It may sound easy, but aluminum provides specific challenges regarding effective surface preparation before plating.
Stephen Rudy, CEFImportant considerations include cycle requirements, designations of alloys and castings, and processing limitations. Aluminum surface preparation is unique in that it employs specific steps not familiar to the typical cycles for the more common ferrous and nonferrous alloys.
Photo courtesy of Advanced Plating Technologies
It is more interesting, since aluminum is the most electropositive element, or least likely for satisfactorily plating directly on to the substrate. I would like to share some of my experiences using different treatment solutions in typical cycles.
Surface Preparation for Plating
An overall surface preparation for plating may follow the following progression:
- Ultrasonic, mechanical, or solvent cleaning to remove buffing and polishing compounds.
- Soak clean in a non-etch, mildly alkaline solution.
- Rinse
- Alkaline or Acid etch
- Rinse
- Zincate
- Rinse
- Optional strip zincate
- Rinse
- Optional Re-zincate
- Rinse
- Proceed to specific electroplating or electroless step
The first step in cleaning employs mechanical action to assist in dislodging tough soils. Special cleaning formulations containing unique solvents and detergency agents are used. Stand-alone organic solvents are an alternative. There has been a departure from the chlorinated types to alternatives, such as the n-propyl bromide types. Traditional soak cleaners in step 2 are formulated, providing a buffer in the working range of 9-10, greatly minimizing or eliminating any etching.
It is especially important that non-etch cleaners are used where contact is made on highly polished surfaces. Naturally, these cleaners are non-silicated and do not contain any caustics. For the most part, they are non-corrosive solutions. Cleaners of this type will remove oils, grease, and shop dirt, as encountered on typically soiled parts. The cleaning mechanism would be either emulsification or displacement.
Rinsing, as applied in each step, is very important. It is not just that rinsing is achieved between treatment steps, but contaminants and dragged-on solutions effectively wash off the surface before the next step. Reviewing the cycle will confirm that alternating alkaline and acid treatments follow one another. That is why multiple rinses, such as double and triple counterflow types, are favored. Fog mists and water sprays are also effective in this regard. Clean, rinsed surfaces are very important as the critical zincate step must be satisfactory, developing a strong adhesive bond to the aluminum surface for subsequent plating.
Etching can be accomplished in alkaline or acidic solutions. The treatment focuses on removing the surface oxide skin to develop an active surface for zincing.
Alkaline Etch
The more common type of alkaline etch is the highly acidic solution, where parts are aggressively etched. Sequestering agents, chelates, defoamers, and wetting agents aid surface conditioning. Removal of the surface aluminum oxide leaves a smutted surface. These are the insoluble oxides of the particular alloying metals. Modifying the operating parameters (time, concentration, and temperature) will affect the rate of aluminum etching. Heavier or prolonged etch develops more smut.
A second type of alkaline etch is usually termed the mildly alkaline etch. This treatment employs a steady, mild, or micro-surface etch. It is preferred where geometric surface conditions require minimal surface metal removal. Other critical surfaces may include areas such as threaded holes. The main focus is to remove the oxide skin while preserving surface integrity. These mild etchants contain wetters, surfactants, and surface conditioners. Formula modifications are available for controlled, uniform etching and cleaning.
A third type of etch is the acidic process. These solutions usually combine two or more mineral acids in balance with organic acids, surface conditioners, and wetting agents. The oxide skin is removed, but usually less aggressively. This is important for polished surfaces or where it is preferred to substitute, as less aggressive, for the alkaline etch. It has also been established that for some electroless nickel applications, acid etching of aluminum develops a surface structure more favorable to this type of deposition. There is another branch of the acid etch that is fluoride-based. This bath, normally non-wetted, mildly etches aluminum, masking or hiding surface imperfections.
Deoxidizing and Desmutting
Acidic solutions effectively deoxidize and desmutt the smut developed in the previous etching step. Depending on the type of alloy or casting being processed, several types of effective solutions are available for this application. Dilute solutions of nitric acid are good for several of the 1000, 3000, and 6000 series alloys.
Using ferric iron-based solutions can eliminate nitric acid and its accompanying handling and safety hazards. Nitric acid-containing fluorides, as are the ferric-based solutions containing fluorides, are effective on silicon and copper smuts. In some instances, the nonwetted peroxygen compounds can also be substituted for mineral acids. Ferric blends containing nitric acid and fluorides are effective on 5000, 6000, 7000 alloys and 380 castings.
The strongest deoxidizer / desmutter is commonly referred to as the universal tri acid. This solution consists of a ratio of three acids (nitric, sulfuric, and hydrofluoric). It appears to be most effective on any aluminum alloys and castings, especially the sometimes troublesome higher alloys (6000 and above), along with 380 and 412 castings. The chromated desmutters are less commonly used, consisting of dichromates, fluorides, and chlorides. These types of solutions have been very effective on wrought and extruded alloys.
Effective de-smutting develops a surface that is whitened (or bleached) in appearance and should be towel-wiped clean. In this step, a micro etching effect significantly increases the surface area, promoting better adhesion to the subsequent zincate.
Stephen F. Rudy, CEF, is president of Chem Analytic and has written extensively about the finishing industry. Visit www.chemanalytic.com or call him at 917-604-5001.
