Oil corrosivity testing determines whether petroleum products contain substances capable of attacking the metal components of engines, fuel systems, transformers, and storage infrastructure. Core test items include copper strip corrosion, acid number, corrosive sulfur, and rust-preventing characteristics, governed by internationally recognized standards such as ASTM D130, ISO 2160, and GB/T 5096. Accredited third-party laboratories perform these analyses to support refinery quality control, product certification, and equipment protection across the fuel, lubricant, and power industries.
Technical Overview
Corrosivity is one of the most critical quality and safety attributes of any oil product, whether a light distillate fuel, a finished lubricant, or an electrical insulating oil. Corrosive behavior is caused primarily by active sulfur compounds, including elemental sulfur, hydrogen sulfide, mercaptans, and certain disulfides, as well as by organic acids such as naphthenic acids, strong inorganic acids, alkaline contaminants, and acidic oxidation products that form during storage or in service.
When corrosive species are present, they attack copper, silver, brass, bronze, steel, and bearing alloys. Consequences include surface staining and metal loss on yellow-metal components, plugging of filters and fuel nozzles by corrosion debris, seizure of fuel-metering and hydraulic valves, accelerated bearing wear, and, in electrical systems, copper sulfide deposition that compromises dielectric performance. Because these failure modes develop quickly and are difficult to reverse, virtually every national and international fuel and lubricant specification contains at least one mandatory corrosivity limit.
Corrosivity testing therefore serves three purposes: verifying that refined products are non-corrosive to the metals they will contact, monitoring the condition of in-service oils, and diagnosing the root cause of component damage. A complete assessment typically combines a direct metal-contact test, such as the copper strip test, with chemical measurements of acidity and active sulfur.
Testing Samples
Aviation and Automotive Fuels
Aviation turbine fuel (jet fuel), aviation gasoline, motor gasoline, and automotive diesel are routinely tested for copper strip corrosion and mercaptan sulfur because fuel pumps, metering units, and engine fuel controls contain copper and silver alloys. Jet fuel specifications generally require a copper strip rating no worse than slight tarnish, typically 1a or 1b.
Lubricating Oils and Hydraulic Fluids
Turbine oils, hydraulic fluids, gear oils, compressor oils, and refrigeration oils are evaluated for corrosiveness to copper and for acid number, both in new condition and after oxidation stability testing, since oxidation continuously generates acidic products that increase corrosivity over the service life.
Electrical Insulating Oils
Transformer and switchgear oils are tested for acid number and corrosive sulfur compounds. Certain sulfur species occurring naturally or introduced by passivation can produce copper sulfide on windings, degrading insulation performance, which is why IEC 60296 places strict limits on corrosive sulfur in new insulating oils.
Marine Fuels, Base Oils, and Refinery Streams
Residual marine fuels, crude oils, intermediate streams, base oils, and additive concentrates are analyzed for acid number, naphthenic acid content, and sulfur corrosivity to manage corrosion risk in refinery units, tankage, pipelines, and marine engines.
Greases and Semi-Solid Products
Lubricating greases are tested for copper strip corrosion and for corrosion-preventive performance against ferrous metals in the presence of water, using standardized bearing rust and water-corrosion procedures.
Testing Items
A comprehensive corrosivity evaluation typically covers the following parameters:
- Copper strip corrosion rating of the sample in contact with polished copper
- Silver strip corrosion, specifically required for aviation turbine fuels
- Total acid number (TAN) and strong acid number (SAN)
- Water-soluble acids and alkalis
- Corrosive sulfur species: elemental sulfur, hydrogen sulfide, and mercaptan sulfur
- Total sulfur content as supporting characterization
- Rust-preventing characteristics of inhibited mineral oils in the presence of water
- Copper corrosion and ferrous corrosion of lubricating greases
- Acid number after oxidation testing, linking corrosivity to oxidative degradation
Testing Methods
Copper Strip Corrosion Test
The copper strip test is the universal screening method for corrosive sulfur compounds. A freshly polished copper strip is immersed in a fixed volume of sample and heated at a specified temperature for a defined period, commonly 50 °C for 3 hours for automotive fuels, 100 °C for 2 hours for aviation turbine fuel, and 100 °C for 3 hours for many lubricating oils. The exposed strip is washed and compared against the standard corrosion scale, producing a rating from 1 (slight tarnish) to 4 (corrosion) with letter sub-designations such as 1a, 1b, 2a, and 3b. Primary reference standards include ASTM D130, ISO 2160, IP 154, EN ISO 2160, JIS K 2513, and GB/T 5096.
