Why an EP Grease That Passed a Copper-Strip Test Can Still Ruin a Bronze Bushing

Last verified: 2026-08-02

Consider a common maintenance scenario — illustrative, not a specific documented incident: a bronze worm wheel running packed in an EP grease chosen partly because its data sheet showed a passing copper-strip corrosion result. When the housing is opened for inspection, the grease has darkened and the bronze surface underneath shows a soft, etched sheen where a solid, unmarked surface should be. The data sheet was accurate. That is exactly the problem, not the reassurance it looks like. A passed copper-strip result tells a specifier almost nothing about whether this particular EP grease is safe on this particular bronze worm wheel — because the test that passed it may not even claim, in its own written scope, to cover grease at all, and no standard found requires an EP grease to declare yellow-metal compatibility in the first place.

Practitioners describe it as green goo, or say the grease "ate" the bushing. Formally, the failure has a name and a chemistry — a sulfurized extreme-pressure additive doing to a copper alloy what it was built to do to steel. The data sheet was clean. That is the finding this article works from: the standards ladder sitting under the phrase "passed copper corrosion test" is narrower, and more fragmented, than the phrase suggests.

The additive doing exactly what it was designed to do

Sulfurized EP additives are not one chemistry. They split into active and inactive sulfur carriers. Active carriers hold labile polysulfide bonds that release sulfur and react even at relatively low temperatures. Inactive carriers hold more stable disulfide or mono-sulfide bonds that react mainly at higher, localised temperatures — the kind generated at asperity contact under boundary lubrication.

On steel, that reactivity is the whole point. Sulfur — often formulated alongside phosphorus anti-wear additives — reacts with the metal to build a sacrificial iron-sulfide or iron-phosphide film at the points of boundary contact, a film that wears away instead of the steel underneath. It is designed to react.

Put the same additive package against a copper alloy instead of steel, and the reaction does not stop being a reaction. It builds copper sulfide instead of a protective film — a corrosion compound that tarnishes and etches the surface, and that can itself flake away as wear debris once it forms.

None of this is the additive malfunctioning. Copper alloys are simply far more reactive with active sulfur than steel is — it is precisely that heightened reactivity that makes copper the indicator metal in the copper-strip family of tests in the first place. The metal underneath the grease is just the wrong one for that chemistry. Heat makes the reaction markedly worse, with no single verified threshold at which it switches on, only a direction of travel.

Three test methods, one shared rating scale

Three standards sit under the phrase "copper corrosion test," and they do not test the same thing.

ASTM D4048-22, active since 5 September 2022, is titled Standard Test Method for Detection of Copper Corrosion from Lubricating Grease. It is written for grease, and it is written for static conditions — the grease sitting still against a copper strip, not circulating through a bearing under load. The method's own limitation language says as much: there are no established correlations with actual field service, which operates mostly under dynamic conditions. It also does not measure the grease's ability to inhibit corrosion arising from causes other than the grease itself, or the grease's long-term stability in copper's presence.

ASTM D130-26, active 12 March 2026, is titled Standard Test Method for Corrosiveness to Copper from Petroleum Products by Copper Strip Test. Its scope names aviation gasoline, aviation turbine fuel, automotive gasoline, Stoddard solvent, kerosine, diesel fuel, distillate fuel oil, lubricating oil, and natural gasoline, plus a catch-all for other hydrocarbons with a vapor pressure not exceeding 124 kPa (18 psi) at 37.8 °C — a liquids-and-fuels threshold that does not describe a semi-solid grease. Grease is not named on that list, and it is not swept in by the catch-all either.

That distinction is not academic. It is the same method our automotive gear oil guide to GL-4 and GL-5 correctly cites for oil-side yellow-metal risk, because gear oil sits squarely inside D130's scope. Carry that answer across to a grease purchase and it stops being correct.

DIN 51811:2017-05, the current edition superseding a withdrawn 1991-09 version, is titled Testing of lubricants; testing of corrosiveness to copper of greases; copper strip tarnish test. This is the method most often printed on non-US supplier data sheets.

All three are read against the same ASTM Copper Strip Corrosion Standard — bands 1a–1b slight tarnish, 2a–2e moderate, 3a–3b dark, 4a–4c corrosion. That shared scale is exactly why a D130 rating and a D4048 rating look interchangeable on a data sheet. They are not testing the same product form.

