The copper strip corrosion test is the internationally recognized method for evaluating the corrosiveness of diesel fuel additives and additive-treated diesel fuel toward copper and copper alloys used in fuel injection systems. Performed in accordance with ASTM D130, ISO 2160, and GB/T 5096, the test detects active sulfur compounds and other corrosive species, assigning a visual classification rating from 1a to 4c. This article explains the technical principles, sample types, testing items, procedures, rating criteria, and industrial applications of copper strip corrosion testing for diesel fuel additives.
Technical Overview
The copper strip corrosion test evaluates the tendency of a petroleum product—in this case, diesel fuel additives and additive-treated diesel fuel—to corrode pure copper under standardized temperature and time conditions. Corrosive behavior is driven primarily by active sulfur compounds, including elemental sulfur, hydrogen sulfide, mercaptans, disulfides, and polysulfides, which react with copper to form copper sulfide films ranging from light tarnish to black corrosion. Organic acids, peroxides generated by fuel oxidation, and certain degraded additive components can also contribute to strip discoloration.
The test is particularly relevant for diesel fuel additives because modern ultra-low-sulfur diesel (ULSD) production removes most naturally occurring corrosion-protective species along with the sulfur, leaving additives responsible for both performance and fuel system compatibility. Fuel injection pumps, unit injectors, transfer pumps, and governors contain copper, brass, and bronze parts that are directly exposed to the fuel; a corrosive additive formulation can therefore cause staining, deposits, and premature component wear in service.
In the standard procedure, a freshly polished copper strip is immersed in the sample and held at 50 °C for 3 hours—the reference condition for middle distillates in ASTM D130, ISO 2160 (EN ISO 2160), GB/T 5096, and JIS K 2513. After exposure, the strip is washed, dried, and compared with the ASTM Copper Strip Corrosion Standard to assign one of fourteen classification ratings from 1a (slight tarnish) to 4c (jet black corrosion). The result is a direct, visually documented index of the corrosiveness of the sample.
Testing Samples
Neat Additive Concentrates
As-supplied additive concentrates are tested to characterize the intrinsic corrosiveness of the formulation before it is blended into fuel. Typical products include cetane number improvers based on 2-ethylhexyl nitrate, lubricity improvers based on fatty acids and esters, cold flow improvers such as ethylene-vinyl acetate copolymers and polymethacrylates, detergent and dispersant packages, antioxidants, demulsifiers, and corrosion inhibitors. Because concentrates contain functional species at high concentration, testing is often performed on diluted samples at one or more treat rates in addition to the neat product.
Additive-Treated Diesel Fuel Blends
The most common industrial sample is base diesel fuel treated with the additive at the manufacturer's recommended treat rate, typically in the range of 50 to 1,000 mg/kg depending on the additive type. These blends represent the real-world exposure of copper components in the fuel system and provide the rating used for specification compliance and product release.
Reference and Base Fuels
Parallel testing of the unadditized base fuel is strongly recommended so that any rating change can be attributed to the additive rather than to the fuel itself. Comparative matrices may include fuels of different crude origin and sulfur levels, biodiesel blends from B5 to B30 covered by EN 590 and ASTM D7467, thermally or oxidatively aged fuels, and retained samples from field corrosion complaints.
Testing Items
Accredited third-party laboratories typically offer the following copper strip corrosion determinations for diesel fuel additives:
- Standard copper strip corrosion rating of additive-treated diesel fuel, 3 hours at 50 °C, per ASTM D130, ISO 2160, and GB/T 5096
- Copper strip corrosion of neat additive concentrate, as supplied and at specified dilution ratios
- Elevated-severity testing, such as 3 hours at 100 °C, for severe service simulation or heavily additized products
- Comparative blank-versus-additized testing to quantify the additive's contribution to the rating
- Corrosion behavior after accelerated storage, thermal stress, or oxidation aging of the treated fuel
- Effectiveness verification of corrosion inhibitors and metal passivators added to protect copper surfaces
- Silver strip corrosion screening as a complementary check for fuel system components plated with silver
Testing Methods
Strip Material and Preparation
The test strip is cut from smooth-surfaced, soft-temper electrolytic copper of at least 99.9 percent purity, with nominal dimensions of 12.5 mm width, 50 to 75 mm length, and 1.5 to 3.0 mm thickness. Immediately before the test, all surface blemishes are removed by polishing with silicon carbide paper or cloth of progressively finer grit, finishing with a 105 μm (240 grit) abrasive. The polished strip is handled only with stainless steel or PTFE-coated forceps and must not touch the skin or any contaminating surface, since fingerprints and residual particles cause false tarnish patterns.
