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1 SURFACE TREATMENT www.aluminiumtoday.com The science of successfully anodizing cast substrates

Die castings pose some of the most challenging problems in A review of several questions received in anodizing. The finish can be too thin, non-uniform and/or have an ‘Products Finishing Magazine’ over the years 2001 - 2009 (1-8) determined unfavourable appearance. generally consistent problems with anodizing die castings. These are common problems with a variety of practical solutions; The problems can be summarized as they are easy to recognize, but in many instances, the source for follows: 1) the anodic finish is too thin to meet the necessary design the problem remains unknown. specification or much thinner when Critical to solving the problems of anodizing die castings is compared to corresponding components manufactured from wrought processes; 2) understanding the die cast substrate and the impact of surface the finish appearance is unfavourable: condition, alloy composition, casting quality and microstructure hazy, muddy, ‘not black enough’, and/or not uniform; and, 3) the on the anodizing process. Substrate quality issues are just as resistance of the anodic oxide is important, maybe more so, than anodizing conditions and insufficient. Review of recommended anodizing technique. Certain optimum anodizing conditions may be used in solutions to the various problems with some cases to help overcome less than advantageous metallurgical anodizing cast alloys determined that they do not always work, indicating that there conditions. These include well known processing tools such as are factors other than the anodizing various pretreatment chemistries, higher anodizing bath process that impact the anodic oxide finish quality. concentration, and higher bath temperatures.These, and other Consideration given to solve these rather recommended solutions are not successful in every case; easy-to-identify problems has illuminated four broad cause areas for discussion: 1) sometimes trial and error testing on actual production parts must alloy selection, 2) substrate quality, 3) be done to find the best processing techniques. surface treatment, and 4) anodizing process parameters. Of the four, perhaps By providing real-life solutions in terms of anodizing theory and the metallurgical factors that impact interfacial science, this paper gives insight to anodizing solutions substrate quality: alloy composition, casting quality, microstructure and surface for die-cast alloys by tying together metallurgical science with quality are dominant in determining anodizing practice. By Jude Runge* and Larry Chesterfield** *Comprehensive Metallurgical Consulting, ** Anodizing Technologies, Inc,

Fig 1 Fig 1a

Fig 1 and 1a Metallographic cross section of anodized die cast alloy AlSi9Cu3Fe (similar to alloy 383). In the micrograph on the left, large hypereutectic primary platelets from the microstructure as well as smaller eutectic needles are incorporated into the anodic oxide. In the Scanning Electron Microscope (SEM) photomicrograph on the right, the surface of the component is documented, showing the cracks dotted with white-appearing silicon as well as deposits appearing as darker-gray haze across the surface and concentrated in areas near the cracks

