip successively with fine silicon carbide paper or cloth and alumina abrasive until a bright, uniform, defect-free surface is obtained, handling the strip only with oil-free filter paper or forceps
• Immediately place the strip or strips into the clean corrosion test cylinder and assemble the vessel
• Purge the cylinder with a small amount of sample, then charge it with liquid LPG so that the strips are fully wetted by the liquid phase
• Immerse the cylinder in the water bath at the specified temperature and hold for one hour
• Depressurize safely, remove the strips promptly, wipe them clean with a suitable solvent and lint-free material
• Compare the exposed strips with the standard corrosion color references under good lighting and report the classification without delay, since the surface can continue to change on standing

Classification of Results

The standard classification scale is divided into four broad categories:

• Class 1 (1a, 1b), slight tarnish: light to dark orange tones close to a freshly polished strip; considered acceptable by most specifications
• Class 2 (2a through 2e), moderate tarnish: red, purple, lavender, and brassy colorations indicating a noticeable reaction with sulfur species
• Class 3 (3a, 3b), dark tarnish: magenta or multicolored iridescent films covering the strip
• Class 4 (4a, 4b, 4c), corrosion: transparent, gray-black, or glossy black films representing severe attack by corrosive sulfur compounds

Factors Affecting Accuracy

The test is sensitive to technique. Common sources of error include inadequate or inconsistent strip polishing, fingerprints or moisture on the strip, free water in the sample, inaccurate bath temperature, incorrect exposure time, delays between exposure and evaluation, and aging or contamination of the sample cylinder. Meticulous surface preparation, strict temperature control, and prompt reading are essential for reliable, repeatable classifications.

Applications

The LPG copper strip corrosion test is applied throughout the gas value chain, from production to the end user:

• Refinery and gas plant quality control, including verification of amine treating and sweetening unit performance
• Import and export inspection at ports and terminals, providing authoritative reports for customs clearance and trade settlement
• Cylinder-filling plant and distributor quality assurance programs
• Automotive LPG (autogas) conformity assessment against EN 589 and equivalent specifications
• Verification of incoming material quality by appliance and equipment manufacturers
• Commercial contract specification checks and resolution of quality disputes
• Regulatory supervision and market surveillance by authorities, with testing performed by accredited third-party laboratories
• Research and development of desulfurization processes, corrosion inhibitors, and additive formulations

FAQ

What does the LPG copper strip corrosion test actually detect?

The test detects the presence of corrosive, chemically active sulfur compounds, chiefly hydrogen sulfide, elemental sulfur, and reactive mercaptans. It does not measure total sulfur content; a gas can contain a significant concentration of non-reactive sulfur compounds such as sulfides and thiophenes and still pass the copper strip test.

Which classification is required for compliant LPG?

Most specifications are strict: GB 11174 requires no worse than Class 1, EN 589 for automotive LPG requires Class 1 per ISO 6251, and ASTM D1835 for commercial LP gases similarly limits the result to Class 1. A result of Class 2 or worse generally indicates the presence of corrosive sulfur compounds and non-conformance.

How does this test differ from the ASTM D130 copper strip test?

ASTM D130 is intended for liquid petroleum products such as fuels, lubricants, and solvents, which can be tested in open tubes at atmospheric pressure. Because LPG is volatile and must be kept liquefied under pressure, ASTM D1838 uses a sealed corrosion test cylinder and a shorter, well-defined exposure of one hour at 37.8 °C, while the classification standards and reference color boards are shared between the two methods.

Why would an LPG sample fail the test?

Common causes include carryover of hydrogen sulfide from incomplete sweetening, contamination with elemental sulfur from degraded scrubbing solutions or corrosion control chemicals, bacterial activity in storage systems generating sulfides, or contact with sulfur-containing materials during handling. Failing results should prompt investigation of the treatment process, tanks, hoses, and sampling equipment.

How much sample is needed and how should it be taken?

A few hundred milliliters of liquid LPG are typically sufficient, since the test cylinder itself is small. The sample must be drawn into a clean, purged stainless steel pressure cylinder in a way that preserves the liquid phase, avoids air and moisture ingress, and minimizes the time between sampling and testing. Trained personnel and appropriate safety precautions are mandatory due to the flammable, pressurized nature of the product.

Can the test quantify the hydrogen sulfide concentration?

No. The copper strip test is a qualitative to semi-quantitative screening method that reports a visual corrosion classification. If a quantitative value is required, complementary methods such as ASTM D2420 for hydrogen sulfide in LP gases, or instrumental total sulfur and speciated sulfur analyses, should be performed alongside the copper strip test.