Exposure Procedure
Approximately 30 mL of the homogeneous sample is placed in a chemically clean, dry test tube; the polished strip is slid in gently, and the tube is closed with a vented stopper. The tube is immersed in a constant-temperature bath maintained at 50 °C ± 1 °C for middle distillates and held for 3 hours ± 5 minutes. After exposure, the strip is withdrawn, drained, washed promptly with a suitable volatile solvent such as isooctane, and blotted dry for immediate inspection. Any delay between polishing, immersion, and evaluation can alter the visual rating, so the sequence is performed without interruption.
Rating and Classification
The washed strip is viewed against the ASTM Copper Strip Corrosion Standard, a set of color reproduction plates, under standardized lighting conditions. Fourteen ratings are assigned within four categories:
- 1a to 1b — slight tarnish, light to dark orange, essentially equivalent to a freshly polished strip
- 2a to 2c — moderate tarnish, claret red, lavender, brassy, or multicolored overcast
- 3a to 3b — dark tarnish, magenta or rainbow-colored, with no gray deposits
- 4a to 4c — corrosion, transparent black, graphite, or glossy jet black surface
For specification purposes, EN 590 automotive diesel requires a rating of Class 1, GB 19147 diesel requires no worse than Class 1, and ASTM D975 diesel grades permit up to No. 3, although commercial contracts frequently impose No. 1 or better. A well-formulated additive must not deteriorate the rating of the base fuel; in practice, the additized fuel should retain a Class 1 finish.
Method Variations for Additive Evaluation
Because additives are judged on their effect at the point of use, laboratories frequently run matrix designs covering multiple treat rates, several base fuels, and pre- and post-aging conditions. Extended-temperature exposure at 100 °C, thermal stability sequences, and combined tests with oxidation procedures help reveal latent corrosive species that form during storage. Repeat determinations on independently prepared strips provide a check on repeatability, which for most middle distillates is within one rating class.
Applications
Copper strip corrosion testing of diesel fuel additives supports the complete product life cycle, from formulation research to field troubleshooting:
- Formulation screening and optimization of additive packages for copper compatibility
- Quality control release testing of additive production batches and additized fuel shipments
- Demonstration of compliance with EN 590, ASTM D975, GB 19147, and related diesel specifications
- OEM and engine manufacturer approval programs for aftermarket and refinery additives
- Investigation of fuel system corrosion, injector staining, and filter deposit field complaints
- Storage stability studies of additized ULSD and biodiesel blends in tanks and distribution systems
- Support of lubricity, detergency, and cold flow development programs where additive interactions are critical
FAQ
What Is the Copper Strip Corrosion Test?
It is a standardized visual method in which a polished copper strip is immersed in a fuel or additive sample at a controlled temperature and time, then compared with color standards to rate the degree of tarnish or corrosion. For diesel and middle distillates, the reference condition is 3 hours at 50 °C according to ASTM D130, ISO 2160, and GB/T 5096.
Why Do Diesel Fuel Additives Need This Test?
Additives can introduce or generate active sulfur species, acids, and peroxides that attack copper and copper alloys in fuel pumps and injectors. The test verifies that an additive package does not increase the corrosiveness of the treated fuel and remains compatible with copper-containing fuel system components.
What Rating Counts as a Pass?
Class 1 (1a or 1b) is required by EN 590 and GB 19147, while ASTM D975 allows up to No. 3. Most additive specifications and customer agreements demand Class 1 to leave a margin against discoloration in service.
What Causes a Dark or Failing Strip?
Common causes include elemental sulfur contamination, mercaptans and polysulfides in additive intermediates, oxidation products such as peroxides and organic acids formed during storage, trace hydrogen sulfide, and insufficient corrosion inhibitor dosage. Poor strip preparation or handling can also produce false positives, which is why polishing discipline and blank controls are essential.
How Should Samples Be Submitted for Testing?
Submit at least 500 mL of homogenized treated fuel, or 100 mL of concentrate together with the recommended treat rate and base fuel details. Accredited third-party laboratories can also perform the blending, run the unadditized blank in parallel, and provide photographic documentation of the rated strips.
Can a Corrosive Fuel or Additive Be Corrected?
In many cases, yes. Options include adjusting the corrosion inhibitor or metal passivator concentration, removing sulfur species by adsorptive treatment or fuel sweetening, improving antioxidant protection to suppress peroxide formation, and tightening raw material specifications for sulfur-containing intermediates. Verification by repeat copper strip testing is always required after any remediation step.