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anodizing conditions, technique and minimum gassing when molten, and resistance, exhibit more complex surfaces, quality. This paper covers each of the four ability to remove the casting from the with less free aluminium, which make cause areas by discussing, in limited detail, mould when solid. Mechanical properties them more difficult to anodize. A cast the impact each has on the interface from are insured first by the integrity of the alloy is rarely designed for subsequent which the anodic oxide originates and casting, therefore, the casting alloy and surface treatment, especially anodizing. grows. process are designed to minimise porosity Although an anodic oxide finish may be Theoretical scientific reasons for why the and maximise surface integrity. Actual desired to impart corrosion protection, problems occur and why most solutions mechanical properties of strength, where resistance to be a decorative finish, succeed and some fail are presented. The hardness, and resistance to wear and the ramifications of the anodizing process limitations as to what can be done from fatigue are produced metallurgically in on complex cast alloys is typically not an anodizing standpoint to overcome the aluminium castings two ways: (1) by solid considered of the design or foundary level. metallurgical condition of a cast substrate solution hardening; that is: by the To include anodizing as a design are presented not as an excuse, but as a substitution of aluminium atoms with condisderation for cast alloys, the call for understanding and communication alloying atoms in the aluminium crystal understanding that alloy elements do not between finishers, component structure and (2) by precipitation anodize is a must; and to maximise the designers who would like to use die hardening: the dispersion of second phase ability for complex alloys to be anodized castings, and the casting houses who constituents or elements in solution and requires that microstructures must be as provide the castings in order to optimise precipitating them out as small fine and homogeneous as possible. product and process and to increase the intermetallic compounds, incoherent with Rarely, if ever, is a cast alloy designed for use of anodized cast aluminium the microstructure, which inhibit material subsequent surface treatment, especially components. deformation. anodizing, because alloying elements do Cast components have limited ductility not anodize. Alloy Selection and can be brittle; therefore, castings are Aluminium die castings have been not usually meant for subsequent Common Cast Alloy Additions commercially available since the beginning deformation processing. Other than The alloy chemistry and the casting of the 20th century. Castings are used for minimal finishing processes such as process affect the level of microstructural a variety of applications, from decorative , a casting is typically produced homogeneity, the defect population and sculptures and jewelry to automotive to function near net shape. Because cast therefore the variation of chemical pistons and engine blocks. components are produced to function potential across a cast component surface. Die casting is a versatile process for near net shape, castings can be alloyed To put cast substrate complexities in producing engineered metal parts by beyond what is typical for wrought context with the anodizing process, it is forcing molten metal under high pressure products; that is, additions of other important to understand the nature of the into reusable steel moulds. These moulds, elements are at a higher % than the cast surface and therefore the interface called dies, can be designed to produce additions for alloys intended for extruded, with the anodizing from which complex shapes with a high degree of rolled or deep drawn product (up to 16% the aluminium anodioc oxide (AAO) accuracy and repeatability. Parts can be total alloy content for castings vs. up to grows. It is important to understand the sharply defined, with smooth or textured 8% for wrought alloys) (10). As such, cast nature of the surface and therefore the surfaces, and are suitable for a wide alloys are metallurgically more complex interface between the component surface variety of attractive and serviceable than their wrought counterparts; and the anodizing electrolyte such that finishes (9). increased alloy additions produce anodizing process parameters can be First, cast alloys are formulated for correspondingly higher levels of solution modified to effect optimum oxide growth. castability, followed by mechanical phases, intermetallic compounds and Silicon is the alloying element that properties and structural integrity. Alloys precipitates. Castings, therefore, in essentially makes the commercial viability are formulated for maximum fluidity, addition to their strength and fatigue of the high volume aluminium casting

Fig 2a Fig 2

Fig 2 and 2a Alloy inhomogeneity coupled with surface connected porosity change the surface quality (chemical potential) and make it more difficult to anodize

