Copper Sulfate Test for HDG Coatings of BS 1387 Steel Tubes

BS 1387 steel tubes are usually furnished with hot dip galvanization (HDG) coatings for improved corrosion protection. When the hot dip zinc coating is required to be tested, one tube per batch of 500 pieces shall be tested in accordance with the copper sulfate method in accordance with appendix C of BS 1387 standard.

The Principle of Copper Sulfate Test

BS 1387 HDG steel tubes with plain ends, medium duty, packed in bundles, stock available.

The copper sulfate test is a commonly used method for evaluating the quality and uniformity of the zinc coating on BS 1387 HDG steel tubes. This test helps determine whether the steel is adequately protected against corrosion. The principle of the copper sulfate test involves a displacement reaction due to the electrochemical potential difference between zinc and copper: Zn + CuSO₄ → ZnSO₄ + Cu. In this reaction, the deposited copper on the zinc coating forms a non-adherent spongy copper layer, which can be rinsed or brushed off from the zinc coating easily. If the bare steel surface is exposed to the copper sulfate solution, another displacement reaction occurs: Fe + CuSO₄ → FeSO₄ + Cu. This reaction results in a more adherent red copper layer on the steel surface. The appearance of this firm red copper layer confirms the absence of the zinc coating in the tested area. The earlier and more extensive the red copper layer appears, the thinner the zinc coating, indicating poorer uniformity of the galvanization.

Materials & Equipment

• Copper Sulfate Solution: A standard solution is prepared by dissolving copper sulfate (CuSO₄) crystals in distilled water, typically at a concentration of around 16-20% by weight.
• Sulfuric Acid (dilute solution): Used for cleaning the surface of the steel tube before testing.
• Distilled Water: For preparing the solution and rinsing the samples.
• Clean Cloth or Sponge: For vigorous scrubbing or wiping.
• Bath: To accommodate the testing tube workpiece to be immersed in the solution.
• Protective Gear: Gloves, goggles, and aprons to ensure safety during the test.

Test Procedure

BS 1387 HDG steel welded tubes with threaded ends according to BS 21. DN50, medium duty.

• Prepare the test sample, which shall comprise a sufficient length of BS 1387 hot dip galvanized steel tube. Degrease the sample and wipe it dry with a clean soft cloth. Dip it into the sulfuric acid solution for 15 seconds and thoroughly rinse it in clean water. Dry the sample by wiping with a clean soft cloth.
• Immerse the test sample into the copper sulfate test solution for a duration of 1 minute. Check that the temperature of the test solution does not fall below 15°C or rise above 21°C. Do not agitate the sample or the solution during immersion. Remove the sample and immediately rinse it in clean running water, removing any black deposit with a brush and taking care to clean all holes and pockets. Wipe the sample dry and immediately return it into the test solution.
• Repeat the procedure until a total of four successive immersions have been made. Wash and dry the sample. After each test or batch of tests discard the test solution. Do not return any used test solution to the stock supply.

Examination & Compliance

A galvanization workshop: The workers are packing the hot-dip galvanized steel tubes.

After the four successive immersions in the test solution, the dry test sample shall be examined for deposits of red metallic copper on the hot dip zinc coating. It shall not show any adherent red deposits of metallic copper on the HDG coating after final rinsing and wiping dry. The manufacturer shall record the results of the test.