Q) Do you have a recommendation for a good reference for: Cleaning Al, specifically relating to deoxidizing, desmutting - Solutions, concentations, temps, etc...Electrocleaning, pluses and minuses, of anodic vs cathodic, typical solutions, set ups etc...Thanks, Terry N.
A) There are two good references that will provide you with information pertaining to both of your topics. These are the Metal Finishing Guidebook (from metalfinishing.com) and The Metals Handbook - Volume 5 - Surface Cleaning Finishing and Coating from the ASM (American Society for Metals).
Your chemical supplier is also a great source for information. They should be able to provide you with the correct chemistries, cycles and operating parameters for your aluminum alloys. This topic is much too long to answer without additional information.
Both anodic and cathodic electrocleaning is used for ferrous and non ferrous materials. Anodic electrocleaning is typically used for cleaning most substrates including ferrous alloys that do not contain nickel. It is suggested that nickel bearing alloys be electrocleaned cathodically to eliminate the generation of oxygen and the subsequent passivation of the material being cleaned. Cathodic electrocleaning is also used on brass substrates to reduce the potential of etching the substrate. The disadvantage of using cathodic electrocleaning is that any positively charged particles in solution could deposit onto the surface of the work and cause plating adhesion failures. It is very important to keep your solution clean, which usually requires that dump schedules for the cleaners be more frequent. Your supplier of cleaners will be able to provide the recommended products for your application.
Q) I would like to know about surface activation using titanium phosphate prior to zinc phosphating, Our current process is:
Alkaline degreasing
Zinc Phosphating
Dipping in bonding solution for rubber
If surface activation is done prior to Zn phosphate, what is its effect on the rubber to metal bonding - will it improve the bonding? - Narasimmaraj
A) The use of titanium activators prior to phosphates has an effect on the grain structure of the phosphate coating. Although both the crystal size and coating weight are reduced using the activator, the amount of crystals per unit area is increased when implementing a titanium activator in the cycle. The additional crystal density provides extra sites for phosphate adhesion to the steel substrate, which it turn, provides additional sites for the rubber to bond to the phosphate coating. These products are designed to enhance the adhesion properties of your subsequent coating.
Ask the supplier of your phosphate product to supply you with the best recommendation for a titanium activator for application.
Q) I have a cleaning problem with preparing a brass part prior to nickel plating. My current process is set up with a soak cleaner, electrocleaner, and acid salt activation followed by nickel and chromium plating. In the past we have used our electrocleaner with anodic current. We experienced problems with etching and were told to use the cleaner with cathodic current. Since we’ve made this change, we now experience a stained appearance after plating. This seems to only happen a couple of weeks after a new make-up of the electrocleaner. Could you please advise me on why this is happening?
A) A properly formulated brass electrocleaner should have adequate proportions of alkaline builders. These types of formulations require the chemistry to contain enough alkalinity for conductivity while using certain alkaline builders that inhibit attack on the substrate. The answer to your problem should be explained with two answers.
First, your current product attacked the substrate while used anodically. I imagine that either you don't have the correct product formulated for brass or the concentration or current density was outside the recommended operating conditions. More than likely the zinc in the brass alloy was etched and partially dissolved causing the attacked appearance. Most brass electrocleaners should be safe to use anodically when used at the recommended operating conditions.
Second, most correctly formulated electrocleaners can be used with cathodic current. Using cathodic current on brass is a safe alternative to anodic cleaning to help reduce the potential for attack to the work. However, there are still other issues that could be presented when operating the cleaner cathodically. Once the cleaner becomes contaminated, positively charged particles will plate onto the work . This positively charged debris can cause the work to exhibit a stained appearance after decorative plating. Under severe situations, adhesion will be compromised. Cleaners that are used with cathodic current require more monitoring, need to be maintained with a lower level of contamination, and are, as a result, usually changed more frequently. You should review your current make-up and dump schedule and adjust accordingly.
I suggest that you contact your chemical supplier to determine if you have the correct chemistry for the application and that you are operating it by their recommended parameters.
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