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industry possible. modifications can refine the cast structure is close to or intersects the surface, the Silicon content between 4% to the by keeping aluminium-silicon phases and ramifications of chemical processing such as eutectic level of 12% reduces scrap losses, primary silicon crystal small, such additions anodizing can develop the pores and create permits production of much more intricate can help or hinder surface finishing. discontinuities in the AAO (Fig 2). designs with greater variations in section thickness and yield castings with higher Substrate Quality Surface Quality surface and internal quality (forms sound Controlling Cast Microstructure Anodic oxide finish thickness variations outer surface layer), because of this, Cast microstructure dictates the after processing, even on components aluminium-silicon cast alloys can be used component quality. Because of the rapidity finished on the same rack can be caused in applications where pressure tightness is of the die casting process, microstructures by variations in substrate . a requirement. are finer than components cast by other Variation in these cases will not correspond These benefits derive from the effects of methods. to component location on the racks or to silicon and aluminium molten mixtures, The size and distribution of the various how the parts are contacted on the racks. which exhibit increased fluidity, reduced phases, compounds and precipitates hinge Furthermore, thickness variation may be cracking and improved feeding to on: alloy chemistry, which includes the measured across the surface of individual minimise shrinkage porosity. Alloys with addition of elemental modifiers to the alloy parts, with the machined surfaces the eutectic composition (Al-12%Si) exhibit chemistry; the casting method; the exhibiting a uniform and continuous finish highest fluidity during casting (11). method and speed by which the casting is thickness. Areas that are not machined (as- The limited solublility of silicon and cooled and whether it is heat treated cast or shot peened or blasted) may exhibit aluminium produces aluminium-silicon, (tempered) after it is finished. discontinuous and unacceptably thin compositions that exhibit approximately Die casting dies are designed with vents anodic oxide finishes. 1% silicon in solid solution as a continuous and cooling lines to facilitate rapid cooling. As-cast surfaces can be contaminated phase, and the rest of the silicon falls out In addition, components can be ejected with residual mould release (parting of solution as essentially pure silicon into a cooling tank to rapidly quench compounds) or degradation/corrosion particles that range in size from small finished components. To keep micro- product from poorly maintained moulds. particles to large needles and platelets. structures fine with complex alloys, the Burrs, laps and the seams that they cause Copper, and are the rate of cooling must be as fast as possible, between the machined defect and the most important secondary alloying ito keep as much of the elements in substrate are well documented sites of elements which impart fluidity during solution. If cooling media is not turned localised anodic charge concentration casting and various phases which impart over as rapidly as components are loaded, which prohibit uniform anodic oxide mechanical properties such as strength, inadequate cooling can develop coarse growth. These are points of resistance corrosion resistance and fatigue resistance. cast microstructures. Precipitation and heating, hyper-growth and ultimately thin Other alloying elements such as , growth of intermetallic compounds, phase discontinuous anodic oxide finishes manganese, and , etc formation and grain coarsening will occur because, when the process current bias is are added to castings to produce second if the casting is slow cooled or allowed to imposed, the burrs stand immediately phase constituents that moidify the remain at a high enough temperature after upright as the charged defects repel the aluminium-silicon structure and increase ejection from the die. Latent heat retained surface (similar charged objects repel one strength and hardness. in castings will cause effects another). As burrs proceed to anodize Alloying elements have varying that are manifested in the microstructure through, breaking off into the electrolyte, characteristic solubility in aluminium, and by the growth large primary silicon crystals or into the AAO, they leave bare how they mix, stay in solution or and coarse precipitates. Microstructural areas of substrate that begin to anodize precipitate as intermetallic compounds effects on the surface are developed by the later in the cycle. Because a blasted surface impact the cast microstructure. All die cast anodizing process (Fig 1). condition may not be uniform from components contain some level of both Because die casting is a closed process, the component to component, the resultant phenomena, to varying levels depending amount of gas dissolved in melt can finish thickness will never be uniform from upon alloy chemistry, casting process and ‘precipitate’ out as porosity. This is typically part to part (Fig 3). any post-cast heat treatment (whether avoided by degassing the melt prior to Inasmuch as substrate consumption is accidental or actually imposed). Deliberate injection. However, in the event that porosity part of anodizing, substrate defects

Fig 3 Fig 3a

Fig 3 and 3a Documentary photograph of the shot peened surface of the die cast component before anodizing and after anodizing. The charge across the surface would be uniform during anodizing. The surface on the left is typical of a shot blasted surface. Note the non-uniformity of the surface. The charge distribution would be exceedingly non uniform, creating a situation where the finish growth would not be uniform, clearly documented by the non-uniform AAO finish thickness on the right