No standard makes the declaration mandatory

Search for a standard that requires an EP grease to state its yellow-metal compatibility as a condition of being called "EP," and none turns up in the standards checked.

DIN 51502, the designation-code standard for greases, assigns the letter "P" to an EP or AW-additised grease on the presence of an EP or AW additive — this is how the code is consistently documented across published designation-code references, though DIN's own full text sits behind a paywall and was not read directly for this article. The coding carries no copper-corrosion clause in any reference checked. The letter tells a buyer that some EP additive is present, not which chemistry, and not what it will do to bronze.

Even within one organisation, the requirement is not consistent — and NLGI's own documentation is explicit about it. NLGI's High-Performance Multiuse (HPM) specification, aimed at multiuse industrial applications, lists copper corrosion by ASTM D4048 among the tests it added beyond the older classification. The older GC-LB Performance Classification, published as ASTM D4950 and originally conceived as an automotive chassis and wheel-bearing specification, carries no copper-corrosion requirement at all — and it remains very much alive, with over 300 products licensed under it by 85 different companies.

That disagreement is itself the evidence. If copper compatibility were a settled baseline for calling a grease "EP," one organisation would not need two different answers.

Two data sheets that read identically

Two greases can both read "NLGI 2, EP, lithium complex" — the grade and thickener terms covered in our EP grease primer — and differ completely on yellow-metal compatibility, because none of those three descriptors says which EP additive chemistry is inside. NLGI grade describes consistency. Thickener family describes the soap or complex holding the oil. "EP" describes only that an extreme-pressure additive is present, not whether it carries active or inactive sulfur.

The one line on a data sheet that speaks to yellow-metal risk is a stated copper-corrosion result together with the method that produced it. Absent that line — method named, rating stated — the data sheet is silent on the question, not reassuring.

Where the reaction meets real hardware

The components at risk are not exotic. Bronze worm gears carry the most consistently documented case, alongside bronze bushings, brass bearing cages and retainers, copper-alloy thrust washers, and marine or pump components running on yellow-metal wear surfaces.

A worm gearset pairing a bronze wheel against a steel worm puts a copper alloy directly in the path of whatever EP chemistry the grease carries — the same underlying reason sulfur-driven attack on yellow-metal synchronizers drives the incident lane covered in mixing GL-4 and GL-5 gear oil, on the oil side.

When the symptoms overlap

Grease darkening and surface pitting or etching are shared symptoms across failure modes with nothing chemically in common. Chemical attack from an active-sulfur EP additive on copper is one path to a dark, etched surface and discoloured grease. Electrical arcing or EDM erosion is another — it blackens grease independently, through carbon particulate thrown off by the arc, with no sulfur chemistry involved at all. Water-driven corrosion is a third path to the same visual result.

What is documented is the overlap in appearance. What is not documented is that inspectors routinely misread one for another — that claim is not sourced, and this article does not make it. The practical point is narrower, and just as useful: a dark, etched yellow-metal surface does not, by sight alone, tell you which of three unrelated mechanisms produced it.

What to put in writing before the grease ships

The acceptance rating — what tarnish band is tolerable — is supplied by a performance category, an OEM specification, or the purchase agreement itself, never by the test method. ASTM D4048 is a detection method and contains no pass/fail limit of its own; neither does D130, on the products inside its own scope.

NLGI's HPM specification shows exactly where such a limit does live: it sets a rating of 1B on ASTM D4048, run for 24 hours at 100 °C, as part of its own core specification. That figure governs greases certified to HPM. It is not a general threshold for grease, and a product outside that certification carries no equivalent number unless someone writes one into the contract.

Four variables decide the real case, and the method alone will not supply any of them: which copper alloy is actually in the application, since bronze, brass and other copper alloys behave differently; the operating temperature range, since the sulfur reaction is markedly accelerated by heat; whether the exposure is static or circulating, given that D4048 does not correlate to dynamic service; and which sulfur chemistry — active or inactive carrier — the specific formulation uses.

So specify the method by name for the product form in question: ASTM D4048 or DIN 51811 for grease, not D130. Request the rating band achieved, not merely the word "pass." Name the alloy and the operating temperature in the specification, because they are what the rating has to be judged against. And put the requirement in the purchase specification itself, since no designation code supplies it automatically — a documented answer, rather than an assumed one. Where sourcing is the next step, that request can go to a supplier through the platform's RFQ process alongside the rest of the data sheet.

Frequently asked questions