For copper and copper alloy wire products, the surface quality is usually very important. Many types of surface defects may occur, and a particular set of defects are the so-called stains, discoloration spots or a discoloration of part of the wire surface.
Where do stains come from?
Stains can have many different causes, and the origin of stains is not always easy to find. To produce wire, the copper or copper alloy undergoes many different process steps from casting, rolling and wire drawing. The appearance of stains may become visible at a different, later stage in the production than where the defect had occurred.
To help identify the root cause of a stain, Q8Oils R&D has developed a test to analyse the stain in more detail. With this test, a stain on the surface of a copper or copper alloy can be investigated and an analysis can be made its composition. Historically, stains on copper alloys typically originate from oxidation of copper, or carbonaceous matter, or a copper sulphur substance. With this test, these different natures can be distinguished from each other.
How stains are investigated?
At Q8Oils R&D facilities the test is conducted with four copper alloy pieces of wire that contain stains spots or larger stain areas. One-piece is always kept untouched as reference and the other three wires are each partly submerged into a chemical.
Each of the three samples is dipped in one of the investigation liquids (A, B & C). Then the three differently treated copper alloy wire samples are visually inspected together with the reference wire to see if the stain is removed or not. In case of fine wire and/or small stain spots a microscope could be required. With the matrix table below the nature of the stain can be determined.
What does the nature of the stain mean?
When the nature of the stain is known, the next step of course is to investigate at which stage the stain is initially formed. Copper sulfide stains may originate from tramp oil contamination or bacterial infection. Copper oxide can be formed at high wire temperatures in the presence of oxygen (air). Examples are:
- wet wire exiting the drawing machine
- insufficient inhibitor in the drawing or annealing solution
- excessive temperature due to high reductions
- high speeds
- not enough lubrication
- inadequate cooling
- or combinations thereof
Stains with carbonaceous nature can be formed when too much carbon containing material is present at the copper alloy surface at high temperatures. An example is the case where too much organic matter is present in the annealing process, resulting in the phenomenon that not all the organic matter can evaporate, and remaining residue starts to carbonize at the wire surface.
Why is Q8Oils your ideal partner regarding lubricants?
Q8Oils works closely with customers to improve their casting, rolling and wire drawing process with optimized lubricants. An important part of improvements is the correct understanding of possible defects such as staining. The test that we provide helps in understanding the root cause of stains on copper alloy wire.
For more information about stains on copper wire, please contact our Rolling and Wire Team at Q8Oils.