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introduced by way of mechanical which gives it a pre- processes will also be consumed, from as crystalline character (12). many surfaces as are exposed to the The anodic oxide electrolyte. AAO formed around alum- structure is comprised inium-based defects are prone to chipping only of aluminium and and cracking. The tips of sharp corners ions coordinated and edges created by laps, folds or cracks as AlO4 - tetrahedra which introduced by finish machining and form the ‘network in the surface blasting are areas of charge conc- network’ of self assembled, highly entration that result in localised high ordered nanoscale columns (13). interfacial heat (increased reactivity) which The ionic solid facilitates ionic may result in outgassing, burning and/or conduction which enables finish the formation of rounded voids in the growth. Excess non-aluminium . If the burr is anodized through its metal ions migrate through cross section, once detached from the interstitial sites of the network substrate it will be included in the anodic lattice. Interstitial Al3+ ions migrate oxide, much like an inclusion. together with electrons from the Non-aluminium abrasive media will not electrolyte during anodizing. The react with the electrolyte, producing gaps electrolyte reacts with these sites at the Fig 4: Representation of anodic oxide structure comprised in the anodic oxide or inclusion-like surface of the pore walls. (Fig 4). of AlO4 tetrahedra with short range order. discontinuities. Next, it is critical to understand the These tetrahedra comprise the internal network of the aluminium substrate and how the alloying individual cells which make up the nanoscale network of Anodizing Complex Alloys elements are present in the aluminium the anodic oxide Anodic oxide formation is an matrix. The aluminium substrate lattice is electrochemical corrosion process that face centered cubic (fcc), which presents a solid solution (such as in die castings). An begins with nucleation at separate and structural mismatch between the substrate increased resistance heating results at the distinct preferential sites across the and the forming anodic oxide’s tetrahedra interface; producing thin, rough finishes substrate surface. Preferred sites are those structure (14). that often exhibit deposits of sooty- which are not electrochemically complex, An substrate lattice appearing reaction by-products (Fig 5). neither chemically (sites are comprised of contains non-aluminum atoms in Inclusions, intermetallic compounds, aluminium only) nor topographically substitutional solution as well as interstitial precipitates and other insoluble alloying (surface is rather continuous with no intermetallic compounds that are not elements that are incoherent with burrs, laps or seams). In short, an ideal coherent with the microstructure. Since substrate microstructure are not anodized; substrate is that which favours aluminium the anodic oxide will not comprise these instead, the primary oxidation reaction oxidation. non-aluminium atoms or intermetallic proceeds around them, taking them up In industry, an ideal substrate is rarely compounds, a com positionalmismatch is into the anodic oxide. The impact of inert encountered. Products cast from alum- also present between the substrate and or insoluble defects such as inclusions and inium alloys exhibit a variety of elemental the anodic oxide. microconstituents that are not soluble in additions that form alloy phases (which A thermodynamic mismatch is present the aluminium matrix (eg lead and are homogeneous with the alloy system), because there are diffusion rate hypereutectic silicon) on the finish is in the intermetallic compounds (a compound of differences between and spacing of the initial corrosion nuclei that two metals that has a distinct chemical (diffusion occurs faster in metals) (15). precede the anodic oxide network and in formula), precipitates (which fall out of Since the substrate is the source for the the coherency of the AAO structure. This alloy solution usually through heat anodic oxide constituents, and the anodic disruption in order can lead to irregular treatment) and many other material oxide is the result of the continued growth and irregular inter-column defects that are on the microscopic and reaction of the electrolyte with the spacing. However, as the surface is atomic level. Examples of atomic level substrate surface, the interface of the consumed and a more ordered surface is defects are: grain boundaries, dislocations electrolyte and the surface is the key to presented for oxidation, the oxide growth and vacancies, which are atomic level gaps continued anodic oxide formation and recovers, but the surface of the oxide and points of structural mismatch in the growth. often replicates the defect, making the metallic crystal structure. The substrate In nature, energy is required to surface rougher. Mass transport of inclu- electrochemical resistance changes with overcome a mismatch. Energy produces sions and insoluble species also occur defects, alloy additions and heat. Non-aluminium metallic ions (cop- through the network; although they are contamination, especially when they per, zinc, and other metal ions in solution never incorporated into the structure itself. intersect the surface, or interface between with the aluminium alloy substrate) ‘pile Side reactions such as these also retard the substrate and the electrolyte. up’ at the interface and must realign the kinetics of the primary aluminium Therefore, all non-aluminium interfacial themselves as they move from their oxidation reaction (16). However, dep- phenomena confound the anodizing positions on the substrate lattice to ending on the amount and distribution of reaction, retard it, and disrupt the anodic interstitial spaces within the anodic oxide incoherent defects, less energy (heat of oxide structure. lattice. Once they find a space, they move reaction) is required at the interface to Critical to understanding the side slower as they ‘hop’ through the spaces in overcome such defects than the energy reactions that occur during anodizing is the anodic oxide network while aluminium required to overcome the effects of the recognition that the anodic oxide is an ion diffusion occurs on the tetrahedral substitutional elements in solution with ionic solid comprised only of hydrated lattice which comprises the forming the substrate on the anodic oxide . The structure exhibits anodic oxide cell. The kinetics of the formation and growth. minimal x-ray diffraction contrast, and is primary oxidation reaction are therefore Complex alloys, such as die castings, therefore designated amorphous but has retarded when anodizing complex alloys offer mixtures of inclusions, phases and been shown to exhibit short range order, that are rich in non-aluminium elements in intermetallic compounds which confound

