The fascinating and rewarding parts of the finishing industry are evident every day; no matter how much time we spend doing and learning, the rewards provide invaluable experience that helps us do our jobs better.
Troubleshooting, process understanding, and the rationale for specific treatments guide decision-making and optimal selections. Knowing how to adapt, what to use, when to, and when not to gives a positive sense of direction. Operating parameters are optimized, and analysis procedures address the key factors for successful metal finishing.
In many instances, we take certain things for granted. The following items are prepared as a possible list of interesting and helpful facts.
Aluminum
Aluminum comprises 8.1% of Earth’s crust. The main sources of aluminum production are bauxite (for primary applications such as aircraft) and recycled scrap. We associate aluminum with its superior strength, similar to steel, but at about half the weight.
Why, then, is this so if aluminum itself is soft and pliable? Raw aluminum is alloyed with metals, such as magnesium and silicon. The product is then baked, which transforms the alloy into a strong, durable material. How the alloy structure forms and how it promotes high strength attest to its durability. First, the element silicon forms a pillar-and-skeleton assembly. On this assembly form particles of an inorganic complex consisting of magnesium/silicon / and aluminum.
By substituting an equal thickness of duplex nickel, the corrosion delay was at least 24 months. It is also notable that the ductility of the semi-bright deposit is at least 500% higher than that of the bright deposit.
This critical structure then forms an additional inorganic species made of magnesium/silicon. It is this structure that inhibits aluminum's tendency to behave as a smooth metal. Now that this heating and annealing process is better understood, even stronger and more workable aluminum alloys may be formed. This leads to many benefits, such as further reducing motor-vehicle weight, resulting in improved fuel economy.
Other major applications include: aircraft, construction, electrical wiring, medical devices, food packaging, and consumer goods. Anodizing is the primary aluminum finishing method, followed by plating.
Duplex Nickel
To achieve optimal corrosion protection for nickel-plated parts with a chrome flash, such as in automotive applications, duplex nickel is most effective.
The duplex nickel deposit consists of a 75% underlayment of semi-bright nickel followed by a 25% topcoat of bright nickel. The tandem provides for excellent corrosion protection. The cathodic protection layer of the semi-bright deposit suppresses corrosion from the base metal. In turn, the bright deposit layer provides a sacrificial anode, protecting the semi-bright layer from corrosion.
Total nickel thickness from both deposits with controlled sulfur in the deposit works together. Controlled corrosion testing confirms that corrosion on copper-bright nickel-chromium-plated zinc die-cast parts occurred within 12-18 months under severe test conditions. By substituting an equal thickness of duplex nickel, the corrosion delay was at least 24 months. It is also notable that the ductility of the semi-bright deposit is at least 500% higher than that of the bright deposit.
Trivalent Chromium Plating
There was a big push to introduce decorative trivalent chrome baths in the 1980s and early 1990s.
This process uses inert anodes, inorganic trivalent chromium salts, conductivity salts, and complexing agents. wetting agents, and grain refiners. The plating solutions are typically blue to blue-green, depending on the proprietary, commercially available bath. All the health and safety hazards associated with hexavalent chromium baths are eliminated. The reduction of trivalent chromium to the metallic state requires three electrons, versus the same reaction that requires six electrons for the hexavalent chromium ion.
Therefore, the trivalent chromium bath is at least twice as efficient as the hexavalent bath (approaching 30% compared to approx. 10-15%). For decorative plating applications, trivalent baths support larger surface areas per flight bar (more parts), resulting in higher production throughput. Trivalent chromium plating tolerates interrupted cathode contacts and is far less sensitive to AC voltage. Deposits, based on the type of bath, range in color from light grey to shades of blue, resembling a hexavalent deposit.
Evaluation of any specific cleaner of interest is strongly recommended, within the specified operating parameters. Post-cleaning observations and appropriate testing should provide sufficient confidence in the choice of soak cleaner.
Trivalent chrome baths deposit the metal to a limiting thickness, up to 40-50 millionths of an inch. This meets decorative applications. There is a point in the deposition process at which the thickness becomes self-limiting, unlike the hexavalent bath, which will continue to build thickness. Trivalent chromium baths are sensitive to metallic impurities. In some baths, metal contaminants are removed by precipitation with a purifier or by ion-exchange technology. Unlike hexavalent baths, trivalent solutions do not passivate unplated steel surfaces (e.g., the inside of tubes). Therefore, chrome-free passivation post-dips or similar applications are required to prevent corrosion.
The current PEL (permissible exposure limit) for hexavalent chromium and all other hexavalent chromium compounds is 5 micrograms per cubic meter of air as an 8-hour time-weighted average. Decorative trivalent chromium baths provide an alternative to hexavalent chromium in this regard. Trivalent chromium deposits meet the ELV directive and comply with RoHS.
Nasty Oils
Certain oils can be very challenging to remove in the soak cleaner, especially if they adversely affect the finishing cycle. Chlorinated and paraffin oils can gum up parts, especially when soaked in caustic (sodium or potassium hydroxide)- based cleaners. Improved cleaning results can be obtained in non-caustic, alkaline cleaners, silicate-based, containing specific ratios of nonionic to anionic surfactants. Molybdenum sulfide lubricating grease can also be difficult to remove. The previously described soak cleaner may also work best.
Mineral, spindle, and water-soluble oils may be optimally removed in a soak cleaner that contains approximately one-quarter caustic and silicates in the concentrated formula, in addition to the ratios of anionic to nonionic surfactants and dispersing agents. Dipropylene glycols provide excellent solvency in effective soak cleaner formulations. Water hardness inhibitors in a formula are essential because they prevent useful surfactants (essential for cleaning) from binding to calcium, magnesium, and iron.
Evaluation of any specific cleaner of interest is strongly recommended, within the specified operating parameters. Post-cleaning observations and appropriate testing should provide sufficient confidence in the choice of soak cleaner.
Rust Spots in Electrocleaning
How often have you noticed steel parts exiting the electrocleaner with a brown, stained appearance? Or worse yet, brown splotches and even high-current-density burning? It happens whether in a rack or barrel operation.
Chances are that either the electrocleaner formulation is incorrect or the correct one is under-concentrated. A lack of reserve alkalinity in the bath prevents dissolution of the iron hydroxide film that forms on the surface during anodic conditioning. This reserve alkalinity is typically in the form of caustic soda (sodium hydroxide). Severe under-concentration of caustic results in poor conductivity that tends to etch and burn the high current densities of parts.
Adjust the operating electrocleaner concentration, or switch to an appropriate concentrate blend.
Corrosion Spots after Electrocleaning
This is another problem that occurs in rack-and-barrel systems. Parts exiting the bath can be examined and found to have black dot etch marks. This problem typically occurs in double cleaning cycles or when racks or barrels cross over between the acid and electrocleaner.
Drag in of the first hydrochloric acid solution into the second electrocleaner introduces chloride as a contaminant. During anodic treatment, chloride ions (negative) are attracted to the anode (positive), forming chlorine gas bubbles (oxidation product). The bubbles remain in place long enough to etch the steel. Corrective measures include switching the acid from hydrochloric to a suitable alternative, such as sulfuric acid.
Otherwise, better rinsing between the acid and the electrocleaner is required. Or switch to a sufficiently inhibited formulation that prevents this corrosive etching on steel.
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.