Silver Strip Corrosion Test
Because some jet engine fuel-system components are silver-plated, aviation fuels are additionally tested with a polished silver strip under the conditions specified by the applicable fuel standard. Recognized methods include IP 227 and SH/T 0023, and results are reported by comparison with a reference classification of silver surface discoloration.
Acid Number Determination
Acid number quantifies acidic constituents that contribute to corrosion of ferrous and yellow metals. In ASTM D664 and GB/T 7304, the sample is dissolved in a mixed toluene–isopropanol–water solvent and titrated potentiometrically with alcoholic potassium hydroxide to determine total and strong acid number in mg KOH/g. ASTM D974, ISO 6618, and GB/T 4945 apply color-indicator titration for oils pale enough for visual end-point detection, while GB/T 264 remains a traditional volumetric method. New finished products typically exhibit very low acid numbers, and a significant rise in an in-service oil is a key condition-monitoring alarm.
Corrosive Sulfur Analysis
The doctor test (ASTM D4952) provides a qualitative screen for mercaptans and hydrogen sulfide, while mercaptan sulfur is quantified by potentiometric titration according to ASTM D3227. Total sulfur is measured by ultraviolet fluorescence (ASTM D5453) or wavelength-dispersive X-ray fluorescence (ASTM D2622). For insulating oils, corrosive sulfur behavior is specifically addressed by DIN 51353 and the IEC 62697 series, which detect and quantify potentially corrosive sulfur compounds.
Rust-Preventing and Grease Corrosion Tests
For inhibited oils, corrosion protection in the presence of water is evaluated by ASTM D665 and GB/T 11143, in which a polished steel rod is immersed in a stirred oil–water mixture at 60 °C for 24 hours and inspected for rust. Greases are assessed by ASTM D4048 for copper corrosion, ASTM D1743 for corrosion-preventive properties on bearings, and the EMCOR water corrosion test ASTM D6138.
Supporting Instrumental Techniques
Modern laboratories complement classical methods with X-ray fluorescence for sulfur mapping, ion chromatography for chloride and sulfate contaminants that accelerate corrosion, inductively coupled plasma spectroscopy for dissolved corrosion metals such as copper, iron, lead, and tin, and infrared spectroscopy for tracking oxidation by-products in used oils.
Applications
- Refinery quality control, product release, and blend certification
- Aviation fuel receipt, storage surveillance, and recertification programs
- Lubricant formulation research, additive evaluation, and batch conformity testing
- In-service transformer and turbine oil monitoring for asset health management
- Marine fuel bunkering verification and contractual dispute resolution
- Import and export commodity inspection and arbitration analysis
- Root-cause failure analysis of fuel pumps, bearings, valves, and turbine hardware
FAQ
What does a copper strip corrosion rating mean?
The scale runs from 1 to 4 with letter sub-designations: 1 indicates slight tarnish, 2 moderate tarnish, 3 dark tarnish, and 4 true corrosion. Most fuel and lubricant specifications require a result of 1a or 1b, or at most 2a, for acceptance.
Why can an oil with low total sulfur still fail the copper strip test?
Corrosivity depends on the chemical form of sulfur, not only its quantity. A few milligrams per kilogram of active sulfur, such as elemental sulfur, hydrogen sulfide, or free mercaptans, can blacken a copper strip even when total sulfur is well within specification.
Does a passing copper strip result guarantee long-term corrosion protection?
No. The copper strip test is a short-term screen for active sulfur. Long-term protection, especially against water and acidic oxidation products, must be confirmed through acid number trending, rust-prevention tests, and scheduled in-service monitoring.
How are acid number and corrosivity related?
Acid number measures the total acidic constituents in an oil, which correlates with the potential to corrode ferrous metals and to promote sludge formation. A steadily increasing acid number in service signals oxidation and usually precedes measurable corrosion damage.
How often should corrosivity testing be performed?
New products should be tested against the applicable specification at production and at custody transfer. In-service oils, such as transformer and turbine oils, are typically tested annually or semi-annually depending on equipment criticality, with frequency increased whenever acid number, moisture, or color trends change unexpectedly.