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important to rinse well between treatment Fig 5a Fig 5b steps. Rinsing is the most important step in any anodizing operation. Anodizing strategy should be developed around the alloy chemistry. Understand what alloying elements are in solution and what intermetallic compounds are present, and that everything ultimately intersects the surface. Work within moderate process parameters: medium range concentration, lower current density and use gentle ramping (turn up the bias/potential slowly to the desired current density/voltage). Interfacial resist- ance of the alloying elements is relieved by the careful imposition of anodizing parameter not by electrically blasting the surface. Medium range acid concentration is recommended because higher acid concentrations corrode the component surface that has just been prepared by Fig 5a and 5b: Schematic of the walls of the anodic oxide at the junction of three anodic oxide cells. An entire cell is cleaning and pretreatment. bracketed with pink dotted lines; the central pore appears gray with repulsive forces represented by a single sphere. The While increased solution conductivity is fcc aluminium structure is at the bottom of the schematic with the current bias represented by bold green arrows. The an effective means to facilitate current forming tetrahedral structure (5a and 5b) exhibits the primary oxidation reaction at the interface between the fcc and the flow through the electrolyte, it should not electrolyte in the pore by the presence of oxygen atoms on the tetrahedral structure. necessarily be viewed as a means to The schematic on the right (5b) shows non-aluminium ions (copper colored) diffusing from the fcc lattice, through the accelerate anodic oxide growth. tetrahedral anodic oxide network, into the central pore where it will be carried into the electrolyte. In fact, the electrolyte can react with Note how the copper atoms ‘pile up’ at the fcc-tetrahedral interface. This is due to the structural/compositional mismatch elemental copper, zinc or magnesium at between the aluminium and the anodic oxide the surface of the component, producing corrosion product barriers to the the anodizing reaction. With as many concentration, temperature and current anodizing reaction. To initiate anodizing, various defects as the die casting contains, density; and 5) consider further process a large input of electrical energy may be various side reactions occur to impede the optimization with a ramp or pulse necessary to break through the corrosion surface reaction. The realignment of the anodizing. product before a normal anodizing non-aluminium substrate elements in As-cast components may exhibit residual reaction can go forward at the surface. solution and mass transport of inclusions dirt or mould release but will always However, since surface corrosion product and insoluble alloy additions such as exhibit an aluminium-rich surface due to layers are typically discontinuous and hypereutectic silicon compete with the the temperature gradient that occurs nonuniform, anodic oxides with high oxidation kinetics, resulting in repeated during solidification. Gentle soap cleaning roughness and perhaps thin or burned restructuring of the substrate surface may be all that is needed to prepare the layers will form. Moderate acid conce- which in turn disrupts the order of the surface for anodizing, provided all surface ntration allows for a anodizing at a slightly anodic oxide. Depending upon the size dirt is removed by cleaning. This may reduced voltage for a given current density and population of insoluble defects, the require frequent turnover of the cleaner and always produces a smoother, more cohesive strength of the finish can be solution. Other chemical pretreatments are uniform finish. reduced and paths for environmental used to minimise the effects of non- Lower current density anodizing slows ingress can be created, reducing the aluminium alloy constituents that intersect down the diffusion rate of electrons and corrosion resistance of the finish; the surface, in other words, these metal ions in the component and the furthermore, the continuity of the porous pretreatments are used to maximise the forming anodic oxide, giving the non- structure will be interrupted, causing a amount of free aluminium at the surface aluminium ions a chance to re-align marked decrease in dye uptake. for anodizing. A note of caution: it is themselves at the casting – AAO interface possible to overdo a good thing, as the pH which in turn reduces resistance heating Overcoming the Barriers of of the cleaner typically indicates what in creating a smoother finish. By the same Anodizing Complex Alloys: the alloy will be attacked/removed. token, ramping and pulse anodizing help Recommended Process Parameters Alkaline etches only remove aluminium, anodize complex alloys by giving non- The following recommendations are leaving behind non-aluminium residues aluminium ions in substrate solution time presented to aid in overcoming the barriers such as copper which require a subsequent to diffuse, thereby relieving interfacial heat to anodizing die castings: 1) inspect and desmut step with or ferrous created by the competing reactions at the evaluate the components as received to to remove. High silicon alloys interface. make sure there are no obvious defects almost always require an acid etch such as porosity or other discontinuities; 2) (fluoride). Ammonium bifluoride etch Summary if the customer does not want to pay for solutions and hydrofluoric acid/nitric acid Review of several questions in Products inspection, demand a premium cast mixtures are often used to minimise the Finishing Magazine regarding the product; 3) alter/improve/make the best of effects of silicon at the surface. anodizing of die castings illuminated four the surface with chemical cleaning and Due to the complexities of the chemical broad areas for discussion: alloy selection, pretreatment; 4) optimize the anodizing pretreatments and the variety of elements substrate surface treatment, anodizing process with proper electrolyte acid and surface dirt that they remove, it is process parameters, and substrate quality.

