Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
CLEANING FOR PLATING
Alkaline cleaning requires the right chemistry, the right temperature, the
right length of time and the right agitation to perform satisfactorily.
Some random thoughts:
How do you know when a surface is clean? Tests consist of "water-break" testing (caution some surfaces will show a wetted (water-break free surface and not be clean)
When blisters, pits or discoloration occurs. (Too late to save the plated parts.) White filter paper can sometimes show poor cleaning by wiping the surface.
Other methods involve more complex instruments such a SEM with EDAX for identifying the particles on a surface, chromatographic for organics, etc. These require expensive analytical tools,
3000 years the Phoenicians mixed animal fate with lye, to form what we call soap, to wash clothes. Today, we use detergents instead of soap for cleaning. The primary functional constituent in cleaners today are surfactants (surface active agents) There are three basic types: cationic (with a positive charge) anionic (with a negative charge) and non-ionic (with no charge)
Commercial cleaners often have a mixture of non-ionic and one of the charged surfactants.
What is in alkaline cleaners?
Alkali such as sodium hydroxide, or rarely, potassium hydroxide for saponification
Carbonates for buffering
Sodium meta-silicate as a dispersant
Phosphates for water softening and sequestering
Borax (sometimes for buffering and as a dispersant)
Other items that may be found are wetting agents (surfactants are also wetting agents)
Deformers to control the foam,
Biocides to inhibit algae and molds
Solvent cleaners are sometimes used. They too can include surfactants, and can be a mixture of different solvents.
A word of caution: Solvent cleaners do a very efficient job of dissolving oils and grease. However, they can leave solid materials behind and once used, can leave diluted oils and grease on the surface that must be removed before the plating preparation and plating process. However, solvent cleaning prior to alkaline cleaning can make the detergent (hot alkaline cleaners) ineffective. Surfactants connect to oils and with the oils; particulate matter is also removed by the surfactants. If solvents are used the alkaline cleaners have little to attach leaving residues that are not removed.
It is a wise plater that finds out what type of lubricant, drawing oils, mold release, and/or cutting oils are used prior to the plating operation.
Be aware that silicone mold releases, silicone cutting oils, or silicone metal working fluids will not be removed by most cleaners, including solvent, emulsion or hot alkaline or any other type of cleaner. Mechanical removal may be required, but also has its shortcomings. The best approach is to ask that no silicone materials ever come in contact with the work to be plated.
Hot alkaline cleaners are effective for steel, stainless steels, titanium, copper, nickel, magnesium, tin-lead and zirconium,
Inhibited alkaline cleaners are used for aluminum, brass, Bronze, tin, and zinc.
Acid cleaner-mild acid, such as phosphoric acid type is used for aluminum, magnesium, stainless steel and many of the other metals except iron and mild steel.
Soak cleaners are usually heavy duty for removing many heavy oils and waxes, and are often used prior to electro-cleaning.
Anodic cleaners (also called reverse cleaners, using the plus connection from the rectifier.) are used for most soils and most metals, except for stainless steels. It is customary to use cathodic (minus connection to the rectifier) Typical current density range from 1 to 12-amps/square decimeter (10-150 amps/sq.ft). Cleaning time 1-6 minutes depending on the type and quantity of soil and the metal being cleaned. The oxygen gas bubbles aid in removing tenacious soils. Metals subject to hydrogen embrittlement should be cleaned in anodic alkaline cleaners. If periodic reverse cleaning is used, remove the work during or after the anodic cycle, never on the cathodic cycle.
Cathodic cleaners are formulated similarly to that for anodic cleaners with a few exceptions regarding the type of surfactants. Cathodic (-) cleaning produces hydrogen gas. The amount of gas bubbles is twice that of anodic cleaning. If the hydrogen gas accumulates in the foam, and exposed to a spark a hydrogen implosions can take place. An implosion takes place when the volume of the interacting constituents produces less pressure (negative pressure) than the original gas. An implosion makes loud noise but is much less harmful than an explosion. An explosion is when more volume is produced in the violent reaction.
Chromium contamination is very bad for cleaners. The result is often loss of adhesion because of the passivating effect that chromium produces. 10 ppm is too much chromium. Lead is also bad for cleaners. Lead peroxide forms on anodic surfaces and plates out on cathodic surfaces causing pits and or loss of adhesion. Change cleaners often. Most alkaline cleaners are tolerant to many other metal contaminants
Review: cathodic cleaning for stainless steel, tin lead brass and magnesium
Anodic cleaners are for buffed nickel, steels
Typical soils: grinding, stamping, vegetable, drawing compounds cutting, machine oils, other lubricants, greases, synthetic lubricants, and soaps, waxes (Waxes require high temperature to remove) marking inks, smut (using anodic) phosphates, fingerprints, etc.
Other types of cleaning include spray, and flooding. In using ultrasonic cleaning, the part shape will dictate the limits. Recessed areas may not clean well. And the parts must be at a specific distance from the transducers to be effective. Etching of softer metals is possible if the energy is too high or the wrong frequency is selected.
Plating chemical suppliers can guide you in selecting the right cleaner for the soils that you need to remove. There is not enough space to cover all possibilities of cleaning technology.
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