What is Copper Sulfate Test?
Copper sulfate test is a chemical test carried out to find the presence of Iron (Fe) in stainless steel by visual verification. The test works on the principle of oxidation reduction chemical reaction that causes the dissolved copper ions to settle or stick to the iron presents on the surface. Copper sulfate solution is applied by swabbing or brushing to the surface of interest and if any iron particles are presents on the surface they will react with the solution and change to brownish color.
This highly sensitive technique is advised for the presence of iron or iron oxide on the surface of austenitic 200 series and 300 Series, duplex stainless steel, the precipitation hardening alloys, and the ferritic 400 Series stainless steels containing 16 % chromium or higher. The test is not advised for the martensitic stainless steel and low chromium ferritic stainless steels of the 400 types since the test may show a +ve reaction on these materials irrespective of the presence or absence of anodic surface contaminants.
Copper Sulfate Standard
Copper sulfate test is defined in the ASTM A 380 standard & MIL-STD-753 (US Military standard).
Materials for Copper Sulfate test
Copper Sulfate test can be carried out on:
- SS200 series,
- S300 Series
- Duplex stainless steel
- Precipitation hardening alloys
- Ferritic Stainless steel, 400 series having more than 16% chromium
During welding of stainless steel components, iron particles can stick to the stainless steel parts and could be the initiation point for corrosion, here copper sulfate test can be used that no iron is present on the stainless steel surface. Or when a carbon steel component is cladded with stainless steel or Inconel alloy and to verify it is free from iron, copper sulfate test can be used.
Preparing the Copper Sulfate Solution
The test solution is prepared by first adding sulfuric acid to distilled water and then dissolving copper sulfate in the following proportion.
Distilled water : 250 mL
95–100 % Sulfuric acid (H2SO4) :1 mL
Copper sulfate pentahydrate (CuSO4·5H2O) : 4 g
Steps for Copper Sulfate test
- Prepare the test solution and mix the chemicals with water in quantity given above. Let it to mix for 4 to 6 minutes. Never use solution which is prepared more than 2 weeks before.
- Apply the test solution to the surface to be inspected by swabbing or brushing, apply additional solution if needed to keep the surface wet during the test duration for six minutes.
- Rinse the surface fully and take care not to disturb copper deposit if any till the part is completely dried.
- Perform the visual inspection for the presence of any copper deposit on the area of test. Any copper deposit indicates the presence of free iron.
Test Result Interpretation
Copper sulfate test is a qualitative test method. The results is interpreted as pass or fail by visual inspection without ad of any other inspection tool. The test principle uses the oxidation by chemical reaction of iron particles if any presents and turning them into copper color as visible in the below picture. The test will not tell how much iron is present on the surface but gives indication in a qualitative manners that iron is certainly presents on the surface being tested.
Copper sulfate test for passivation parts
Copper sulfate test for passivation parts is carried out by applying the copper sulfate solution as described in similar procedure above.
Copper sulfate test for Clad material removal inspection
Copper sulfate test is also used for the repair of clad and weld overlay applications to ensure the full removal of the clad layer before the start of welding on the carbon steel material.
-For clad restoration or clad removal due to wrong welding clad filler, after the removal of clad, the area shall be etched by nitric acid or copper sulfate solution to confirm the removal of clad material.
– Before the back cladding butt joints, apply right precautions to avoid cross-contaminating the cladding and backing material weld metal. Any suspected areas of cross-contamination shall be etched with either a nitric acid or copper sulfate solution to confirm the location of the clad/ substrate interface.