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Through extensive literature research, aluminium; acid cleaners ‘desmut’ the Conclusion scientific evaluation, both theoretical and surface, removing alloying elements exposed There is no ‘trick’ to anodizing complex applied, through failure analysis, and, with during alkaline cleaning. Acid-based fluoride alloys; there are ways, grounded in practical experience, the conclusion follows etches or buffered bifluoride formulations scientific reason, to approach challenges. that obtaining anodic finishes (anodized act to remove silicon. All steps are intended • Start with as homogeneous a substrate aluminium) of acceptable quality on die to yield as much free aluminium, and as surface as possible. This means a clean cast alloys is dependent primarily on the uniform charge distribution at the surface substrate, free of oil and dirt, without substrate quality of the die casting. for anodizing, as possible. casting defects such as porosity These include the of the intersecting the surface, or laps, burrs or substrate, alloy composition, casting Anodizing Parameters: It is commonly seams created by machining, quality, microstructure and surface quality. viewed that increased solution conductivity or blasting of the surface. Substrate quality, therefore, is just as (reducing the solution resistance) will aid in important, maybe more so, than anodizing successful anodizing of complex alloys. • Secondly, as much free aluminium conditions and technique. This is not to be confused with the coil should be present at the surface as Die castings are selected as the manu- anodizing process in which increased possible. This can mean after cleaning, facturing process of choice usually because solution conductivity is necessary to match that the surface is alkaline treated to a number of complex components can be the speed with which the coil is exposed to remove a small amount of aluminium at produced rapidly that will function near the electrolyte. Maximum solution cond- the surface in order to expose alloying net shape. uctivity has been measured at 375 g/l for elements such as copper and silicon. Alloys are first formulated such that they (17) and successful These elements can be removed can be cast; next, that they yield the anodizing is routinely carried out at through a nitric acid desmut operation necessary mechanical properties for the concentrations much less than that of (to remove the copper) and/or a application. Surface finish is not very well maximum conductivity, typically 180 g/l to treatment to remove the silicon such as considered, and although an anodic oxide 220 g/l. a dilute hydrofluoric acid etch or an finish may be desired to impart corrosion With die cast aluminium alloys with ammonium bifluoride treatment. protection and wear resistance or to be a elements such as copper in solution decorative finish, the ramifications of the (phases, not intermetallic compounds or • After a good rinse to prevent dragging anodizing process on a complex alloy is inclusions), it is best to recognise that time these undesired elements into the not typically considered at the design or is required for diffusion of the non- anodizing electrolyte, or leaving them to foundry level. Each factor has an impact on aluminium constituent(s) through the redeposit on the component surface, the interface from which the anodic oxide hydrated aluminium oxide ionic solid as it anodize in an electrolyte whose acid originates and grows: forms. This is achieved by beginning the concentration is efficient enough to process with an incrementally increasing carry the current without corroding the Alloy Selection: The elements which ramp to the appropriate anodizing volt- surface. Depending upon the alloy comprise the various cast aluminium alloys age, if anodizing potentiostatically; or to complexity, this is typically from 180 to play a role in each of the three aspects of the appropriate current density, if anod- 220 grams/liter H2SO4. The current alloy selection: cast-ability, mechanical izing galvanostatically. bias should be applied gently, via a properties, and surface finishing. A basic It has been shown that complex alloys ramp or with intermittant pulses, understanding of the alloys, which can be anodized to sufficient thickness if because time is required to allow for the elements are in solution (like copper, anodized in electrolytes of typical diffusion of the non-aluminium magnesium and zinc), which form concentration, at intermediate to high elements during anodized aluminium intermetallic compounds, precipitates or current densities (1.8 – 3.2 amps/dm2 [16 oxide (AAO) growth. A slower AAO fall out of solution as primary crystals (like – 30 amps/ft2]) at intermediate or room growth will always lead to a more silicon) is critical for developing effective temperature, without compromising uniform anodized finish. approaches to finishing complex die cast hardness. • It may be desired to follow finishing components. with a dilute HNO3 rinse to remove any Substrate Quality: When the anodizing interstitial copper from the AAO Surface Treatment: Interfacial defects process is under control, if things go surface, to enable dye uptake and to are also introduced by external processes wrong with finishing a component, it is prevent isolated spots of copper such as machining or residues from reasonable to conclude that the source for corrosion product from the surface. previous processes. the problem is not the anodizing. It is , grit blasting or other important to know how to proceed to • Rinsing is imperative between process mechanical finishing techniques that are troubleshoot effectively. steps to keep alloying elements from performed to clean and/or place the Understanding component failures from building up in the process tanks. substrate in compression may leave residue the point of view that anodizing develops that can also introduce interfacial the surface of the part is important It is also important to know and contamination and will impede anodic because metal finishing is the last step of understand the component history should oxide formation. Laps, seams and burrs the manufacturing process. Therefore, if anodizing problems arise to connect created in the surface by these processes something unfavourable develops, all eyes manufacturing variations to variations in have distinct charge distribution effects on will be on the metal finisher. anodic oxide appearance or function. the substrate. Protrusions will exhibit At times like these, substrate quality Such understanding enables best possible charge concentrations and roots of needs to be metallurgically evaluated. A alloy selection for manufacturing and substrate discontinuities will exhibit metallurgical engineer need not be empl- metal finishing; appropriate adjustments decreased reactivity due to repulsion forces oyed by the metal finishing plant; however, to the anodizing process, or target areas of set up by the proximity of like charges it is essential to know when one is needed the casting/manufacturing processes during the anodizing cycle. to investigate a root cause for failure. and/or the metal finishing process for Chemical pretreatments are element corrective action. ᔢ specific: Alkaline cleaners remove

