Metal Bath Copper Strip Corrosion Tester

Theory & Test Method

The copper strip corrosion test is based on the fundamental chemical reactivity between active sulfur compounds present in petroleum products and the metallic copper surface. Petroleum fuels, lubricating oils, and related hydrocarbon products may contain a variety of sulfur species arising from crude oil processing, blending, or additive use. These sulfur compounds range from relatively inert forms (such as thiophenes and sulfides in adequately hydrotreated products) to highly reactive active sulfur species — including elemental sulfur, hydrogen sulfide, mercaptans (thiols), and polysulfides — that react aggressively with copper and copper alloys at elevated temperatures.

The corrosion mechanism involves two primary reaction pathways. First, direct chemical attack: reactive sulfur species react with the copper surface to form copper sulfide (Cu₂S, CuS) and copper mercaptide compounds, producing the characteristic dark gray, black, or multicolored tarnish observed on the tested copper strip. Second, oxidative corrosion acceleration: dissolved oxygen and acidic species in the petroleum sample can accelerate the copper sulfide formation reaction and contribute to additional surface tarnishing. Both mechanisms are active during the metal bath heating cycle of the copper strip corrosion test.

The ASTM D130 test procedure employs a freshly polished, reagent-grade copper strip of standardized dimensions (approximately 12.5 × 1.5 × 75 mm) as the test coupon. The surface of the copper strip is prepared by sequential abrasion with 240, 400, and 600-grit sandpaper, followed by solvent cleaning with absolute ethanol, to produce a bright, contaminant-free surface that is maximally reactive with corrosive sulfur compounds in the test sample. The prepared copper strip is immersed in a specified volume of the petroleum product sample in a sealed glass test tube or stainless steel pressure bomb, which is then placed in the TT-130J’s metal bath at the temperature prescribed by the applicable product standard.

After the specified test period, the copper strip is removed, washed with absolute ethanol to remove adhering sample, and immediately compared with the ASTM Copper Strip Corrosion Standard Color Chart under standard lighting conditions. The degree of tarnishing and discoloration is classified into one of four corrosion rating categories:

• Class 1 — Slight Tarnish: Fresh copper color, slight orange discoloration, or faint straw color. Indicates low or no active sulfur content. Acceptable for most petroleum product specifications.
• Class 2 — Moderate Tarnish: Dark orange, claret, or lavender tarnish. Indicates moderate active sulfur levels. May or may not meet product specifications depending on the standard.
• Class 3 — Dark Tarnish: Multicolored tarnish (magenta, dark blue-purple). Indicates significant active sulfur content. Typically fails petroleum product specifications.
• Class 4 — Corrosion: Transparent black, graphite black, or jet black copper strip. Indicates high active sulfur content and significant copper corrosion. Always fails petroleum product specifications.

The TT-130J’s integrally cast aluminum metal bath delivers the critical requirement of this test: uniform, stable temperature at every test position throughout the full test duration. Unlike open oil baths — which generate vapors, require oil level management, and exhibit temperature gradients between positions — the TT-130J metal bath uses matched-power electric heating elements embedded in the solid aluminum block to maintain equal temperature at all 8 test wells simultaneously, with a bath accuracy of ±0.1°C. This thermal uniformity is essential for between-position and between-laboratory reproducibility of copper strip corrosion test results, and is a key advantage of the metal bath copper strip corrosion tester design over liquid bath alternatives.

Test Procedure Overview

• Copper Strip Preparation — Polish the copper strip sequentially with 240, 400, and 600-grit sandpaper to produce a bright, uniform surface. Clean with absolute ethanol and degreasing cotton immediately before use. Handle with forceps only — do not contact the polished surface with bare hands.
• Sample Loading — Place the prepared copper strip into a clean glass test tube or stainless steel test bomb. Add the specified volume of petroleum sample (approximately 30 mL for test tube, or as specified by the applicable standard), completely immersing the copper strip. Seal the test tube or bomb.
• Temperature Selection & Pre-Heat — Select the required test temperature on the TT-130J touch screen (40°C, 50°C, 100°C, 120°C, or 150°C per the applicable product standard). Press Heating to bring the metal bath to the set point. Wait for temperature stabilization before loading samples.
• Sample Immersion & Timing — Place the loaded test tube or test bomb into one of the 8 metal bath wells. Set the test timer per the standard (typically 3 hours for most petroleum products). Press Timer to begin the countdown. The TT-130J’s real-time temperature curve confirms bath stability throughout the test.
• Endpoint & Strip Removal — On timer alarm, immediately remove the test tube or bomb from the metal bath. Remove the copper strip using forceps and wash with absolute ethanol to remove adhering sample.
• Corrosion Rating — Under standard lighting, compare the copper strip’s discoloration against the ASTM Copper Strip Corrosion Standard Color Chart. Assign the ASTM Class rating (1 through 4) based on the closest matching color standard.
• Result Recording & Compliance Assessment — Record the corrosion class rating and sub-rating. Compare the result against the maximum permissible corrosion rating specified in the applicable product standard (e.g., ASTM D4814, ASTM D1655) to determine pass/fail compliance.