Inadequate cleaning is the source of too many plating rejects. Here’s how to improve preplate electrocleaning.
Electrocleaning is the normal way to prepare parts for electroplating. It’s so well accepted that it is easy to take for granted. But as with any other chemical process, there are often ways to achieve better results.
Perhaps the most basic decision you have to make about electrocleaning is whether the item to be prepared is better cleaned by anodic (reverse-curent), cathodic (direct-current) or a sequence involving both types of electrocleaning.
Whenever possible, use anodic electrocleaning for final cleaning. When you do, the metal surface is actually being dissolved as well as cleaned. This deplating action removes metallic smuts and prevents the deposition of non-adherent metallic films and particles. At the interface of the anodic part’s surface and the solution, oxygen is liberated. The scrubbing of these gas bubbles also helps to remove soil.
For anodic cleaning, low voltage (3–12 volts) is the norm. Current densities vary from 10–15 asf, depending on the metal being cleaned and cleaning time allowed. Cleaning times of 1/2 – 2 minutes generally suffice. Higher current densities may be used when cleaning times must be shorter.
While reverse-current alkaline electrocleaning is best for preparing many metals, don’t use it for cleaning aluminum, chromium, tin, lead, brass, magnesium or any basis metal that is soluble or is etched by alkaline electrocleaners. Direct-current cleaning is generally used to clean these metals and to clean buffed nickel prior to chromium plating. Anodic cleaning would leave a passive nickel oxide film, which prevents the proper deposition of chromium.
The work is actually being “plated” in a direct-current electrocleaner. Therefore any positively charged particles or ions in the solution will be attracted to and may be reduced and deposited on the surface of the part. Metallic films deposited this way usually are non-adherent but can be difficult to detect and remove. Such films can cause poor adhesion, roughness and/or staining of subsequently applied electroplated metals. Consequently, direct-current cleaners must be discarded and re-made more frequently than reverse-current cleaners. And you may want to follow direct-current electrocleaning with a final reverse-current clean.
In processing brass and zinc die castings, you should avoid prolonged reverse-current cleaning as well as high-current densities, high temperatures and low cleaner concentrations, to prevent dezincification and over etching.
The same equipment, voltages and current densities specified for anodic cleaning are generally satisfactory for cathodic cleaning. Hydrogen rather than oxygen is liberated at the surface of the work. The volume of hydrogen liberated at the cathode is twice that of oxygen liberated at the anode, for a given current density. Therefore, more gas scrubbing is achieved at the cathode than at the anode.
Chromium contamination of cleaners is sometimes unavoidable, since the same rack is used in cleaning, chromium plating and other electroplating. Direct-current cleaning is more susceptible to staining from chromium-contaminated cleaners than is reverse-current cleaning.
Parts marred by heat treating, welding or other sources of oxides often may require a double cleaning cycle, depending on the degree of oxidation. A mineral acid dip usually follows the final cleaner, to neutralize the alkaline film on the metal surface.
Avoiding Hydrogen Embrittlement
Any work negatively affected by hydrogen embrittlement (e.g. spring steel) should not be cleaned cathodically unless adequate steps are taken after processing to relieve the hydrogen. Generally, heat treatment for one hour at 400°F immediately after processing will remove the hydrogen embrittlement. Parts with hardness exceeding 40 Rockwell C can be embrittled and must be baked after plating.
Periodic-Reverse Cleaning (PR)
In periodic-reverse cleaning, the work is made alternately cathodic and anodic, using a current of 6–15v. PR cleaning in alkaline solutions containing sequestering or chelating agents removes smut, oxide and scale from ferrous metals. When PR is the final electroclean, the parts should leave this station during the reverse-current part of the cycle.
Work may be PR cleaned on racks or in a barrel. One of the advantages of periodic-reverse cleaning to replace acid pickling is elimination of acid trapped by certain types of work such as hinges. In such applications acid from pickling can bleed out after alkaline electroplating (brass, copper, zinc, cadmium, tin). Oxides also may be removed without the danger of etching or the development of the smut usually associated with acid pickling.
Interrupted-Current (IR) Cleaning
The theory behind IR cleaning is simple and logical. At the interface of the soil on the part and the cleaning solution, a reaction is occurring. This reaction depletes the concentration of the cleaning chemicals at the interface. By turning off the power momentarily, the reaction ceases and the cleaner concentration is restored. When the current comes back on, the solution concentration is what it should be at the interface. A typical cycle would be 8–9 seconds with current applied, followed by 1–2 seconds with power off.
This technique is widely used in processes such as electrochemical deburring (ECD), electrochemical machining (ECM), electropolishing and electroforming. If a company is using the same power source for cleaning as it is using for these processes, rather than rerack, the finisher might also use current interruption in cleaning.
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