(Today's post is the Conclusion of an article, "The science of successfully anodizing die castings substrates," which is a review of several questions received by PRODUCTS FINISHING Magazine. It was published by www.aluminumtoday.com. You can download the full article here or use this link.)
There is no ‘trick’ to anodizing complex alloys; there are ways, grounded in scientific reason, to approach challenges.
Fig 5a and 5b: Schematic of the walls of the anodic oxide at the junction of three anodic oxide cells. An entire cell is bracketed with pink dotted lines; the central pore appears gray with repulsive forces represented by a single sphere. The fcc aluminium structure is at the bottom of the schematic with the current bias represented by bold green arrows. The forming tetrahedral structure (5a and 5b) exhibits the primary oxidation reaction at the interface between the fcc and the electrolyte in the pore by the presence of oxygen atoms on the tetrahedral structure.
The schematic on the right (5b) shows non-aluminium ions (copper colored) diffusing from the fcc lattice, through the tetrahedral anodic oxide network, into the central pore where it will be carried into the electrolyte.
Note how the copper atoms ‘pile up’ at the fcc-tetrahedral interface. This is due to the structural/compositional mismatch between the aluminium and the anodic oxide
* Start with as homogeneous a substrate surface as possible. This means a clean substrate, free of oil and dirt, without casting defects such as porosity intersecting the surface, or laps, burrs or seams created by machining, peening or blasting of the surface.
• Secondly, as much free aluminium should be present at the surface as possible. This can mean after cleaning, that the surface is alkaline treated to remove a small amount of aluminium at the surface in order to expose alloying elements such as copper and silicon. These elements can be removed through a nitric acid desmut operation (to remove the copper) and/or a treatment to remove the silicon such as a dilute hydrofluoric acid etch or an ammonium bifluoride treatment.
• After a good rinse to prevent dragging these undesired elements into the anodizing electrolyte, or leaving them to redeposit on the component surface, anodize in an electrolyte whose acid concentration is efficient enough to carry the current without corroding the surface. Depending upon the alloy complexity, this is typically from 180 to 220 grams/liter H2SO4. The current bias should be applied gently, via a ramp or with intermittant pulses, because time is required to allow for the diffusion of the non-aluminium elements during anodized aluminium oxide (AAO) growth. A slower AAO growth will always lead to a more uniform anodized finish.
• It may be desired to follow finishing with a dilute HNO3 rinse to remove any interstitial copper from the AAO surface, to enable dye uptake and to prevent isolated spots of copper corrosion product from the surface.
• Rinsing is imperative between process steps to keep alloying elements from building up in the process tanks.
It is also important to know and understand the component history should anodizing problems arise to connect manufacturing variations to variations in anodic oxide appearance or function. Such understanding enables best possible alloy selection for manufacturing and metal finishing; appropriate adjustments to the anodizing process, or target areas of the casting/manufacturing processes and/or the metal finishing process for corrective action.