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Acknowledgement ASM International, Materials Park, Ohio, 1993. 16 Runge, J., “Interfacial Phenomena and The authors would like to acknowledge Ms 11 Davis, J.R., editor, Alloying, Understanding Anodizing: Ramifications and Process Joy Kaufman of Joyjoycreations, Inc for her the Basics, ASM International, Materials Park, Solutions”, Proceedings of the 17th Annual AAC illustrations that help to explain the Ohio, 2001. Conference, San Francisco, California, 2008. diffusion and structural differences that 12 Kneip, R.; Lamparter, P.; Steeb, S.; Journal 17 Brace, A., The Technology of Anodizing occur during anodic oxide formation. of Applied Chemistry, 101. 1989, 7., 975 – 977. Aluminium, 3rd Edition, Interall Srl, 2000. 13 Runge, J., “Formation of Porous Anodic References Oxide Finishes – A New Approach and Theory”, Contact 1 – 8 Chesterfield, Larry. 2001 – 2009 Proceedings, Aluminium 2000, 2007. Comprehensive Metallurgical Consulting, Aluminum Anodizing Clinic, Products Finishing, 14 Van Vlack, L., Elements of Materials Science Lombard, Illinois, USA, Vol. 65 No. 11 - Vol. 74 No. 2 and Engineering, 4th Edition, Addison-Wesley Email [email protected], 9 North American Die Casting Association, Publishing Company, Reading, MA, 1980. www.compmetconsulting.com www.diecasting.org, FAQ, Introduction. 15 Shewmon, P., Diffusion in Solids, 2nd Anodizing Technologies, Inc, Indianapolis, Indiana, 10 Davis, J. R., editor, ASM Specialty Edition, McGraw-Hill Series in Materials Science USA, Email [email protected], Handbook, Aluminum and Aluminum Alloys, and Engineering, 1989. www.anodizingtechnologies.com

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