Parts Cleaning Clinic: Cleaning Oxygen-Free Copper

Cleaning Oxygen-Free Copper

Q. In our company there is a manual chemical cleaning line for chemical polishing our oxygen-free copper parts. Due to complex shape (blind-hole) there is relatively poor cleaning in this area. Could you share with me how to lower or eliminate this defect and how to utilize automatic cleaning?

A. I am not sure of the parts you are cleaning, but design for manufacturing can never be over emphasized. If at all possible, you should discuss part geometry with your design engineers in an attempt to eliminate recesses and blind holes. These will always be problematic for effective cleaning, pretreatment and coating. 

If the blind holes and recesses cannot be eliminated or minimized, then I recommend you consider the use of ultrasonics. As long as the material can be effectively wetted by the cleaning solution, the ultrasonics will aid in the removal of contaminants from the interior surfaces. However, good rinsing will still be important, and multiple rinses will likely be necessary. Also, since you indicated it is a manual process, you could use that to your advantage to direct operators to use a spray wand or other device to direct cleaning solution and rinse water to this specific area. 

Regarding an automated cleaning line process, it would be difficult to advise you how to set that up in the space of this column. However, some important things to keep in mind would be: maximum part or basket size, understanding your process boundaries (current minimum and maximum soil levels), expected maximum throughput (parts or baskets per shift or day), degree of automation desired (full or semi-automatic), available floor space and total available capital, to name a few. Once you have determined some of these factors, I recommend reviewing potential equipment suppliers at PFonline.com.

 

Spray Zinc Phosphate

Q. Where can I find a distributor for a spray method of zinc phosphate?

A. I am not as familiar with spray zinc phosphating as I am with immersion. While spray isn’t impossible to find, immersion zinc phosphating is much more prevalent due to the time it usually takes at each of the process steps, particularly the zinc phosphate stage. This step is responsible for building the zinc phosphate conversion coating to the required weight. The zinc phosphate coating weight is significantly higher than what is required for iron phosphate, by an order of magnitude.

For this reason, the amount of time spent in the zinc phosphate stage is significantly higher than the amount of time spent in any of the other steps. With an immersion phosphate line, you can simply leave your parts or basket in that stage a longer time to insure the correct residence time is achieved in order to meet the required coating weight. In the case of the spray zinc phosphate, you will have a fixed stage length and line speed. In order to achieve the desired coating weight, you will typically have to make this stage two to five times as long as any of the other stages. As a result, this will take up considerably more floor space and will give you limited flexibility to modify the coating weight. The primary means to do this will be to adjust the line speed of the entire conveyor. 

Also, health and safety issues become a greater concern with sprayable zinc phosphate since the zinc phosphate tank operates at a lower pH than an iron phosphate tank will. That will drive additional cost into the materials for construction of the canopy, tank and piping. Insuring good ventilation of this stage will also be important for worker safety. Although it is less prevalent, it is not impossible to find suppliers for spray zinc phosphate. I found some of the suppliers listed under the Cleaning and Pretreatment area on PFonline.com to provide spray zinc phosphating chemicals. 

 

Phosphate Coating Problems

Q. We have a phosphating plant and are facing the following issues: 1) The coating is non-uniform; 2) After the phosphate coating and stearate coating are applied, the material develops rust spots; 3) Coated material occasionally makes sounds while being wire drawn to a smaller size but we don’t know why; 4) While forging, (producing roller) a muck formation problem is observed; and 5) We don’t know what the phosphate coating thickness on coils should be to have better lubrication and extend tool life.

A. You have several issues related to the phosphate line. Some problems are to be expected and others can be improved. Related to the non-uniform coating, I assume you are performing zinc phosphate coating followed by a stearate coating onto wire for drawing purposes. It is a common technique to zinc phosphate and zinc stearate coat metals for severe forming operations. The crystalline zinc phosphate coating is a good repository for the zinc stearate lubricant. When forming or drawing, this will provide lubricity by acting as a sacrificial coating between a tool and workpiece. 

A non-uniform coating would typically be due to either poor maintenance of bath chemistry, resulting in a spotty coating, or because the existing wire spools are wound too tightly to allow the free flow of zinc phosphate solution onto all surfaces. If it were related to bath chemistry, the spotty coating would likely be related to a low concentration of the zinc phosphate chemical, low activator concentration or too low a pH. For material handling and minimizing storage space, it would not be unrealistic to find the coil wound too tightly for good chemical flow. 

The formation of rust spots is consistent with the spotty coating you mention initially. A low pH on a poorly phosphate coated spot will be likely to induce corrosion locally. Poor rinsing will make this condition worse. Once the non-uniform coating issue above is resolved, the rust spots will also resolve.

I don’t know why the coated wire produces a sound when drawing it. I believe it has nothing to do with the coating, but rather vibration in the wire as it is feeding through.

The problem of “muck” formation is to be expected. As mentioned, the zinc phosphate coating with stearate acts as a sacrificial interface between the die and workpiece (wire). This coating will tend to spall off as the base material is deformed, which will eventually lead to a buildup of residue, particularly on the die.

I am not aware of a specification requirement for minimum zinc phosphate coating weights for forming or drawing. Because it is needed to support significant deformation, it will be quite a bit greater than the requirements for a paint pretreatment. A typical level for a forming pretreatment would be five to ten grams per square meter. Translated to typical units for the U.S. would be roughly 500 to 1,000 milligrams per square foot.